Habitat loss and fragmentation increase realized predator-prey body size ratios

In the absence of predators, habitat fragmentation favors large body sizes in primary consumers with informed movement due to their high gap-crossing ability. However, the body size of primary consumers is not only shaped by such bottom-up effects, but also by top-down effects as predators prefer prey of a certain size. Therefore, higher trophic levels should be taken into consideration when studying the effect of habitat loss and fragmentation on size distributions of herbivores.\n\nWe built a model to study the effect of habitat loss and fragmentation within a simple food web consisting of (i) a basal resource that is consumed by (ii) a herbivore that in turn is consumed by (iii) a predator. Our results highlight that predation may result in local accumulation of the resource via top-down control of the herbivore. As such, the temporal and spatial variation of the resource distribution is increased, selecting for increased herbivore movement. This results in selection of larger herbivores than in the scenario without predator. As predators cause herbivores to be intrinsically much larger than the optimal sizes selected by habitat fragmentation in the absence of predators, habitat fragmentation is no longer a driver of herbivore size. However, there is selection for increased predator size with habitat fragmentation as herbivores become less abundant, favoring gap-crossing ability of the predator. Since herbivore and predator body size respond differently to habitat loss and fragmentation, realized predator-herbivore body size ratios increase along this fragmentation gradient. Our model predicts the dominance of top-down forces in regulating body size selection in food webs and helps to understand how habitat destruction and fragmentation affect overall food web structure.