||Elizabeth A. Morrison
Chapter 23: Populations
I.Ecology: the study of the interactions between organisms of the same species, other organisms of different species, and the non-living (abiotic) environment. Complex and allows scientists to answer the big questions, but it's flaw is it's complexity and immensity.
A.Population: a group of organisms from the same species, living in a connected habitat. This habitat is non-specified in size.
As Malthus and Darwin realized, all species have the capacity for unlimited growth
1.Population size. We are accustomed to this, but ecologists rarely use it.
2.Population density: estimate. Ecologists use this most frequently
a.Evenly spaced: organisms repel each other: territoriality, tall trees. Even if food supply is more than needed, will not share.
b.Clumped: organisms attract each other -- cattle, or do not distribute their offspring evenly: many flowers, uneven environment. Herding animals and humans.
c.Random: completely unaffected by each other and uniform environment: very rare.
4.Growth rate (increases or decreases).
III.Demography: study of distribution of sex and age groups
A.Age-sex structure of populations: population Pyramid.
B.Stable population: size and structure remain the same through time.
IV.Life histories: another way of looking at populations
A.Mortality and survivorship curves. Age distributions typical of a given population that has been in a stable environment for several generations. Survivorship curves are upside down mortality (death) curves.
1.Type I: low mortality, then high. Human, elephant.
2.Type II: constant. Hydra, birds.
3.Type III: high, then low. Oysters, insects, plants.
1.Current reproduction reduces capacity for future (residual) reproduction. This is the cost of reproduction. If reproduction happens now, less likely to reproduce later
2.Clutch size. How many they have each time.
3.Number of reproductive events per lifetime. Most designed to reproduce once.
4.Age at first reproduction.
A.Factors related to growth
1.Rate of population increase = r = fecundity (natality) + immigration - (mortality + emigration)
2.If r = 0, zero population growth. Everything stays the same.
3.Biotic potential: innate capacity to increase. Intrinsic rate of natural increase (maximum r)
4.Carrying capacity (K) and environmental resistance to population growth. K = ability to carry species: maximum population density. What happens when population gets too high = environmental resistance.
B.Models of growth
1.Exponential growth followed by die back. J-shaped curve. Each generation is larger than the last, and has even more offspring when it reaches sexual maturity. (ex. E coli). Characteristics:
a.Exponential growth phase
b.Overshoot: may occur because a limiting factor is effective only close to the carrying capacity.
d.Stabilization. Depends on how damaged the environment is, if too much, k becomes lower, or species is wiped out there.
e.Opportunistic. Species that take advantage of short-lived habitat. Grow rapidly, try to give offspring chance to get to the next habitat.
2.Logistic growth: environmental resistance increases with population size. Ideal situation. Sigmoid ("sigma") growth curve
a.Stabilize around carrying capacity, since the environment is not damaged
b.Oscillations may still be large.
3.R and K strategists
a.R-strategists, evolved to maximize growth. Many offspring, little care, short lived. Take advantage of unstable environment. J-shaped growth.
b.K-strategists, evolved to maximize survival when the population is near its environmental carrying capacity. Few offspring, much care, long lived, reproduce many times. Sigmoid growth (type I survivorship).
C.Factors controlling population growth
1.Density dependent factors: disease, competition, predation, stress
2.Density independent factors: earthquake, storm. Rare.
D.Human population growth
1.Three surges in growth
a.Cultural revolution: Start of humans
b.Agricultural revolution: domestication of plants and animals.
c.Industrial revolution: medicine, sanitation, nutrition. Big surge
d.Post-industrial societies: cost, benefits of child rearing reduce growth. Wealthy societies. Costs of raising children are great, and most women chose not to have many.
2.Unevenness in growth: population pyramids. Negative growth, zero growth, rapid growth increase.
3.The Earth's carrying capacity. What is it?
4.Could the carrying capacity drop unexpectedly?
a.Failure of a staple crop
b.Pollution and extinction: how resilient are ecosystems? 20% of species are critical.
d.Disease. Science may control.
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