Community Ecology

Community Ecology Chapter 54

Community • an assemblage of populations of various species living close enough for potential interaction

Interspecific Interactions interactions between species in a community

Interspecific Interactions – (SYMBIOSIS)

Competition (–/–) • interspecific competition – occurs when species compete for a particular limited resource • strong competition can lead to the elimination of 1 of the 2 competing species = competitive exclusion • b/c one species will use the resource(s) more efficiently and, thus, reproduce more rapidly than the other • competition affects an organism’s niche = its ecological role (how it “fits into” an ecosystem) • fundamental niche = an organism’s potential niche • realized niche = the niche an organism actually occupies due to competition for limited resources

Competitive Exclusion Principle • The concept that when populations of two similar species compete for the same limited resource, one population will use the resources more efficiently and have a reproductive advantage that will eventually lead to the elimination of the other population.

The Niche A niche is the full range of physical and biological conditions in which an organism lives and the way in which the organism uses those conditions An organism’s niche includes: Its place in the food web The range of temperatures that the organism needs to survive The type of food the organism eats, how it obtains that food, and which other species use the organism as food When and how the organism reproduces

Niche v/s Habitat If an organism’s habitat is its address, its niche is its occupation. The combination of biotic and abiotic factors in an ecosystem often determines the number of different niches in that ecosystem. No 2 species can share the same niche in the same habitat, but different species can occupy niches that are very similar (competitive exclusion principle).

resource partitioning • the modification of one species niche that allows two species with identical niches to coexist • character displacement • tendency for characteristics of populations of the same species to be more divergent in sympatric populations (geographically overlapping) than in allopatric populations (geographically separate) b/c of competition

Predation (+/–) • relationship in which one species (the predator) kills and eats the other (the prey) • includes seed predators • predators possess adaptations for securing prey • acute senses • claws, teeth, fangs, stingers, poison • prey have adaptations that help them avoid predators • behavioral defenses (hiding, fleeing, self-defense, alarm calls) • cryptic coloration (camouflage) • aposematic coloration (bright warning coloration) • Batesian mimicry (harmless species mimics a harmful one) • Müllerian mimicry (two unpalatable species mimic each other)

Herbivory (+/–) • an herbivore eats part of a plant or alga

Defense Mechanisms to Predation in Plants • Chemical toxins • Strychnine produced by Strychnos toxifera • Morphine produced by opium poppy • Nicotine produced by tobacco plants • Mescaline produced by peyote cacti • Tannins produced by a variety of plant species • Spines & thorns

Parasitism (+/–) • symbiotic relationship in which one organism (the parasite) derives its nourishment from another organism (the host) which is harmed in the process • types of parasites: • endoparasites – live within the body of their host • ectoparasites – feed on the external surface of their host • parasitoidism – insects lay eggs on or in living hosts and, then, the larvae feed on the host

Disease (+/–) • disease-causing agents = pathogens • typically bacteria, viruses, or protists • most microscopic • many inflict lethal harm

Mutualism (+/+) • symbiotic relationship that benefits both species involved • ex: ants & acacia tree

Commensalism (+/0) • symbiotic relationship that benefits one of the species involved and neither helps nor harms the other species • difficult to document a true example

Co-evolution and Interspecific Interactions • Coevolution refers to reciprocoal evolutionary adaptations of two interacting species. • A change in one species acts as a selective force on another species, in which counteradaptation in turn acts as a selective force on the first species. • There is actually LITTLE evidence for coevolution in most cases of interspecific interactions.

Community Structure

Species Diversity • the variety of different kinds of organisms that make up a community • two components: • species richness = total # of different species in the community • relative abundance = the proportion of the total # of individuals in the community that each species represents • measuring species diversity is essential for understanding community structure & for conserving biodiversity

Trophic Structure • feeding relationships between organisms in a community • food chain = the transfer of food energy up the trophic levels • food chains are usually only a few links long (≤5) • two hypotheses why: • energetic hypothesis– suggests that the length of a food chain is limited by the inefficiency of energy transfer along the chain • dynamic stability hypothesis– proposes that long food chains are less stable than short chains because population fluctuations at lower trophic levels are magnified at higher levels • most of the data available support the energetic hypothesis • another factor that may limit food chain length is that animals in a food chain tend to be larger at successive trophic levels • food webs – made up of linked food chains

food web

Dominant Species • those species in a community that are most abundant or have the highest biomass • exert a powerful control over the occurrence and distribution of other species • ex: sugar maples are the dominant plant species in many North American forest communities

Keystone Species • species that are not necessarily the most abundant but exert strong control on community structure by their pivotal ecological roles (niches)

Foundation Species • organisms that influence community structure by causing physical changes in the environment that affect the structure of the community • act as facilitators that have positive effects on the survival and reproduction of some of the other species in the community

Two Models of Community Organization • bottom-up model • postulates a unidirectional influence from lower to higher trophic levels • nutrient availability  plant #s  herbivore #s  predator #s • thus, to change the community structure, you need to alter biomass at lower trophic levels • top-down model • postulates that it is mainly the predators that control community organization • predators limit herbivores which limit plants which limit nutrient levels • a change at the higher trophic levels moves down the trophic structure as a series of +/– effects (trophic cascade)

Disturbance an event that changes a community, removes organisms from it, & alters resource availability

Intermediate Disturbance Hypothesis • suggests that moderate levels of disturbance can create conditions that foster greater species diversity than low or high levels of disturbance b/c: • high levels of disturbance reduce species diversity by creating environmental stresses that exceed the tolerance of many species • frequent disturbances may also exclude slow colonizing or slow growing species • low levels of disturbance can reduce species diversity by allowing competitively dominant species to exclude less competitive species

Ecological Succession • in a process called ecological succession, a disturbed area may be colonized by a variety of species which are, in turn, replaced by other species until a climax community is reached • two types: • primary succession– occurs in a virtually lifeless area, where there were originally no organisms and where soil has not yet formed • secondary succession– occurs where an existing community has been cleared by some disturbance that leaves the soil intact

Primary Succession Biodiversity increases as succession proceeds: (plants | animals | decomposers

The Pioneers of Succession • Pioneer species – first individuals to inhabit an area of primary succession – break down rock, erode surfaces, and turn it into soil. • Lichen and moss • Climax Community – the final community of any succession.

What causes increase/decrease in biodiversity as succession progresses? • Some populations facilitate biodiversity/succession (by developing conditions more suitable for other species and/or developing conditions less suitable for their progeny. • Some populations inhibit biodiversity/succession (by developing conditions less suitable for other species and/or developing conditions more suitable for their progeny. • Increase in plant stratification (increased layering of plants; ex: canopy, understory. • More niches/habitats formed (plants, animals, decomposers). • Pioneer plant species – dominants (more shade-tolerant plants emerge). • Increase in producer diversity brings about increase in consumer diversity. • Shift from more opportunistic (r) to more equilibrium (K) species.

Abiotic Factors Can Cause Succession

Successions Can Have Immediate & Long-Term Effects • Examples:

Biogeographic Factors geographic location and size are correlated with species diversity

Size • species-area curve • shows that, with all other factors being equal, the larger the geographic area of a community, the greater the number of species • likely explanation is that larger areas offer greater diversity of habitats and microhabitats than smaller ones

Community Ecology