Interactions in Populations and Communities

Like humans, individual weeds live in contact and interact with a myriad of individuals of their own species and other species. We have already alluded to this in previous chapters where we mentioned pollination and occasionally mentioned the influence of other organisms via competition and her-

bivory (especially in Chapter 7), though we have yet to explore what this all means. Perhaps the most important implication is that the fate of an individual cannot really be disentangled from its interactions with others. Interactions are complex as they benefit, harm, or have no effect and will vary amongst the many, varied individuals found in populations and communities. While based on the environmental variation and

© 2003 CAB International. Weed Ecology in Natural and Agricultural Systems (B.D. Booth, S.D. Murphy and C.J. Swanton)

Table 8.1. Summary of interactions that might occur between two species.




Neutralism 0

Competition (amensalism) 0/-

Allelopathy 0

Herbivory +

Mutualism +

Commensalism +

Parasitism +

Neither species affected

Both species inhibited, or one species affected, the other not

Species A releases a chemical then inhibits species B Species A (animal) consumes part of species B

(plant) Both species benefit

Species A benefits while species B is not affected Species A (parasite) exploits species B (host) by living on or in it

individual characteristics like germination, growth rate and death rate, the sheer number of possible interactions and outcomes complicates the dynamics and structures of populations and communities beyond what we have discussed so far.

Because of the complexity of interactions, they may be categorized and presented as if only two individuals were interacting at one time (Table 8.1). Again, in reality, many individuals (too numerous to actually count) are involved but it is easier for us to visualize and discuss interactions between a pair of individuals. In this chapter, we focus on interactions where at least one individual is negatively affected. These are called competition (both individuals are negatively affected, but one individual less so) and amensalism (only one individual is negatively affected and the other neither benefits nor is harmed). Because determining whether an interaction inhibits one or both of the individuals is difficult, we group amensalism with competition (Lawton and Hassel, 1981; Connell, 1983).

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