Env Sci 4.3 ppt.pptx

Environmental Science Chapter 4, Lesson 3: Population Growth  OBJECTIVES: 1. Describe the factors that influence a population’s growth rate. 2. Explain exponential growth and logistic growth. 3. Explain how limiting factors and biotic potential affect population growth. F...

Environmental Science Chapter 4, Lesson 3: Population Growth  OBJECTIVES: 1. Describe the factors that influence a population’s growth rate. 2. Explain exponential growth and logistic growth. 3. Explain how limiting factors and biotic potential affect population growth. FACTORS THAT DETERMINE POPULATION GROWTH Birth and death rates ○ Influenced by population size, density, distribution, sex ratio, and age structure ○ Natality: the rate at which individuals are born ○ Mortality: the rate at which individuals die FACTORS THAT DETERMINE POPULATION GROWTH Birth and death rates (continued) ○ Natality and mortality are usually expressed as the number of births or deaths per 1000 individuals over a given time If birth rate is greater than death rate, population size increases. If death rate is greater than birth rate, population size decreases.  FACTORS THAT DETERMINE POPULATION GROWTH Birth and death rates (continued) ○ Survivorship curves show how the likelihood of death varies with age. Type I curve: organisms have higher mortality at older ages. (Example: humans) Type II curve: indicates a population with equal mortality at all ages. (Example: many bird species) Type III curve: death is less likely at an older age than at a very young age (Example: golden toads produce large numbers of young that suffer high death rates) FACTORS THAT DETERMINE POPULATION GROWTH  FACTORS THAT DETERMINE POPULATION GROWTH Birth and death rates (continued) ○ Immigration and Emigration Immigration: arrival of individuals from outside a given area Emigration: departure of individuals from a given area Migration: seasonal movement into and out of an area ○ Calculating Population Growth (often expressed as %) (Individuals added) - (individuals subtracted) OR (birth rate + immigration rate) - (death rate + emigration rate)  HOW POPULATIONS GROW Exponential growth: when a population increases by a fixed percentage each year, it’s said to undergo exponential growth ○ Often occurs when starting population is small and environmental conditions are ideal ○ Represented by J-shaped curve Logistic growth: describes how a population’s initial exponential increase is slowed and finally stopped by limiting factors-- represented by S- shaped curve ○ Limiting factors: characteristics of the environment that limit population growth ○ Carrying capacity: largest population size a given environment can sustainably support  HOW POPULATIONS GROW Population growth in nature: as limiting factors in an environment change, so does its carrying capacity ○ Example: plants in the understory of a dense forest may be limited by the low amount of sunlight available, but if a large tree dies and sunlight comes through, then the carrying capacity for understory plants in the area may increase LIMITING FACTORS AND BIOTIC POTENTIAL Density-dependent factors: influence changes with population density ○ Example: high population density increases competition for resources such as food and water Density-independent factors: limiting factors whose influence is not affected by populations ○ Example: floods, fires, landslides-- it doesn’t matter if the original population was dense or not, the result is the same: dramatic and sudden reduction in population size  LIMITING FACTORS AND BIOTIC POTENTIAL Biotic potential: maximum ability to produce offspring in ideal conditions ○ Biotic potential influenced by: Gestation time: how long it takes for an embryo or fetus to develop and by “born” Generation time: the span from an organism’s birth to the time it has its own offspring