Community Ecology and Ecosystem Energy Flow: Study Notes

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

Community Ecology

Definition of a Community

A community is a group of populations of different species living close enough to interact with each other. These interactions shape the structure and dynamics of the community.

Interspecific interactions: Relationships between individuals of different species in a community.
Types of interactions: Competition, predation, herbivory, parasitism, mutualism, and commensalism.

Coral reef community with diverse fish species

Interspecific Competition

Interspecific competition occurs when individuals of different species compete for a resource that limits their growth and survival. This is a (-/-) interaction because both species are negatively affected.

Competitive Exclusion Principle: Two species competing for the same limiting resource cannot coexist permanently; one will outcompete the other.
Species with a reproductive or resource-use advantage will eliminate the other.

Paramecium competition experiment graph

Ecological Niche

An ecological niche is the sum of a species' use of the biotic and abiotic resources in its environment.

Fundamental niche: The full range of environmental conditions a species could theoretically occupy.
Realized niche: The portion of the fundamental niche actually occupied due to competition and other biotic factors.

Diagram of fundamental and realized niche

Predation

Predation is an interaction in which one species, the predator, kills and eats the other, the prey. This is a (+/-) interaction.

Predators have adaptations for capturing prey; prey have adaptations for avoiding predators.

Praying mantis eating prey

Defensive Adaptations Against Predation

Prey species have evolved various adaptations to avoid being eaten:

Cryptic coloration: Camouflage that makes prey difficult to spot.
Aposematic coloration: Bright warning coloration in animals with effective chemical defenses.
Batesian mimicry: A harmless species mimics a harmful one.
Müllerian mimicry: Two or more unpalatable species resemble each other.

Cryptic coloration: leaf-tailed gecko camouflaged among leaves Aposematic coloration: brightly colored poison frog Batesian mimicry: Monarch and Viceroy butterflies

Herbivory

Herbivory is an interaction in which a herbivore eats parts of a plant or alga. This is a (+/-) interaction.

Plants have evolved mechanical and chemical defenses against herbivores.

Elk grazing on grass

Parasitism

Parasitism is a (+/-) interaction where one organism (the parasite) derives nourishment from another organism (the host), which is harmed in the process.

Parasites can significantly affect host populations and community structure.

Tomato hornworm caterpillar with braconid wasp larvae Braconid wasp laying eggs on caterpillar

Mutualism

Mutualism is an interspecific interaction that benefits both species (+/+).

Examples include pollination, seed dispersal, and cleaning symbioses.

Red-billed oxpecker on impala

Commensalism

Commensalism is an interaction where one species benefits and the other is neither helped nor harmed (+/0).

Example: Cattle egrets feeding on insects flushed by grazing cattle.

Egret (commensal) on cattle (host)

Species Diversity

Species diversity is a measure of the number of different species (species richness) and the relative abundance of each species in a community.

Communities with higher evenness and richness are more stable and resilient to disturbances.

Diagram comparing two communities with different species diversity

Trophic Structure

The trophic structure of a community describes the feeding relationships among organisms. Energy flows from primary producers to various levels of consumers.

Primary producers: Autotrophs (e.g., plants, algae)
Primary consumers: Herbivores
Secondary, tertiary, quaternary consumers: Carnivores at increasing trophic levels

Trophic pyramid showing producers and consumers

Food Chains and Food Webs

Food chains show the linear transfer of energy from one organism to another, while food webs illustrate the complex network of feeding relationships in a community.

Food webs provide a more accurate representation of energy flow in ecosystems.

Food web diagram Simple food chain with four trophic levels

Energy Transfer and the 10% Rule

Energy is lost at each transfer in a food chain, primarily as heat. Only about 10% of the energy at one trophic level is transferred to the next level. This limits the length of food chains.

Energy loss occurs due to metabolic processes, waste, and death.

Energy transfer and heat loss along a food chain Energy pyramid showing 10% rule and heat loss Trophic level and energy pyramid with heat loss

Dominant and Keystone Species

Dominant species are those that are most abundant or have the highest biomass in a community. Keystone species have a disproportionately large effect on community structure relative to their abundance.

Removal of a keystone species often leads to dramatic changes in community structure.

Disturbance and Ecological Succession

A disturbance is an event that changes a community by removing organisms or altering resource availability. Ecological succession is the sequence of community changes after a disturbance.

Primary succession: Occurs in lifeless areas where soil has not yet formed.
Secondary succession: Occurs where a disturbance has cleared an existing community but left the soil intact.

Early stage of primary succession with pioneer plants

Ecosystems and Energy Flow

Definition of an Ecosystem

An ecosystem consists of all the organisms living in a community as well as the abiotic factors with which they interact. Ecosystem ecology focuses on energy flow and chemical cycling.

Primary Producers and Consumers

Primary producers (autotrophs) support all other organisms in the ecosystem by converting solar energy into chemical energy. Consumers (heterotrophs) obtain energy by eating other organisms.

Herbivores: Primary consumers
Carnivores: Secondary, tertiary, and higher-level consumers
Detritivores (decomposers): Obtain energy from detritus (nonliving organic material) and recycle nutrients back to producers

Food Webs in Ecosystems

Food webs illustrate the complex feeding relationships and energy flow in ecosystems, showing how producers, consumers, and decomposers are interconnected.

Food web diagram with producers, consumers, and decomposers

Energy Flow and Chemical Cycling

Energy flows through ecosystems in one direction—from the sun to producers to consumers and finally to decomposers. Nutrients cycle within ecosystems, moving between biotic and abiotic components.

Energy is not recycled; it is lost as heat at each trophic level.
Nutrients such as carbon, nitrogen, and phosphorus are recycled by decomposers.

Summary Table: Types of Interspecific Interactions

Interaction Type	Effect on Species 1	Effect on Species 2	Example
Competition	-	-	Paramecium species competing for food
Predation	+	-	Praying mantis eating grasshopper
Herbivory	+	-	Elk grazing on grass
Parasitism	+	-	Braconid wasp and tomato hornworm
Mutualism	+	+	Oxpecker and impala
Commensalism	+	0	Cattle egret and cattle