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Does interspecific aggression influence elevational limits in closely related species that replace one another along the elevational gradient?

Asymmetric interspecific aggression in New Guinean songbirds that replace one another along an elevational gradient. Freeman, B.G., Class Freeman, A.M. & Hochachka M. 2015. IBIS. DOI: 10.1111/ibi.12384 VIEW

A riddle: What is the difference between the forests located at 1,650 m and 1,700 m along the northwest ridge of Mt. Karimui, a New Guinean mountain?

Is it:

A) 50 m of vertical elevation.
B) Forest type; There is a major transition just below 1,700 m, with tall foothill forest below and mossy cloud forest above.
C) Human disturbance; secondary forest predominates just below 1,700 m, primary forest above.
D) The species of robin; Each elevation has a common species of understory robin, but the species are different, White-eyed Robins Pachycephalopsis poliosoma at the lower site, and Slaty Robins Peneothello cyanus at the upper.

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Figure 1 Primary forest blankets the slops of Mt. Karimui; the mountaintop is located at 2,520 m
© Ben Freeman

While A) is of course correct, the best answer is D). White-eyed Robins are common at 1,650 m but absent higher, where the related Slaty Robin is common. That these two robins share an elevational limit is notable, as the primary forests below and above look similar (if not identical), at least to this ornithologist. These robins can thus be considered “elevational replacements”; ecologically similar species that inhabit largely non-overlapping elevational zones. Elevational replacements are common in New Guinean birds, and, more broadly, in tropical montane floras and faunas worldwide.

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Figure 2 White-eyed Robin, Pachycephalopsis poliosoma (left), and Slaty Robin, Peneothello cyanus (right) © Ben Freeman

So why don’t White-eyed Robins live at higher elevations, or Slaty Robins at lower elevations? A longstanding hypothesis is that competition between the two species of elevational replacements prevents such range expansions. But testing this hypothesis is difficult. One method would be to conduct a removal experiment: Get rid of all the Slaty Robins on Mt. Karimui, and observe whether White-eyed Robins begin to colonize the vacated terrain (and vice versa). Though ideal from a scientific viewpoint, such experiments are problematic from ethical and logistical perspectives.

For territorial birds, a better option is to use playback experiments to measure behavioral interactions between the two species. The logic is that interspecific competition between territorial birds should manifest as interspecific aggression towards the other species. Researchers can thus broadcast song to simulate an interloping intruder, and measure how the territory owner responds. We conducted a series of playback experiments on Mt. Karimui for White-eyed and Slaty Robins, as well as four other species-pairs of elevational replacements.

Listen to the songs of our study species on The Macaulay Library:
White-eyed Robin song
Slaty Robin song

As predicted by the interspecific competition hypothesis, some of the species show interspecific aggression. For example, we found that White-eyed Robins respond quite aggressively to Slaty Robin song. However, Slaty Robins ignored White-eyed Robin song, an asymmetry that suggests White-eyed Robins are behaviorally dominant. In addition, only White-eyed Robins that lived near their upper range limit (close to Slaty Robins) showed interspecific aggression, while individuals at lower elevations did not. This observation suggests interspecific aggression by White-eyed Robins towards Slaty Robins is a learned response to the presence of Slaty Robins, rather than a case of mistaken identity or a genetically evolved response.

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Figure 3 Aggression scores in response to playback trials White-eyed and Slaty Robins. Larger aggression scores indicate more aggressive responses to playback. Dots depict raw data of territorial birds’ response to playback trials, and trendlines with 95% confidence intervals are shown for each trial type.

We found similar patterns of asymmetric interspecific aggression, with the lower elevation species more aggressive, in two additional species-pairs of elevational replacements on Mt. Karimui. But in the final two species-pairs, we did not detect any interspecific aggression. This may be because, in these latter examples, there was a “no man’s land” gap between the ranges of the upper and lower species such that the two species did not actually interact with one another.

In sum, we found asymmetric interspecific aggression in three out of five species-pairs of elevational replacements on Mt. Karimui. These results are consistent with the interspecific competition hypothesis, and indicate that behavioral interactions can be important factors shaping species’ distributional limits. That biotic interactions influence the distributions of elevational replacements (and other species) may explain why species responses to global warming vary substantially. For example, our previous research demonstrated that birds on Mt. Karimui are generally shifting upslope associated with recent temperature increases. However, birds are not shifting upslope in lockstep unison: some species are moving far upslope while others are lagging behind. Behavioral interactions potentially explain some of this variation. Aggressive lower elevation species, like the White-eyed Robin, have shifted upslope substantially on Mt. Karimui, while the two examples of lower elevation species that did not exhibit interspecific aggression have failed to shift upslope. This possibility, however, is based on the very small sample size of five species-pairs and thus remains speculative—a riddle for another day.

Further reading

Freeman, B. & Class Freeman, A.M. 2014. Rapid upslope shifts in New Guinean birds illustrate strong distributional responses of tropical montane species to global warming. Proceedings of the National Academy of Sciences 111: 4490-4494. View.

This article describes the results of our recent resurveys of Mt. Karimui’s birds, and compares species’ modern elevational distributions to those documented by Jared Diamond in 1965.

Freeman, B. 2015. Competitive interactions drive elevational divergence in tropical montane birds. The American Naturalist 186: 470-479. View.

I used a comparative analysis to show that competition between closely related species likely explains the origin of elevational replacements. Thus, ecological competition can have evolutionary repercussions on the evolution of species’ geographic distributions.

Freeman, B. & Montgomery, G. 2016. Interspecific aggression by Swainson’s Thrush (Catharus ustulatus) may limit the distribution of the threatened Bicknell’s Thrush (Catharus bicknelli) in the Adirondack Mountains. The Condor: Ornithological Applications 118: 169-178. View.

Elevational replacements are not a purely tropical phenomenon. This paper documents asymmetric aggression in elevational replacements of thrushes in the temperate mountains of northeastern North America.

Freeman, B. 2016. Strong asymmetric interspecific aggression between two sympatric New Guinean robins. Ibis 158: 75-81. View.

An example of strong asymmetric aggression between two species that inhabit the same forest (i.e. are NOT elevational replacements).

Jankowski, J.E., Robinson, S.K. & Levey, D J. 2010. Squeezed at the top: Interspecific aggression may constrain elevational ranges in tropical birds. Ecology 91: 1877-1884. View.

The inspiration for the current study; Jankowski et al. used playback experiments to measure interspecific aggression between three pairs of elevational replacements in Costa Rica.

Pasch, B., Bolker, B.M. & Phelps, S.M. 2013. Interspecific dominance via vocal interactions mediates altitudinal zonation in Neotropical singing mice. American Naturalist 182: E161-E173. View.

Birds are not the only taxa where you find territorial species-pairs of elevational replacements. Pasch et al. investigated behavioral interactions between an elevational replacement species-pair of singing mice. Impressively, they combined playback experiments and small-scale removal experiments (the joys of working with small rodents!); the results of both approaches were similar.


Image credit

Top image: White-eyed Robin Pachycephalopsis poliosoma © Ben Freeman

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