Allelopathic species can alter biodiversity. Using simulated assemblages that are characterised by neutrality, lumpy coexistence and intransitivity, we explore relationships between within-assemblage competitive dissimilarities and resistance to allelopathic species. An emergent behaviour from our models is that assemblages are more resistant to allelopathy when members strongly compete exploitatively (high competitive power). We found that neutral assemblages were the most vulnerable to allelopathic species, followed by lumpy and then by intransitive assemblages. We find support for our modeling in real-world time-series data from eight lakes of varied morphometry and trophic state. Our analysis of this data shows that a lake's history of allelopathic phytoplankton species biovolume density and dominance is related to the number of species clusters occurring in the plankton assemblages of those lakes, an emergent trend similar to that of our modeling. We suggest that an assemblage's competitive power determines its allelopathy resistance.

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Recent advances in phytoplankton modelling have used species-rich, self-organising assemblages. These models have shown that phytoplankton with complementary life-history traits related to resource exploitation assemble into stable states of lumpy coexistence when resources fluctuate where species’ niches occur in clusters along resource gradients. They have also shown that a high degree of competitive dissimilarity between clusters arises, and that this relates to the incidence of monospecific blooms of allelochemical-producing taxa, i.e. some harmful algal bloom (HAB) species. These findings further suggest that the mode (sudden v. gradual changes) under which limiting resources fluctuate plays an important role in determining the emergent properties of the assemblage. For example, productivity, biodiversity and the number of species clusters (and, therefore, resistance to HABs) are all enhanced when switches in resource supplies are gradual, compared with when they are sudden. These theoretical findings, as well as others discussed herein, are of particular interest in watersheds where human activities, such as dam construction, have the capacity to dramatically alter natural-resource fluctuation patterns.

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