9 Population Growth and Regulation 9 Population Growth and Regulation • Case Study: Human Population Growth • Life Tables • Age Structure • Exponential Growth • Effects Of Density • Logistic Growth • Case Study Revisited • Connections in Nature: Your Ecological Footprint Figure 9.2 Explosive Growth of the Human Population Introduction One of the ecological maxims is “No population can increase in size forever.” Ecologists try to understand the factors that limit or promote population growth. Figure 9.3 Dash to the Sea Life Tables Concept 9.1: Life tables show how survival and reproductive rates vary with age, size, or life cycle stage. Information about births and deaths is essential to predict trends or future population size. Life Tables A survivorship curve is a plot of the number of individuals from a hypothetical cohort that will survive to reach different ages. Survivorship curves can be classified into three general types. Figure 9.5 Three Types of Survivorship Curves Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 1) Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 2) Figure 9.6 Species with Type I, II, and III Survivorship Curves (Part 3) Age Structure Concept 9.2: Life table data can be used to project the future age structure, size, and growth rate of a population. A population can be characterized by its age structure—the proportion of the population in each age class. Age structure influences whether a population will increase or decrease in size. Figure 9.7 Age Structure Influences Growth Rate in Human Populations Age Structure Life table data can be used to predict age structure and population size. Figure 9.8 A Growth of a Hypothetical Population Age Structure The growth rate (λ) can be calculated as the ratio of the population size in year t + 1 (Nt+1) to the population size in year t (Nt). N t 1 Nt Figure 9.8 B Growth of a Hypothetical Population Age Structure When age-specific survival and fecundity rates are constant over time, the population ultimately grows at a fixed rate. The age structure does not change from one year to the next—it has a stable age distribution. Loggerheads – protect eggs, but even w/ 100% protection, species would still decline. Have to protect juveniles and adults. Exponential Growth Concept 9.3: Populations can grow exponentially when conditions are favorable, but exponential growth cannot continue indefinitely. Exponential Growth Geometric growth: N t 1 N t λ = geometric growth rate; also known as the (per capita) finite rate of increase. Exponential Growth Geometric growth can also be represented by N t N 0 t This predicts the size of the population after any number of discrete time periods. Exponential Growth In many species, individuals do not reproduce at discrete time intervals, they reproduce continuously The growth in these populations is exponential growth. Exponential Growth Exponential growth is described by dN rN dt dN dt = the rate of change in population size at each instant in time. r is the exponential population growth rate or the (per capita) intrinsic rate of increase. Figure 9.9 A Geometric and Exponential Growth Figure 9.9 B Geometric and Exponential Growth Figure 9.10 How Population Growth Rates Affect Population Size Effects Of Density Concept 9.4: Population size can be determined by density-dependent and density-independent factors. Under ideal conditions, λ > 1 for all populations. But conditions rarely remain ideal. What factors cause λ to fluctuate over time? Figure 9.12 Weather Can Influence Population Size Figure 9.13 Comparing Density Dependence and Density Independence Effects Of Density In an experiment where eggs of the flour beetle Tribolium confusum were placed in glass tubes, death rates increased as the density of eggs increased. Logistic Growth Concept 9.5: The logistic equation incorporates limits to growth and shows how a population may stabilize at a maximum size, the carrying capacity. Logistic growth: Population increases rapidly at first, then stabilizes at the carrying capacity (maximum population size that can be supported indefinitely by the environment). Figure 9.17 An S-shaped Growth Curve in a Natural Population Figure 9.18 Logistic and Exponential Growth Compared Figure 9.20 Faster than Exponential (Part 2) Figure 9.21 United Nations Projections of Human Population Size Figure 9.22 The Human Carrying Capacity Connection in Nature: Your Ecological Footprint The environmental impact of a population is called its ecological footprint. Ultimately, every aspect of our economy depends on the ecosystems of Earth. Connection in Nature: Your Ecological Footprint The ecological footprint approach highlights the fact that all of our actions depend on and affect the natural world. Find yours at http://www.conservation.org/act/live_gree n/Pages/ecofootprint.aspx