Population and Population and Community EcologyCommunity Ecology

Chapter 35Chapter 35

Population DensityPopulation Density

Ecologists often Ecologists often describe a describe a population in terms population in terms of its of its densitydensity..

Population density is the is the numbernumber of of individuals of a individuals of a particular particular speciesspecies per unit area or per unit area or volume. volume.

exponential growth

the population multiplies by a the population multiplies by a constant factor at constant constant factor at constant timetime intervals.intervals.

Carrying CapacityCarrying Capacity In nature, a population may start In nature, a population may start growinggrowing

exponentially, but eventually one or exponentially, but eventually one or more environmental factors will more environmental factors will limitlimit its its growth. growth.

The population then stops growing or The population then stops growing or may even begin to may even begin to decreasedecrease. Space is . Space is one example of a one example of a limiting factor, a , a condition that can condition that can restrictrestrict a population's a population's growth. growth.

Other limiting factors include Other limiting factors include diseasedisease and and availability of availability of foodfood..

Carrying CapacityCarrying Capacity When such environmental factors limit a When such environmental factors limit a

population's growth rate, the population is population's growth rate, the population is said to have reached its said to have reached its carrying capacity. .

The carrying capacity is the The carrying capacity is the numbernumber of of organisms in a population that the organisms in a population that the environment can environment can maintainmaintain

As a growing population approaches As a growing population approaches carrying capacity, the carrying capacity, the birthbirth rate may rate may decrease or the decrease or the deathdeath rate may increase rate may increase (or both), until they are about equal. (or both), until they are about equal.

CompetitionCompetition This competition within a single species limits This competition within a single species limits

the the growthgrowth of the population. of the population.

Within a community, Within a community, interspecific competition (competition (competition betweenbetween species) takes place species) takes place when when twotwo or more species rely on the same or more species rely on the same limited resource. limited resource.

NichesNiches Within a community, each species has a Within a community, each species has a

unique unique livingliving arrangement called its arrangement called its niche. niche.

A A niche includes an organism's living includes an organism's living place (place (habitathabitat), its food sources, the time ), its food sources, the time of day it is most active, and many other of day it is most active, and many other factors specific to that organism's way of factors specific to that organism's way of life. life.

The local loss of a The local loss of a speciesspecies due to due to competitive exclusion is most likely to competitive exclusion is most likely to occur if two species have occur if two species have nichesniches that are that are very similar. very similar.

predation an interaction in which an interaction in which

one organism one organism eatseats another. another.

Predators – animals that hunt and eat other animals.

Prey- animals that get eaten by the predator.

Symbiotic RelationshipsSymbiotic Relationships A A symbiotic relationship is a is a closeclose

interaction between species in which interaction between species in which one of the species one of the species liveslives in or on the in or on the other. other.

There are three main types of There are three main types of symbiotic relationships: parasitism, symbiotic relationships: parasitism, mutualismmutualism, and commensalism., and commensalism.

Parasitism is a relationship in which one is a relationship in which one organism, the parasite, obtains its organism, the parasite, obtains its foodfood at the expense of another organism, the at the expense of another organism, the hosthost. .

Examples include blood-sucking Examples include blood-sucking mosquitoes, mosquitoes, tapewormstapeworms that live and that live and feed in the intestines, feed in the intestines, waspswasps laying eggs laying eggs on caterpillaron caterpillar

In mutualism, both organisms In mutualism, both organisms benefitbenefit from the symbiotic from the symbiotic relationship. relationship.

Clownfish and sea anemones present an example of facultative mutualism. The clownfish benefits by having a protected home territory.

CommensalismCommensalism is a relationship in which one organism is a relationship in which one organism

benefitsbenefits, while the other organism is , while the other organism is neither neither harmedharmed nor helped significantly. nor helped significantly.

This shows the symbiotic relationship commensalism, the bird gets food from the back of the cow but the cow is not affected

Ecological SuccessionEcological Succession Communities may change drastically as a result of a Communities may change drastically as a result of a

disturbancedisturbance..

A variety of A variety of speciesspecies may colonize the disturbed may colonize the disturbed

area. area.

These species may be replaced as yet other species These species may be replaced as yet other species move into the area later. move into the area later.

This process of community change is called This process of community change is called ecological succession.ecological succession.

Primary Primary SuccessionSuccession

When a community When a community arises in a lifeless area arises in a lifeless area that has no that has no soilsoil, the , the change is called change is called primary succession. primary succession.

Examples of such Examples of such areas are new islands areas are new islands created by erupting created by erupting volcanoesvolcanoes, or the bare , or the bare rockrock left behind a left behind a retreating glacier.retreating glacier.

Primary SuccessionPrimary Succession AutotrophicAutotrophic microorganisms are first organisms to microorganisms are first organisms to

appear. appear. Then Then lichenslichens and and mossesmosses.. Soil develops gradually from the action of these early Soil develops gradually from the action of these early

colonizers and from their colonizers and from their decomposeddecomposed remains. remains. Once soil is present, Once soil is present, grassesgrasses, shrubs, and , shrubs, and treestrees grow. grow. Primary succession from Primary succession from barrenbarren ground to a ground to a

community such as a deciduous forest can take community such as a deciduous forest can take hundreds or even thousands of years. hundreds or even thousands of years.

Secondary Secondary SuccessionSuccession

When a disturbance damages an existing community but leaves When a disturbance damages an existing community but leaves the soil intact, the change that follows is called secondary the soil intact, the change that follows is called secondary succession. succession.

An example is a forested area that has been cleared for An example is a forested area that has been cleared for farmingfarming.. Grasses may grow from Grasses may grow from seedsseeds carried by animals or the carried by animals or the windwind.. They may be replaced by They may be replaced by shrubsshrubs, and eventually by , and eventually by treestrees similar to similar to

those in the original forest. those in the original forest.