Empirical evidence for a stabilizing “Type 1.5” functional response

Background/Question/Methods

              One of the most pervasive ecological models, the Rosenzweig-MacArthur model, predicts that simple communities should become unstable under moderate nutrient enrichment; however, natural communities often fail to match these predictions.  One of the main components of the Rosenzweig-MacArthur model, the Type 2 functional response, assumes that predator consumption will reach an asymptote, such that at sufficiently high prey densities, increases in prey density will not lead to increases in consumption.  In natural systems, certain biological mechanisms can lead to nonlinear monotonically increasing functional responses that do not approach an asymptote for any natural prey density (what we refer to as a Type 1.5 functional response). 

We ask whether the Type 2 functional response describes most predator-prey systems.  Specifically, we ask, does the Type 2 functional response fit experimental data better than the Type 1.5?  And, if the Type 1.5 does occur, what are the implications concerning community dynamics?

To answer our first question, we performed a meta-analysis on approximately 200  functional response experiments .  Using a sum of squares analysis, we compared the statistical fit of Type 1.5 and Type 2 functional responses.  To understand the dynamical consequences of Type 1.5 functional responses, we used numerical simulations and analytical methods.

Results/Conclusions

               Our meta-analysis shows that approximately 50% of published functional responses that are attributed a Type 2 form are fitted as well, or better, by a Type 1.5 functional response. Although we don’t mean to infer that consumption is actually unlimited, we argue that for many natural systems, prey density may not (or hardly ever) reach a point where consumption approaches an asymptote.  Our numerical and analytical investigations show that the use of a Type 1.5 functional response creates Strong Stabilization: a phenomenon where dynamics are stable throughout a significant non-narrowing range of mortalities, even at infinitely high nutrient levels.  As a consequence, the destabilizing potential of the Rosenzweig-MacArthur and similar models diminishes for a large class of non-asymptotic functional responses.

Our modification to the functional response helps resolve some of the discrepancy between theoretical models and live communities.  Our study also highlights the importance of rechecking accepted dogmas in ecology.  All ecological models are built upon other models, if we do not fully understand low complexity models, how can we expect to predict dynamics in natural communities?