Ecology, Ecosystems, and Biodiversity: Study Notes

Population Ecology

Introduction to Population Ecology

Population ecology examines the factors that affect population size, distribution, and dynamics over time. It is a key field for understanding how species interact with their environment and with each other.

• Population: A group of individuals of the same species living in the same area 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, including births, deaths, immigration, and 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 older individuals (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 exist between fecundity, growth rate, age at maturation, number of offspring, parental investment, and body size.

• Organisms allocate limited resources to maximize fitness in their environment.

Population Growth Models

• Growth Rate (r): The per capita rate of increase. If r = 0, population is stable; r > 0, population increases; r < 0, population decreases. rmax is the maximum possible growth rate.

• Exponential Growth: Density-independent, J-shaped curve. Occurs when resources are unlimited.

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

$\frac{dN}{dt} = rN$ (Exponential growth)

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

Population Regulation

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

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

Human Population Dynamics

• Population structure varies by economy and development.

• 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, increasing extinction risk.

Community Ecology

Species Interactions

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

• Competition (-/-): Both species are harmed by shared resource use.

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

• Mutualism (+/+): Both species benefit.

• Interactions are conditional and context-dependent.

Competition Types

• Intraspecific: Competition within a species.

• Interspecific: Competition between species.

• Exploitative: Indirect competition via resource depletion.

• Interference: Direct interactions (e.g., aggression, territoriality).

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.

Defense Mechanisms and Mimicry

• Passive Defenses: Cryptic coloration (camouflage).

• Escape: Rapid movement or flight.

• Chemical Defenses: Toxins or repellents.

• Schooling: Safety in numbers.

• Mechanical Defenses: Physical structures (spines, shells).

• Intimidation: Displays to deter predators.

• Batesian Mimicry: Harmless species mimics harmful one.

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

Keystone Species and Trophic Cascades

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

• Trophic Cascade: Effects that spread across multiple trophic levels due to changes in keystone species.

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: Degree to which one or few species predominate.

• 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: Variation within species.

• Species Diversity: Number and relative abundance of species.

• Ecosystem Diversity: Variety of ecosystems in a region.

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

• Key factors: temperature, precipitation, sunlight, wind, soil, site history.

• Temperature and precipitation are most important for determining organism distribution.

Climate and Biomes

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

• Precipitation is influenced by atmospheric circulation (Hadley cells), rain shadows, and ocean currents.

• Terrestrial biomes are defined by dominant vegetation, temperature, and precipitation.

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

Aquatic Biomes

• Determined by salinity, depth, flow, and nutrient content (upwellings, turnover).

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 from lack of fire and 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 energy used in plant respiration.

Food Chains and Food Webs

• Food Chain: Linear sequence of energy transfer.

• Food Web: Complex network of feeding relationships.

• Aboveground food chains 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, water, organisms.

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

• Human impact: fertilizer use, fossil fuel combustion, industrial fixation.

Dead Zones

• Areas of low oxygen in aquatic systems due to nutrient loading (mainly nitrogen and phosphorus).

• Sources: agricultural runoff, sewage.

• Consequences: fish kills, loss of biodiversity.

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

Phosphorus Cycle

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

• Processes: weathering, uptake by plants, decomposition.

• No significant gaseous phase; considered a local cycle.

• Human impact: mining, fertilizer use.

• Peak Phosphorus: Point at which maximum global phosphorus production is reached.

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, ocean acidification.

Reducing Carbon Footprint

• Use renewable energy, reduce energy consumption, eat less meat, use public transport, plant trees.

Biodiversity and Conservation

Threats to Biodiversity

• Human activities are causing the 6th mass extinction.

• Main threats: habitat destruction, introduced species, over-harvesting, 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

• Provisioning: Food, water, raw materials.

• Regulation: Climate, disease, water purification.

• Cultural: Recreation, spiritual value.

• Pollination and Pest Control: Essential for agriculture.

• Ethical considerations for conservation.

Conservation Strategies

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

• Individual actions: citizen science, reducing resource use, supporting conservation efforts.

Regulatory Successes

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

Summary Table: Key Ecological Concepts

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
Type I Survivorship	High juvenile survival, late mortality	Humans
Mutualism	Both species benefit (+/+)	Bees and flowering plants
Primary Succession	Succession on new surfaces (no soil)	After volcanic eruption
Biomagnification	Pollutant concentration increases up food chain	DDT in birds of prey
Provisioning Service	Direct products from ecosystems	Food, timber