Simply stated, 'amensalism' is the term used to describe the (0,—) term in the familiar two-species-interaction matrix. In plain English, it is the two-species interaction in which the impact of one species on the other is negative, but where there is no detectable impact of the second species on the first.
It is much less familiar than the predation (+,—), competition (—,—), mutualism (+,+) or, even, commens-alism (+,0) interactions; nevertheless, the idea of amensalism as a phenomenon of interest to ecologists was raised as early as the 1950s by Odum, borrowing on similar concepts in the social sciences. Other later textbooks, notably Williamson, restated and expanded on Odum's statements.
May picked up on Williamson's mathematical view of amensalism and directed serious attention to the process as part of his lucid summaries of the varieties of Lotka-Volterra equations potentially of interest to theoretical ecologists. May conceptualized amensalism as the limiting case of the two-species system modeled by the simultaneous equations representing competition, viz.:
where, of the interaction coefficients, a is positive and ft close to or equal to zero.
It was this formulation which led Lawton and Hassell to rename the process as 'asymmetrical competition'. It was their influential paper that represented the start of renewed interest in the process of amensalism (or asymmetric competition - from this point on to be considered as interchangeable terms). In a wide review of insect-insect interactions, they showed that so-called strong asymmetrical competition - where the negative impact of one species upon another far outweighed the reciprocal negative interaction - was more common by a factor of 2:1 in nature than the more traditional competitive interactions.
Dodds explored this imbalance among interaction types by constructing a simple null model of interactions that were positive (+), negative (—), or neutral (0). His model predicted that of all possible interactions within ecological communities the (0,0), (0,—), and (0,+) examples should be far more common than (—,—), (—,+), and (+,+) types and that this imbalance would be particularly marked when smaller species assemblages were considered. He concluded, in consequence, that ecologists spend relatively far too much effort in investigation on competitive and predatory interactions.
This last observation represents the key question for studies of amensalism. Although there is little doubt that when multispecies assemblages are considered, amensa-listic interactions are both frequent and important, their close study holds much less interest or significance for those whose endeavors lie within population ecology.
This divide is well illustrated by two examples.
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