Further Reading

Loechler EL (1996) The role of adduct site-specific mutagenesis in understanding how carcinogen-DNA adducts cause mutations: Perspective, prospects and problems. Carcinogenesis 17: 895-902.

Norbury CJ and Hickson ID (2001) Cellular responses to DNA damage. Annual Reviews of Pharmacology and Toxicology 41: 367-401.

Rosenberg SM (2001) Evolving responsively: Adaptive mutation. Nature

Reviews Genetics 2: 504-515. Sarasin A (2003) An overview of the mechanisms of mutagenesis and carcinogenesis. Mutation Research 544: 99-106. Shugart LR, Theodorakis CW, Bickham AM, and Bickham J (2002) Genetic effects of contaminant exposure and potential impacts on animal populations. In: Calow P (ed.) Handbook of Ecotoxicology, pp. 1129-1148. Oxford: Blackwell Science. Wang Z (2001) DNA damage-induced mutagenesis: A novel target for cancer prevention. Molecular Interventions 1: 269-281. Wirgin I and Theodorakis CW (2002) Molecular biomarkers in aquatic organisms: DNA- and RNA-based endpoints. In: Adams SM (ed.) BiologicalIndicators of Aquatic Ecosystem Health, pp. 73-82. New York: American Fisheries Society.

Mutualism

J N Holland, Rice University, Houston, TX, USA J L Bronstein, University of Arizona, Tucson, AZ, USA © 2008 Elsevier B.V. All rights reserved.

Introduction Mutualism Defined Historical Study of Mutualism Benefits and Costs of Mutualism

Functional Responses and Population Dynamics Conditional and Context-Dependent Outcomes Evolutionary Ecology of Mutualism Further Reading

Introduction

Fundamental to the discipline of ecology is understanding how and why interactions between populations of different species (i.e., interspecific interactions, species interactions) influence the growth, abundance, dynamics, and stability of the interacting populations. Interspecific interactions occur when the actions, traits, or density of individuals of one population result in a change in some attribute of another species' population. Population attributes may include, for example, (per capita) reproduction, survival, recruitment, mortality, population growth, population size, population density, and mean character (trait) values of individuals comprising the population. Almost all, if not all, species are involved in at least one interspecific interaction, and most are involved in multiple interspecific interactions at any one time. For example, an individual plant may simultaneously interact with pollinators, seed dispersers, root symbionts, herbivores, and plant competitors.

Interspecific interactions are most commonly classified according to the outcomes or effects of interactions between individuals of different species. The effect or outcome of any given interaction on a population attribute can be positive (+), negative (—), or neutral (0). Thus, there are six different pairwise outcomes: predation (+, —), competition (—, —), mutualism (+, +), commens-alism (+, 0), neutralism (0, 0), and amensalism (—, 0) (Figure 1). Although this classification is based on discrete (+, —, 0) effects on each of the interacting populations, as Figure 1 depicts, they actually range continuously among one another; for example, a very small positive effect (+) ranges into a neutral (0) and then a negative (—) effect.

Mutualisms are increasingly recognized as fundamental to patterns and processes of ecological systems. Mutualisms occur in habitats throughout the world, and ecologists now acknowledge that almost every species on Earth is involved directly or indirectly in one or more mutualism (Table 1). Examples include animal-mediated pollination and seed dispersal, which can be particularly

Mutualism

Mutualism

Competition

Figure 1 A compass of interaction outcomes that classifies interspecific interactions into one of six general forms based on their effects or outcomes on the interacting populations. Moving from the center toward the periphery of the compass increases the strength or magnitude of the interaction outcome, but does not alter the sign of the effect of the interaction for either of the interacting species. On the other hand, moving around the periphery of the circumference changes the sign and type of interspecific interaction.

Competition

Figure 1 A compass of interaction outcomes that classifies interspecific interactions into one of six general forms based on their effects or outcomes on the interacting populations. Moving from the center toward the periphery of the compass increases the strength or magnitude of the interaction outcome, but does not alter the sign of the effect of the interaction for either of the interacting species. On the other hand, moving around the periphery of the circumference changes the sign and type of interspecific interaction.

prominent in tropical forests; the plants benefit by having pollen and seeds transported by animals, while the animals are generally attracted to and rewarded by food (nectar and fruit, respectively). Nitrogen-fixation mutualisms are important in many habitats, notably including deserts and agroecosystems. In these interactions, root-associated bacteria fix nitrogen to a form that can be used by plants, and obtain carbon from the plants in return. Nutrient exchanges also occur between root-associated mycorrhizal fungi and plants in grasslands, which are common in grasslands; between fungi and algae that constitute lichens (prominent in tundras and early succes-sional communities); between coral and the zooxanthellae that inhabit them in marine systems; and between microbes in deep-sea vents of oceans. Other common mutualisms involve relationships between animals that protect plants or other animals from harsh abiotic environments and from natural enemies. For example, ants defend many plants from attack by herbivores, in

Table 1 Some examples of mutualisms, types of species involved in the interactions, and associated benefits and costs

Mutualism

Partners

Benefits

Costs

Lichen

Fungi

Algal photosynthates

Nutrients, water

Algae

Nutrients, water

Algal photosynthates

Coral

Corals

Algal photosynthates

Nutrients, nitrogen

Zooxanthellae

Nutrients, nitrogen

Algal photosynthates

Mycorrhizal

Plants

Nutrients, phosphorus

Root exudates, carbon

Mycorrhizae

Root exudates, carbon

Nutrients, phosphorus

Nitrogen Fixation

Plants

Nitrogen

Root exudates, carbon

Rhizobia

Root exudates, carbon

Nitrogen

Ant agriculture

Ants

Fungus-food resource

Ant-collected leaves

Fungus

Ant-collected leaves

Fungus-food resource

Digestive symbiosis

Termites

Protozoa-digested food

Food for termites (?)

Protozoa

Termite-ingested cellulose

Digesting food (?)

Pollination

Plants

Pollen dispersal, pollination

Nectar and/or pollen

Animals

Nectar and/or pollen

Time/energy

Seed dispersal

Plants

Seedling recruitment

Disperser food resource

Animals

Seed/fruit food resource

Time/energy

Ant-plant protection

Plants

Herbivore protection

Nectar, food bodies

Ants

Nectar, food bodies

Time/energy protecting

Ant-insect protection

Insects®

Natural enemy protection

Food secretions/excretions

Ants

Insect food provision

Time/energy protecting

aLycaenid caterpillars, homopterans.

aLycaenid caterpillars, homopterans.

exchange for food and living space. These mutualisms are particularly well known in tropical environments, although they occur in habitats worldwide.

Influences of mutualism transcend levels of biological organization from cells to populations, communities, and ecosystems. Mutualisms are now thought to have been key to the origin of eukaryotic cells, as both chloroplasts and mitochondria were once free-living microbes. Mutualisms are crucial to the reproduction and survival of many plants and animals, and to nutrient cycles in ecosystems. Moreover, the ecosystem services mutualists provide (e.g., seed dispersal, pollination, and carbon, nitrogen, and phosphorus cycles resulting from plant-microbe interactions) are leading mutualisms to be increasingly considered a conservation priority.

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