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Population Ecology
Ecology is the study of
interactions among organisms
and their environment
• Not concerned with individuals
• Populations - same area, same time
• Population ecology - looks at the dynamics
of populations that are similar between
• Population density - number of individuals
of a given species in a specific area at a
given time
• Range - geographic area or limit of a
• Dispersion - frequency or patterns of
individuals within a range:
• uniform
• random
• clumped
Population change
• Growth rate = births - deaths +immigration
- emigration
• Doubling rate (time it takes for a population
to double) = 0.7/growth rate (see page 911)
• Biotic potential = maximum rate of
population growth given ideal
Limits on growth
• Environmental resistance - combination of
the limiting factors and effects
• Limiting Factors - any resource or
requirement that acts to limit population
when in short supply
• Carrying capacity (K) - the greatest
population that can be maintained
indefinitely by a given system or place
Density dependent factors
• As population increases the rate of growth
is slowed by density dependent factors
either by increasing the death rate or
decreasing the birth rate
• predation, disease, intraspecifc (within a
species) competition and interspecific
Density independent factors
• Weather events
• natural disasters
Survival tactics - reproduction
• r strategies - (r=growth rate) - small body
size, large brood, short life span, may be
opportunistic and found in disturbed or
variable environments
• K strategies - (K=carrying capacity) - large
body small brood, long life, care for young,
constant or stable environments
Human population growth
See page 45-9
1800 human population reaches 1 billion
1930 - in 130 years 2 billion
1960 - in 30 years 3 billion
1975 - in 15 years 4 billion
1987 - in 12 years 5 billion
1999 - in 12 years 6 billion
What are the density dependent
limits to growth acting on human
Demographic transition
• Preindustrial stage - high birth and death
rates - slow population growth
• transitional stage - lower death rate but birth
rate remains high - rapid population growth
• industrial stage - birth rate declines - rate of
growth slows
• post industrial stage - low birth and death
Communities of organisms
Chp 46
• Producers - autotrophs
• Consumers - heterotrophs
• primary and secondary
• Decomposers - heterotrophs
No organism lives in isolation.
Every living thing is part of a
• Predation - pursuit and ambush
• Defenses - camoflage, chemical defense,
mimicry - (batesian mimicry or mullerian
• A symbiotic relationship
• both partners benefit
– rhizobium bacteria and plants
– pilot fish and sharks?
• One benefits , the other is neither harmed
nor helped
– epiphytes
– sea ducks and sting rays
• Parasite and host
• pathogen - parasite that causes disease
• The ecological role of an organism is its
• fundamental niche- potential niche
• realized niche- actual
• interspecific competition leads to
competitive exclusion - absolute overlap
cannot exist in nature
Diversity in communities
• Isolated or places with harsh environments
have less diversity
• edge effect - diversity is usually greatest at
the margins
• old communities (tropical rainforests) tend
to be more diverse than new communities
(Canadian shield, artic)
• Primary succession - change in species
composition over time in a habitat not
previously inhabited by organisms
• Pioneer community - first community to
• Secondary succession - change in species
composition over time in a habitat already
modified by previous organisms
• Climax community -