A further general observation on studies of amensalism is one that could well be applied to some other two-species interactions. This is the elusiveness of obtaining realistic numerical estimates for the interaction coefficients (a and p) which are so central to mathematical representations and analyses of the processes. Essentially, this demands a figure for the per capita reduction in reproductive success of a species as produced by a unit increase in abundance of the other. If such numbers are elusive in the study of more or less 'symmetrical' competition, they will be even more so in asymmetrical competition where one of the interaction terms will be very small. So what are realistic aims for future studies of the process of amensalism? Two goals in particular are apparent.
First, and following on from the work of Dodds, there is scope for community-level studies of the sets of interactions which occur in the so-called species matrix as part of an expanding agenda in food web studies. Simple communities in discrete habitat units such as phytotel-mata, temporary pools, dung, carrion, or fungus fruiting bodies would be excellent starting points in that respect. Another type of system that lends itself well to studies of this nature is represented by food webs of arthropods on perennial crops, which can be manipulated for biological control. One example is the mite complex on apple orchards. Usually, two phytophagous mites feed on apple foliage, but in many areas, only one, the European red mite, is a major pest - the apple rust mite is a minor pest and the interaction between the two pests is amensalistic in that the European red mite outcompetes the rust mite. However, if the predator Zetzellia mali is present early in the season, it feeds on the rust mite, building up a dense predator population, which subsequently decimates the red mite population. Indirectly, the rust mite outcompetes the red mite through the fostering of the predator population, reversing the amensalism when seen in a more diverse food web.
Second, the often highly imbalanced population-level interactions between vertebrates and invertebrates would reward further study. The oft-quoted interactions between elephants and ants, cattle and collembola, or large predators and carrion beetles await quantification. However, a recent study on the interaction between an ungulate and four different herbivorous insects, all feeding on Erysimum medio-hispanicum, a common small plant in the montane regions of Spain, quantified strongly asymmetrical competition between the ungulate and the insects, that is, amensalism. The ungulate in question was the ibex (Caprapyrenaica), and the insects were a guild of several species of sap-sucking bugs that fed on the flowering stalks of the plant, a stem-boring larva of a weevil (Lixus ochraceus), a seed-predatory larva of another weevil (Ceutorhynchus chlorophanus), and a gall midge (Dasineura sp.). The competitive effects of the ibex on the different insects were both indirect, mediated through the deceased plant tissue resulting from the ibex's grazing, and directly, as the stem borers, seed predators, and midge larvae trapped in their galls were inadvertently eaten by the ibex. The sap-sucking bugs are sufficiently mobile to avoid being eaten by the ibex. Since the insect species were generally oflow abundance, their effects on one another and on the ibex were negligible. There are of course also examples of insects that become temporarily so abundant that massive defoliation of its food plant would have a strong negative effect on certain mammals. A spectacular example of this form of amensalism was the outbreak of the gypsy moth after its introduction into North America, which caused complete defoliation of oak trees. As a result, the oaks produced no acorns, which led to a major drop in the population of squirrels and jays.
See also: Aquatic Organisms; Coexistence; Commensalisms; Ecological Network Analysis, Energy Analysis; Ecological Niche; Gause's Competitive Exclusion Principle; Grazing; Grazing Models; Plant Competition.
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