Abstract
It is widely believed that when populations of two species of animals compete, each has an adverse effect on the other1,2. In other words, each zero growth isocline (dNi/dt = 0 where Ni is the population density of species i) is assumed to be some negative function of the population density of the other species. Cases where one species has a marked effect on the other, but there is no detectable reciprocal effect are sometimes distinguished as ‘amensalism’2,3. This is regarded as unusual. Standard textbooks of ecology either do not mention it1,4–6, define it but do not discuss it2, or occasionally give a brief account3. Ricklefs7 states that “competition can manifest itself by reducing the numbers of one or both competing species” without saying which is the more usual. Here we show that for insects in natural conditions, strongly asymmetrical competition (amensalism or near amensalism) is the norm rather than the exception by a ratio of at least 2:1.
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