Bio1b Summer 2008 Ecology Lecture 3 Eric Harris Page 1 of 2 Updated on 06/30/08 ECOLOGY LECTURE 3: POPULATION BIOLOGY I – DEMOGRAPHY & LIFE HISTORY Reading: 7th ed., 1136–1143; 8th ed., 1174–1181. A population = a group of species of the same species in the same general area (recall: population genetics) A. Population Structure i) Density = number of individuals/area or volume Difficulty of measuring directly Estimation (e.g., mark-recapture method) – assumes the proportion of marks in the second sample is equal to the proportion of marks in the population as a whole. Knowing the number marked (x) and recaptured (n) in the second sample and the number initially marked (m) we can calculate the population size (N) → x/n = m/N therefore N= (mn)/x Density is the result of births/deaths/immigration/emigration ii) Distribution (or dispersion) Random (rare in nature) Regular (or overdispersed) – frequently caused by competition Clumped (or patchy) – many possible causes, distribution of resources for example. iii) Age Structure – organisms may change in many ways as they age, (e.g., food source, predators and competitors may all change.) Comparison of human populations Graphical presentations B. Demography = the study of the factors that influence population growth and decline i) Life tables Cohort – follows a group of individuals born at the same time (hard to do but accurate) Static – looks at all the individuals present during one time period (easier but harder to interpret)Bio1b Summer 2008 Ecology Lecture 3 Eric Harris Page 2 of 2 Updated on 06/30/08 ii) Survivorship curves Type I (e.g., humans) – mortality is low until most individuals die late in their lifespan. Type II (e.g. some bird species) – a constant proportional mortality. Type III (e.g. many insect and fish species) – most individuals die young, the few that survive may then live for much longer. C. Life History Traits i) Life history components Semelparity vs. iteroparity (reproduce once or repeatedly) Age at first reproduction (precocity vs. delay) Clutch size Size as an adult (and growth rates to reach that size) ii) Life history trade-offs All life history variables cannot all be maximized at the same time. (e.g., current reproduction may reduce future growth or
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