Population Ecology Overview

Population Ecology

• Population ecology studies factors influencing population size and change over time.
• Key factors impacting population size include births, deaths, immigration, and emigration.
• Over time, populations may stabilize, fluctuate irregularly, or experience cyclical changes.

Defining a Population

• A population is a group of individuals of the same species living within a specific area.
• Populations are defined by boundaries, which can be natural (e.g., a lake) or arbitrary (e.g., a county).
• Population size is determined by the number of individuals in the group.

Populations Interact

• Individuals within populations reproduce and compete for resources.
• Intra-specific competition occurs between individuals of the same species.
• Inter-specific competition occurs between individuals of different species competing for the same resources.

Population Concepts

• Populations have structure, which includes age and sex structure.
• Populations are regulated over time.
• Populations are dynamic over time.

Population Structure

• Population age and sex structure is visualized using a population pyramid.
• Births add individuals to the base of the pyramid, while deaths remove individuals at all ages after birth.

Age-Related Survivorship Patterns

• Survivorship patterns differ among species and can be categorized into Type I, Type II, and Type III.
• Type I: High mortality in older age classes, common in long-lived species in stable environments.
• Type II: Constant mortality throughout life, observed in species with relatively stable environments.
• Type III: High mortality in juveniles, common in short-lived species in unpredictable environments.

Spatial Structure of Populations

• Population dispersion (spatial arrangement) is shaped by the ecology of the species.
• Random dispersion: position of individuals is independent of others, occurs due to the absence of strong competition or uniform distribution of resources.
• Uniform dispersion: individuals are evenly spaced, common in species with territorial behavior or competition for resources.
• Clumped dispersion: individuals aggregate in patches, occurs in areas with high resource availability or suitable conditions required for mating, predation, or predator avoidance.

Population Regulation

• Populations increase (or decrease) depending on the rate of change of each individual at each moment in time.
• Exponential population growth occurs in the absence of limiting factors; populations grow by a constant proportion at each moment.
• Exponential population growth can be described as dN/dt=rN.
  • r = per capita rate of increase(instantaneous)
  • N = population size
  • dN/dt = change in population over time.
• The logistic population growth model accounts for limitations of resources or carrying capacity (K) that influence population growth.
• In the logistic growth model, population growth approaches zero as the population size approaches K – The logistic equation describes this as:
  • dN/dt=rN(1-(N/K))
    • K = carrying capacity – As population size (N) increases, per capita growth rate rN decreases.

Density Dependent Factors-

• Influence population growth rate depending on density or population size and regulate the population in the environment.
  • Competition for resources, territoriality, disease, toxic wastes, and intrinsic factors.

Density Independent Factors

• Influence population growth without regard to population size and are often related to climate events or natural disasters.
  • These factors may also cause changes over time.

Summary of Population Regulation

• Both density-dependent factors (influence by population size) and density-independent factors (influence regardless of size) influence population regulation.
• Different species exhibit varying regulation patterns.
• Density-dependent factors often limit population growth and prevent overpopulation.

Population Dynamics

• Populations experience fluctuations and cycles with respect to time.
• Various factors, including food availability, diseases, predation, and competition, influence population cycles.

Summary

• Population ecology studies populations, which are groups of individuals of the same species living in the same general area and described by their boundaries and size.
• Populations have structure, can be regulated, and experience dynamics.