Community Ecology - Basic Biology - Lecture Slides, Slides for Biology. Baddi University of Emerging Sciences and Technologies
kumara11 January 2013

Community Ecology - Basic Biology - Lecture Slides, Slides for Biology. Baddi University of Emerging Sciences and Technologies

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Its the important key points of Basic Biology are: Community Ecology, Sense of Community, Biological Community, Assemblage of Populations, Potential Interaction, Interspecific Interactions, Competitive Exclusion, Ecologi...
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Chapter 54 • Community Ecology

Overview: A Sense of Community

• A biological community is an assemblage of populations of various species living close enough for potential interaction

Concept 54.1: Community interactions are classified by whether they help, harm, or

have no effect on the species involved • Ecologists call relationships between species in a

community interspecific interactions • Examples are competition, predation, herbivory,

and symbiosis (parasitism, mutualism, and commensalism)

• Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)


Interspecific competition (–/– interaction)occurs when species compete for a resource in short supply

Competitive Exclusion

• Strong competition can lead to competitive exclusion, local elimination of a competing species

• The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place

Ecological Niches

• The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche

• An ecological niche can also be thought of as an organism’s ecological role

• Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community

• As a result of competition, a species’ fundamental niche may differ from its realized niche

Character Displacement

Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species

• An example is variation in beak size between populations of two species of Galápagos finches


Predation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey

• Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

• Prey display various defensive adaptations • Behavioral defenses include hiding, fleeing,

forming herds or schools, self-defense, and alarm calls

• Animals also have morphological and physiological defense adaptations

Cryptic coloration, or camouflage, makes prey difficult to spot

• Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration

• Predators are particularly cautious in dealing with prey that display such coloration


Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga

• It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores


Symbiosis is a relationship where two or more species live in direct and intimate contact with one another


• In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process

• Parasites that live within the body of their host are called endoparasites; parasites that live on the external surface of a host are ectoparasites

• Many parasites have a complex life cycle involving a number of hosts

• Some parasites change the behavior of the host to increase their own fitness


• Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species

• A mutualism can be – Obligate, where one species cannot survive without

the other – Facultative, where both species can survive alone


• In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected

• Commensal interactions are hard to document in nature because any close association likely affects both species

Concept 54.2: Dominant and keystone species exert strong controls on

community structure • In general, a few species in a community exert

strong control on that community’s structure • Two fundamental features of community

structure are species diversity and feeding relationships

Species Diversity

Species diversity of a community is the variety of organisms that make up the community

• It has two components: species richness and relative abundance

Species richness is the total number of different species in the community

Relative abundance is the proportion each species represents of the total individuals in the community

• Two communities can have the same species richness but a different relative abundance

• Diversity can be compared using a diversity index – Shannon diversity index (H):

H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + …]

• Determining the number and abundance of species in a community is difficult, especially for small organisms

• Molecular tools can be used to help determine microbial diversity

Trophic Structure

Trophic structure is the feeding relationships between organisms in a community

• It is a key factor in community dynamics • Food chains link trophic levels from producers

to top carnivores

Food Webs

• A food web is a branching food chain with complex trophic interactions

• Species may play a role at more than one trophic level

• Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community

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