POSITIVE INTERACTIONS IN PLANT-COMMUNITIES AND THE INDIVIDUALISTIC-CONTINUUM CONCEPT

The individualistic nature of communities is held as a fundamental eco logical tenet by many ecologists. The empirical rationale for the indi vidualistic hypothesis is largely based on gradient analyses in which plant species are almost always found to be arranged independently of one another in ''continua'' along environmental gradients. However, co ntinua are correlative patterns and do not identify the processes that determine them, and so they do not necessarily preclude the possibili ty of interdependent interactions within plant communities. For exampl e, the common occurrence of positive interactions suggests that plant species may not always be distributed independently of each other. If the distributions and abundances of species are enhanced by the presen ce of other species, their organization is not merely a coincidence of similar adaptation to the abiotic environment. Interpretations of gra dient analyses also appear to assume that interactions among species s hould be similar at all points along environmental axes, and that grou ps of species should be associated at all points on a gradient if inte rdependence is to be accepted. However, virtually all types of ecologi cal interactions have been shown to vary with changes in the abiotic e nvironment, and a number of field experiments indicate that positive e ffects become stronger as abiotic stress increases. Furthermore, inter actions among plants have been shown to shift from competition to faci litation along environmental continua. Thus, significant interdependen ce may occur even when species do not fully overlap in distribution. H igher-order, indirect interactions between animals and plants, and amo ng plants, also suggest that interdependence within communities occurs . Eliminating a species involved in an indirect interaction may not ne cessarily mean that its beneficiary will be eliminated from a communit y, but the prospect that the distribution and abundance of any species in a plant community may be positively affected by the effects that o ther species have on their competitors suggests that communities are o rganized by much more than ''the fluctuating and fortuitous immigratio n of plants and an equally fluctuating and variable environment'' as s tated by Henry Gleason. The ubiquity of direct and indirect positive i nteractions within plant communities provides a strong argument that c ommunities are more interdependent than current theories allow.