Tyrone Lavery Ecology

Biogeography of mammals in the Solomon Islands and Papua New Guinea

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The thousands of islands that span the southwest Pacific provide one of the world’s most valuable natural laboratories. Northern Melanesia’s islands are particularly useful. The Solomon, Bismarck, Trobriand, Admiralty and D’Entrecasteaux archipelagos are biologically diverse, and they are also relatively undisturbed with very few species extinctions having occurred.

Across Northern Melanesia there are ‘snapshots’ of speciation visible in birds. Small differences in the plumage of species on adjacent islands signify speciation in action. By studying these visible stages of evolution, researchers like Ernst Mayr and Jared Diamond developed some of the most influential theories of biogeography and evolution.

Much of their collaborative work is compiled in the The birds of Northern Melanesia: speciation, ecology and biogeography. Within this book, Mayr and Diamond addressed important questions such as why some species diversify faster than others, and whether former land bridges are influential on island species richness. They also illuminated broad patterns of species diversity, endemism, and the influences of oceanic barriers between islands. Effectively, Mayr and Diamond established Northern Melanesian birds as a model system for speciation, against which other groups of animals or plants can be compared. The colour plates and species lists they provided formed a rich database immensely valuable to other researchers.

In comparison with birds, northern Melanesia’s mammals have been rarely studied. Although species lists for many islands (e.g. New Britain and Solomon Islands) are improving, many islands (large and small) still remain to be surveyed by mammalogists. Because of the lack of survey data, there have been no attempts to analyse biogeographic patterns in mammals in a way similar to the analyses performed by Mayr and Diamond. In our most recent paper published in Mammal Review, using compiled species lists for 75 of Northern Melanesia’s islands and a range of analyses, we do exactly this.

We used the published literature, museum databases and my own survey data to develop mammal species lists for 75 islands from the Solomon, Bismarck, Trobriand, Admiralty and D’Entrecasteaux archipelagos. We then used direct comparisons of family compositions, and Ochiai and endemism indices to compare mammal assemblages between island groups and a defined New Guinea source pool region. Generalised linear models and non-parametric multivariate analyses were used to identify, and rank, the importance of island variables for mammalian species richness.

Island size proved to be the island attribute most influential on mammal species richness. Among the smaller islands below 200km² in size, their level of geographic isolation became a more influential factor. When we analysed the data on bats independently, island size was again the most important factor. However, for all other mammals (rats and marsupials) the distance of the island from New Guinea was most important in determining how many species were present. This reflects the fact that these mammals do not have the abilities of bats to cross oceanic barriers.

When we examined the numbers of endemic species on different islands and archipelagos, Solomon Islands stood out as a region with a very high proportion of endemics. This is because there have been small radiations of unique genera of mammals in this archipelago. The monkey-faced bats (Pteralopex – 5 species) and Solomon giant rats (Solomys – 4 species) that are both unique to Solomon Islands contribute a large part of this endemism.

Endemism indices were calculated for islands and island groups using a modified version of the method employed for birds by Ernst Mayr and Jared Diamond. Higher values indicate a greater proportion of endemic species.

Our results also revealed some islands that are in need of survey focus because their mammal inventories were much lower than expected for their size. We suggest that large islands should remain the focus for future surveys and particularly the higher altitudes of large islands.

By defining biogeographic groups and island clusters with high endemism, we have offered a useful starting point for mammalian conservation strategies in Northern Melanesia. The large number of islands within Northern Melanesia offers many options for conservation of the region’s mammals. It is clear that more surveys and taxonomic research will help to further develop these types of analyses and potentially highlight subtle differences in nearby islands that will be important for conservation of Melanesia’s extraordinary mammals.

 

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