Diana H Wall Richard D Bardgett Alan P Covich and Paul VR Snelgrove

Many species of plants and animals live in soils and sediments, and they play crucial roles in providing ecosystem services for human well-being. A comprehensive synthesis of existing information on which habitats, taxa, and ecological functions in soil and freshwater and marine sediments are most essential is urgently needed if we are to maintain or restore their low-cost natural ecosystem services. This is of increasing importance when we recognize that more than 90 percent of the energy that flows through an ecosystem eventually passes through the food webs in the below-surface system. Therefore, the consequences of the loss of species and their functional roles may have far-reaching effects. Relatively little is known, however, about how species loss in soils and sediments will have direct and indirect effects on these functions and associated ecosystem services, or how this will feed back to below-surface systems (all chapters, this volume; Wall et al. 2001b).

This book addresses this vast underworld ecosystem and considers the consequences of global changes on the capacity of a rich diversity of organisms to provide ecosystem services. It is the first rigorous synthesis of the ability of the biodiversity both within and across soils and sediments to provide ecosystem services. To bring existing information together, international scientists specializing in the biodiversity and ecosystem functioning of one of the three domains examined the biota, habitats, and ecosystem functions provided by the biota. They further analyzed biotic interactions, abiotic factors, and the ecosystem services provided by the biota. This baseline was then incorporated into a preliminary appraisal of how the biota and services in each domain will be affected at var-

Table 10.1. Threats to soil biodiversity and to their ability to provide critical ecosystem services.

Category

Terrestrial

Freshwater

Marine

Exotic species

Imported soils

Ballast water

Ballast water

Imported plants

Aquaculture

Aquaculture

Pollution

Mining

Agriculture

Sewage effluents

Agriculture

Industrial waste

Oil spills

Industrial waste

Aquaculture

Industrial waste

Harvesting

Agriculture

Water extraction

Fisheries

L°ggmg

Ocean bottom trawling

Habitat

Clearcutting

Water diversion

Coastal

destruction

reclamation

Agriculture

Dam building

Dredging

Urbanization

Channelization

Fishing

Global climate

Altered vegetation

Severe drought

Circulation

change

changes

Multiple climatic

Severe flooding

Species

events of drought, rain

Bank erosion

compression

ious spatial scales by global changes (e.g., land-use change, invasive species, atmospheric change) (see Table 10.1). The ecologists offer an in-depth appraisal of these issues by discussing specific case studies. They provide useful analytical frameworks that will enable readers to systematically consider the vulnerabilities of the organisms to global changes and the subsequent effects (positive or negative, direct or indirect) on other biota, the habitat, biogeochemical cycling, and ecosystem services. This approach provides powerful tools in the form of frameworks or templates that will allow scientists, land managers, and others to consider how understanding the below-surface system can provide management options for longer-term provision of ecosystem services.

This book fills a crucial gap in scientific knowledge by amplifying information on the critical roles of soil and sediment biota in the operation of the Earth's system. The observation that overexploiting one service can diminish another service emphasizes that trade-offs among alternative ecosystem management goals within and among domains can be important and should be considered in any analysis of complex ecosystem relationships. Linear approaches to management are often too focused on a single ecosystem service. For example, increasing crop or fish production by increased use of nitrogen-

and phosphorus-rich fertilizers often has detrimental downstream effects on water quality and recreational uses of lake and river ecosystems (Covich et al., Chapter 3; Giller et al., Chapter 6). Given these detailed and forward-thinking analyses about the sus-tainability of soils and sediments, what are some of the important findings?

A wide diversity of habitats exists within each domain. Soils and sediments are heterogeneous within small (centimeters to meter) and large (meter to kilometers) scales, with mosaics of physical and chemical habitats derived over geologic history and modified by climate, weathering (e.g., water, erosion), and above- and below-surface biodiversity. In other words, habitats within each of the soils, freshwater sediment, and marine sediment domains are often not physically or chemically alike. Also, the biological species that have evolved in these heterogeneous habitats have a range of life-history characteristics that determine their ability to withstand and recover from different global environmental changes.

The below-surface biota are intimately connected to other ecosystems. If we are to appreciate their role in providing ecosystem services, it is imperative that we integrate interdisciplinary research to examine these connections to adjacent ecosystems. Soils and sediment domains connect physically, chemically, and biologically, in three major ways: (1) below-surface, (2) above- and below-surface, and (3) laterally, above-surface (Table 10.2; Chapter 1, Figure 1.1). The biota in each domain, including soils, are fundamental to the regulation of groundwater quality and quantity and to the transfer of materials to adjacent domains. The food webs in the habitats of each domain become linked in a below-surface network that rapidly transforms and transfers materials, particles, and organisms across spatial scales that cover centimeters to kilometers. The feedback linkages between the below-surface components that control these material fluxes are understudied relative to above- and below-surface connections (Wardle et al. 2004). What is evident is that, for soils and sediments, the role of the biota in providing ecosystem services declines as human use intensifies. For example, the rapid increase in the rate of soil biotic habitats (soil types) (Amundson et al. 2003) and land area lost to urbanization and agricultural expansion is a major global-change driver affecting soil biodiversity and its provision of ecosystem services (van der Putten et al., Chapter 2; Wardle et al., Chapter 5; Wall et al. 2001a).

Another key finding is that there are similarities in the ecosystem processes (e.g., nutrient and energy pathways) (Table 10.2) and services, and in the functional groups (Chapter 1, Table 1.3), but the biodiversity of the organisms differs greatly across domains. Ecosystem processes such as decomposition, primary production, and hydro-logical cycling are major components of the below-surface system of sediments and soils, and bioturbators, or ecosystem engineers, appear to be critical taxa in many ecosystems for stabilizing both soil and sediments. Our knowledge, however, about the distribution, diversity, and role of larger organisms in driving ecosystem processes is better understood than it is for the smaller fauna.

Table 10.2. Below-surface connectivity and examples of similar ecosystem processes.

The connectivity of each domain (soil, freshwater sediments, or marine sediments) determines physical and biological states of adjacent terrestrial and aquatic ecosystems. The biota living below the surface regulate the movement and fate of materials and are integrally connected to physical and chemical environments and to other biota.

Interconnectivity Similarities of Below-surface Habitats in Ecosystem Processes

Below surface

Soils, freshwater sediments, and marine sediments are linked by feedbacks in various cycles

Each soil and sediment domain is linked to groundwater

Above and below surface

Each soil and sediment component is linked to dynamic interfaces and to the atmosphere

Lateral: above surfaces Ocean ^ Freshwater Freshwater ^ Land Land ^ Ocean

Primary productivity, leaching of nutrients, transfer of water, nutrients, particles and organisms; specialized biota in interface habitats

Recapture/loss of nutrients, particles, and organisms from/to groundwater

Nutrient cycling through decomposition and primary productivity, secondary production; atmosphere connections; biodiversity and food web dependence

Erosion and deposition of inorganic and organic particles, hydrologic cycling, energy transfers, nutrient cycling; transfer of organisms, and particles

Where Do We Go From Here?

There is an abounding biodiversity in soils and sediments, although we still do not know many of the species present or whether there are hot spots of biodiversity in different domains. This limitation is partly because much of our global knowledge of biodiversity and ecosystem functioning in soils, freshwater sediments, and ocean sediments is gained from studies primarily at small scales in northern temperate ecosystems. We have little evidence that biodiversity at the species level is directly related to ecosystem functioning, particularly for marine and freshwater sediment organisms where there have been fewer experiments than there have been for soils (Covich et al., Chapter 3; Weslawski et al., Chapter 4; Giller et al., Chapter 6; Snelgrove et al., Chapter 7). However, assemblages of species and their interactions in food webs appear to be key components in regulating ecosystem processes. Less understood is the extent to which specific interactions—such as facilitative interactions, symbioses of microbes with different metabolic functions, parasites of invertebrates, or predator-prey relationships— are critical to the provision of ecosystem services in or among soils and sediments. The resistance and recovery of assemblages of soil organisms to a disturbance or stress, or to multiple stresses such as with global changes, is also not well known and can be very context dependent. Studies of ecosystem functioning and the provision of ecosystem services need to include these and many abiotic variables that affect biodiversity patterns and their linkages within and across below-surface domains to establish a more comprehensive understanding for sustainable management and conservation. Quantitative information gained at the multi-species level from a number of robust experiments at small and large spatial scales and longer temporal scales must be conducted regionally and globally for a greater predictive capability concerning threats to, and controls on, different ecosystems and their services.

Global changes affect soil and sediment organisms, directly and indirectly, through connections to other ecosystems (Table 10.2). There is sufficient evidence to indicate that diversity of species decreases with habitat disturbance (climatic or direct human intervention), leading to the dominance of communities by a few species. Therefore, measures of biodiversity other than species richness, such as evenness of species abundances, are needed as indicators of disturbances that impact soil and sediment biodiversity with a subsequent loss of services. Different, but relevant, indicators of soil and sediment biodiversity are critical if we are to expand management options, which presently are based on relatively few species in microcosm and small-scale field experiments, to more diverse assemblages and to larger spatial and longer temporal scales. Because organisms and their regulation of ecosystem processes occur across a range of scales within and among domains, the impact of global changes will be manifested on ecosystem services across multiple scales (Lavelle et al., Chapter 8; Ineson et al., Chapter 9). A priority for future research is incorporating effects of multiple stressors into experiments regionally.

The information presented here provides sufficient evidence that soils and sediments and their biota must no longer be considered as isolated systems that are less urgently in need of study. It is imperative for ecosystem sustainability at local, regional, and global levels that all of society considers the vertebrate, invertebrate, and microbial biodiversity of soils and sediments as having a crucial role in the provision of ecosystem services. It is apparent, based on the brief summary above, that in order to determine whether valued below-surface ecosystem services will be lost under global changes, we must determine through a quantitative, systematic approach how critical species respond to disturbances, and identify which biota and processes are most sensitive.

Literature Cited

Wall, D.H., G.A. Adams, and A.N. Parsons. 2001a. Soil biodiversity. In: Global Biodiversity in a Changing Environment: Scenarios for the 21st Century, edited by F.S. Chapin, III, O.E. Sala, and E. Huber-Sannwald, pp. 47-82. New York, Springer-Verlag.

Wall, D.H, P.V.R. Snelgrove, and A.P. Covich. 2001b. Conservation priorities for soil and sediment invertebrates. In: Conservation Biology, edited by M.E. Soule and G.H. Orians, pp. 99-123. Washington, DC, Island Press. Wardle, D.A., R.D. Bardgett, J.N. Klironomos, H. Setala, W.H. van der Putten, and D.H. Wall. 2004. Ecological linkages between aboveground and belowground biota. Science 304:1629-1633.

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