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Delayed compensatory responses in a guild of ant-followers Touchton, Janeene M.

Abstract

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.

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