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. It 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, focusing on broader spatial scales.

Island Area, Isolation, and Species Richness

Terrestrial and Aquatic Islands

Islands, whether terrestrial or aquatic, serve as natural laboratories for studying species richness and the effects of area and isolation on biodiversity.

• Species-Area Relationship: Larger islands tend to support more species than smaller islands.

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

Examples:

• Fewest bird species live on the smallest islands; most species are found on the largest islands.

• Mountain islands: Once-continuous forests (Pleistocene) are now fragmented, with high-elevation habitats isolated like islands.

• Lakes as islands: The number of fish species increases with lake area; lakes differ in their degree of isolation.

Marine Islands

• Isolation affects species richness: Distant islands (e.g., Azores) support fewer bird species than islands closer to the mainland (e.g., Channel Islands).

• Some groups, like ferns, may not show the same pattern as birds, indicating differences in dispersal ability or ecological requirements.

Isolation and Habitat Islands on Continents

• Negative Relationship: The number of mammal species on mountaintops in the American Southwest declines with increasing isolation from source populations.

Equilibrium Model of Island Biogeography

MacArthur and Wilson's Model

This model explains patterns of species diversity on islands as a balance between immigration and extinction rates.

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

• Extinction Rate: Increases as the number of species increases due to smaller population sizes and increased competition.

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

Key Factors Affecting Extinction Rates:

• Island size (larger islands have lower extinction rates)

• Isolation (closer islands have higher immigration rates)

Model Predictions:

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

Latitudinal Gradients in Species Richness

Patterns and Hypotheses

Species richness generally increases toward the equator, with the tropics supporting more species than temperate or polar regions. Several hypotheses have been proposed to explain this pattern:

• 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 stress and allow more species to coexist.

• Niche Breadth and Interspecific Interactions: Biological processes may play a secondary role; ultimate causes are often physical differences.

• 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

• Cape Floristic Region (South Africa): Despite its small area, the Cape has about twice the species diversity of similar Mediterranean climates in California and southwestern Australia.

• Factors Increasing Diversity:

  • Selection for arid-adapted plants began north of the Cape during the late Tertiary (26 million years ago), providing a large area of origin.

  • As Africa drifted northward, arid-adapted plants migrated south to the Cape.

  • Speciation promoted by a dissected landscape, diverse soils, and climate fluctuations during the Pleistocene.

  • Many refuges reduced extinction rates.

• Temperate Forest Trees: East Asia, Europe, and North America have similar areas of temperate forest, but East Asia has about three times as many species as North America and six times as many as Europe.

• Historical Component: Most temperate tree taxa originated in Asia and migrated to Europe and North America. Barriers to dispersal and ongoing speciation in Asia have led to higher diversity there.

Technologies in Geographic Ecology

Global Positioning System (GPS)

• GPS uses satellites to determine precise latitude, longitude, and altitude, aiding in mapping and ecological studies.

Remote Sensing

• Remote sensing satellites are equipped with sensors that scan multiple bands of the electromagnetic spectrum, allowing for the assessment of vegetative biomass, land cover, and marine primary production from space.

Geographic Information System (GIS)

• GIS is a computer-based system for storing, sorting, analyzing, and displaying geographic data. It can handle large datasets and is essential for modern ecological research.

• GIS integrates multiple layers of information (e.g., topography, land cover, zoning) to analyze spatial patterns and relationships.

Summary Table: Key Concepts in Geographic Ecology

Concept	Description	Example/Application
Species-Area Relationship	Larger areas support more species	Bird diversity on Caribbean islands
Isolation Effect	More isolated habitats have fewer species	Mammal diversity on mountaintops
Equilibrium Model	Species number is a balance of immigration and extinction	Island biogeography theory
Latitudinal Gradient	Species richness increases toward the equator	Tropical rainforests vs. temperate forests
GIS/Remote Sensing	Technologies for mapping and analyzing ecological data	Mapping vegetative biomass, marine productivity

Additional info: The notes expand on the original slides by providing definitions, context, and examples for each concept, as well as summarizing key models and hypotheses relevant to geographic ecology.