Essentials of BiologySylvia S. Mader

Chapter 31Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31.1 Ecology of Communities

• A community is a collection of different species of living organisms.

• In some communities, some populations affect the evolution of other populations (coevolution).

• An ecosystem consists of a community and the physical environment.

Community Composition and Diversity

• Two characteristics can be used to describe a community.– Species richness includes the variety of

different plant species in a community.– Diversity includes the richness and relative

abundance of individuals of different species.

Ecological Succession

• As communities change over time, they typically go through an ordered process of change called ecological succession.

• The climax-pattern model of succession maintains that the climate of an area leads to the same stable climax community.

Two Types of Succession

• There are two types of succession.– Primary succession occurs where soil has not

formed.

– Secondary succession occurs on a disturbed site with soil (such as an old agricultural field) that returns to a natural pattern of community change.

Two Types of Succession (cont.)

Interactions in Communities

• The species in a community can interact in a variety of ways.– Competition occurs when species compete for the

same resource.– In predation, one species preys upon another.– In parasitism, one species is a parasite on another.– In commensalism, one in which the interaction

benefits one species, but the other is not affected.– In mutualism, the interaction benefits both species.

Interactions in Communities (cont.)

Interaction Expected Outcome

Competition (-, -) Abundance of both species decreases

Predation (+, -) Predator abundance increases, prey abundance decreases

Parasitism (-,-) Parasite abundance increases, host abundance decreases

Commensalism (+, 0) Abundance of one species increases; the other does not

Mutualism (+, +) Abundance of both species increases

Ecological Niche

• The spatial location where a species is found is called its habitat.

• The ecological niche of a species is the combination of its role in a community, its habitat, and interactions with other organisms.

Competition

• Competition for limiting resources contributes to the niche of each species and community structure.

• The competitive exclusion principle states that since no two species can occupy the same niche, one species will eventually die out.

Competition (cont.)

Competition (cont.)

• Competition for resources can also lead to resource partitioning, meaning that the two species utilize different aspects of the niche so they can both survive.

• Character displacement can also occur as organisms evolve different characters to adapt to aspects of a niche.

Competition (cont.)

Mutualism

• In mutualism, both species benefit from the interaction.

• Mutualism leads to an intricate web of interdependency critical to community structure.

Community Stability

• Communities rely upon one or more species called keystone species that stabilize the structure of a community.

Native Versus Exotic Species

• Native species are those indigenous to an area.

• Invasive species, sometimes called exotic species, are introduced into an area and displace native species.

31.2 Ecology of Ecosystems

• The populations of an ecosystem are described in terms of their food source.

• Autotrophs, also called producers, produce their own food.– Photoautotrophs use the sun to make their

own food.– Chemoautotrophs obtain energy to make food

by oxidizing inorganic compounds.

31.2 Ecology of Ecosystems (cont.)

• Heterotrophs (consumers) must consume the nutrients synthesized by autotrophs to survive.– Herbivores graze on plants.

– Carnivores eat meat.

– Omnivores eat both plants and meat.

– Decomposers break down dead organic matter (detritus).

Energy Flow and Chemical Cycling

• The autotrophs and heterotrophs of a community create a flow of energy and nutrients through an ecosystem.

• As energy flows from one level of an ecosystem to the next, a large fraction (about 90%) is lost, generally as heat.

Energy Flow and Chemical Cycling (cont.)

Energy Flow

• The energy flow through an ecosystem can be illustrated as a food web or food chain.– A grazing food web begins with plants.– A detrital food web begins with detritus.

• The level of nourishment within a food web or chain is called a trophic level.

• Since energy is lost as energy moves from one trophic level to the next, the flow of energy can be depicted as an ecological pyramid.

Energy Flow (cont.)

Energy Flow (cont.)

Chemical Cycling

• The pathways by which organisms move nutrients within ecosystems create biogeochemical cycles.– For a sedimentary cycle, nutrients originate in

sediments or soils and cycle from autotrophs to heterotrophs before being returned by decomposers.

– In a gaseous cycle, the nutrient originates in and is returned as a gas.

Chemical Cycling (cont.)

• A large fraction of nutrients may be found in reservoirs that are unavailable to organisms.

• Organisms generally acquire nutrients from an accessible source called an exchange pool.

• Human activities have both positive and negative effects on the pools of nutrients.

Chemical Cycling (cont.)

Phosphorus Cycle

• Phosphorus in the environment is released as rocks and weather, releasing phosphate.

• Producers such as plants and algae utilize phosphate in their metabolism.

• Consumers acquire phosphate by eating producers.

• Phosphate is returned to the ecosystem during decomposition of producers and consumers.

Phosphorus Cycle (cont.)

Nitrogen Cycle

• Producers can use different forms of nitrogen from the environment.– Ammonium (NH4

+) found naturally or derived from N2 by nitrogen fixation

– Nitrate (NO3-), produced from ammonium by

nitrification

• In the nitrogen cycle, nitrification is counterbalanced by denitrification and nitrogen fixation.

Nitrogen Cycling (cont.)

Carbon Cycle

• There are two primary sources of carbon for the carbon cycle. – Gaseous or dissolved carbon dioxide– Inorganic carbonate

• Much of the Earth’s carbon is tied up in the bodies of organisms or in limestone and carbonate shells.

Carbon Cycling (cont.)

31.3 Ecology of Major Ecosystems

• The Earth’s biosphere is composed of both aquatic and terrestrial ecosystems.– Aquatic ecosystems are both freshwater and

marine.– The different terrestrial ecosystems are called

biomes.

Primary Productivity

• Ecosystems can be compared on the basis of their primary productivity, which is the rate by which producers capture and store energy.

• Several factors influence the primary productivity of an ecosystem.– Temperature– Moisture– Nutrient availability

Primary Productivity (cont.)