Positive interactions in a novel annual plant community

Background/Question/Methods

Species introductions often lead to novel species associations and interactions, which in turn affect local community structure in recipient ecosystems. An important goal in modern ecology is to quantify how these novel communities function differently from those they are replacing. Despite widespread acknowledgment that novel communities will become increasingly common under global change, empirical evaluations are still needed to understand their ecological significance.

Using a guild of common annual plants found in a fragmented woodland ecosystem in Western Australia, we explored biotic interactions in native and novel communities containing both native and introduced annuals. We evaluated (by comparing biomass yields) complementarity and selection effects, two components encapsulating patterns of productivity and functioning in multispecies assemblages, and how these components differed between community types.

Results/Conclusions

Both native and novel assemblages tended to overyield due to a combination of positive complementarity and selection effects. Positive complementarity values revealed that species average yields in mixture were greater than in monoculture, suggestive of positive interactions or niche partitioning. Positive selection arose because particular species contributed more to community biomass than predicted by their relative abundance alone, a trend driven by consistent overyielding of a native forb, Waitzia acuminata, in multispecies assemblages. Surprisingly, both complementarity and selection effects were more positive in novel assemblages than in native assemblages, and may be due to the presence of a diminutive exotic annual grass, Aira caryophyllea.