WLF 448: Fish & Wildlife Population Ecology

III. Introduction to Populations (Intropop.cpl)

A. What is a population?

Webster's Third New International Dictionary -

• "The total number or amount of things especially within a given area."

• "The organisms inhabiting a particular area or biotype."

• "A group of interbreeding biotypes that represents the level of organization at which speciation begins."

Krebs (1972:139)

• A group of organisms of the same species occupying a particular space at a particular time.

La Monte Cole (1957)

• A biological unit at the level of ecological integration where it is meaningful to speak of birth rate, death rate, sex ratios, and age structure in describing properties or parameters of the unit.

B. Level of Aggregation

1. Biome
2. Landscape
3. Ecosystem
4. Community
5. Population
6. Individual
7. Organ
8. Cell

C. Hierarchical Aggregations

Groupings of individuals driven by demography, geography, and genetics:

• Deme - A group of individuals were breeding is random. Continuous distribution geographically. One "patch" of habitat.

• Population - A collection of demes with strong connections between adjacent demes. Geographically a collection of patches without great expanses of non-habitat intervening. Genetically closely related. High rates of dispersal between demes. High correlations in demographic rates between adjacent demes.

• Metapopulation - A collection of populations. Possible low correlations in demographic rates (which produces high levels of independence). Possible low rates of dispersal (which can produce genetic differences). Populations may act as possible sources for recolonization.

• Subspecies - A collection of metapopulations in a region. Very rare dispersals maintain genetic similarity. Demographic independence may be nearly complete. Occupied patches may be separated by large areas of non-habitat.

• Species - The collection of subspecies encompassing the entire distribution of the species. Defines the entire geographic range of the species. May encompass substantial differences in phenotypes (habitat, physiology, behavior) and genotypes.

D. Hierarchy of Methods

• Species: presence-absence

• Subspecies: relative abundance

• Metapopulation: density

• Population: survival, fecundity

• Deme: immigration/emigration

E. Population Unit

First step in making statements and predictions about a population is to delimit the population unit.

Goal is to delimit a population unit that is as discrete as possible but which still meets your research/management objectives.

Ideally, chances of mating within this unit should be randomly distributed.

F. Steps to Delimit a Population

1. State objectives clearly.

2. Determine distribution.

3. Determine patterns of movement and barriers to movement.

4. Determine levels of genetic/phenotypic similarity among subunits.

5. Identify associations in demographic rates between subunits.

6. Integrate all this information to outline the most discrete unit(s) possible, which still meet(s) objectives.

G. "Unit Stock"

Cushing (1981)

• Many fish do not disperse.

• Approaches:

Ricker (1972)

• "During the first 30 years or so of this century it was customary to regard all populations of a fish species as more or less uniform, at least in respect to biologically important particulars. Observed differences between stocks were ascribed to differences in their environments."

What is a "stock"?

• A group of fish spawning in a particular lake or stream at a particular season, which to a substantial degree do not interbreed with any other such group.

ESU

• Evolutionary Significant Unit

• Endangered Species Act requires protecting a population if it is an ESU.

• ESU criteria:

1. Reproductively isolated from other conspecific population units.

2. Represents an important component in the evolutionary history of a species.

Implications of "Unit Stock" Concept:

1. Provides genetic perspective.

2. Two key concepts: (a) fish are subdivided into local populations, and (b) genetic differences between local populations are adaptive.

3. Cause: heterogeneity of resources.

4. Selective processes are most effective if populations are subdivided in local populations (Sewall Wright 1929).

5. Dispersal does not equal gene flow necessarily.

6. Ricker (1972) concluded that most transplants with salmonids reduced survival to maturity.

7. Maximum production from a complex of stocks with local adaptation of subpopulations.

Stock Differentiation:

1. Population parameters
2. Marking
3. Physiological/behavioral characters
4. Morphometric/meristic characters.
5. Calcareous structures.
6. Cytogenic characters.
7. Biochemical characters.
8. Immunogenetics.
9. DNA

H. Population Characteristics, Processes, and Environment

Population processes

a. birth
b. death
c. immigration (ingress)
d. emigration (egress)

Population characteristics

a. abundance
b. composition (e.g., sex and ratios)
c. distribution (pattern, scale)
d. movement (migratory, nonmigratory)

"Environment"

a. food
b. cover
c. water conditions
d. nest sites
e. disease
f. predators
g. competitors
h. weather

Interactions

a. within a population
b. between populations
c. between populations and environment

I. Uses of Population Dynamics

1. Endangered or rare species
2. Harvested species
3. Controlling harmful species
4. Predicting changes in non-harvested populations

J. Early Population Demographers and Their Ideas

The following list very briefly outlines the contribution of some early demographers to the understanding of population processes. You will note that at least three fundamental concepts were soon recognized:

1. There is a marked tendency for population to increase.
   
2. Increase tends to be prevented by certain limiting factors acting on birth and death rates.
   
3. Population processes are influenced by crowding; i.e., there are density effects.

• Plato - attributed population regulation to the balance between fertility and immigration on one side, and mortality and emigration on the other. He noted wars and pestilence as leading causes of mortality, and mentioned relative sterility due to social conditions.

• Machiavelli (1523) - realized the danger of human populations increasing beyond their means of subsistence on limited areas. He stated that under such circumstances, they were then controlled by want and disease.

• Giovanni Botero (1588) - published "The Greatness of Cities", in which he stated that populations were prevented from growing by a lack of resources to support them. He mentioned famine, disease, wars, and various catastrophes as sometimes halting population growth, but felt that the fundamental limitation was caused by want or starvation ("the fruits of the earth do not suffice to feed a greater number"). He recognized that increased density tended to intensify mortality due to disease and various aggressive acts such as war, murder, etc.

• John Graunt (1662) - constructed a table of mortality and survival in a cohort of 100 persons by decades. This was the first life table. Cole (1957) considers Graunt the father of demography. Graunt also estimated the potential rate of population growth for the city of London, stating that it could double in 64 years without immigration.

• Dewitt (Holland) and Halley (England) - constructed life tables based on frequency distribution of age at death. Both worked during the late 1600s.

• Hale (1677) - was the first to indicate that human populations tended to increase at a geometric rate. He also estimated that the human population could potentially double itself in 35 years, but would soon exceed its means of subsistence. He ascribed population limitation to famine, disease, war, and catastrophes.

• Thomas Malthus - Essay on Human Populations (late 1700s)

a. Integrated previous views and popularized them.

b. Focused problems.

c. Human population was increasing exponentially while food was increasing arithmetically:

 d. Influenced Darwin's thinking.

• Quetelet (1835) - was a Belgian mathematician who proposed that populations are stabilized by a balance between their potential of geometric growth and resistance to this growth which increases as the square of the growth rate.

• Verhulst (1838) - was also a Belgian and a student of Quetelet. He developed an equation describing population growth. He called the curve resulting from this equation the "logistic curve". Verhulst tested the goodness of fit of this curve against data for a few human populations in western Europe. His equation was apparently forgotten until Pearl and Reed (1920) independently hit upon the same equation, and then discovered Verhulst's work in a search through the literature.

• Doubleday (1841) - believed that when a species was endangered through lack of food, the fertility increased and more offspring were produced. Conversely, he believed that when food is abundant, fertility drops. Doubleday called his theory "The True Law of Population". His ideas probably arose from observations of rich vs. poor human social strata, and of certain plant characteristics. Doubleday thought that reduced fertility was attributable, probably through some direct physiological mechanism, to overfeeding per se.

• William Farr (1843) - studied human populations in England, and found that within limits, mortality increased as the sixth root of density. Work as late as the early 20th century has tended to corroborate Farr's findings.

K. Student Responsibilities:

• Be able to give a definition of a population and cite the reference if it is not your own.

• Know the definitions given in the handout and the differences between closely related terms.

• Where can we use population dynamics? Where is it not so useful?

• What are the interactions and feedback mechanisms in population dynamics?

• What criteria are used to delimit a population?

• Know the fundamental concepts illustrated in the works of the early demographers.

• What were some of the early demographers noted for?

L. References:

Adams, L. 1970. Population ecology. Dickenson Publ. Co., Inc. Belmont, California. pp. 1-7.

Begon, M. and M. Mortimer. 1981. Population ecology. Sinauer Assoc., Inc., Sunderland, Mass. 296pp.

Begon, M., J. L. Harper and C. R. Townsend. 1996. Ecology: Individuals, populations and communities. 3rd ed. Blackwell Scientific Ltd., Cambridge, Mass. 1068pp.

Caughley, G. 1977. Analysis of vertebrate populations. John Wiley and Sons, New York. pp. 1-7.

Cushing, D. H. 1981. Fisheries biology: a study of population dynamics. University of Wisconsin Press, Madison. (on reserve at library)

Hutchinson, G. E. 1978. An introduction to population ecology. Yale Univ. Press, New Haven, Conn. pp. 1-21.

Quick, H. 1974. Population ecology. Pegasus. Indianapolis. 185pp. Preface ix-xi.

Voute, A. D. 1970. in Osterbech, ed. Adv. Inst. Dynamics Numbers Pop.. pp.19-29.

Revised: 25 August 2011