 A community is any assemblage of populations in an area or habitat.  Species richness- the number of species contained within a community  Relative abundance- a measure of how common or rare a species is in a community

 Contrasting views of communities are rooted in the individualistic and interactive hypotheses Two views on the question of how to account for species found together as members of a community emerged in the 1920s and 30s based on observation of plant distribution An individualistic hypothesis of community structure was proposed by H.A. Gleason it asserts that members of a community are found together simply because they share similar abiotic requirements such as temperature, amount of rainfall and type of soil The Interactive hypothesis was advocated by F.E. Clements who insisted that a community was a closely intertwined group of individuals who were linked by mandatory biotic interactions that cause the community to work together as a unit

The individualistic hypothesis predicts that communities should generally lack discrete geographic boundaries because each species will be distributed according to its tolerance ranges for abiotic factors, and communities should change continuously with the addition or subtraction of any particular specie The interactive hypothesis predicts that species should be clustered into discrete communities with noticeable boundaries due to the presence of one species greatly influencing the presence of another species

 The Rivet Model Proposed by Paul and Anne Ehrlich,the rivet model of communities is an extension of the interactive model, it suggests that most of the species in a community are associated tightly with other species in a web of life. Species are analogous to rivets because not all the rivets are required to hold the wing together, but if someone started removing rivets, we’d be concerned with the welfare of the next flight.

 The Redundancy Model According to this model, most of the species in a community are not tightly associated, the web of life is loose. An increase or decrease in one species has little effect on other species which operate independently Most communities lie somewhere in the middle of these two extremes

 Interspecific interactions are relationships between species of a community  Populations may be linked by competition predation mutualism and commensalism InteractionEffects on Population Density Competition (-/-) The interaction is detrimental to both species Predation (+/-) Including parasitism The interaction is beneficial to one species and detrimental to the other Mutualism (+/+)The interaction is beneficial to both species Commensalism (+/0)One species benefits from the interaction but the other is unaffected

 Interspecific competition for resources can occur when resources are in short supply  If two populations do compete for a resource the result may be a reduction in the density of one or both species or the elimination of one of the two competitors

 The competition exclusion principle G.F. Gause studied the effects of interspecific completion and determined that two species cannot coexist in the same community if there niches are identical The ecological niche is the sum total of a species’ use of the biotic and abiotic resources in its environment or the organism’s ecological role Ecologically similar species can live in the same community if their niches differ in one or more specific ways

Resource partitioning is the differentiation in niches that enables similar species to coexist in a community Character displacement is the tendency for characteristics to be more divergent in sympatric populations of the same two species Species adapt to eating different types of seeds

 Herbivory- herbivore eats part of a plant  Parasitism-a parasite lives on or in its host and depends on the host species for nutrition  Predation is a large factor in adaptive evolution, natural selection refines the adaptations of both predator and prey

 Predator Adaptations Most predators have acute senses that enable them to locate and identify potential prey Claws, teeth, fangs, stingers, and poison help catch and subdue prey  Plant Defenses Against Herbivores Chemical toxins often in combination with antipredator spines and thorns are a plant’s main defense against being eaten into extinction Examples include morphine, opium, nicotine, and mescaline. Also, plants produce flavors that may be distasteful to herbivores like cinnamon, and peppermint

Behavioral defenses include alarm calls which bring in many individuals of the same species Feeing is a common antipredator response Cryptic coloration is a passive defense that makes potential prey difficult to spot in its surroundings Some animals have mechanical of chemical defenses against predators like porcupines and skunks or frogs that can synthesize toxins Aposematic coloration is a warning to predators, many animals with chemical defenses are brightly colored

In Batesian mimicry a harmless species copies the appearance of a harmful model In Müllerian mimicry two or more harmful species resemble each other, the predator learns more quickly to avoid prey with this appearance due to the large amount of them, and both species benefits

Parasitism is a symbiotic relationship in which the parasite derives its nourishment from another organism, it’s host which is harmed in the process Parasites that live within their host are endoparasites Parasites that feed on the external surface of the host are ectoparasites In parasitoidism insects lay their eggs on a living host the larvae then feed on the body of the host eventually killing it

 Mutualism is an interspecific interaction that benefits both species Examples include nitrogen fixation by bacteria in the root nodules, and the digestion of cellulose by micro organisms in the digestive systems of termites

 Commensalism is an interaction between species that benefits only one species for example, algae or barnacles that attach to whales

 Coevolution refers to reciprocal evolutionary adaptations of two interacting species. A change in one species acts as a force on another species, in which counteradaptation in turn acts as a selective force on the first species

 Food chains link the trophic levels from producers to top carnivores. Branching food chains form food webs. A communities total energy input limits the length of its food chains

 Dominant species are the most abundant species in a community and dominance is achieved if a species has a great deal of competitive ability  Keystone species are relatively rare species that exert a disproportionate influence on the community structure

 The bottom up model proposes that nutrients and produces are the main determinants of community structure  The top down model proposes that predators control herbivores who in turn control producers, thus power comes from the trophic level above

 Most communities are in a state of nonequilibrium Evidence suggests that disturbance and nonequilibrium instead of stability and equilibrium are normal for most communities  Humans are the most widespread agents of disturbance Humans create the greatest disturbances in communities usually reducing species diversity. Humans also prevent some naturally occurring disturbances like fire

Primary succession occurs when no soil exists when succession begins; secondary succession will begin in an area where soil remains after a disturbance

 Community biodiversity measures the number of species and their relative abundance The simplest measure of biodiversity is the number of species in a community (species richness) species may be rare or common in a community so relative abundance is a factor in biodiversity

 Species richness generally declines along an equatorial-polar gradient Species richness is much greater in the tropics than in the temperate and polar regions. Climate is the best explanation for this biodiversity gradient through its impact on energy and water  Species richness is related to a community’s geographic size  Species richness on islands depends on island size and distance from the mainland A hypothesis of island biogeography maintains that species richness on an ecological island levels off at some equilibrium point where new immigrations are balanced by extinctions The hypothesis predicts that species richness is directly proportional to island size and inversely proportional to distance of the island from the source of colonizers