Controls on soil biodiversity: insights from extreme environments
Introduction
Knowledge of the distribution and diversity patterns of soil species and their influence on ecosystem processes is currently inadequate, partially due to the high species richness and complexity of below-ground ecosystems (Brussaard et al., 1997). Previous research has addressed this problem by pooling species into functional groups to reduce the complexity, and approach that helps us to describe the critical tasks that organisms perform in ecosystems, e.g. decomposition, nitrogen fixation, etc. However, this approach tells us nothing about interactions between, and redundancy of, functions among species. Another possible approach is to examine species in ecosystems with naturally reduced complexity, such as extreme environments. These studies on individual species within soil communities can be used to unravel mechanisms that are hidden in more complex ecosystems, and to predict how ecosystems will respond to increasing environmental change and disturbance (Freckman and Virginia, 1998).
Environmental changes, e.g. atmospheric composition and climate, have increased in importance today because of the rate and intensity, and the global scale at which they are occurring (Vitousek et al., 1997). These changes will determine the above- and below-ground diversity of plants, animals and microbes that will exist in the future (Swift et al., 1998; Wardle et al., 1998). In geological time, climate changes have resulted in the extinction of many animal and plant species. Today, we see, as a result of human activities, an accelerated frequency and magnitude of both catastrophic and milder environmental events (e.g. soil erosion and salinisation, pollution of air, soil and water) at local, regional and global scales, resulting in extinction of entire plant and animal communities (Hoffman and Parsons, 1997; Vitousek et al., 1997).
The consequences of these accelerated changes for soil biotic communities and ecosystem functioning must be identified and evaluated. Research is focusing on the effects on soil biota (Smith et al., 1998; Wardle et al., 1998) of increased atmospheric nitrogen deposition (Holland and Lamarque, 1997), deforestation (Bloemers et al., 1997), agricultural practices (Matson et al., 1997) ecosystem reclamation and restoration (Zink and Allen, 1998), pollution (Bongers, 1990) and desertification (Freckman and Virginia, 1989), as well as the consequences of soil biodiversity loss, for sustainable land use.
These natural and human-induced changes may represent new environmental extremes or challenges for many organisms, and concerns are raised about their response and survival. Yet, extreme conditions exist in natural ecosystems around the world where low-diversity soil communities have reproduced and survived. The study of these systems can provide insights into the response of soil species to perturbation and environmental change (Hoffman and Parsons, 1997). This paper uses soil nematodes as a model for the soil biota to provide some of the insights from these extreme ecosystems on the responses of soil biodiversity, soils and ecosystem functioning to changes in the environment. We propose these insights will be useful (a) to elucidate similar mechanisms and relationships that occur, but may be masked, in more complex ecosystems, and (b) to predict the effects of environmental change on soil organisms and communities.
Section snippets
Soil nematodes
The impacts of a changing environment, or other perturbation, on soil organisms, as measured by their diversity, abundance, survival or extinction, and how their response may affect ecosystem processes, is much more difficult to measure than for above-ground biota. Microscopic soil organisms, including invertebrates such as nematodes, represent a major component of the Earth's biodiversity, one whose importance is well acknowledged, but whose taxonomic dimensions remain obscure (May, 1988).
Extreme environments
Examples of naturally occurring soil environments that can be considered extreme include, but are not limited to, caves, deserts, high-altitude and high-latitude ecosystems, and saline soils and sediments. In addition, there are precipitous events, such as fire, hurricanes and floods, that allow us to examine how the soil biota and the ecosystem responds to sudden changes in the environment. Our discussion is limited to two desert systems. The biota in these two water-limited soil ecosystems
Decomposition-based food webs can be very simple
The main difference between the polar deserts of Antarctica and the Jornada is the presence in the Jornada of woody vegetation that acts to concentrate biological activity in discrete patches. The factors responsible for creating suitable soil habitats are not visible in the Dry Valleys, but rather result from long-term dynamics of glaciers, lakes and their influences on primary productivity (Virgina and Wall, in press). Antarctic Dry Valleys represent the most extreme soil habitat on earth (
Conclusion
These two studies of nematode communities in the extreme environments of the Antarctic and Chihuahuan deserts show that, in simple systems, effects of disturbance on individual species in soil communities could have a high impact on ecosystem processes (e.g. decomposition, herbivory, energy flow). Korthals et al. (1996), (see also Niles and Freckman, 1998) noted that closely related nematode genera have different responses to pollutants. Research on feeding behaviours and life histories (Ferris
Acknowledgements
The authors wish to thank Dr. Gina Adams for her critical and helpful review, anonymous reviewers, and Mr. Dan Bumbarger, Mr. Mark St. John and Dr. Andy Parsons for their help with this manuscript. D. Wall wishes to acknowledge Dr. D.R. Strong and the Bodega Marine Laboratory for their support. This research was supported by National Science Foundation grants OPP 96 24743 to D. Wall and R.A. Virginia, and NSF DEB 96 26813 to D. Wall, and is a contribution to the McMurdo LTER (NSF OPP 92 11773)
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