August 8, 2017

Apex predators, or organisms at the top of the food chain, are a prominent example of how animal movement can significantly impact the health of an ecosystem. These predators hunt to remove diseased or weak individuals from the habitat. They also ensure that prey populations do not exceed a level that can be sustained by the ecosystem. Scientifically, this is known as “top-down trophic regulation” (trophic levels refer to levels of the food chain).


Because the ecosystem cannot support them in large numbers, apex predators have evolved their own mechanisms of population limitation. These include slow growth and low reproductive rates. As a result, apex predators are very vulnerable to population decimation. Furthermore, their removal can result in cascading impacts throughout the ecosystem.


One phenomenon of recent focus is “mesopredator release”. Mesopredators are organisms that occupy the middle of the food chain. Mesopredator release occurs when their populations spike in the absence of the predator. This leads to the decline of species further down in the food chain. For example, Myers et al. (2007) noted a dramatic loss of 11 large shark species in the northwest Atlantic. Many of these species consumed other elasmobranchs (i.e. smaller sharks and rays). Their absence has allowed a particular mesopredator, the cownose ray, to flourish. Because these rays feed on scallops, their abundance resulted in a massive decline in scallops. In this case, the loss of apex predators ultimately led to the termination of a scallop fishery (Myers et al., 2007).


Mesopredator release is only one of many detrimental ecosystem effects that can occur when apex predator populations decline. There are many others that are less obvious, perhaps entirely unknown. Yet there is one certainty regarding these species: they play a vital role in the ecosystems in which they inhabit. Consequently, their presence or absence, driven by movement, can be a significant indicator of ecosystem health.