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Chapter 53Community EcologyPowerPoint Lectures forBiology, Seventh EditionNeil Campbell and Jane Reece

Overview: What Is a Community? A biological community is an assemblage ofpopulations of various species living close enoughfor potential interaction Animals and plants surrounding a watering hole insouthern Africa are members of a savannacommunity

Concept 53.1: A community’s interactions includecompetition, predation, herbivory, symbiosis, and disease Ecologists call relationships between species in acommunity interspecific interactions Interspecific interactions affect species survivaland reproduction Examples are competition, predation, herbivory,symbiosis (parasitism, mutualism, andcommensalism), and disease

Competition Interspecific competition occurs when speciescompete for a resource in short supply Strong competition can lead to competitiveexclusion, local elimination of a competing species

The Competitive Exclusion Principle The competitive exclusion principle states that twospecies competing for the same limiting resourcescannot coexist in the same place

Ecological Niches The total of a species’ use of biotic and abioticresources is called the species’ ecological niche Ecologically similar species can coexist in acommunity if there are one or more significantdifferences in their niches As a result of competition, a species’ fundamentalniche may differ from its realized niche

LE 53-2ChthamalusBalanusHigh tideHigh tideChthamalusrealized nicheChthamalusfundamental nicheBalanusrealized nicheOceanLow tideOceanLow tide

Resource Partitioning Resource partitioning is differentiation ofecological niches, enabling similar species tocoexist in a community

LE 53-3A. insolitususually percheson shady branches.A. ricordiiA. distichusperches onfence postsand othersunnysurfaces.A. insolitusA. alinigerA. distichusA. christopheiA. cybotesA. etheridgei

Character Displacement Character displacement is a tendency forcharacteristics to be more divergent in sympatricpopulations of two species than in allopatricpopulations of the same two species An example is variation in beak size betweenpopulations of two species of Galapagos finches

LE 53-4G. fortisG. fuliginosaPercentage of individuals in each size classBeakdepthSanta María, San CristóbalSympatricpopulations40200Los Hermanos40G. fuliginosa,allopatric200Daphne40G. fortis,allopatric20081012Beak depth (mm)1416

Predation Predation 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, selfdefense, and alarm calls Animals also have morphological andphysiological defense adaptations

Cryptic coloration, or camouflage, makes preydifficult to spot

Animals with effective chemical defense oftenexhibit bright warning coloration, calledaposematic coloration Predators are particularly cautious in dealing withprey that display such coloration

In some cases, a prey species may gainsignificant protection by mimicking the appearanceof another species

In Batesian mimicry, a palatable or harmlessspecies mimics an unpalatable or harmful model

LE 53-7Green parrot snakeHawkmoth larva

In Müllerian mimicry, two or more unpalatablespecies resemble each other

LE 53-8Cuckoo beeYellow jacket

Herbivory Herbivory refers to an interaction in which anherbivore eats parts of a plant or alga It has led to evolution of plant mechanical andchemical defenses and adaptations by herbivores

Parasitism In parasitism, one organism, the parasite, derivesnourishment from another organism, its host,which is harmed in the process Parasitism exerts substantial influence onpopulations and the structure of communities

Disease Effects of disease on populations andcommunities are similar to those of parasites Pathogens, disease-causing agents, are typicallybacteria, viruses, or protists

Mutualism Mutualistic symbiosis, or mutualism, is aninterspecific interaction that benefits both species

Commensalism In commensalism, one species benefits and theother is apparently unaffected Commensal interactions are hard to document innature because any close association of twospecies likely affects both

Interspecific Interactions and Adaptation Coevolution is reciprocal evolutionary adaptationsof two interacting species The term is often used too loosely in describingadaptations within a community There is little evidence for true coevolution in mostinterspecific interactions

Concept 53.2: Dominant and keystone speciesexert strong controls on community structure In general, a few species in a community exertstrong control on that community’s structure Two fundamental features of community structureare species diversity and feeding relationships

Species Diversity Species diversity of a community is the variety oforganisms that make up the community It has two components: species richness andrelative abundance Species richness is the total number of differentspecies in the community Relative abundance is the proportion each speciesrepresents of the total individuals in the community

Two communities can have the same speciesrichness but a different relative abundance A community with an even species abundance ismore diverse than one in which one or twospecies are abundant and the remainder are rare

LE 53-11ABCDA: 25%A: 80%Community 1B: 25% C: 25%Community 2B: 5% C: 5%D: 25%D: 10%

Trophic Structure Trophic structure is the feeding relationshipsbetween organisms in a community It is a key factor in community dynamics Food chains link trophic levels from producers totop carnivores

LE rimaryproducersPlantA terrestrial food chainPhytoplanktonA marine food chain

Food Webs A food web is a branching food chain with complextrophic interactions

LE rill)Phytoplankton

Food webs can be simplified by isolating a portionof a community that interacts very little with therest of the community

LE 53-14Sea nettleJuvenile striped bassFish larvaeFish eggsZooplankton

Limits on Food Chain Length Each food chain in a food web is usually only afew links long Two hypotheses attempt to explain food chainlength: the energetic hypothesis and the dynamicstability hypothesis

The energetic hypothesis suggests that length islimited by inefficient energy transfer The dynamic stability hypothesis proposes thatlong food chains are less stable than short ones Most data support the energetic hypothesis

Number of species6No. of species5No. of ivityLowNumber of trophic linksLE 53-15

Species with a Large Impact Certain species have a very large impact oncommunity structure Such species are highly abundant or play a pivotalrole in community dynamics

Dominant Species Dominant species are those that are mostabundant or have the highest biomass They exert powerful control over the occurrenceand distribution of other species

One hypothesis suggests that dominant speciesare most competitive in exploiting resources Another hypothesis is that they are mostsuccessful at avoiding predators

Keystone Species In contrast to dominant species, keystone speciesare not necessarily abundant in a community They exert strong control on a community by theirecological roles, or niches

Field studies of sea stars exhibit their role as akeystone species in intertidal communities

Number of speciespresentLE 53-1620With Pisaster (control)1510Without Pisaster (experimental)501963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73

Observation of sea otter populations and theirpredation shows how otters affect oceancommunities

LE 53-17Otter number(% max. count)100806040200Sea otter abundanceNumber per0.25 m2Grams per0.25 m24003002001000Sea urchin biomassFood chain beforekiller whaleinvolvement inchain10864201972 1985 1989 1993 1997YearTotal kelp densityFood chain afterkiller whales startedpreying on otters

Ecosystem “Engineers” (Foundation Species) Some organisms exert influence by causingphysical changes in the environment that affectcommunity structure For example, beaver dams can transformlandscapes on a very large scale

Some foundation species act as facilitators thathave positive effects on survival and reproductionof some other species in the community

LE 53-19Number of plant species86420Salt marsh with s

Bottom-Up and Top-Down Controls The bottom-up model of community organizationproposes a unidirectional influence from lower tohigher trophic levels In this case, presence or absence of mineralnutrients determines community structure,including abundance of primary producers

The top-down model proposes that control comesfrom the trophic level above In this case, predators control herbivores, which inturn control primary producers

Long-term experimental studies have shown thatcommunities can shift periodically from bottom-upto top-down controls

LE 53-20Percentage ofherbaceous plant cover10075502500100200Rainfall (mm)300400

Pollution can affect community dynamics Biomanipulation can help restore pollutedcommunities

LE 53-UN1171Polluted StateRestored bundantRare

Concept 53.3: Disturbance influences speciesdiversity and composition Decades ago, most ecologists favored the viewthat communities are in a state of equilibrium Recent evidence of change has led to anonequilibrium model, which describescommunities as constantly changing after beingbuffeted by disturbances

What Is Disturbance? A disturbance is an event that changes acommunity, removes organisms from it, and altersresource availability Fire is a significant disturbance in most terrestrialecosystems It is often a necessity in some communities

LE 53-21Before a controlled burn.A prairie that has not burned forseveral years has a high proportion of detritus (dead grass).During the burn. The detritusserves as fuel for fires.After the burn. Approximatelyone month after the controlledburn, virtually all of the biomassin this prairie is living.

The intermediate disturbance hypothesis suggeststhat moderate levels of disturbance can fosterhigher diversity than low levels of disturbance The large-scale fire in Yellowstone National Parkin 1988 demonstrated that communities can oftenrespond very rapidly to a massive disturbance

LE 53-22Soon after fire. As this photo taken soon after the fireshows, the burn left a patchy landscape. Note theunburned trees in the distance.One year after fire. This photo of the same general areataken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants,different from those in the former forest, cover the ground.

Human Disturbance Humans are the most widespread agents ofdisturbance Human disturbance to communities usuallyreduces species diversity Humans also prevent some naturally occurringdisturbances, which can be important tocommunity structure

Ecological Succession Ecological succession is the sequence ofcommunity and ecosystem changes after adisturbance Primary succession occurs where no soil existswhen succession begins Secondary succession begins in an area wheresoil remains after a disturbance

Early-arriving species and later-arriving speciesmay be linked in one of three processes:– Early arrivals may facilitate appearance of laterspecies by making the environment favorable– They may inhibit establishment of later species– They may tolerate later species but have noimpact on their establishment

Retreating glaciers provide a valuable fieldresearch opportunity for observing succession

dPacific 1892187919131860Reid Gl.Johns HopkinsGl.McCaBrseidementGlGl.LE 53-2318790GlacierBayMiles505 10 15Kilometers183017801760Pleasant Is.McBride glacier retreating10

Succession on the moraines in Glacier Bay,Alaska, follows a predictable pattern of change invegetation and soil characteristics

LE 53-24Pioneer stage, with fireweed dominantDryas stage60Soil nitrogen (g/m2)50403020100PioneerDryasAlder SpruceSuccessional stageNitrogen fixation by Dryas and alderincreases the soil nitrogen content.Sprucestage