Important words and concepts
from Chapter 52, Campbell & Reece, 2002 (3/25/2005):
by Stephen T. Abedon (abedon.1@osu.edu)
for Biology 113 at the Ohio State University
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Course-external links are
in brackets Click [index] to access site index Click here to access
text’s website Vocabulary
words
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(1) Chapter title: Population Ecology
(a)
[population ecology
(Google Search)]
[index]
(a)
Population ecology studies organisms from the point of view of the size
and structure of their populations
(b)
A population ecologist studies the interaction of organisms with their
environments by measuring properties of populations rather than the behavior of
individual organisms
(c)
Properties of populations include
(i)
Population size (size)
(ii)
Population density (density)
(iii)
Patterns of dispersion (dispersion)
(iv)
Demographics (demographics)
(v)
Population growth (growth)
(vi)
Limits on population growth (limits)
(d)
Note that all of these properties are not those of individual organisms
but instead are properties which exist only if one considers more than one
organism at any given time, or over a period of time (i.e., they are emergent properties)
(e)
"The characteristics of a population are shaped by the
interactions between individuals and their environments on both ecological and
evolutionary time scales, and natural selection
can modify these characteristics in a population."
(f)
Thus, population ecology also goes beyond consideration of just
population parameters and additionally considers how the characteristics of
individual organisms impact on population parameters
(g)
[population ecology
(Google Search)]
[index]
(a)
A population in an ecological sense is a group of organisms, of the
same species, which roughly occupy the same geographical area at the same time
(b)
Individual members of the same population can either interact directly,
or may interact with the dispersing progeny of other members of the same
population (e.g., pollen)
(c)
Population members interact with a similar environment and experience
similar environmental limitations
(d)
[population (Google Search)]
[index]
(a)
A population's size depends on how the population is defined
(b)
If a population is defined in terms of some degree of reproductive
isolation, then that population's size is the size of its gene pool
(c)
If a population is defined in terms of some geographical range, then
that population's size is the number of individuals living in the defined area
(d)
Ecologists typically are more concerned with the latter means of
defining a population since this is both easier to do and is a more practical
measure if one is interested in determining the impact of a given population on
a given ecosystem, or vice versa
(e)
“Although we can determine an average population size for many species,
the average is often of less interest than the year-to-year or place-to-place
trend in numbers.” (p. 1166, Campbell & Reece, 2002)
(f)
[population size, "population size"
and "population ecology" (Google Search)]
[index]
(a)
Given that a population is defined in terms of some natural or
arbitrarily defined geographical range, then population density may be defined
as simply the number of individual organisms per unit area
(b)
Different species, of course, exist at different densities in their
environments, and the same species may be able to achieve one density in one
environment and another in a different environment
(c)
Population densities may additionally be determined in terms of some
measure other than population size per unit area such as population mass per
unit area
(d)
[population density,
"population density"
and "population ecology" (Google Search)]
[index]
(a)
Individual members of populations may be distributed over a
geographical area in a number of different ways including
(i)
Clumped distribution (attraction)
(ii)
Uniform distribution (repulsion)
(iii)
Random distribution (minimal interaction/influence)
(b)
See Figure 52.2, Patterns of dispersion within a population’s
geographical range
(c)
Clumping may result either from individual organisms being attracted to
each other, or individual organisms being attracted more to some patches within
a range than they are to other patches; the net effect is that some parts of
the range will have a large number of individuals whereas others will contain
few or none
(d)
A uniform distribution means that approximately the same distance may
be found between individual organisms; uniform distributions result from
individual organisms actively repelling each other
(e)
A random distribution means that where individual organisms are found
is only minimally influenced by interactions with other members of the same
population, and random distributions are uncommon;
"Random spacing occurs in the absence of strong attractions or repulsions
among individuals of a population."
(f)
Note that both clumping and uniform distributions suggest that
individual organisms are either interacting with one another (actively seeking
each other out or actively avoiding each other), or are all competing with one
another for the same limited resources, regardless of the overall population
(g)
[patterns of disperson
(Google Search)]
[index]
(a)
A population's demographics are its vital statistics, particularly
those statistics which can impact on present and future population
size
(b)
Two statistics that are of particular import are a population's age
structure and a population's sex ratio
(c)
Additional considerations (in human populations and for example) are
considered to the right à
(d)
[demographics (Google Search)]
[Center for Demography and Ecology] [index]
(a)
Age structure refers to the size of cohorts within a population
(b)
Parameters related to age structure include
(i)
Fecundity (birth rate)
(ii)
Generation time
(iii)
Death rate
(c)
See Figure 52.22, Age structure pyramids for the human population of
(d)
Below is the actual and predicted age structure the Dutch civil service
in 2000, 2005, 2010, and 2015:
(f)
[age structure (Google Search)]
[index]
(a)
A cohort is a group of individuals all of whom have the same age
(b)
In a typical population, the size of cohorts will vary with age
(c)
For example, in a typical population, younger cohorts will be larger
(i.e., more individuals per cohort) than older cohorts, all else being equal
(a)
Fecundity refers to the average birth rate associated with a population
(b)
The greater a population's fecundity, all else held constant, the
faster a population will increase in size
(c)
Note that fecundity typically varies with the age of individuals
(d)
[fecundity, birth rate (Google Search)]
[index]
(a)
Generation time is simply the average span between the birth of
individuals and the birth of their offspring
(b)
"Other factors being equal, a shorter generation time will result
in faster population growth."
(c)
Note that species which are capable of reproducing more than once will
display an overlapping of generations which basically means that parental cohorts
and progeny cohorts can be alive (and potentially competing with one another)
at the same time
(d)
Note that another way of saying this is that when life expectancies
exceed the minimum time between generations, generations will overlap
(e)
[generation time (Google Search)]
[index]
(a)
Death rate is the rate at which individuals of a certain age die
(b)
Note that death rates often vary with age with either the very young or
the very old displaying the greatest death rates
(c)
Note additionally that population growth
occurs when overall birth rates exceed overall death rates
(d)
[death rate, "death rate" and
"population ecology" (Google Search)]
[index]
(a)
More often than not the rate at which a population may grow is
dependent on the sex ratio in the population; the fewer females, the slower the
rate of population growth
(b)
This, of course, is because uteruses are limiting and males often can
inseminate more than one female
(c)
This generalization falls apart, however, when males are limited in
their ability to inseminate more than one female, or when males contribute
significantly to the raising of offspring
(d)
Below are sex ratios (
(e)
(f)
[sex ratios, "sex ratios" and
"population ecology" (Google Search)]
[index]
(a)
Observing age structure graphically can provide insights into a
species' (or a population's) ecology
(b)
Survivorship curves graph cohort size
against relative age
(c)
See Figure 52.3, Idealized
survivorship curves
(d)
The typical survivorship curve shows cohort size declining with age
(e)
There exist three general types of survivorship curves
(i)
Type I
(ii)
Type II
(iii)
Type III
(f)
Note
in the following survivorship curves that the y axis is logarithmic!!!
(h)
[survivorship curves
(Google Search)]
[index]
(15) Type II
survivorship curves
(a)
The simplest type of decline is exponential, i.e., the death rate for
every cohort is the same
(b)
These survivorship curves graph as a straight line on
semi-logarithmic graph paper (i.e., as presented in a typical survivorship
curve)
(c)
The individuals in populations that display a type II curve are those
that both do not age and are born as fully fit as adults, e.g., hydra
(d)
Individuals are lost in these populations mostly to accidents and
predation
(e)
[(Google Search)]
[index]
(16)
(a)
Because individuals tend to die exponentially due to accidents or
predation, it often is a good strategy to reproduce relatively early in a life
span rather than relatively late
(i)
That way individuals achieve reproduction while they still have a
reasonable likelihood of being alive
(ii)
This is assuming, of course, that the goal is a Darwinian one, i.e.,
maximizing one's reproductive output
(iii)
Note that how such a strategy works is complicated if individual fecundity
increases with age
(b)
Very often for a given species there will be some age at which
individuals are maximally fecund
(c)
Species that combine maximum fecundity with early ages typically do so
at the expense of their ability to survive long periods (i.e., this is an
example of the principle of allocation)
(d)
(e)
Humans, of course, have a type I survivorship curve; evolution makes us
get married young and have lots of babies before a saber toothed tiger comes
along and picks us off, i.e., à
(f)
[(Google Search)]
[index]
(17) Type III
survivorship curves
(a)
The other side of the survivorship coin is the degree of investment in
individual progeny
(b)
Some organisms invest a great deal in each offspring and those
organisms are (ideally at least) rewarded with relatively high survivorship at
early ages
(c)
Other organisms invest little in individual offspring, and display very
low early-age survivorship (which they make up for by producing buckets of
offspring)
(d)
Organisms that produce large numbers of cheap progeny and which display
minimal declines in fecundity with age, if they survive their youth, display
type III survivorship curves
(e)
Examples include sea turtles and trees
(f)
That is, type III survivorship species have a very large rate of
mortality when young, but should they survive their youth, they put significant
energy into continued survival since the longer they survive, the more progeny
they will produce
(g)
[(Google Search)]
[index]
(a)
“The traits that affect an organism’s schedule of reproduction and
survival (from birth through reproduction to death) make up its life history.”
(p. 1156, Campbell & Reece, 2002)
(b)
The study of life history characteristics is the detailed study of
those ecological and evolutionary parameters that impact on survivorship curves
(c)
"In many cases there are trade-offs between survival and traits
such as clutch size (number of offspring per reproductive episode), frequency
of reproduction, and investment in parental care. The traits that affect an
organism's schedule of reproduction and death make up its life history. Of
course, a particular life history pattern, like most characteristics of an
organism, is the result of natural selection operating over evolutionary
time."
(d)
In other words, the Darwinian goal is to maximize lifetime
reproductive output, and this can be achieved by having babies more rapidly
or living longer, or some combination of the two, as well as by varying many
additional details having to do with survival and reproduction
(e)
However, different combinations of these life history parameters will
result in organisms producing different numbers of surviving
offspring—evolution will tend to maximize the representation in a population of
those individuals who display those combinations of life history traits that
maximize the number of surviving progeny they produce
(f)
[life history (Google Search)]
[index]