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Plant Mutualism: protect your producers

May 22, 2012

Plant Mutualism: Plant-Animal Mutualism

Plant mutualism and facilitation with animals is a widespread phenomenon, most often taking the form of pollination and/or seed dispersal for a plant which provides rewards for an animal often in the form of nutrient resource. Another mutualism phenomena is the protection and sheltering between ant population and plants. If a plant and animal population are dependent on each other to persist than it follows that what hinders the success of one population hinders the other. This means the mutulaistic relationships important to an areas ecosystem dynamics especially if the populations forming mutualisms are a significant proportion of an area’s diversity.

Plant-animal mutualisms are widespread and diverse in their history of development. Mutualisms have been gained and lost through evolutionary history (Bronstein, J. L. 2003). Plants have adapted to different pollinators or have diverged to take advantage of a more beneficial abiotic dispersal mechanism. The benefit from plant-animal mutualisms is based on the fact that plans are sessile and the animal is mobile, so that the plant can facilitate the dispersal of seed or pollen. Thinking of animals as a resource for these actions plants compete for them by offering reward or presenting an attractive floral display. Some plants trick their pollinators by attracting them and making it appear as though a reward is present that is actually not, this interaction does not harm the pollinator but benefits the plant, which falls under commensalism and is not mutualism (Bronstein, J. L. 2003).  Most plant-animal interactions are asymmetric, meaning that specialists usually form a mutualism with a generalist, and vice versa. A general pattern in networks created by the interactions of mutualists is that as networks grow larger the number of connections between a given species and others decreased (Vazquez. et al. 2012).

Fig. 1 – A male Lipotriches sp. bee of family Halictidae and subfamily Nomiinae, commonly called Sweat bee. The bee was about 6mm long. Pictured in Dar es Salaam, Tanzania.

When we think about plant-animal mutualism the first thing that probably comes to mind is pollination by insects. During reproduction stages plants create floral displays and produce rewards so that animals, mostly insects, will carry their pollen to other plants. The number of plant-pollinator mutualisms is enormous, some plants use abiotic methods of seed dispersal, but many have bees, wasps, flies, butterflies, moths, beetles, birds, bats, or  a combination of them serve as a means of transporting their pollen using nectar as a reward. Plants range between being specialists to generalists in their use of pollinators. Flower morphology may reflect the pollinator the plant is trying to attract especially if using a specialist approach. Floral arrangements may have colors that are easily distinguished by the pollinator, and the shape may form in a way that is easily accessible to the desired pollinator and not to other pollinators that may be less effective. Plants adapted to a generalist strategy might produce a floral arrangement that will attract as many of the most common or abundant pollinators and is likely to be as open as possible to allow many pollinators to access it. Plants occupy a very large continuum of specificity to generalist methods in pollinator attraction (Johnson S.D., Steiner K.E. 2000). It’s important to recognize that a flower’s morphology is not based alone on promoting the mutualistic relationship between pollinator and plants. A given plants evolutionary history, climate, nutrient availability, and susceptibility to herbivory all may influence floral morphology (Galen. C. 1999).

Along with pollination, many plants use rewards to attract dispersers as a mechanism to create an effective method of dispersing their seeds. Plants using animals instead of biotic factors have two dispersal syndromes that they may exhibit. One method known as frugivory is to produce a fleshy nutrient rich fruit body around the seed to attract an animal that will consume it and then dispose of or defecate the seed elsewhere. The other method known as scatter-hoarding is the production of seeds meant to be taken by animals that will take the seeds for consumption but store most of them. There are some plants that exhibit both methods in different phases of dispersal which is known as diplochory. Ecologically the difference in these dispersal mechanism mutualisms is important as they provide different benefits at the cost of others (Vander Wall S.B., Beck M.J. 2012). Animal populations may rely heavily on the fruits and/or seeds of the plants in the community, and the dispersal of the seeds by these animals are often dramatically important to the establishment of seedlings for a given plant. The reliance on the food source or on seed dispersal has contributed to some of the diversification for animals and plant species respectively especially seen in tropical areas. However the reliance on dispersal from other organisms in the community comes with the danger of being subject to the dispersers population flux. Thus the losses of dispersers, especially when the plant does not have a high diversity of dispersers, can have serious ecological consequences (Willson M.F., Traveset A. 2000).

Acacia cornigera. Enlarged thorns as domatia to house ants.

Another interesting plant-animal mutualism found in abundance is the protection provided by ants for rewards produced by the plant. One approach to gain protection is a generalist strategy where plants secrete nectar near developing structures. Though it may attract a variety of insects the ants drawn by the nectar will attack other insects and herbivores attempting to harvest the plant as a food source. More than a quarter of all plant families have members that secrete nectar in this manner. Plants adopting a rarer and more specialist strategy will create cavities in enlarged stems and thorns known as domatia to shelter the ants and provide a rich food source, preserving an ant population in and around themselves which will drive away other damaging insects and herbivores and may provide additional resources to the plant in their waste. The plants that form these relationships are called myrmecophytes and are mostly found in tropical regions (Bronstein, J. L. 2003). The benefits from this mutualism must be important as plants have lost chemical defenses in order to allow occupancy by the ants, and some plants have methods to attract ants to develop a mutualistic relationship with. The fact that these relationships are widespread, persistent, and have evolved exclusive of each other indicate that they are selected for in ecosystems that can support them. A plant family with cosmopolitan distribution also uses ants as seed dispersers. The fruits produced attract ants will be carried back to their nest sites, the ants discard the seeds near the nest site without damaging it (Bronstein, J. L. 2003).

So many plants around the world have mutualisms formed with animals. These relationships are often vital to the success and/or persistence of a plant and yet an enormous amount of variation is seen. This shows us how connected the animal and plant world are beyond just the producer and consumer relationship that they occupy. The complexity these mutualistic relationships give to the ecology of the world around us is amazing, and their importance shouldn’t be forgotten when considering the natural world in which they operate.

Sources

Vazquez D.P. et al. (2009) Uniting pattern and process in plant–animal mutualistic networks: a review. Annals of Botany 103. pp. 1445–1457, 2009

Bronstein, J. L., et al. (2006), The evolution of plant–insect mutualisms. New Phytologist, 172. pp. 412–428 2006

Johnson S.D., Steiner K.E. (2000) Generalization versus specialization in plant pollination systems. TREE vol. 15, no. 4 April. pp. 140-143 2000

Galen C. (1999) Why do flowers vary?. BioScience Vol. 49, No. 8. pp. 631-640. 1999

Vander Wall S.B., Beck M.J. (2012) A comparison of Frugivory and Scatter-Hoarding Seed-Dispersal Syndromes. The Botanical Review. Vol. 78. No. 1. pp. 10-23. 2012.

Willson M.F., Traveset A. (2000) The Ecology of Seed Dispersal. Seeds: The Ecology of Regeneration in Plant Communities, 2nd edition. Ch. 4. pp. 85-110. 2000

Figures

Fig. 1 –Muhammad Mahdi Karim. Lipotriches sp. bee pollinating flowers. 2009. Photograph. http://en.wikipedia.org/wiki/File:Lipotriches_sp..jpg. Web. 19 April. 2009.

Fig. 2 – user: kafka4prez@flickr.com. Acacia cornigera. 2006. Photograph. http://en.wikipedia.org/wiki/File:A-cornigera.jpg#filelinks. Web. 7 Apr. 2007.

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