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Geographic Ecology. Chapter 22. Outline. Introduction Island Area, Isolation, and Species Richness Terrestrial Aquatic Equilibrium Model of Island Biogeography Latitudinal Gradients in Species Richness Historical and Regional Influences. Introduction.

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Geographic Ecology

Chapter 22


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Outline

  • Introduction

  • Island Area, Isolation, and Species Richness

    • Terrestrial

    • Aquatic

  • Equilibrium Model of Island Biogeography

  • Latitudinal Gradients in Species Richness

  • Historical and Regional Influences


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Introduction

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

    • Geographic ecology is the study of ecological structure and process at large geographic scales.


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Island Area and Species Richness

  • On islands and habitat patches on continents, species richness increases with area and decreases with isolation.


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Island Area and Species Richness

  • Prestonfound fewest bird species live on smallest islands and most species on largest islands.


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Island Area and Species Richness

  • Nilsson et.al. found island area was best single predictor of species richness among woody plants, carabid beetles, and land snails.


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Habitat Patches on Continents: Mountain Islands

  • As Pleistocene ended and climate warmed, forest and alpine habitats contracted to the tops of high mountains across American Southwest.

    • Woodlands, grasslands, and desert scrub, invaded lower elevations.

    • Once continuous forest converted to series of island-like fragments associated with mountains: Montane.


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Lakes as Islands

  • Lakes can be considered as habitat islands.

    • Differ widely by degree of isolation.

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

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


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Island Isolation and Species Richness

  • There is often a negative relationship between the isolation of an island and the number of species it supports.

    • An island that is very isolated for one group of organisms may be completely accessible to another group.


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Marine Islands

  • MacArthur and Wilson found isolation reduces bird diversity on Pacific Islands.


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Marine Islands

  • Williamsonsummarized data from relationship between island area and species richness in Azore Islands:

    • Birds show clear influence of isolation on diversity, pteridophytes do not.

    • Land birds fly across water barriers, and pteridophytes produce large quantities of light spores easily dispersed in the wind.


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Isolation and Habitat Islands on Continents

  • Lomolino et.al. found a strong negative relationship between isolation and the number of montane mammal species living on mountaintops across the American Southwest.


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Equilibrium Model of Island Biogeography

  • MacArthur and Wilson: Model explaining patterns of species diversity on islands as result of immigration and extinction rates.

    • Rates of immigration would be highest on new island with no organisms.

      • As species began to accumulate, rate of immigration would decline since fewer arrivals would be new species.


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Equilibrium Model of Island Biogeography

  • They predicted the rate of extinction would rise with increasing number of species on an island for three reasons:

    • Presence of more species creates a larger pool of potential extinctions.

    • As number of species increases, population size of each must diminish.

    • As number of species increases, potential for competitive interactions between species will increase.


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Equilibrium Model of Island Biogeography

  • Point where two lines cross predicts the number of species that will occur on an island.

  • Proposed rates of extinction on islands would be determined mainly by island size.

    • Large near islands will support highest number.

    • Small far islands will support lowest number.

    • Small near and large far will support intermediate number.


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Equilibrium Model of Island Biogeography

  • Predictions of the equilibrium model:

    • Island diversity is the outcome of a highly dynamic balance between immigration and extinction.

    • Rates of immigration and extinction are determined mainly by the isolation and area of islands.

  • Predicts that the species composition on islands is not static – it changes over time.

    • Species Turnover


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Species Turnover on Islands

  • Diamond found birds in nine California Channel Islands in a stable equilibrium as a result of immigration and extinction.


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Experimental Island Biogeography

  • Simberloff and Wilson studied insect recolonization in Florida Keys.

    • They chose 2 stands of mangroves as control islands, and 6 others as experimental islands which were defaunated.


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Experimental Island Biogeography

  • Followed recolonization for 1 yr.

    • Species number stayed constant, but composition changed considerably.


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Colonization of New Islands by Plants

  • Rydin and Borgegardfound variation in species richness correlated positively with island area and accounted for 44-85% of variation in species richness among islands. Small and medium islands continued to accumulate species.

    • Large islands attained equilibrium of immigration and extinction.

    • Difficult to separate effects of habitat diversity from area effects.


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Manipulating Island Area

  • Simberloff tested effect of island area on species richness.

    • In all cases where area was reduced, species richness decreased.

      • Richness on control island increased slightly.

    • Islands with reduced area lost species with each reduction in area.

      • Showed area has positive influence on species richness.


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Island Biogeography Update

  • Brown and Kodric-Brown found higher immigration rates to near islands can reduce extinction rates.

  • Lomolinofound island area can have a significant effect on immigration rates.

  • Area and isolation are only two of several environmental factors affect island species richness.


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Latitudinal Gradients in Species Richness

  • Species richness generally increases from middle and high latitudes to the equator.

    • Most groups of organisms are more species-rich in the tropics.


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Latitudinal Gradients in Species Richness

  • Brown grouped hypotheses that explain geographic gradients into six categories:

    • Time Since Perturbation

      • More species in the tropics because tropics are older and disturbed less frequently.

      • More time for speciation, and less frequent disturbance reduces extinction rate.


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Latitudinal Gradients in Species Richness

  • Productivity

    • High productivity contributes to high species richness.

      • More energy to divide among population.

  • Environmental Heterogeneity

    • More heterogeneity, thus more potential habitat areas and niches.


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Latitudinal Gradients in Species Richness

  • Favorableness

    • Tropics have more favorable environments.

      • No extremes to limit diversity.

  • Niche Breadth and Interspecific Interactions

    • Brown suggests biological processes must play secondary role.

      • Ultimate causes must by physical differences.


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Latitudinal Gradients in Species Richness

  • Differences in Speciation and Extinction Rates

    • Tropical species richness is greater because the tropics have experienced higher rates of speciation and/or lower rates of extinction.


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Area & Latitudinal Gradients in Species Richness

  • Terborgh and Rosenzweig proposed that the greater species richness of the tropics can be explained by the greater area covered by tropical regions.


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Area & Latitudinal Gradients in Species Richness

  • In addition, temperatures are more uniform across this tropical belt.


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Continental Area and Species Richness

  • Brown analyzed the number of non-flying mammals on five major continents plus Madagascar and New Guinea.

    • Strong positive relationship between mammalian richness and the area of continents or large islands.


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Continental Area and Species Richness

  • Rosenzweig found a strong positive relationship between area and species diversity in the tropics.


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Historical and Regional Influences

  • Long-term historical and regional processes can significantly influence species richness and diversity.

    • Exceptions to the common patterns.


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Historical and Regional Influences

  • Latham and Ricklefs: Reported striking contrast in diversity of temperate zone trees that cannot be explained by area effect.


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Historical and Regional Influences

  • Temperate forest biome in Europe, Eastern Asia, and Eastern North America all have roughly equitable area, but support vastly different levels of biological diversity.

    • Eastern Asia: 3x North America and 6x Europe.


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Historical and Regional Influences

  • Ralph found a negative correlation between foliage height diversity and bird species diversity.


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Cape Floristic Region of South Africa

  • Bond and Goldblatt attributed unusually high species richness of the Cape floristic region to several historic and geographic factors.

    • Continental drift

    • Wide variety of soil types.

    • Repeated expansion, contraction, and isolation of plant populations.

    • Refuge areas


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Diversity of Temperate Trees

  • Latham and Ricklefs: Must examine conditions trees in these regions faced during the last glacial period.

    • Mountains in Europe form east-west oriented barriers.

    • During last ice age, temperate trees had southward retreat largely cut-off.

    • Lower species richness as consequence of higher extinction rate.


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Historical and Regional Influences

  • Appalachian Mountains in North America run north-south, thus temperate trees had an avenue of retreat as temperatures became colder.

    • Also no mountain barriers in Asia.

  • Concluded from various lines of evidence that most temperate tree taxa originated in Eastern Asia and dispersed to Europe and North America.

    • After dispersal lines were cut, speciation continued in Asia.


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    Bird Diversity in the Beech Forests of South America

    • Shrub habitats contain more bird species than the beech forests with a greater diversity of heights.

      • Shrub habitats occupy a much larger area than beech forest.

      • Beech forests isolated from tropical forests by arid habitat.


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