17 Ecology and Ecosystems

17.1 Learning Objectives

By the end of this chapter, you should be able to:

Define ecology and distinguish between different levels of ecological organization
Explain how abiotic factors influence the distribution and abundance of organisms
Analyze population growth models and factors regulating population size
Describe different types of species interactions and their ecological consequences
Trace energy flow and nutrient cycling through ecosystems
Explain patterns of biodiversity and factors affecting community structure
Analyze human impacts on ecosystems and conservation strategies
Apply ecological principles to real-world environmental problems

17.2 Introduction

Ecology is the scientific study of interactions between organisms and their environment. This chapter explores how living organisms interact with each other and with their physical surroundings, from individual populations to entire ecosystems. Understanding ecology is essential for addressing pressing environmental challenges such as climate change, biodiversity loss, and sustainable resource management. Ecology connects the biological principles from previous chapters to the larger scale of populations, communities, and ecosystems, showing how energy and information flow through living systems at planetary scales.

17.3 Introduction to Ecology

17.3.1 Defining Ecology

Ecology: Scientific study of interactions between organisms and their environment

Ernst Haeckel (1866): Coined term “ecology” (oikos = house, logos = study)

17.3.2 Levels of Ecological Organization

Organismal ecology: How individuals interact with environment
Population ecology: Dynamics of populations
Community ecology: Interactions among species
Ecosystem ecology: Energy flow and nutrient cycling
Landscape ecology: Patterns across multiple ecosystems
Global ecology: Biosphere-level processes

17.3.3 Ecological Questions

Distribution: Where organisms occur and why

Abundance: How many organisms and why

Interactions: How organisms affect each other

Energy flow: How energy moves through systems

Nutrient cycling: How elements are recycled

17.3.4 Historical Development

Early naturalists: Linnaeus, Darwin, Wallace

20th century: Quantitative approaches, systems thinking

Modern era: Global change ecology, conservation biology

17.4 The Physical Environment

17.4.1 Climate and Weather

Climate: Long-term patterns of temperature, precipitation

Weather: Short-term atmospheric conditions

Factors determining climate: 1. Solar radiation: Latitude, season 2. Atmospheric circulation: Hadley, Ferrel, Polar cells 3. Ocean currents: Heat distribution 4. Topography: Rain shadows, elevation effects 5. Continental position

17.4.2 Biomes

Definition: Major ecological communities determined by climate

Terrestrial biomes:

Tropical forest: High rainfall, high biodiversity
Savanna: Seasonal rainfall, grasses with trees
Desert: Low precipitation, specialized adaptations
Chaparral: Mediterranean climate, fire-adapted
Temperate grassland: Moderate rainfall, fertile soils
Temperate forest: Seasonal, deciduous/coniferous
Boreal forest (taiga): Cold, coniferous
Tundra: Permafrost, low vegetation

Aquatic biomes:

Freshwater: Lakes, rivers, wetlands
Marine: Ocean zones (intertidal, neritic, oceanic, benthic)

17.4.3 Abiotic Factors

Temperature: Affects metabolism, distribution

Water availability: Critical for all life

Sunlight: Energy for photosynthesis, behavior

Wind: Dispersal, evaporation, structure

Soil characteristics: Texture, pH, nutrients

Disturbance: Fire, storms, floods

17.4.4 Physiological Ecology

Adaptations to environment:

Temperature regulation: Endothermy vs. ectothermy
Water balance: Osmoregulation, water conservation
Energy acquisition: Photosynthesis, chemosynthesis

Tolerance ranges: Organisms have limits for each factor

Liebig’s Law of the Minimum: Growth limited by scarcest resource

17.5 Population Ecology

17.5.1 Population Characteristics

Population: Group of same species in same area

Density: Number per unit area

Dispersion: Spatial pattern (clumped, uniform, random)

Demography: Study of population dynamics

17.5.2 Population Growth Models

Exponential growth: Unlimited resources

dN/dt = rN
N: Population size
r: Per capita growth rate
J-shaped curve

Logistic growth: Limited resources

dN/dt = rN(1 - N/K)
K: Carrying capacity
S-shaped (sigmoid) curve

Factors affecting growth:

Density-dependent: Competition, predation, disease
Density-independent: Weather, natural disasters

17.5.3 Life Tables and Survivorship

Life table: Age-specific survival and reproduction

Survivorship curves:

Type I: High survival until old age (humans, large mammals)
Type II: Constant mortality (birds, rodents)
Type III: High early mortality (plants, marine invertebrates)

Reproductive strategies:

r-selected: Many offspring, little care (opportunists)
K-selected: Few offspring, much care (competitors)

17.5.4 Population Regulation

Top-down control: Predation, herbivory

Bottom-up control: Resource limitation

Metapopulations: Linked populations in patchy habitat

Source-sink dynamics: Some patches produce excess individuals

Human population growth:

Current: ∼8 billion
Growth rate: Slowing but still positive
Demographic transition: High birth/death → low birth/death

17.6 Community Ecology

17.6.1 Species Interactions

Competition (-/-):

Intraspecific: Within same species
Interspecific: Between different species
Competitive exclusion: One species outcompetes another
Resource partitioning: Different use of resources
Character displacement: Evolution of differences

Predation (+/-):

True predators: Kill prey immediately
Herbivores: Eat part of plant
Parasites: Live on/in host
Parasitoids: Lay eggs in host, larvae kill host

Defenses against predation:

Cryptic coloration: Camouflage
Aposematic coloration: Warning colors
Mimicry: Batesian (harmless mimics harmful), Müllerian (harmful mimic each other)

Mutualism (+/+):

Obligate: Required for survival
Facultative: Beneficial but not required
Examples: Pollination, mycorrhizae, nitrogen-fixing bacteria

Commensalism (+/0):

One benefits, other unaffected
Examples: Epiphytes on trees, barnacles on whales

Amensalism (0/-):

One harmed, other unaffected
Examples: Allelopathy, trampling

17.6.2 Community Structure

Species richness: Number of species

Species evenness: Relative abundance

Diversity indices: Shannon, Simpson

Keystone species: Disproportionate effect on community

Ecosystem engineers: Modify environment (beavers, corals)

Succession: Change in community composition over time

Primary: On new substrate (rock, lava)
Secondary: After disturbance (fire, clear-cut)
Pioneer species: Early colonists
Climax community: Stable endpoint

Disturbance: Event that changes community

Intermediate disturbance hypothesis: Max diversity at intermediate disturbance

17.6.3 Food Webs and Trophic Structure

Trophic levels:

Primary producers: Autotrophs
Primary consumers: Herbivores
Secondary consumers: Carnivores
Tertiary consumers: Top carnivores
Decomposers/detritivores: Break down dead matter

Energy transfer efficiency: ∼10% between levels

Explanation: Heat loss, incomplete digestion, metabolic costs
Consequences: Limits food chain length (typically 4-5 levels)

Biomass pyramids: Usually upright, but can be inverted (aquatic)

17.7 Ecosystem Ecology

17.7.1 Energy Flow

Primary production: Energy captured by autotrophs

Gross primary production (GPP): Total energy fixed
Net primary production (NPP): GPP minus respiration
Global patterns: Highest in tropics, coastal zones

Secondary production: Energy in consumer biomass

Ecological efficiency: Energy transfer between trophic levels

Productivity measurements:

Biomass: Dry weight per area
Turnover rate: How quickly biomass is replaced

17.7.2 Nutrient Cycling

Biogeochemical cycles: Movement of elements between living and non-living

Reservoirs: Where elements are stored

Fluxes: Movement between reservoirs

Major cycles:

Water cycle: Evaporation, precipitation, transpiration
Carbon cycle: Photosynthesis, respiration, combustion, ocean uptake
Nitrogen cycle: Fixation, nitrification, denitrification, ammonification
Phosphorus cycle: Weathering, sedimentation, no gaseous phase
Sulfur cycle: Volcanic emissions, bacterial transformations

Human impacts: Accelerated cycling, pollution

17.7.3 Ecosystem Services

Categories:

Provisioning: Food, water, timber, fiber
Regulating: Climate, flood control, water purification
Cultural: Recreation, aesthetic, spiritual
Supporting: Nutrient cycling, soil formation, primary production

Economic value: Estimated $125 trillion/year globally

17.8 Landscape and Global Ecology

17.8.1 Landscape Ecology

Study of: Spatial patterns and processes across large areas

Patches: Discrete areas of habitat

Corridors: Strips connecting patches

Matrix: Dominant habitat type

Edge effects: Differences between edge and interior

Habitat fragmentation: Breaking continuous habitat into patches

Consequences: Reduced area, increased edge, isolation

17.8.2 Biogeography

Study of: Geographic distribution of species

Island biogeography theory (MacArthur & Wilson):

Species richness increases with island size
Decreases with distance from mainland
Equilibrium between immigration and extinction

Continental drift: Explains distribution patterns

Dispersal: Movement to new areas

Vicariance: Geographical separation

17.8.3 Global Change Ecology

Climate change:

Causes: Greenhouse gas increase, land use change
Effects: Temperature rise, sea level, precipitation changes, phenology shifts

Biodiversity loss:

Current extinction rate: 100-1000× background
Causes: Habitat loss, climate change, invasive species, overexploitation, pollution

Nitrogen cycle disruption: Fertilizer use → eutrophication

Ocean acidification: CO₂ absorption → pH decrease

17.9 Conservation Biology

17.9.1 Biodiversity Crisis

Estimates: 8.7 million eukaryotic species, 1.5 million described

Threats: Habitat loss (greatest threat), climate change, invasive species

Hotspots: Areas with high endemism and threat

17.9.2 Conservation Strategies

Protected areas: Parks, reserves, marine protected areas

Restoration ecology: Reestablishing degraded ecosystems

Species-focused conservation:

Captive breeding: For reintroduction
Genetic rescue: Increasing genetic diversity
Conservation genetics: Managing small populations

Ecosystem management: Considering whole systems

Sustainable use: Balancing human needs and conservation

17.9.3 Ethical Foundations

Instrumental value: Useful to humans

Intrinsic value: Value independent of human use

Stewardship: Responsibility to care for nature

Intergenerational equity: Future generations’ rights

17.9.4 Success Stories

Bald eagle: DDT ban, habitat protection

California condor: Captive breeding, reintroduction

Gray whale: International protection

Yellowstone wolves: Reintroduction, trophic cascade effects

17.10 Applied Ecology

17.10.1 Resource Management

Fisheries: Maximum sustainable yield, aquaculture

Forestry: Sustainable harvest, certification

Agriculture: Integrated pest management, organic farming

Water resources: Watershed management, wetlands protection

17.10.2 Environmental Problem-Solving

Pollution control: Bioremediation, phytoremediation

Invasive species management: Prevention, control, eradication

Climate change mitigation: Carbon sequestration, renewable energy

Urban ecology: Green infrastructure, sustainable cities

17.10.3 Ecological Economics

Valuing ecosystem services

Cost-benefit analysis of environmental policies

Payment for ecosystem services (PES): Compensating conservation

17.10.4 Citizen Science and Education

Public participation in data collection

Environmental education: Building ecological literacy

Policy engagement: Science-informed decision making

17.11 Chapter Summary

17.11.1 Key Concepts

Ecology: Study of organism-environment interactions at multiple scales
Physical environment: Climate, biomes, abiotic factors influence distributions
Population ecology: Growth models, regulation, life history strategies
Community ecology: Species interactions, diversity, succession
Ecosystem ecology: Energy flow, nutrient cycling, productivity
Landscape/global ecology: Spatial patterns, biogeography, global change
Conservation biology: Biodiversity protection, sustainable management
Applied ecology: Solving environmental problems, managing resources

17.11.2 Population Growth Models

Model	Equation	Assumptions	Pattern
Exponential	dN/dt = rN	Unlimited resources	J-shaped
Logistic	dN/dt = rN(1-N/K)	Limited resources	S-shaped

17.11.3 Species Interactions

Type	Effect on Species 1	Effect on Species 2	Example
Competition	-	-	Lions and hyenas
Predation	+	-	Wolf and deer
Mutualism	+	+	Bee and flower
Commensalism	+	0	Barnacle and whale
Amensalism	0	-	Walnut tree and other plants

17.11.4 Energy Transfer Efficiency

Trophic Level	Energy Available (kcal/m²/yr)	% Transfer
Producers	20,000	-
Primary consumers	2,000	10%
Secondary consumers	200	10%
Tertiary consumers	20	10%

17.11.5 Major Biogeochemical Cycles

Cycle	Key Processes	Human Impacts
Carbon	Photosynthesis, respiration, combustion	CO₂ increase, climate change
Nitrogen	Fixation, nitrification, denitrification	Fertilizer runoff, eutrophication
Phosphorus	Weathering, sedimentation	Mining, fertilizer use
Water	Evaporation, precipitation, transpiration	Dams, irrigation, climate change

17.11.6 Conservation Status Categories (IUCN)

Category	Criteria	Example
Extinct	No individuals remain	Dodo
Critically Endangered	Extreme risk of extinction	Sumatran rhino
Endangered	Very high risk	Tiger
Vulnerable	High risk	Polar bear
Near Threatened	Close to qualifying	Emperor penguin
Least Concern	Widespread, abundant	House sparrow

17.12 Review Questions

17.12.1 Level 1: Recall and Understanding

Define ecology and list the different levels of ecological organization.
What are the main abiotic factors that influence the distribution of organisms?
Compare exponential and logistic population growth models.
List and define five types of species interactions.
What is the 10% rule in energy transfer between trophic levels?

17.12.2 Level 2: Application and Analysis

A population of 1000 birds has a per capita growth rate of 0.1 per year. What will be the population size after one year if resources are unlimited? If carrying capacity is 2000?
Explain why food chains are typically limited to 4-5 trophic levels.
How does the intermediate disturbance hypothesis explain patterns of species diversity?
Trace a carbon atom from the atmosphere through photosynthesis, consumption, respiration, and back to the atmosphere.
Why are invasive species often so successful in new environments?

17.12.3 Level 3: Synthesis and Evaluation

Evaluate the statement: “Human population growth is the root cause of all environmental problems.”
How does island biogeography theory inform conservation planning for habitat fragments?
Design an experiment to test the competitive exclusion principle using two species of microorganisms.
Propose a comprehensive conservation strategy for an endangered species considering both ecological and socioeconomic factors.

17.13 Key Terms

Ecology: Scientific study of interactions between organisms and their environment
Population: Group of individuals of the same species living in the same area
Community: All populations of different species living in a particular area
Ecosystem: Community of organisms plus their physical environment
Biome: Major ecological community type determined by climate
Carrying capacity (K): Maximum population size an environment can sustain
Competition: Interaction between organisms for limited resources
Predation: Interaction where one organism kills and eats another
Mutualism: Interaction where both species benefit
Succession: Process of community change over time
Food web: Network of feeding relationships in an ecosystem
Trophic level: Position in a food chain determined by number of energy transfers
Primary production: Rate at which energy is converted to organic compounds
Biogeochemical cycle: Movement of elements between living and non-living components
Biodiversity: Variety of life at all levels of organization
Conservation biology: Scientific study of biodiversity protection

17.14 Further Reading

17.14.1 Books

Begon, M., Townsend, C. R., & Harper, J. L. (2014). Ecology: From Individuals to Ecosystems (5th ed.). Wiley-Blackwell.
Odum, E. P., & Barrett, G. W. (2005). Fundamentals of Ecology (5th ed.). Brooks/Cole.
Wilson, E. O. (1992). The Diversity of Life. Harvard University Press.

17.14.2 Scientific Articles

Hutchinson, G. E. (1957). Concluding remarks. Cold Spring Harbor Symposia on Quantitative Biology, 22, 415-427.
MacArthur, R. H., & Wilson, E. O. (1967). The Theory of Island Biogeography. Princeton University Press.
Vitousek, P. M., et al. (1997). Human domination of Earth’s ecosystems. Science, 277(5325), 494-499.

17.14.3 Online Resources

Encyclopedia of Life: https://eol.org
IUCN Red List: https://www.iucnredlist.org
NASA Earth Observatory: https://earthobservatory.nasa.gov
Ecological Society of America: https://www.esa.org

17.15 Quantitative Problems

Population Growth: A population of rabbits starts with 100 individuals and has an intrinsic growth rate r = 0.5 per year.
1. Calculate population size after 5 years with exponential growth.
2. If carrying capacity K = 1000, calculate population size after 5 years with logistic growth.
3. At what population size is growth rate maximized in logistic model?
Energy Flow: An ecosystem has primary production of 10,000 kcal/m²/yr.
1. How much energy is available to primary consumers? Secondary consumers?
2. If humans eat at the secondary consumer level, how many people could be supported per km² if each needs 2000 kcal/day?
3. What if humans eat at the primary consumer level?
Island Biogeography: For an island 100 km from mainland, immigration rate I = 10 - 0.1S, extinction rate E = 0.05S.
1. What is equilibrium species richness?
2. If island area doubles, extinction becomes E = 0.025S. New equilibrium?
3. If a bridge is built, immigration becomes I = 15 - 0.1S. New equilibrium?

17.16 Case Study: Yellowstone Wolf Reintroduction

Background: Wolves were reintroduced to Yellowstone National Park in 1995 after being absent for 70 years.

Questions:

How did wolf reintroduction trigger a trophic cascade?
What changes occurred in elk behavior and vegetation?
How did other species (beavers, songbirds, scavengers) benefit?
What does this case study teach us about ecosystem management?
What are the socioeconomic implications (tourism, livestock)?

Data for analysis:

Wolf population: From 0 to ∼100 in 20 years
Elk population: Decreased from ∼20,000 to ∼5,000
Aspen recruitment: Increased where wolves present
Beaver colonies: Increased from 1 to 12
Scavengers: More stable food supply from wolf kills

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