The tortoise and the hare

Dean Pearson is a Research Ecologist based at the Rocky Mountain Research Station, USDA Forest Service. In this blog post, he gives some background to his recent Journal of Ecology paper; The tortoise and the hare: reducing resource availability shifts competitive balance between plant species.

This week is Invasive Species Week – Dean’s paper was chosen to be included in our special Invasive Species Virtual Issue which features some of the finest research on invasives across all the BES Journals


Ecology is the science of nature – its directive is to explain the complex processes underlying the wide and wonderful array of patterns occurring in nature. Accordingly, ecology is best served when scientific questions derive explicitly from natural history observations, such that the science is based on, and feeds back to explain the processes underlying the patterns observed.

JEC-Invasive-Species-300dpiOur study ‘The tortoise and the hare’ was initiated based on the simple observation that spotted knapweed, a highly invasive non-native plant that has overrun grasslands of western Montana, seemed to wither and die during and immediately following severe drought events. At the same time, Montana’s state grass, bluebunch wheatgrass, seemed to thrive at the knapweed’s expense under these extreme conditions. These observations suggested that increased drought conditions might be shifting the balance of competition between these two dominant combatants.

However, complicating the story was the fact that a species of weevil that feeds on knapweed in its native range had been introduced as a biological control agent in Montana a decade earlier in an effort to suppress knapweed’s populations. As a result, many biocontrol practitioners attributed declines in knapweed to increasing effectiveness of this biological control insect. These different perspectives have instigated debates regarding the real cause underlying the knapweed declines.

The tortoise and the hare

From left to right; Bluebunch wheatgrass (Pseudoroegneria spicata), Root boring weevil (Cyphocleonus achates), Spotted knapweed (Centaurea stoebe)

In reality, either the drought or the insect could have been causing the knapweed declines. In fact, we suspected that both drought and insect attacks were important, with these forces combining to generate the observed declines in knapweed populations. The challenge in revealing the true story behind this natural history observation was to create an experiment that simulated drought, manipulated the weevils, and also manipulated competition between the two plant species to see which factors were driving declines in knapweed and increases in bluebunch.

Seven years of experimentation indicated that drought was a key driver of competitive outcomes. Whilst the biocontrol agent appeared to add some negative effects under drought and competition, these effects were relatively minor. The winner of the plant competition battles was predicted based on the life history strategy of the players – the slow “tortoise” winning when water was limited and the fast “hare” winning when water was more abundant. The fact that neither species responded to drought when grown without their competitor was surprising. This latter result demonstrates that biotic interactions are important in determining the effects of climate change on species and communities.

Dean Pearson, USDA Forest Service

Read the full paper here.

Photos via Wikimedia Commons. 

 

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