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impacts of recent climate change on populations and species. It is also increasingly apparent that not all species respond in identical ways, creating the poten- tial for mismatches in the timing of events or spatial associations within ecosystems. Regional differenc- es in the magnitude of climate change may further complicate the population dynamics of certain species, such as long distance migrants, that depend on the environmental conditions of more than one area (Stenseth and Mysterud 2002). Phenological events, such as the timing of flowering, the onset of breeding, or the timing of migration, have typically evolved through natural selection to match environmental conditions. These seasonal life cycle events are generally tied to environmental cues, and a growing number of studies have documented changes in phenology in response to recent climate change (e.g., see McCarty 2001, Walther et al. 2002, Parmesan and Yohe 2003, Root et al. 2003, Parme- san 2006 for reviews). However, as local conditions drive phenological events, it is regional changes in climate, rather than global changes, that are likely to be more relevant in the context of species and habitat responses to climate change (Walther et al. 2002). Differences in the rate and magnitude of change across the globe will contribute to heterogeneity in ecological dynamics across systems, potentially disrupting interactions across trophic levels as well as co- evolved relationships such as pollination and seed dispersal. Long -term data sets from Europe and North Amer- ica document phenological changes across taxa, including timing of flowering and leaf out in plants, first appearance of butterflies, initiation of breed- ing in birds, timing of phytoplankton blooms, and choruses or spawning of amphibians (McCarty 2001, Parmesan 2006), which are generally associ- ated with warmer temperatures and earlier onset of growing seasons in northern latitudes. Across species and regions, these observed advances in phenological timing range from a day or less to several weeks per decade (McCarty 2001, Table 1 -1). However, not all species will have the capacity to respond rapidly to climate change, and this variability in response has the potential to disrupt correlations with other ecological factors. For example, population declines in the migratory pied flycatcher (Ficeclula hypoleu- ca) in the Netherlands have been associated with a mismatch between the timing of breeding and their main food supply (Both et al. 2006). Populations have declined by 90% in areas in which the peak in caterpillar abundance in spring has started earlier than the birds' breeding date. The laying dates of resident great tits (Tarus major) have not advanced in concert with the availability of insects and peak food demands for food and thus face a similar mismatch (Visser et al. 1998). Shifts in the timing of emergence or arrival in response to climate change may also have repercus- sions on competitive interactions within and among populations of species. For example, Winkler et al. (2002) found that laying dates in tree swal- lows (Tachycineta bicolor) were more constricted in warmer years. Greater synchrony of hatching dates among nests in warmer years may result in increased