Many anthropogenic changes are suspected to influence the population and range size of many species. One way to assess the state of a population or community is via abundance-occupancy (AO) analysis. Interspecific AO relationships – in which the average abundance across the range typically increases with area of occupancy – can give advance signals about declining community health, giving policy-makers time to react. Researchers study both an interspecific form of the AO relationship, and intraspecific forms. In the intraspecific case, an individual species increases its area of occupancy with increasing average abundance across the range. Negative intraspecific AO relationships are rare, but it is theoretically possible for negative intraspecific AO relationships to become more common. One reason might be that species are undergoing abrupt, large range increases (population explosion, resulting in large area of occupancy but lowered average abundance across the range). A second reason: a species declining in abundance might be expanding its range because local resources have become limiting. For this study we calculated intraspecific AO relationships for North American terrestrial birds using data from the Breeding Bird Survey (BBS) for the years 1990-2016. We hypothesized that declining species would show a negative intraspecific AO relationship.
Results/Conclusions
Out of 397 species analyzed, 121 were significantly decreasing in abundance. Of those, 34 (25%) exhibited a significant negative AO relationship. A negative AO relationship indicates that species that are declining in abundance are increasing their area of occupancy. We discuss possible drivers for the expanding area of occupancy contemporaneous with declines in abundance, and present simulations to assess which drivers are more likely to be causal mechanisms.