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- may also indicate where
  a species could be found. Ex. Adult pupfish
  tolerate a larger range of temeperatures in their
  environment, but juvenile pupfish only tolerate a
  small range, therefore reproduction will only
  occur within the small range.

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Adaptation

• Adapt is used in two ways
• Range of physiological modifications available to
  individual organisms. This is called
  physiological modifications or acclimation.
• Inheritance of specific genetic traits allowing a
  species to live in a particular environment. This
  is the process of evolution by natural selection.
  
• 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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Galapagos Finches
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Evolution

• Divergent Evolution - Mutations and different
  selective pressures cause populations to evolve
  along dissimilar paths. (variety of finches which
  can no longer interbreed)
• Convergent Evolution - Unrelated organisms evolve
  separately to cope with environmental conditions
  in the same fashion.
• Look alike (fruit eating finches look like
  parrots) Act alike

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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 a process in which species exert
  selective pressure on each other- prey develops
  defenses against the predator and then the
  predator develops mechanisms to overcome the
  defenses.

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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. Both benefit
• Wasp Beetle

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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.
• Productivity of a forest, coral reef or estuary
  will be higher than a desert due to lack of
  water, each ecosystem might have a limiting
  factor whether it be water or nutrients.
• Only a small percent of the sunlight that is
  absorbed by a producer is used for photosynthesis

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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. Productivity is
  related to abundance and diversity which depend
  on the total resource available in an ecosystem.

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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 in composition or
  function)
• Inertia (Resistance to perturbation, a deviation
  from its normal state)
• Renewal (Ability to repair damage)
• MacArthur (1955) proposed complex, interconnected
  communities would be more stable and resilient in
  the face of disturbance.
• Controversial depending on the disturbance or
  removal of a keystone member

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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
• Indistinct boundaries - Open communities
• Animals may interconnect the ecosystems by
  feeding in one and make their home in the other.
• Conservationists recognize that the edge effects
  associated with habitat fragmentation are
  generally detrimental to biodiversity. Preserving
  large habitat blocks and linking smaller blocks
  with migration corridors may be the best way to
  protect rare and endangered species

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Edges and Shapes
No significan t core habitiat since no area is
far enough from the edge of the ecosystem
Significant core habitat
Temp, wind, humidity will differ at the edge of a
habitat
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COMMUNITIES IN TRANSITION

• Ecological Succession- the gradual change in the
  environment over time by creating soil, shade,
  shelter or increasing humidity
• Primary Succession - A community begins to
  develop on a site previously unoccupied by living
  organisms.
• Pioneer Species (microbes, mosses, lichens)
  creating ecological development ( a modification
  of the environment)
• 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, usually by
  periodic fires.

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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