Important to determining population dynamics, community composition, and ecosystem function

I.  General Categories
Assigned by the effect it has on the two individual organisms interacting:

+, + = mutualism; e.g. N2 fixation, mycorrhizae
+, 0 = commensalism; e.g. nurse plants
- , 0 = amensalism; e.g. allelopathy
-, + = parasitism, herbivory (predation)
-, - = competition

Interactions can be more complicated than direct interactions between 2 individuals; can be mediated through a third individual or species

II. Inference of species interactions in the field: spatial patterning
If species interactions are important to plant species it should be reflected in the spatial patterns of individuals (inter and intraspecific); e.g. if mutualisms among plant species occurs, there should be a  positive association- i.e. they should occur closer together than predicted by chance (random)

Occurrence of spatial patterns is correlative: does not indicate causation; experimentation is  required to support a particular interaction as a determinant of spatial patterning and /or species abundances
includes:
1) species removals or introductions in the field
2) greenhouse/ growth chamber studies with plant combinations, resource supply, and/or densities of plants varied

Facilitative and inhibitive interactions may occur simultaneously, masking each others effect
 

rev 10/23/05