Variation In Species Losses From Islands: Artifacts, Extirpation Rates, Or Pre‐Fragmentation Diversity?

Species are being lost from isolated reserves as predicted by ecological theory, prompting calls for larger reserves with higher species immigration rates. However, some large islands have lost a large proportion of their species, whereas some small islands have not lost any. Conservation efforts would be more efficient if the cause of such variation in the relationships among number of species lost, island size, and immigration rate were known. Observed species losses could be affected by the time since islands were isolated, species immigration rates, species extirpation rates, the pre‐fragmentation diversity of the region relative to steady state, or overestimation of the pre‐fragmentation diversity of islands. To test the last three hypotheses, I compared the intersection points of the island, intraprovincial, and interprovincial species–area relationships of terrestrial mammals from nine archipelagos of land‐bridge islands and terrestrial habitat isolates. Species losses from three archipelagos were greater than expected due to reduced immigration rates alone, although I could not resolve if this was due to increased extirpation rates or overestimation of the pre‐fragmentation diversity of the islands. Analysis of six archipelagos indicates that the diversity of mammals in two regions of North America is currently below steady state, probably due to the extinction of mammals and glacial retreat during the late Pleistocene. These results have direct implications for reserve planning. When provincial diversity is below steady state, some combinations of reserve size and species immigration rate will allow reserves to maintain their pre‐isolation diversity. However, the diversity of provinces relative to steady state is likely to vary, so conservation of a given proportion of a province may not always conserve the same proportion of its species. I present a new species–area relationship for islands formed by fragmentation that replaces the parameter c (fitted constant) with a rotation point. Estimation of this rotation point will allow reserve planners to separate the effects of extirpation and immigration rates on species losses from islands, identify provinces that are below steady‐state diversity, and estimate the combinations of reserve size and immigration rate that will prevent loss of species from reserves.