1.3.5.5 Species community changes and ecosystem processes

In many parts of the world, species composition has changed (Walther et al., 2002), partly due to invasions and distributional changes. The assemblages of species in ecological communities reflect interactions among organisms as well as between organisms and the abiotic environment. Climate change, extreme climatic events or other processes can alter the composition of species in an ecosystem because species differentially track their climate tolerances. As species in a natural community do not respond in synchrony to such external pressures, ecological communities existing today could easily be disaggregated (Root and Schneider, 2002).

Species diversity in various regions is changing due to the number of species shifting, invading or receding (Tamis et al., 2001; EEA, 2004) (see Sections 1.3.5.2 and 1.3.5.3). Average species richness of butterflies per 20 km grid cell in the UK increased between 1970-1982 and 1995-1999, but less rapidly than would have been expected had all species been able to keep up with climate change (Menendez et al., 2006). In non-fragmented Amazon forests, direct effects of CO2 on photosynthesis, as well as faster forest turnover rates, may have caused a substantial increase in the density of lianas over the last two decades (Phillips et al., 2004). Although many species-community changes are also attributable to landscape fragmentation, habitat modification and other non-climate drivers, many studies show a high correlation between changes in species composition and recent climate change, also via the frequency of weather-based disturbances (Hughes, 2000; Pauli et al., 2001; Parmesan and Yohe, 2003). Examples of altered or stable synchrony in ecosystems via multi-species interactions, e.g., the pedunculate oak–winter moth–tit food chain, are still fairly rare (van Noordwijk et al., 1995; Buse et al., 1999).