Data from: An empirical test of the mechanistic underpinnings of interference competition

Interspecific competition is an essential element of the evolution of
species and can strongly influence the abundance and distribution of
species. Where competition interacts with anthropogenic habitat
modification, this natural ecosystem process can become a threatening
process. Understanding the mechanisms behind competition in such cases is
essential for the formulation of cost-effective management responses for
biodiversity conservation. According to the resource availability
hypothesis of competition, interference competition limits access to
resources by species vulnerable to aggression and wastes energy in evasive
responses. Studying competition is notoriously difficult, however,
empirical evidence of the resource availability hypothesis is limited, and
there are few published experimental studies showing the effect at larger
scales. We present the results of a controlled, replicated empirical study
of interference competition at a landscape scale. We removed an
aggressive, overabundant bird, Manorina melanocephala, whose interference
competition is a threatening process for small woodland birds in the
highly modified agricultural landscapes of eastern Australia. We monitored
foraging and harassment rates of small woodland birds before and after
removal to indicate if levels of interference competition changed. Due to
unexpected immediate recolonisation, abundance of Manorina melanocephala
in treatment sites declined by only 34% relative to control sites.
Twenty-four bird species displayed aggressive behaviour towards other
birds and 41 bird species were victims of aggression. Manorina
melanocephala was responsible for 66% of all aggressive interactions.
After removal, we recorded a doubling in foraging rate of small woodland
birds in treatment compared to control sites. This appears to confirm the
resource availability hypothesis of competition. Paradoxically, increased
foraging was not accompanied by a decline in harassment. Low detection
rates of harassment of small woodland birds, combined with the modest
reduction in Manorina melanocephala abundance, make it difficult to draw
firm conclusions about the relationship between rates of harassment and
rates of foraging.