Ecology of Mutualisms

Abstract

Mutualisms are interactions among individuals of different species that benefit both sides and encompass a wide diversity of interspecific exchanges of resources or services. The effects of mutualisms pervade multiple levels of biological organisation. At the individual level, mutualisms provide fitness benefits for interacting partners, creating novel metabolic pathways and providing dispersal services, trophic rewards or defence against natural enemies. At the population level, the positive effects of mutualisms have the potential to increase population densities above the limits imposed by resource availability. At the community level, mutualisms form networks of interacting species that impact the persistence of local populations as well as their evolutionary and ecological dynamics. The broader implications of mutualisms to biodiversity are illustrated by the fact that mutualistic interactions are the backbone of species‐rich ecological communities, such as tropical rainforests and coral reefs, and by how mutualisms fueled the spreading of humankind around the world.

Key Concepts

  • Mutualisms are ecological interactions among individuals of different species that result in positive net benefits for both sides.
  • Individuals of most species are involved in mutualistic interactions.
  • The natural history of mutualisms is highly diverse, ranging from short‐term associations among partners to interactions in which individuals are biologically attached for most of their lives.
  • Some mutualisms are symbiotic, that is, individuals show tight biological integration, but symbiosis and mutualisms are not synonyms.
  • All animals, plants and fungi are built on mutualisms between eukaryote cells and intracellular symbionts that have became organelles, such as mitochondria and chloroplasts.
  • Mutualisms have the potential to create positive feedbacks leading to large population densities, but these effects are offset by other ecological interactions, such as competition and predation, and also by the costs of mutualisms.
  • Reciprocal specialisation is rare in species‐rich mutualisms.
  • At the community level, mutualisms form multispecies ecological networks that show recurrent structural patterns.
  • A variety of structural patterns observed in mutualistic networks have disparate effects on system dynamics, which can affect the long‐term persistence of species and communities.
  • The most diverse ecosystems on Earth are shaped by mutualistic interactions.

Keywords: ant–plant interactions; cheater; cleaning station; ecological network; lichen; Müllerian mimicry; mycorrhiza; pollination; population growth; seed dispersal; symbiosis

Figure 1. A few terrestrial and marine mutualisms. (a) Symbiotic mutualisms between fungi and algae form lichens that are able to colonise a variety of environments. Photo by M. A. R. Mello. (b) Workers of ant species that tend extrafloral nectaries can minimise the effects of herbivores, as illustrated by these individuals of Camponotus rubritorax attacking a caterpillar close to extrafloral nectaries of Acacia constricta (Fabaceae). Photo by W. Dátillo. (c) Clownfishes (Amphiprion perideraion) benefit from protection provided by the tentacles of sea anemones (Heteractis magnifica), which in turn benefit from the fish leftovers. Photo by J. P. Krajewski. (d) Frugivorous animals, such as some bats, play a key role in plant life cycles by affecting the fate of seeds. Photo by M. A. R. Mello. (e) Pollinators, such as bees, also provide key services to plants by moving plant gametes and ensuring their sexual reproduction. Photo by M. M. Pires. (f) Cleaner species are widespread in marine ecosystems, such as this shrimp of the genus Lysmata, here interacting with its client, the dark‐spotted moray eel Gymnothorax fimbriatus. Photo by J. P. Krajewski.
Figure 2. A schematic view of the diversity of mutualisms based on interaction intimacy. The bar at the bottom denotes increasing interaction intimacy. Geometric figures with different colours indicate different mutualist species. Squares and cycles depict labour division, that is, interacting individuals provide highly complementary resources or services to each other. (a) Mutualisms without division of labour that involve no physical interaction or exchange between free‐living species, for example, Müllerian mimetism and mixed‐species bird flocks in which birds forage together. (b) Facultative mutualisms between free‐living organisms with division of labour. Examples include a wide variety of mutualistic associations characterised by rapid exchanges between species, such as interactions between plant and pollinators, plant and animals dispersing seeds, cleaner species and their clients and protective ants tending plants with extra‐floral nectaries. (c) Mutualisms between free living species that involve sustained interactions with physical association between the species during at least the majority of the life cycle of one of the organisms, such as occur in mutualistic associations between fig and fig wasps, anemone and anemonefishes and between protective ants and their host myrmecophyte plants. (d) Ectosymbiotic mutualisms that involve interactions characterised by strong methabolic and physiological attachment between small, often microscopic symbiont and their mutualist hosts, such as occur in association between ruminants and the cellulose‐digesting microbiota that inhabits the rumen. (e) Endosymbiotic mutualisms in which the symbiont lives inside the cells of their hosts, such as widespread associations between protozoans and bacteria. (f) The most extreme case of intimate mutualisms is the association between eukaryotic cells and endosymbionts that have became key intracellular organelles – the mitochondria and chloroplasts – that are genetically and metabolically integrated to the host.
Figure 3. Theoretical populations dynamics between mutualists. (a) The positive effects of mutualism between two populations lead them to exponential growth, resulting in unbounded densities. (b) The positive effect of mutualism is limited by saturation of the benefit gained, resulting in bounded population densities. After a given threshold of mutualistic partner density, the interaction does not convert into population growth, reaching the density equilibria of both populations. (c) The presence of cheaters in one of the populations limits the benefit the other population may gather with the interaction. The negative (or null) effect of cheaters bounds the density of the partner that in turn regulates the density of the mutualistic partners and cheaters. Populations oscillate until reaching the equilibria of bounded densities of coexistence of mutualists and cheater.
Figure 4. Mutualistic interactions are embedded in complex interaction networks. Even species that show a high degree of morphological specialisation such as some hawk moths (Sphingidae) and orchids (Orchidaceae) that often establish interactions with other species in the community. In the theoretical plant‐pollinator network depicted here species are represented as nodes and links represent interactions between them. The size of each node is proportional to the number of interactions each species establish.
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Further Reading

Bascompte J and Jordano P (2013) Mutualistic Networks. Princeton: Princeton University Press.

Boucher DH, James S and Keeler KH (1982) The ecology of mutualism. Annual Review of Ecology and Systematics 13: 315–347.

Bronstein JL (2015) Mutualism. Oxford: Oxford University Press.

Douglas A (1994) Symbiotic Interactions. Oxford: Oxford University Press.

Karplus I (2014) Symbiosis in Fishes: The Biology of Interspecific Partnerships. Hoboken, NJ: Wiley‐Blackwell.

Thompson JN (2005) The Geographic Mosaic of Coevolution. Chicago: Chicago University Press.

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How to Cite close
Guimarães, Paulo R, Pires, Mathias M, Marquitti, Flavia MD, and Raimundo, Rafael LG(Apr 2016) Ecology of Mutualisms. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0026295]