Latitudinal and Elevational Range Shifts under Contemporary Climate Change 600 The Mechanics of Range Shifts: Distinction between Range Core and Range Periphery Lo ng de itu itu t La itu La de (a) Lo ng de titu de (b) Figure Representation of a species’ range (red dots) across the three geographic dimensions (latitude, longitude, and elevation) of a given landscape in the Northern Hemisphere ( the ‘‘volcano’’ data set available in R has been used (R Development Core Team, 2010) for illustrative purpose) and at two different time periods: t1 (a) and t2 (b) Note that the illustrative species shifted its geographic range upward and poleward between t1 and t2 order to understand the mechanics of range shifts under climate change Given this conceptual framework, a given species may respond differently to the environmental changes over time It may (1) relocate toward newly suitable geographic locations, completely or partially tracking its environmental requirements (range shift) (Lenoir et al., 2008); (2) remain within the same locations and acclimate or adapt to its new environment (niche shift) (Davis and Shaw, 2001); (3) go extinct (Svenning, 2003); or some combination of these responses In short: move, adapt, or die (Maggini et al., 2011) How Do Species Shift Their Ranges? Species’ ranges are not static over time, but constantly shift either more or less stochastically by means of population dynamics within a steady-state environment or directionally through population dynamics within a shifting environment that is spatially autocorrelated (Figure 2) Although many human-induced changes in the global environment, such as land-use change, climate change, nitrogen deposition, biological invasions, and atmospheric CO2 increase (Sala et al., 2000) may trigger species range shifts, here the focus is on the effect of contemporary climate change In a steady-state environment, stochastic range shifts are the net result of growth, decline, colonization, and extinction processes that are tuned to different temporal scales (Breshears et al., 2008) Whereas local populations grow and decline over relatively short time periods, local populations colonize and go extinct over much longer time periods, especially for longlived species Additionally, growth, decline, colonization, and extinction processes are tuned differently across a species’ range (Figure 2(a)) Notably, there may be a strong core–periphery distinction leading to lower population densities near the peripheral area in comparison to the core area (Brown and Lomolino, 1998) Here the term ‘‘core’’ instead of ‘‘center’’ is explicitly used when referring to the geographic location of the abundance peak within a species’ range Indeed, the geographic location of the highest population density or abundance of a given species within its range is not necessarily located at the geometric center or centroid of its range (Murphy et al., 2006), even though the core area of a species’ range may be represented at the range center for simplicity (Figure 2(a)) At the core area of a species’ range, sometimes called species’ optimum to avoid confusion with the geometric center, local populations are generally thought to experience favorable conditions leading to enhanced fitness and population density In other words, at the core, birth and emigration rates may exceed death and immigration rates, respectively, leading to sink populations (Pulliam, 2000) Such populations not only are likely to grow within the suitable localities already occupied, but are also likely to colonize and establish populations in suitable localities not previously occupied by the species in the core area of its range and even beyond, if dispersal and biotic interactions are not constraints Decline and extinction processes might occur there as well, but to a lesser extent, unless there is a drastic shift in environmental conditions At the peripheral area of a species’ range, local populations may experience unfavorable conditions, leading to reduced fitness and reduced population density In other words, at the periphery, death and immigration rates may exceed birth and emigration rates, respectively, leading to sink populations (Pulliam, 2000) Such rangeedge sink populations will tend to decline and eventually go extinct, except when maintained by a ‘‘rescue effect’’ through constant immigration from source populations, typically located close to the core area of the species range Growth and colonization processes might occur there as well, but to a lesser extent, unless there is a drastic shift in environmental conditions Directional Range Shifts under Climate Change Let us now consider shifting conditions in the environment, such as the temperature and precipitation changes observed during contemporary climate change (IPCC, 2007b) Any shift in climatic conditions in a given location will impact the individuals living there and thus affect population dynamics locally For instance, species sensitive to temperature may respond to a warmer climate through local changes in growth, decline, colonization, and extinction rates On the one hand,