Dispersal limitation predicts the spatial and temporal filtering of tropical bird communities in isolated forest fragments

The link between dispersal traits and patterns of community assembly
remains a frontier in understanding how vertebrate communities persist in
fragmented landscapes. Using experimental release trials and intensive
field surveys of bird communities in fragmented forests of the Peruvian
and Colombian Andes, we demonstrate that morphological traits related to
movement (1) predict experimental flight performance and (2) exhibit
dispersal-mediated environmental filtering at the community scale. After
correcting for body size, four traits hypothesized to influence flight
ability (wing length, wing pointedness, wing loading, and eye size)
predicted distance flown across a hostile experimental landscape, with
successful species having significantly longer pointed wings, carrying
less mass per unit wing area (i.e., lower wing loading), and having
smaller eyes. Species with larger eyes also displayed increased flight
latency, potentially due to disability glare. At the community scale we
detected a gradient of dispersal-mediated environmental filtering in
fragments compared to reference forest within the same landscape, with
relative differences in trait values explained by the temporal and spatial
extent of patch isolation. In the Colombian landscape where fragments had
been isolated for > 60 years, communities were filtered for species
with long and narrow wings and small eyes, especially within the most
spatially isolated fragments. We observed the opposite pattern in the more
recently fragmented Peruvian landscape (15-30 years): communities within
fragments tended to have shorter and more rounded wings compared to those
in nearby contiguous forests, suggesting that dispersal-limited species
accumulate in the initial years following patch isolation due to
“restricted dispersal” and represent an extinction debt yet to be paid.
Our results (1) experimentally validate the use of morphological traits as
proxies for movement ability in fragmented landscapes, (2) demonstrate
that visual acuity functions as a novel dimension of dispersal limitation,
and (3) quantify how the spatial and temporal components of patch
isolation produce a gradient in dispersal-mediated environmental filtering
and extinction debt for communities inhabiting fragments.