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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.
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.
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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