Biotope Niche and Ecosystem Processes

Assuming that the boundaries of a biotope are described in an adequate manner in terms of scale, structure, or composition, or a specific combination of environmental factors, the explanation of the presence of a small or large number of species (the alpha diversity) is an open question that is intimately connected with the perception of the biotope. Among the major theories explaining variations of species diversity across environmental gradients, the so-called dynamic or energy theory well matches the view of a biotope as an entity of available resources such as water, nutrients, and energy. These resources determine the total biomass and the population size of the biota, and then short-term ecological processes determine the species number while long-term evolutionary processes determine structural properties such as niche widths and species packing.

Biotope Space, Species Richness, and Ecosystem Functioning

When diversity (species richness, taxonomic composition, and abundance distribution) is considered as an 'ecosystem property' driven by the variation in the biotope's variables (abiotic and biotic) - such as productivity or soil nutrient content - then a unimodal or humpback relationship between them is considered as widely valid. The humpback relationship between ecosystem productivity and diversity, produced from field survey data, varies among taxa, between spatial scales, and within as against between biotopes.

Effects of plant species diversity on plant productivity can be analyzed in comparative studies in which a set of natural communities is selected to represent different levels of diversity, being as similar as possible in other characteristics except productivity, or in experiments where plant communities of different diversities were constructed. These experiments treat diversity as the independent variable-driving variation in ecosystem processes, considered as the dependent or response variables. Recent field experiments (e.g., cross-European BIODEPTH experiment or biodiversity experiment at Cedar Creek, Minnesota, set up by David Tilman), as well as theoretical models, suggest that the processes maintaining ecosystem functioning could be adversely affected by the loss of diversity due to both selection and complementarity effects.

In 1957, George Hutchinson discriminated niches as properties of species and biotope as the physical space in which all, part, or none of a species' niche may occur:''... niche may be regarded as a set of points in an abstract ยป-dimensional N space. If the ordinary physical space B of a given biotope be considered, it will be apparent that any point p(N) in N can correspond to a number of points p(B) in B, at each one of which the conditions specified by p(N) are realized in B'' (Figure 1). Hutchinson presents a simple lake system as an example of a biotope that is a ''segment of the biosphere with convenient upper and lower boundaries, which is horizontally homogeneously diverse.''

According to niche theory, species can coexist in the same particular biotope as long as their niches do not fully overlap within this biotope. If species differ in their niches (i.e., more complete utilization of available resources) and the niches are considerably smaller than the entire biotope space (i.e., niche axes length), then communities richer in species may have higher productivity and other process intensities than species-poor mixtures or monocultures because of complementarity. These diverse communities would not become as strongly dominated by

Figure 1 The biotope and the niche. (a) A pair of variables (X1, X2) define two-dimensional areas that represent the niche of species A (NA(X1A, X2A)) and of species C (NC(X1C, X2C)), respectively. (b) The ordinary physical space B(X1, X2) of a given biotope. (c) Part of species niches (NA and NC) represented within this biotope. Overlap of species niches in the biotope shows the amount of interspecific competition (gray box).

individual species as expected under the selection effect. Figure 2 illustrates mechanisms driving different biodiversity-ecosystem functioning relationships.

Does reducing biotope space weaken the strength of the diversity-ecosystem functioning relationship? In the example of soil volume, several species with different rooting architecture and depths may be combined, provided the biotope is sufficiently large and deep to fully accommodate these species niches. Therefore, complementarity and beneficial biodiversity effects should increase with biotope space. Recent experimental approaches, using constructed plant communities of various diversities growing on a gradient of increasing soil depths and volumes that offer increased rooting space to species and related to the niche dimension of nutrient acquisition by the roots of plants, showed that biodiversity effects due to species complementarity increased linearly with biotope space. Scaled up to agricultural systems, this means that benefits of intercropping may be greater in deep soils and that soil erosion may reduce intercropping benefits. Comparatively, the strength of the positive relationship between species richness and productivity appeared to decrease in disturbed biotopes, as in grassland ecosystems where horse trampling and drought stresses were induced, indicating that reductions in biotope space through environmental stresses decrease the potential for positive effects of niche separation among species.

Community Succession and Biotope Changes

Biotopes are not constant in time; as they are intimately linked with a biocoenosis, they undergo gradual changes in a series of ecological factors generated by the biotic

Mixture of all

Mixture of all

Performance e.g., resource use

Number of species

Niche axis

V. Positive interactions

IV. Full complementarity

II. No complementarity Dominance hierarchies

I. No complementarity No dominance hierarchies

Niche axis

Species 1

V. Positive interactions

Species 2

Species 1

IV. Full complementarity

Partial complementarity

II. No complementarity Dominance hierarchies

I. No complementarity No dominance hierarchies

Figure 2 Mechanisms explaining various responses of ecosystem processes in changing diversity. Two plant species performance curves along a niche axis represented in a particular biotope were used to depict potential niche separation and overlap. Species performances are hypothesized to be directly correlated with competitive ability. Each case (I-V) represents a different rule based on which species were assembled, shaping a different biodiversity-ecosystem functioning relationship. When a mixture of mechanisms operate together, the expected relationship is shown. From Kinzig AP, Pacala SW, and Tilman D (eds.) (2002) Functional Consequences of Biodiversity: Empirical Progress and Theoretical Extensions. Princeton, NJ: Princeton University Press.

components of the community. In turn, these changes influence the nature of the biocoenosis modifying its species composition and therefore the structure of the entire ecosystem, and so on. In a given area, these interactions determine a sequence of successive biocenoses until equilibrium is reached. Various explanations of the causes of succession have been proposed, emphasizing key environmental (biotope) factors as determinants of the theoretical 'end' stage of the succession. However, they converge in the hypothesis that when a biotope is physically modified at the limits of the capacities of the organisms composing the community, then compositional stability is reached.

There is a long tradition in ecology of studying the composition of plant communities. At the beginning of the twentieth century, two contrasting views concerning plant community assembly were introduced by Fredrick Clements and Henry Gleason. Clements considered communities as well-organized associations of coevolved species, whereas Gleason viewed communities as more or less random aggregations of species in time and space. When species distributions are plotted along a biotope gradient, communities appear to have sharp boundaries based on Clements' model, while the Gleason model predicts that species are arranged independently of one another and thus no distinct communities exist (Figure 3). Compared with real communities, the two models can be considered as the extremes of a continuum. The Gleason vision seems to fit better communities of early secondary succession. These communities often show geometric dominance-diversity curves, indicating that the dominant species takes a certain proportion ofthe total resources offered by the biotope, the second most dominant species the same proportion of the remainder, and so on. In contrast, dominance hierarchies are less strongly developed in late-successional communities, indicating less niche overlap and more niche separation, presumably due to there being more time for coevolu-tionary processes in shaping species interactions. The Clements model seems to better fit these late-successional communities. The species diversity and composition of a community is, in both models, treated as a function of succession.

Biotope gradient

Figure 3 Species distributions along a biotope gradient. (a) Communities (A, B) appear to have clear boundaries and are separated by intermediate zones (ecotones). (b) Species are arranged independently of one another, resulting in no distinct communities.

Biotope gradient

Biotope gradient

Figure 3 Species distributions along a biotope gradient. (a) Communities (A, B) appear to have clear boundaries and are separated by intermediate zones (ecotones). (b) Species are arranged independently of one another, resulting in no distinct communities.

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