Experimental Evidence for the Population-Dynamic Mechanisms Underlying Extinction Cascades of Carnivores

Dirk Sanders,Rachel Kehoe,F.J Frank Van Veen

doi:10.1016/j.cub.2015.10.017

Abstract

Species extinction rates due to human activities are high, and initial extinctions can trigger cascades of secondary extinctions, leading to further erosion of biodiversity. A potential major mechanism for secondary extinction cascades is provided by the long-standing theory that the diversity of consumer species is maintained due to the positive indirect effects that these species have on each other by reducing competition among their respective resource species. This means that the loss of one carnivore species could lead to competitive exclusion at the prey trophic level, leading to extinctions of further carnivore species. Evidence for these effects is difficult to obtain due to many confounding factors in natural systems, but extinction cascades that could be due to this mechanism have been demonstrated in simplified laboratory microcosms. We established complex insect food webs in replicated field mesocosms and found that the overharvesting of one parasitoid wasp species caused increased extinction rates of other parasitoid species, compared to controls, but only when we manipulated the spatial distribution of herbivore species such that the potential for interspecific competition at this level was high. This provides clear evidence for horizontal extinction cascades at high trophic levels due to the proposed mechanism. Our results demonstrate that the loss of carnivores can have widespread effects on other species at the same trophic level due to indirect population-dynamic effects that are rarely considered in this context.

Highlights

Sustained harvesting of the parasitoid A. megourae (Figure 1) led to its functional extinction and markedly increased the extinction rate of the two other indirectly linked parasitoid species Lysiphlebus fabarum and Aphidius ervi (Figure 2; Table S1; Cox proportional hazards model harvesting effect z = 3.53, p < 0.001), but only in the treatments with a uniform aphid distribution (Figure 2; Table S1; harvesting 3 aphid distribution z = 3.53, p = 0.0310)
We assembled replicate food webs, consisting of herbivorous aphids and carnivorous insect species (Figure 1), in 40 field mesocosms. We applied to this a factorial experiment manipulating (1) human impact on one carnivore species and (2) the potential for interspecific competition among herbivores, by having all aphid species either uniformly distributed or aggregated (Figure 1)
Interspecific competition was manipulated without altering food web structure

Summary

Introduction

We assembled replicate food webs, consisting of herbivorous aphids and carnivorous insect species (Figure 1), in 40 field mesocosms. We applied to this a factorial experiment manipulating (1) human impact on one carnivore species (harvesting of the parasitoid wasp Aphidius megourae leading to functional extinction, which is defined as a significant reduction in its abundance that markedly weakens the interaction strength with its host) and (2) the potential for interspecific competition among herbivores, by having all aphid species either uniformly distributed or aggregated (Figure 1).

Results

Conclusion