Species interactions limit the occurrence of urban-adapted birds in cities

Urbanization represents an extreme transformation of more natural systems. Populations of most species decline or disappear with urbanization, and yet some species persist and even thrive in cities. What determines which species persist or thrive in urban habitats? Direct competitive interactions among species can influence their distributions and resource use, particularly along gradients of environmental challenge. Given the challenges of urbanization, similar interactions may be important for determining which species persist or thrive in cities; however, their role remains poorly understood. Here we use a global dataset to test among three alternative hypotheses for how direct competitive interactions and behavioral dominance may influence the breeding occurrence of birds in cities. We find evidence to support the Competitive Interference Hypothesis: behaviorally dominant species were more widespread in urban habitats than closely-related subordinate species, but only in taxa that thrive in urban environments (hereafter, urban-adapted), and only when dominant and subordinate species overlapped their geographic ranges. This result was evident across diverse phylogenetic groups, but varied significantly with a country’s level of economic development. Urban-adapted, dominant species were more widespread than closely-related subordinate species in cities in developed, but not developing, countries; countries in economic transition showed an intermediate pattern. Our results provide evidence that competitive interactions broadly influence species responses to urbanization, and that these interactions have asymmetric effects on subordinate species that otherwise could be widespread in urban environments. Results further suggest that economic development might accentuate the consequences of competitive interactions, thereby reducing local diversity in cities.

城市化是对自然生态系统的极端改造。多数物种种群随城市化进程衰退甚至消失，但仍有部分物种得以存续，甚至在城市中繁衍生息。那么，究竟是什么因素决定了哪些物种能在城市生境中存续或繁盛？
物种间的直接竞争互作可影响其分布与资源利用，尤其在环境挑战梯度下。鉴于城市化带来的多重挑战，类似的竞争互作或许对决定城市物种的存续与繁盛至关重要，但其具体作用仍鲜为人知。
本研究依托全球数据集，针对三项备选假说开展检验，以探究直接竞争互作与行为优势如何影响鸟类在城市中的繁殖出现情况。
研究结果支持竞争干扰假说（Competitive Interference Hypothesis）：行为优势物种在城市生境中的分布范围相较于近缘从属物种更广，但这一模式仅存在于城市适应型类群（urban-adapted）中，且仅当优势与从属物种的地理分布范围存在重叠时成立。
该结果在多样的系统发育类群中均有体现，但会随国家经济发展水平出现显著差异：在发达国家，城市适应型类群中的优势物种的城市分布范围显著大于近缘从属物种；而在发展中国家则无此现象，经济转型国家则呈现中间过渡模式。
本研究证实，竞争互作会广泛影响物种对城市化的响应，且对从属物种存在不对称影响——这些从属物种本可在城市环境中拥有更广的分布。研究结果进一步表明，经济发展可能会加剧竞争互作的后果，进而降低城市的本地物种多样性。