Vulnerability of Ecosystems to Future Change

State-of-the-art assessments of likely future changes in ecosystems due to global change are typically not extrapolated from the observations reviewed above. This is because most ecosystem processes are driven by multiple factors, making the net result a highly nonlinear function of GHG increase or warming. A key issue for many ecosystems is whether there is a threshold that might be reached at which the intrinsic capacity of the ecosystem to adapt to the changing environment is exceeded. Another problem occurs where time lags of response exist, after which particularly strong impacts must be expected (i.e., due to failed reproduction of some important species). Many ecosystem processes show strong hysteresis, for example, the rate of response to reduced resources may be quick, while the recovery with increased resources could be much slower. Finally, changes in variability (e.g., extreme weather event frequencies) may be more important for ecosystems than gradual changes toward warmer or drier/wetter conditions. For all these and other reasons, the different drivers of change are typically investigated using numerical models, mostly based on experimental or observational evidence of

Figure 1 Changes in global vegetation distribution 2100-2000. Simulated changes (2100-2000) of the fractional cover of deciduous woody (top), evergreen woody (middle), and nonwoody (bottom) vegetation functional types for a strong climate change scenario (SRES-A2, HadCM3). Simulations with the LPJ-DGVM. Reproduced from LuchtW, Schaphoff S, ErbrechtT, Heyder U, and Cramer W (2006) Terrestrial vegetation redistribution and carbon balance under climate change. Carbon Balance and Management 1: 6.

Figure 1 Changes in global vegetation distribution 2100-2000. Simulated changes (2100-2000) of the fractional cover of deciduous woody (top), evergreen woody (middle), and nonwoody (bottom) vegetation functional types for a strong climate change scenario (SRES-A2, HadCM3). Simulations with the LPJ-DGVM. Reproduced from LuchtW, Schaphoff S, ErbrechtT, Heyder U, and Cramer W (2006) Terrestrial vegetation redistribution and carbon balance under climate change. Carbon Balance and Management 1: 6.

key mechanisms controlling ecosystem functioning. The accumulated observations of recent and historical changes serve as important data sources for the validation of these models. Only where process understanding is mostly lacking, correlative approaches are still being used, for example, for the estimation of changing geographic distribution of many species.

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