Community Structure

• Community – An assemblage of all the populations

interacting in an area

• The ecological community is the set of plant

and animal species that occupy an area

• Questions central to ecological studies:– What controls the relative abundance of species within the

community?

– How do the component species interact with each other?

– How do communities change through time?

– How do different communities on the larger landscape

interact?

Community Ecology

• A broad definition of community is a group of

species that occupy a given area, interacting

either directly or indirectly

– A spatial concept

• A more restrictive definition of community is a

subset of species, such as a plant, bird, small

mammal, or fish community

– Food chain vs. food web

Community Structure

• A community has attributes that differ from

those of its components

– Number of species

– Relative abundance of species

– Nature of species interactions

– Physical structure

Community Structure

• Species richness (S) is the count of the

number of species occurring within the

community

• Relative abundance represents the percentage

each species contributes to the total number of

individuals of all species

Relative Abundance and Diversity

• The patterns of species richness and relative

abundance can be compared between

communities

Relative Abundance and Diversity

• A common method for comparing patterns of

species richness and abundance between

communities is to plot the relative abundance

of each species against rank (called a rank-

abundance diagram)

• Species evenness indicates the distribution of

species richness

– A community with a greater species evenness

would have a more gradual slope of the rank-

abundance curve

Relative Abundance and Diversity

• Diversity indexes provide a way to quantify

the relationship between species number and

relative abundance

• Simpson’s index (D) = (ni/N)2

– = summation for all species

– ni = number of individuals of species i

– N = total number of individuals of all species

– D ranges between 0 and 1 and as both species’

richness and evenness increase, the value

approaches 0

Relative Abundance and Diversity

• Simpson’s index (D) = (ni/N)2

• Simpson’s index of diversity = 1 – D

– The greater the value of D, the lower the

diversity

• Simpson’s reciprocal index or Simpson’s

diversity index = 1/D

– The lowest possible value is 1, representing a

community containing only one species

– The maximum value is the number of species in

the community (species richness: S)

Relative Abundance and Diversity

• Shannon (or Shannon-Weiner) index = H =

(pi)(log2 pi)

– Relative abundance of each species = pi = ni / N

– pi = proportion of species i

• In the absence of diversity, where only one

species is present, H = 0

• Hmax = ln S, occurs when all species are present

in equal numbers

Relative Abundance and Diversity

• When a single or few species predominate

within a community, these species are referred

to as dominants

• Dominance is the converse of diversity.

– When the basic Simpson index (D) is 1, the 1

represents total dominance–only one species present

in the community

• Dominant species are usually defined separately

for different taxonomic or functional groups of

organisms within the community (e.g., tree

versus herbaceous plant species)

Species Dominance

• Dominance can reflect the number of

individuals, size of individuals, or some

combination of characteristics that include both

the number and size of individuals

• Dominant species are typically the dominant

competitors under the prevailing environmental

conditions

– Other factors may determine dominance within

communities

Species Dominance

• The adaptation of organisms to the physical

environment and species interactions need to

be integrated to explain the processes that

control community structure

Structure of Communities

Communities are affected by:

• Available living space

– habitat

• Resource Availability

– niche

• Species interactions

– Symbiosis – “living together”

• commensalism

• mutualism

• parasitism

– Competition

– Predation

• Coevolution

Habitat – an organism’s mailing addressthe type of place where the individuals normally live

Available Living Space

Resources

• Resources: Features of the environment

required for growth, survival, or

reproduction, and that can be consumed to

the point of depletion.

Resources• Examples of resources:

• Food

• Water in terrestrial habitats

• Light for plants

• Space, especially for sessile organisms

• For mobile animals, space for refuge, nesting, etc.

Space Can Be a Limiting Resource

Habitat and Resources

• Species are also influenced by features of

the environment that are not consumed,

such as temperature, pH, salinity.

• These factors are not consumed and are

not considered to be resources.

• Competition reduces availability of

resources.

Niche –

an organism’s

profession (role)

in the

community

Resource Availability

• All living organisms have a range of

environmental conditions under which they

can successfully survive, grow, and reproduce

• This range of environmental conditions is not

the same for all organisms

Niche Constrains Community

Structure

• Environmental conditions vary in both time

and space

• The fundamental niches of species vary along

environmental gradients

• The distribution of fundamental niches along

the environmental gradient represents a

primary constraint on community structure

Niche Constrains Community

Structure

• As environmental conditions change from

location to location, the possible distribution

and abundance of species will change–in turn

changing the community’s structure

• Geographic distributions of species reflect the

occurrence of suitable environmental

conditions

Niche Constrains Community

Structure

• The null model assumes that the presence and

abundance of species are solely a result of the

independent responses to the prevailing

physical environment

– Interactions among species have no significant

influence on community structure

Niche Constrains Community

Structure

• Actual community patterns are compared to

the null model to establish the role (if any) of

species interactions

• A great deal of evidence indicates that species

interactions influence both the presence and

abundance of species within a wide variety of

communities

– Interspecific competition

– Facilitation

– Mutualism

Niche Constrains Community

Structure

• Ecological studies underestimate species

interactions because such interactions are often

diffuse, involving a number of species

Species Interactions Are Diffuse

Species Interactions

Symbiosis

• Living together for at least some part of the life cycle

• Most interactions are neutral; they have no effect on either species

– Commensalism

– Mutualism

– Parasitism

Commensalism

One species benefits and has no effect on the other

Mutualism

• Both species benefit

• Some are obligatory; partners depend

upon each other (coevolution)

– Yucca plants and yucca moth

– Mycorrhizal fungi and plants

– Anemone fish and anemone

Mutualism

Protection

photograph © Alex Wild 2004

Mutualism

Birds get food, help plant disperse seeds

Mutualism

Pollination

Yucca and Yucca Moth

• Example of an obligatory mutualism

• Each species of yucca is pollinated only by

one species of moth

• Moth larvae can grow only in that one

species of yucca

Obligate MutualismYucca Plant

Yucca Moth

Mycorrhizae

• Obligatory mutualism

between fungus and

plant root

• Fungus supplies

mineral ions to root

• Root supplies sugars

to fungus

Parasitism

• Parasites

– draw nutrients from hosts

• live on or in host body

• Vectors

– Convey a parasite from host to host

• Parasitoids

– Develop inside another species

– Consume and kill host

Micro Parasites

bacteria

fungus

Protozoan - flagellate

Macro parasites

tick

flea

Macro parasite –lamprey

Parasites

Parasites as biological controls

• Some parasites and parasitoids are used as

biological controls

• Adapted to specific host and habitat

– Good at locating host

– High population growth rate

– Offspring disperse

Biological Controls

• Wasp and aphid

Social Parasite

Brown-headed cowbird

Competition

• Interspecific: among different species– Exploitative competition

– Interference competition

• Intraspecific: between members of the same species– Intraspecific competition is most intense

• Territoriality

• Allelopathy

• Some species have eliminated ways of avoiding competition:– Resource partitioning

InterspecificCompetition

Intraspecific Competition

Intraspecific Competition

Territoriality

Intraspecific Competition

Allelopathy (sibling harming)

Eucalyptus oils harm young saplings of same species.

Can also affect other species

Forms of Competition

• Exploitative competition

– Species have equal access to resource; compete

to exploit resource

• Interference competition

– One species prevents another from using

resource

• usually through aggressive behavior

Interference Competition

Competitive Exclusion Principle

When two species compete for identical

resources, one will be more successful and

will eventually eliminate the other

Gause’s Experiment

Paramecium caudatum

Paramecium aurelia

Species grown together

Resource Partitioning

• Apparent competitors

may have slightly

different niches

• May use resources in a

different way or time

• Minimizes competition

and allows coexistence

Predation

• Predators

– animals that feed on other living

organisms

– free-living

– do not reside on their prey

• Carnivores and omnivores

Predator – Prey Relationships:

Canada Lynx and the

Showshoe Hare

• Species are limited by

the number of

available prey

• In some cases

predators limit a prey

species

Coevolution

• Two or more species exert selection

pressure on each other

– Prey defenses evolve

– Predator responses to prey evolve

Prey Defenses

• Camouflage

• Warning coloration

• Mimicry

Camouflage

Warning Coloration & Mimicry

Predator Responses

• Predators counter prey defenses with new

adaptations

– stealth

– camouflage

– avoidance of chemical repellents

Predator Responses