ECOLOGY-2 Interactions In Communities. An Ecological Community is… All populations of organisms…...

ECOLOGY-2

Interactions

In

Communities

An Ecological Community is…

• All populations of organisms…• …inhabiting a common environment…• …and interacting with one another.

Competition

• Individuals of the same species which interact for resources are said to show intraspecific competition

• Individuals of different species which interact for resources are said to show interspecific competition

• Both may limit the supply of resources

• Affects the reproductive success of the population

Types of Competitors

• Producers – plants– Compete with other plants for sunlight & water

• Herbivores – animals that eat plants & algae– Compete with other herbivores for food, etc.

• Carnivores – animals that eat other animals– Compete with other carnivores for food, etc.

Competitive Exclusion

• G.F. Gause formulated the competitive exclusion principle in 1934:– “if two species are in competition for the same

limited resource, one or the other will be more efficient at utilizing or controlling access to this resource and will eventually eliminate the other in situations in which the two species occur together.”

In Other Words…

• No two similar species can occupy the same niche at the same time!

Competitive Exclusion

• Where two species “overlap” in the same niche, one will exclude the other

What’s a Niche?• The total environment and way of life of all

members of a particular species of organism in the community

• An ecological niche is the role that an organism plays in its environment

• By analogy, a niche is roughly equivalent to an organism’s profession, as opposed to its address.

Niche

• Sum total of all biological and physical factors which define the “space” in which a particular species serves its function in the ecosystem

Environmental Niche

• Description includes:– Physical factors

• Temperature range• Moisture requirements

– Biological factors• Nature & amount of required food sources• Pattern of movement & behavior• Seasonal & daily activity cycles

Gause’s Experiments

Experiment Analysis• Fastest growing species is not always the victor:

• Lemna gibba grows more slowly, but always replaces L. polymorpha because of its flotation air sacs – it forms a mass over the other species and cuts off its supply of light!

Resource Partitioning

• When two similar species inhabit the same area and have similar ecological requirements

• Close examination shows that they “divide up” the resources to avoid competition

• Exact cause of resource partitioning is subject to debate

Resource Partitioning in Warblers

Resource Partitioning in Lizards

Resource Partitioning in Plants

Resource Partitioning in Seabirds

Experimental Studies of Competition• Barnacle competition studied along the

Scotland coast

• Chthamalus is usually found only in upper intertidal (smaller – slower growing)

• Balanus usually found in lower intertidal (larger – grow faster)

• If Balanus removed, Chthamalus will move into the lower intertidal zone

• Balanus however doesn’t invade the upper zone, because it cannot survive the drier conditions

Barnacle Competition

Fundamental vs. Realized Niche

• Fundamental niche – describes the physiological limits of the organism (maximum tolerance for temperature, desiccation, etc.)

• Realized niche – that portion of the fundamental niche actually used; determined by physical factors and interactions with other organisms

Winner Takes All

• A successful competitor may actually displace its rival completely!

• Example: introduced starlings replace bluebirds

Predation

• Defined as the consumption of live organisms– Plants by animals– Animals by animals– Animals by plants or fungi

Computer Model

Lynx – Hare Predation

Predation & Species Diversity

• Predation holds competition down and more resources remain, more species can coexist

• R.T.Paine did an experiment to test the hypothesis that predators actually INCREASE the community DIVERSITY!

Paine's work in intertidal communities of the Pacific Northwest

In the undisturbed (control) areas:

15 prey species coexist

In the starfish removal areas:

8 prey species remain and community dominated by mussels

Community Interactions

Concept of Feedback Loops

Direct and Indirect Interactions Control PopulationsDirect and Indirect Interactions Control Populations

Oaktrees

Acornproduction

Deermice

Gypsymoths

Oaktrees

Acornproduction

Deermice

Gypsymoths

Predators

Oak trees produce large crops of acorns

every few years.

Mouse populations increase greatly in years of heavy

acorn production.

Dense mouse populations keep gypsy moth populations low by eating

gypsy moth pupae.

Healthy oaks produce

more acorns.

Gypsy moth infestations defoliate oak trees, reducing acorn population.

Oak trees produce large crops of

acorns every few years.

Mouse populations

increase greatly in years of

heavy acorn production.

Predators on mice reduce mouse populations, allowing

recovery of gypsy moth populations.

Deer mice eat relatively few gypsy moth pupae; moth population

remains high.

Symbiosis

• Parasitism– One species benefits, the other is harmed

• Commensalism– One species benefits, the other not affected

• Mutualism– Both species benefit

Symbiosis• HOST • SYMBIONT

PARASITISM

COMMENSALISM

MUTUALISM

Examples of Symbiosis

• Parasitism– Usually smaller

than the host– May be animal on

animal; animal on plant; plant on plant


• Parasitism– Usually smaller

than the host– May be animal on

animal; animal on plant; plant on plant

– Nest parasitism


• Parasitism• Commensalism

– Scale worms live in the grooves of starfish arms




– Barnacles on a scallop shell





– Anemone fish





– Anemone fish– Epiphytes ( plants

which grow using other plants

or objects for support )


• Parasitism• Commensalism• Mutualism

– Ants & aphids



– Ants & aphids– Ant & Acacia



– Ants & aphids– Ant & Acacia– Insects & plants



– Ants & aphids– Ant & Acacia– Insects & plants– Mycorrhizal fungi



– Ants & aphids– Ant & Acacia– Insects & plants– Mycorrhizal fungi– Cleaner shrimp

Community Composition:

Community Stability

And

Equilibrium

Island Biogeography Model• R. MacArthur & E.O. Wilson (1963)

• Used small islands as models to study species composition & stability of communities

• Findings:– The NUMBER of species was relatively

CONSTANT– The SPECIES COMPOSITION CONSTANTLY

CHANGES

• Known as the Equilibrium Hypothesis

Equilibrium Hypothesis

Intermediate Disturbance Model

• Tropical rainforests & coral reefs long thought to be stable, equilibrium communities (diversity thought to be a function of stability)

• Evidence now suggests that diversity is a function of the frequency and magnitude of the disturbances a community is subject to

Mode for medium bouldersMode for

large

Wayne Sousa noticed that small and large boulders tended to have fewer species of algae on them than boulders of intermediate size. Sousa guessed that that small boulders were more likely to roll during storms (“scouring” the algae off them) than medium boulders. Medium boulders, in turn, were more likely to roll than large boulders.

Intertidal boulder fields on the California coast

Sousa tested this hypothesis by cementing small boulders to the substrate so they could not roll.

The figure above shows that small rocks (“unstable small rocks”) are normally dominated by a single species of alga (Ulva, sea lettuce). Similar rocks that are cemented to the substrate (“stabilized small rocks”) eventually develop a richer algal community.

Ecological Succession

Relatively long, gradual changes in community composition following initial colonization

Temporal Succession

• Temporal distributions of plant species remains in pack rat nests (“middens”) in Texas since the last glaciation.

• Each horizontal graph is a different plant species.

• Time before present is plotted left (past) to right (present).

• Plant community changes almost continuously

Spatial Succession• First colonists gradually crowded out by

successive groups of organisms.

Theories of Succession

• Facilitation model– Each stage “prepares the way” for next

• Inhibition model– Each stage prevents colonization of next

• Tolerance model– Existing species neither inhibit nor promote

the colonization of next

Succession Terminology

• Pioneer species– Early colonists; grow rapidly (“weedy”) and

fully occupy available area

• Seral stages (or communities)– Intermediate assemblages of species;

populations may vary significantly in size

• Climax community– Group of organisms likely to remain in the

area until a disturbance

Climax vs. Continuum• The classical concept of successional climax communities

has been superceded by the continuum concept, in line with the acceptance that most communities are relatively “open”: each species responds individually to climatic conditions.

The key feature to remember is that communities are DYNAMIC in their COMPOSITION; their makeup will change as conditions change!

ECOLOGY-2 Interactions In Communities. An Ecological Community is… All populations of organisms…...

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