Over the last decade, observers at a network of long-term monitoring sites have documented a steady decline in the number of individuals of a particular forest-dwelling songbird within a large reserve. Point counts conducted during breeding season show fewer territorial males each year, and fledgling surveys indicate reduced nesting success in fragmented patches along the reserve's edges. By standard measures of abundance, the local population appears to be shrinking. Yet laboratory analyses of blood samples taken from captured birds tell a different story about the population's composition: genetic diversity, as measured by heterozygosity across several neutral markers, has increased over the same interval. Conservation biologists often worry that small, isolated populations will lose genetic variation, making the simultaneous trends—fewer birds but more genetic diversity—seem paradoxical. Field biologists note that the reserve has experienced bouts of drought and a series of intense storms that downed canopy trees, altering microhabitats and concentrating predators along new edges. At the same time, satellite imagery shows continued loss of similar forest types in agricultural districts surrounding the reserve, including to the north and west. The monitoring team has not observed a corresponding rise in reproductive success within the reserve that could account for the genetic pattern. How, then, can a population that is numerically declining display rising genetic diversity? An adequate explanation must reconcile the decline in local counts and nesting success with the increase in measured genetic variation among the birds that remain within the reserve.
Which of the following, if true, most helps explain how both statements can be true?
- The most recent surveys used automated recording units instead of human listeners at some points.
- A rare mutation affecting plumage color became more common in the reserve during the study period.
- Habitat loss in surrounding areas has displaced birds from multiple distinct source populations into the reserve, increasing genetic mixing even as overall numbers within the reserve have fallen due to limited nesting sites and higher predation. (correct answer)
- Populations of nest predators in the reserve have increased markedly in recent years.
- Several of the reserve's interior forest patches experienced temporary flooding that reduced nesting success.
Explanation: Immigration from multiple external sources can boost genetic diversity while total numbers drop if the reserve cannot support all newcomers. The other options either rest on methodology (A) or only explain reduced success or demographic decline without accounting for higher genetic variation (B, D, E).