Our Future: Hot n’ Tasty?

Climate change. It’s going to wreak no small amount of havoc on mother nature (and if you’re reading this but think all of this climate change stuff is poppycock, please visit Skeptical Science and then come back). How good of a guide is our intuition for what will happen?

This is a great question when it comes to predator-prey relationships and the food chain. One may well think that, heck, from first principles we know that adding heat to a system speeds things up. So, you know, things should continue just as they are – just faster, and maybe with some range-limits and annual timing moved around a bit.

This is quite a sensible proposition. It’s also wrong.

Biomass of phytoplankton and zooplankton under different temperature regimes. Images from Wikipedia.

Recent work from Mary O’Connor and colleagues in PLoS Biology as well as Oliver Beveridge and colleagues in the Journal of Animal Ecology points out some intriguing and non-intuitive effects of heating up food webs – in two completely different systems no less.

O’Connor et al. examine the relationship between phytoplankton and their predators under nutrient enriched conditions. While the little green guys grow more quickly (per capita primary production goes up with increasing temperature), this doesn’t matter to voracious copepods. Their metabolism is sped up even faster, leading to more and more copepods and less and less phytoplankton at higher temperatures.

But what about layering a little complexity on top of that? Beveridge et al. look at a three-level food chain, with bacteria at the bottom, a protozoan consumer, and a ciliate top predator. They cross a manipulation of number of links in the food chain against an increase in temperature and let things run for 6 weeks. While cranking up the heat leads bacteria activity to pick up markedly (and even moreso when their consumer is around), temperature crossed with food chain length does some funny things.

Density of different organisms under conditions of differing food chain length and temperature. Note, if you like the knitted bacteria, you can find the patterns at loxosceles - http://loxosceles.org/crafty/bacterium.html.

Depending on the number of links in the food chain, the relationship between temperature and bacterial density is positive, negative, or U-shaped. In contrast, their consumer, Colpidium, increases where it’s warm when there are no predators, but markedly declines with temperature when it’s predator is around. It’s predator, Didinium, increases in density only at intermediate temperatures. Again, shifts in metabolism and predation rates throughout the food chain appears to be key.

Together, the two studies suggest a dynamic interplay between metabolic activity, rates of predation, and population dynamics. Different levels of a food chain can be affected in very different ways. The simple faster-herbivore-kill-kill scenario is sadly discredited. Rather, we need an understanding of how warming will affect different types of organisms’ rates of growth, death, and predation. Only then can we determine climate change’s dynamic impact on food webs.

O’Connor, M., Piehler, M., Leech, D., Anton, A., & Bruno, J. (2009). Warming and Resource Availability Shift Food Web Structure and Metabolism PLoS Biology, 7 (8) DOI: 10.1371/journal.pbio.1000178

Beveridge, O., Humphries, S., & Petchey, O. (2010). The interacting effects of temperature and food chain length on trophic abundance and ecosystem function Journal of Animal Ecology, 79 (3), 693-700 DOI: 10.1111/j.1365-2656.2010.01662.x

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