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                               CONCEPTS IN POPULATION ECOLOGY




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



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.




|                    population



        N    |

|                                      food









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



2.  In 1920, this growth curve was rediscovered and developed independently by Pearl and Reed.  This gave

      rise to Quantitative Population Ecology. 






|                                        oscillations

| ------------------------------------------------------  

| still vacant

|    places

        N    |

|                                      occupied








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





|                         prey


|                                      predator


        N    |











With Immigration                            



|                                                         prey reintroduced

|                                                         into system





        N    |










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



     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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