Prioritisation of Philippine Island avifaunas for conservation: a new combinatorial measure

Given the ubiquitous fact that resources for preserving whole faunas are limited, prioritisation of areas is called for. The popularity of the complementarity approach for setting priorities derives from their representing the maximum number of species of a taxon with a minimum number of areas, e.g. islands in an archipelago like the Philippines. While the logic of the approach is impeccable, it lacks dynamism. Here a new approach to prioritisation is exemplified with avifaunas in the Philippines. A combinatorial, linear model permits assigning to each island avifauna a priority index that allows ranking of islands in terms of prioritisation. The model is (1) dynamically proactive in that it takes into account the island-specific ongoing and future dieback of the avifaunas at stake; by applying the well-established species-area relationship to the remaining forests as the new islands; (2) it is accountable by operationalising the weighting of its parameters by introducing the new quantity ‘opportunity for choice’; (3) it is combinatorial, being thereby more realistic. The model's results for the six largest Philippine islands were reasonably well validated based on a ranking of island-specific number of endangered endemics. The results lead to assigning the Greater Negros faunal region (Negros, Panay) top priority, similar to more species-rich Greater Luzon and Greater Mindanao, thus substantially changing the ranking of islands derived from the rather static one-parameter complementarity approach.