FILE: <ent129.4.htm> Comprehensive
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I. Introduction (Note:
Instructor to draw curves on graphs)
A. The following discussion is brief, highly
generalized, with the various subjects slated for further
development in advanced courses in
biological control and other specialized courses on this campus.
B. When insects, plants, or microorganisms
become sufficiently abundant to compete with humans for
food and fiber; i.e., to cause economic
losses, they are termed pests. Thus, in economic entomology and
biological control, we are concerned
with pest population densities and how these densities change and
can be
changed.
II. Terms
A. Population
= any group of individuals of the same species that occupies a given area at a
given time.
1. can be broken down into smaller units or demes.
2. gene flow occurs among the individuals of a
population.
3. a population must have a certain minimum
size and occupy an area that contains all its need resources
(ecological requisites) before it can
display fully such characteristics as growth,
dispersal, genetic
variability, and continuity in time.
4. populations also possess such unique
characteristics as birth rates, death rates, sex ratios and age
structure.
B. Demography
= the study of populations.
C. Population Ecology = a phase of demography.
D. Population Dynamics = applied to that aspect of population
ecology that deals with the forces affecting
changes in population density (i.e.,
with the forces affecting the form of population growth).
1. population equilibrium = refers to the tendency
of a single species population to return to its average
density, or equilibrium level, after
some outside external force has
temporarily caused it to depart from that
level.
This tendency has also been termed homeostasis
or balance.
E. Ecosystem
= used to designate the interacting system comprised of all the living
organisms of an area
and their nonliving environment. This area must be large enough or contain
enough resources to permit
energy flows associated with the
perpetuation of its component organisms.
F. Control
= to control a pest is to reduce or maintain its densities below the so-called economic injury level.
G. Biological Control = through the importation, augmentation, and
manipulation of the pest's natural enemies,
seeks to create an environment
permanently unsuited to the pest's development.
III. To appreciate the complexities
involved in permanent shifts of the equilibrium level of a pest, decades of
carefully
examined studies are often required of
the natural populations themselves. The
majority of entomologists apparently
find this an unsurmountable task and are
satisfied to merely recognize that the complexities exist.
Some of us who comprehend many of the
forces are daring to communicate this knowledge to colleagues
and students. Language (i.e., fixed terminology), is the
first necessary step in the process.
The following t
erms are suggested to
enable communication on matters of population dynamics:
Competition = the interference between to or more
organisms seeking the same requisites.
Two kinds exist:
interspecific and intraspecific.
Limiting Factor = a factor whose input into a given
ecosystem is independent of a given
population, yet sets
the maximum density at which that
population can exist. Examples are
nesting sites, protective niches, quality
of available food, etc.
Regulating Factor(s) = the one (or sometimes
more) element(s) in the ecosystem that is (are) primarily
responsible for the level of the
population density. Examples are as
follows:
single factor = Rodolia cardinalis,
Metaphycus helvolus, Aphytis melinus,
Trioxys pallidus, Cactoblastis,
Dactylopius,
Chrysolina spp., Myxomytosis, Ceratocystis, Endothia,
etc.
(in Ceratocystis the beetle
Scolytus multistriatus
is probably better designated the key regulating factor).
two factors = Rodolia
and Cryptochaetum in the
Riverside, California area.
three or more factors = found especially in
multivoltine species and probably true of the common house fly.
Control = the manipulation by humans of population determining
factors to maintain a given pest population
at noneconomic levels.
IV. Until relatively
recent times, population study was largely confined to human demography. Early humans
undoubtedly counted their domestic
animals, but most of the written speculations on population growth prior to the
19th Century dealt with humans.
A. Censuses were taken by the ancient
Egyptians, Babylonians, Greeks, Romans and Chinese.
B. William Derham in England published Physico-Theology in 1713, which entitles him to a place in the
history of population ecology.
1. asserted that various species of animals
differed in their structure and modes of life because "... the surface
of the globe is covered with different
soils, with hills and vales, with seas, rivers, lakes, and ponds, with diverse
trees and plants," and that various
species of animals were "manifestly adapted" for the places in which
they
live and for the ways in which they
live.
2. Derham also stated that, "the whole
surface of our globe can afford room and support only to such a number
of all sorts of creatures. And if by their doubling, trebling, or any
other multiplication of their kind, they should
increase to double or treble that number,
they must starve, or devour one another."
This is prevented, he stated
by "balancing the number of
individuals of each species of creatures in that place appointed thereto."
C. Robert Malthus (1803)
1. published commentary on social problems of
his day and ours that generated considerable controversy and
criticism, yet which became a major
biological concept: The Malthusian Principle.
2. he was antedated by Giovanni Botero who conceived and published the same concept two
centuries earlier.
Malthus had the advantage of living in a
time when England was worrying about overpopulation.
3. both Botero and Malthus suggested that human
population increased more rapidly than their means of
subsistence, until they are checked by famine, disease, or war.
4. Malthusian Principle = populations tend to
increase geometrically, that is, by successive doublings
per successive equal time intervals, and
their means of subsistence increase only arithmetically.
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5. Malthus' ideas have been criticized because
he was considering humans, who along among animals can
consciously and drastically alter their
environments to favor their wholesale displacement of other species.
Humans alone can also control their own birth rates.
6. A weakness inherent in the Malthusian
concept, is the notion that the means of subsistence increase
arithmetically ad infinitum. Were this so, populations would indeed
increase without limit. All
environments,
however, have limited resources and no
species populations can increase indefinitely.
D. Charles Darwin <PHOTO> - Origin
of Species published 1859.
1. called attention to the fact that
"There is no exception to
the rule that every organic being increases at so
high a rate, that, if not destroyed, the
earth would be covered by the progeny of a single pair."
2. He also recognized that the abundance of
plant and animal species was limited not only by other organisms
that directly exploited them as food,
but also by competition: competition among their own kind and between
other species for food, space, shelter
and other such resources of the environment.
3. Darwin's considerable insight into these
matters of competition and exploitation is attested to his use of
the terms "struggle for existence,"
"survival
of the fittest" and "to eat and to be eaten."
4. In his Origin
of Species, Darwin presented numerous examples of how mortality
caused by other organisms
served to limit populations of various
species.
5.. In his distinction between that is today called interspecific
and intraspecific competition,
Darwin provided
a scheme of ecological thinking which
advanced and profoundly influenced population theory.
E.
Spencer
(1897).
1. introduced the concept of homeostasis which he defined as the
tendency of living systems to maintain
by their own regulatory devices, an
internal stability.
2. The Spencerian concept implies that the more
stable biological systems are those which are more complex;
i.e., the greater the kinds of organisms
present in a community, the more reliable is the system of checks and
balances against excessive fluctuations
in abundance. This notion was explored
in depth by the English ecologist, Elton.
3. This concept also led Harry Scott Smith in
1929 to suggest that a complex of natural enemies, rather than a
single species, should be imported for
biological control, an idea that is still debated in the current literature
[the so-called "Canadian"
versus "Californian" approaches].
F. Verhulst (1838).
1. calculated that population growth followed a
characteristic S-shaped curve, which
he termed the logistic
curve.
2. In 1920, this growth curve was rediscovered
and developed independently by Pearl
and Reed. This gave
rise to Quantitative Population Ecology.
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oscillations
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3. Logistic
Relation = if, under physically constant conditions, the beginnings of a
population of organisms
is introduced into a favorable environment,
growth will start slowly, then tend to increase geometrically, and
finally to progressively decrease,
becoming more and more retarded, until population growth ceases, at which
point the population density is in equilibrium
with a given environment.
G. Chapman (1931)
1. coined the term Environmental Resistance to designate the total effect of all
factors tending to limit the
growth of populations.
2. environmental resistance retards population
growth at the upper asymptote of the logistic curve.
3. Therefore, the logistic curve expresses
quantitatively the idea that the growth of a population of organisms
is at every moment of time determined by
the relationship between the potential rate of increase, designated
the biotic
potential by Chapman, and the environmental resistance.
4. Environmental
resistance is composed of:
a. biotic
factors (i.e., other organisms involved).
b. abiotic factors (e.g., weather, soil,
air, space and light).
5. The population density is influenced by many
forces; it is difficult to separate the roles of biotic versus
abiotic factors.
H. Howard and Fiske (1911).
1. were the first to
develop a scheme based on action or effect (i.e., functional relationships).
2. They separated the causes of mortality in
insects into two categories: catastrophic and facultative.
a. catastrophic
= destruction of a constant percentage regardless of the abundance of insects.
b. facultative
= destruction of a percentage which increased when numbers of the host
increased. In other
words, facultative mortality factors
are responsible to changes in host density.
I. Harry Scott Smith (1935) <PHOTO>.
1. proposed that these groups of factors be
called density-independent (=
catastrophic) and density-dependent
(= facultative).
2. suggested that density-dependent factors
were mostly the function or actions of biotic agents, whereas
density-independent factors were mainly functions of physical or
abiotic components of the environment
and were often associated with climate.
3. Smith also recognized that intra- and
interspecific competition for food, space, and other requisites were
density-dependent factors.
J. Lotka and Volterra (1920).
1. devised a mathematical model not tied to the mathematics of
geometric progression, but one which resulted
in a periodic or cyclic, host-parasitoid
relationship.
2. Host mortality was viewed as a function of
both parasitoid numbers and host numbers.
3. They
proposed that for each value of host abundance, there is a corresponding
value for parasitoid
abundance: as the host density increases, so does that of the parasitoid.
4. The increase in parasitoid numbers results
in a fall in host numbers, followed by a fall in parasitoid
numbers, and so on, cycle after cycle.
K. Gause (1930).
1. experimentally studied
the interactions of populations of two species of protozoans, Didinium nasutum
which fed on Paramecium caudatum.
2. Hist laboratory systems were handicapped
because of spatial limitations, so that he usually got extermination
of the prey species instead of the
fluctuations we see in nature.
Predator-Prey
Interaction
Without Immigration
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With Immigration
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reintroduced
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system
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3. The curves depict the characteristic sequence
of events when a predatory of a parasitic species regulates
the population of a prey species.
4. Gause at first was concerned by the fact
that he always got extermination (Graph I).
5. He subsequently overcame this difficulty by
periodically introducing new individuals of the prey into his
systems, simulating prey
immigration. This technique resulted in
the cyclic fluctuations shown in Graph II.
L. Nicholson and Bailey (1933-35).
1. extended the Lotka-Volterra model.
2. Its basic premise was that populations of animals search at random for such requisites as food,
mates, and
suitable places in which to live, even if
the individuals which comprise these
populations do not!
i.e., by random searching, the success of a group of animals in finding a
requisite of food, etc., is a simple
function of the product of density
of animals and the density of the object sought.
3. Theory of Balance
The premise that the density of animals
themselves governs the degree by which the inherent trend to increase
in numbers (biotic
potential) is greater or less than the repressive forces (Chapman's components
of
environmental
resistance) of the environment.
repressive components
= a. inter- and intraspecific
competition; and b. action of
natural enemies.
V. Contemporary Concepts in Population Regulation
A. Early workers such as Verhulst, Pearl, Lotka
and Volterra, recognized that the problems of predator-prey
relationships were distinctly
mathematical.
B. Experimental ecology subsequently evolved as
a means of providing the data for mathematical formulation
and a means of testing deductive
(general to specific) models in the laboratory, such as was begun by Gause
in the 1930's.
C. During the late 1920's to 1930's, several
distinct but related lines of thought evolved.
Three important
ones were:
1. physical factor ecology.
2. production ecology.
3. population ecology.
C. Physical factor ecology evolved as a
reaction to what was felt to be undue emphasis on Darwin's ideas
about the "struggle for
existence," competition, balance and on the normal regulation of insect
population
densities by natural enemies (i.e., the
ideas of Bodenheimer).
1. Bodenheimer adhered strongly to this
concept.
2. He believed that the abiotic factors,
principally climate regulate the
numbers of individuals of a species
population.
3.
His view, as later admitted by
himself, was a gross oversimplification, in that it failed to account for the
adaptiveness of organisms to change
and for the ability of individuals to interact with all components of
their environment, both biotic and
abiotic.
D. Elton
1. inspired the idea of "Production
Ecology."
2. proved to be a more durable line of
ecological thought.
3. Production ecology has the objective of the
study of the more complex life communities as trophic
associations or as food cycles. It is concerned mainly with the dynamics of the systems by which
regulation
is effected, rather than with the
actual operation of the regulatory process on individuals within populations.
3. The term production ecology stems from the
preoccupation of workers in this area of ecology with the
supply or production of food, and
with the flow or exploitation of energy within food cycles. It remains a
viable lines of ecological thought
and inquiry.
E. Population Ecology
1. may be defined as the study of events and
processes which determine the distribution, abundance
and persistence of species
populations.
2. Four theories on natural control are:
a. facultative or density-dependent factors
play a key role in the determination of population numbers
by operating as stabilizing or
regulatory mechanisms (e.g., A. J. Nicholson).
b. density-dependent processes are generally of
minor or secondary importance and play no part in
determining the abundance of
some species (W. R. Thompson, Andrewartha & Birch).
c. a middle course between the first two
viewpoints (e.g., Milne).
d. stress is placed on the influence of the genetic factor in the determination of
population densities
(e.g., Chitty, Pimentel, etc.).
Various other viewpoints have been
given by Franz, Wellington, etc.
F. Elaboration of 4 Main Theories
1. Nicholson
a. populations exist in a state of balance in
their environments, as a result of density-dependent factors,
such as the action of natural
enemies and self-governing action associate with intraspecific competition.
b. Nicholson believed that populations are
self-regulating, self-governing systems.
c. He suggested that they regulate their own
densities by depleting "requisites" in their environment.
Requisites are those essential
items that are necessary for the growth and multiplication of organisms.
d. Thus, Nicholson believed that the mechanism
for density governance or regulation is most always
intraspecific competition, either
among organisms for a critically important requisite, or among natural
enemies for which the host
organisms become the requisite.
e. Nicholson was criticized for:
(1). his preoccupation with density-related
processes.
(2). for the scant consideration he gave to
populations as they exist in nature (his experiments were
largely laboratory bound).
(3). for the limited attention he gave the
density-independent factors.
2. Thompson
a. the primary factors that control the density
of populations are extrinsic, density-independent, and
mainly climatic and edaphic in
nature.
b.
he believed that a species in
intrinsically limited in abundance only because it can eat only certain
things and thrive only under certain
conditions.
c. he maintained that populations are not
self-governed, that they merely vary within the limits set by
the physical environment. Since this environment is constantly
changing from one of increased to decreased
favorability, and vice versa, the
organism is never allowed to increase indefinitely or to decrease to zero.
d. he saw no need for invoking
density-dependent actions in population regulation.
e. Thompson is criticized because it is
difficult to imagine how density-independent mechanisms alone
could function in regulating
population densities and maintaining balance.
Also, if a stable ecosystem is
exactly balanced on the average as to periods of climatic and edaphic
favorability
versus unfavorability, what is to prevent a
slight change in the environment from rendering the resulting
environment slightly
more favorable to this organism and facilitating its indefinite increase in
numbers?
Also, by definition,
density-independent factors are unresponsive
to changes in organism density. How,
then, could a trend towards indefinite
increase be reversed in order that population balance be restored?
3. Milne
a. The theory derived from his work with cattle
ticks involves the following:
(1). Density-Independent Factors
(a). the actions of
physical factors, mainly weather.
(b). actions of other animals,
such as their indiscriminate browsing, grazing, fouling and treading on
vegetation and thus
causing casual predation.
(2). Imperfectly Density-Dependent Factors
(a). actions of natural
enemies.
(b). interspecific
competition for the same resources.
(3). Perfectly Density-Dependent Factor: only one,
intraspecific competition.
b.
A hybridization of the Thompson-Andrewartha & Birch ideas with those
of Nicholson.
c. Milne
summarized the theory as follows:
"For the most part, control of increase in populations is due
to the combined actions of
density-independent and imperfectly density-dependent environmental
factors.
In the relatively rare instances
where this combined action fails, increase to the point of collective suicide
is prevented by intraspecific
competition. Decrease of population
numbers to zero is prevented ultimately
by density-independent factors
alone."
Milne's argument for distinguishing
between perfectly density-dependent factors and imperfectly density-
dependent factors, was
that the responses of a natural enemy population to changes in host density are
not
determined by host
density per se, but are also influenced by
intrinsic factors peculiar to the natural enemy
or to those extrinsic
environmental factors experienced by both host and natural enemy, but affecting
each
to different degrees.
d.
Milne's critics recognized that he tried to blend the ideas of Nicholson
and Thompson by attempting
to distinguish between the action
of intraspecific competition, which is automatic,
and the action of density-
dependent natural enemies, which is
more governed by laws of probability.
Milne's use of the terms
"perfect" and "imperfect" is misleading. The fact that intraspecific competition is
largely automatic in action does not mean
that it can prevent population increase to extinction. Also, the
fact that the density-related responses of a
particular predator to change in numbers of its prey is, to some
degree, probabilistic,
does not mean that it cannot serve as a reliable, if not infallible, density
stabilizing action.
4. Chitty
a. worked with voles (small rodents).
b. He suggested that populations are
numerically self-regulating, which comes about through genetically-
induced changes in the average
vitality of individuals, as population densities changed.
c. He hypothesized that all species are capable
of regulating their own population densities without
destroying the renewable resources
of their environment.
d. Species did not necessarily require natural
enemies or unfavorable weather to keep them from
destroying these resources.
e. Under appropriate circumstances, indefinite
increase in population density is prevented through a
deterioration in the genotypic
quality of the population.
f. As population numbers rise, the average
quality of individuals deteriorates, partly because of a limited
increase in the proportion of the
individuals of weaker genotypes (= genetic
shift) and partly because of a
subsequent decrease in the
capability of all genotypes to survive.
g. Chitty stated that the existence of such a
mechanism would not mean that it is always efficient nor that
species do not also occur in
environments where the mechanisms seldom, if ever, come into effect.
5. Pimentel
a. He emphasized mutual adaptation between
inherent properties of species and those of their food plants
and natural enemies.
b.
He suggested that in the course of
evolution, density-stabilizing or density-dependent mechanisms of
the sort described by Nicholson,
tend to be replaced by genetic feed-back
processes between predators
and prey species, which then serve
the same purpose.
c. In other words, the population density of a
species may sometimes be controlled not by its "genetic
shift," but by the genetic
shift it causes in the population of the organism on which it feeds.
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Principles and
Application
of Biological Control. Academic Press,
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Bellows,
T. S. & R. van Driesche. 1999. Construction and analysis of life tables in
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In: Chapter 3, Principles and Application of
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