POPULATION DISTRIBUTIONS
I. ECOLOGICAL THEORY OF POPULATION DISTRIBUTIONS
“Underlying species-environment models is the premise that predictable relations exist between the occurrence of a species and certain features of its environment and that the distributions of species have adaptive significance.” (Heglund 2002:35)
From an evolutionary perspective, where an animal is likely to occur, can be thought of as the result of adaptations to certain biotic and abiotic factors that predispose and animal to occur in one area as opposed to another (Morrison et al. 1998).
Elton (1927) - First used the term to describe an animal's place in the biotic community (e.g., herbivore, carnivore)
Hutchinson (1957) - n-dimensional hypervolume representing all conditions under which a species can persist.
1. Fundamental – the set of resources a species can utilize in the absence of competition and other biotic interactions
Andrewartha and Birch (1954) - Abiotic factors such as climate are the primary force determining the distribution of a species
2. Realized – a species actual usage of resources after competition and biotic interactions
MacArthur (1958) - Biotic Factors such as competition, predation, and disease are the main factors controlling species' distributions.
B. Dynamic Relationships Through Time
1. Environmental variation
a) Radically fluctuating environments prevent close tracking by populations
b) Populations may follow average conditions more closely
2. “Ghosts of habitats past” (Knick and Rotenberry 2000) – Populations may not respond immediately to environmental change in the short term
3. Acclimation - Animals may respond differently depending on capacities for acclimatization
C. Factors Limiting Distributions
The environment for a particular species will contain factors that promote occurrence and factors that will deem an area incompatible.
1. Limiting Factor Concepts
Types of environmental gradients
Indirect – no direct physiological effect on population dynamics. Usually correlated with something that does (e.g., altitude)
Direct – have a direct effect on population dynamics but are not consumed (e.g., physiological tolerances)
Resource – consumed by animals
2. Limiting Factors - Mobility/Dispersal
If an animal is not found in an area it might be because it was unable to disperse there. An inability to disperse to an area be the simplest explanation for an observed distribution and should be investigated 1st.
3 Types of dispersal
a) Diffusion - gradual movement of a population across hospitable terrain.
b) Jump dispersal - movement across long distances followed by successful establishment of a population (e.g., island colonization)
c) Secular dispersal - diffusion over geologic time. Usually accompanied by species divergence.
Example: North American Darters, Cutthroat trout
Role of Dispersal in Metapopulation Dynamics - This may be one of the most important considerations for metapopulation dynamics (see Weins 1997)
3. Limiting Factors - Habitat
Generally, if an animal is able to disperse to an area but is still not found in that area it is because of habitat selection. For an organism to occur in an area, the environment must provide the minimum requirements for life and must contain no influence incompatible with life (Clarke 1954).
Temperature, Moisture, Light, Oxygen
Light - Limits the distribution of animals mainly through indirect effects on vegetation
Temperature - All species have characteristic lethal minimum and maximum temperatures.
Moisture - Mainly through indirect effects on vegetation.
Oxygen - Dissolved oxygen is critical for fishes
Vegetation, Aquatic or Lithic Substrates
Frequently related to food resources and/or escape cover
The occurrence of an animal can be affected by other organisms because of 2 opposing tendencies (1) the tendency for individuals to be attracted to others of the same or different species, and (2) the tendency to avoid each other.
1. Intraspecific dynamics
a) Fretwell and Lucas’ (1969) “Ideal Free Distribution” – Organisms will distribute themselves spatially and temporally to so that all have the same fitness
Hierarchy of habitats – Following the IFD, animals will occupy the best habitats at low density but as population density increases, the best habitats become overcrowded and suboptimal but less crowded habitats might have the same fitness
b) "Ideal Despotic Distribution" - IFD constrained by aggressive behavior of other individuals (see Van Horn 1983)
2. Interspecific dynamics
a) Competition
Gause's Competitive Exclusion (Gause 1934) - when species too similarly use the same kinds of resources, they cannot coexist very long (e.g., redwing and tricolored blackbirds; Orians and Collier 1963)
Character displacement - sympatric species show more divergence than when allopatric
2 Types of Competition:
Interference
Exploitative
b) Predation
Restriction of prey by predators (e.g., young salmon and predatory birds)
Restriction of predators by prey (e.g., snowy owls and lemmings)
c) Disease and Parasitism
Liebig’s law of the minimum
In 1840 he suggested that the growth of crop plants was often limited by the essential element that was in shortest supply relative to requirements.
Example: essential elements and reproduction (e.g., selenium and elk)
Problem with law: Interactions among factors
Hierarchy of Scales
Forman’s (1964) distributional hierarchy
Different factors and combinations of factors are limiting at the various levels of aggregation
Each factor affecting the distribution of a species operates at different scales
Most ecological studies suggest biotic factors are more important at a local scale but abiotc factors may be important on a continent-wide scale.
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