Passerine assemblages show niche expansion from natural to urban areas

Pictures of six common passerine species in our study locations. Species were increasingly tolerant to habitat transformation from left to right. Left-up: Red-billed Leiothrix Leiothrix lutea. Left-down: Grey-cheeked Fulvetta Alcippe morrisonia. Middle-up: Streak-breasted Scimitar-Babbler Pomatorhinus ruficollis. Middle-down: Rufous-capped Babbler Stachyris ruficeps. Right-up: Red-whiskered Bulbul Pycnonotus jocosus. Right-down: Japanese White-eye Zosterops japonicus.

By Emilio Pagani-Nunez

Recently, there has been a burst of research activity investigating the impact that habitat transformation has on biodiversity patterns, individual fitness, species’ traits and community structure. Overall, most of these studies stress the negative consequences that habitat transformation has on biodiversity. Two main effects are of special interest for our research. First, at a regional scale there is a sharp decrease in biodiversity accompanying habitat transformation. Second, as a result of this process of biodiversity loss, persisting species are more similar to each other and thus redundant, i.e. there is functional homogenization.

A key question is how these environmental changes affect species’ and assemblages’ niches. Traditionally, niche “packing” and “expansion” are the main ways in which niche changes across environmental gradients are interpreted. Niche packing means that, with increasing diversity, species should have narrower or more overlapped niches, with the assemblage niche space staying relatively constant. Niche expansion means that, with increasing diversity, species’ niche width and overlap may remain constant, and hence assemblage niche space may increase.

In our study in southern China, we tested whether niche packing or expansion was the main mechanism explaining niche changes in passerine birds across a habitat transformation gradient, from natural forests to urban areas (Fig. 1). We had to reformulate the paradigm to fit this situation. Here we expected that species’ niche width would steeply increase from natural to urban areas because species in natural habitats are specialists, while species in transformed habitats are habitat and diet generalists. Moreover, because our study was conducted in a subtropical region, we also expected that alpha diversity would remain relatively high even in human-disturbed areas. Hence in this situation, niche packing would occur if niche overlap got greater in transformed habitats, counteracting greater niche width, whereas niche expansion would occur if niche overlap was constant, creating a larger assemblage niche (Fig. 2). We attempted to distinguish between these patterns by measuring stable isotopes (δ15N and δ13C) in claws of 28 species and 608 individuals (bird pictures at top of blog post).

Figure 1. Pictures of the different habitat types in which we carried out our study. Above: heavy rain in the mixed broadleaf subtropical forest in Damingshan (Guangxi, South China). Middle: agricultural area besides the Nujiang River surrounded by Gaoligong Mountains and close to Baoshan town (Yunnan, Southwest China). Below: overview of the Guangxi University campus in Nanning city (Guangxi, South China).​

Figure 2. Graphical representation of how niche characteristics can change from natural to human-made habitats. P1: There could be a progressive increase in species’ niche width and overlap from natural to human-made habitats without affecting overall assemblage niche space, resulting in niche packing. P2: Alternatively, although niche width may increase, niche overlap could be rather constant across this gradient, resulting in niche expansion.​

Our results supported niche expansion, rather than packing, from natural to human-made habitats. We found that species’ niche width strongly increased from natural to urban habitats, while niche overlap among species at each assemblage remained rather unchanged across this gradient. The fact that niche overlap among species was nearly constant suggests that passerines at each assemblage were composed of complementary species, regardless of human disturbance. While at regional scales the loss of specialized species in human-made habitats is evident (lesser beta diversity), at each location species assemblages can remain relatively diverse, especially in the tropics/subtropics. Thus, although at a regional scale there may be progressive functional homogenization with habitat transformation, at the scale of the local assemblage, species may remain diverse in their foraging niches.

We obviously have to protect natural habitats because they comprise many wonderful species structured in extremely complex trophic webs. Yet since transformed habitats are human constructs, there is room to improve the way in which we manage these ecosystems, too, to increase and maintain taxonomic and functional diversity. Our study is important in stressing that when considering the ecological consequences of habitat transformation, such as functional homogenization, it is very important to discriminate between regional and local processes. We may conclude by suggesting that in most cases, in any assemblage, species will tend to be complementary rather than redundant in their foraging niches.

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