Download Understanding Our Environment

Biological Communities and Species Interactions
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Outline
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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