The relationship between the intelligent but weak George Milton and the retarded but strong Lennie Small is the focal point of Steinbeck's novella, and a surface reading strongly suggests that "friendship" or "personal commitment" is one of this work's salient themes. As the half-witted Lennie dutifully intones, the two men are distinguished from all of the other characters in the story "because I got you to look after me, and you got me to look after you, and that's why." (p.15). The initial interview by the ranch boss underscores the unusual quality of this bond, and the jerkline skinner Slim later echoes his employer's bewilderment when he says to George, "'Funny how you an' him string along together.'" (p.43). George confides that he and Lennie are not, in fact, cousins, but we learn that they have known each other since grammar school. They are linked together by a shared past, by a dream of the future, and by current circumstances. All of this implies a substratum of mutual affection.
Yet theirs is a symbiotic relationship. The two men are forced together by common necessity rather than genuine emotional attachment. Lennie, of course, depends entirely upon his long-time comrade, and the very thought of George abandoning him sends the childlike giant into a state of panic. It is evident from the start that Lennie could not possibly function in the harsh world that they inhabit without George, who holds his companion's work card and always does the talking for him. The stable buck Crooks is unsparingly accurate in his assessment that without George's continual guidance, Lennie would wind up chained like a dog in an institution for the feeble-minded. Lennie wears the same clothes as George and even imitates his gestures. The extent of Lennie's psychological integration with the George is acutely apparent in the novel's concluding chapter when the giant rabbit of his stricken conscience mouths George's words in Lennie's own voice.
By the same token, just as Lennie needs mice and pups and rabbits to take care of, George needs Lennie to tend. As George discloses to Slim, the incident that sealed the bond between the duo came when he told his utterly compliant friend to jump in the rushing Sacramento River and was then forced to save the huge man from drowning. Lennie furnishes George with an object for his own lower-case ennoblement. George also uses Lennie as an excuse for the menial hardships that he must endure. He repeatedly claims that life would be "so easy" for him were it not for the burden of caring for Lennie. This is plainly an expression of wishful thinking. With or without Lennie in tow, George would still be compelled to eke out a meager, inane existence as a lowly ranch hand. But most of all, George needs Lennie to concur with and to prop up his "dream" of owning a little farm and thereby preserve it from dissolving under the brutal force of reality. It is a web of dependencies, not brotherly love, which binds the two men together.
A profound, primordial isolation runs through the lives of all of the characters in Of Mice and Men, and it is this separateness that constitutes the novel's predominate theme. George and Lennie are adrift and, at bottom, on their own in the world that Steinbeck depicts. Although this lack of anchorage is particularized as an historical manifestation of the Depression Era, people in this story are basically divided by a timeless and universal feature of the human condition, a distrust born of vulnerability . As Slim muses, the reason that ranch hands are loners is that "'everybody in the whole damn world is scared of each other.'" (p.38). In one of the novel's most touching episodes, the black stable worker Crooks (set even further apart from his fellows by virtue of his race) tells Lennie that lacking someone to share his experience, he can't even tell if what he sees before him is real or merely a dream. (p.80).
Curley's wife is there to remind Crooks that his subordinate status is all too real when she responds to a felt insult: "'Nigger, I could bet you strung up on a tree so easy it ain't even funny'" (p.89). As a black man, Crooks is clearly liable to such false...
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This article is about the biological term. For the economic theory and other uses, see Mutualism (disambiguation).
Mutualism is the way two organisms of differentspecies exist in a relationship in which each individual benefits from the activity of the other. Similar interactions within a species are known as co-operation. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the "expense" of the other. Symbiosis involves two species living in close proximity and includes relationships that are mutualistic, parasitic, and commensal. Symbiotic relationships are sometimes, but not always, mutualistic.
A well-known mutualism is the relationship between ungulates (such as bovines) and bacteria within their intestines. The ungulates benefit from the cellulase produced by the bacteria, which facilitates digestion; the bacteria benefit from having a stable supply of nutrients in the host environment. This can also be found in many different symbiotic relationships.
Mutualism plays a key part in ecology. For example, mutualistic interactions are vital for terrestrial ecosystem function as more than 48% of land plants rely on mycorrhizal relationships with fungi to provide them with inorganic compounds and trace elements. In addition, mutualism is thought to have driven the evolution of much of the biological diversity we see, such as flower forms (important for pollination mutualisms) and co-evolution between groups of species. However mutualism has historically received less attention than other interactions such as predation and parasitism.
Measuring the exact fitness benefit to the individuals in a mutualistic relationship is not always straightforward, particularly when the individuals can receive benefits from a variety of species, for example most plant-pollinator mutualisms. It is therefore common to categorise mutualisms according to the closeness of the association, using terms such as obligate and facultative. Defining "closeness," however, is also problematic. It can refer to mutual dependency (the species cannot live without one another) or the biological intimacy of the relationship in relation to physical closeness (e.g., one species living within the tissues of the other species).
The term mutualism was introduced by Pierre-Joseph van Beneden in his 1876 book Animal Parasites and Messmates.
Types of relationships
Mutualistic relationships can be thought of as a form of "biological barter" in mycorrhizal associations between plant roots and fungi, with the plant providing carbohydrates to the fungus in return for primarily phosphate but also nitrogenous compounds. Other examples include rhizobia bacteria that fix nitrogen for leguminous plants (family Fabaceae) in return for energy-containing carbohydrates.
Service-resource relationships are common.
Pollination in which nectar or pollen (food resources) are traded for pollen dispersal (a service) or ant protection of aphids, where the aphids trade sugar-rich honeydew (a by-product of their mode of feeding on plant sap) in return for defense against predators such as ladybugs.
Phagophiles feed (resource) on ectoparasites, thereby providing anti-pest service, as in cleaning symbiosis. Elacatinus and Gobiosoma, genera of gobies, also feed on ectoparasites of their clients while cleaning them.
Zoochory is the dispersal of the seeds of plants by animals. This is similar to pollination in that the plant produces food resources (for example, fleshy fruit, overabundance of seeds) for animals that disperse the seeds (service).
Strict service-service interactions are very rare, for reasons that are far from clear. One example is the relationship between sea anemones and anemone fish in the family Pomacentridae: the anemones provide the fish with protection from predators (which cannot tolerate the stings of the anemone's tentacles) and the fish defend the anemones against butterflyfish (family Chaetodontidae), which eat anemones. However, in common with many mutualisms, there is more than one aspect to it: in the anemonefish-anemone mutualism, waste ammonia from the fish feed the symbioticalgae that are found in the anemone's tentacles. Therefore, what appears to be a service-service mutualism in fact has a service-resource component. A second example is that of the relationship between some ants in the genus Pseudomyrmex and trees in the genusAcacia, such as the whistling thorn and bullhorn acacia. The ants nest inside the plant's thorns. In exchange for shelter, the ants protect acacias from attack by herbivores (which they frequently eat, introducing a resource component to this service-service relationship) and competition from other plants by trimming back vegetation that would shade the acacia. In addition, another service-resource component is present, as the ants regularly feed on lipid-rich food-bodies called Beltian bodies that are on the Acacia plant.
In the neotropics, the ant, Myrmelachista schumanni makes its nest in special cavities in Duroia hirsute. Plants in the vicinity that belong to other species are killed with formic acid. This selective gardening can be so aggressive that small areas of the rainforest are dominated by Duroia hirsute. These peculiar patches are known by local people as "devil's gardens".
In some of these relationships, the cost of the ant’s protection can be quite expensive. Cordia sp. trees in the Amazonian rainforest have a kind of partnership with Allomerus sp. ants, which make their nests in modified leaves. To increase the amount of living space available, the ants will destroy the tree’s flower buds. The flowers die and leaves develop instead, providing the ants with more dwellings. Another type of Allomerus sp. ant lives with the Hirtella sp. tree in the same forests, but in this relationship the tree has turned the tables on the ants. When the tree is ready to produce flowers, the ant abodes on certain branches begin to wither and shrink, forcing the occupants to flee, leaving the tree’s flowers to develop free from ant attack.
The term "species group" can be used to describe the manner in which individual organisms group together. In this non-taxonomic context one can refer to "same-species groups" and "mixed-species groups." While same-species groups are the norm, examples of mixed-species groups abound. For example, zebra (Equus burchelli) and wildebeest (Connochaetes taurinus) can remain in association during periods of long distance migration across the Serengeti as a strategy for thwarting predators. Cercopithecus mitis and Cercopithecus ascanius, species of monkey in the Kakamega Forest of Kenya, can stay in close proximity and travel along exactly the same routes through the forest for periods of up to 12 hours. These mixed-species groups cannot be explained by the coincidence of sharing the same habitat. Rather, they are created by the active behavioural choice of at least one of the species in question.
Humans are involved in mutualisms with other species: their gut flora is essential for efficient digestion. Infestations of head lice might have been beneficial for humans by fostering an immune response that helps to reduce the threat of body louse borne lethal diseases.
Some relationships between humans and domesticated animals and plants are to different degrees mutualistic. For example, agricultural varieties of maize provide food for humans and are unable to reproduce without human intervention because the leafy sheath does not fall open, and the seedhead (the "corn on the cob") does not shatter to scatter the seeds naturally.
In traditional agriculture, some plants have mutualist as companion plants, providing each other with shelter, soil fertility and/or natural pest control. For example, beans may grow up cornstalks as a trellis, while fixing nitrogen in the soil for the corn, a phenomenon that is used in Three Sisters farming.
Boran people of Ethiopia and Kenya traditionally use a whistle to call the honeyguide bird, though the practice is declining. If the bird is hungry and within earshot, it guides them to a bees' nest. In exchange the Borans leave some food from the nest for the bird.
A population of bottlenose dolphins in Laguna, Brazil coordinates, via body language, with local net-using fishermen in order for both to catch schools of mullet.
One researcher has proposed that the key advantage Homo sapiens had over Neanderthals in competing over similar habitats was the former's mutualism with dogs.
Mathematical treatments of mutualisms, like the study of mutualisms in general, has lagged behind those of predation, or predator-prey, consumer-resource, interactions. In models of mutualisms, the terms "type I" and "type II" functional responses refer to the linear and saturating relationships, respectively, between benefit provided to an individual of species 1 (y-axis) on the density of species 2 (x-axis).
Type I functional response
One of the simplest frameworks for modeling species interactions is the Lotka–Volterra equations. In this model, the change in population density of the two mutualists is quantified as:
- N and M=the population densities.
- r=intrinsic growth rate of the population.
- K=carrying capacity of its local environmental setting.
- β=coefficient converting encounters with one species to new units of the other.
Mutualism is in essence the logistic growth equation + mutualistic interaction. The mutualistic interaction term represents the increase in population growth of species one as a result of the presence of greater numbers of species two, and vice versa. As the mutualistic term is always positive, it may lead to unrealistic unbounded growth as it happens with the simple model. So, it is important to include a saturation mechanism to avoid the problem.
The type I functional response is visualized as the graph of vs.M.
Type II functional response
In 1989, David Hamilton Wright modified the Lotka–Volterra equations by adding a new term, βM/K, to represent a mutualistic relationship. Wright also considered the concept of saturation, which means that with higher densities, there are decreasing benefits of further increases of the mutualist population. Without saturation, species' densities would increase indefinitely. Because that isn't possible due to environmental constraints and carrying capacity, a model that includes saturation would be more accurate. Wright's mathematical theory is based on the premise of a simple two-species mutualism model in which the benefits of mutualism become saturated due to limits posed by handling time. Wright defines handling time as the time needed to process a food item, from the initial interaction to the start of a search for new food items and assumes that processing of food and searching for food are mutually exclusive. Mutualists that display foraging behavior are exposed to the restrictions on handling time. Mutualism can be associated with symbiosis.
Handling time interactions In 1959, C. S. Holling performed his classic disc experiment that assumed the following: that (1), the number of food items captured is proportional to the allotted searching time; and (2), that there is a variable of handling time that exists separately from the notion of search time. He then developed an equation for the Type II functional response, which showed that the feeding rate is equivalent to
- a=the instantaneous discovery rate
- x=food item density
- TH=handling time
The equation that incorporates Type II functional response and mutualism is:
- N and M=densities of the two mutualists
- r=intrinsic rate of increase of N
- c=coefficient measuring negative intraspecific interaction. This is equivalent to inverse of the carrying capacity, 1/K, of N, in the logistic equation.
- a=instantaneous discovery rate
- b=coefficient converting encounters with M to new units of N
This model is most effectively applied to free-living species that encounter a number of individuals of the mutualist part in the course of their existences. Wright notes that models of biological mutualism tend to be similar qualitatively, in that the featured isoclines generally have a positive decreasing slope, and by and large similar isocline diagrams. Mutualistic interactions are best visualized as positively sloped isoclines, which can be explained by the fact that the saturation of benefits accorded to mutualism or restrictions posed by outside factors contribute to a decreasing slope.
The type II functional response is visualized as the graph of vs.M.
Structure of networks
Mutualistic networks made up out of the interaction between plants and pollinators were found to have a similar structure in very different ecosystems on different continents, consisting of entirely different species. The structure of these mutualistic networks may have large consequences for the way in which pollinator communities respond to increasingly harsh conditions and on the community carrying capacity.
Mathematical models that examine the consequences of this network structure for the stability of pollinator communities suggest that the specific way in which plant-pollinator networks are organized minimizes competition between pollinators, reduce the spread of indirect effects and thus enhance ecosystem stability and may even lead to strong indirect facilitation between pollinators when conditions are harsh. This means that pollinator species together can survive under harsh conditions. But it also means that pollinator species collapse simultaneously when conditions pass a critical point. This simultaneous collapse occurs, because pollinator species depend on each other when surviving under difficult conditions.
Such a community-wide collapse, involving many pollinator species, can occur suddenly when increasingly harsh conditions pass a critical point and recovery from such a collapse might not be easy. The improvement in conditions needed for pollinators to recover, could be substantially larger than the improvement needed to return to conditions at which the pollinator community collapsed.
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