54 Populations in Space and Time The individuals of a species in a given area is a population. The...
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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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54 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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54 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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54 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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54 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.
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Table 54.1 Life Table of the 1978 Cohort of the Cactus Finch on Isla Daphne (Part 1)
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54 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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54 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.
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Figure 54.2 Age Distributions Change over Time
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54 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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54 Types of Ecological Interactions
predator–prey and parasite–host interactions (+/– interactions).
Many examples.
competition (–/– interaction). Countless examples.
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54 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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54 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.
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Figure 54.5 Introduced Zebra Mussels Have Spread Rapidly
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54 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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54 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.
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Figure 54.6 Exponential Population Growth (Part 1)
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54 Fluctuations in Population Densities
• Exponential growth can be represented mathematically:
N/t = (b – d)N
• N = the change in number of individuals
• t = 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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54 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
/t = rmaxN
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54
RABBIT/LYNX ACTIVITY