Abundance-occupancy (A-O) relationships are widely documented for many organismal groups and regions, and have been used to gain an understanding of regional population and community trends. Monitoring changes in abundance and occupancy over time may be what is required to document changes in conservation status and needs for some species, communities or areas. We hypothesize that if there is a higher proportion of declining species in one group of species compared to another (e.g., migratory species vs. permanent residents), then a consequence of that difference will be vastly different abundance-occupancy relationships. If this difference persists through time, then the resulting A-O relationships between the groups will continue to diverge. For neotropical migrants, short-distance migrants and permanent resident birds of North America, we assess the numbers of declining species over 1969-2009. We further test for differences in the A-O relationship across these three groups, and in rates of change of abundance and occupancy separately.
Results/Conclusions
We found significant differences in numbers of declining species across the migratory groups, a significant decline/significant increase/nonsignificant decline in the A-O relationship for permanent residents/neotropical migrants/short-distance migrants respectively over the 40 years. Further, abundances are not changing at different rates but occupancies are consistently greater over time for neotropical migrants vs. permanent residents, likely driving the changes in A-O relationships observed. In these analyses, we documented changing A-O trends for different groups of species, over a relatively long time period for ecological studies, one of only a few studies to examine A-O relationships over time. Further, we have shown that a temporally-unvarying abundance-occupancy relationship is not universal, and we posit that variability in A-O relationships is due to human impacts on habitats, coupled with variation in species’ abilities to respond to human impacts.