Population Ecology Overview

Population Ecology

• Population ecology studies the distribution and abundance of organisms, and how and why populations change over time.
• It aims to understand spatial and temporal patterns in population abundance and distribution.
• Population ecology is the science of population dynamics in space and time.
• Population = total number of individuals of a given biological species found in one place at one time.

Describing Populations

• Population size: the total number of individuals.
• Population density: the number of individuals per unit area.
• Population dispersion: how individuals are arranged spatially relative to one another (e.g., clumped, uniform, random).
• Population occupancy: whether a species or population member is present in a given habitat.
• Population distribution: where a population occurs in space.
• Geographic range: the furthest geographic limits of a species' occurrence.

Dispersion Patterns

• Dispersion patterns, also known as distribution patterns, summarize the spatial relationships between population members within a habitat at a specific time.
• Clumped dispersion: most common, often due to resource availability, social interaction, or mating behaviors.
• Uniform dispersion: less common, usually due to competition or territoriality.
• Random dispersion: rare, with individuals distributed independently of each other.

Population Growth

• Population growth is affected by births, deaths, immigration, and emigration.
• Growth rate = (Birth rate + Immigration rate) - (Death rate + Emigration rate).
• Ignoring immigration and emigration, population growth is primarily influenced by birth and death rates.
• Births and immigration increase population size.
• Deaths and emigration decrease population size.

Age Structure

• Age structure is the relative numbers of organisms of each age within a population.
• Age determination is done using annual growth rings, scales in fishes, and horn and teeth structure in some organisms.

Age Structure and Population Size

• Wide base in age structure diagrams indicates many young, high reproduction, and rapid population growth.
• Even age distribution indicates births equal deaths, creating a stable population.

Population Growth: Exponential Growth

• Exponential growth occurs when a population's growth rate remains constant.
• A population increases by a fixed percentage of a large number.
• A fixed percent of a large number produces a large increase.
• Not sustainable indefinitely.
• Occurs in nature in small populations with optimal conditions.
• Does not typically last long.

Population Growth: Logistic/Sigmoid Growth

• Logistic growth occurs when density-dependent limiting factors cause population growth to slow before reaching a maximum level.
• Growth levels off, becoming stable.
• Limiting factors are physical, chemical, and biological characteristics that restrain growth. Examples include water, space, food, predators, and disease.
• Carrying capacity: maximum population size a species can sustain in its environment.

Population Regulation

• Population regulation refers to the ecological processes (biotic and abiotic factors) that limit the population growth from birth and death rates.
• Limiting factors are categorized into density-dependent and density-independent factors.

Density-Dependent Limiting Factors

• Impact populations based on their density.
• These factors will usually cause the growth rate to drop as the population gets larger.
• These factors cause population size to level off at the carrying capacity.
• This is known as logistic growth.
• Examples include intraspecific competition, interspecific competition, increased disease, parasitism, and predation.

Density-Independent Limiting Factors

• Impact a population's growth rate regardless of the population's density.
• Their impact is often catastrophic to smaller populations with limited geographic ranges.
• Examples include natural disasters (e.g., forest fires).

Top-Down Population Regulation

• Higher trophic levels control lower trophic levels.
• Apex predators affect a population.
• Examples include mountain lions and mule deer.

Bottom-Up Population Regulation

• The population is dependent on the resources of an ecosystem.
• Factors such as vegetation and food sources influence populations.
• Examples include plant die-off leading to starvation impacting other species.

Population Regulation in Humans

• Exponential population growth has occurred due to technology development, abundance of food, medicine, increased accessibility to contraception, etc
• Density-dependent factors such as climate change, widespread resources depletion, and disease are outpacing the increase in carrying capacity.

Life History Strategies

• Life history is the series of events that characterize an organism's life cycle.
• Factors influencing life history include natural selection, competition for resources, and environmental conditions.
• Organisms make trade-offs between their individual survival and reproduction.

Life History Traits

• Traits include size, growth rate, age at reproduction, duration of possible reproduction, number of offspring per reproductive event, and lifespan.

Survivorship Curves

• Graphic representations of life table data showing the number of individuals surviving at each stage of an organism's life.
• Three types: Type I (low mortality in early life, high in older age), Type II (constant mortality rate throughout life), and Type III (high mortality in early life, low in older age).

Life History Strategies: r-strategists and k-strategists

• R-strategists: reproduce quickly with many offspring, have little parental care, and fast maturation. Examples include insects and mice.
• K-strategists: reproduce slowly, have fewer offspring, invest heavily in parental care, and have slow maturation times. Examples include elephants and whales.

Population Size Estimate

• Complete census: counting all individuals in a population
• Population size estimation methods: using models, capture-recapture, camera trapping, quadrat sampling
• Population index: using data correlated with actual abundance to estimate population size. Examples include vocalizations, fecal pellets, and animal tracks.

Population Density

• The number of individuals per unit area.
• It's a measure of relative abundance.
• Can be converted to rough population size estimates.
• Larger organisms usually have lower population densities due to increased resource needs.