Biological Communities and Species Interactions

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Biological Communities and Species Interactions
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Outline

• Critical Factors and Tolerance Limits
• Adaptation
• Natural Selection
• Speciation
• Ecological Niche
• Species Interactions
• Community Properties
• Succession
• Introduced Species and Community Change

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Critical Factors and Tolerance Limits

• Von Liebig proposed the single factor in shortest
  supply relative to demand is the critical
  determinant in species distribution.
• Shelford later expanded by stating each
  environmental factor has both minimum and maximum
  levels, tolerance limits, beyond which a
  particular species cannot survive.

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Tolerance Limits
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Critical Factors and Tolerance Limits

• For many species, the interaction of several
  factors, rather than a single limiting factor,
  determines biogeographical distribution.
• For some organisms, there may be a specific
  critical factor that mostly determines abundance
  and distribution.
• Species requirements and tolerances can also be
  used as useful indicators.
• Environmental indicators

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Adaptation

• Adapt is used in two ways
• Range of physiological modifications available to
  individual organisms.
• Inheritance of specific genetic traits allowing a
  species to live in a particular environment.
• Population level
• Explained by process of evolution.

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Natural Selection

• Natural Selection - Members of a population best
  suited for a particular set of environmental
  conditions survive and reproduce more
  successfully than competitors.
• Acts on pre-existing genetic diversity.
• Limited resources place selective pressures on a
  population.

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Speciation

• Given enough geographical isolation or selective
  pressure, members of a population become so
  different from their ancestors that they may be
  considered an entirely new species.
• Alternatively, isolation of population subsets,
  preventing genetic exchange, can result in
  branching off of new species that coexist with
  the parental line.

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Evolution

• Divergent Evolution - Mutations and different
  selective pressures cause populations to evolve
  along dissimilar paths.
• Convergent Evolution - Unrelated organisms evolve
  separately to cope with environmental conditions
  in the same fashion.
• Look alike - Act alike

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Galapagos Finches
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Ecological Niche

• Habitat - Place or set of environmental
  conditions where a particular organism lives.
• Ecological Niche - Description of the role a
  species plays in a biological community, or the
  total set of environmental factors that
  determines species distribution.
• Generalists - Broad niche
• Specialists - Narrow niche

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Ecological Niche

• Fundamental Niche - Full range of resources or
  habitat a species could exploit if there were no
  competition with other species.
• Realized Niche - Resources or habitat a species
  actually uses.

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Resource Partitioning

• Law of Competitive Exclusion - No two species
  will occupy the same niche and compete for
  exactly the same resources for an extended period
  of time.
• One will either migrate, become extinct, or
  partition the resource and utilize a sub-set of
  the same resource.
• Given resource can only be partitioned a finite
  number of times.

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Resource Partitioning
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SPECIES INTERACTIONS

• A predator is an organism that feeds directly
  upon another living organism, whether or not it
  kills the prey in doing so.
• Prey most successfully on slowest, weakest, least
  fit members of target population.
• Reduce competition, population overgrowth, and
  stimulate natural selection.
• Co-evolution

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Keystone Species

• Keystone Species - A species or group of species
  whose impact on its community or ecosystem is
  much larger and more influential than would be
  expected from mere abundance.
• Often, many species are intricately
  interconnected so that it is difficult to tell
  which is the essential component.

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Competition

• Interspecific - Competition between members of
  different species.
• Intraspecific - Competition among members of the
  same species.
• Often intense due to same space and nutritional
  requirements.
• Territoriality - Organisms defend specific area
  containing resources, primarily against members
  of own species.
• Resource Allocation and Spacing

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Symbiosis

• Symbiosis - Intimate living together of members
  of two or more species.
• Commensalism - One member benefits while other is
  neither benefited nor harmed.
• Cattle and Cattle Egrets
• Mutualism - Both members benefit.
• Lichens (Fungus and Cyanobacterium)
• Parasitism - One member benefits at the expense
  of other.
• Humans and Tapeworms

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Defensive Mechanisms

• Batesian Mimicry - Harmless species evolve
  characteristics that mimic unpalatable or
  poisonous species.
• Mullerian Mimicry - Two unpalatable species
  evolve to look alike.

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COMMUNITY PROPERTIES

• Primary Productivity - Rate of biomass
  production. Used as an indication of the rate of
  solar energy conversion to chemical energy.
• Net Primary Productivity - Energy left after
  respiration.

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Abundance and Diversity

• Abundance -Total number of organisms in a
  community.
• Diversity - Number of different species,
  ecological niches, or genetic variation.
• Abundance of a particular species often inversely
  related to community diversity.
• As general rule, diversity decreases and
  abundance within species increases when moving
  from the equator to the poles.

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Complexity and Connectedness

• Complexity - Number of species at each trophic
  level, and the number of trophic levels, in a
  community.
• Diverse community may not be complex if all
  species are clustered in a few trophic levels.
• Highly interconnected community may have many
  trophic levels, some of which can be
  compartmentalized.

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Resilience and Stability

• Constancy (Lack of fluctuation)
• Inertia (Resistance to perturbation)
• Renewal (Ability to repair damage)
• MacArthur proposed complex, interconnected
  communities would be more stable and resilient in
  the face of disturbance.
• Controversial

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Edges and Boundaries

• Edge Effects - Important aspect of community
  structure is the boundary between one habitat and
  others.
• Ecotones - Boundaries between adjacent
  communities.
• Sharp boundaries - Closed communities
• Gradual or indistinct boundaries - Open
  communities

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Edges and Shapes
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COMMUNITIES IN TRANSITION

• Ecological Succession
• Primary Succession - A community begins to
  develop on a site previously unoccupied by living
  organisms.
• Pioneer Species
• Secondary Succession - An existing community is
  disrupted and a new one subsequently develops at
  the site.

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Primary Succession
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Ecological Succession

• Ecological Development - Process of environmental
  modification (facilitation) by organisms.
• Climax Community - Community that develops and
  seemingly resists further change.
• Equilibrium Communities (Disclimax Communities) -
  Never reach stable climax because they are
  adapted to periodic disruption.

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Introduced Species and Community Change

• If introduced species prey upon or compete more
  successfully than native populations, the nature
  of the community may be altered.
• Human history littered with examples of
  introducing exotic species to solve problems
  caused by previous introductions.
• Mongoose and Rats in Caribbean

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Summary

• Critical Factors and Tolerance Limits
• Adaptation
• Natural Selection
• Speciation
• Ecological Niche
• Species Interactions
• Community Properties
• Succession
• Introduced Species and Community Change

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