Communities and Ecosystems: Biodiversity and Species Interactions

Chapter 20: Communities and Ecosystems

Introduction to Communities and Ecosystems

This chapter explores the structure and function of biological communities and ecosystems, focusing on biodiversity, species interactions, and the factors that influence ecosystem health and stability.

Biodiversity

Definition and Importance

• Biodiversity is the variety of life on Earth, including different genes, species, and ecosystems.

• Each species plays a role in maintaining the health of the planet.

• Loss of biodiversity can disrupt ecosystem services such as food production, oxygen generation, and disease regulation.

Example: Bees pollinate crops, Tasmanian devils provide antibiotic-resistant proteins, and sharks are studied for cancer research.

Genetic Diversity

• Genetic diversity within a species is crucial for adaptation to environmental changes.

• Low genetic diversity increases vulnerability to diseases and environmental changes.

• Example: Most cultivated bananas are genetically identical, making them susceptible to a single disease.

Ecosystem Diversity and Services

• An ecosystem includes all living and nonliving components in an area.

• Ecosystem services are functions ecosystems perform, such as providing clean air, water, and climate regulation.

Factors Leading to Biodiversity Decline

• Habitat Destruction & Fragmentation: Deforestation, urbanization, and road construction reduce available habitats.

• Invasive Species: Non-native species outcompete native species for resources (e.g., zebra mussels in the Great Lakes).

• Overexploitation: Overfishing, hunting, and excessive harvesting reduce populations.

• Pollution: Contaminants in air, water, and soil can harm organisms and disrupt ecosystems (e.g., Deepwater Horizon oil spill).

Community Structure and Species Interactions

Community Definition

• A community consists of all populations of different species living and interacting in a specific area.

Interspecific Interactions

• Interspecific interactions are relationships between species in a community.

Types of Interspecific Interactions

• Competition (-/-): Different species compete for the same resources (e.g., lions and hyenas for prey).

• Competitive Exclusion Principle: Two species with similar niches cannot coexist indefinitely; one will outcompete the other.

• Mutualism (+/+): Both species benefit (e.g., crocodiles and birds).

• Predation (+/-): One organism (predator) eats another (prey). Prey may evolve defenses such as quills, camouflage (cryptic coloration), or warning coloration.

• Mimicry: One species resembles another for protection (e.g., non-venomous snakes mimicking venomous ones).

• Herbivory (+/-): Animals eat plant parts; plants may develop thorns, toxins, or bad taste as defenses.

• Parasitism and Pathogens (+/-): Parasites live on/in hosts and steal nutrients; pathogens cause disease (e.g., tapeworms, influenza virus).

• Commensalism (+/0): One species benefits, the other is unaffected (e.g., birds nesting in trees).

Trophic Structure and Energy Flow

Food Chains and Food Webs

• Trophic structure describes feeding relationships among species.

• Food chain: Energy flows from producers to herbivores to predators.

• Food web: Interconnected food chains showing complex feeding relationships.

Trophic Levels

• Producers: Plants and algae that make food via photosynthesis.

• Primary consumers: Herbivores that eat producers.

• Secondary consumers: Carnivores that eat herbivores.

• Tertiary consumers: Eat secondary consumers.

• Quaternary consumers: Top predators in the food chain.

• Decomposers: Fungi and bacteria that break down dead material, recycling nutrients.

Species Diversity in Communities

Species Richness and Relative Abundance

• Species richness: Number of different species in a community.

• Relative abundance: Proportion of each species in the community.

• High diversity means both high richness and even abundance.

Keystone Species

• Keystone species have a disproportionately large effect on their ecosystem relative to their abundance.

• Removal of a keystone species can cause dramatic changes in community structure.

Disturbances and Succession

Disturbances

• Events like storms, fires, or floods that disrupt ecosystems and alter resource availability.

• Small-scale disturbances can create new habitats and increase diversity.

Ecological Succession

• Primary succession: Occurs in lifeless areas with no soil (e.g., after a volcanic eruption).

• Secondary succession: Occurs where a disturbance destroys a community but leaves soil intact (e.g., after a wildfire).

Ecosystem Ecology: Energy Flow and Chemical Cycling

Energy Flow

• Energy flows through ecosystems from sunlight to producers to consumers and is eventually lost as heat.

Chemical Cycling

• Elements like carbon, nitrogen, and phosphorus cycle between living and nonliving parts of ecosystems.

• Biogeochemical cycles ensure the reuse of essential nutrients.

Key Biogeochemical Cycles

• Carbon Cycle: Movement of carbon through photosynthesis, respiration, and decomposition.

• Phosphorus Cycle: Movement of phosphorus through rocks, water, soil, and living organisms.

• Nitrogen Cycle: Movement of nitrogen through the atmosphere, soil, and living things.

Conservation and Restoration

Biodiversity Hotspots

• Regions with high numbers of unique and endangered species (e.g., Madagascar).

• Protecting hotspots is crucial for preventing extinctions.

Landscape Ecology and Corridors

• Landscape: A large area with multiple connected ecosystems.

• Movement corridors: Strips of habitat that connect isolated ecosystems, allowing species to migrate safely.

Bioremediation

• Using living organisms to clean up pollution (e.g., bacteria that degrade oil, plants that absorb toxins).

Summary Table: Main Factors for Biodiversity Decline

Factor	Description	Example
Habitat Destruction	Loss and fragmentation of natural habitats	Deforestation, urban development
Invasive Species	Non-native species outcompete natives	Zebra mussels in the Great Lakes
Overexploitation	Excessive harvesting of species	Overfishing, poaching
Pollution	Contamination of air, water, soil	Oil spills, industrial emissions

Key Equations

• Energy Transfer Efficiency:

$\text{Energy Transfer Efficiency} = \frac{\text{Energy at Higher Trophic Level}}{\text{Energy at Lower Trophic Level}} \times 100\%$

• Population Growth (for context):

$\text{Population Growth Rate} = (\text{Births} - \text{Deaths}) + (\text{Immigrants} - \text{Emigrants})$

Additional info: This guide covers the core concepts of Chapter 20 (Communities and Ecosystems) from Campbell Essential Biology, including biodiversity, species interactions, trophic structure, and ecosystem processes. It is suitable for college-level General Biology exam preparation.