Species up a mountain

After a short hiatus, we are back!
Anand Krishnan continues with the next post in his series Specimen Tales ~

In continuation with the previous post’s theme of how species distribute themselves, we deal here with altitudinal separation of species. Many species tend to replace each other over a gradient of elevations, such as that of the Andes of South America. Habitat types change as altitude increases; forests decrease in height, average temperatures are lower, and overall species diversity decreases as fewer and fewer lowland species can make it all the way up a high mountain. In many cases among birds, the montane populations become isolated from their sister species in the lowlands, and develop into new species.

The jewel-babblers of New Guinea (Ptilorrhoa) are a great example of this phenomenon. Three species occurring across the island separate out from each other over the altitude gradient. The Blue jewel-babbler (Ptilorrhoa caerulescens), occupies lowland rainforest, the Chestnut-backed jewel-babbler (Ptilorrhoa castanonota) occupies mid-mountain forests, and the Spotted jewel-babbler (Ptilorrhoa leucosticta) is found in high-elevation montane forest. You might think this was simple, right? Species occupying different habitats at different elevations? Well, it turns out it’s a bit more complicated than that. A study by Cornell University’s Benjamin Freeman on species pairs in the Neotropics, Himalayas and New Guinea (including two of the species above) found that this elevational divergence was only observed if related species occupied the same geographic location. In other words, if these jewel-babblers had occupied different regions of New Guinea, they might all be found at similar elevations, but instead, they overlap geographically. What happens then is that so-called ‘secondary contact’ occurs, resulting in elevational divergence to avoid competition. These interesting findings serve to highlight the general theme: where a species is found is actually a complex result of many ecological interactions. Additionally, these interactions are dynamic, so species distributions could change over time. Imagine, hypothetically, if one of these jewel-babblers were to disappear from a region of overlap? Would the others expand their ranges to occupy its niche? Food for thought!

jewel_babbler
Museum specimens of Blue (bottom), Chestnut-backed (middle) and Spotted (top) jewel-babblers, from the collections of the Smithsonian Museum of Natural History, Washington D.C.

 

Reference:
Freeman, B.G. (2015). Competitive Interactions upon secondary contact drive elevational divergence in tropical birds. American Naturalist 186(4):470-79.

 

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