Interspecific competition (like intra-specific competition) is frequently highly asymmetric - the consequences are often not the same for both species. For instance, with Connell's barnacles, Balanus excluded Chthamalus from their zone of potential overlap, but any effect of Chthamalus on Balanus was negligible: Balanus was limited by its own sensitivity to desiccation. An analogous situation is provided by two species of cattail (reedmace) in ponds in Michigan; Typha latifolia occurs mostly in shallower water whilst T. angustifolia occurs in deeper water. When grown together (in sympatry) in artificial ponds, the two species mirror their natural distributions, with T. latifolia mainly occupying depth zones from 0 to 60 cm below the water surface and T. angustifolia mainly from 60 to 90 cm (Grace & Wetzel, 1998). When grown on its own (allopatry), the depth distribution of T. angustifolia shifts markedly towards shallower depths. In contrast, T. latifolia shows only a minor shift towards greater depth in the absence of interspecific competition.
On a broader front, it seems that highly asymmetric cases of interspecific competition (where one species is little affected)
generally outnumber symmetric cases (e.g. Keddy & Shipley, 1989). The more fundamental point, however, is that there is a continuum linking the perfectly symmetric competitive cases to strongly asymmetric ones. Asymmetric competition results from the differential ability of species to occupy higher positions in a competitive hierarchy. In plants, for example, this may result from height differences, with one species able to completely over-top another and preempt access to light (Freckleton & Watknson, 2001). In a similar vein, Dezfuli et al. (2002) have argued that asymmetric competition might be expected between parasite species that occupy sequential positions in the gut of their host, with a stomach parasite reducing resources and adversely influencing an intestinal parasite further downstream, but not vice versa. Asymmetric competition is especially likely where there is a very large difference in the size of competing species. Reciprocal exclusion experiments have shown that grazing ungulates (domestic sheep and Spanish ibex Capra pyrenaica) reduce the abundance of the herbivorous beetle Timarcha lugens in Spanish scrubland by exploitation competition (and partly by incidental predation). However, there was no effect of beetle exclusion on ungulate performance (Gomez & Gonzalez-Megias, 2002).
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