Determinants of bird species richness, endemism, and island network roles in Wallacea and the West Indies: is geography sufficient or does current and historical climate matter?

Abstract

Island biogeography has greatly contributed to our understanding of the processes determining species' distributions. Previous research has focused on the effects of island geography (i.e., island area, elevation, and isolation) and current climate as drivers of island species richness and endemism. Here, we evaluate the potential additional effects of historical climate on breeding land bird richness and endemism in Wallacea and the West Indies. Furthermore, on the basis of species distributions, we identify island biogeographical network roles and examine their association with geography, current and historical climate, and bird richness/endemism. We found that island geography, especially island area but also isolation and elevation, largely explained the variation in island species richness and endemism. Current and historical climate only added marginally to our understanding of the distribution of species on islands, and this was idiosyncratic to each archipelago. In the West Indies, endemic richness was slightly reduced on islands with historically unstable climates; weak support for the opposite was found in Wallacea. In both archipelagos, large islands with many endemics and situated far from other large islands had high importance for the linkage within modules, indicating that these islands potentially act as speciation pumps and source islands for surrounding smaller islands within the module and, thus, define the biogeographical modules. Large islands situated far from the mainland and/or with a high number of nonendemics acted as links between modules. Additionally, in Wallacea, but not in the West Indies, climatically unstable islands tended to interlink biogeographical modules. The weak and idiosyncratic effect of historical climate on island richness, endemism, and network roles indicates that historical climate had little effects on extinction-immigration dynamics. This is in contrast to the strong effect of historical climate observed on the mainland, possibly because surrounding oceans buffer against strong climate oscillations and because geography is a strong determinant of island richness, endemism and network roles.

Keywords: Birds; Caribbean; Wallacea; West Indies; current climate; endemism; historical climate; island biogeography; modularity; species richness.

Figure 1

Four bird species representing a nonendemic and an endemic species to Wallacea (left) and the West Indies (right). Upper left: Olive-flanked Whistler (Hylocitrea bonensis) endemic to Sulawesi, the largest island in Wallacea. Lower left: Brown-throated Sunbird (Anthreptes malacensis), a nonendemic species found on Sulawesi and other islands in the western part of Wallacea and throughout much of South-East Asia. Upper right: Yellow-faced Grassquit (Tiaris olivacea) is a nonendemic West Indian species mainly found on large Greater Antillean islands, or nearby satellite islands, and in Central America and the northwestern South America. Lower right: Narrow-billed Tody (Todus angustirostris) is endemic to Hispaniola, the second largest and the most mountainous islands of the West Indies. Illustrations by Jon Fjeldså.

Figure 2

Map of Wallacea (left) and the West Indies (right) showing (A) total species richness, (B) richness of nonendemics, and (C) richness of regional endemics. Notice the logarithmic scale.

Figure 3

The relationship between island biogeographical network roles (l and r) and total species richness, richness of nonendemics, and richness of regional endemics for (A) Wallacea and (B) the West Indies. Notice that in both archipelagos local topological linkage l correlated strongest with richness of regional endemics, whereas regional topological linkage r correlated strongest with richness of nonendemics.