Community EcologyAP Biology

Introduction

• Community = An assemblage of populations interacting with one another within the same environment

• Example: A forest• Many different populations of:

• Fungi• Plants• Invertebrates• Birds• Small mammals• Etc.

How do we compare communities?

Community Characteristics

• Composition• Listing the species in the community

• Diversity• Includes both the species richness (the number of

species) and evenness (the relative abundance of individuals of different species)

Microbial Communities of the Small and Large Intestines

Community Structure

• Communities are made of a dynamic system of interacting species• Examples of interactions: predator-prey, parasite-host,

cooperation

• Species interactions are based on the competition for limited resources

Habitat vs. Niche

Each species occupies a particular position in the community

• Spatially (where it lives)

• Functionally (what role it plays)

Habitat = Where an organism lives and reproduces

Ecological Niche = the role an organism plays in its community, including its habitat, interactions with other organisms, and resources

Feeding Niches for Wading Birds

Flamingos feed on small mollusks, crustacean and vegetable matter strained from mud pumped through their bills by their powerful tongues

Dabbling ducks feed by tipping, tail up, to reach aquatic plants, seeds, snails, and insects

Avocets feed on insects, small marine invertebrates, and seeds by sweeping their bills from side to side in shallow water

Competition

• Ecological niches overlap, and organisms need to compete for resources

• Interspecific competition = when members of different species try to utilize a resource that is in limited supply

• Intraspecific competition = when members of the same species try to utilize a resource that is in limited supply

Q: Do you think that interspecific competition or intraspecific competition is usually the most fierce? Why?

Competitive Exclusion Principle

• This principle states that no two species can occupy the same niche at the same time. Why?

• One of the two species is going to be better (more efficient) at utilizing the limited resource, and will dominate the other, which is eliminated

How can organisms avoid competitive exclusion?• One way is resource partitioning/niche

partitioning

• In a different study, two different species of paramecium were able to share one tube • One species fed on bacteria at the bottom of the tube

and the other fed on bacteria suspended in solution

Species Interactions: Predation

Predation occurs when one living organism, called the predator, feeds on another, called the prey

Predator-Prey Population Dynamics• We use graphs of population density to show the

interactions between predators and their prey

• Predators and prey both affect each other’s population density (it’s a “two-way street”)

Species Interactions: Symbiotic Relationships• Symbiosis = interactions in which there is a close

relationship between members of two populations

• There are many types of symbiosis that each affect the fitness of the individuals differently• Parasitism

• Commensalism

• Mutualism

Types of Symbiosis: Parasitism• Parasitism = an organism called a parasite derives

nourishment from another called the host• Similar to predation, where the predator derives

nourishment from the prey

• The effects of parasites can range from mild damage to death of the host

• Example: Viruses, such as HIV, reproduce inside an organism and are always parasitic

• Example: Varroa destructor, a mite that can only reproduce in a honey bee colony. It attaches to the body of the bee and weakens the bee by sucking hemolymph (a fluid analogous to blood)

Types of Symbiosis: Commensalism• Commensalism = a relationship between two

species in which one species is benefited and the other is neither benefited nor harmed

• Example: Barnacles attach themselves onto whales which provide a home and transportation, but the barnacles do not harm the whales

Types of Symbiosis: Mutualism

• Mutualism = a relationship in which both members of the association benefit

• Example: Bacteria in the human intestinal tract• We provide the bacteria with a place to live and food

• They help us in a lot of ways, such as making vitamins that we cannot synthesize on our own

Community Development

• Community structures change over time, often due to abiotic factors• Ex. storms, volcanic eruptions

• Also human activities

• After a disturbance, the community may or may not return to how it was before

Ecological Succession

• Ecological succession is a change within a community following a disturbance

• Primary succession happens in areas where there is no soil formation, such as following a volcanic eruption or glacial retreat

• Secondary succession happens in areas where soil is present

Secondary Succession in a Forest

The first species to colonize an area after a disaster

In a particular area, succession will always lead to the same kind of community