Transcript Table of Contents - Milan Area Schools

54
Populations in Space and Time
• The individuals of a species in a given area is a
population.
• The distribution of the ages of individuals in a
population and the way those individuals are
distributed over the environment describe the
population structure.
• The number of individuals of a species per unit of
area (or volume) is its population density.
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Populations in Space and Time
• Density of terrestrial organisms is measured as
number of individuals per unit area.
• Density of aquatic organisms is measured as
individuals per unit volume.
• For some species such as plants, the percentage
of ground covered may be a more useful measure
of density than the number of individuals.
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Populations in Space and Time
• The structure of a population changes continually
because of demographic events—births, deaths,
and movment in and out of the population.
• Population dynamics is the change in population
density through time and space.
• Demography is the study of birth, death, and
movement rates that give rise to population
dynamics.
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Populations in Space and Time
• Population dynamics can be represented by:
• N1 = N0 + B – D + I – E
 N1 = number of individuals at time 1
 N0 = number of individuals at time 0
 B = number of individuals born between time 0
and time 1
 D = number of individuals that died between
time 0 and time 1
 I = number of individuals that immigrated
 E = number of individuals that emigrated
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Populations in Space and Time
• Life table information can be used to predict
future trends in populations.
• A cohort is a group of individuals that were born
at the same time.
• A life table can be constructed by determining the
number of individuals in a cohort that are still alive
at specific times, called survivorship.
Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)
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Populations in Space and Time
• In some populations (e.g., humans in the U.S.),
most individuals survive for most of their potential
life span and die at about the same age.
• In some (e.g., songbirds), the probability of
surviving over the life span is the same once
individuals are a few months old.
• In species that produce a large number of
offspring and provide little parental care, high
death rates for the young are followed by high
survival rates during the middle of the life span.
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Populations in Space and Time
• The age distribution of individuals in a population
reveals much about the recent history of births
and deaths.
• For example, in the U.S., population size
increased during the “baby boom” of the 1950s
and again during the “baby boom echo” of the
1980s.
Figure 54.2 Age Distributions Change over Time
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Types of Ecological Interactions
• Species interactions fall into several categories.
 mutualism (+/+ interaction).
 Ex. Termites have protists in their gut that
digest cellulose; they provide the protists,
in turn, with nutrients.
 commensalism (+/0 interaction).
 Ex. Epiphytes living on other plants.
 amensalism (0/– interaction).
 Falling limbs damage smaller plants
beneith them.
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Types of Ecological Interactions
 predator–prey and parasite–host
interactions (+/– interactions).
 Many examples.
 competition (–/– interaction).
 Countless examples.
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Factors Influencing Population Densities
• Species that use abundant resources often reach
higher population densities than species that use
scarce resources.
• Species with small individuals generally reach
higher population densities than species with
large individuals.
 Ex. Cockroaches, ants. Best example is
bacteria!!!
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Factors Influencing Population Densities
• Newly introduced species often reach high
population densities.
• An example is species introduced into a region
where their normal predators and diseases are
absent.
• Zebra mussels whose larvae were carried from
Europe in the ballast water of ships now occupy
much of the Great Lakes and Mississippi River
drainage.
• Complex social organizations (e.g., ants, termites,
humans) may facilitate high densities.
Figure 54.5 Introduced Zebra Mussels Have Spread Rapidly
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Fluctuations in Population Densities
• If a single bacterium were allowed to grow and
reproduce in an unlimited environment, explosive
population growth would result.
• Within a month, the bacterial colony would weigh
as much as the visible universe and would be
expanding outward at the speed of light.
• But while populations do fluctuate in density, even
the most dramatic fluctuations are less than what
is theoretically possible.
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Fluctuations in Population Densities
• All populations have the potential for explosive
growth because, as the number of individuals in
the population increases, the number of new
individuals added per unit of time accelerates.
• If births and deaths occur continuously and at
constant rates, a graph of the population size over
time forms a J-shaped curve that describes a form
of explosive growth called exponential growth.
Figure 54.6 Exponential Population Growth (Part 1)
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Fluctuations in Population Densities
• Exponential growth can be represented
mathematically:
DN/Dt = (b – d)N
• DN = the change in number of individuals
• Dt = the change in time
• b = the average per capita birth rate (includes
immigrations)
• d = the average per capita death rate (includes
emigrations)
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Fluctuations in Population Densities
• The difference between per capita birth rate (b)
and per capita death rate (d) is the net
reproductive rate (r).
• When conditions are optimal, r is at its highest
value (rmax), called the intrinsic rate of increase.
• rmax is characteristic for a species.
• The equation for population growth can be written
D/Dt = rmaxN
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RABBIT/LYNX ACTIVITY