A long-standing ecological question of particular relevance to resource utilization marginal areas is whether or not intensity of competition is a direct function of habitat productivity. It can be argued that as productivity rises so will the intensity of competition increase. The obverse situation would be that communities with low productivity are made up of species that have lower growth rates and therefore make lower demands on their habitat for resources and create communities with lower levels of competition intensity (Grime, 2001). However, the contrary effect can also be argued. If competition for light increases there should be a reduction in competition for other resources such as nutrients and water so that the total intensity of competition will be independent of productivity (Tilman, 1987). The nature of this argument is conceptual and depends on how competition intensity is to be measured. Two possible variants exist depending on whether absolute competition or relative competition is a true reflection of the intensity of competition (Table 3.1). Comparison of a number of data sets using these two indices tends to support the view that competition intensity increases with productivity if absolute competition intensity is calculated, while the view that competition intensity is independent of productivity is true if relative competition intensity is calculated (Grace, 1993). This distinction, however, does not apply with equal clarity to all studies. The use of the term performance in many studies introduces an element of ambiguity as it may mean growth or biomass accumulation in one study and flowering or seed production in another. The ambivalence of just how competition should be assessed underlines the principle that it is not a physical reality and that what is observed is highly dependent on how the concept of competition has been defined and how survival is assessed (see also Section 3.6.2).
Communities of plants and animals are usually dynamic with constantly changing species assemblages that result from continuing and long-term competitive processes. Unfortunately, it is only rarely that the processes of competition can be observed in action in natural plant communities. Consequently, most competition studies are carried out on crops where variation is minimal, conditions controlled, and the problem is confined either to the effects of density on crop yield, or to the interaction of usually not more than two species in carefully controlled replacement experiments. Such studies can be used in estimating maximal sustainable yields as well as increasing resource-use efficiency in crops. They may also prove helpful in combating pollution by finding the optimal planting densities for nutrient uptake and thus reducing the rate at which artificial fertilizers need to be applied to agricultural land. Human life spans are usually too short for long-term observation of competition in natural systems and most examples are restricted to monitoring the effects of invasive and aggressive alien species (see Section 4.6).
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