Ecological Communities species diversity and community structure

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Ecological Communities-species diversity and
community structure

• Lecture 17

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Ecological communities are often highly complex
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Questions about food webs and ecological
communities

• Why are there so many species?
• What contributes to ecosystem stability?
• What factors determine patterns of species
  diversity?

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First-some basic definitions

• Community two or more populations that live in
  the same geographic area
• Food chain a sequence of organisms through which
  energy flows
• Food web a series of interconnected food chains

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Ecological communities are often highly complex
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Species diversity

• Early ecologists were puzzled by the diversity of
  species
• If competition was so prevalent shouldnt the
  best adapted species out-compete the others?
• In early lab experiments, Gause inoculated
  experimental ecosystems with pairs of organisms
• One species eventually took over and eliminated
  the other
• Which species prevailed depended on the initial
  densities

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Competitive exclusion

• Term used to describe this process
• If competitive exclusion occurred in the lab, why
  are there so many species in the wild?
• G. R. Hutchinson (1959) asked Why are there so
  many kinds of animals? in the title of a famous
  paper

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The answer to this question

• Is complex
• And has a direct bearing on much of the practical
  conservation work that will face us over the next
  several decades
• Still the subject of a great deal of research
• Theoretical modeling
• Empirical research

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Before we examine examples

• We need to define species diversity
• We need to understand how it is measured in the
  field

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Species diversity has two components

• Species richness
• The total number of different species
• Species evenness
• Measure of the relative proportions of the
  different species

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Species diversity has two components

• Two communities
• Community A
• 1 Red-winged blackbird
• 9 Marsh wrens
• Community B
• 5 Red-winged blackbirds
• 5 Marsh wrens
• Species richness (number of different species)
• Equal for the two communities
• Species evenness
• Evenness is higher in community B
• Community B is more diverse

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Diversity indices

• A variety of indices have be developed to measure
  species diversity
• They incorporate both the richness and evenness
  components of species diversity
• Example Shannon-Weaver diversity index
• s
• H - S pilnpi
• i1

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Species diversity in fertilized and unfertilized
fields
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Factors influencing species diversity include

• Area
• Latitudinal gradients
• Productivity
• Habitat diversity

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Island Biogeography

• Some of the early attempts to understand species
  diversity and the effects of area focussed on the
  study of diversity patterns on islands

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Species diversity on islands
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Numbers of plant genera
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Area effects may be due to

• increased habitat diversity as area increases
• the effects of area itself
• Larger target of colonizers to find
• Lower extinction rates for larger populations on
  large islands

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Island Biogeography

• The study of patterns of species diversity and
  community structure on islands
• Is important when considering the design of
  nature reserves
• Nature reserves can often be thought of as
  islands of protected habitat in a sea of urban or
  agricultural landscapes
• Island biogeography can also be applied to forest
  patches in an urban landscape etc.

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Conservation biologists

• Must decide whether to protect a small number of
  large areas
• Or a large number of small areas
• The best approach will depend on the species that
  the reserves are designed to protect

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Making the best decision requires

• Information about species area relationships
• Home range requirements
• Dispersal ability
• Extinction probabilities

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Species area relationships

• Also have implications for measuring and
  comparing species diversity at different sites
• As sampling time, effort, or area searched
  increases so does the number of species detected

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Area effects
Cumulative of species
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Species area curves
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Species area curves correct sampling design
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Species area curves incorrect sampling design
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Latitudinal gradients
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Latitudinal gradients
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Nutrients and productivity

• Based on theoretical models
• some have argued that as net primary productivity
  increases, species diversity should increase
• others have argued that as net primary
  productivity increases, species diversity or
  richness should decrease

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Net primary productivity

• Is the rate of production of new biomass per
  individual (or area) per unit time
• e.g. 1100g C /ha/Yr
• Or 50 g C/plant/year
• The rate at which nutrients and energy is
  converted to biomass

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Theoretical arguments for increasing species
richness with increasing productivity

• As the number of species increases, so does the
  probability of including a highly productive
  species, i.e. one that is very efficient at
  converting nutrients into biomass
• of species is driving the rate primary
  production, i.e., more species greater
  productivity

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Theoretical arguments for increasing species
richness with increasing productivity
Complementary resource use

• As the number of species increases, so does the
  probability of including species that can use all
  the resources
• i.e. if only one species were present, some
  niches would remain unoccupied and resources
  would remain unused

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Evidence

• Support for the hypothesis that species diversity
  increases as productivity increases
• Tilman and Downing examined species richness and
  productivity in plots they fertilized with N
• After severe drought productivity declined less
  in high diversity plots
• Productivity also recovered more quickly in high
  diversity plots
• These results suggest that the of species is
  driving productivity

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Contd

• Naeem (1994,1995, 1996) compared productivity in
• High and low diversity plant communities
• Low diversity communities were subsets of the
  high diversity
• Productivity was highest in the high diversity
  communities

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Increasing diversity as productivity decreases

• As latitude decreased so does primary net
  productivity
• Tropical forests have low net primary
  productivity and high species diversity
• Temperate forests have intermediate net primary
  productivity and low species diversity

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This seems counter intuitive

• But new production in tropical forests are
  minimal
• Soil is nutrient poor
• Nutrients are tied up in standing crop

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Increasing diversity as latitude (productivity)
decreased
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Pattern holds for a variety of taxa

• Birds
• Bats
• Mammals
• Insects
• Plants

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Explaination

• May be that low productivity means that
  succession and competitive exclusion occur slowly
• Eventually disturbance resets succession

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Effect of habitat diversity
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Niche theory

• Proposes that different species have different
  ecological requirements and tolerances
• Each occupying a different ecological niche
• Many species co-occur because they occupy
  different niches
• They can out-compete competitors in their
  preferred niche but are poor competitors outside
  their preferred conditions

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MacArthurBird species diversity

• Argued that forests with more complex physical
  structures should have more available niches
• He correlated foliage height diversity with bird
  species diversity

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Foliage height diversity vs bird species
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Patterns of species diversity

• Influenced by several factors including
• Area
• Habitat diversity
• Productivity