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Speciation is a complex process, in part because it can be caused by a multitude of different mechanisms. One aspect of speciation that has received much attention lately is the importance that variation among environments may play in driving speciation, i.e., ecological speciation. However, attention has been largely limited to the role of divergent natural selection as a consequence of such variation. Nonetheless, variation among environment may also result in flexible individual responses: phenotypic plasticity, adjustment of the environment, and selection of the environment (incl. habitat choice). Here I discuss and give examples of how these other consequences of variation among environments can generate ecology-driven speciation, including when they are interacting with each other or with natural selection. I propose that such a bottom-up approach to speciation may help us uncover neglected aspects of the speciation process, including when speciation may not occur.

Climate change has been shown to be an increasingly important driver of changes in the distribution, abundance and life history of bird species, causing changes of biodiversity and community compositions, e.g., measured by climate change indicators. Especially changes in the distributional ranges of species have been demonstrated in many cases, with some species being driven towards their altitudinal or latitudinal limits leading to shrinking populations. But also changes of phenology, genetics, and population sizes have been proposed to be caused by climate change. Although there is recent evidence only for regional climate-driven extinction events for birds, climate change can be considered among the major risk factors that might lead to the complete extinction of bird species. Together with (and sometimes contradicting) land-use change and demographic effects, climate change is shown to be a risk factor especially for cold-dwelling, restricted-range, and slowly adapting species. However, indirectly, by means of climate change mitigation measures modifying land-use patterns, also widespread generalist species are becoming increasingly threatened. Despite demonstrated niche conservatism in several cases, also adaptation of species to climate change is taking place, changing their multidimensional niche spaces. Birds are not sufficiently tracking climate change, nor do northern and southern range limits shift at the same pace. Thus, altered migration phenology and distance have been proven, often proposed to result from phenotypic adaptation. In a few cases, however, genetic predisposition and/or microevolution appear to shape the impact climate change has on bird species. There is broad consensus that climate change will lead to a reshuffling of communities and altered selection pressures both within and among species. The extent of the projected changes largely differs among studies owing, e.g., to the uncertainty inherent in climate change predictions. Nevertheless, niche space is expected to undergo future changes for many species, which, in turn, can have beneficial or detrimental effects on the survival of the respective species or communities and may also lead to changes in the effectiveness of bird protection which in turn should be adjusted to climate change impacts.

Urban habitats and landscapes are markedly different from nonurban “natural” habitats. The major difference is the transformation of the land, from natural green areas to anthropogenic structures and impervious surfaces. To survive in the urban habitat, birds are forced to either accept or avoid the new conditions. In addition, the urban sprawl has led to a highly fragmented landscape, with islets of suitable bird habitat surrounded by highways and buildings that frequently act as barriers, even for mobile creatures such as birds. These altered conditions have changed the avifauna dramatically, with many species vanishing once an area is urbanized, thus resulting in a significant loss of local biodiversity. However, some species seem to thrive in the city, and these urban-dwelling species often show pronounced phenotypic differences (e.g., in behavior, physiology, and morphology) to their rural conspecifics. These phenotypic changes have been linked to specific urban selective drivers such as air pollution, artificial light at night, noise, different kinds of food, different predation pressures, and human disturbances. However, these drivers are often confounded, and it is hard to separate one urban factor as the main driver for the differentiation. Although the urban habitat is a large threat to biodiversity, it is also an exciting environment for studies of population divergence, evolutionary responses, and ultimately speciation in real time.