Geographic Ecology: Patterns of Diversity and Distribution

Geographic Ecology

Introduction to Geographic Ecology

Geographic ecology is the study of large-scale patterns of biological diversity and the geographic range of species. This field seeks to understand how plant and animal life can be mapped and how these patterns are influenced by various ecological and evolutionary processes.

• Definition: MacArthur defined geographic ecology as the "search for patterns of plant and animal life that can be put on a map."

• Scope: Geographic ecology operates above the level of landscape ecology, encompassing vast spatial scales.

Island Area, Isolation, and Species Richness

Terrestrial and Aquatic Islands

Islands, both terrestrial and aquatic, serve as natural laboratories for studying the relationship between area, isolation, and species richness.

• Species-Area Relationship: Larger islands tend to support more species than smaller islands. This pattern is observed in both birds and beetles, among other taxa.

• Habitat Patches as Islands: Mountain tops and lakes can function as 'islands' of habitat, isolated from similar environments by unsuitable terrain.

• Empirical Evidence:

  • Studies by Tonn and Magnuson found that the number of species increases with the area of an insular environment.

  • Barbour and Brown found a positive relationship between area and fish species richness in lakes.

Isolation Effects

• Marine Islands: Birds show a clear influence of isolation on diversity; more isolated islands have fewer species. However, some groups like ferns are less affected by isolation.

• Continental Habitat Islands: Lomolino et al. found a strong negative relationship between isolation and the number of mammal species on mountaintops in the American Southwest.

Equilibrium Model of Island Biogeography

Model Overview

The equilibrium model of island biogeography, developed by MacArthur and Wilson, explains patterns of species diversity on islands as a balance between immigration and extinction rates.

• Immigration: Highest on new islands with no organisms; declines as more species accumulate because fewer arrivals are new species.

• Extinction: Increases as the number of species increases due to:

  • Larger pool of potential extinctions

  • Smaller population sizes per species

  • Increased competition

• Equilibrium Point: The intersection of immigration and extinction rates predicts the number of species present at equilibrium.

Model Predictions:

• Rates of extinction are mainly determined by island size (lower on large islands, higher on small islands).

• Rates of immigration are higher on islands closer to the source of colonists.

Summary Table: Effects of Island Size and Isolation

Island Type	Immigration Rate	Extinction Rate	Species Richness
Large, Near	High	Low	Highest
Small, Near	High	High	Intermediate
Large, Far	Low	Low	Intermediate
Small, Far	Low	High	Lowest

Key Equation:

At equilibrium, the rate of immigration equals the rate of extinction:

$I(S) = E(S)$

Where $I(S)$ is the immigration rate as a function of the number of species $S$, and $E(S)$ is the extinction rate as a function of $S$.

Latitudinal Gradients in Species Richness

Patterns and Hypotheses

Species richness generally increases toward the tropics. Several hypotheses have been proposed to explain this pattern, grouped by Brown into six categories:

• Time Since Perturbation: Tropics are older and less disturbed, allowing more time for speciation and lower extinction rates.

• Productivity: High productivity in the tropics provides more energy, supporting more species.

• Environmental Heterogeneity: Greater habitat diversity creates more niches.

• Favorableness: Stable, favorable climates in the tropics reduce environmental extremes that limit diversity.

• Niche Breadth and Interspecific Interactions: Various biological processes, though Brown suggests physical differences are the ultimate cause.

• Differences in Speciation and Extinction Rates: Higher speciation or lower extinction rates in the tropics may contribute to greater diversity.

Historical and Regional Influences

Case Studies and Patterns

• Cape Floristic Region (South Africa):

  • Mediterranean climate, similar to California and SW Australia.

  • Has about twice the species diversity of similar regions, despite a smaller area.

  • Diversity increased by historical selection for arid-adapted plants, migration patterns, and climatic fluctuations promoting speciation and reducing extinction.

• Temperate Forest Trees:

  • East Asia, Europe, and North America have similar areas of temperate forest, but East Asia has much higher species richness.

  • Historical migration and barriers to dispersal have led to greater diversity in Asia.

Technologies in Geographic Ecology

Modern Tools for Studying Patterns

• Global Positioning System (GPS): Uses satellites to determine precise locations (latitude, longitude, altitude) on Earth.

• Remote Sensing: Satellites equipped with electro-optical sensors scan multiple bands of the electromagnetic spectrum to collect data on vegetation, land cover, and marine productivity.

• Geographic Information System (GIS): Computer-based systems for storing, sorting, analyzing, and displaying geographic data. GIS can handle large datasets and integrate multiple layers of information (e.g., topography, land use, vegetation).

Example: Remote sensing can be used to estimate vegetative biomass (e.g., red color in satellite images) and marine primary production from space.

Additional info: These technologies are essential for mapping biodiversity, monitoring environmental changes, and informing conservation strategies.