Temperature and competition determine formation of bird communities: study

In the natural world, when two or more species come together in the same area for a certain time, they form a community. The birds that visit your backyard in the winter or the fish in the nearby lake before a high-rise was built on it, are examples of species communities. For bird communities, it is usually temperature or competition that determines the composition of the communities. But a recent study finds that both temperature and competition together determine formation of bird communities.

The composition of communities was known to be determined by abiotic factors such as rainfall or temperature; or biotic factors such as competition, the number of species found in an area and the way the species have evolved. In temperate areas that have distinct seasons and fewer species, scientists believe that temperature influences the way bird communities are structured. While in tropical regions where temperatures do not vary greatly through the year and there is a rich haul of species, competition determines the composition of bird communities.

This study is among the first “to really compare and contrast two dominant frameworks for explaining variation in abiotic and  biotic factors in defining how communities are formed,” said Paul R. Elsen, co-lead of the study and, at the time, a postdoctoral researcher at the Woodrow Wilson School of Public and International Affairs, (WWSPIA), Princeton University, USA.

One framework suggests that temperature determines how birds group together in areas that have well-defined seasons. Within this consideration another hypothesis — known as the climate variability hypothesis — propounds that in areas that have poorly defined seasons, the species have adapted to fewer fluctuations in seasonal variation making them more sensitive to temperature.

The other framework suggests in environments that have poorly-defined seasonal patterns, competition governs how species form communities.

“Our study shows that neither of these frameworks is sufficient, and that temperature and competition do not operate independently,” said Elsen. “We show that species in non-seasonal environments are likely more sensitive to temperature, but that seasonal changes in temperature also influence their response to competition.”

“The study highlights the roles of climate and competition in structuring communities. People certainly believe that both must be involved, but this study is perhaps the first to put them together with clear tests,” said Trevor Price, a scientist at the University of Chicago, who also studies birds in the Himalayas, but was not involved in this study.

Himalayas provided ideal habitat for study

The researchers wanted to find out under what conditions either temperature or competition takes precedence over the other in restricting bird species to specific range limits and therefore in community formation.

To investigate these conditions, the researchers carried out their study in the Himalayas. Across the mighty mountain chain, western Himalayas resemble temperate regions while eastern Himalayas are more tropical in nature. Thus, in the east, the temperature between seasons doesn’t vary greatly and the region has more species. Moving towards the west, the seasons become pronounced with cold winters and warm summers, and the number of species vary across the seasons. The Himalayas hence provide ideal grounds for the researchers to test how temperature and competition influence the formation of bird communities.

The researchers surveyed birds by walking along transects in the Great Himalayan National Park that lies in Himachal Pradesh and in the Eaglenest Wildlife Sanctuary located in Arunachal Pradesh.

“We walked up mountain slopes in summer and winter, recording birds we encountered on our walks. At the same time, we had placed temperature recorders on the mountainside, so that we knew what temperature each bird was at when we saw or heard it,” said Umesh Srinivasan, postdoctoral researcher at WWSPIA and first author of the study.

The researchers tested predictions for three scenarios: how temperature, competition and both taken together asserted formation of bird communities.

Under the first assumption that temperature determines formation of bird communities, the researchers expected the birds, in the western Himalayas, to seek comfort of their preferred temperature ranges in both winter and summer by migrating between different elevations.

As the differences between the winter and summer temperatures are not considerable in eastern Himalayas, the birds are thought to have evolved low tolerance for large fluctuations in temperature, also making them move to areas with suitable temperatures.

Additionally, small birds were expected to seek temperatures that made them snug, with greater gusto, compared to largebodied birds that are able to regulate their body temperatures better.

With the other assumption, that competition influences formation of bird communities, the researchers expected birds closely-related to each other and that compete severely — two species of laughing thrushes for instance — to seek areas with different temperatures. Such a scenario is more likely in the eastern Himalayas as it has more number of species which fester more competition.

Further, within closely related birds, species of similar size are also expected segregate into areas with different temperatures.

And finally, with the hypothesis that temperature and competition together influence bird communities, the researchers predicted competition to increase in winter when food resources are scarce and to relax in summer when food is more readily available. Closely-related species were expected to segregate into areas with different temperature zones. As the temperature differences in seasons are more pronounced in the western Himalayas, such patterns are more likely.

Study results differ from established and expected outcomes

The analysis of the data lobbed interesting results. “Small species in tropical areas appear to be highly temperature-sensitive and adapted to survive in a pretty narrow temperature range,” noted Srinivasan. This proved to be consistent with the hypothesis that birds, in areas that have poorly defined seasons, have evolved low tolerance towards huge temperature variations. But it contradicts the prediction for areas with well defined seasons.

The study found the prediction for competition determining composition of bird communities in eastern Himalayas to hold true, as potentially competing species segregated.

The study also found the third prediction of increase in competition in winter to be true. Winter influenced competition differently in the eastern and western Himalayas. “We had hypothesised that the coming of winter can reduce the amount of food available to birds, appearing to lead to more fierce competition between species. In summer, food is likely to be abundant, allowing similar species to coexist in the same area,” Srinivasan said. This pattern was observed in western Himalayas.

Meanwhile in the east, in both seasons, similar species coexisted. However, in winter, birds closely related but of different sizes coexisted. In summer, closely related birds of similar sizes coexisted. Even though similar sized birds were expected to compete fiercely, their coexistence suggested abundance of resources.

All the results taken together find that bird communities in non-seasonal areas are influenced more by temperature. Paucity of resources in winter promotes competition in both temperate and tropical regions.