CHAPTER 6 COMPETITIVE EFFECTS AND RESPONSES OF SELECTED SPECIES IN
6.1 Introduction
Since the early twentieth century, nine basic interactions between individuals in an ecological community have been recognized. These include amensalism, commensalism, competition (direct interference type), competition (resource-use type), mutualism, neutralism, parasitism, predation and proto-cooperation (Clements et al. 1929; Burkholder 1952; Haskel 1949). Among these, resource-use-type competition has long been recognized as the ‗dominant law of relationships‘ between individuals and species in plant communities (Tansley 1920). There is therefore a consensus that botanical composition and productivity of any vegetation is largely determined by competitive interactions (e.g. Newman 1983; Wilson 1988).
Competitive interactions of a species may not only explain the relative abundance of a species in a given community, but may also provide some insight into the nature of forces that structure such a community (Miller and Werner 1987).
Competition is important in both natural and agricultural plant communities.
Understanding how competition varies with productivity is thus vital for differentiating among models of plant community structure and function. Models that do not include mechanisms of interactions among organisms can describe the phenomenology of population interactions, but cannot predict the community dynamics or the outcome of these interactions (Tilman 1990). Therefore there is need to include environmental variables and mechanisms of interactions in efforts to develop predictive theories of competition and succession in different plant communities.
6.1.1 Concepts and theories of plant competition
The ability of a species to persist and prosper in a community is often determined by its competitive interactions with other species (Connell 1983). Competition generally implies exploitation of limited resources that are essential for their life, primarily light (above-ground), water and nutrients (below-ground). Because nutrients are heterogeneously distributed, the fate of a plant largely depends on its local environmental settings (Aerts 1999; Berger et al. 2008). Although root and shoot functions are fundamentally integrated (McPhee and Aarssen 2001), the spatial
division between them and the differences in the microclimate experienced by each has received considerable attention in recent efforts to understand the mechanisms of competition and the bases of differences in competitive abilities among species.
Three models or hypotheses are commonly used in plant ecology to predict changes in competition intensity with standing crop along principal natural gradients. The first model is that competition for both light and soil resources increases with productivity (Grime 1977; Chapin 1980; Keddy 1989). The second is that competition for light increases with standing crop but competition for belowground resources declines (Grubb 1985; Tilman 1988), and the third states that competition intensity is minimal in plant communities of low (but not the lowest) productivity (Oksanen 1993;
Sammul et al. 2000). Experimental evidence supporting these models has been derived through work on both natural and experimental resource gradients that are characterized by turnover in species composition or life form (Peltzer et al. 1998). It is conceded that species in natural communities are organized into transitive competitive hierarchies, and that these hierarchies vary among environments (Keddy et al. 1994).
Habitat fertility and disturbance are the two major variables that have been recognized as major underlying determinants of plant community organization (Grime 1977;
Tilman 1988), while competition determines species distribution and abundance along fertility gradients (Grime 1979; Austin 1990). However, the influence of competition on extremely low and high fertility gradients is highly contentious (Fynn et al. 2005).
6.1.2 Competitive response and competitive effect
Most ecologists agree that competition is an important structuring factor in plant communities, but researchers disagree on the circumstances where it is most intense, and on which traits can be considered to contribute to competitive ability in different species. Competitive ability has two components: competitive effect and competitive response. The distinction between a species' effect on resources and its response to reduced resource levels might help to solve these questions. Competitive effect is defined as the ability to depress the growth or reproduction of neighbours; and competitive response as the ability to withstand the negative effects of neighbours (Goldberg 1990). These two components attempt to predict species interactions relative to resources, which are often in short supply. Competitive effect predicts how
an individual depletes recourses, and therefore how it influences their amount available to other organisms, and how it performs and survives in response to depleted resources (Goldgerg and Werner, 1983; Suding et al. 2004). Successful species must have either a low response to the abundance of other species, and/or a large enough effect that the abundance of other species is significantly reduced (Miller and Werner 1987). Therefore, a species may either have a low response and a high effect, or any other combination of effect and response.
Competition is a result of plant density and size relative to available resources (Zedaker 1982). Essential resources such as nitrogen and phosphorus can limit primary production of individual plants (Tilman 1982; Ingestad and Agren 1995), and can play a role in plant community assembly (Daufresne et al. 2005). Therefore understanding plant-herbivore, plant-plant and plant-nutrient interactions are challenges for plant community ecology and ecosystem biology. The process of competition is not measured directly. The presence and intensity of competition are inferred from stand and plant-level attributes measured within populations grown under different density and site conditions and how these attributes change over time.
These density-induced effects are assumed to be the outcome of competition between individual plants.
Patterns of competitive effects and responses are particularly challenging in plant communities due to the heterogeneity of their natural habitats and resource availability. To address these challenges, competitive abilities of species can be determined experimentally by growing species in additive mixtures and measuring the reduction in their performances in mixtures relative to controls (Keddy et al. 1994;
Sammul et al. 2000). Making such comparative measures is one of the several research strategies in plant ecology. In this study, eight species (seven grasses and one woody plant) were being explored and ranked relative to their competitive effects and responses in different environments.
6.1.3 Rationale and Objectives
In the False Thornveld of the Eastern Cape, most of the agricultural land is used for pastoral farming with cattle, sheep, goats, and to a lesser extent, game farming. One
of the problems facing farmers and range managers is compositional change whereby the more acceptable grass species are replaced by less desirable grasses or even trees and shrubs (primarily Acacia karroo).
This study is aimed at investigating competitive interactions between selected species in a simulated non-selective grazing environment across a soil fertility gradient.
The following key questions were asked: