Introduction to Ecology and Populations: Key Concepts and Models

Introduction to Ecology and Populations

Ecology: The Study of Home

Ecology is the scientific study of how organisms interact with each other and with their environment. It is a multidisciplinary field that addresses questions about adaptation, species interactions, and the impact of human activities.

• Ecology: Study of relationships between organisms and their environment.

• Key Questions: How do adaptations shape interactions? How do species affect each other? How do human activities influence biodiversity?

• Human Impacts: Human activities can alter ecological processes and biodiversity.

Basic Ecological Concepts

Environmental Factors and Organism Distribution

Organisms live in different habitats, and their distribution is influenced by both biotic and abiotic factors.

• Abiotic Factors: Non-living components such as temperature, water, and nutrients.

• Biotic Factors: Living components including competition, predation, and symbiosis.

• Range of Conditions: Each species has a range of environmental conditions it can tolerate.

• Geographic Distribution: The area where a species is found.

Example: Temperature tolerance curves show how species are distributed according to their optimal temperature range.

Levels of Ecological Study

Hierarchy of Ecological Organization

Ecology can be studied at multiple levels, from individuals to the biosphere.

• Organismal Ecology: Individual organisms and their adaptations.

• Population Ecology: Groups of individuals of the same species.

• Community Ecology: Interactions among different species.

• Ecosystem Ecology: Energy flow and nutrient cycling.

• Biosphere: Global ecological processes.

Populations

Population Ecology

A population is a group of individuals of the same species living in a specific area. Population ecology studies the factors that affect population size, distribution, and structure.

• Population Range: The geographic area where a population occurs.

• Population Structure: The pattern of spacing and demographic makeup.

• Demography: Study of population changes over time.

Population Ranges and Changes Over Time

Population ranges can expand or contract due to environmental changes, human activities, or species interactions.

• Range Expansion: Example - Cattle egret range has expanded.

• Range Contraction: Example - Moose range has contracted.

Dispersion Patterns

Dispersion refers to how individuals are spaced within a population.

• Clumped: Individuals aggregate in patches; common in nature due to resource availability.

• Uniform: Even spacing, often due to territorial behavior or competition.

• Random: Unpredictable spacing; rare in nature.

Demography

Population Growth and Regulation

Demography studies the factors that determine population size and structure, including births, deaths, immigration, and emigration.

• Population Growth: Increase due to births and immigration.

• Population Reduction: Decrease due to deaths and emigration.

Equation for Population Change:

$N_{t+1} = N_t + B - D + I - E$

Where: $N_{t+1}$ = population size at time t+1 $N_t$ = population size at time t $B$ = number of births $D$ = number of deaths $I$ = number of immigrants $E$ = number of emigrants

Determining Population Sizes

Several methods are used to estimate population sizes:

• Line transect

• Quadrat sampling

• Radio or GPS tracking

• Mark and recapture

Population Dynamics

Factors Affecting Population Dynamics

Population dynamics are influenced by life span, mortality, age structure, fecundity, generation time, and age at first reproduction.

• Life Span: Duration of an organism's life.

• Mortality: Rate of death in a population.

• Fecundity: Reproductive rate.

• Generation Time: Time between generations.

Life Tables and Survivorship Curves

Life tables summarize survival and reproductive rates for cohorts. Survivorship curves graphically represent the proportion of individuals surviving at each age.

Type	Description
Type I	High survivorship until old age (e.g., humans)
Type II	Constant rate of survival (e.g., birds)
Type III	High mortality early in life (e.g., oysters)

Life History Strategies

Life history refers to how organisms allocate resources to growth, reproduction, and survival. Trade-offs are common, such as between reproduction and growth.

• Semelparity: Organism reproduces once, then dies.

• Iteroparity: Organism reproduces multiple times during life.

• Trade-offs: Quantity vs. quality of offspring; current vs. future reproduction.

Population Growth Models

Exponential Growth

Exponential growth occurs when resources are unlimited and population size increases rapidly.

Exponential Growth Equation:

$\frac{dN}{dt} = rN$

Where: $N$ = population size $r$ = intrinsic rate of increase

Exponential growth is density-independent and cannot continue indefinitely due to resource limitations.

Logistic Growth

Logistic growth incorporates carrying capacity, the maximum population size an environment can support.

Logistic Growth Equation:

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

Where: $K$ = carrying capacity $N$ = population size $r$ = intrinsic rate of increase

Growth rate slows as population approaches carrying capacity.

Density-Dependent and Density-Independent Factors

Population size is regulated by both density-dependent and density-independent factors.

• Density-Dependent Factors: Effects increase with population density (e.g., competition, disease, predation).

• Density-Independent Factors: Effects are unrelated to population density (e.g., weather, natural disasters).

K-Selected vs. r-Selected Species

K-Selected Species	r-Selected Species
Stable environments, few offspring, high parental care	Unstable environments, many offspring, low parental care
Long life span	Short life span
Type I or II survivorship	Type III survivorship

Population Interactions and Human Populations

Species Interactions

Population dynamics can be affected by interactions with other species, such as predation and competition.

Human Population Growth

Human populations have grown rapidly due to technological advances and improved health care. Age structure diagrams predict future growth and have social and economic implications.

• Demographic Transition: Shift from high birth and death rates to low birth and death rates.

• Age Structure: Distribution of age and sex in a population.

Ecological Footprint

The ecological footprint measures the amount of land and resources required to support an individual's lifestyle. It is a key concept in environmental science and conservation.

• Population size affects resource use and environmental impact.

• Developed countries typically have larger ecological footprints.

• Resource consumption, pollution, and land use are major factors.