The web of our life is of a mingled yarn, good and ill together.
The present chapter deals with another feature of interactions between species: the degree of antagonism. Interactions between two species can be classified according to the fitness effects of the interaction on each species. Most people's understanding of ecological communities is that they are full of exploiter-victim relationships, where one species benefits at the expense of another ( + ,— ). Predation, herbivory, and parasitism are examples, and are of course ubiquitous in communities: they are the building blocks of food webs. Competition involves negative effects on both parties ( —,—) and is also common between members of the same trophic level. Again, such relationships are to be expected in a Darwinian world where individual's primary concern is its own reproductive success, rather than that of another individual of a different species. There are also more nearly neutral relationships, such as commensalisms (+ ,0) and amensalism (— ,0).In these one species is affected by the interaction, but the other is not affected. Commensalism is common where one species incidentally benefits from another species' activity: humans have their own community of such species that includes house sparrows, house mice, house spiders, and most garden weeds,which have little effect on human fitness most of the time.Amensalism is found when one species consistently excludes another (asymmetric competition), as found in many plant and insect communities.
Finally, there are mutualistic relationships ( + , + ) where both parties benefit from each other. Mutualistic relationships of this kind are ubiquitous and of great ecological and evolutionary significance (Chapters 2 and 3). They include plants and their pollinators; plants and their fruit and seed dispersers; plants and their mycorrhizal fungi; corals and their symbiotic algae; lichens (symbioses between fungi and algae); leguminous plants and rhizobia bacteria; and many animal species with nutritional or digestive symbioses with microbes; including cows, termites, aphids, and humans. Put simply, these mutualistic relationships are essential components of most modern communities.
One striking fact that has emerged from the study of species interactions is that they must sometimes evolve pretty rapidly: different populations of the same species can be either mutualistic or parasitic, and closely related species likewise. In contrast, other relationships are apparently ancient and stable. What conditions favour such changes or else help maintain the status quo? Our understanding of these questions has particularly benefited from two types of study; studies of parasite-host relationships and studies of the evolution of cooperation, which, as we will see, share considerable conceptual overlap.
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