In a new study published in Ecology Letters, Luke Frishkoff, assistant professor of biology at The University of Texas at Arlington, explores an age-old question: Why do some locations have more species than others?
"Imagine you travel to two mountains, each on a different island," Frishkoff said. "The local environment on each of the mountains is the same. They are at the same elevation, they have the same climate and the same types of vegetation. Based on the similarities, you might expect the number of species to be the same, but often in nature that's not what scientists find."
Frishkoff is determined to understand why. One classic hypothesis states that local diversity is determined by local interactions between species and the environment. Only so many niches exist in nature, and once all these niches are full then competition between those species limits biodiversity from increasing further.
But an alternative hypothesis states that local interactions are of minimal import and instead regional forces predominate. That is, if a region as a whole has evolved many species in it, then this regional richness will bleed into local communities, overwhelming any diversity cap that would otherwise be imposed by competition.
To seek answers, Frishkoff teamed up with University of Toronto collaborators Luke Mahler, assistant professor of ecology and evolutionary biology, and doctoral student Gavia Lertzman-Lepofsky. They traveled to Jamaica and Hispaniola, two Caribbean islands with similar environments, and collected data on individual communities of lizards by searching 15-mile radius plots across each. They found that despite the drastic difference in the overall size of the two islands and the difference in the total number of species that evolved in each (more than 45 on Hispaniola versus only six on Jamaica), in lowland environments both islands only had about five lizard species that co-occurred in local communities.
Results showed that despite evolving independently, the overall size and shape of the lizards in the lowlands of both islands were also extremely similar. This indicates that local lizard communities in that environment will gravitate toward having the same number and types of species. These findings suggest the primacy of the local environment and competition setting caps on community diversity.
However, the lowlands of both islands are very large. Where regional area was limited, the team found different results. In the extensive highlands of Hispaniola, there were four or five lizard species, all of which were found exclusively in the mountains. In Jamaica, where the highlands are much smaller, only two to three species were found at the same elevations.
"Even though the environments in the highlands are essentially identical, the lack of sufficient area in Jamaica seems to have prevented speciation from being able to fill up local communities," Frishkoff said.
Indeed, only one of these species specialized in the highlands; the other two bled over from the lowlands. "Presumably these primarily lowland species are able to persist in the Jamaican highlands only because there aren't enough highland specialists that are able to outcompete them," Frishkoff said.
The study helps better explain how the local and regional hypotheses work together to set limits on biodiversity. Large areas are able to evolve many species, which then compete and set caps on local diversity. Small areas can't evolve local specialists, and so their local diversity is more strongly influenced by regional diversity instead of the local environment. The findings inform fundamental ecological knowledge that could support scientists' efforts in conservation, the mitigation of species invasions and ecosystem restoration.
"As an ecologist with an evolutionary bent, I want to be able to explain the natural world around me based on first principles," Frishkoff said. "This work hints that if the right factors are considered, we might be able to turn ecology and evolutionary biology into a more predictive science."
