Demography to infinity and beyond!

One of my favourite manuscripts provides a detailed account as to why evolutionary biologists should be demographers (Metcalf & Pavard 2007). The authors argue that, because the propagation of genes into future generations depends on the st/age-specific vital rates of survival, fecundity and migration of individuals within and between populations, and such rates are precisely at the core business of demography, a formal link exists between both disciplines… but that this link has not been explored to its full potential. Based on the strong belief that the ramifications of demography are indeed much broader than “just” the population, and that they reach out to evolutionary biology, and also to genetics, physiology, conservation biology, and community and landscape ecology, we (below) organised the symposium “Demography beyond the Population”, which took place two weeks ago (March 24-26 2015) in Sheffield, UK. This symposium was supported by the British Ecological Society, and brought together ca. 100 delegates from all four corners of the world (e.g. UK, USA, Russia, French Guiana, Brazil, Australia, Mexico).

BES merchandise

In case you were not able come to Sheffield or follow us on Twitter (Alden Griffith (@alden_griffith) and myself (@DRobcito) tweeted frequently using #BeyondDemog), allow me to virtually walk you through some of the highlights of the symposium. This demographic extravaganza actually started on Monday March 23rd with five workshops on integral projection models (Metcalf et al. 2013, Merow et al. 2014), age-by-stage matrix population model decompositions (Caswell 2012; Caswell & Salguero-Gomez 2013), continuous physiological models (de Roos 2008) and Bayesian survival analyses (Colchero et al. 2012). These four workshops showcased the vast richness of analytical approaches to analyze individual-level demographic records, even in the presence of uncertainty (BaSTA). To me, however, the greatest highlight of that day was a mini-workshop ran over lunch by Marco Visser on the importance of programming efficiently for demographic analyses. Marco and his colleagues recently published a manuscript with great tips for ecologists and evolutionary biologists (Visser et al. 2015), resulting from a working group organized at the Max Planck Institute for Demographic Research in 2012. Marco’s presentation, based on this publication, emphasized the need to think very carefully how to expedite calculations: when should you get a more powerful computer?, when to bother with parallel core processing?, or simply how does one track which lines of code are taking most RAM (and how to make them faster)? I have learned a lot from the useful advice from Marco, and I have already started to apply it to my own programming. Certainly Marco is an early career ecologist to keep in the radar due to his already important scientific and computational contributions to the field (https://github.com/MarcoDVisser).

The official start day of the symposium, Tuesday, and the rest of it, took place at Cutler’s Hall. To say that I’ve never been to a conference venue like would be quite an understatement… I’ve never been to a wedding venue like this one either! Great corridors, a broad, red-carpeted stair with a large painting of the queen welcoming us at the end, and then the English grandeur of the conference room.

Cutlers Hall

The symposium covered a wide range of topics at the forefront of demography and its application. The invited speakers included Yvonne Buckley, Johan Ehrlén, Hal Caswell, Steve Ellner, Elizabeth Crone, Jordan Golubov, Dave Hodgson, Eelke Jongejans, Frank Schurr, Maria del Carmen Mandujano, Shripad Tuljapurkar, Mark Rees and María Uriarte. Collectively, they highlighted the importance of including major axes of variation in demography (variation through space, through ecological time, through evolutionary time, and through disciplines) to better understand the context in which demographic processes operate and how far one can project in time before projections become too uncertain. I was particularly excited to see Elizabeth Crone’s presentation about how to model the variation in vital rates as a function of time, space and plant’s “personalities”. The latter, personalities, is a term commonly used in human demography (Chen et al. 2006) whose analyses are finally making their way into non-human ecology and evolution. Maria del Carmen Mandujano & Jordan Golubov gave a combo-presentation drawing from phylogenetic analyses and demographic field data in the Cactaceae to evaluate the role of seedbanks (one of the big unknowns in plant demography), and a novel approach to incorporate genetic structure into matrix population models in an Opuntia species.

One of the main premises in demography is that individuals contribute to the population differently as a function of their age, stage or size. Honouring this fundament of demography, and having admittedly only briefly covered above some contributions by more senior scientist contributed to the symposium, I have the full intent to focus what follows in early career folks exclusively. Demography has in the last years experienced a rather prominent recruitment event (yep, pun intended!), whereby a sizable number of early career ecologists and evolutionary biologists are making very important, dynamic contributions. As a result of their oral presentations and accompanied posters, three students were recognized with awards sponsored by Journal of Ecology and the British Ecological Society. The two runner-ups were Julia Barthold and Maria Paniw. Julia and Maria presented respective state-of-the-art research lines on the estimation of unobserved demographic processes such as mortality rates in migrant male lions (Barthold), and seedbank rates of another carnivorous (plant) creature: Drosophylum lusitanicum (Paniw). The winner of the student award was Edgar Gonzalez, based on his work on the “inverse demographic problem”. This challenge consists on the determination of individual-level vital rates based on the observation of population structures, and it is one of the long-standing unresolved questions in our field. Interestingly, and perhaps not by coincidence, Edgar, Julia and Maria’s approaches were based on Bayesian statistics, further supporting my thought that we will all soon be talking priors and posteriors. If you haven’t taking at least an intro course in Bayesian statistics… you might soon become rusty in population studies!

Other very interesting presentations included the decomposition of individual heterogeneity effects onto whole-population dynamics by Merel Jansen, based on a publication in Journal of Ecology (Jansen et al. 2012), bee demography using integral projection models (IPMs) by Natalie Kerr, the examination of which climate variables affect most the demography of a rare orchid also using IPMs by Sascha van der Meer, or the decomposition of plant-animal interactions on a plant population using (you guessed it!) IPMs by Zdenek Janovsky. Matrix Population Models (MPMs; Caswell 2001) and IPMs (Easterling et al. 2000, Metcalf et al. 2013, Merow et al. 2014) were indeed a recurrent tool throughout the symposium, as it was the recently launched COMPADRE Plant Matrix Database , published this year in Journal of Ecology (Salguero-Gómez et al. 2015), and used by Dave Hodgson, Shaun Coutts, Anna Csergo, Yvonne Buckley and myself on separate talks to explore questions as broad as the structuring factors of transient dynamics, the role of phylogenies and geographic proximity on population dynamics, or the connection between traits and life history strategies.

An important advancement in the field of plant population ecology is underway. I already commented in an earlier blog on the need to replicate plots through space more extensively to really encompass the range of demographic responses and underlying mechanisms that (plant) species may display. Suzzane Lommen has been conducting a European-wide project in the last years coordinating data collection of the invasive ragweed by researchers. Maria Begoña García shared with us her valuable experiences in organizing and engaging with the general public to collect demographic information on various endangered plant species in NE Spain. Last by not least, Yvonne Buckley ran a break-out group discussion on the recently launched PlantPopNet, a globally replicated demographic census of the cosmopolitan Plantago lanceolata with the ultimate goals of trying to understand the local, regional and global determinants of population dynamics in the wild (http://plantago.plantpopnet.com/).

But of course most science developed not in the main hall, but away from oral presentations. We, the organizers, made sure to have ample and frequent coffee breaks and to work with our local co-organizer, Dylan Childs, to give the symposium participants a true Sheffield experience. Furthermore, some social events were planned throughout the symposium, which included an informal networking event at a local pub sponsored by the BES journals and a brewery tour on the Wednesday night. I have since heard great things about the tour of the brewery!

The Fat Cat

Ultimately, I hope that this symposium will have been thought-provoking to all its participants and that we will all go back to our home institutions with a better understanding of what the big questions are, and what directions we should push next to take “demography beyond the population”. And now that we have almost recovered from this symposium, all co-organizers are working with five BES journals (Journal of Ecology, Journal of Animal Ecology, Journal of Functional Ecology, Journal of Applied Ecology and Methods in Ecology & Evolution) to put together the first-ever cross-BES-journals special feature… stay tuned!

I must also express my greatest gratitude to Lauren Sandhu, Andrea Baier, Amelia Simpson, Amy Everard, Richard English and the rest of the fantastic BES staff team for the excellent support prior to and during the symposium. It has been a pleasure working with the BES. I can’t wait for the 13-16 December BES annual meeting!

Rob Salguero-Gómez

Associate Editor, Journal of Ecology

Information about the symposium organizers:

Here is a picture that Amelia took of all the symposium organizers at the end of the symposium.

Organisers 2

(L-R)
Rob Salguero-Gómez (Associate Editor Journal of Ecology)
The University of Queensland (Australia)
Max Planck Institute for Demographic Research (Germany)
Cory Merow
Fish & Wildlife Service (USA)
University of Connecticut (USA)
Alden Griffith
Wellesley College (USA)
Dylan Childs (AE Journal of Animal Ecology)
Sheffield University (UK)
Jess Metcalf (AE Methods in Ecology & Evolution)
Princeton University (USA)
Sean McMahon (AE Methods in Ecology & Evolution)
Smithsonian Environmental Research Center (USA)

References:

H Caswell (2012) Matrix models and sensitivity analysis of populations classified by age and stage: a vec-permutation matrix approach. Theoretical Ecology
5: 403–417.

H Caswell & R Salguero-Gómez (2013) Age, stage and senescence in plants. Journal of Ecology 101: 585-595.

Y-F Chen, C-M Wang & H-J Lin (2006) Explore the relationships among demography, personality traits and self-directed learning. The Journal of Human Resource and Adult Learning Nov: 141-150

F Colchero, OR Jones & M Rebke (2012) BaSTA: an R package for Bayesian estimation of age-specific survival from incomplete mark-recapture/recovery data with covariates. Methods in Ecology and Evolution 3: 466–470.

MR Easterling, SP Ellner & PM Dixon (2000) Size-specific sensitivity: applying a new structured population model. Ecology 81: 694-708.

M Jansen, PA Zuidema, NPR Anten & M Martinez-Ramos (2012) Strong persistent growth differences govern individual performance and population dynamcis in a tropical forest understory palm. Journal of Ecology 100: 1224-1232

C. Merow, JP Dahlgren, CJE Metcalf, DZ Childs, MEK Evans, E Jongejans, S Record, M Rees, R Salguero-Gómez, SM McMahon (2014) Advancing population ecology with integral projection models: a practical guide. Methods in Ecology and Evolution 5: 99-110.

CJE Metcalf & S Pavard (2007) Why evolutionary biologists should be demographers. TRENDS in Ecology and Evolution

CJE Metcalf, SM McMahon, R Salguero-Gómez & E Jongejans (2013) IPMpack: an R package for integral projection models. Methods in Ecology and Evolution 4: 195-200.

AM de Roos (2008) Demographic analysis of continuous-time life-history models. Ecology Letters 11: 1-15.

R Salguero-Gómez, OR Jones, CR Archer, YM Buckley YM, J Che-Castaldo, H Caswell, A Scheuerlein, DA Conde, A Baudisch, E Brinks, H de Buhr, C Farack, F Gottschalk, A Hartmann, A Henning, G Hoppe, G Römer, J Runge, T Ruoff, J Wille, S Zeh, D Vieregg, R Altwegg, F Colchero, M Dong, D Hodgson, H de Kroon, J-D Lebreton, CJE Metcalf, M Neel, I Parker, T Takad, T Valverde, LA Vélez-Espino, GM Wardle, M Franco & JW Vaupel (2015) The COMPADRE Plant Matrix Database: an online repository for plant population dynamics. Journal of Ecology 103: 202-218.

MD Visser, SM McMahon, C Merow, PM Dixon, S Record & E Jongejans (2015) Speeding up ecological and evolutionary computations in R; Essentials of high performance computing for biologists. PLOS Computational Biology DOI:10.1371/journal.pcbi.1004140

Eminent Ecologist Virtual Issue: In Honour of Deborah Goldberg

I am honored by the Editors’ decision to put together this Virtual Issue of some of the papers I have published in the Journal of Ecology and grateful for this opportunity to reflect on them—it has been many years since I have read most of these papers. I have been fortunate to work with an extremely talented group of students, postdoctoral fellows, and collaborators over the years; much of this work is more due to their efforts than my own. The ten papers provide a record of the major themes we have pursued and, without too much distortion, can be put into three groups.

1. Defining competitive ability and discerning associated traits

Goldberg, D.E. and L. Fleetwood. 1987. Competitive effect and response in four annual plants. Journal of Ecology 75:1131-1143.
Goldberg, D.E. and K. Landa. 1991. Competitive effect and response – hierarchies and correlated traits in the early stages of competition. Journal of Ecology 79:1013-1030.
Baraloto, C., P.M. Forget, and D.E. Goldberg. 2005. Seed mass, seedling size and neotropical tree seedling establishment. Journal of Ecology 93:1156-1166.

My earliest papers in Journal of Ecology were associated with my attempt to clarify the definition of competitive ability by distinguishing between competitive effect and competitive response. As part of my dissertation work at the University of Arizona investigating the mechanisms underlying vegetation patterns in the Sierra Madre of northern Mexico, I had tested the hypothesis that evergreen species were poorer competitors than deciduous species by comparing response of seedlings of both to the presence/absence of surrounding vegetation in field experiments. While this made obvious sense in the context of the problem I was addressing, it didn’t match with many experiments in the literature where competitive ability was measured as the ability to suppress other plants. Musing about this led to the obvious-in-retrospect distinction between competitive effect and response. Although sometimes portrayed as distinct measurements, they are better defined in terms of how the same measurement is compared. Competitive response compares the degree to which different “target” taxa are suppressed by neighbours, while competitive effect compares the degree to which different “neighbor” taxa suppress targets. I first published this distinction in 1983 in the American Journal of Botany, where I also argued that competitive effects and responses should be measured and compared on a per-unit size basis using an additive design and that different traits would likely be correlated with competitive effect vs response. The 1987 Journal of Ecology paper was my first attempt to assess the latter and was based on an experiment with four species conducted with Linda Fleetwood, an undergraduate student at the time. The 1990 paper was a more comprehensive experiment, looking at all combinations of 7 species as both target and neighbor, as well as measurements of 6 different traits that had been proposed to relate to effect or response. Again, this experiment involved undergraduates; this time an entire class of students in the introductory ecology course I taught. The students designed small experiments involving subsets of species and Keith Landa, the graduate student course coordinator, realized that with a few additional suggestions to students, we could also get a complete competitive matrix to compare effect and response hierarchies and associated traits. As expected, these studies showed plant size was a primary determinant of per-individual competitive effect and RGRmax was a primary determinant of per-gram competitive effect. In contrast, competitive response showed the reverse trends and was uncorrelated with competitive effect. This work laid the foundation for the next step in this research theme, digging deeper into the mechanisms of resource competition, how effect and response related to resource use and then how this distinction between individual-level effect and response could help clarify the “Grime-Tilman” debate about the community consequences of competition in plants—the latter explored in a book chapter around that time (Goldberg 1990).

Also related to this set of papers is the work of my PhD student Chris Baraloto, who eschewed the old fields and wetlands of the North America Midwest and the deserts of Israel where I have done most of my field work in favor of the tropical forests of French Guiana. Chris was interested in the processes maintaining diversity and focused on various traits related to seedling establishment in different microhabitats—essentially the traits related to competitive response. Chris’s 2005 paper in Journal of Ecology in 2005 shows a clear advantage to large seeds for seedling survival and size across 8 species regardless of microhabitat, contrary to some models of coexistence based on seed size variation.

2. Community level consequences of interactions

Goldberg, D.E. and G.F. Estabrook. 1998. Separating the effects of number of individuals sampled and competition on species diversity: an experimental and analytic approach. Journal of Ecology 86:983-988.
Zamfir, M. and D.E. Goldberg. 2000. The effect of initial density on interactions between bryophytes at individual and community levels. Journal of Ecology 88:243-255.
Rajaniemi, T.K., V.J. Allison, and D.E. Goldberg. 2003. Root competition can cause a decline in diversity with increased productivity. Journal of Ecology 91:407-416.
Farrer, E.C. and D.E. Goldberg. 2011. Patterns and mechanisms of conspecific and heterospecific interactions in a dry perennial grassland. Journal of Ecology 99:265-276.
Herben, T. and D.E. Goldberg. 2014. Community assembly by limiting similarity vs. competitive hierarchies: testing the consequences of dispersion of individual traits. Journal of Ecology 102:156-166.

Another major theme in my research has been around the linkage between individual-level interactions, especially competition, and the community-level consequences of those interactions. I was motivated by two realizations about the large experimental literature on plant. First, almost all experiments on plants measured the consequences of competition for components of individual fitness (Goldberg and Barton 1992), yet the theory of competition was largely focused on coexistence and maintenance of diversity at equilibrium and therefore represented the longer-term population dynamic outcome of interactions. Second, the typical field experiment on competition quantified competitive response to diffuse competition from all neighbor species and so could not compare intra- and interspecific competition, which is fundamental for considering co-occurrence in communities (Goldberg and Barton but see Farrer’s experiments below). Thus, we had (and still have to some degree) a strong discrepancy between the questions asked by experiments on species interactions and by the theory of species interactions. In one strand of my response to this problem, I proposed two different experimental approaches to quantifying the community level effects of competition (Goldberg 1994, Goldberg et al. 1995). Three papers in Journal of Ecology use or expand those approaches. Manuela Zamfir used both approaches as part of her dissertation on competition in bryophytes at Uppsala University, and found that the community density series produced more consistent results than the combined monoculture approach. George Estabrook, a colleague at the University of Michigan, helped me deal with the problem that increasing total community density to increase competition also inevitably can increase diversity by sampling effects. And my students Tara Rajaniemi and Victoria Allison used the combined monocultures approach to separate the community-level effects of root vs shoot competition along a productivity gradient. Rajaniemi et al. (2003) showed that root competition alone could be responsible for decreasing diversity under fertilized conditions—a quite surprising result for many of us who expected that only size-asymmetric light competition could cause this result.

A fourth paper in Journal of Ecology took a different approach to understanding community level consequences. Emily Farrer, then a Ph.D. student at the University of Michigan, directly tested whether intra-specific competition was consistently greater than inter-specific competition in a series of elegantly-designed field experiment in a dry sand prairie. She found that this necessary criterion for coexistence was generally met for interactions among adults, although not for effects of adults on seedlings.

Finally, a fifth paper in this set, written with Tomas Herben of Charles University and the Czech Academy of Sciences, took an entirely different approach to understanding the community consequences of interactions that also incorporates the focus on traits related to competitive ability from the first set of papers described above. Tomas and I were both frustrated with the many papers that inferred mechanisms of community assembly from patterns of over- or under-dispersion of traits but did not often even discuss evidence for the assumed function of those traits. We used an approach I am increasingly convinced is an essential tool for understanding the community and ecosystem consequences of individual-level traits: conducting in silico experiments with highly-parameterized models of real plant communities. In this case, we used validated models of montane grassland and Michigan fens, manipulated the degree of dispersion of one trait at a time, and examined the long-term effect on coexistence and diversity. This kind of experiment is simply not possible with real plants, yet can reveal much about what traits actually do in real communities.

3. Competition along productivity gradients

Goldberg, D.E. and A. Novoplansky. 1997. On the relative importance of competition in unproductive environments. Journal of Ecology 85:409-418.
Suding, K.N. and D.E. Goldberg. 1999. Variation in the effects of vegetation and litter on recruitment across productivity gradients. Journal of Ecology 87:436-449.

Because some of the most consistent patterns in vegetation occur along productivity gradients, understanding how the role of species interactions changes along such gradients is a fundamental challenge in plant ecology. Two papers we published in the Journal of Ecology bear on this problem. Designing experimental treatments to mimic rainfall regimes along a productivity gradient from a desert to a Mediterranean climate brought home to me the importance of considering both frequency and amounts of resource supply. I had also begun to realize that survival and growth responded quite differently to neighbors, with survival more often facilitated and growth more often inhibited. (As an aside, this contrast is yet another reason why individual-level experiments are difficult to scale up to population dynamics.) Together with a postdoctoral fellow from Israel, Ariel Novoplansky, we wove these ideas together into the two-phase resource dynamics hypothesis that predicts quite different patterns for competition along productivity gradients, as well as different patterns for gradients mediated by nutrients and by water.

As a first-year Ph.D. student in my lab, Katie Suding, addressed quite a different aspect of productivity gradients that she argued had been largely overlooked in considering experimental results on competition intensity: whether or not litter was also removed along with living vegetation. She manipulated litter and/or live plants along productivity gradients in several different community types and found that either one alone had largely facilitative effects along the gradient, while the combination of the two resulted in largely negative net effects across most of the gradient.
This 1999 paper was also the beginning of a theme that isn’t well represented in any Journal of Ecology papers, but has become an important part of my thinking about plant communities: the importance of non-trophic interactions (i.e., not involving either consuming resources or being consumed as a resource). It is now well-recognized that facilitation is common in plants, especially in more stressful habitats. But is perhaps less well-appreciated that the nontrophic mechanisms that cause net positive effects are probably much more widespread—what changes among environments is not whether facilitation OR competition occurs, but the relative magnitude of positive (always non-trophic) and negative (often but not always resource competition) mechanisms and so a change in the net balance of those mechanisms. Further, the prevalence of nontrophic mechanisms of interactions involving plant-soil and plant-microclimate feedbacks also implies functional linkages between community and ecosystem ecology. Developing theory that can accommodate the diverse nontrophic mechanisms of interactions yet allows the development of generalizations about plant community and ecosystem dynamics is one of the great challenges for the next generation of ecologists.

Deborah E. Goldberg

Literature cited
Goldberg, D.E. and P.A. Werner. 1983. Equivalence of competitors in plant communities: a null hypothesis and an experimental approach. American Journal of Botany 70:1098-1104.
Goldberg, D.E. 1990. Components of resource competition in plant communities. Pages 27-49 in: J. Grace and D. Tilman (eds.). Perspectives in Plant Competition. Academic Press.
Goldberg, D. E. 1994. On testing the importance of competition for community structure. Ecology 75:1503-1506.
Goldberg, D.E., R. Turkington, and L. Olsvig-Whittaker. 1995. Quantifying the community-level effects of competition. Folia Geobotanica and Phytotaxonomica 30:231-242.

Greetings from… Kagoshima!

This is a guest post by Associate Editor Rich Shefferson, from the University of Tokio, who attended the Annual Meeting of the Ecological Society of Japan last week in Kagoshima.

Greetings from Kagoshima, the site of the 62nd Annual Meeting of the Ecological Society of Japan! Kagoshima is located at the southern end of Kyushu, the third largest and westernmost of the four major islands in the archipelago. Quite far from Tokyo, it represents a unique cultural and historical identity within Japan. It was the political capital of a sprawling and often uncontrollable area of sea and islands between Honshu and Okinawa, was key to Japan’s trade with continental Asia, and was even home to Japan’s homegrown pirate culture at around the time of the European Renaissance. It also served as a stronghold of the samurai counter-revolution against the Meiji Restoration, which modernized Japan in the 1860s through the removal of the shogun and the creation of a parliamentary constitutional monarchy under the Meiji Emperor.

Sakurajima_IMG_3504

Active volano Sakurajima

Ecologically, the city is quite interesting for the presence of Sakurajima – a very active volcano close enough to the city to occasionally spew boulders on the buildings (the city’s various museums are eye-opening in their documentation of such events), and to acidify Kagoshima Bay. Although the last truly major eruption was in 1914, the volcano has had minor eruptions for the last 6 years. Lava flows are under study by ecologists interested in ecological succession and other processes, and by geologists interested in the growth of volcanic islands. Kagoshima is also the gateway to the amazing islands to the south, including Yakushima, a UNESCO World Heritage Site famous for its old-growth forest of Japanese cedars (Cryptomeria japonica), some of which are over 2000 years old.

This year’s meeting has strong symposia and organized sessions, some very much in line with the setting. For example, a session on volcanic ecology documented fascinating research on life on the roughly 10% of the world’s volcanoes that fall within the country’s borders, and the abundance of islands in the country (>6000 total) made island and marine ecology very common themes. Japan’s urban culture also made sessions devoted to urban ecology and ecosystem management particularly abundant and fascinating, and the Fukushima disaster yielded a strong outpouring of research in radio-ecology (e.g., the impacts of cesium contamination on ecological processes at all levels). Journal of Ecology readers would no doubt have been fascinated by strong sessions on more global themes as well, including plant-soil

Highschool ecology poster session at the Annual Meeting of the Ecological Society of Japan

Highschool ecology poster session at the Annual Meeting of the Ecological Society of Japan

interactions, plant and animal evolutionary ecology, regional perspectives on the management of ecosystem services and processes, eco-evolutionary dynamics, speciation, and the management of invasive species. And the Friday poster sessions included one gymnasium devoted to research conducted by high school students – I’ve never seen so many high school students present so much good science!

Very strong science by Japan’s up and coming stars was on display. Of particular interest to me, and potentially to other readers of Journal of Ecology, were the talks and posters devoted to eco-evolutionary dynamics. Masato Yamamichi, who has recently authored a number of papers in American Naturalist, Ecology, and Evolution dealing with the community and ecosystem impacts of rapid evolution and integrated timescales to ecological and evolutionary processes, won one of the 4 Suzuki Prizes, given to young investigators for outstanding work. Shunsuke Utsumi, whose work on eco-evolutionary dynamics in plant-herbivore systems is slated for possible inclusion in an upcoming special issue of Journal of Ecology, also presented his work. And Kenji Suetsugu also won a Suzuki Award for his work on the ecology and evolution of mycoheterotrophic plants – plants that have lost the ability to photosynthesize and instead parasitize mycorrhizal fungi.

As a Japanese speaker, and an ecologist living and working in Japan, I find the ESJ meetings to be informative and fun opportunities to advance ecology, and more selfishly, my own research. Similarly to BES meetings, the participants are not afraid to have fun, and this year’s shochu-tasting contest (in which Journal of Ecology editor Richard Bardgett and I both participated) was typical in how much fun ESJ can be. Although ESJ meetings themselves are typically in Japanese, Japanese ecologists are a very inviting and fun-loving bunch, and so even non-Japanese speakers can enjoy themselves immensely. But the society is also currently considering how to bring more ecologists from around the world to Japan for its annual meetings, and for collaborative research in general.

The society is progressively making efforts to open the meetings to ecologists from around the world, and so more than 10% of talks at this year’s meeting were in English. There is hope that this will increase with time, and the society is now particularly open to input on how to achieve a greater international showing. This year, special guests were on the bill, including the Journal of Ecology’s very own Richard Bardgett, who gave a fascinating lecture on plant-soil feedback impacts on ecosystems processes (the Journal of Ecology editorship was well represented!). We anticipate that this trend will continue to bring in more great ecologists from around the world over the coming years.

Next year’s meeting will be held in Sendai, which was the site of some of the worst damage during the 2011 tsunami. This is the largest city in northern Honshu, and serves as a gateway to the Tohoku region. Ecologists wishing to see mainland Japan’s remnant old-growth birch forests and spectacular volcanic mountains, and to learn about research on ecological themes related to the tsunami disaster, should come next year to experience Japanese science at its finest, and to network and collaborate on a global scale.

Rich Shefferson
Associate Editor, Journal of Ecology

Should ecologists be banned from using p-values?

Note from Executive Editor: The following post is written by Journal of Ecology Associate Editor Caroline Brophy in response to an announcement in a psychology journal regarding the use of p-values. The Journal of Ecology will continue to judge the appropriateness of the statistics in submitted manuscripts on a case-by-case basis.


When I saw that a journal was banning p-values and hypothesis testing I felt a momentary fear that my career as a frequentist statistician might be nearing an end. I then paused and reflected on what was really going on here. Are there problems with p-values and confidence intervals in the context of hypothesis testing? Yes, there can be, however, these problems often stem from misguided usage. So please hold off throwing all the statistics books you have lying around your office on the bonfire, for the moment at least.

The journal in question is Basic and Applied Social Psychology and they have banned the use of null hypothesis significance testing procedure (NHSTP, see http://www.tandfonline.com/doi/full/10.1080/01973533.2015.1012991#abstract). Since this is a Psychology journal are there any implications for Ecologists?

Ecologists regularly perform experiments and the statistical analyses they perform will help to tell the story of their data. If a person who knows little about statistics and inferential theory chooses a statistical test for their data arbitrarily, finds a significant p-value and uses this p-value as support for what they were trying to prove (even if a different hypothesis altogether was actually tested) then their conclusions are not likely to be valid. However, if the person understands their data, carries out a preliminary screening of their data through graphical or other summary methods, understands the test they choose to apply (i.e. knows it is appropriate for their data, has validated its assumptions and is aware of its limitations) and presents the results in graphical or tabular form to illustrate the story of the data, then the p-value is a useful tool to quantify the probability of getting a test statistic as extreme or more extreme than what was observed, given the null hypothesis. Are there problems with this? Generally not!

Going back to the question: what are the implications for Ecologists? In summary, if you have good understanding of the statistical tools you are using then there are no implications because you already know that a p-value is just part of a data analysis package, not the be all and end all. If however, you know that you want p<0.05 but not why or what that means, or whether or not your test is appropriate for your data and your hypothesis, then perhaps you should consider a self-imposed ban from using p-values! At least until you have signed up for some statistical courses to improve your basic understanding.

This is of course quite a simplistic view and there are many well documented deeper discussions on the usage of p-values and other outputs of hypothesis testing (see for example the p-value and model selection forum in Ecology at http://www.esajournals.org/toc/ecol/95/3) and your opinion on using (or not using) NHSTP may also be related to your personal statistical philosophy (e.g. Bayesian or frequentist). One thing is for sure, this recent ban has generated a lot of discussion (see for example http://andrewgelman.com/2015/02/26/psych-journal-bans-significance-tests-stat-blogger-inundated-with-emails/) and perhaps this was one of the goals of the journal?! Are ecological journals likely to follow suit? Personally I see the move by BASP as a bad one and think ecological journals are unlikely to make similar bans but rather will continue to respect their contributors’ judgement of their own statistical capabilities.

Caroline Brophy
Associate Editor, Journal of Ecology
Maynooth University
caroline.brophy@nuim.ie
@carolinebrophy5

 

Roberto Salguero-Gómez interviews Hal Caswell

Last year Roberto Salguero-Gómez (Associate Editor, Journal of Ecology) interviewed Hal Caswell and you can listen to the interview in its entirety below.

hal caswell tai chi

Hal doing tai chi. Listen right to the end of the interview for the context

Roberto and Hal are co-authors on “The COMPADRE Plant Matrix Database: an open online repository for plant demography“, which was published in Journal of Ecology in issue 1 of 2015.

 

Roberto is one of the co-organisers of the British Ecological Society’s “Demography Beyond The Population” symposium which is being held in Sheffield in March. Registration is currently open.

Editor’s Choice 103:2

Issue 103:2 of Journal of Ecology will be online very soon. The Editor’s Choice paper for this issue is Early human impact (5000–3000 BC) affects mountain forest dynamics in the Alps by Schwörer et al. One of Journal of Ecology’s Editors, Amy Austin, has written a commentary about the paper below.

Editor’s Choice 103:2

When we think about human impact on the landscape, visions of deforestation in the Amazon, conversion of grasslands for crop cultivation, or even urbanization of rural areas with sprawling housing developments, often come to mind.  But the truth of the matter is that we, as human beings, have been modifying the natural landscape for thousands of years, and in some places much more intensively than others. The Editor’s Choice paper for this issue of the Journal of Ecology (103:2) explores the effects of human activity on forest biodiversity in the European Alps, but the time frame is not what one might expect – humans were altering biodiversity in these forests more than 5000 years ago!

The alpine Lake Iffigsee (2065 m asl.) in the Swiss Alps

The alpine Lake Iffigsee (2065 m asl.) in the Swiss Alps

And what did the humans do?  They burned and cleared the forest, and then burned some more and then grazed their animals. Moreover, all of this happened long before industrial civilization took hold of Europe.  These dramatic land-use changes resulted in alterations in the dominant vegetation, from Abies alba (silver fir), a fire-sensitive species, to broad scale expansion of Picea abies (Norway spruce) between 5000 and 3000 years ago. While simultaneous climate changes were happening, Schwörer et al. were able to link fire frequency and extent, resulting from human activity, to the expansion of P. abies in the palaeoecological record.

Coring platform on Lake Iffigsee that was used to retrieve the sediment cores.

Coring platform on Lake Iffigsee that was used to retrieve the sediment cores.

How did they discover this?  A combination of approaches, including sediment cores for pollen and microscopic charcoal analysis, macrofossils, and radiocarbon dating of vegetation remains, with intensive sampling in two different locations in the Northern Swiss Alps, was used.  The question that comes to mind is, as with many studies exploring human impact, how can we be sure it was humans and not climatic variability that caused these shifts?  What this study presents is a step forward in the empirical demonstration of human impact. Due to the number of other studies in the region examining climatic change and vegetation distribution (Heiri et al., 2003), the authors were able to show that shifting treeline at the upper elevation site was due to human clearing and fire, rather than natural responses to regional climatic variation.  Other studies have evaluated the importance of large herbivores in determining forest openness and vegetation structure in lowland Europe (Mitchell, 2005; Sandom et al., 2014), but this was the first studies to show how human activity related to pastoralism resulted in detectable changes in forest community composition. The authors even went a step further, suggesting that based on their analysis, moderate human activity in the form of grazing and controlled fires could counteract effects of climate change, and keep silver fir from invading the alpine grasslands as global temperatures rise.

Lake Lauenensee (1382 m asl.)

Lake Lauenensee (1382 m asl.)

This study is being published at a time when there is a renewed interest in understanding and using palaeoecological changes, both to establish the magnitude of human-environment interactions in the recent past, but also to use this information as a tool for predictions of human activity in the future. This application of palaeoecological tools was highlighted as priority in a recent review in Journal of Ecology that focuses on 50 key questions for the discipline (Seddon et al., 2014). For this reason, it is a timely and interesting contribution. Take a closer look!

Amy Austin

Editor, Journal of Ecology

References 

Heiri, O., Lotter, A. F., Hausmann, S. & Kienast, F. (2003) A chironomid-based Holocene summer air temperature reconstruction from the Swiss Alps. Holocene, 13, 477-484.

Mitchell, F. J. G. (2005) How open were European primeval forests? Hypothesis testing using palaeoecological data. Journal of Ecology, 93, 168-177.

Sandom, C. J., Ejrnaes, R., Hansen, M. D. D. & Svenning, J. C. (2014) High herbivore density associated with vegetation diversity in interglacial ecosystems. Proceedings of the National Academy of Sciences of the United States of America, 111, 4162-4167.

Seddon, A. W. R., Mackay, A. W., Baker, A. G., Birks, H. J. B., Breman, E., Buck, C. E., Ellis, E. C., Froyd, C. A., Gill, J. L., Gillson, L., Johnson, E. A., Jones, V. J., Juggins, S., Macias-Fauria, M., Mills, K., Morris, J. L., Nogués-Bravo, D., Punyasena, S. W., Roland, T. P., Tanentzap, A. J., Willis, K. J., Aberhan, M., van Asperen, E. N., Austin, W. E. N., Battarbee, R. W., Bhagwat, S., Belanger, C. L., Bennett, K. D., Birks, H. H., Bronk Ramsey, C., Brooks, S. J., de Bruyn, M., Butler, P. G., Chambers, F. M., Clarke, S. J., Davies, A. L., Dearing, J. A., Ezard, T. H. G., Feurdean, A., Flower, R. J., Gell, P., Hausmann, S., Hogan, E. J., Hopkins, M. J., Jeffers, E. S., Korhola, A. A., Marchant, R., Kiefer, T., Lamentowicz, M., Larocque-Tobler, I., López-Merino, L., Liow, L. H., McGowan, S., Miller, J. H., Montoya, E., Morton, O., Nogué, S., Onoufriou, C., Boush, L. P., Rodriguez-Sanchez, F., Rose, N. L., Sayer, C. D., Shaw, H. E., Payne, R., Simpson, G., Sohar, K., Whitehouse, N. J., Williams, J. W. & Witkowski, A. (2014) Looking forward through the past: Identification of 50 priority research questions in palaeoecology. Journal of Ecology, 102, 256-267.

 

Special Feature: Peat’s muddy past

What is so special about peat?  To the untrained eye, these ecosystems appear to be desolate swamps, with limited value, biodiversity- or other-wise.  To the seasoned wetland ecologist, the more apt question is what isn’t special about peat?  These long-neglected ecosystems are vital reservoirs of fresh water for us thirsty humans; they contain 10 times as much carbon as all of the world’s forests, whilst occupying only 3% of the Earth’s surface; and house a rich diversity of species found nowhere else.  Yet as we begin to learn more about the world’s peatlands, we master the technologies needed to exploit them rapidly and irreversibly (Fig. 1).

Fig 1_cole

Fig. 1 A drained peatland in Indonesian Borneo, with young oil palm plants in the foreground and heavily degraded peat swamp forest in the background.

To be scientifically accurate, the irreversible component of peatland conversion is an assumption, wanting of sufficient evidence from “the field” due to the recent nature of large-scale exploitation.  But any ecosystem we see today is a product of its evolving past; a period over which it has encountered disturbances and presented a response.  From these patterns of responses, we can measure the resilience of the ecosystem (Cole et al., 2014a) and develop hypotheses as to how it may respond to future disturbances.  In its simplest form, resilience is described as the ability of an ecosystem to maintain its structure and function despite perturbation (Holling, 1973).

How resilient are peatlands?  Specifically, how have the tropical peat swamp forests of Southeast Asia responded to disturbance in the past?  We sought to answer these questions for the coastal peatlands of Sarawak, in Malaysian Borneo (Cole et al., 2015) (Fig. 2).

The plug is rapidly being pulled on these sweaty, mosquito-ridden jungles as industrial-scale agriculture spreads like wildfire across the region.  Dipterocarp forests, rich in a variety of fruit-bearing trees, ‘black-water’-adapted fish and nimble mammals, are being drained, flattened and converted into monoculture landscapes where oil palms (Elaeis guineensis) can quickly bring economic profit to even the inexperienced farmer.  Though wild-fires themselves are in fact a rare phenomenon in intact peatlands, the recent elevation in burning has been blamed primarily on the recent expansion of agriculture, and brought huge environmental, health and political challenges to the region.  But how frequent were fires in the past?  And what impact did they have on the vegetation?

Unlike in the temperate zone, tropical peat swamps are naturally forested.  It is the pollen grains and fern spores produced by this vegetation that provide the answers to our questions and insights into the resilience of these ecosystems.  Fossilised grains, deposited tens to millions of years in the past, are one of the primary datasets available in palaeoecology.  Often referred to as (the less archaic-sounding) long-term ecology, the discipline extends the scope of ‘short-term’ ecology through using deposited remains to study plants and animals and their interactions with the environments of the past.

We used pollen grains, fern spores and fossil charcoal to explore drivers and impacts of disturbance in three peatland areas in northern Borneo.  Peat cores were collected from three coastal sites in Sarawak (Fig. 2), where peat extends over approximately 13% of the States’ land surface.  The depths of the cores ranged from c. 1.5 to 3m, and radiocarbon dating of sediment samples from each demonstrated that they covered a period of 2000 to 7000 years before present (BP).

fig 2_cole

Fig. 2 The three degraded peatland sites (red circles; DPL, PSF & CPL) from which cores were collected for this palaeoecological study, in Sarawak, Malaysian Borneo. Sarawak’s peatlands are shown in brown and its major towns in blue.

Once I’d spent many more hours than I’d like to remember counting and identifying microscopic pollen grains, I was able to look for answers to our key research questions:

  • How has the vegetation in these peatland ecosystems changed through time?
  • What factors disturbed the peat swamp forest vegetation?
  • How did these ecosystems respond to the different disturbances?

Vegetation change

Our data demonstrated that peat swamp forest vegetation has persisted in these coastal peatlands since the onset of ecosystem development c. 4000yrs BP.  At this time, coastal progradation, resulting from sea-level fall, provided land suitable for peat to accumulate.  Apart from fluctuations between pioneer and mature peat swamp forest species over this period, reflecting local disturbances and dynamic internal responses (Fig. 3), the only significant vegetation change observed was shown in the last 500 years in two of the cores.  Increases in plant taxa associated with degraded peatlands suggested the introduction of humans and land use change to these coastal ecosystems.

fig 3_cole

Fig. 3 Relatively intact peat swamp forest patch, near to the site where the “PSF” core was extracted (Fig. 2).

Disturbances

There were three drivers of vegetation change that we focused on in this study, each with its associated palaeoecological proxy: climatic variability, such as El Niño Southern Oscillation (ENSO) activity, identified through a literature review; local and regional fire inferred from fossil macro- and microcharcoal respectively, and anthropogenic activity indicated by pollen and spores of plants common in open areas, such as grasses (Poaceae) and sedges (Cyperaceae).  The literature reported increasing intensity of ENSO events in this region over the late Holocene, subjecting northern Borneo to arid conditions.  Burning, both from local and regional fires, occurred throughout the past in all three sites, though elevated dramatically in the last c. 500 years in parallel with greater levels of open vegetation indicators.

Vegetation response

Only within the recent past, from c. 200-500 years BP, did the peat swamp forest vegetation show signs of being ‘disturbed’.  Prior to this period, episodes of more intense ENSO and burning did not appear to correspond with notable declines in the peat swamp forest taxa, suggesting ecosystem resilience to these forms of perturbation.  However, given that elevated levels of open vegetation indicators and charcoal do correlate with the declines observed in the recent past, and in the period when literature and interviews suggest humans started exploiting these environments, it is likely that anthropogenic activities are responsible.  Though a lack of sufficient data prevents us from inferring whether these changes equate to a recent loss of ecosystem resilience, we have made several conclusions:

  • These peat swamp forests have shown resilience to natural disturbances in the past;
  • Levels of disturbance within the last c. 500 years have exceeded those recorded in the previous 5000 years, and humans are the main culprits;
  • Recent, coincident instability and declines in peat swamp forest taxa suggest a notable anthropogenic impact on this ecosystem, potentially challenging the future persistence of these forests.

Back to the future

So, what more do we need to know about these vital* ecosystems?  Our study has provided some baseline data and information on the functioning of the coastal peat swamp forests of northern Borneo, but there are many other patches of peat around the island, and indeed the whole region to investigate.  In order to design more sustainable management practices for these unique ecosystems, it is important that we find out more about their ecology, past and present, and in particular their ability to respond to different disturbances.  With fire posing a major threat to the persistence of Southeast Asian peatlands, and the resultant carbon emissions posing a major threat to us, we need to gather insights from patterns of past recovery and build an understanding of peat swamp forest resilience.  And fast, before there’s no mud left.

*I hope I have convinced you of their extreme importance by now!

Lydia Cole

 

References

Cole, L.E.S., Bhagwat, S.A. & Willis, K.J. (2014a) Recovery and resilience of tropical forests after disturbance. Nature Communications, 5:3906, 1-7. Doi: 10.1038/ncomms4906.

Cole, L.E.S., Bhagwat, S.A. & Willis, K.J. (2015) Long-term disturbance dynamics and resilience of tropical peat swamp forests. Journal of Ecology – Special Issue on Forest Resilience. Doi: 10.1111/1365-2745.12329.

Gaveau et al. (2014) Major atmospheric emissions from peat fires in Southeast Asia during non-drought years: evidence from the 2013 Sumatran fires. Nature, 4:6112. Doi: 10.1038/srep06112.

Holling, C.S. (1973) Resilience and stability of ecological systems. Annual Review of Ecology and Systematics, 4, 1–23.

Page, S.E., Siegert, F., Rieley, J.O., Boehm, H.-D.V., Jaya, A. & Limin, S. (2002) The amount of carbon released from peat and forest fires in Indonesia during 1997. Nature, 420, 61–65.


New Special Feature: Forest Resilience, Tipping Points and Global Change Processes

Web ad_forest resilience
How capable are forests in bouncing back from disturbances? Are they becoming less resilient during this time of rapid environmental change? Can forests deal with the changes, or are they approaching critical thresholds (i.e., tipping points) that will result in a permanent shift towards a different ecosystem state? These critical questions in forest research have increasingly been receiving global attention.

Incidentally, we humans are largely responsible for the current local and global changes that are taking place, which are affecting forests in manifold ways. We are, however, at the same time very much dependant on a large range of forest products and services that are critical for our existence. Knowing how forests will change and how this will have a knock-on effect on the services forests provide is therefore of vital importance for our sustainable future.

This Special Feature brings together contributions from the INTECOL 2013 conference and presents research that addresses these important questions and issues across a range of spatio-temporal scales. The papers presented in this Special Feature include plot-level observational (Camarero et al. 2015; Jakovac et al. 2015; Standish et al. 2015), experimental (Holmgren et al. 2015), paleo-ecological (Cole et al. 2015), and global modelling (Steinkamp & Hickler 2015) studies, as well as a synthesis paper covering the current state of affairs in forest resilience and tipping point research (Reyer et al. 2015).

The contributions to this Special Feature foster a deeper understanding of forest resilience and potential tipping points under local and global change. This Special Feature shows that it remains largely unclear how and if local and global change processes reduce resilience and/or whether they can lead to abrupt vegetation composition and/or species shifts. We have to develop a better understanding of the mechanisms and feedback loops involved in forest resilience and potential tipping points, and the aforementioned contributions will proof useful in guiding further research to safeguard sustainable forests.

Niels Brouwers, Christopher Reyer, Anja Rammig & Fanny Langerwisch

Special Feature Guest Editors, Journal of Ecology

#BESsfe on reflection by Ignasi Bartomeus

Last week quite a few members of Journal of Ecology’s Editorial team were in Lille for the joint BES/SFE Annual Meeting. It was a fantastic few days. Associate Editor Ignasi Bartomeus has written about his highlights below.

#BESsfe

Quick summary of the talks that surprised me most:

Lots of Pollination biology talks: the most surprising finding was from K. Ohashi who showed with comparative analysis across plant species and elegant experiments in the lab that old flowers change their colour (reflectance) quite often, especially when these plants are visited by bees (which are central place foragers, and re-visit sites often). Matthias Schleuning also had a nice talk showing strong trait matching between plant floral traits and hummingbird pollinator traits. I wondered why bee-flower matching is not as strong (presumably).

Also lots of networks stuff: M. Dheling showed an amazing method to relate plant and pollinator traits via their interactions in a network. The 4rth corner analysis! I had the kind of moment where you wonder why you didn’t come up with it before, as it seems so obvious once he explained it. The plenary talk by Pedro Jordano was also a great summary of the potential that a network approach has for explaining a variety of questions, ranging from responses to global change to evolutionary process. On a more methodological point, Marta Sales-Pardo tempted me with an algorithm to detect under-sampled links. I may try it soon.

BEF talks: I skipped most of them this time, but I really liked Rudolf Rohr’s talk using a series of Lotka-Volterra models to show that BEF is positive only when no competitive exclusion and no environmental filtering is modelled. Otherwise the relationship is broken. That fits really well with our observation that dominance patterns and differential extinction risks in natural pollination systems also weaken the experimental BEF relationship.

And as always, the best part is the people that you meet and talk with.

Ignasi Bartomeus
Associate Editor, Journal of Ecology

Editor’s Choice 103:1

Editor’s Choice 103:1

Issue 103:1 of the Journal will be online soon. The latest Editor’s Choice paper is “Earthworm invasion, white-tailed deer and seedling establishment in deciduous forests of north-eastern North America” by Dobson & Blossey. Associate Editor Frank S. Gilliam has written a commentary on the paper below.

The herbaceous layer of eastern hardwood forests: small of stature, large on importance

To those of us who do research on the dynamics of the herbaceous layer in forest ecosystems, the essential role of this lowest of vegetation strata in maintaining the structure and function of those ecosystems is no mystery.  To others, however, it may come as a surprise that, despite that its relative biomass is far less than 0.1% of total standing above-ground biomass of forests; the herb layer serves a function that belies its diminutive stature.  For example, up to 90% of more of the plant biodiversity of forest communities is found in the herb layer.  Furthermore, it can represent 5% of net primary productivity and produce nearly 20% of litterfall, material which tends to have far higher concentrations of N, P, K, and Mg, thus facilitating rapid recycling of macronutrients (Gilliam 2014).

photo 4

Geranium maculatum seedling (photo credit: Annise Dobson)

Forest disturbance ecology: a complexity of agents

Another characteristic of the herb layer of forests is its sensitivity to a wide variety of endogenous and exogenous disturbances of both natural and anthropogenic origin.  The more intense and extensive of these disturbances, such as forest harvesting or fire, can initiate secondary forest succession, which actually can be mitigated by composition of the herb layer itself, wherein the herb layer serves as a regeneration filter for seedlings of eventual canopy dominant species.  Other disturbances are perhaps more subtle (since they do not directly impact the forest overstorey), yet are no less profound in their effects.  Essentially all of these are of anthropogenic origin.  These can include plant species invasions (Ehrenfeld 2010), earthworm activity (Hale et al. 2006), and browsing by white-tail deer (Waller 2014).  As is virtually always the case involving human activities that interfere with ecosystem function, these effects often occur simultaneously (Whitfeld et al. 2014) .

Interactive effects of forest disturbance:  deer browsing and earthworm invasion

soil profile

The forest floor profile from as forest invaded (above) and uninvaded (below) by earthworms (photo credit: Grant Dobson)

The Editor’s Choice in this issue of Journal of Ecology is “Earthworm invasion, white-tailed deer and seedling establishment in deciduous forests of northeastern North America,” by Annise Dobson and Bernd Blossey.  The authors tackle the challenge of simultaneous effects of anthropogenic disturbance head-on by establishing a field experiment that manipulates deer browsing and earthworm activity in a factorial design in the Finger Lakes Region of New York.  Among the compelling aspects of this study is that they used transplants of 15 prominent herb-layer species in the temperate hardwood forests of the region, essentially allowing herb composition to be an additional manipulated variable.  Thus, part of the novel aspect of this study was in combining these variables in an experimental setting in the field.

Enemies above versus enemies below

Although most plant ecologists are aware of the deleterious effect that overbrowsing by burgeoning populations of deer can have on forests (Waller 2014), I suspect that many who might read this are at least somewhat surprised to consider earthworms as agents of disturbance in eastern hardwood forests of the United States.  I recall learning as a child that earthworms serve an important role in ‘tilling’ the soil, simultaneously aerating and fertilizing the soil.  The reality, however, is that they can increase decomposition to the extent that there is minimal humus layers in impacted forests, and can enhance herbivore and pathogen attacks (Hale et al. 2006).  Dobson’s and Blossey’s results suggest that earthworm activity (using the surrogate metric of earthworm biomass) decreased seedling survivorship in virtually all herbaceous species (12 out of 15).  In addition, the negative effects of earthworms exceeded that of deer browsing. Interestingly, there were no interactive effects of deer and earthworms.

Lumbricus terrestris and Annise Dobson (photo credit: Grant Dobson)

Lumbricus terrestris and Annise Dobson (photo credit: Grant Dobson)

Some surprising findings:  other trophic interactions

I found this paper to be intriguing for a variety of reasons. As previously stated, it tackles the challenge of multiple stressors by examining them interactively.  Furthermore, the use of transplants of important herb layer species eliminates confounding that can be unavoidable in field studies, wherein ‘replicate’ sites are often not true replicates of each other because of natural spatial variation in herb composition. I also found quite interesting the novel findings regarding other trophic interactions: earthworms increased likelihood of insect attacks on herbs, whereas deer browsing decreased extent of rodent attacks.

Frank S. Gilliam

Associate Editor, Journal of Ecology

References 

Ehrenfeld, J.G. (2010) Ecosystem consequences of biological invasions.  Annual Review of Ecology, Evolution, and Systematics, 41, 59-80.

Gilliam, F.S. (2014) The Herbaceous Layer in Forests of Eastern North America, 2nd edition. Oxford University Press, New York.

Hale, C.M., Frelich, L.E. & Reich, P.B. (2006) Changes in cold-temperate hardwood forest understory plant communities in response to invasion by European earthworms. Ecology, 87, 1637-1649.

Waller, D.M. (2014) Effects of deer on forest herb layers. The Herbaceous Layer in Forests of Eastern North America, 2nd edition (ed F.S. Gilliam), pp.369-399. Oxford University Press, New York..

Whitfeld, T.J.S., Roth, A.M., Lodge, A.G., Eisenhauer, N., Frelich, L.E. & Reich, P.B. (2014) Resident plant diversity and introduced earthworms have contrasting effects on the success of invasive plants. Biological Invasions, 16, 2181–2193.