Lecture 19 - Community Ecology

8/11/2019 Lecture 19 - Community Ecology

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Community Ecology


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What is a Community? A community is defined as an assemblage of

species living close enough together forpotential interaction.

Communities differin their speciesrichness , thenumber of speciesthey contain, andthe relativeabundance of

different species.

Introduction


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The rivet model of communities is areincarnation of the interactive model.

The redundancy model states that mostspecies in a community are not closelyassociated with one another.


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1. Populations may be linked bycompetition, predation,

mutualism and commensalism


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Competition. Interspecific competition for resources

can occur when resources are in shortsupply. There is potential for competition between

any two species that need the same limitedresource.

The competitive exclusion principle :

two species with similar needs for samelimiting resources cannot coexist in thesame place.


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The ecological niche is the sum total ofan organisms use of abiotic/biotic

resources in the environment. An organisms niche is its role in the

environment.

The competitive exclusion principle canbe restated to say that two species cannotcoexist in a community if their niches areidentical.


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Classic experiments confirm this.


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Resource partitioning is thedifferentiation of niches that enables two

similar species to coexist in a community.

Fig. 53.2


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Character displacement is the tendency for characteristicsto be more divergent in sympatric populations of twospecies than inallopatricpopulationsof the same twospecies.

Hereditary changesevolve that bringabout resourcepartitioning.


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Predation. A predator eats prey. Herbivory , in which animals eat plants. In parasitism , predators live on/in a host

and depend on the host for nutrition. Predator adaptations: many important

feeding adaptations of predators are bothobvious and familiar. Claws, teeth, fangs, poison, heat-sensing

organs, speed, and agility.


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Plant defenses against herbivores includechemical compounds that are toxic.

Animal defenses against predators. Behavioral defenses include fleeing, hiding, self-

defense, noises, and mobbing. Camouflage includes cryptic coloration ,

deceptive markings.


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Mechanical defenses include spines. Chemical defenses include odors and toxins

Aposematic coloration is indicated bywarning colors, and is sometimesassociated with other defenses (toxins).


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Mimicry is when organisms resemble otherspecies. Batesian mimicry is where a harmless species

mimics a harmful one.


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Mllerian mimicry is where two or moreunpalatable species resemble each other.


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Parasites and pathogens as predators. A parasite derives nourishment from a host,

which is harmed in the process. Endoparasites live inside the host and

ectoparasites live on the surface of thehost.

Parasitoidism is a special type of parasitismwhere the parasite eventually kills the host.

Pathogens are disease-causing organisms

that can be considered predators.


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Mutualism is where twospecies benefit from theirinteraction.

Commensalism iswhere one speciesbenefits from theinteraction, but otheris not affected. An example would

be barnacles thatattach to a whale.


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Coevolution and interspecific interactions. Coevolution refers to reciprocal

evolutionary adaptations of twointeracting species. When one species evolves, it exerts

selective pressure on the other to evolve tocontinue the interaction.


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The trophic structure of a community isdetermined by the feeding relationshipsbetween organisms.

The transfer of food energy from its source inphotosynthetic organisms through herbivoresand carnivores is called the food chain .

2. Trophic structure is a keyfactor in community dynamics


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Charles Elton firstpointed out that thelength of a food chain is

usually four or five links,called trophic levels .

He also recognizedthat food chains are

not isolated units butare hooked togetherinto food webs .


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Food webs. Who eats whom in a

community? Trophic relationships

can be diagrammed ina community.

What transformsfood chains intofood webs?

A given species mayweave into the web atmore than one trophiclevel.


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Dominant species are those in a communitythat have the highest abundance or highestbiomass (the sum weight of all individuals ina population). If we remove a dominant species from a

community, it can change the entirecommunity structure.


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Keystone species exert an importantregulating effecton other speciesin a community.


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If they are removed, community structure is greatlyaffected.


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Consider three possible relationshipsbetween plants ( V for vegetation) andherbivores ( H ). V H V H V H

Arrows indicate that a change in biomass of one trophic levelcauses a change in the other trophic level .

The bottom-up model postulates V H

linkages, where nutrients and vegetation controlcommunity organization. The top-down model postulates that it is mainly

predation that controls community organizationV H .

Other models go between the bottom-up and top-down extreme models.


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Disturbances affect community structure andstability. Stability is the ability of a community to

persist in the face of disturbance.

Introduction


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Disturbances are events like fire, weather, or humanactivities that can alter communities. Some are routine.

1. Most communities are in a state of

nonequilibrium owing to disturbances


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Marine communities are subject todisturbance by tropical storms.


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We usually think that disturbances have anegative impact on communities, but in

many cases they are necessary forcommunity development and survival.


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Human activities cause more disturbancethan natural events and usually reducespecies diversity in communities.

2. Humans are the most widespread agentsof disturbance


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Ecological succession is the transition inspecies composition over ecological time.

Primary succession begins in a lifelessarea where soil has not yet formed.

3. Ecological succession is the sequence ofcommunity changes after a disturbance


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Mosses and lichens colonize first andcause the development of soil.

An example would be after a glacier hasretreated.


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Secondary succession occurs where anexisting community has been cleared by

some event, but the soil is left intact. Grasses grow first, then trees and other

organisms.


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Soil concentrations ofnutrients show

changes over time.


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The variety of different kinds of organismsthat make up a community has twocomponents. Species richness , the total number of species in

the community.

Relative abundance of the different species. Imagine two small forest communities with 100

individuals distributed among four different treespecies.

1. Community biodiversity measures thenumber of species and their relative

abundance


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Speciesrichness

may beequal,but relativeabundancemaybe different.


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Counting species in a community to determine theirabundance is difficult, especially for insects andsmaller organisms.


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Tropical habitats support much largernumbers of species of organisms than dotemperate and polar regions.

2. Species richness generally declinesalong an equatorial-polar gradient


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What causes these gradients? The two key factors are probably

evolutionary history and climate. Organisms have a history in an area where

they are adapted to the climate.

Energy and water may factor into thisphenomenon.


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The species-area curve quantifies whatmay seem obvious: the larger the geographicarea, the greaterthe numberof species.

3. Species richness is related to acommunitys geographic size


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Because of their size and isolation, islandsprovide great opportunities for studying someof the biogeographic factors that affect thespecies diversity of communities. Imagine a newly formed island some distance

from the mainland.

Robert MacArthur and E. O. Wilson developed ahypothesis of island biogeography to identify thedeterminants of species diversity on an island.

4. Species richness on islands depends onisland size and distance from the mainland

T f ill d i h b f


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Two factors will determine the number ofspecies that eventually inhabit the island. The rate at which new species immigrate to the island.

The rate at which species become extinct.


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Studies of plants onmany island chains

confirm theirhypothesis.

Lecture 19 - Community Ecology

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