BIO226 : Community : Community Patterns : Lesson

Glossary terms that are important in this lesson: Alpha diversity, anthropogenic extinction, aspect diversity, background extinction, beta diversity, biodiversity, continental drift, convergence, density compensation, ecological diversity, ecological release, endemic, escape space, gamma diversity, genetic diversity, habitat expansion, mass extinctions, morphological volume, niche, niche breadth, niche overlap, niche volume, Pangaea, pseudoextinction, species pool, trophic level.

I. Biodiversity

1. Introduction
   • Patterns - community properties are regulated by physical and ecological processes
   • Thermodynamics: energy available to each trophic level decreases at each step up the food chain; results in fewer predators than herbivores
   • Species diversity: large islands support more species than small islands (diversity is related to area); there are more species in the tropics than at higher latitudes in communities of similar productivity
   • Hypothesis 1: diversity increases with time - tropical habitats are older than temperate and arctic habitats and so have greater diversity
   • Hypothesis 2: the number of species is a balance between the rate at which new species appear and the rate at which they disappear; these rates differ systematically from place to place

2. Geographic patterns of species diversity
   • Most taxonomic groups increase in number of species toward the equator: ants, breeding birds, marine organisms
   • In a latitudinal belt, productivity, structural heterogeneity and physical conditions influence species diversity
   • Habitat structure is more important than productivity: deserts have higher habitat and species diversity than more productive marshes
     • Examples: birds, spiders, lizards
   • In North America, mammals and breeding land birds are most diverse in lower latitudes and in the topographically heterogeneous western mountains
   • Diversity decreases towards the tip of a peninsula (e.g., Baja California)

3. Diversity and niche relationships
   • Niche - the ranges of conditions and resources within which an organism can survive and reproduce; it is multidimensional e.g., temperatures between 10° and 30°C, prey sizes of 4 and 12 mm, diurnal
     • Competition is implied when niches overlap
     • Niche relationships of species reflect the structural organization of communities
     • Total number of niches (and species diversity) depends on total niche space, breadth of each niche, and niche overlap
   • Higher diversity in the tropics results from more ecological roles, i.e., greater community niche volume
     • Greater numbers of fruit and nectar-eating birds and more sit and wait insectivorous birds
     • More bats in the tropics
   • Species diversity is directly related to total morphological volume occupied
   • Species diversity is correlated with ecological diversity: diversity in streams and rivers increases from headwaters to the mouth
   • Escape space - adaptations to avoid predation
     • Expect predation pressure to diversify prey
     • Aspect diversity - variety of cryptic patterns, increases at low latitudes
   • Simultaneous changes in diversity, niche breadth, and niche overlap make it difficult to identify determinants of species diversity
   • Habitat specialization and diversity
     • Gamma diversity equals alpha diversity times beta diversity
     • Usually fewer species on islands than on comparable mainland areas, but island species often have greater densities
     • Species usually occupy fewer habitats on the mainland than on islands

4. The time hypothesis
   • Fossil record
     • Inherent problems
       • Abundance and quality of fossils increase toward the present
       • Regional diversity is easier to assess than local diversity
       • It is difficult to distinguish fossils at the species level
       • Habitats that favor preservation (shallow embayments vs arid or montane areas) display a more complete record
     • Conclusions
       • Cenozoic diversity has remained constant in some groups but increased in others: flowering plants, fishes, birds, mammals
       • Not apparent whether other groups declined simultaneously
       • Not apparent if local equilibria achieved as diversity increased globally
       • Numbers of taxa remained stable while taxonomic composition changed
   • Tropics
     • Tropical habitat existed longer than temperate habitat - more time for evolution in the tropics
     • Tropical zones contain more area than temperate zones
     • Tropical environments expanded and contracted with climatic shifts
     • Periods of glacial expansion brought low rainfall and cooler temperatures in the tropics
       • Restriction and fragmentation could have facilitated speciation: Australian avifauna, Eucalyptus

5. Equilibrium theories of diversity
   • Local processes
     • Competitive exclusion sets limits on the ecological similarity of species: the more similar the species, the more intense is competition between them
     • Appearance of a new species is compensated by the loss of a close competitor
     • Equilibrium number of species affected by quality of the local habitat
   • Diversity on islands
     • Equilibrium theory of island biogeography (Robert MacArthur and E.O. Wilson)
       • The number of species on an island balances regional emigration against local extinction
       • Species pool comes from the adjacent mainland
       • Equilibrium number of species occurs where rates of immigration and extinction are equal
       • Extinction curves (rates) are higher on smaller islands
       • Immigration curves (rates) are lower for far islands
       • Changes in species on Krakatau were consistent with the theory
     • Equilibrium theory in continental communities
       • Regional production of new species by speciation augments immigration

6. Competition and diversity
   • An increasing number of niche dimensions may allow more species to be packed into a community, e.g., wet tropics
   • In harsh environments, a few physical factors may establish a small number of niche dimensions

7. Plant species diversity: tropics vs temperate regions
   • Hypotheses
     • Environmental heterogeneity in space: tropical forest trees are specialized to soil and climate conditions; such specialization can account for some but not all the difference
     • Environmental variability in time: variation in reproductive rates may prevent competitive exclusion; coral reef fishes may conform to the lottery hypothesis
     • Environmental heterogeneity produced by disturbance: intermediate levels of disturbance open space for colonization and initiate a cycle of succession of species resulting in a mosaic of habitat patches
   • Consumer pressure
     • Selective predation on superior competitors may allow competitively inferior species to persist in a community
     • Introduction of a predator can transform a community of primary consumers and producers
     • Increased success of trees in germination and establishment with increased distance from the parent may result in part from herbivore pressure: predict species more widely dispersed than clumped
   • Rates of species production
     • Tropical forests are more diverse than temperate forests at higher taxonomic levels as well as at lower levels
     • Causes of differences in diversity have acted over very long periods

II. History and biogeography

1. Continental drift
   • About 200 million years ago all the continents were joined in a single land mass (Pangaea), allowing organisms alive at that time to disperse to all continents
   • After subsequent separation, the plants and animals in each continental region developed distinctive characteristics independently of evolutionary changes in other regions
   • Regions that remain connected share organismal groups; those that were not connected developed unique flora and fauna

2. Climate history
   • Location of the continents influenced the circulation of the oceans and the distribution of heat over the surface of the earth
   • Continents (and organisms) experienced tropical periods as well as cooler and drier periods, which influenced the distribution of plants and animals

3. Catastrophes in earth's history
   • Mass extinctions eliminated most of the species on several occasions in geologic time, greatly reducing biological diversity
   • Evolutionary responses to new conditions replaced species and reestablished community diversity

4. Organismal convergence
   • Unrelated species living under similar ecological conditions in different locations came to resemble one another more than their ancestors did
   • Hypothesis 1: Adaptations of organisms to their environments obey general rules of structure and function
   • Hypothesis 2: Structure and function of organisms have resulted from their histories rather than from environments

5. Community convergence
   • Expect independently derived communities that occupy similar habitats in different regions to have similar diversity

III. Extinction and conservation

1. Human impact
   • Human population is now 6 billion and increasing at 2% a year
   • 35% of land surface used for crops and more for grazing
   • Tropical forests are being cut at 17 million ha per year
   • Deserts are spreading
   • Water and atmosphere are polluted

2. Biological diversity
   • More than 1.4 million species of plants and animals have been described
     • Insects are half of all described species
   • There may be 10-30 million species on earth

3. Value of diversity
   • Economic and recreational benefits
   • Stabilizing ecosystem function
   • Indicate environmental change

4. Types of extinction
   • Background extinction
   • Mass extinction
   • Anthropogenic extinction
   • Pseudoextinction

5. Causes of extinction
   • Climate change
   • Reduction of habitat
   • Declining habitat quality
   • Overexploitation

6. Strategies for conservation of species
   • Minimum viable population - can sustain itself in the face of environmental change
   • Quantity and quality of habitat is critical
   • Protect species from the human population

When you have completed this lesson, go on to Review Questions