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Abstract
Closely related tropical bird species often occupy mutually exclusive elevational ranges, but the mechanisms generating and maintaining this pattern remain poorly understood. One hypothesis is that replacement species are segregated by interference competition (e.g. territorial aggression), but the extent to which competition combines with other key factors such as specialization to distinct habitats remains little studied. Using vegetation surveys and reciprocal playback experiments, we explored the dynamics of interspecific aggression between two Nightingale‐Thrushes Catharus sp. in Central America. We show that lower‐elevation Black‐headed Nightingale‐Thrushes Catharus mexicanus are aggressive towards higher‐elevation Ruddy‐capped Nightingale‐Thrushes Catharus frantzii where they meet at contact zones. However, interspecific aggressive responses were weak and unidirectional, and the two species were associated with different habitats. We conclude that the contact zone is maintained and located primarily by habitat selection, and is probably reinforced by interspecific aggression. This has important implications for understanding how montane species will respond to climate change because the pace and extent of range shifts will not depend solely on habitat shifts or interspecific competition, but instead on interactions between these two factors.
Highlights
Tropical mountains offer unique opportunities to study the drivers of community composition owing to their exceptional biodiversity and rapid changes in environmental factors within relatively small spatial scales (Whittaker 1975, Jankowski et al 2013a)
Two long-standing hypotheses for these contact zones on tropical mountains are that they are maintained by abrupt habitat transitions or interspecific competition
Behavioural studies focusing on pairs of related bird species in tropical mountains have shown interspecific aggression at contact zones in Costa Rica (Jankowski et al 2010), Ecuador (Dingle et al 2010), Colombia (Caro et al 2013), Malaysia (Boyce & Martin 2019) and New Guinea (Freeman et al 2016)
Summary
Tropical mountains offer unique opportunities to study the drivers of community composition owing to their exceptional biodiversity and rapid changes in environmental factors within relatively small spatial scales (Whittaker 1975, Jankowski et al 2013a). Behavioural studies focusing on pairs of related bird species in tropical mountains have shown interspecific aggression at contact zones in Costa Rica (Jankowski et al 2010), Ecuador (Dingle et al 2010), Colombia (Caro et al 2013), Malaysia (Boyce & Martin 2019) and New Guinea (Freeman et al 2016) This is generally interpreted as evidence that interspecific competition maintains spatial parapatry of populations through aggressive territorial behaviour, a form of ‘behavioural interference’ (Grether et al 2017). Aggressive interactions of this kind are thought to drive or maintain divergence in habitat preferences between species pairs in the tropical and temperate lowlands (Robinson & Terborgh 1995, Martin & Martin 2001a, 2001b), but this possibility has rarely been examined in montane systems
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