As alluded to previously, MacArthur and Wilson (1967) did consider evolutionary processes when they developed their Theory of Island Biogeography. Specifically, in situ evolutionary diversification (i.e. speciation) may contribute substantially to the species diversity of an island and should be considered in any general attempt to model species-area relationships on islands. Building on Rand’s 1969 paper studying the ecological determinants of species richness in Caribbean anoles, Losos (1996) incorporates an evolutionary perspective into the Caribbean Anolis species-area story.
Losos first updated Rand’s list of species to include a few more islands and to reflect the latest understanding of Anolis taxonomy. He used Rand’s island type classifications but added two more for six total classifications: (1) Greater Antilles, (2) Greater Antillean satellites, (3) Great Bahama bank, (4) Great Bahama satellites, (5) Lesser Antilles, and (6) oceanic islands.
Using MacArthur and Wilson’s equation for the species-area relationship, s = caz where s is species number, a is island area, and c and z are constants, Losos found a significant relationship between area and species richness (z = 0.13 from a log-log regression), although this is much lower than findings published for other organisms. Like Rand, Losos found that there is heterogeneity in the species-area relationship among island classes. There is no relationship between species number and area for oceanic islands and Great Bahama satellites, while only a weak relationship exists for the Lesser Antillean islands. Note that these islands wouldn’t be connected to other islands during sea level minima.
In contrast, area and species number are strongly related for the Greater Antilles, Greater Antillean satellites, and the Great Bahama Bank, again, similar to Rand’s findings. Losos invoked some of the same explanations as Rand for this heterogeneity, including competitive exclusion, extinction, and role of land bridges for colonization during sea level minima (explained in more depth in the previous post).
However, Losos also considered whether evolutionary diversification affects the species-area relationship in anoles. Using newly available phylogenetic hypotheses to infer sister-species relationships, Losos concluded that in situ speciation is absent on islands smaller than Puerto Rico, the smallest of the Greater Antilles. On the Greater Antilles, however, Losos estimates that at least 70% of the species arose through in situ diversification. Part of this in situ diversification is likely due to greater opportunity for geographic isolation and allopatric speciation with increased area; indeed, some species groups on Cuba and Hispaniola have produced a large number of geographically isolated species.
However, on the Greater Antilles, Anolis species also partition their niches more finely than their oceanic island brethren, diverging in body size, structural habitat use, and thermal biology. Six ecomorphological habitat specialists have evolved that occupy different parts of the structural niche. These ecomorph clades then further diversified along body size and thermal niche axes, such that an ecomorph type often has multiple representatives within an island.
Additionally, on the biggest islands (Cuba and Hispaniola), Losos argued that the presence of so many species in many different niches has driven the evolution of additional unique, non-ecomorph, habitat specialists (e.g. rock dwellers, stream dwellers), allowing for even more species to coexist sympatrically, and adding further to the species richness of the largest islands. Thus, in situ diversification allows for species richness to be higher than that predicted from ecological processes alone.
- DNA Sequencing from Formalin-Preserved Specimens - December 16, 2014
- Sans the Claws, Anoles Present Under The Trees? - December 5, 2014
- Out Damned Spot! Or, How They’d Be Loving If They Weren’t Fighting - November 18, 2014
1 Pingback