Resource availability affects competition and predation. If suitable resources (plants or animal prey) become more abundant, resource discovery becomes easier and populations of associated consumers grow. The probability of close contact and competition among consumers increases, up to a point at which the superior competitor(s) suppress or exclude inferior competitors. As a result, the intensity of interspecific competition may peak at intermediate levels of resource availability, although the rate of resource use may continue to rise with increasing resource availability (depending on functional and numeric responses). Population outbreaks reduce resource availability and also reduce populations of competing species.
Interactions are affected by the heterogeneity of the landscape. Potential competitors, or predators and their prey, often may not occur simultaneously in the same patches, depending on their respective dispersal and foraging strategies. Sparse resources in heterogeneous habitats tend to maintain small, low-density populations of associated species. The energetic and nutrient costs of detoxifying current resources or searching for more suitable resources limits growth, survival, and reproduction (see Chapters 3 and 4). Under these conditions, potentially interacting species are decoupled in time and space, co-occurring infrequently on a particular resource. Hence, competition is minimized and predator-free space is maximized in patchy environments. In contrast, more homogeneous environments facilitate population spread of associated species and maximize the probability of co-occurrence.
Palmer (2003) explored the effect of termite-generated heterogeneity in resource availability on the competitive interactions of four ant species that reside on acacia, Acacia drepanolobium, in East Africa. Only one ant species occupied an individual tree at any given time, and violent interspecific competition for host trees by adjacent colonies was common. Acacia shoot production and densities of litter invertebrates increased with proximity to termite mounds. The competitively dominant ant, Crematogaster sjostedti, displaced other acacia ants, C. mimosae, C. nigriceps, and Tetraponera penzigi, near termite mounds, whereas the probability of subordinate species displacing C. sjostedti increased with distance from termite mounds. This variation in the outcome of competition for acacia hosts appeared to result from differential responses among the ant species to resource heterogeneity on the landscape.
Species interactions also can affect habitat heterogeneity or resource availability. Cardinale et al. (2002) manipulated composition of three suspensionfeeding caddisfly species at the same total density in experimental stream mesocosms. They reported that the total consumption of suspended particulate food was 66% higher in mixtures compared to single-species treatments. Facilitation of food capture by these potentially competing species in mixture resulted from increased stream bed complexity (reflecting variation in silk catchnet size), which in turn increased eddy turbulence and near-bed velocity, factors controlling the rate of food delivery.
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