4.3 Population Distribution and Abundance: Ecological Limits and Dispersal

Population Distribution and Abundance

Introduction

Understanding how populations are distributed and how abundant they are is a central theme in ecology. This section explores the factors that determine where populations are found, the limits to their ranges, and the ecological and evolutionary processes that shape these patterns.

Species Geographic Range

Definition and Examples

• Geographic range: The entire area over which a species is found.

• Example: In the Sonoran Desert, Carnegiea gigantea (saguaro cactus) has a more restricted range compared to the creosote bush (Larrea tridentata), largely due to temperature constraints.

• Physical barriers, such as freezing temperatures, can sharply limit a species' range.

Limits to Population Ranges

Environmental and Biological Constraints

• Abiotic factors: Non-living environmental factors such as temperature, salinity, pH, elevation, sunlight, moisture, and nutrient availability.

• Biotic factors: Interactions with other species, including competition, predation, and mutualism.

• Geographic barriers: Physical features like mountains, rivers, or unsuitable habitats that prevent movement and gene flow between populations.

• Dispersal ability is crucial for overcoming barriers and expanding ranges.

Habitat Suitability

Definition and Determinants

• Habitat: The type of environment in which an organism or group normally lives or occurs.

• Habitat suitability is determined by both abiotic and biotic factors.

• Example: The spotted owl (Strix occidentalis) is found in old-growth forests, while the sage grouse (Centrocercus urophasianus) inhabits sagebrush steppe.

Abiotic and Biotic Factors

• Abiotic: Temperature, moisture, sunlight, nutrients, etc.

• Biotic: Presence of competitors, predators, mutualists, and other interacting species.

Population Distributions: Case Study

Barnacle Zonation in the Intertidal Zone

• Joseph Connell's 1961 study in Scotland examined two barnacle species: Semibalanus balanoides (formerly Balanus balanoides) and Chthamalus stellatus.

• Distribution patterns:

  • Semibalanus adults and larvae occupy the entire intertidal zone, but adults are limited by desiccation at higher levels.

  • Chthamalus adults are restricted to higher intertidal zones due to competition with Semibalanus at lower levels, despite greater desiccation tolerance.

• Both abiotic (desiccation) and biotic (competition) factors determine species distributions.

Ecological Niche

Definition and Concepts

• Ecological niche: The set of physical and biological conditions a species needs to grow, survive, and reproduce.

• Alternative definitions:

  • Krebs: "The role or 'profession' of an organism in the environment; its activities and relationships in the community."

  • Hutchinson: "A region in a multi-dimensional space of environmental factors that affect the welfare of a species."

  • Wikipedia: "How an organism makes a living."

• Pronunciation note: "Niche" (French origin) is technically correct, but "nitch" is also commonly used.

Habitat vs. Niche

• Organisms may share a habitat but occupy different niches (e.g., black bear and mule deer in Yellowstone).

Generalists vs. Specialists

• Generalists: Species with wide niches, able to survive and reproduce under many conditions (e.g., rats, creosote bush).

• Specialists: Species with narrow niches, requiring specific conditions (e.g., koalas, saguaro cactus).

Fundamental vs. Realized Niche

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

• Realized niche: The actual conditions under which a species exists, limited by competition, predation, and other biotic factors.

• The realized niche is typically smaller than the fundamental niche.

Historical Factors Affecting Distribution

Continental Drift and Speciation

• Species are not always found in all suitable habitats due to historical events such as continental drift and speciation.

• Example: Polar bears (Ursus maritimus) are found only in the north, while penguins (Spheniscidae) are found in the south, due to their evolutionary origins and dispersal barriers.

Dispersal

Definition and Barriers

• Dispersal: The movement of organisms or propagules from their birthplace to new locations.

• Barriers to dispersal include unsuitable habitats (e.g., land for fish, open areas for butterflies), mountains, water bodies, and human-caused habitat fragmentation (e.g., roads, urban development).

• Dispersal ability varies among species (e.g., amphibians are limited by water availability, bald eagles can cross many habitats).

Reasons for Dispersal

• Finding new mates and increasing genetic variation (reducing inbreeding).

• Accessing new food resources.

• Avoiding crowding, resource depletion, disease, and predation.

• Expanding the species' range.

Factors Influencing Dispersal

• Species' dispersal mechanisms (e.g., apple seeds fall near the parent tree, while some seeds are dispersed by animals).

• Resource distribution: Clumped resources can lead to clumped dispersal patterns.

• Individual behavior: Territoriality (e.g., three-spined stickleback) can limit dispersal; facilitation by other species (e.g., birds dispersing seeds) can enhance it.

Testing Dispersal Limits

• Ecologists can test if dispersal limits a species' range by transplanting individuals outside their current range.

• If the transplanted individuals survive and reproduce, dispersal is the limiting factor; if not, other environmental or biotic factors are also involved.

Dispersal vs. Migration

Key Differences

• Dispersal: One-way movement of individuals to new locations (immigration and emigration).

• Migration: Seasonal, round-trip movement between locations.

Example: Monarch Butterfly

• The northern range of monarch butterflies (Danaus plexippus) is limited by the distribution of milkweed plants.

• Monarchs migrate seasonally, traveling thousands of miles between breeding and overwintering grounds (e.g., from northern U.S. and Canada to central Mexico).

• Environmental factors such as temperature and host plant availability determine their seasonal ranges.