Transcript Table of Contents - Milan Area Schools

54
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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Fluctuations in Population Densities
• Real populations do not grow exponentially for long
because of environmental limitations.
• Environmental limitations include food, nest sites,
shelter, disease, and predation.
• The carrying capacity of an environment (K) is
the maximum number of individuals of a species it
can support.
Figure 54.7 Logistic Population Growth
54
Fluctuations in Population Densities
• The mathematical representation of this type of
growth (logistic growth) is:
DN/Dt = r[(K – N)/K]N
• The equation for logistic growth indicates that the
population’s growth slows as it approaches its
carrying capacity (K).
• Population growth stops when N = K.
54
Fluctuations in Population Densities
• Per capita birth and death rates usually fluctuate
in response to population density; that is, they are
density-dependent.
 Competition for resources
 Easier for disease to spread.
• Factors that affect birth and death rates in a
population independent of its density are said to
be density-independent.
• For example, a severely cold winter may kill large
numbers of a population regardless of its density.
Figure 54.9 Population Sizes May Be Stable or Highly Variable
54
Population Fluctuations
• Densities of populations that depend on limited
resources fluctuate more than those that use a
greater variety of resources.
• Why does this make sense?
54
Population Fluctuations
• Predator–prey interactions generate fluctuations
because predator population growth lags behind
growth in prey and the two populations oscillate.
 Lynx-Hare activity.
• Experiments with Canada lynx and snowshoe
hares revealed that the oscillating cycle of their
populations was driven by both predation and
food supply for the hares.
Figure 54.11 Hare and Lynx Populations Cycle in Nature (Part 1)
54
Managing Populations
• A general principle of population dynamics is that
the total number of births and the growth rates of
individuals tend to be highest when a population
is well below its carrying capacity.
• If we wish to maximize the number of individuals
that can be harvested from a population, that
population should be managed so that its
population is far below its carrying capacity.
• Hunting seasons are established with this
objective in mind.
54
Managing Populations
• Populations with high reproductive capacities can
sustain their growth despite a high rate of harvest.
• Fish are an example of a population with high
reproductive capacity.
 High number of eggs produced by each.
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Managing Populations
• The whaling industry engaged in excessive
harvests that almost caused the extinction of blue
whales.
• Management of whale populations is difficult
because they reproduce at a low rate.
• Since whales are distributed worldwide, their
management is dependent on cooperative action
by all whaling nations (which is difficult to
achieve).
54
Managing Populations
• To reduce the size of populations of undesirable
species, removal of resources is more effective
than large-scale killing.
• By removing resources, the species will have a
reduced carrying capacity and therefore lower
numbers.
• Killing large numbers of the species would simply
reduce them to a population size that grows more
rapidly to reach its carrying capacity.
• Conversely, if a rare species is to be preserved,
the most important step usually is to provide it
with suitable habitat.
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Managing Populations
• Humans have introduced many species to new
habitats outside their native ranges.
 Rabbits in Austrailia
 Opuntia cactus in Austrailia.
Figure 54.19 Biological Control of a Pest
54
Managing Populations
• For many thousands of years, Earth’s carrying
capacity for humans was set at a low level by food
and water supplies and by disease.
• What caused the increase?
 Medicine
 Agriculture
 Others
• Earth’s carrying capacity is currently limited by:
 “Waste” removal
 Willingness to destroy other species.
Figure 54.20 Human Population Growth