Identifying the mechanisms that promote local species coexistence is a long-standing problem in ecology, as it would help us understand not only if, but how, and why biological diversity is maintained or lost. Local species coexistence can be achieved, in part, when differences between species cause them to experience stronger demographic effects of increasing conspecific than heterospecific densities. When present, these ‘stabilizing’ effects give species a demographic advantage when rare and can therefore stave off competitive exclusion. Much of our understanding for how species might coexist via stabilizing effects has focused on ecological interactions. Yet, there is no pre-requisite that ecological interactions alone are what shapes the ability for species to locally coexist. This might especially be the case for species with complex life cycles, where different mechanisms can act during early vs. later life stages. We performed field experiments manipulating species relative frequencies in larvae and adults of two species of damselflies across three lakes and tested the effects of different mechanisms that potentially affect their coexistence: competition, predation, and sexual conflict. We then parameterized a demographic model that incorporates carry-over effects across the life cycle and determined the relative effect of each mechanism generating frequency dependent population growth.
We found that local species coexistence is infrequent and constrained. This emerges because trade-offs associated with different mechanisms regulating populations across the life cycle vary in their effects between species. In one species,larval growth, and adult fecundity consistently act in opposition to larval mortality, collectively decreasing population growth rate when both common and rare. However, in the second species, adult fecundity, larval growth, and mortality act synergistically, increasing population growth when rare. Thus, while trade-offs are often thought to promote coexistence, our results reveal that trade-offs among different vital rates across life stages can instead constrain coexistence. These results also suggest that to fully understand why species do not locally coexist, we need to determine how different mechanisms can interact across the life cycle. In species with complex life cycles this task is particularly important, as ontogenetic environmental switches can lead to cascading effects between juvenile and adult stages, highlighting the importance of testing multiple mechanisms. Greater consideration of both ecological and non-ecological species interactions will be necessary in this endeavor.