Broadly-inflicted human threats reduce ecological redundancy and resilience by changing the competitive landscape

Background/Question/Methods

There are several ways in which humans inflict mortality simultaneously on multiple species in a single community, toxic pollution and generalist harvesting methods (e.g. trawling and seining) being notable examples.  In addition to the direct impacts of these activities, there is mounting evidence that they can also have indirect effects on species and ecosystems by influencing biotic interactions.  However, these indirect effects can be difficult to predict, as they are diverse and often context dependent.  Here, I use a combination of analytical methods and simulations to explore the indirect impacts of broadly-inflicted harm on competitive communities, with two broad objectives: 1) to identify species- and ecosystem-level impacts that are found broadly across different models; and 2) to develop simple heuristic approaches to minimizing these impacts that can be applied in empirical contexts with little data.       

Results/Conclusions

I show, in a variety of different models, that inflicted mortality levels that would have minimal direct impacts are often sufficient to eliminate species indirectly by making them uncompetitive, particularly when species with very similar ecological niches (i.e. needs and limiting factors) are impacted asymmetrically.  This suggests that broadly-inflicted anthropogenic stresses on ecosystems could reduce ecological redundancy by limiting the functional similarity of species in a community, likely leading to reduced ecological resilience.  In multi-species fisheries, I show that the likelihood of indirectly-caused extinctions can be dramatically reduced by using fishing gear that is niche-selective in addition to being size-selective.  This ensures that ecologically similar species are more symmetrically impacted.  Using bait in fisheries targeting top-predators is a possible example of such a method of niche-selectivity.