Population Distribution and Abundance (Population Ecology: Chapter 9)

Population Distribution and Abundance

Introduction to Population Ecology

Population ecology examines the distribution and abundance of organisms, focusing on where populations live, how many individuals they contain, and how these numbers change over time. Understanding these patterns is essential for fields such as epidemiology, conservation biology, and resource management.

• Population: A group of individuals of a single species inhabiting a specific area.

• Populations are characterized by their size (number of individuals) and density (individuals per unit area).

• Other important characteristics include age distribution, growth rates, and spatial distribution.

Distribution Limits

Factors Limiting Geographic Distribution

The geographic distribution of a species is determined by a combination of physical and biological factors. These factors set the boundaries within which populations can persist.

• Physical environment: Includes temperature, moisture, soil type, and other abiotic factors. Organisms can only compensate so much for environmental variation.

• Biological factors: Include interactions with other species such as competition, predation, and disease.

• Limiting factor: Any factor that restricts the presence, growth, or abundance of an organism in an environment.

Niches

Definition and Types of Niches

The concept of the niche summarizes the range of environmental conditions and resources within which a species can survive, grow, and reproduce.

• Niche: The sum of all biotic (living) and abiotic (non-living) factors that influence a species' survival and reproduction.

• Fundamental niche: The full range of environmental conditions under which a species can survive and reproduce, in the absence of competitors and predators.

• Realized niche: The actual conditions under which a species exists, which are often restricted by interactions such as competition.

Distribution Patterns

Small-Scale Patterns

On small spatial scales, individuals within a population may be distributed in different patterns, depending on resource availability and interactions among individuals.

• Random: Each individual has an equal probability of occurring anywhere. Resources are evenly distributed, and interactions are neutral.

• Regular (Uniform): Individuals are uniformly spaced, often due to negative interactions such as competition or territoriality. Resources are evenly distributed.

• Clumped: Individuals are found in groups or patches, often due to patchy resource distribution or positive interactions (e.g., social behavior, mutual attraction).

Table: Small-Scale Dispersion Patterns

Pattern	Resource Distribution	Interactions	Example
Random	Even	Neutral	Some forest trees
Regular	Even	Negative (e.g., competition)	Nesting seabirds
Clumped	Patchy	Positive or neutral	Schools of fish, herds of mammals

Large-Scale Patterns

At larger spatial scales, such as continents, most species exhibit clumped distributions. This is often due to environmental heterogeneity, habitat gradients, and historical factors.

• Clumped dispersion: Most common at large scales due to uneven distribution of suitable habitats and resources.

• Habitat gradients: Gradual changes in environmental factors (e.g., moisture, temperature) across a landscape.

• Historical contingencies: Past events (e.g., glaciation, migration) that influence current distribution patterns.

Case Studies in Distribution Patterns

Distribution of Tropical Bee Colonies (Hubbell and Johnson, 1977)

• Studied five species of stingless bees in Costa Rica.

• Mapped suitable nest trees; found more nest sites than colonies, so nest site was not limiting.

• Distribution of suitable trees was random.

• Prediction: Aggressive bee species would show regular distributions (due to territoriality), while non-aggressive species would show random or clumped distributions.

Distribution of Desert Shrubs (Phillips and MacMahon, 1981)

• Traditional theory: Desert shrubs are regularly spaced due to competition.

• Findings: Young shrubs are clumped due to germination at safe sites, limited seed dispersal, and asexual reproduction.

• As shrubs grow, competition increases mortality in clumps, leading to more regular spacing among mature plants.

Population Density and Organism Size

Relationship between Size and Density

There is a general inverse relationship between organism size and population density: larger organisms tend to have lower population densities.

• Observed in both animals and plants.

• Example: Damuth found that population density of herbivorous mammals decreases as body size increases.

• Plant example: Duckweed (Lemna) has high density; redwoods (Sequoia sempervirens) have low density.

Commonness and Rarity

Classification of Commonness (Rabinowitz Framework)

Rabinowitz proposed that commonness and rarity can be classified based on three factors:

• Geographic range: Wide vs. restricted

• Habitat tolerance: Broad vs. narrow

• Local population size: Large vs. small

Table: Forms of Commonness and Rarity

Geographic Range	Habitat Tolerance	Local Population Size	Example
Extensive	Broad	Large	American crow
Restricted	Narrow	Small	Endemic island species
Restricted	Broad	Large	Some widespread but locally abundant plants
Extensive	Narrow	Small	Specialist species

Additional info: Table entries inferred from Rabinowitz's framework and slide images.

Conservation Implications

• Species with more attributes of rarity are at higher risk of extinction.

• Identifying forms of rarity helps prioritize conservation strategies, such as habitat preservation, restoration, and protection from exploitation.

• "Island" organisms (not just oceanic islands, but any isolated habitat) are especially at risk because they often combine all three forms of rarity.