Population Ecology: Structure, Dynamics, and Growth

Population Ecology

Introduction to Ecology and Biodiversity

Ecology is the scientific study of how organisms interact with each other and their environment. These interactions occur at multiple scales and are fundamental to understanding the diversity of life on Earth, known as biodiversity. Biodiversity results from evolutionary processes shaped by ecological contexts across time and space.

• Ecology: Study of interactions among organisms and their environment.

• Biodiversity: The variety of life, shaped by evolutionary and ecological processes.

• Ecology and evolution are tightly linked, as ecological interactions drive evolutionary change.

Levels of Ecological Study

Scales of Study in Ecology

Ecologists investigate biological systems at several hierarchical levels, each with distinct questions and methods:

• Organismal Ecology: Focuses on the adaptations (morphological, physiological, behavioral) that enable individuals to survive and reproduce in their environment. Example: Salmon migration between saltwater and freshwater environments.

• Population Ecology: Examines changes in the number and distribution of individuals of the same species over time. Example: Number of eggs produced by salmon, geographic range of a fish species.

• Community Ecology: Studies interactions among populations of different species in the same area. Example: Competition for food between salmon and other aquatic organisms.

• Ecosystem Ecology: Investigates how organisms interact with abiotic (non-living) components of their environment. Example: Decomposition rates of salmon in streams, nutrient cycling.

• Global Ecology: Explores processes affecting the biosphere, including all ecosystems and the lower atmosphere. Example: Effects of climate change on salmon habitats worldwide.

These levels are integrated in Conservation Biology, which aims to study, preserve, and restore genetic, species, and ecosystem diversity.

Biodiversity: Ecological and Evolutionary Scales

• Genetic Diversity: Number and frequency of alleles in populations; informs about adaptive capacity.

• Species Diversity: Richness and evenness of species in a community; relates to ecosystem stability and function.

• Ecosystem Diversity: Variation in species and functional roles, and their interactions with abiotic factors.

Population Ecology: Structure and Dynamics

Definition of Population

A population is a group of organisms of the same species living in the same area at the same time.

Population Ecology

• Studies how and why the number of individuals in populations changes over space and time.

• Integrates organismal traits and adaptations to predict population responses to environmental change.

• Links to evolutionary mechanisms by identifying traits that confer higher fitness.

Distribution and Abundance

Key questions in population ecology include where a species is found (distribution) and how many individuals are present (abundance).

• Distributions are often mapped as geographic ranges, which are dynamic and influenced by abiotic (e.g., temperature, precipitation, elevation) and biotic (e.g., competition, predation) factors.

• Population density (number of individuals per unit area or volume) varies across space and time.

Spatial Distribution Patterns

Spatial Distribution	Proximate Explanation	Ultimate Explanation	Example
Random	Dispersal of seeds, gametes, or larvae; resources are not clustered	Average fitness is not influenced by interactions	Dandelions are randomly distributed by wind
Clumped	Individuals associate in social groups or where resources are abundant	Fitness increases in groups	Fish schooling, plants growing in patches
Uniform	Individuals distance themselves evenly	Competition for resources reduces fitness if too close	Penguins nesting, territorial animals

Population Demography and Life-History

Population Change: Births, Deaths, Immigration, Emigration

• Population size increases via births and immigration (arrival of new individuals).

• Population size decreases via deaths and emigration (departure of individuals).

Demography and Life Tables

• Demography: Study of factors that determine the size and structure of populations over time.

• Key data: Age structure and generation time, summarized in life tables.

• Life tables: Summarize the probability of survival and reproduction for individuals at each age interval.

• Constructed by monitoring individuals or surveying age structure at intervals.

Life Table Example

Age Class (x)	Number of Survivors (lx)	Survivorship (lx)	Age-Specific Fecundity (mx)	Average Births/Female
0	100	1.0	0.0	0.0
1	80	0.8	1.2	0.96
2	50	0.5	2.0	1.0
3	10	0.1	3.5	0.35

Additional info: Table values are illustrative; actual values depend on species and study.

Survivorship Curves

• Graphical representations of the number of survivors at each age.

• Three general types:

  • Type I: High survivorship until old age (e.g., humans).

  • Type II: Steady survivorship throughout life (e.g., birds).

  • Type III: Low survivorship early in life (e.g., many fish, plants).

Life-History Trade-Offs

• Populations differ in life table elements: some have high fecundity but low survivorship, others the opposite.

• Individuals cannot maximize both survivorship and fecundity due to fitness trade-offs (limited resources, time, energy).

• Life-history traits: Strategies for allocating resources to growth, reproduction, and survival; vary among species and populations.

• r-selected species: High fecundity, low survivorship (e.g., many offspring, little parental care).

• K-selected species: Low fecundity, high survivorship (e.g., few offspring, high parental care).

Population Growth: Exponential vs. Logistic

Population Growth Models

Population growth is determined by births, deaths, immigration, and emigration, but models often focus on births and deaths.

• Per capita rate of increase (r): Difference between birth rate (b) and death rate (d) per individual.

Equations:

• Population growth rate: $\frac{\Delta N}{\Delta t} = (b - d)N$

• Or, using per capita rate of increase: $\frac{\Delta N}{\Delta t} = rN$

If $b > d$, then $r > 0$ (population grows). If $b < d$, then $r < 0$ (population declines). If $r = 0$, population size is stable.

Exponential Growth

• Occurs when $r$ does not change over time and resources are unlimited.

• Equation: $\frac{dN}{dt} = rN$

• Growth accelerates as $N$ increases (instantaneous rate of increase).

• Under optimal conditions, $r$ reaches a maximum value, $r_{max}$ (intrinsic rate of increase).

• Exponential growth is density independent: population size does not limit growth.

• Occurs during colonization of new habitats or recovery after disturbance.

Logistic Growth and Carrying Capacity

• In nature, resources become limited as population density increases.

• Carrying capacity (K): Maximum number of individuals a habitat can support over time.

• As $N$ approaches $K$, growth slows due to increased competition, disease, etc.

• Equation for logistic growth: $\frac{dN}{dt} = rN \left(\frac{K-N}{K}\right)$

• When $N$ is small, growth is nearly exponential. As $N$ approaches $K$, growth rate declines to zero.

• Logistic growth is density dependent: growth rate depends on population size.

Limiting Factors in Population Growth

• Density-independent factors: Affect birth and death rates regardless of population size (e.g., weather, natural disasters).

• Density-dependent factors: Change in intensity as population size changes (e.g., competition, predation, disease).

Human Population Growth

• Human populations are subject to the same ecological principles as other species.

• Carrying capacity for humans is influenced by technology, resource use, and environmental impact.

Additional info: For more details, refer to textbook sections 49.1 and 51.3-51.5.