Understanding the direct and indirect impacts of temperature variation on a tri-trophic food chain: A theoretical approach

Background/Question/Methods

How will food webs respond to temporal fluctuations in temperature? Temperature can directly impact population abundances of ectotherms, by changing their vital rates. Moreover, populations are indirectly effected by temperature through their interactions with other populations (eg. temperature increasing the capture rate of a species' predator). How will temporal variation in temperature impact food webs made entirely of ectotherms (eg. plankton, insects, reptiles, etc), where all populations' vital rates are fluctuating? Moreover, the goal of this research is to increase our understanding of how environmental variance will propagate through trophic levels.

We modeled a tri-trophic food chain, where population’s vital rates were size dependent, in an environment with fluctuating temperature. Per capita growth and mortality rates were a function of a population’s average body size ie. smaller bodied individuals grew faster than their larger counterparts. An allometric approach reduced the dimensionality of the system allowing for increased interpretability. Consumer functional responses and the intrinsic rate of increase of the producer were a unimodal function of temperature. We generated temperature time series with different mean, variance, and autocorrelation and in a series of numerical simulations measured the population responses to different temperature regimes. We measured the mean, variance, and autocorrelation of each trophic level and saw if body size changd how a trophic level would express variation. 

Results/Conclusions

We found that temperature variation impacted all trophic levels, however body size determined which trophic level experienced the most variation. Faster growing predators showed faster fluctuations that buffered against fluctuations in the lower trophic levels. The autocorrelation of population abundances was also body size dependent with small bodied consumers removing all the low frequency variation from other trophic levels. For example, a fast growing top predator's population dynamics were dominated by fast fluctuations while its prey showed highly autocorrelated fluctuations in population size, even in highly autocorrelated environments.

In conclusion, our model gives us increasing insight into future impacts of climate change. Our use of an allometric scaling reduced the high dimensionality and increased biologically realism of our model, and instead allowed us to focus on the direct and indirect effects of temperature fluctuations. We found that small body sized individuals will experience the most variance but that the variation a population exhibits is dependent on the interactions with other species.