Evidence of habitat partitioning and competitive exclusion in an amphibian at its range edge

Background/Question/Methods

In order to avoid extinction due to competitive exclusion, species will often resort to habitat partitioning, in which species use different habitats, or different parts of a habitat, in order to reduce interspecific competition. It can be difficult to determine if different habitat use is due to niche partitioning, or habitat selectivity, once species have established in different portions of their habitat. At the edge of ranges, however, species may naturally co-occur and compete strongly. With global change, more species may be interacting with competitors at range edges, as ranges shift or previous habitats become unavailable. In Long Point, Ontario, the Fowler’s toad (Anaxyrus fowleri) is at the northern edge of its range and overlaps with the more common American toad (Anaxyrus americanus). Amphibian species are expected to compete strongly at their tadpole stage, when they are in the highest densities. Thus, to determine if these species compete strongly in local pond environments, we reared both species together, and apart, in mesocosms mimicking natural habitats in 2018 and 2021. We measured survivorship to, weight at, and time to metamorphosis for both species, which are proxies of fitness for these animals.

Results/Conclusions

We determined that the presence of A. americanus always had a detrimental effect on A. fowleri, even though this effect presented itself differently across years. Our study suggests that competitive exclusion is occurring at the edge of A. fowleri’s range, and demonstrates the importance of studying interactions across multiple years. In 2018, the presence of A. americanus tadpoles reduced A. fowleri tadpole survivorship by half and significantly increased A. fowleri time to metamorphosis but did not influence A. fowleri weight at metamorphosis. In 2021, the presence of A.americanus tadpoles significantly reduced A. fowleri tadpole weight at metamorphosis and, while not significant, also increased A. fowleri time to metamorphosis, but did not influence A. fowleri survivorship. In both years, A. americanus tadpoles had a detrimental effect on A. fowleri tadpole fitness, though the appearance of this affect was different across the different years, as the overall dynamics of the mesocosms are slightly different each year, mimicking natural pond changes. The reduced survivorship, weight, and time to metamorphosis of A. fowleri when in the presence of A. americanus indicate that these toadlets had lower fitness, and A. americanus may be competitively excluding A. fowleri from local ponds.