C OMMUNITY E COLOGY By: Mary E. Towns Kelsee Elders Blake Wilson

A C OMMUNITY ????? A community is an assemblage of populations of various species living close enough for potential interaction.

I NTERSPECIFIC I NTERACTIONS They are interactions that occur between populations of just two species in a community. (+/+) benefit both species. E.g. mutualism (-/-) can be detrimental to both species E.g. competition (+/-) benefits one species and is detrimental to the other E.g. parasitism (+/0) benefits one species and other is unaffected E.g. commensalism

I NTERSPECIFIC I NTERACTIONS : C OMPETITION Interspecific competition occurs when species compete for a particular resource that is in short supply. The competitive exclusion principle says that two species that rely on the same limited resource cannot exist in the same community. This principle predicts that the less efficient competitor will be locally eliminated.

C OMPETITION CONTINUED An organism’s ecological niche is described as its role in an ecosystem- its habitat and use of biotic and abiotic resources. Realized niche- what a species actually occupies Fundamental niche- the niche potentially occupied by that species. Resource partitioning is the differentiation that enables similar species to coexist in a community. Character displacement of some morphological trait or resource use allows closely related sympatric species to avoid competition.

P REDATION Predation involves a predator killing and eating prey. Potential prey may use camouflage in the form of cryptic coloration to blend in with the background. Bright, conspicuous, aposematic coloration warns predators not to eat animals with chemical defenses.

M IMICRY Sometimes, one prey species may gain significant protection by mimicking the appearance of another. Batesian mimicry - a harmless species mimics a harmful model. Müllerian mimicry - two or more harmful species mimic each other.

H ERBIVORY An herbivore eats parts of a plant or alga.

P ARASITISM In parasitism, a parasite obtains its nourishment from its host. Types of parasites: ectoparasites endoparasites

D ISEASE Pathogens, such as bacteria, viruses, protists, or even fungi or prions, are like microscopic parasites. Pathogens may kill their hosts; parasites do not usually cause lethal harm to the host on which they feed.

M UTUALISM In mutualism, interactions between species benefit both participants. Mutualistic interactions may involve the evolution of related adaptations in both species.

C OMMENSALISM In commensalism, only one member appears to benefit from the interaction.

I NTERSPECIFIC I NTERACTIONS AND A DAPTATIONS Coevolution may be defined as reciprocal adaptations of two species that involve genetic changes in both interacting populations.

S PECIES D IVERSITY The species diversity of a community is determined both by species richness (# of different species present) and by relative abundance (# of individuals in each species).

T ROPHIC S TRUCTURE The trophic structure of a community is its feeding relationships. A food chain shows the transfer of food energy from one trophic level to the next: from producers to primary consumers on to secondary, tertiary, and quaternary consumers and eventually to decomposers.

F OOD W EBS A food web diagrams the complex trophic relationships within a community. Within a food web, each food chain usually consists of 5 or fewer links. Energetic hypothesis - food chains are limited by the inefficiency of energy transfer (only about 10%) from one trophic level to the next.

F OOD W EBS The dynamic stability hypothesis suggests that short food chains are more stable than long ones.

S PECIES WITH A LARGE IMPACT Species In a community that have the highest abundance or largest biomass are a major influence on the occurrence and distribution of other species. A species may become a dominant species due to its competitive use of resources or success at avoiding predation or disease. Invasive species may reach high biomass due to the lack of natural predators and pathogens.

S PECIES WITH A LARGE IMPACT A keystone species has a large impact on community structure as a result of its ecological role. Ecosystem “engineers” or foundation species influence community structure by changing the physical environment which are facilitators.

B OTTOM -U P & T OP -D OWN C ONTROLS According to the bottom-up model of community organization, N –> V –> H –> P, an increase in mineral nutrients (N) yields an increase in biomass at each succeeding trophic level: vegetation, herbivores, and predators (P). The top-down model, N –> V –> H –> P, assumes that predation controls community organization, with a series of +/- effects cascading down the trophic levels. According to the trophic cascade model, increasing predators will decrease herbivores, which results in increased vegetation and then lowered levels of nutrients.

D ISTURBANCE I NFLUENCES S PECIES & C OMPOSITION The ability of a community to reach and maintain this relatively constant species composition and to return to this steady state following a disturbance is known as stability.

W HAT IS A DISTURBANCE ? Disturbances change resource availability, reduce of eliminate some populations, and may create opportunities for new species. According to the intermediate disturbance hypothesis, small-scale disturbances may enhance environmental patchiness and help maintain species diversity.

H UMAN D ISTURBANCE A common result of human disturbance is a reduction in species diversity.

E COLOGICAL S UCCESSION The sequential transitions in species composition in a community, usually following some disturbance, are known as ecological succession. Primary succession : a type of ecological succession that occurs in a virtually lifeless area, where there were originally no organisms and where soil had not yet formed. Secondary succession occurs when an existing community is disrupted by fire, logging, or farming, but the soil remains intact.

E QUATORIAL -P OLAR G RADIENTS Evapotranspiration is the amount of water evaporated from soil and transpired by plants, and is determined by solar energy, temperature, and water availability (actual evapotranspiration) or just solar radiation and temperature (potential evapotranspiration).

A REA E FFECTS A species-area curve illustrates the correlation between the size of a community and the number of species found there.

I NTEGRATED AND I NDIVIDUALISTIC H YPOTHESES Integrated hypothesis : the concept that a community is an assemblage of closely linked species, locked into association by mandatory biotic interactions that cause the community to function as an integrated unit, a sort of superorganism. Individualistic hypothesis : the concept that a plant community is a chance assemblage of species found in the same area simply because they happen to have similar abiotic requirements.

R IVET AND R EDUNDANCY M ODELS The rivet model of communities suggests that changing the composition or abundance of species in a community would affect many of the members of that interwoven community. The redundancy model views communities as loosely connected assemblages, in which species are redundant—a disappearing species will be replaced by another species that fills the same role in the community.