The guild of inverte­brate herbivores on a plant species comprises of a small number of abundant species and a larger number of scarce species. Time is perhaps the least important factor affecting guild structure: in the U.K., the alien but wide­spread Acer pseudoplatanus has more enemy species of leafhoppers than the long established but more local A. campestre (Claridge and Wilson, 1981).

Joern and Lawlor (1981) have defined guilds as functional groupings (for example, foliage-gleaning insectivorous birds) of co-existing species which use resources in a very similar fashion. Guilds are groups of organ­isms that interact strongly among themselves for the use of resources but interact only weakly with members of other groups. Several guilds may form an ecological community. There are strong indications to the effect that guilds strongly influence the dynamic functioning of an ecosystem by increasing stability or by affecting the pattern of interaction (Holmes et al., 1979). In competitive communities, guilds constitute arenas of strong inter­ specific competition, with strong interactions within guilds but only weak interactions between individuals of different guilds.

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In a study in Texas, Joern and Lawlor (1981) have described the guild structure of grasshoppers. They observed the existence of much guild struc­ture in assemblages of grasshopper species for both diet and microhabitat, and recorded similar patterns of guild structure of assemblages from differ­ent sites which shared only a portion of the same type of grasshopper. Their observations also suggested that guilds were not a result of random process­es. Most species were found to belong to a guild based on either food use or microhabitat use or sometimes both. Another interesting observation was the common occurrence of hierarchical arrangements of species into guilds of increasing size.

The possible significance of guild structure in maintaining community diversity has been discussed by Pianka (1980). In some circumstances, pairs of potential competitors may increase, instead of decrease, one another’s densities if both share other competitors. Such competitive mutualism often operates when two weakly competing species have a common strong com­petitor, since the two weak competitors inhibit the third species strongly; each has a beneficial net effect on the other even though their direct pairwise interaction is negative. It shows two target species X and Y (members of two different guilds) interacting negatively by one direct but weak interaction; this means that the species X and Y are competitors in the absence of other species. However, X has many strong direct and negative interactions due to its high overlap with other members of its own guild A. Hence, species X considerably inhibits the population of other species of the guild A.

These decreased population densities are reflected back as many weak, indirect, beneficial interactions on species Y in the guild B. The Y also exerts an indirect helping action on species X via its own strong direct interactions within the guild B. According to Pianka, this is how guild structure may be a significant factor in maintaining community diversity.