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 members of a particular species of organism in the community of all • 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.

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

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

into the lower intertidal zone will move •

Balanus

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

Barnacle Competition

Fundamental vs. Realized Niche • Fundamental niche physiological limits – describes the 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

Healthy oaks produce more acorns.

Direct and Indirect Interactions Control Populations

Gypsy moths Oak trees Gypsy moth infestations defoliate oak trees, reducing Acorn production Oak trees produce every few years.

Oak trees Oak trees produce large crops of Gypsy moths Acorn production Deer mice Deer mice eat relatively few gypsy moth pupae; moth Predators on mice reduce mouse populations, allowing greatly in years of heavy acorn production.

populations.

Deer mice Mouse populations increase greatly in years of Predators production.

Symbiosis • Parasitism – One species benefits, the other is harmed • Commensalism – One species benefits, the other not affected • Mutualism – Both species benefit

• HOST Symbiosis • SYMBIONT

PARASITISM COMMENSALISM MUTUALISM

Examples of Symbiosis • Parasitism – Usually smaller than the host – May be animal on animal; animal on plant; plant on plant

Examples of Symbiosis • Parasitism – Usually smaller than the host – May be animal on animal; animal on plant; plant on plant

Examples of Symbiosis • Parasitism – Usually smaller than the host – May be animal on animal; animal on plant; plant on plant

Examples of Symbiosis • Parasitism – Usually smaller than the host – May be animal on animal; animal on plant; plant on plant – Nest parasitism

Examples of Symbiosis • Parasitism • Commensalism – Scale worms live in the grooves of starfish arms

Examples of Symbiosis • Parasitism • Commensalism – Scale worms live in the grooves of starfish arms – Barnacles on a scallop shell

Examples of Symbiosis • Parasitism • Commensalism – Scale worms live in the grooves of starfish arms – Barnacles on a scallop shell – Anemone fish

Examples of Symbiosis • Parasitism • Commensalism – Scale worms live in the grooves of starfish arms – Barnacles on a scallop shell – Anemone fish – Epiphytes ( plants which grow using other plants or objects for support )

Examples of Symbiosis • Parasitism • Commensalism • Mutualism – Ants & aphids

Examples of Symbiosis • Parasitism • Commensalism • Mutualism – Ants & aphids – Ant & Acacia

Examples of Symbiosis • Parasitism • Commensalism • Mutualism – Ants & aphids – Ant & Acacia – Insects & plants

Examples of Symbiosis • Parasitism • Commensalism • Mutualism – Ants & aphids – Ant & Acacia – Insects & plants – Mycorrhizal fungi

Examples of Symbiosis • Parasitism • Commensalism • Mutualism – 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 CONSTANT of species was relatively – 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

Intertidal boulder fields on the California coast

Mode for large Mode for medium boulders

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.

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!