Range contraction and extinction vulnerability: what is natural?

Abstract

The potential of species to generate new taxa or go extinct may be a consequence of their evolutionary history. Here, parameters involved in the natural contraction of species ranges, in particular whether distributional ranges have a phylogenetic component, and to what extent the pattern of distributional change is affected by the scale at which populations differentiate, are examined. It is argued that, for taxa in which local differentiation is impossible, the primary opportunity for taxonomic divergence will only occur subsequent to colonization of a new habitat, at which time taxa are also likely to expand their geographic and ecological range. In such a scenario, the most ancestral species will be relictual, with the narrowest ranges and greatest vulnerability to extinction. For taxa in which local population subdivision is possible subsequent to colonization of a land mass, subdivided taxa become progressively more restricted in geographic and ecological range, In this second scenario the more derived species will be confined to ever-decreasing ranges, their probability of extinction coincidentally enhanced. Representatives of various lineages from the Hawaiian Islands are used to test these concepts. First, for spiders in the genus Tetragnatha, the phylogeny of two clades that differ in vagility is compared. As predicted, more derived species have broader ranges in the more vagile clade. In the more sedentary clade, the more derived species appear to be more restricted. For flies in the genus Drosophila, and plants in the Hawaiian silversword alliance, genus Dubuatia on Kauai, known molecular data sets were used to examine the history of species in terms of population size. Applying recent theory based on coalescence, it is shown that the Drosophila lineage has been expanding its range through its history, as expected from a group that requires colonization of a new land mass to form new species. On the other hand, the Dubautia lineage has suffered range contraction through its history, which is also as predicted because the group differentiates extensively within the same mountain mass. The results provide a method with which to predict how species will respond to conservation action. This reasoning suggests that species that are confined to small ranges as a result solely of their phylogenetic legacy are unlikely to be vulnerable to immediate extinction. However, species that have been confined to small ranges as a result of recent ecological modification, such as alien invasion, are highly vulnerable to extinction.