Mutualism

Mutualistic interactions benefit both partners (positive effects on each) and therefore represent cooperative or mutually exploitative relationships. One member of a mutualism provides a resource that is exploited by the other (the symbiont). The symbiont, in turn, unintentionally provides a service to its host.

Mutualism Pictures

| Commensalism: an unidentified mite in an ambrosia beetle, Trypodendron lineatum, mine in Douglas-fir. A variety of predaceous and detritivorous mites exploit resources in bark and ambrosia beetle mines.

| Commensalism: an unidentified mite in an ambrosia beetle, Trypodendron lineatum, mine in Douglas-fir. A variety of predaceous and detritivorous mites exploit resources in bark and ambrosia beetle mines.

For example, plants expend resources to attract pollinators, ants (for defense), or mycorrhizal fungi, which perform a service to the plant in the process of exploiting plant resources. Similarly, bark beetles provide nourishment to their symbiotic microorganisms that improve resource suitability for their host as a consequence of being transported to new resources (see later in this section). Gut symbionts of many insects, and other animals, provide nourishment as a consequence of exploiting resources in the host gut. Some mutualisms require less sacrifice of resources by either member of the pair. For example, aphids attract ants to their waste product, honeydew, and benefit from the protection the ants provide.

Mutualisms have received considerable attention, and much research has focused on examples such as pollination (see Chapter 13), ant-plant and mycor-rhizae-plant interactions, and other conspicuous mutualisms. Nevertheless, Price (1997) argued that ecologists have failed to appreciate mutualism as equal in importance to predation and competition, at least in temperate communities, reflecting a perception, based on early models, that mutualism is less stable than competition or predation (e.g., Goh 1979, May 1981, M. Williamson 1972). However, as Goh (1979) noted, such models did not appear to reflect the widespread occurrence of mutualism in ecosystems. As a cooperative relationship, mutualism can contribute greatly to the presence and ecological function of the partners, but the extent to which such positive feedback stabilizes or destabilizes interacting species populations remains a topic of discussion.

Mutualistic interactions tend to be relatively specific associations between co-evolved partners and often involve modification of host morphology, physiology, or behavior to provide habitat or food resources for the symbiont. In return, the symbiont provides necessary resources or protection from competitors or predators. Although the classic examples of mutualism often involve mutually dependent (obligate) partners (i.e., disappearance of one leads to demise of the other) some mutualists are less tightly coupled. However, Janzen and Martin (1982) suggested that some mutualisms might reflect substitution for an extinct co-evolved symbiont by an extant symbiont, by virtue of similar attributes (see Chapter 13). To some degree, herbivores on plants often may function as mutu-alists, pruning and permitting reallocation of resources to more productive plant parts in return for their resources. Many insect species engage in mutualistic interactions with other organisms, including plants, microorganisms, and other insects.

Among the best-known mutualisms are those involving pollinator and ant associations with plants (Feinsinger 1983, Huxley and Cutler 1991, Jolivet 1996). The variety of obligate relationships between pollinators and their floral hosts in the tropics perhaps has contributed to the perception that mutualism is more widespread and important in the tropics. As discussed in Chapter 13, the prevalence of obligate mutualisms between plants and pollinators in the tropics, compared to temperate regions, largely reflects the high diversity of plant species, which precludes wind pollination between nearest neighbors. Sparsely distributed or understory plants in temperate regions also tend to have mutualistic association with pollinators. Other mutualistic associations (e.g., insect-microbial association; see later in this section) may be more prominent in temperate than in tropical regions. Many plants provide nest sites or shelters (domatia) (e.g., hollow stems or pilose vein axils) for ants or predaceous mites that protect the plant from herbivores (O'Dowd and Willson 1991). Other plant species provide extrafloral nectaries rich in amino acids and lipids that attract ants (e.g., Dreisig 1988, Jolivet 1996, Oliveira and Brandao 1991, Rickson 1971, Schupp and Feener 1991, Tilman 1978). In addition to defense, plants also may acquire nitrogen or other nutrients from the ants (Fischer et al. 2003).

Clarke and Kitching (1995) discovered an unusual example of a mutualistic interaction between an ant and a carnivorous pitcher plant in Borneo. The ant, Camponotus sp., nests in hollow tendrils of the plant, Nepenthes bicalcarata, and is capable of swimming in pitcher plant fluid, where it feeds on large prey items caught in the pitcher. Through ant-removal experiments, Clarke and Kitching found that accumulation of large prey (but not small prey) in ant-free pitchers led to putrefaction of the pitcher contents and disruption of prey digestion by the plant. By removing large prey, the ants prevent putrefaction and accumulation of ammonia.

Seed-feeding ants often benefit plants by assisting dispersal of unconsumed seeds. This mutualism is exemplified by myrmecochorous plants that provide a nutritive body (elaiosome) attached to the seed to attract ants. The elaiosome usually is rich in lipids (Gorb and Gorb 2003, Jolivet 1996). The likelihood that a seed will be discarded in or near an ant nest following removal of the elaio-some increases with elaiosome size, perhaps reflecting increasing use by seed-disperser, rather than seed-predator, species with increasing elaiosome size (Gorb and Gorb 2003, Mark and Olesen 1996, Westoby et al. 1991). The plants benefit primarily through seed dispersal by ants (Horvitz and Schemske 1986, Ohkawara et al. 1996), not necessarily from seed relocation to more nutrient-rich microsites (Horvitz and Schemske 1986,Westoby et al. 1991;see Chapter 13).This interaction has been implicated in the rapid invasion of new habitats by myrme-cochorous species (J. M. B. Smith 1989).

Gressitt et al. (1965, 1968) reported that large phytophagous weevils (Coleoptera: Curculionidae) in the genera Gymnopholus and Pantorhytes host diverse communities of cryptogamic plants, including fungi, algae, lichens, liverworts, and mosses, on their backs. These weevils have specialized scales or hairs and produce a thick waxy secretion from glands around depressions in the elytra that appear to foster the growth of these symbionts. In turn, the weevils benefit from the camouflage provided by this growth and, possibly, from chemical protection. Predation on these weevils appears to be rare.

Insects exhibit a wide range of mutualistic interactions with microorganisms. Parasitoid wasps inoculate their host with a virus that prevents cellular encapsulation of the parasitoid larva (Edson et al. 1981, Godfray 1994; see Chapter 3). Intestinal bacteria may synthesize some of the pheromones used by bark beetles to attract mates (Byers and Wood 1981). Most aphids harbor mutualistic bacteria or yeasts in specialized organs (bacteriomes or mycetomes) that appear to provide amino acids, vitamins, or proteins necessary for aphid development and reproduction (Baumann et al. 1995). Experimental elimination of the microbes results in aphid sterility, reduced weight, and reduced survival. Many homopter-ans vector plant pathogens and may benefit from changes in host condition induced by infection (Kluth et al. 2002). Leaf-cutting ants, Atta spp. and Acromyrmex spp., cultivate fungus gardens that provide food for the ants (e.g., Currie 2001, Weber 1966).

Virtually all wood-feeding species interact mutualistically with some cellulose-digesting microorganisms. Ambrosia beetles (Scolytidae and Platypodidiae) are the only means of transport for ambrosia (mold) fungi, carrying hyphae in specialized invaginations of the cuticle (mycangia) that secrete lipids for fungal nourishment, and require the nutrition provided by the fungus. The adult beetles carefully cultivate fungal gardens in their galleries, removing competing fungi. Their offspring feed exclusively on the fungus, which derives its resources from the wood surrounding the gallery, and collect and transport fungal hyphae when they disperse (Batra 1966, French and Roeper 1972).

Siricid wasps also are the only means of dispersal for associated Amylostereum (decay) fungi, and larvae die in the absence of the fungus (Morgan 1968). The adult female wasp collects fungal hyphae from its gallery prior to exiting. The wasp stores and nourishes the fungus in a mycangium at the base of the ovipositor, then introduces the fungus during oviposition in the wood. The fungus decays the wood around the larva that feeds on the fungal mycelium, destroying it in the gut, and passes decayed wood fragments around the body to combine posteriorly with its frass. Phloem-feeding bark beetles transport mycangial fungi and bacteria as well as opportunistic fungi. Ayres et al. (2000) reported that mycan-gial fungi significantly increased nitrogen concentrations in phloem surrounding southern pine beetle, Dendroctonus frontalis, larvae, compared to uncolonized phloem. Opportunistic fungi, including blue-stain Ophiostoma minus, did not concentrate nitrogen in phloem surrounding larvae, suggesting that the apparent antagonism between this fungus and the bark beetle may reflect failure to enhance phloem nutrient concentrations (see later in this chapter). Termites similarly depend on mutualistic bacteria or protozoa in their guts for digestion of cellulose (Breznak and Brune 1994).

Many mutualistic interactions involve insects and other arthropods. A well-known example is the mutualism between honeydew-producing Homoptera and ants (Fig. 8.9). Homoptera excrete much of the carbohydrate solution (honey-dew) that composes plant sap so as to concentrate sufficient nutrients (see Chapter 3). Aphid species are particularly important honeydew producers. A variety of species are tended by ants that harvest this carbohydrate resource and protect the aphids from predators and parasites (Bristow 1991, Dixon 1985, Dreisig 1988). This mutualism involves only about 25% of aphid species and varies in its strength and benefits, perhaps reflecting plant chemical influences or the relative costs of defending aphid colonies (Bristow 1991). Ant species show different preferences among aphid species, and the efficiency of protection often varies inversely with aphid and ant densities (Bristow 1991, Cushman and Addicott 1991, Dreisig 1988).

Dung beetles (Scarabaeidae) and bark beetles often have mutualistic association with phoretic, predaceous mites. The beetles are the only means of

Mutualistic Association With Insects

long-distance transport for the mites, and the mites feed on the competitors or parasites of their hosts (Kinn 1980, Krantz and Mellott 1972).

Although mutualism usually is viewed from the perspective of mutual benefits, this interaction also can be viewed as mutual exploitation or manipulation. The structures and resources necessary to maintain the mutualism represent costs to the organisms involved. For example, the provision of domatia or extrafloral nectaries by ant-protected plants represents a cost in terms of energy and nutrient resources that otherwise could be allocated to growth and reproduction. Ants may provide nitrogen or other nutrients, as well as defense, for their hosts (Fischer et al. 2003). Therefore, plants may lose ant-related traits when the benefit from the ants is removed (Rickson 1977).

Models of mutualistic interactions have lagged behind models for competitive or predator-prey interactions, largely because of the difficulty of simultaneously incorporating negative (density-limiting) and positive (density-increasing) feedback. The Lotka-Volterra equations may be inadequate for extension to mutualism because they lead to unbounded exponential growth of both populations (May 1981, but see Goh 1979). May (1981) asserted that minimally realistic models for mutualists must allow for saturation in the magnitude of at least one of the reciprocal benefits, leading to a stable equilibrium point, with one (most often both) of the two equilibrium populations being larger than that sustained in the absence of the mutualistic interaction. However, recovery from perturbations to this equilibrium may take longer than in the absence of the mutualistic interaction, leading to instability (May 1981). May (1981) presented a simple model for two mutualistic populations:

in which the carrying capacity of each population is increased by the presence of the other, with a and b representing the beneficial effect of the partner, K1 ^ K1 + aN2, K2 ^ K2 + bN1 and ab < 1 to limit uncontrolled growth of the two populations. The larger the product, ab, the more tightly coupled the mutualists. For obligate mutualists, a threshold effect must be incorporated to represent the demise of either partner if the other becomes rare or absent. May (1981) concluded that mutualisms are stable when both populations are relatively large and increasingly unstable at lower population sizes, with a minimum point for persistence.

Dean (1983) proposed an alternative model that incorporates density dependence as the means by which two mutualists can reach a stable equilibrium. As a basis for this model, Dean developed a model to describe the relationship between population carrying capacity (ky) and an environmental variable (M) that limits ky:

where Ky is the maximum value of ky and the constant a is reduced by a linear function of ky. This equation can be integrated as follows:

where Cy is the integration constant. Equation (8.11) describes the isocline where dY/dt = 0.

For species Y exploiting a replenishable resource provided by species X, Equation (8.11) can be rewritten as follows:

where Nx is the number of species X. The carrying capacity of species X depends on the value of Y and can be described as follows:

where Ny is the number of species Y. Mutualism will be stable when the number of one mutualist (Ny) maintained by a certain number of the other mutualist (Nx) is greater than the Ny necessary to maintain Nx. When this condition is met, both populations grow until density effects limit the population growth of X and Y, so that isoclines defined by Equations (8.12) and (8.13) inevitably intersect at a point of stable equilibrium. Mutualism cannot occur when the isoclines do not intersect and is unstable when the isoclines are tangential. This condition is satisfied when any value of Nx or Ny can be found to satisfy either of the following equations:

Ky (1 - e(-aN┬ąCy VKy) >-(Cx + Kx [ln(Kx - Nx) - lnKx ])/b (8.14)

Kx (1 - ebNy+CxvKx) >-(Cy + Ky [ln(Ky - Ny) - lnKy ])/a (8.15)

The values of the constants, Cx and Cy, in equations (8.13) and (8.14) indicate the strength of mutualistic interaction. When Cx and Cy > 0, the interacting species are facultative mutualists; when Cx and Cy = 0, both species are obligate mutual-ists; when Cx and Cy < 0, both species are obligate mutualists and their persistence is determined by threshold densities (Fig. 8.10).

The growth rates of the two mutualists can be described by modified logistic equations as follows:

What Threshold MutualismPlant Growth Equations

The effect of integration constants in Dean's (1983) model on the form of mutualism (see text for equations) over a range of densities (X and Y) for two interacting species. A: When Cx and Cy > 0, the interacting species are facultative mutualists; B: when Cx and Cy = 0, both species are obligate mutualists; and C: when Cx and Cy < 0, both species are obligate mutualists and have extinction thresholds at densities of B. Reprinted with permission from the University of Chicago. Please see extended permission list pg 571.

The effect of integration constants in Dean's (1983) model on the form of mutualism (see text for equations) over a range of densities (X and Y) for two interacting species. A: When Cx and Cy > 0, the interacting species are facultative mutualists; B: when Cx and Cy = 0, both species are obligate mutualists; and C: when Cx and Cy < 0, both species are obligate mutualists and have extinction thresholds at densities of B. Reprinted with permission from the University of Chicago. Please see extended permission list pg 571.

where ry and rx are the intrinsic rates of increase for species Y and X, respectively. However, ky and kx are not constants but are determined by equations (8.12) and (8.13).

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Responses

  • tesfalem
    What are mutualists species?
    6 years ago

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