Why so many species

Samples of soil are notoriously rich in biodiversity. A handful of soil will contain numerous species from many phyla (Chapter 1). We have seen examples of this diversity and abundance in previous chapters, and more examples are outlined here. The abundance of individuals in a population (of any one species) depends on the duration that adequate growth conditions persist. It also depends on the interactions with other species, through predation, symbiosis or competition. However, the diversity of species depends on niche diversity. A niche describes the combination of abiotic parameters (such as pH, ion concentrations, temperature and moisture) as well as the range of tolerance for each parameter, that affect the activity and survival of a species. The niche is also delineated by specific nutrient requirements and biotic parameters. For example, syntrophy with other species requires that one or more other species be active at the same time or in close succession. Ecological theory predicts that species number reflects niche diversity (for a full discussion, see Krebs, 2001). Also, when food resources and space are not limiting, the carrying capacity increases. Both the available space and resources are allocated to more species. For example, a desert soil with limited resources will contain fewer species than a pasture with more abundant resources. However, the number of species can be grouped into a smaller number of functional groups (Chapter 4). Both the desert soil and the pasture would have the same functional groups, although with fewer species each in the desert soil. Functional groups cross taxo-nomic boundaries, unlike guilds, which represent species within the same functional group and the same taxonomic group that occur in the same habitat. Therefore, an ecosystem provides a range of abiotic conditions, resources and a spatial habitat for species. The space and other resources are allocated to species, which occupy a niche. Species can be grouped into a smaller number of functional groups which fulfil the same ecological function. Through the year, with seasonal cycles, the abiotic conditions, food resources and space vary in quality and quantity. Therefore, the niche occupied by a species becomes more or less abundant, or restricted with time. These changing conditions cause species to become quiescent at regular intervals. Their activity is replaced by other species which become active on the opportunities vacated. The shift in active species through time displays a succession of adaptations to changing abiotic parameters and resources. Species that become quiescent are not replaced by species that fill exactly the same niche. In part, it is because the resources change in quality and abundance as a result of decomposition by previous species. There is a stochastic element in which species are activated and become dominant.

The soil habitat is also notoriously heterogeneous in structure. With depth, the composition of the physical environment changes (Chapter 2). Horizontally, there is variation in litter composition at the landscape scale down to microscopic distances at the submillimetre scale. With depth, it is not just the litter decomposition state that varies. The depth profile also reflects the history of the origin of litter deposition. At a precise sampling point, this year's litter is followed by litter from previous years. However, the composition of litter at that location may be different between years (Fig. 5.4). This creates heterogeneity of resources at the microscale with depth and reflects the horizontal litter accumulation differences. Microscale heterogeneity creates microhabitats which provide conditions for different species activity, and different species succession.

The coexistence of many nematode species was considered in an important paper, in relation to soil spatial heterogeneity (Ettema, 1998). Comparing nematode species composition between adjacent samples usually reveals little species overlap. Species composition similarity is low between samples short distances apart. This suggests a patchy distribution over several centimetres. Each soil core contains a small number of dominant species, with many rare species. The dominant species are different between soil cores. However, the same species occur across the site, at a larger scale (forest, grassland and agricultural field), so that 10-100 species represent that site. These observations reinforce the idea of many overlapping patches (microhabitats) in close proximity. Each microhabitat then supports different guilds of nema-todes that reflect species composition. Some will be competing for the same resources, and with species outside of the guild in similar functional groups. This competition further reflects individual abundance between species. Those species that compete best and survive predation best are more numerous, given a set of abiotic and resource parameters. As conditions change with time in the microhabitat, the succession of species in each is also different. The succession pattern is then determined by activation of quiescent individuals in the vicinity, immigration of other species and predators, and changes in the abiotic parameters (that include weather changes). Therefore, there is an element of sto-chasticity in activity patterns, succession and abundance, even at the microscale level.

The following example attempts to demonstrate how one can reduce the apparent large number of species down to a variety of niche allocations. Let us postulate that we observe that a taxonomic group is

Fig. 5.4. Microhabitat development with depth through successive years.

represented by 100 species at a field site (total number of species at that site). Let us then observe that 30 species are active in summer, 40 in autumn, ten in winter and 20 in spring. Let us observe further that in each season, some species are active only on moist and warmer days, others on moist and cooler days, others on drier days. This series of observations already reduces the number of active species to a smaller number on each given day. If we then consider that some species are active in the top horizon, and others in successive horizons, we obtain a further spatial reduction in the number of coexisting species. Next, we have to note that species belong to different functional groups. The remaining coexisting species can be delineated further into a niche by considering their functional group and food preferences. By considering these parameters, along temporal and spatial stratification, accounting for succession in microhabitats is possible, without being burdened by too many competing species. The remaining species in the same functional group that appear to compete can be segregated further developmentally. For example, the development rate to maturity and resource requirements can be out of synchrony between competing species (Vegter, 1987).

In summary, within core samples, observed coexisting species can be differentiated into a variety of niches. Niche delineation requires consideration of microhabitat preference, functional group, development rate, dispersal strategy and other pertinent parameters (Krebs, 2001). Arguments to this effect were proposed for nematodes (Bongers and Bongers, 1998; Ettema, 1998), and species were proposed to fit into five basic categories. These could be differentiated further by considering spatial and temporal segregation in the habitat, e.g. activity periods in relation to weather and season, pore space restrictions, and food preferences within functional groups. Field sampling can reflect niche allocation of species only if sample analysis is stratified sufficiently, at the correct spatial and temporal scales (see Chapter 3). The interpretation of data and understanding niche differentiation between species are limited by how much biology is known about the interacting organisms.

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