🌾World Biogeography Unit 4 – Dispersal & Distribution in Biogeography

Key Concepts

• Dispersal refers to the movement of individuals or propagules from their place of origin to new locations
• Distribution patterns describe the spatial arrangement of species or populations across a geographic area
• Barriers to dispersal include physical (oceans, mountains), ecological (climate, habitat), and biological (competition, predation) factors that limit the spread of organisms
• Vicariance occurs when a population is split into two or more geographically isolated subpopulations due to the formation of a barrier
• Dispersal can be active (organisms move themselves) or passive (organisms are transported by external agents such as wind, water, or other animals)
  • Active dispersal includes walking, flying, or swimming
  • Passive dispersal includes wind-borne seeds, floating coconuts, or animals hitching rides on other organisms
• The distance and direction of dispersal are influenced by the dispersal mechanism, the characteristics of the organism, and environmental factors
• Successful establishment after dispersal depends on the suitability of the new habitat and the ability of the organism to adapt to new conditions

Mechanisms of Dispersal

• Wind dispersal involves lightweight seeds, spores, or other propagules being carried by air currents
  • Dandelion seeds have a parachute-like structure that allows them to be easily carried by the wind
  • Some spiders use silk threads to "balloon" through the air and disperse to new areas
• Water dispersal occurs when organisms or their propagules are transported by water currents
  • Coconuts can float on ocean currents and establish on distant shores
  • Many aquatic plants have seeds that can remain viable after being submerged in water for extended periods
• Animal-mediated dispersal involves organisms being transported by other animals, either internally (endozoochory) or externally (epizoochory)
  • Birds and mammals can disperse seeds after consuming fruits and excreting the seeds in new locations
  • Seeds or small organisms can attach to the fur, feathers, or feet of animals and be carried to new areas
• Human-mediated dispersal has become increasingly important, with humans intentionally or unintentionally transporting species across the globe
  • The introduction of non-native species for agriculture, horticulture, or as pets has led to the spread of many organisms beyond their natural ranges
  • Accidental transport of organisms in shipping containers, ballast water, or on vehicles has also contributed to long-distance dispersal

Barriers to Dispersal

• Geographic barriers such as oceans, mountain ranges, and deserts can prevent the spread of organisms that cannot cross these obstacles
  • The formation of the Isthmus of Panama about 3 million years ago allowed the exchange of terrestrial fauna between North and South America, which had previously been separated by a marine barrier
• Ecological barriers include unsuitable habitats or climatic conditions that prevent the establishment of dispersing organisms
  • A tropical species dispersing to a temperate region may not survive the colder temperatures and seasonal changes
  • Soil type, moisture availability, and other abiotic factors can limit the distribution of plants and soil-dwelling organisms
• Biological barriers involve interactions with other organisms that prevent successful establishment after dispersal
  • Competition with resident species for resources can hinder the success of dispersing organisms
  • Predation or herbivory can reduce the survival and reproduction of dispersed individuals
• Anthropogenic barriers such as roads, cities, and agricultural land can fragment habitats and limit the dispersal of some species
  • Habitat fragmentation can isolate populations and reduce gene flow, leading to decreased genetic diversity and increased vulnerability to local extinctions

Distribution Patterns

• Cosmopolitan distribution refers to species that are found across a wide range of geographic regions and habitats
  • Humans and some domesticated animals (dogs, cats) have a cosmopolitan distribution due to their association with human activities
• Endemic distribution describes species that are restricted to a specific geographic area, often due to unique ecological conditions or evolutionary history
  • The lemurs of Madagascar are endemic to the island and found nowhere else in the world
  • Many island species are endemic due to their isolation and unique evolutionary trajectories
• Disjunct distribution occurs when populations of a species are separated by a geographic barrier, often resulting from historical vicariance events
  • The monkey puzzle tree (Araucaria araucana) has a disjunct distribution, with populations in Chile and Argentina separated by the Andes Mountains
• Continuous distribution refers to species whose populations are connected across their range, without significant gaps or barriers
  • Many widespread species, such as the red fox (Vulpes vulpes), have a continuous distribution across their range
• Clinal variation describes gradual changes in a species' characteristics (morphology, genetics) across its geographic range, often in response to environmental gradients
  • The body size of some bird species may vary clinally, with larger individuals found in colder climates and smaller individuals in warmer areas (Bergmann's rule)

Factors Influencing Distribution

• Climate, including temperature, precipitation, and seasonality, plays a crucial role in determining the distribution of species
  • The distribution of biomes (tundra, desert, rainforest) is largely determined by global climate patterns
  • Species' thermal tolerances and moisture requirements often define their geographic ranges
• Topography and elevation influence distribution by creating local variations in climate and habitat conditions
  • Mountainous regions often have distinct elevational zones with different communities of organisms adapted to the changing conditions
  • The rain shadow effect can create arid regions on the leeward side of mountain ranges, influencing the distribution of drought-adapted species
• Soil type and quality affect the distribution of plants and soil-dwelling organisms
  • Serpentine soils, derived from ultramafic rocks, host unique plant communities adapted to the high metal content and low nutrient availability
• Biotic interactions, such as competition, predation, and mutualism, can shape the distribution of species
  • The presence of a competitively dominant species may exclude others from a particular area
  • The distribution of plants can be influenced by the presence or absence of their pollinators or seed dispersers
• Disturbance events, such as fires, floods, and storms, can create opportunities for dispersal and colonization of new areas
  • Some plant species, such as the lodgepole pine (Pinus contorta), have serotinous cones that open and release seeds after a fire, allowing them to recolonize burned areas

Case Studies

• The Galápagos Islands provide an excellent example of how dispersal and isolation have led to the evolution of unique endemic species
  • The Galápagos finches, studied by Charles Darwin, show adaptive radiation and specialization to different ecological niches on the islands
  • The Galápagos giant tortoises have diverged into distinct subspecies on different islands, each adapted to the specific conditions of their habitat
• The Great American Biotic Interchange (GABI) demonstrates the consequences of removing a geographic barrier to dispersal
  • The formation of the Isthmus of Panama allowed the exchange of terrestrial fauna between North and South America, which had been separated by a marine barrier for millions of years
  • Many species, such as armadillos and opossums, dispersed from South to North America, while others, like horses and camels, moved in the opposite direction
• The spread of the cane toad (Rhinella marina) in Australia highlights the potential ecological impacts of human-mediated dispersal
  • Introduced to control agricultural pests, the cane toad has become an invasive species, spreading rapidly and causing declines in native predators that attempt to eat the toxic amphibian
  • The cane toad's success in colonizing new areas is facilitated by its high reproductive output, generalist diet, and lack of natural predators in Australia

Ecological Implications

• Dispersal allows species to colonize new habitats, expand their ranges, and escape unfavorable conditions in their original location
  • Range shifts in response to climate change depend on the ability of species to disperse to suitable habitats as conditions change
• Successful dispersal and establishment can lead to increased genetic diversity and adaptability of populations
  • Gene flow between populations can introduce new alleles and reduce the risk of inbreeding depression
• Invasive species, often introduced through human-mediated dispersal, can have significant impacts on native ecosystems
  • Invasive species may outcompete native species for resources, alter habitat structure, or introduce novel diseases or parasites
  • The ecological and economic costs of invasive species management are substantial, highlighting the importance of preventing unwanted dispersal
• The ability of species to disperse and track suitable habitats is crucial for their persistence in the face of anthropogenic climate change and habitat fragmentation
  • Species with limited dispersal abilities or specialized habitat requirements may be more vulnerable to extinction under changing environmental conditions
• Dispersal and distribution patterns influence the assembly and structure of ecological communities
  • The relative importance of dispersal limitation and environmental filtering in shaping community composition varies across spatial scales and ecosystems

Current Research and Future Directions

• Advances in genetic and genomic techniques allow researchers to study dispersal and population connectivity in unprecedented detail
  • Molecular markers, such as microsatellites and single nucleotide polymorphisms (SNPs), can be used to infer dispersal patterns and gene flow between populations
  • Landscape genetics combines genetic data with spatial and environmental information to understand how landscape features influence dispersal and population structure
• Remote sensing and satellite imagery provide new tools for mapping and monitoring species distributions across large spatial scales
  • High-resolution satellite data can be used to identify suitable habitats, track changes in vegetation cover, and monitor the spread of invasive species
• Ecological niche modeling and species distribution modeling are increasingly used to predict the potential distribution of species under current and future environmental conditions
  • These models combine occurrence data with environmental variables to estimate the ecological niche of a species and project its potential distribution
  • Niche models can inform conservation planning, invasive species risk assessments, and predictions of range shifts under climate change scenarios
• Understanding the role of dispersal in the response of species and communities to global change is a key challenge for future research
  • Integrating dispersal processes into models of species' responses to climate change and habitat fragmentation will improve predictions and inform conservation strategies
  • Studying the eco-evolutionary dynamics of dispersal, such as the evolution of dispersal traits in response to changing environments, will provide insights into the long-term persistence of species
• Collaborative research efforts and data sharing are essential for addressing the complex questions surrounding dispersal and distribution in biogeography
  • Initiatives such as the Global Biodiversity Information Facility (GBIF) and the Group on Earth Observations Biodiversity Observation Network (GEO BON) facilitate the integration and accessibility of biodiversity data from around the world
  • Interdisciplinary approaches, combining expertise from fields such as ecology, genetics, geography, and remote sensing, will be necessary to advance our understanding of dispersal and distribution patterns in a changing world