Climate, habitat and demography predict dispersal by an endangered bird in a fragmented landscape

Worldwide, biodiversity is threatened by human-induced habitat degradation
and fragmentation. Dispersal, particularly long-distance dispersal between
isolated habitat fragments, is key for population connectivity and species
persistence in the face of environmental change. However, we lack
understanding of how habitat fragmentation and degradation itself affect
the dispersal process. To identify conditions that promote or constrain
connectivity, we need to reveal how habitat, demographic, and climatic
conditions drive dispersal success and distance. This is challenging,
however, because detecting dispersal events in wild animals, especially
over long distances, is notoriously difficult. Here we address this in the
Endangered purple-crowned fairy-wren, Malurus coronatus, a small
cooperatively breeding songbird in which individuals can opt to delay
natal dispersal, and we are able to consistently detect dispersal by
colour-marked individuals, including over long distances. Thus, an
assessment of drivers of dispersal success and distance is achievable.
Using data from a 17-year study, we analyse dispersal probability for 796
individuals, and distance of 206 dispersal events (up to 68 km). We show
that poorer quality and isolated habitat is associated with more frequent
dispersal, and over greater distances. Additionally, low population
density promotes dispersal, while higher rainfall (which increases food
availability for several months), especially during the wet season,
increases dispersal distance, particularly at high population density.
This indicates that, when the population is saturated, favourable
conditions may allow birds to successfully disperse over the long
distances required to connect fragments. Policy implications: Our findings
suggest that while habitat fragmentation and degradation may promote
dispersal, such dispersal, especially over greater distances, may require
benign climatic conditions, presumably to limit its cost. Our results
highlight the need to integrate the impacts of climate change when
assessing how habitat degradation and fragmentation affect population
connectivity. Additionally, we recommend conservation management actions
targeted at increasing source population density as an important first
step in promoting connectivity between habitat fragments via long-distance
dispersal.