Squeezed at the Top: Interspecific Aggression May Constrain Elevational Ranges in Tropical Birds

The effect that climate change will have on many endangered species is currently a hot topic in the field of environmental analysis. While many sophisticated predictions are being made in terms of the abiotic impact climate change will have, rarely do researchers take biotic effects into account.  Jankowski et al. (2010) performed an experiment that explored one possible biotic aspect that will influence species survival in the event of a temperature change, namely, niche habitats and the species interactions that maintain them. The researches studied tropical birds in the Cloud Forests of Costa Rica, who are known for their strict elevational distribution. By playing pre-recorded bird songs to target birds, the researchers tested the hypothesis that the birds use interspecific competitive interactions to enforce their spatial boundaries. The results the researchers obtained indicated that the bird’s ranges were enforced as a result of interspecies competition, and that when combined with the effects of climate change, this dynamic could lead some species to extinction far sooner than anticipated through abiotic considerations alone.¾Emily Grace Cole

Jankowski, J., Robinson, S., Levey, D., 2010. Squeezed at the top: interspecific aggression may constrain elevational ranges in tropical birds. Ecology 91(7), 1877-1884.

  While the mechanism the birds use to maintain their range is still in debate, this is the first study to provide experimental evidence supporting the theory that the birds use interspecific competition to maintain their elevational boundaries rather than phenotypic specialization or some other attribute. Jankowski et al. experimented with five species of bird, two of which were known to inhabit lower forest elevations: the White-breasted Wood-Wren and the Orange-billed Nightingale-Thrush, two of which inhabited higher forest elevations: the Gray-breasted Wood-Wren and the Slaty-backed Nightingale-Thrush, and one, the Black-headed Nightingale-Thrush, which inhabited a narrow strip between the other two Thrushes. Each of these birds has been known to maintain strict elevational boundaries and is highly subject to species replacement.
In order to discern whether these birds were maintaining their elevational distribution through species interactions, the researchers played pre-recorded birdcalls, both from birds of the same species and from birds of a different species but the same genus, to the target birds. The target bird’s behavior was observed and recorded before the recordings, after the first recording, and after the second recording. The researchers established the extent of each species’ territory by listening to birdsong and watching individual birds’ movements. They played the recordings to birds at varying distances from the edge of their territory, to see if they exhibited a distance-based change in behavior. A recording of either the Golden-crowned Warbler or the Lesser Greenlet, which are both known to cohabitate at many elevations, was also used as a control. No differences were found between the target bird’s behavior before this recording and after it however, so these behavioral observations were added to the general pool of uninfluenced target bird behavior.
          The researchers found that the birds of each genus responded aggressively to the songs of birds from the other genus, indicating that there was interspecies competition. In addition, in almost all the species, aggressive reactions grew stronger closer to the edge of the territory where the chances of meeting an outside bird increased, indicating that the aggressive behavior was learned, and therefore flexible, allowing for responses to change based on population density, etc. Finally, it was detected that certain species showed more aggression in their reactions toward the birdsong of other species than vice versa. This demonstrates a certain interspecific dominance, which could have large repercussions should the climate change. If, for example, the higher elevation species was subordinate and warming were to allow lower dominant species to expand into higher elevations, the subordinate upper elevation species would then be forced into an even smaller amount of land, possibly too small to sustain it’s population. In an alternate scenario a dominant upper elevation species could prevent the expansion of a lower elevation subordinate species, squeezing it between the dominant species’ territory and the increasing temperature. In this case at least, biotic factors play a key role in anticipating the effects of global warming on endangered species.    

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