Ecology, Ecosystems, and Biodiversity: Study Guide

Population Ecology

Introduction to Population Ecology

Population ecology examines the factors that affect the distribution, abundance, and dynamics of populations—groups of individuals of the same species living in a defined area at a specific time. It explores how populations interact with their environment and with other species.

• Population: A group of individuals of the same species in the same place at the same time. Boundaries are defined by the researcher.

• Population Attributes: Size (N), density (N/area), dispersion, genetic characteristics, demography, and fertility.

Population Dispersion Patterns

• Uniform: Individuals are evenly spaced due to territoriality or competition.

• Clumped: Individuals aggregate in patches, often around resources.

• Random: Position of each individual is independent of others.

Demography and Life Tables

• Demography: Study of factors that determine the size and structure of populations over time (births, deaths, immigration, emigration).

• Life Tables: Summarize survival and reproductive rates of individuals in specific age groups.

• Cohort: Group of individuals of the same age tracked over time.

• Generation Time: Average time between the birth of individuals and the birth of their offspring.

Survivorship Curves

• Type I: High juvenile survival, most mortality occurs in old age (e.g., humans).

• Type II: Constant mortality rate throughout life (e.g., birds).

• Type III: High juvenile mortality, few survive to adulthood (e.g., many fish, plants).

Fitness Trade-Offs

• Trade-offs between fecundity, growth rate, maturation age, number of offspring, parental investment, and body size.

• Example: Species with high fecundity often have low parental investment per offspring.

Population Growth Models

• Growth Rate (r): If $r = 0$, population is stable; $r > 0$, population increases; $r < 0$, population decreases. $r_{max}$ is the maximum growth rate.

• Exponential Growth: Density-independent, J-shaped curve. Population grows without limits.

• Logistic Growth: Density-dependent, S-shaped curve. Growth slows as population approaches carrying capacity (K).

Exponential Growth Equation: $\frac{dN}{dt} = rN$

Logistic Growth Equation: $\frac{dN}{dt} = rN \left(1 - \frac{N}{K}\right)$

Population Regulation

• Density-Independent Factors: Affect populations regardless of size (e.g., climate, natural disasters).

• Density-Dependent Factors: Intensify as population increases (e.g., competition, disease, predation, waste accumulation, social behavior).

Human Population Dynamics

• Population structure varies by economy (developed vs. developing countries).

• Zero Population Growth (ZPG): When birth rate equals death rate.

• Ecological Footprint: Measure of human demand on Earth's ecosystems.

• Carrying Capacity: Maximum population size an environment can sustain.

Extinction Vortex

• Reduced genetic diversity leads to lower offspring viability, smaller populations, and further loss of genetic diversity—a downward spiral toward extinction.

Community Ecology

Species Interactions

• Intraspecific Competition: Competition among individuals of the same species.

• Interspecific Competition: Competition between different species.

• Exploitive Competition: Indirect competition through resource use.

• Interference Competition: Direct interactions, such as fighting for resources.

Types of Species Interactions

• Commensalism (0/+): One benefits, other unaffected.

• Competition (-/-): Both species are harmed.

• Consumption (+/-): One benefits, other is harmed (includes predation, herbivory, parasitism).

• Mutualism (+/+): Both benefit.

• These interactions are conditional and context-dependent.

Niches and Niche Exclusion

• Niche: The range of resources and conditions a species uses and tolerates.

• Fundamental Niche: Full range of conditions a species can use.

• Realized Niche: Actual range used, limited by competition.

• Competitive Exclusion Principle: Two species cannot occupy the same niche indefinitely.

Prey Defense Mechanisms

• Passive: Cryptic coloration (camouflage).

• Escape: Rapid movement or flight.

• Chemical: Toxins or repellents.

• Schooling: Safety in numbers.

• Mechanical: Physical defenses (spines, shells).

• Intimidation: Startle displays or mimicry.

Mimicry

• Batesian Mimicry: Harmless species mimics a harmful one.

• Müllerian Mimicry: Two or more harmful species resemble each other.

Keystone Species and Trophic Cascades

• Keystone Species: Species with a disproportionately large effect on community structure (e.g., sea otters).

• Trophic Cascade: Effects that ripple through trophic levels when a keystone species is removed or added.

Disturbance and Succession

• Disturbance: Disruption of community structure (e.g., fire, storm).

• Succession: Recovery after disturbance.

• Primary Succession: Occurs on newly exposed surfaces (no soil).

• Secondary Succession: Occurs where soil remains after disturbance.

Species Diversity

• Richness: Number of species present.

• Dominance: Most abundant species.

• Evenness: Relative abundance of species (measured by Shannon-Weaver Index).

Island Biogeography

• Biodiversity increases with island size and decreases with distance from mainland.

• Concept applies to terrestrial habitat fragments as well.

Levels of Biodiversity

• Genetic Diversity

• Species Diversity

• Ecosystem Diversity

Biodiversity Patterns

• Estimated ~30 million extant species.

• Endemic Species: Found only in a specific location.

• Biodiversity Hotspots: Areas with high levels of endemic species and threat.

• Biodiversity is generally higher at the equator and varies with geography.

Ecosystem Ecology

Abiotic Constraints

• Temperature, precipitation, sunlight, wind, soil, and site history.

• Temperature and precipitation are most important.

Climate and Biomes

• Temperature varies with latitude (solar input) and season (Earth's tilt).

• Precipitation: Warm air rises, cools, and drops water. Influenced by Hadley cells, rain shadows, and ocean currents.

• Terrestrial biomes determined by temperature and precipitation; characterized by dominant vegetation.

• Terrestrial biomes: 22% of Earth's surface; aquatic biomes: 78%.

• Aquatic biomes determined by salinity, depth, flow, and nutrients (upwellings, turnover).

Example: Tallgrass Prairie (Iowa Biome)

• Hot summers, cold winters, moderate precipitation.

• Grass-dominated; over 50% of biomass belowground, contributing to fertile soils.

• Endangered due to agriculture (99% converted) and woody encroachment (lack of fire, large herbivores).

Ecosystem Size and Energy Flow

• Ecosystem size is variable and defined by the researcher.

• Energy flows through ecosystems; nutrients cycle.

• Solar energy → Gross Primary Productivity (GPP, ~1%) → Net Primary Productivity (NPP, ~45% of GPP).

• NPP excludes plant respiration.

Food Chains and Food Webs

• Food Chain: Linear sequence of energy transfer.

• Food Web: Complex network of feeding relationships.

• Aboveground vs. decomposer food chains.

Energy Transfer Efficiency

• Only ~10% of biomass is transferred to the next trophic level.

• Efficiency varies by age, metabolism, and organism type (higher in young and ectotherms).

Biomagnification

• Pollutants become more concentrated at higher trophic levels.

Nutrient Cycling

Nitrogen Cycle

• Major reservoirs: atmosphere, soil, organisms.

• Processes: nitrogen fixation, nitrification, assimilation, ammonification, denitrification.

• Human impact: fertilizer use, fossil fuel combustion.

Dead Zones

• Caused by nutrient loading (mainly nitrogen and phosphorus) leading to oxygen depletion in aquatic systems.

• Remedies: reduce fertilizer use, restore wetlands, improve wastewater treatment.

Phosphorus Cycle

• Major reservoirs: rocks, soil, water, organisms.

• No gaseous phase; considered a local cycle.

• Human impact: mining, fertilizer use.

• "Peak phosphorus": point at which phosphorus production reaches maximum rate.

Carbon Cycle

• Major reservoirs: atmosphere, oceans, terrestrial biomass, fossil fuels.

• Processes: photosynthesis, respiration, decomposition, combustion.

• Human impact: fossil fuel burning, deforestation.

• Consequences: global climate change.

Reducing Carbon Footprint

• Use renewable energy, reduce energy consumption, support sustainable practices.

Biodiversity and Conservation

Threats to Biodiversity

• Human activities are causing the 6th mass extinction.

• Major threats: habitat destruction, introduced species, overharvesting, pollution, climate change.

Ecosystem Benefits of Biodiversity

• Resource Use Efficiency: Diverse communities use resources more efficiently.

• Facilitation: Some species enhance the survival of others.

• Resilience: Ability to recover from disturbance.

• Resistance: Ability to withstand disturbance.

Ecosystem Services Provided by Biodiversity

• Provisioning: Food, water, raw materials.

• Regulation: Climate, disease, water purification.

• Cultural: Recreation, spiritual value.

• Pollination and Pest Control

• Ethical considerations for conservation.

Conservation Strategies

• Education, ex situ conservation (zoos, seed banks), genetic intervention, corridors, restoration, reserves.

• Citizen science and individual actions can contribute to conservation.

Regulatory Successes

• Examples: DDT ban (US), ozone hole recovery, removal of lead from gasoline, reduction of acid rain.

Summary Table: Key Concepts in Ecology

Concept	Definition	Example
Population	Group of same species in same place/time	All deer in a forest in 2020
Carrying Capacity (K)	Maximum population size environment can sustain	Number of wolves a park can support
Keystone Species	Species with large effect on community	Sea otters in kelp forests
Primary Succession	Succession on new surfaces (no soil)	After volcanic eruption
Biomagnification	Increase in pollutant concentration at higher trophic levels	DDT in birds of prey
Biodiversity Hotspot	Area with high endemic species and threat	Madagascar

Additional info: Some explanations and examples were expanded for clarity and completeness based on standard ecology textbooks.