I drink your milkshake!

Perhaps the thing about climate change that worries me the most are “unknown unknowns” — those feedback loops and unpredictable shifts in the way the biological and physical worlds function together that could potentially doom an entire ecosystem. The analogy of a plane has been used to describe biodiversity loss, for example, where each species represents a rivet in the plane. The loss of one probably won’t affect the system, but once enough rivets have been lost — and we have no idea how many that may be — the jet comes apart in mid-air.

I’m not particularly moved by the plane analogy — species do not, in fact, hold ecosystems together like rivets. But a recent paper in the journal *Ecology* has introduced an entirely new kind of unintended consequence: increased predation due to drought.

The science of community ecology — those interactions between and among species at different levels of the food chain — has generally been analyzed along nutrient and energy pathways. As an example, we know that an average wolf consumes a few pounds of meat per day. If we introduce that wolf to a new ecosystem, we can expect a decline in the number of small, furry creatures roughly equivalent to that wolf’s nutrient needs. Such “exchange” of energy and nutrients is basic community ecology.

The authors of this new paper studied a relatively unexplored nutrient pathway: water. Through a pretty clever experimental design involving new and old leaf litter, crickets, and wolf spiders, the authors discovered that wolf spiders tended to eat more crickets when water was scarce, and the crickets preferred to eat fresher leaves over older ones. Conversely, when water supplies were plentiful, crickets didn’t particularly care how old the leaves were and wolf spiders ate far fewer crickets.

The leaves, crickets, and wolf spiders, in ecology-speak, exhibited strong interactions in a drought situation and weak interactions when water resources were plentiful. At both trophic levels (spiders are predators here and crickets are herbivores), the critters ate more of their preferred food type in response to a decline in water availability. In other words, at some point, the spiders and crickets ate more because they had less to drink. Interestingly, we had no idea that these species would respond to drought in this way, which is why the study got published in the top ecological journal in the world.

As the climate changes and precipitation patterns shift around the world, there are many interactions between species and trophic levels that may be altered. We have limited ability to predict these interactions. Should species at higher trophic levels eat their way further down the food chain as a response to water scarcity, the earth’s food webs may prove to be more fragile than we previously considered. What scares me about climate change are the things we don’t know — and that happens to be a whole lot.

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tim

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  1. richard - August 12, 2009

    clearly, they were both after the amount of water required for their own biology. greener leaves = more liquid for the crickets, more crickets = more liquid for the spiders.
    does this imply that because meat provides less liquid to a human diet we will naturally go to a more vegetable diet as meat ‘costs’ more as water to feed animals ‘costs’ more?

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