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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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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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1999. Theory and mechanisms of
natural control of populations. In: Chapter 2, Principles and
Application of Biological Control.
Academic Press, San Diego CA.
1046 p.
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In: Chapter 3,
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232 p.
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