Biodiversity is assessed quantitatively at different levels—notably at the genetic level (i.e., the richness of genetically different types within the total population), the species level (i.e., the richness of species in an area), and the landscape level (i.e., the richness of ecosystem types within a given area). Overall, biodiversity is forecast to decrease in the future as a result of a multitude of pressures, particularly increased land-use intensity and associated destruction of natural or semi-natural habitats (Heywood and Watson, 1996). The most significant processes are habitat loss and fragmentation (or reconnection, in the case of freshwater bodies); introduction of exotic species (invasives); and direct effects on reproduction, dominance, and survival through chemical and mechanical treatments. In a few cases, there might be an increase in local biodiversity, but this usually is a result of species introductions, and the longer term consequences of these changes are hard to foresee.

These pressures on biodiversity are occurring independent of climate change, so the critical question is: How much might climate change enhance or inhibit these losses in biodiversity? There is little evidence to suggest that processes associated with climate change will slow species losses. Palaeoecology data suggest that the global biota should produce an average of three new species per year, with large variation about that mean between geological eras (Sepkoski, 1998). Pulses of speciation sometimes appear to be associated with climate change, although moderate oscillations of climate do not necessarily promote speciation despite forcing changes in species' geographical ranges.

Dukes and Mooney (1999) conclude that increases in nitrogen deposition and atmospheric CO₂ concentration favor groups of species that share certain physiological or life history traits that are common among invasive species, allowing them to capitalize on global change. Vitousek et al. (1997b) are confident that the doubling of nitrogen input into the terrestrial nitrogen cycle as a result of human activities is leading to accelerated losses of biological diversity among plants adapted to efficient use of nitrogen and animals and microorganisms that depend on them. In a risk assessment of Switzerland alpine flora, Kienast et al. (1998) conclude that species diversity could increase or at least remain unchanged, depending on the precise climate change scenario used.

5.2.3.1. Global Models of Biodiversity Change

Several general principles describe global biodiversity patterns in relation to climate, evolutionary history, isolation, and so forth. These principles continue to be the subject of considerable ecological theory and testing; the Global Biodiversity Assessment (Heywood and Watson, 1996) and the Encyclopaedia of Biodiversity (Levin, 2000) contain detailed reviews.

Kleidon and Mooney (2000) have developed a process-based model that simulates the response of randomly chosen parameter combinations ("species") to climate processes. They demonstrate that the model mimics the current distribution of biodiversity under current climate and that modeled "species" can be grouped into categories that closely match currently recognized biomes. Sala et al. (2000) used expert assessment and a qualitative model to assess biodiversity scenarios for 2100. They conclude that Mediterranean climate and grassland ecosystems are likely to experience the greatest proportional change in biodiversity because of the substantial influence of all drivers of biodiversity change. Northern temperate ecosystems are estimated to experience the least biodiversity change because major land-use change already has occurred.

Modeling to date demonstrates that the global distribution of biodiversity is fundamentally constrained by climate (see Box 5-2). Future development along these lines (e.g., adding competitive relations and migration processes) could provide useful insights into the effect of climate change on biodiversity and the effects of biodiversity on fluxes of carbon and water on a global scale.

There has been considerable progress since the SAR on our understanding of effects of global change on the biosphere. Observational and experimental studies of the effects of climate change on biological and physical processes have increased significantly, providing greater insights into the nature of the relationships. Greater biological realism has been incorporated into models of small patches of vegetation (point models), and more realistic biological representations have been incorporated into regional and global change models. The main improvement has been development of dynamic representations of biological processes that respond directly to climate. Nevertheless, several major challenges remain before fully effective models of the interaction between climate and biophysical processes will be available.

5.2.4.1. Landscape Processes

Most vegetation models still treat patches of vegetation as a matrix of discrete units, with little interaction between each unit. However, modeling studies (Noble and Gitay, 1996; Rupp et al., 2000) have shown that significant errors in predicting vegetation changes can occur if spatial interactions of landscape elements are treated inadequately. For example, the spread of fires is partly determined by the paths of previous fires and subsequent vegetation regrowth. Thus, the fire regime and vegetation dynamics generated by a point model and a landscape model with the same ignition frequencies can be very different. There has been considerable progress in modeling of spatial patterns of disturbances within landscapes (Bradstock et al., 1998; He and Mladenoff, 1999; Keane et al., 1999), but it is not possible to simulate global or regional vegetation change at the landscape scale. Thus, the challenge is to find rules for incorporating landscape phenomena into models with much coarser resolution.

Another challenge is to develop realistic models of plant migration. On the basis of paleoecological, modeling, and observational data, Pitelka and Plant Migration Workshop Group (1997) conclude that dispersal would not be a significant problem for most species in adapting to climate change, provided that the matrix of suitable habitats was not too fragmented. However, in habitats fragmented by human activities that are common over much of the Earth's land surface, opportunities for migration will be limited and restricted to only a portion of the species pool (Björkman, 1999).

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