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Range contraction in declining north american bird populations

Rodríguez, Jon Paul
Ecological applications 2002 v.12 no.1 pp. 238-248
birds, breeding, conservation areas, endangered species, geographical distribution, home range, population dynamics, probability, surveys
Significant population declines are typically used to diagnose the level of threat faced by endangered species. In contrast, conservation strategies for these species often focus on setting aside parts of their ranges into protected areas, without regard for the distribution of population abundances within those ranges. Since abundances are not uniformly distributed within a species' range, different portions of the range may be of different conservation value. This article examines the relationship between the decrease in abundance of a taxon and the resulting pattern in the contraction of its geographical distribution. Three possible types of decline are considered. High‐abundance‐biased (HAB) declines occur when the overall decline in abundance occurs predominantly in the portions of the range where abundances are highest. Here, while the total abundance may significantly decline, the range can remain unchanged. Conversely, in low‐abundance‐biased (LAB) declines, the decline occurs predominantly in areas where abundances are lower, thus leading to a relatively faster decline in range than in abundance. In a range‐wide decline (RW), all areas are equally likely to experience a decline, and the relative changes in both range size and total abundance depend on the initial abundance profile. Data for 27 declining bird species, obtained from the North American Breeding Bird Survey, show that the majority exhibit patterns consistent with an HAB decline during the last three decades. This suggests that efficient strategies for preventing or slowing a decline should concentrate conservation efforts in high‐abundance areas. The designation of a nature reserve in a portion of the range where the species is simply “present” may coincide with an area with low probability of long‐term persistence, and thus be an inefficient allocation of limited conservation resources.