I studied the influence of competition on the maintenance of a dominance
structured multi-species guild of ant-following birds. I explored the numerical and
behavioral responses of bicolored (Gymnopithys leucaspis) and spotted (Hylophylax
naevioides) antbirds several generations after the extirpation of the dominant ocellated
(Phaenostictus mcleannani) antbird on Barro Colorado Island, Panama. I compared the
abundances and behavior of these species to data collected by E.O. Willis and others
prior to the decline of ocellated antbirds on Barro Colorado, and to a nearby mainland
control in Parque Nacional Soberania, where the complete guild of these ant-followers
still exists.
Populations of bicolored and spotted antbirds increased in density on Barro
Colorado, completely compensating in combined biomass for the loss in overall biomass
by ocellated antbirds. Historical records suggest that complete population turnover of
these species occurred before density compensation was detectable. At ant swarms on
Barro Colorado, the numbers of spotted antbirds doubled from historical records and in
comparison to Soberania. The increased proportion of biomass of spotted antbirds at
swarms on Barro Colorado compensated for the reduced proportion of biomass of
ocellated antbirds. No shifts in microhabitat use by bicolored antbirds was observed after
the loss of the dominant ocellated antbird. Bicolored antbirds foraged at similar rates,
showed similar aggression towards conspecifics, and equal activity at ant swarms on
Barro Colorado and in Soberania. Rates of aggression between bicolored and spotted
antbirds on Barro Colorado, however, increased. Ocellated antbirds rarely interacted
directly with spotted antbirds in Soberania, consistent with historical observations. Thus,
the limited swarm use by spotted antbirds historically on Barro Colorado and in
Soberania likely results from indirect competitive pressure promoted by ocellated
antbirds and mediated through direct interactions with bicolored antbirds.
My results suggest that interspecific competition actively maintains guild
structure in this complex tropical foraging association through direct and indirect
interactions. Behavioral adaptations in guilds may occur over several generations,
delaying the onset of compensatory responses. Detailed long-term experiments and/or
comparative analyses are needed to fully understand the role of competition in the
structuring of multi-species guilds in tropical forests.Science, Faculty ofZoology, Department ofGraduat