This is the last post (for a while, anyways) about species richness patterns in anoles. Unlike the previous papers and discussions (found here, here, and here), Algar and Losos (2010) zoom out from the Caribbean and take aim at the entire Anolis radiation.
Islands are often species depauperate relative to mainland settings, likely because their small size makes them difficult to colonize and those species that do make it are more susceptible to extinction. Yet, islands also house many iconic evolutionary examples of adaptive radiation. Algar and Losos (2010) point out that this discrepancy stems mainly from the role that in situ evolutionary diversification plays in these two scenarios. In their paper, Algar and Losos, using anoles of course, explore how in situ diversification on islands affects the relationship between island and mainland species richness.
They predict that: (1) small islands where speciation has not occurred will conform to the commonly observed pattern of reduced diversity relative to similar mainland areas; (2) that substantial in situ speciation could result in islands that are essentially small mainlands; and (3) that radiation into empty niches could produce exceptional species richness and result in faster diversification on islands than mainlands.
To document mainland species richness, the authors built range maps from 4,748 locality records downloaded from the literature, HerpNET, and the Global Biodiversity Information Facility. 35% of these localities had geographical coordinates. Based on locality descriptions, an additional 1,929 localities were georeferenced. After some additional quality control, the final dataset included 3,670 georeferenced occurrences for 203 mainland species. To quantify island species richness, the authors generated species tallies for 39 Caribbean islands for which environmental data were available. The authors obtained climate and environmental data from WorldClim. It goes without saying that this was a huge amount of work!
Algar and Losos compared species richness on the mainland vs. species richness on islands by using a spreading-dye model to randomly generate 975 non-overlapping sub-regions on the mainland (MSRs). Each of the 39 islands was represented by 25 MSRs of the same area on the mainland. Environmental data and species richness were sampled from each of these MSRs.
The authors found a threshold area below which speciation doesn’t occur in MSRs that was close to that for islands from this paper and from Losos and Schluter (2000).
Compared to the mainland, small islands below the speciation area threshold had significantly fewer species than their corresponding MSR. However, on large islands above the speciation area threshold, islands have more species and a significantly steeper species-area relationship than corresponding MSRs. Last, the authors found that climate affects island anoles differently than mainland anoles but that climate is not a very good predictor of species richness differences between the islands and the mainland.
The authors conclude that in situ speciation has qualitatively altered the relationship between island and mainland species diversity. Diversification within islands can lead to elevated richness relative to the mainland. This conclusion is strengthened by the observation that the small islands of the Caribbean that do not have in situ diversification have lower species richness than the mainland, as predicted when colonization is the main source of new species. This result was not due to climatic differences between the islands and the MSRs; islands with in situ diversification have more species than expected based on the mainland climate-richness relationship.
So, what have we learned in this series of posts? Well, satellite islands in the Caribbean do follow SAR relationships predicted by island biogeography theory because these islands are connected directly to source populations during glacial maxima and experience faunal relaxation through extinction once the islands reform during glacial mimima. Oceanic islands, on the other hand, do not show a strong species-area relationship because their faunal buildup is dominated by competition-driven reductions in successful colonizations. The biggest islands in the Caribbean, the Greater Antilles, are large enough to support substantial in situ diversification and show an increasing diversification rate with island area. In fact, in situ diversification allows the Greater Antilles to surpass the mainland in species richness. Thus, evolutionary processes, in addition to ecological ones, must be considered in any analysis of species richness and area.
- 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
Ambika Kamath
Do the Caribbean islands in this study include the Lesser Antilles?
Yoel Stuart
Yes.