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Ecological Succession
– ecological succession - ecosystem change from an early,
immature, transient stage to a more mature and final stage
(climax community)
– examples: clearcuts to mature forest, burned prairies to
mature grasslands
– seral stages – a temporary, distinct stage in succession
– primary succession - on sites previously unoccupied by
living organisms; lava flows, volcanic islands
– secondary succession - on sites where organisms existed;
burned or cut areas
– structure and function of ecosystem becomes more
complex with each seral stage
– disclimax - system held at a stage below climax by some
disturbance; longleaf pine forests, some prairie areas
– biologists are interested in which stages support different
organisms; some depend on early stages (quail) some on
old-growth stages (woodpeckers, goshawk); many animals
rely on several stages for food or shelter
Niche and Environment
– niche - role or function of an organism in the biotic
community; includes trophic level (feeding level), foraging
location (forest, pasture, etc.), where it feeds (canopy,
mid-story, ground, etc.), what it eats (insects, seed, etc.),
what size food it eats (large or small seeds)
– habitat - set of environmental conditions under which an
individual, species, or community exists; can have seasonal
habitats
– environment - habitat of an organism plus all surrounding
conditions (flooding regimes, regular weather patterns, etc.)
– microhabitat - small area of intensive use within the habitat
of an organism; dead logs for salamanders, nest sites for
birds
– guild - group of organisms that use resources in similar
patterns and often have much niche overlap (bottom feeder
fish, insectivorous birds, grazers on the African plains)
Competition
– occurs when 2 or more organisms living in the same area
try to use the same resource that is in limited supply
– 2 types
1. interspecific - between 2 or more species; density
dependent
2. intraspecific - within a species; highly density dependent
For interspecific (between 2 or more species):
– niche segregation - species can overlap on several niche
dimensions and still not have competition; hawk and owl
(same area and food but different time of day)
– competitive exclusion principle (Gause’s principle) - no 2
species can completely occupy the same niche indefinitely;
one species will win (outcompete) and the other perish
Examples: sunfish partition food in a pond, they’ll feed in
different levels of the water column; yellow and red-winged
blackbirds will partition nesting sites when using the same
area of a marsh
– in natural, undisturbed systems, interspecific competition
(niche partitioning) has been worked out by co-evolution;
problems usually arise when we introduce exotic species
into a system (giant marine toad, fish from SE U.S. moved to
west coast U.S., many plant problems)
Gause’s paramecium experiment
Niche partitioning by warblers in a New England forest
For intraspecific (within a species):
– species sometimes partition habitat by sexual dimorphism
(smaller male hawks take smaller prey than females)
2 categories:
1. exploitative competition - individuals divide resource and
suffer same ill effects (reduced birth rates, poor body
condition); most fish populations
2. contest (interference) competition - some individuals do
better than others in obtaining a resource (dominance, larger
body size, faster, etc.); typical with carnivores
Home Range and Territory
– home range - the area within which an individual animal
normally travels in its daily activities
– geographic
range (species range) - boundary of a species
– individuals change home range seasonally due to weather,
habitat condition, food availability, and reproductive
condition
– you can identify seasonal or annual home range; useful to
know when an animal needs specific resources
Examples: turkeys need nesting, feeding, and roosting
habitat; deer in cold regions have summer grazing lands and
wintering grounds in forested bottoms
– territory - portion of home range defended against others
of the same or closely related species; deals with space
requirements
Examples: most large carnivores defend a hunting area;
birds use displays and songs to delineate a nesting and
foraging area during the breeding season
– pack territory - area defended by several individuals acting
as a single unit; wolf packs
– territories can be seasonal if individuals must group
together during certain seasons; still have individual
distance requirement that they will defend
Examples: migrating birds travel as flocks, deer group
together on wintering grounds