Chapter 9 Population distribution and abundance Distribution patterns --Main types of distribution patterns: Random distribution: Individuals within a population have an equal chance of living anywhere within an area. Regular distribution: Individuals are uniformly spaced. Clumped distribution: Individuals have a much higher probability of being found in some area than in others. Fig.9.10

--Factors affecting distribution patterns (1) Interactions between individuals within a population. eg. mutual attraction creates a clumped pattern; individuals avoiding each other result in a regular pattern; neutral responses contribute to a random pattern. Fig.9.11 (2) Physical environment. eg. an environment with patched distributions of nutrients, water, nesting sites etc., fosters clumped distribution pattern.

(3) Combination of interactions and environment. --Change of distribution patterns of a same species eg. The creosote bush (a desert bush).

Clumped pattern----random pattern----regular pattern (Young plants) (Medium-size plts) (Large plts) A B C Reasons: A. Seeds dispersal around mother shrubs, seeds germinating clumpedly, asexually produced offspring around parent plants. B. Self-thinning because of density. C. More individuals die because of competition especially for water. Nearby neighbors compete more severely and show higher mortality, which results in regular distribution.

--Distribution of individuals on large scales Populations usually show clumped distribution patterns. eg. Bird populations distribute across North America. At the continental scale, bird populations show clumped pattern (for both winter count and summer count). Fig.9.15, 9.16. Reasons for the pattern: birds clump in areas where the environment are favorable.

Organism size and population density --Animal population density decreases with body size. Fig.9.19 --Plant population density decreases also with body size. Fig.9.21 Rarity and extinction --Relation between rarity and extinction Species that are rare seem to be more vulnerable to extinction. --Seven forms of rarity. Fig.9.22