hand-out - community ecology

8/11/2019 HAND-OUT - Community Ecology

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HAND-OUT 3

C O M M U N I T Y E C O L O G Y

COMMUNITY ECOLOGY

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Study of the organization and functioning of communities, which are assemblages of interacting populations

of the species living within a particular area or habitat.

What is a Community?

A community is defined as an assemblage of species living close enough together for potential interaction.

Communities differ in their species richness, the number of species they contain, and the relative abundance

of different species.Importance of Community Ecology

Community ecology seeks to explain the underlying mechanisms that create, maintain, and determine the fate

of biological communities.

Typically, patterns are documented by observation, and used to generate hypotheses about processes, which

are tested.

Not all science is experimental. Hypotheses tests can involve special observations, or experiments.

Models of Community Ecology

1.

The rivet modelof communities is a reincarnation of the interactive model.

2.

The redundancy modelstates that most species in a community are not closely associated with one another.

Emergent Properties of a Community

1.

Scale

2.

Spatial and Temporal Structure

3.

Species Richness

4.

Species Diversity

5.

Trophic structure

6.

Succession and Disturbance

1. Scaleis the size of a community.

Provided that the area or habitat is well defined, a community can be a system of almost any size, from a drop of

water, to a rotting log, to a forest, to the surface of the Pacific Ocean.

2.

Spatial and Temporal

a.

Spatial Structureis the way species are distributed relative to each other.

Some species provide a framework that creates habitats for other species. These species, in turn create habitats

for others, etc.

Example:Trees in a rainforest are stratified into several different levels, including a canopy, several

understories, a ground level, and roots. Each level is the habitat of a distinct collection of species. Some places,

such as the pools of water that collect at the base of tree branches, may harbor entire communities of theirown.

b. Temporal structureis the timing of the appearance and activity of species. Some communities, i.e., arctic

tundra and the decay of a corpse, have pronounced temporal species, other communities have less.

Example: Many desert plants and animals are dormant most of the year. They emerge, or germinate, in

response to seasonal rains. Other plants stick around year round, having evolved adaptations to resist drought.

3. Species Richness- is the number of species in a community. Clearly, the number of species we can observe is

function of the area of the sample. It also is a function of who is looking. Thus, species richness is sensitive to

sampling procedure

4.

Diversityis the number of species in the community, and their relative abundances.

Species are not equally abundant, some species occur in large percentage of samples, others are poorly

represented. Some communities, such as tropical rainforests, are much more diverse than others, such as the great basin

desert.

5. Trophic structure

The trophic structure of a community is determined by the feeding relationships between organisms.

The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is

called the food chain.

6. Succession, Disturbance and Change

In terms of species and physical structure, communities change with time.

Ecological succession, the predictable change in species over time, as each new set of species modifies

the environment to enable the establishment of other species, is virtually ubiquitous.

Example; a sphagnum bog community may persist for only a few decades before the process of ecological

succession changes transform it into the surrounding Black Spruce Forest.

A forest fire may destroy a large area of trees, clearing the way for a meadow. Eventually, the trees take over

and the meadow is replaced.

Disturbances are events such as floods, fire, droughts, overgrazing, and human activity that damage

communities, remove organisms from them, and alter resource availability.

SOME AGENTS OF DISTURBANCE

Fire

Floods

Drought

Large Herbivores

Storms

Volcanoes

Human Activity


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FACTS ABOUT COMMUNITY

1. Populations may be linked by competition, predation, mutualism and commensalism

A. COMPETITION

- Interspecific competitionfor resources can occur when resources are in short supply.

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There is potential for competition between any two species that need the same limited

resource.

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The competitive exclusion principle: two species with similar needs for same limiting resources cannot

coexist in the same place. The ecological nicheis the sum total of an organisms use of abiotic/biotic resources in the environment.

An organisms nicheis its role in the environment.

The competitive exclusion principle can be restated to say that two species cannot coexist in a

community if their niches are identical.

Classic experiments confirm this.

Resource partitioningis the differentiation of niches that enables two similar species to coexist in a

community.

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Character Displacementis the tendency for characteristics to be more divergent in sympatric populations of

two species than in allopatric populations of the same two species.

Hereditary changes evolve that bring about resource partitioning.

B.

PREDATION

A predator eats prey.

Herbivory, in which animals eat plants.

In parasitism, predators live on/in a host and depend on the host for nutrition.

Predator adaptations: many important feeding adaptations of predators are both obvious and

familiar.

Claws, teeth, fangs, poison, heat-sensing organs, speed, and agility.

Plant defenses against herbivores include chemical compounds that are toxic.

Animal defenses against predators.

Behavioral defenses include fleeing, hiding, self-defense, noises, and mobbing.

CAMOUFLAGE includes cryptic coloration, deceptive markings.

Mechanical defenses include spines.

Chemical defenses include odors and toxins1. Aposematic colorationis indicated by warning colors, and is sometimes associated with other

defenses (toxins).

2. Mimicryis when organisms resemble other species.

1. Batesian mimicryis where a harmless species mimics a harmful one.

2. Mllerian mimicryis where two or more unpalatable species resemble each other.

C. PARASITISM- Parasites and pathogens as predators.

A parasite derives nourishment from a host, which is harmed in the process.

Endoparasiteslive inside the host and ectoparasites live on the surface of the host.

Parasitoidismis a special type of parasitism where the parasite eventually kills the host.

Pathogensare disease-causing organisms that can be considered predators.

D.

MUTUALISMis where two species benefit from their interaction.E.

COMMENSALISMis where one species benefits from the interaction, but other is not affected.

An example:would be barnacles that attach to a whale.

COEVOLUTION AND INTERSPECIFIC INTERACTIONS

Coevolutionrefers to reciprocal evolutionary adaptations of two interacting species.

When one species evolves, it exerts selective pressure on the other to evolve to continue the

interaction.

2. Trophic structure is a key factor in community dynamics

The trophic structureof a community is determined by the feeding relationships between organisms.

The transfer of food energy from its source in photosynthetic organisms through herbivores and carnivores is

called the food chain.

Charles Eltonfirst pointed out that the length of a food chain is usually four or five links, called trophic

levels. He also recognized that food chains are not isolated units but are hooked together into food

webs.

Food webs

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Who eats whom in a community?

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Trophic relationships can be diagrammed in a community.

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What transforms food chains into food webs?

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A given species may weave into the web at more than one trophic level.


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Dominant speciesare those in a community that have the highest abundance or highest biomass (the sum

weight of all individuals in a population).

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If we remove a dominant species from a community, it can change the entire community structure.

Keystone speciesexert an important regulating effect on other species in a community.

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If Keystone species are removed, community structure is greatly affected.

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Consider three possible relationships between plants (Vfor vegetation) and herbivores (H).

- VH VH VH

Arrows indicate that a change in biomass of one trophic level causes a change in theother trophic level.

1.

The bottom-up model postulatesVHlinkages, where nutrients and vegetation control community

organization.

2.

The top-down model postulatesthat it is mainly predation that controls community organization VH.

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Other models go between the bottom-up and top-down extreme models.

COMMUNITY DISTURBANCES

Disturbances affect community structure and stability.

- Stability is the ability of a community to persist in the face of disturbance.

1. Most communities are in a state of nonequilibrium owing to disturbances

Disturbancesare events like fire, weather, or human activities that can alter communities.-

Some are routine.

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Ex. Marine communities are subject to disturbance by tropical storms.

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We usually think that disturbances have a negative impact on communities, but in many cases they are

necessary for community development and survival.

2. Humans are the most widespread agents of disturbance

- Human activities cause more disturbance than natural events and usually reduce species diversity in

communities.

3. Ecological succession is the sequence of community changes after a disturbance

Ecological successionis the transition in species composition over ecological time.

1.

Primary successionbegins in a lifeless area where soil has not yet formed.

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Mossesand lichens colonize first and cause the development of soil.-

An example would be after a glacier has retreated.

2.

Secondarysuccession occurs where an existing community has been cleared by some event, but the soil is

left intact.

Grasses grow first, then trees and other organisms.

Soil concentrations of nutrients show changes over time.

4. Community biodiversity measures the number of species and their relative abundance

The variety of different kinds of organisms that make up a community has two components.

Species richness, the total number of species in the community.

Relative abundanceof the different species.

Imagine two small forest communities with 100 individuals distributed among four different tree

species.

Species richness may be equal, but relative abundance may be different. Counting species in a community to determine their abundance is difficult, especially for insects and

smaller organisms.

5.

Species richness generally declines along an equatorial-polar gradient

Tropical habitats support much larger numbers of species of organisms than do temperate and polar regions.

What causes these gradients?

The two key factors are probably evolutionary history and climate.

Organisms have a history in an area where they are adapted to the climate.

Energy and water may factor into this phenomenon.

1. Species richness is related to a communitys geographic size

The species-area curve quantifies what may seem obvious: the larger the geographic area, the greater

the number of species.2. Species richness on islands depends on island size and distance from the mainland

Because of their size and isolation, islands provide great opportunities for studying some of the biogeographic

factors that affect the species diversity of communities.

Imagine a newly formed island some distance from the mainland.

Robert MacArthur and E. O. Wilsondeveloped a hypothesis of island biogeography to identify the

determinants of species diversity on an island.

Two factors will determine the number of species that eventually inhabit the island.

1.

The rate at which new species immigrate to the island.

2.

The rate at which species become extinct.


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A.

QUESTIONS:

1. Differentiate population ecology from community ecology?

2. How would you interpret Rivet Model and Redundancy Model of community? How they differ?

3. Explain the different emergent properties of community.

4. What are the different agents that may disturb the community? How they affect the community?

5.

Differentiate competitions, predation, mutualism, amensalism, symbiosis and commensalism. Cite atleast 3 examples each.

6. What are the different forms of animal camouflage?

7. What is aposematic coloration and give examples.

8. How Batesian Mimicry differ from Mullerian Mimicry? Cite examples of these forms of Mimicry.

9. How parasitisms affect the host?

10.What is ecological succession? How primary succession and secondary succession differ?

11.Expound the idea of species richness.

B.

QUESTIONS FOR RESEARCH

1. Explain the Gauses Principle of Competitive Exclusion.

2. Is the term niche synonymous with the term habitat? How does and organisms fundamental niche differ

from realized niche?

3. What morphological defenses do plants use to defend themselves against herbivores?

4. How do some animals defend themselves against predators?

5. Consider the aposematic coloration, cryptic coloration and Batesian Mimicry.

a. Which would be associated with an adult viceroy butterfly? Why

b. Which would be associated with a larval monarch butterfly? Why

c. Which would be associated with a larval viceroy butterfly? Why

6. Why do eliminating predators a bad idea for species richness?

7. How do predations interact in regulating species diversity of community?

8. Why do scientists altered the concept of final, climax vegetation in a given ecosystem?

9. What type of organisms associated with early stages of succession? Why?

10.What is the role of disturbances in succession?

xxx END OF LECTURE xxx

Prepared by:

NELSON R. BELLO, MAST

Instructor

hand-out - community ecology

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