Relative Vulnerability ofDifferent Biotic Components ofCommunities

Subsets of the soil biota may differ considerably in terms of how they are affected by global change phenomena. Of these phenomena, the best understood for the soil biota is land use change, including intensification of land management practices. The degree





Figure 5.1. Different response dynamics of soil biota to disturbance (from Brown et al. 2001). The effect of a disturbance can result in changes to soil biomass, density, or diversity with very different results over time. This will affect the ecosystem services provided by the biota.

of impact of global change phenomena, such as land use change, is dependent on the organism's ability to withstand change, the organism's and ecosystem's resilience and resistance to the imposed changes, and the extent of the changes/disturbance imposed (difference from original environment). These can generally follow the different response strategies shown in Figure 5.1. Some organisms are susceptible to certain land management practices and become locally extinct, while others are opportunistic and take advantage of the modified conditions to increase their abundance, biomass, and activity. For example, the conversion of Amazonian rainforest to pastures north of Manaus led to the elimination of many (morpho-) species and groups of macrofauna, while one species of earthworm (Pontoscolex corethruruSs) became the dominant soil macroorganism, reaching biomass values of up to 450 kg ha-1 (Barros 1999). This led to the progressive accumulation of macro-aggregates (worm castings) on the soil surface, dramatically decreasing soil macroporosity down to a level equivalent to that produced by heavy machinery, rendering it anaerobic and increasing methane emission and de-nitrification (Chauvel et al. 1999).

Some organisms may increase or decrease in numbers and biomass for only a short period (temporary or elastic) but then return to predisturbance proportions, while others remain unchanged or only slightly unchanged (persistent or resistant). For instance, if new land-use practices imposed maintain enough similarities to the previous ecosystem (e.g., conversion of native grass savanna to pastures), many soil organisms may resist the change, while some of those negatively affected in the land-preparation and conversion phase (tillage and seeding or transplanting) may eventually recover in the new system once proper soil cover and plant organic matter inputs are re-established (Jiménez & Thomas 2001).

Conversion from forest to agriculture usually results in an overall reduction in microarthropod populations (Crossley et al. 1992), but the response dynamics of component taxa varies. Continuous cultivation, rotations, monoculture, and application of pesticides soon eliminate species susceptible to damage, desiccation, and destruction of their microhabitats, especially those with a life cycle longer than one year, such as many oribatid mites. In contrast, practices such as drainage, irrigation, manuring, and fertilizer use encourage seasonal multiplication of species of prostig-matid mites and Collembola, and their predators, mesostigmatid mites (Crossley et al. 1992).

Several studies reveal that increases in the intensity of agriculture lead to reductions in the diversity of soil biota (Siepel 1996; Yeates et al. 1997). This diversity loss is not a random process. For instance, Siepel (1996) showed that declines in the diversity of soil microarthropods with increasing agricultural intervention were accompanied by dramatic shifts in the life-history characteristics and feeding guilds of the community. Loss of diversity in low-input agricultural systems was explained by the disappearance of drought-intolerant species because low-input grasslands are cut in summer, thereby increasing the chance of drought in the litter layer. However, the loss of species in high-input grassland was explained by the elimination of fungal-feeding grazers that were replaced by opportunistic bacterial-feeders. Moreover, abandoned high-input sites still lacked fungal-feeding mites, even after 20 years of management for nature conservation, due to the low population growth and dispersal rate of these species (Sie-pel 1996).

Effects of land-use practices on soil biota in turn exert profound effects on key ecosystem processes that they perform, and ultimately on the delivery of ecosystem services. Enhanced disturbance regimes tend to favor the bacterial-based energy channel of the soil food web over the fungal-based channel, and management practices which are known to favor the bacterial channel include conventional (vs. non) tillage (Hendrix et al. 1986), nitrogen fertilization (Ettema et al. 1999) and forest clear-cutting (Sohlenius 1996). Domination of soil food webs by bacterial energy channels leads to greater short-term mineralization rates of carbon and nutrients, leading to greater net losses of soil organic matter and reduced retention of nutrients in the soil in the longer term. The tendency of agricultural intensification practices to favor soil animals of smaller body sizes probably also has similar effects on ecosystem nutrient losses (Wardle 1995).

Therefore, effects of land-use practices on the composition of soil biota may have important flow-through effects on key ecosystem services provided by soil organisms, in particular those relating to the maintenance of soil fertility and plant-available nutrient supply.

Unfortunately, given the overwhelming diversity of soil organisms and their functions and interactions in soils, information on the response dynamics of various groups/taxa and species of soil organisms to land-use change and its possible effects on soil function is not available for many sites/land uses. Furthermore, there can also be important differences in the effects on the same organism/function to forces of change, depending on local climate or soil conditions. Processes occurring locally or influenced by organisms at a small scale are less likely to be influenced by change at a large scale, unless it undermines their populations or ability to maintain activity. However, cascading effects of loss of a particular organism or function within the ecosystem could have consequences far beyond the small scales at which they were initially operating.

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