Community [Compatibility Mode]

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COMMUNITY ECOLOGY

A community is an assemblageof plant and animal populationsthat live in a particular area orhabitat.

Populations of the various speciesin a community interact and form asystem with its own emergentproperties.

Pattern vs. Process

Patternis what we can easily observedirectly - vegetation zonation, specieslists, seasonal distribution of activity, andassociation of certain species.

Processgives rise to the pattern-herbivory, competition, predation risk,nutrient availability, patterns ofdisturbance, energy flow, history, andevolution.

Goals of Community Ecology Community ecology seeks to explain the

underlying mechanisms that create, maintain,and determine the fate of biologicalcommunities.

Not all science is experimental.Community ecologists describecommunities (descriptive science) and then

develop hypotheses to explain why theyare the way they are, what regulates them,and how they change.

Both tasks have their roots in nineteenthcentury natural history.

Emergent Properties of a Community

Scale

Spatial and Temporal Structure

Species Richness

Species Diversity

Interactions Among Members Trophic structure

Succession and Disturbance

Scale

Scale is the size of a community.

Provided that the area or habitat iswell defined, a community can be a

system of almost any size, from adrop of water, to a rotting log, to aforest, to the surface of the PacificOcean.

Spatial Structure

Spatial Structure is the way speciesare distributed relative to each other.

Some species provide a framework

that creates habitats for otherspecies. These species, in turncreate habitats for others, etc.


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Example of spatial structure

Trees in a rainforest are stratified intoseveral different levels, including a canopy,several understories, a ground level, androots. Each level is the habitat of a distinctcollection of species. Some places, suchas the pools of water that collect at thebase of tree branches, may harbor entirecommunities of their own.

Temporal Structure

Temporal sturcture is the timing of theappearance and activity of species. Somecommunities, i.e., arctic tundra and thedecay of a corpse, have pronouncedtemporal species, other communities haveless.

Example: Many desert plants and animals aredormant most of the year. They emerge, orgerminate, in response to seasonal rains. Otherplants stick around year round, having evolvedadaptations to resist drought.

Species Richness

Species Richness - is the number ofspecies in a community. Clearly, the

number of species we can observe isfunction of the area of the sample. Italso is a function of who is looking.Thus, species richness is sensitive tosampling procedure

Diversity

Diversity is the number of species in thecommunity, and their relative abundances.

Species are not equally abundant, some speciesoccur in large percentage of samples, others arepoorly represented.

Some communities, such as tropical rainforests,

are much more diverse than others, such as thegreat basin desert.

Species Diversity is often expressed using

Simpsons diversity index: D=1- (pi)2

Example Problem A community contains the following species:

Number of Individuals

Species A 104

Species B 71

Species C 19

Species D 5

Species E 3

What is the Diversity index of this Community?

Total Individuals= (104+19+71+5+3)=202

PA=104/202=.51 PB=19/202=.09

PC=71/202=.35 PD=5/202=.03PE=3/202=.02

D=1-{(.51)2+(.09)2+(.35)2+(.03)2+(.02)2}

D=1-.40=.60


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The Niche

The niche is one of the most important concepts inecology. Paradoxically, it is also one of the hardest todefine (Ecology is still a young science).

In essence, an organisms niche is how it makes aliving: the environmental conditions it tolerates, theimportant resources it needs to survive, and its waysof obtaining those resources.

In obtaining energy, nutrients, etc.. a populations ofone species frequently interact with populations ofother species.

Competition and the Niche

An ecological niche can be thought of in termsof competition. The fundamental niche is the set of resources

and habitats an organism could theoreticallyuse under ideal conditions.

The realized niche is the set of resources andhabitats an organism actually used: it isgenerally much more restricted due tointerspecific competition (or predation.)

Two organisms cannot occupy exactly the same

niche.

This is sometimes called Gaussesrule(although Gausse never put it exactlythat way).

-Experiments by Gausse (Paramecium), Peter Frank(Daphnia), and Thomas Park (Triboleum) have confirmed it

for simple laboratory scenarios.

-This creates a bit of a paradox, because so many speciesexist in nature using the same resources.

-The more complex environments found in nature mayenable more resource partitioning.

Resource Partitioning

Species that share the same habitat andhave similar needs frequently use

resources in somewhat different ways -

so that they do not come into direct

competition for at least part of the

limiting resource. This is called resource

partitioning.

Succession, Disturbance and

Change

In terms of species andphysical structure,communities change with time.Ecological succession, the predictable

change in species over time, as each new setof species modifies the environment to enablethe establishment of other species, is virtuallyubiquitous.

Examples of Change Example, a sphagnum bog community

may persist for only a few decades beforethe process of ecological successionchanges transform it into the surroundingBlack Spruce Forest.

A forest fire may destroy a large area oftrees, clearing the way for a meadow.Eventually, the trees take over and themeadow is replaced.


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Disturbance

Disturbances are events such asfloods, fire, droughts, overgrazing,

and human activity that damage

communities, remove organisms

from them, and alter resource

availability.

Some Natural Agents of

Disturbance

Fire Floods

Drought

Floating Logs

Large Herbivores

Storms

Volcanoes

Disturbance, Invasion, Succession

Disturbance creates opportunities for newspecies to invade an area and establishthemselves.

These species modify the environment, andcreate opportunities for other species toinvade. The new species eventually

displace the original ones. Eventually, theymodify the environment enough to allow anew series of invaders, which ultimatelyreplace them, etc.

Succession

Disturbance of a community is usuallyfollowed by recovery, called ecologicalsuccession.

The concept of succession developed by twoearly ecologists, Warming and Cowles.

Cowles studied dune succession along Lake

Michigan between 1898 and 1911. He emphasized the dynamic nature of vegetation,

and the process by which pioneer speciesgradually modify their environment to enablereplacement by their competitors.

Two Types of Succession

Primary

Secondary

Primary succession is thesequence of species on newlyexposed landforms that have notpreviously been influenced by acommunity, e.g., newly formedsand dunes, lava flows, areasexposed by glacial retreat.

Sequence of succession isdriven by the interactionsamong dispersal, ecologicaltolerances, and competitiveability.


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Early successional species are generally excellentdispersers.

As ecological succession progresses, they arereplaced with species which are superior

competitors, (but not as good at dispersing).

Early successional species frequently modify theirenvironment in such a way as to make it possible

for the next round of species. These, in turn,

make their own replacement by superior

competitors possible.

Secondary succession occurs incases which vegetation of an area

has been partially or completelyremoved, but where well developedsoil, seeds, and spores remain.

Thus, the resulting sequence ofspecies is driven principally byinteractions such as competitionand herbivory, e.g., familiar old-field succession.

CLIMAX COMMUNITY

A climax community is a moreor less permanent and final

stage of aparticularsuccession, often characteristicof a restricted area.

Climax communities arecharacterized by slow rates of

change, compared with moredynamic, earlier stages.

They are dominated by speciestolerant of competition for resources.

STREAM COMMUNITY

Stream lotic water system

Streams unidirectional current (due togravity)

most important environmental feature

the amount and quality of load, and flowvelocity determine the characteristics of thestreambed, rate of nutrient replenishment,adaptations of organisms


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Regions of the stream categorized according to

flow rate:

RIFFLE (Fast Velocity) rocky streambed; rocks hindermovement of water causing decrease in volume ofpassing water, therefore causing turbulence, thus fastermovement of the shallow water; turbulence causesincrease in dissolved oxygen (DO)

Medium Velocity combination of small rocks and somesand

POOL (Slow Velocity) sandy/silty/muddy substrate,slow-moving water will allow sedimentation of humusand other organic matter from upstream; deeper waterthus less DO.

Physico-chemical parameters

Temperature affected by depth of water, current velocity,

bottom materials, temperature of entering

tributary water, exposure to direct sunlight,

degree of shading, time of day

air temp > water

Light Intensity

affected by shading by nearby vegetation,time of day, weather condition

affects temperature and primary productivity

pH

pH 6-7 is the normal range

influences the availability of nutrientsparticularly CO2

Affects density and diversity of organismsliving in the stream

Velocity

affected by shape of channel, roughness ofchannel, size/width, slope/steepness, depth,

wind factor, and intensity of rainfall

affects types of organisms present, only thosew/ favorable structures, adaptations can

withstand the constant turbulence

Electrical conductivity

indicates presence of dissolved salts and metalions in water

affected by pH

high velocity , high EC


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Dissolved oxygen

affected by temperature, velocity, abundanceof primary producers

higher velocity, higher DO ; lowertemperature, higher DO

COASTAL MARINE COMMUNITY (Intertidal

Zone)

Recall:

the sun is the basic source of energy of theplanet

energy is utilized by autotrophs duringphotosynthesis

light can only penetrate the water surface

until a certain level only (light compensationpoint)

Intertidal zone (Eulittoral zone) - transitionarea between marine and terrestrial

environment.

Intertidal Zone communities are defined by:

Tidal exposure low tides expose organisms to dryair, high temp., predation by land animals, salinity

fluctuations

Bottom type fine/loose sediment vs. solidsubstrate; flat vs. terrained

Wave action smashes/tears away objects, dispersesorganisms, stirs up sediments, mixes gases

Physico-chemical parameters

Temperature

affected by climate, season, condition of thesky, time of the day

may result to desiccation of exposedorganisms

~27C on the surface of tropical waters


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Light intensity

Affects water surface temperature Affects primary productivity

Controls distribution of producers, vision,rhythms, rate of photosynthesis

abundance of producers

pH

7.5-8.4 is the normal range basic pH due topresence of strongly alkaline ions (Na, K, Ca)

maintenance of a buffer system

affects abundance and type of organisms

Salinity

~35 ppt; dissolved organic salts such as Cl, Na,SO4, Mg, Ca, K

factors affecting salinity areprecipitation/runoff, evaporation, mixing

Dissolved oxygen

Not limiting in the intertidal zone due to waveaction

High DO is favorable for the heterotrophs

Electrical conductivity

Indicates presence of ions related tobiogeochemical cycles occurring in the marine

community

Microhabitats in marine community

Comparison of the 2 microhabitats

Rocky: less of the sea grasses, more of thebrittle stars, starfish, algae

Sandy: less of the echinoderms, more of thesea grasses

Index of Similarity depends on the situationbut usually ranges from 50-90% due to wave

action


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Adaptations

Rocky: animal - attachment, hiding in rock crevices Plant/algae - holdfasts, flexible thallus, gas-filled

bulbs for floatation of thallus

Sandy: animal - burrowing, highly mobile, attach to sea

grasses, suspension/filter feeders, grazers of seagrasses, detritus feeders

plant - extensive rhizomes hold the sand,flattened leaf blades glide through

the waves

Intra- and Interspecific interactions

Intraspecific: competition for food, space,and light

Interspecific: grazing, predation, competition,plant- animal interactions

Productivity

Productivity is high for both rocky and sandymicrohabitats

Sea Grasses

Contributes to high primary productivity

Stabilizes and builds-up shoreline bottom

Act as shelters and nursery grounds for otherorganisms

Leaves act as protection against UV light anddesiccation

Wave action

Disperses organisms, stirs up sediments, mixesgases

Tides bring food and carry away metabolites

Reduces predatory activity against sessileorganisms

more even distribution of light moreefficient PS

Open up space for colonization, thus reducingstrong interspecific competition

Thus, wave action, being the disturbance whichinfluences community structure so much, is alsothe root of intertidal productivity

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