INTRODUCTION TO POPULATIONS
1. Ecological - A group of organisms of the same species occupying a particular space at a particular time (Krebs 2001:116).
2. Statistical – Group to which inferences are to be made from a sample of that group
B. Hierarchical Aggregations of Groups of Individuals (Garton 2002)
1. Deme – A group of individuals were breeding is random. Continuous distribution geographically. Occupy one “patch” of habitat.
2. 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 dispersa.l between demes. High correlations in demographic rates between adjacent demes.
3. 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.
4. 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.
5. Species - The collection of all individuals encompassing the entire distribution and geographic range of the species.
C. Other Terms for Groups of Individuals
1. Stock
Cushing (1981) noted that many fish do not disperse much
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
2. "Species" as defined by Endangered Species Act 1972
"any distinct population segment of any species of vertebrate fish or wildlife that breeds when mature"
3. Evolutionary Significant Unit (ESU)
Endangered Species Act assigns NMFS responsibility for determining if anadromous fish species are endangered or threatened
NMFS established the Evolutionary Significant Unit to define a "distinct population segment"
ESU criteria:
Reproductively isolated from other conspecific population units
Represents an important component in the evolutionary history of a species
D. Methods to Determine Level of Aggregation
1. State objectives clearly
What level of aggregation is of interest?
Is demographic similarity more important that genetic similarity?
2. Determine distribution
Determine distribution of individuals or groups
Identify significant geographical structures that might restrict movement
3. Determine movement
Determine amount of movement between groups (i.e., dispersal rates) by marking individuals
4. Determine phenotypic similarity
Physiological/behavioral characters
Morphometric/meristic characters
Calcareous structures (fish)
5. Determine genetic similarity
Gene frequencies
6. Determine demographic similarity
Determine demographic rates and identify degree of correlations among different groups
7. Integrate 1-6 to outline most discrete unit(s) possible, which still meet objectives
E. Population Characteristics and Processes
Understanding the interplay between population characteristics, processes and the environment is the key to understanding fish and wildlife population ecology.
1. Population characteristics
a) Distribution (extent, pattern)
random
uniform
clumped
b) Abundance/Density
c) Composition (e.g., sex ratios, age distribution)
2. Population dynamics (processes)
a) Natality (births)
b) Mortality (deaths)
c) Immigration
d) Emigration
3. Environment
a) Food
b) Cover
c) Disease
d) Predators
e) Competitors
F. Uses of Population Dynamics
If we can understand these relationships the we can predict future fluctuations in population distribution and abundance to...
1. Conserve endangered or rare species
2. Manage harvested species
3. Control harmful species
4. Predict changes in non-harvested populations
G. 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 3 fundamental concepts were soon recognized:
· There is a marked tendency for populations to increase
· Increase tends to be prevented by certain limiting factors acting on birth and death rates
· 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 beond their means of subsistence on limited areas. He stated that under such circumstances, they were then controlled by want and disease.
Givanni 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. 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 1st life table. Cole (1957) considers Graunt the father of demography. Graunt also estimated the potential rate of population grouth for the city of London, stating that it could double in 64 years without immigration.
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.
Verhulst (1838) - 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 literature search.
Cushing, D. H. 1981. Fisheries biology. The University of Wisconsin Press, Madison, WI, USA.
Garton, E. O. 2002. Mapping a chimera? Pages 663-666 in J. M. Scott et al. editors. Predicting species occurrence. Island Press, Washington.
Updated 31 July 1996