Positive Interactions in Community Organisation


Positive interactions are ubiquitous in ecological communities and can be facultative or obligate mutualisms or commensalisms. Recent decades of work have developed a framework for understanding positive interactions and their role in community structure and function. The stress‐gradient hypothesis was the first conceptual model developed to explain positive interactions and predicted that the frequency of positive interactions increases at physical and biological stress extremes. More recent work has expanded this perspective to habitat‐forming foundation species and the hierarchical organisation of communities. Currently positive interactions are being incorporated into the most utilised models in ecology – niche theory and the Menge–Sutherland Model – expanding the accuracy and predictive value of these models. Simultaneously, the role of positive interactions is being recognised and incorporated into evolutionary theory.

Key Concepts

  • Positive interactions are ubiquitous under stressful conditions in communities
  • Mechanistically, positive interactions alleviate physical and biotic stresses
  • Positive interactions are slowly being incorporated into community models
  • Foundation species often provide multiple positive interactions to communities
  • Physical and biotic stresses select for the evolution of mutualisms
  • Evolutionary biology has more proactively incorporated positive interactions into their theoretical framework than community ecology

Keywords: stress‐gradient hypothesis; positive interactions; facilitation cascades; foundation species; niche theory; Menge–Sutherland Model; community organisation

Figure 1. Examples of ecological and evolutionary positive interactions include (a) coral reefs with algal symbionts, (b) facultative mutualisms between salt marsh cordgrass and ribbed mussels, (c) clownfish and sea anemone mutualisms, (d) plant–pollinator associations, (e) cells and their mitochondrial machinery and (f) lichens, the coevolved association between algae and fungi.
Figure 2. The stress‐gradient hypothesis as proposed by Bertness and Callaway (). The SGH predicts that the frequency of positive interactions will increase at physical stress extremes in the form of habitat amelioration and at biological stress extremes in the form of associational defences. Reproduced with permission from Bertness MD and Callaway R (1994) © Elsevier.
Figure 3. Conceptual model incorporating positive interactions into niche theory. In contrast to traditional niche theory, this model predicts that the realised niche will be larger than the fundamental niche in physically stressful habitats through habitat amelioration, and will be identical to the fundamental niche in biologically stressful habitats through associational defences. The areas enclosed by thick dashed lines represent the fundamental niche space. Gray areas outlined by solid black lines represent realised niche space along physical and biological stress gradients. Reproduced with permission from Crotty and Bertness (in print) © Ecological Society of America.
Figure 4. Incorporation of SGH predictions into the Menge–Sutherland model. These models predict the relative importance of negative (predation and competition) and positive interactions (associational defences and habitat amelioration) across a gradient of environmental stress given high recruitment of prey species: (a) as originally proposed; (b) including positive intraspecific interactions and (c) including positive interspecific interactions. Reproduced with permission from Bruno et al. (2003) © Elsevier.


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Further Reading

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Crotty, Sinéad M, and Bertness, Mark D(Nov 2015) Positive Interactions in Community Organisation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026293]