Mutualisms in Nature

Welcome to this week’s blog post on mutualisms! This post is a collaboration with fellow science blogger Anna from Search for Science so if you enjoy what you read here check out her site!

Mutualism is the technical term used to describe a relationship between organisms from which both parties benefit. For example, bees benefit from the food that flowers provide as nectar. The flowers themselves are pollinated by the spread of their pollen to other flowers. It differs from symbiosis, a word which is often used alongside mutualism, as symbiosis instead is defined as the living together of unlike organisms [1]. There are several examples of mutualism in nature – but these relationships are threatened by external influences, such as climate change and anthropogenic development.  This post will take a look at some examples of mutualism between animals, plants and fungi, and discuss some of the ways through which these associations are being affected.

Clownfish and sea anemones

One textbook example of mutualism in the oceans is the relationship between clownfish (a type of anemonefish) and sea anemones, made famous by the film Finding Nemo. Sea anemones are some of the most venomous organisms on earth [2], yet anemonefish can tolerate them, forming associations which provide protection from predators, protection of fish eggs, removal of parasites and nourishment from anemone tentacles [2]. In return, the anemone hosts have enhanced defences from predators such as the butterflyfish (who are fiercely chased away by the clownfish, [3]), improved survivorship and improved asexual reproduction [2].

Clownfish swimming through an anemone
Image Credit: Jan Derk (public domain)

Clownfish are protected from the venom of anemones by a protective mucous coat which prevents contact [3]. This protection, and the mutualistic relationship with anemones, has a huge role to play in the lifespan of the fish: though an average fish of that size might live to its fifth birthday, a clownfish will still be swimming about at 30 [3]. However, this relationship is changing: long-term declines in habitable sea anemones have been observed across the world, negatively affecting the fish which depend on them [4]. These declines are linked to widespread coral bleaching and ocean acidification, as climate change stresses the corals to the extent that they expel the organisms which support them and give them their colour [2].

“As giant sea anemone populations continue to shrink in the Red Sea and around the world, the organisms that interact with them both directly and indirectly will likely also experience local to regional extinctions, as the available habitat that these reef hosts provide diminishes.”

McVay, 2015 [4]

Mycorrhizal fungi and Plants

An example of mutualism from the non-animal realm is the relationship between mycorrhizal fungi and plants [5]. Connections are made between the roots of the plant and the hyphae, which are similar root-like structures used by the fungi. Most of the time, plants exchange carbon from photosynthesis for nitrogen from the roots of their fungal friends, however, during early seed development, the fungi may also supply carbon [6]. Around 10% of plant species are dependent on this initial carbon supply from the fungi for their survival [7].

Both plant and fungus have a fine level of control over their exchange and can reciprocally reward or punish cooperation – for example, if a fungus tries to take too much from the plant, it can reduce its oxygen supply and suffocate it [8]. This fine-tunable regulation that plants have is matched by fungi. In times of lower or more variable water conditions, fungi can moderate and even increase the plant’s nutrient acquisition in response [9]. This moderation can help increase plants’ resilience to changes in rainfall and temperature, and in a world that is currently warming, this is vital to survival [9].

Mycorrhizal fungi
Image Credit: Public domain

Dwarf mongoose and hornbill

Mutualistic relationships between social vertebrates are rare, which is why the association between dwarf mongooses and hornbills is so interesting [10]. In Kenya, dwarf mongooses work with the birds when searching for prey, and to alert each other to danger. For example, when a raptor is spotted, mongooses will guard hornbills to protect them from harm. In exchange, hornbills vocally warn their four-legged friends about predators, even when they themselves would not be hunted [10]. Such behaviours are altered depending on the number of birds and the numbers of mongooses, so that each group can provide the best protection to the other [10].

Because vocal calls are vital to the connection between these two species, anthropogenic noise pollution is a critical threat to their defences [11]. Mongooses also listen to the warnings of chacma baboons and tree squirrels for alarm signals, so when they are distracted by other noises, the warnings could be ignored. Anthropogenic noise can mask alarm signals, diverting attention from authentic calls and increasing perceived danger, causing stress to the mongoose and subsequent behavioural changes [11].

Common Dwarf Mongoose
Image Credit: Mathias Appel (public domain)

For this mutualistic relationship, anthropogenic noise must be controlled or eliminated. One method of reducing the impacts of noise is by using noise barriers, which are vegetation or physical barriers which have been shown to significantly reduce noise levels [12]. But where noise levels are associated with human activity, the economic value of the activity will almost always be prioritised over mitigation measures.

Mutualism is a vital aspect of many species survival, and without this collaboration, they would suffer.

Humans are also reliant on some of these mutualisms, and understanding these links can also bring benefits to us. For example, the mycorrhizal fungi aid carbon storage, as when their deep hyphal (root) networks decompose, the carbon remains in the soil for up to decades [13].Fungal networks can also be crucial for food production. Engineering crops to form mutualisms with mycorrhizal fungi may enable them to out compete non-cooperative weed species, reducing the need for herbicides and fertilisers [14]. It is of vital importance that we protect the interactions and species on which we rely, for our own sake as much as for theirs.

Search for Science is an environment-based blog run by Anna, a Masters in Environmental Sciences Student in the UK. Her blog covers themes of land management, conservation, veganism, sustainability and food security – popular articles include “Veganuary: going cold tofu on meat and dairy” and “What makes a sustainable diet?”. Check it out!

References

[1] de Bary, A. (1879) Die Erscheinung Der Symbiose. Strassburg: Verlag von Karl J. Trübner.
[2] da Silva, K.B. & Nedosyko, A. (2016) Sea anemones and anemonefish: a match made in heaven. In The Cnidaria, past, present and future, pp. 425-438
[3] Litsios, G., Sims, C.A., Wüest, R.O., Pearman, P.B., Zimmermann, N.E. & Salamin, N. (2012) Mutualism with sea anemones triggered the adaptive radiation of clownfishes. BMC Evolutionary Biology, 12(1), p.212
[4] McVay, M.J. (2015) Population dynamics of clownfish sea anemones: patterns of decline, symbiosis with anemonefish, and management for sustainability. [pdf] Available at: http://etd.auburn.edu/handle/10415/4940 (Accessed: 04/08/2020)
[5] Hoeksema, J., Chaudhary, V., Gehring, C., Johnson, N., Karst, J., Koide, R., Pringle, A., Zabinski, C., Bever, J., Moore, J., Wilson, G., Klironomos, J. & Umbanhowar, J. (2010) A meta-analysis of context-dependency in plant response to inoculation with mycorrhizal fungi. Ecology Letters, 13(3), pp. 394-40
[6] Cameron DD, Johnson I, Read DJ, & Leake JR (2008) Giving and receiving: measuring the carbon cost of mycorrhizas in the green orchid, Goodyera repens. New Phytologist 180: 176-184.
[7] Leake J.R. & Cameron DD. (2010) Physiological ecology of mycoheterotrophy. New Phytologist, 185 pp.601-605
[8] Kiers E.T., Rousseau R.A., West S.A., & Denison R.F. (2003) Host sanctions and the legume–rhizobium mutualism. Nature 425, pp.80-8
[9] Bowles, T., Jackson, L. & Cavagnaro, T. (2017) Mycorrhizal fungi enhance plant nutrient acquisition and modulate nitrogen loss with variable water regimes. Global Change Biology, 24(1), pp.e171-e182
[10] Anne, O. & Rasa, E. (1983) Dwarf mongoose and hornbill mutualism in the Taru Desert, Kenya. Behavioral Ecology and Sociobiology, 12(3), pp.181-190
[11] Morris-Drake, A., Bracken, A.M., Kern, J.M. & Radford, A.N. (2017) Anthropogenic noise alters dwarf mongoose responses to heterospecific alarm calls. Environmental pollution, 223, pp.476-483
[12] Slabbekoorn, H. & Ripmeester, E.A.P. (2008) Birdsong and anthropogenic noise: implications and applications for conservation. Molecular ecology, 17(1), pp.72-83
[13] Treseder, K. & Holden, S., (2013) Fungal Carbon Sequestration. Science, 339(6127), pp.1528-1529
[14] Cameron DD (2010) Arbuscular mycorrhizal fungi as (agro)ecosystem engineers. Plant and Soil 333: 1-5