What happens after an island subspecies colonizes the mainland? A case study of non-migratory Allen’s Hummingbird

Blog written by Brian Myers. Read the full paper here. Featured image shows the breeding distribution of non-migratory Allen’s Hummingbird, pictured in (a), and (b) shown in comparison to the southern distribution of migratory Allen’s Hummingbird, with the area of breeding overlap between the two subspecies circled in red.

Off the coast of southern California, there is a series of islands known as the Channel Islands that are home to a diverse array of endemic species and subspecies. Among these is one of two subspecies of the Allen’s Hummingbird (Selasphorus sasin sedentarius). This hummingbird colonized the mainland of southern California in the 1960s, in Los Angeles County. When a species (or in this case a subspecies) colonizes a new area, a founder effect is likely to be observed.

A founder effect occurs when a colonizing population consists of a small subset of individuals from a much larger population. In this scenario, the colonizing population shows relatively low genetic diversity, and does not reflect the extent of the genetic diversity found in the original population. This phenomenon occurs because the entire population the founding individuals came from is not entirely represented in the new, colonized area at the genetic level. Given the relatively recent colonization of non-migratory Allen’s Hummingbird, we hypothesized that we would observe founder effects, but we did not. We may not have observed founder effects for a couple of reasons.

In the mid 2000s, non-migratory Allen’s Hummingbird began to expand its range northward and inland on the mainland. Eventually, it reached the breeding range of the other Allen’s Hummingbird subspecies (S. s. sasin). We know from other work (and our own unpublished data) that these two subspecies now interbreed extensively, and that many individuals in non-migratory Allen’s Hummingbird populations on the southern California mainland now have genes from migratory Allen’s Hummingbird populations (and vice versa) as a result. Thus, genes from migratory Allen’s Hummingbird might explain why we have elevated genetic diversity on the mainland. Additionally, gene flow may have continued between the Channel Islands and the mainland ever since the initial colonization event. In short, gene flow has the potential to erase founder effects because it can add genetic variation to a population.

Given that we know non-migratory Allen’s Hummingbird has recently expanded its range, to estimate where it might occur in the future, we implemented ecological niche modeling, which uses environmental conditions in areas where a species is actually found to project where it might be in the future. Interestingly, we found that non-migratory Allen’s Hummingbird is projected to expand its range north along the coast, possibly as far north as the San Francisco Bay area, and inland in southern California, potentially into Kern, San Bernardino, and Riverside counties.

We also investigated what might have contributed to the successful colonization and subsequent expansion of non-migratory Allen’s Hummingbird, which appears to be outcompeting migratory Allen’s Hummingbird where the two subspecies overlap in breeding range. Our ecological niche model made it clear that the environment on the southern California mainland supports non-migratory Allen’s Hummingbird, prompting the following question: what might contribute to the apparent success of non-migratory Allen’s Hummingbird over migratory Allen’s Hummingbird on the southern California mainland?

First, habitat suitability measures, which estimate how suitable a given area is for a species to occupy, were higher (more favorable) for non-migratory than migratory Allen’s Hummingbird across southern California, so the environment does appear to favor the non-migratory subspecies. However, additional ecological factors might be involved. For example, an extended breeding season found within non-migratory Allen’s Hummingbird gives a female enough time to fledge at least four nests per year. A single migratory Allen’s Hummingbird female has a shorter breeding season and can fledge a maximum of only two nests per year, giving non-migratory Allen’s Hummingbird a reproductive advantage (Clark 2017, Clark and Mitchell 2013). Further, Clark (2017) hypothesized that there is a subtle ecological or behavioral difference between migratory and non-migratory Allen’s Hummingbird that allows non-migratory Allen’s Hummingbird to better-exploit urban habitat. A potential topic of future work is to evaluate what ecological factor(s) give non-migratory Allen’s Hummingbird an advantage over its migratory counterpart in urban habitat across southern California.

We also uncovered patterns of genetic variation among populations on the Channel Islands. We found that non-migratory Allen’s Hummingbird on Santa Cruz Island was the most diverged, even though Santa Cruz Island is the closest island geographically to the mainland. Interestingly, this pattern has already been observed across many different species, including birds and mammals. There is not currently a consensus explanation as to why this pattern is consistently observed. The fact that different markers (types of genetic data used for a study, for example, individual genes, short genetic sequences, entire genomes, and mitochondrial data) are implemented across different studies makes it more difficult to come to general conclusions about the dynamics of the Channel Islands because some markers are more reliable than others for different scientific questions. Future studies that sample additional taxa found on the Channel Islands and the mainland might also provide some clarification.

The objectives and results of our study, like others, address topics that are foundational to ecology and evolutionary biology. What are the patterns of divergence and genetic diversity across geographic space? What drives the colonization and expansion of species? Why might one species outcompete another in the same geographic area? These are three fundamental questions that bind together the closely related fields of ecology and evolution, and depending on the outcome, can even have conservation implications. The more we can understand the interactions and trajectories of different species, the deeper our knowledge of evolution and the processes that shape it becomes.

Citations

Clark CJ, Mitchell DE. 2013. Allen’s Hummingbird (Selasphorus sasin), The Birds of North America online. Ithaca, NY: Cornell Laboratory of Ornithology. Web. 15 September 2020.

Clark CJ. 2017. eBird records show substantial growth of the Allen’s Hummingbird (Selasphorus sasin sedentarius) population in urban Southern California. The Condor 119: 122-130.

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