Island are often a haven for speciation. However, accepted theory doesn't always stand up. Many mainland species have developed to be endemic in an isolated area too. If we compare the rate of species turnover and allow for environmental gradients and other factors, we can see how mainland competes against these theoretical island assemblages.
Anolis lizards and Terrarana frogs are ideal genera to study as Caribbean islands and neotropical mainland have harboured their species well. If we briefly glance at the Hawaiian islands, silverswords and honeycreepers are classic adaptive radiations there.
If we want to compare "islands" trapped by mountains or environmental gradients, then we need to quote Himalayan valley species or desert genera such as some cacti. This study The island-mainland species turnover relationship, has been published today in the Proceedings of the Royal Society B: Biological Sciences. Authors Yoel E. Stuart, Jonathan B. Losos and Adam C. Algar of Harvard Cambridge and Nottingham Universities attempted to compare rates of species turnover within:
Long distances often seem to cause high species turnover because dispersal is difficult and gene flow is limited. If there is an island effect apart from this geographical or any environmental "dissimilarity, it could be due to increased ecological opportunity or gene flow effects.
This magnificent silversword is located on top of a Hawaiian mountain - Carabus violaceus; Credit: © Shutterstock
400 Anolis and 850 Terrareana species have radiated from common ancestors around the Caribbean. Both insectivorous egg layers, they have arboreal, terrestrial and semi-aquatic representatives. Species turnover was determined by saying that two identical assemblages would score 0, with 1 indicating no shared species.
M-M species turnover increased with geographical or environmental distance between species assemblages. This is common in most vertebrates and invertebrates worldwide. Range size probably restricted the Anolis turnover which was slow.
M-I species turnovers were similar for both genera. It was higher than predicted with saltwater separation being held responsible for some of this pattern.
I-I species turnover was significantly higher than predicted for M-M models for both groups. They were much more influenced by distance (both geographical and environmental) than M-I species turnovers. The explanation could be that islands share a geological history and were once connected landmasses like the Great Bahamas Bank. Island geographical distances were sometimes quite low too.
The ultimate result is that islands do indeed have unique "island effects" on their fauna. An island biota is unique, as we have known admittedly for many years, because of certain well-known island species. Now we know reasons such as reduced dispersal and gene flow, higher ecological opportunities and more speciation generated by the oversea barrier. Islands have their problems but novel species and species turnover are certainly their true forte.