15.1 Biogeography and Species Distribution

Biogeography explores how species are distributed across the globe. It looks at patterns of species richness and diversity, examining factors like climate, topography, and human impact. Understanding these patterns helps us grasp the complex web of life on Earth.

Species distribution is shaped by both natural and human-driven forces. From historical events like continental drift to current challenges like climate change, biogeography reveals how life adapts and thrives in different environments. This knowledge is crucial for conservation efforts and predicting future ecological changes.

Species Distribution Patterns

Spatial Patterns of Species Richness and Diversity

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• Species are not evenly distributed across the globe, with different regions exhibiting varying levels of species richness (number of species) and diversity (variety of species)
• Biodiversity hotspots are areas with exceptionally high concentrations of endemic species (species found nowhere else) that are under threat from human activities (tropical Andes, Madagascar, Caribbean)
• Latitudinal diversity gradients show a general trend of increasing species richness from the poles to the equator across many taxa (plants, animals, microorganisms)
• Elevational diversity gradients describe how species richness changes with altitude, often peaking at intermediate elevations and declining at higher elevations

Species-Area Relationship and Biogeographic Patterns

• The species-area relationship states that the number of species in a given area increases with the size of that area, used to predict species richness in different regions
• Island biogeography theory explains the factors that affect the species richness of isolated natural communities (islands, mountaintops, lakes)
  • The number of species on an island is determined by the balance between immigration and extinction rates
  • Larger islands tend to have more species due to lower extinction rates and higher habitat diversity
• Ecoregions are distinct areas of the Earth's surface characterized by unique combinations of climate, geology, and species assemblages (Amazon rainforest, Sahara desert, Great Barrier Reef)

Factors Influencing Distribution

Abiotic Factors

• Climate, particularly temperature and precipitation, plays a significant role in determining species distribution, with each species having a specific range of climatic conditions in which it can survive and reproduce
• Topography, including factors such as elevation, aspect, and slope, can create microclimates that influence species distribution at local scales (north-facing vs. south-facing slopes)
• Soil type and chemistry affect the distribution of plant species, which in turn influences the distribution of animal species that depend on those plants
• Availability of resources, such as water, nutrients, and sunlight, can limit species distribution in certain areas (deserts, polar regions)

Biotic Interactions and Dispersal

• Biotic interactions, such as competition, predation, and mutualism, can shape species distribution patterns by facilitating or inhibiting the establishment of others
  • Competition for resources can limit the distribution of species with similar ecological niches (Darwin's finches)
  • Predation can control the population sizes and distribution of prey species (wolves and elk)
  • Mutualistic relationships can expand the distribution of partner species (fig trees and fig wasps)
• Dispersal abilities of species, including their capacity for movement and the presence of dispersal barriers, determine their potential range and ability to colonize new areas
  • Active dispersal involves the intentional movement of organisms (animal migration, plant seed dispersal)
  • Passive dispersal relies on external factors to transport individuals or propagules (wind, water, other organisms)
• Human activities, such as land use change, habitat fragmentation, and the introduction of invasive species, can significantly alter species distribution patterns

Historical Influences on Distribution

Geological and Evolutionary Processes

• Plate tectonics and continental drift have played a crucial role in shaping global species distribution patterns over long time scales by facilitating speciation and influencing the distribution of ancestral lineages
  • The separation of Gondwana into Africa, South America, Australia, and Antarctica led to the unique flora and fauna of these continents
• Mass extinction events, such as the Cretaceous-Paleogene extinction that wiped out the dinosaurs, have dramatically reshaped species distribution patterns by eliminating certain lineages and creating opportunities for others to diversify
• Adaptive radiation, the rapid diversification of a single ancestral species into multiple descendant species adapted to different ecological niches, has contributed to the unique species assemblages in isolated regions (Hawaiian honeycreepers, Galápagos finches)

Climate Change and Biogeographic Barriers

• Past climate change events, such as ice ages and periods of global warming, have caused shifts in species distribution as organisms tracked their preferred climatic conditions
  • During the Pleistocene glaciations, many species were restricted to refugia, isolated areas where they could survive, serving as sources for recolonization when the climate warmed
• The formation of biogeographic barriers, such as the Isthmus of Panama or the Bering Land Bridge, has allowed for the exchange of species between previously isolated regions, influencing current distribution patterns
  • The Great American Biotic Interchange occurred when the Isthmus of Panama connected North and South America, allowing species to migrate between the continents
• Sea level changes have altered the connectivity between landmasses and created or eliminated dispersal routes for species (land bridges during glacial periods)

Endemic vs Cosmopolitan vs Invasive Species

Endemic Species

• Endemic species are native to and restricted to a particular geographic area, often the result of unique evolutionary histories and adaptations to specific local conditions
  • Examples include lemurs (Madagascar), kangaroos (Australia), and giant tortoises (Galápagos Islands)
• Endemic species are often more vulnerable to extinction due to their limited range and specialized adaptations, making them sensitive to environmental changes and human impacts

Cosmopolitan Species

• Cosmopolitan species have a wide global distribution and can be found in many different regions and habitat types, often generalists with broad environmental tolerances
  • Examples include humans, rats, and dandelions
• Cosmopolitan species are typically more resilient to environmental changes and can adapt to a variety of conditions, contributing to their widespread distribution

Invasive Species

• Invasive species are non-native organisms introduced to a new area, often through human activities, that have spread rapidly, causing ecological or economic harm
  • Invasive species can outcompete native species, alter ecosystem processes, and disrupt food webs
  • Examples include cane toads (Australia), zebra mussels (North America), and kudzu vine (southeastern United States)
• The success of invasive species is often attributed to their high reproductive rates, lack of natural predators in the introduced range, and ability to exploit disturbed habitats
• The prevention, early detection, and management of invasive species are critical for minimizing their negative impacts on native biodiversity and ecosystems