Chapter 8 Population Ecology Chapter Overview Questions Ø What are the major characteristics of populations? Ø How do populations respond to changes in environmental conditions? Ø How do species differ in their reproductive patterns? 1
Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? Ø They were overhunted to the brink of extinction by the early 1900 s and are now making a comeback. Figure 8-1 Core Case Study: Southern Sea Otters: Are They Back from the Brink of Extinction? Ø Sea otters are an important keystone species for sea urchins and other kelpeating organisms. Figure 8-1 2
POPULATION DYNAMICS AND CARRYING CAPACITY Ø Most populations live in clumps although other patterns occur based on resource distribution. Figure 8-2 Changes in Population Size: Entrances and Exits Ø Populations increase through births and immigration Ø Populations decrease through deaths and emigration 3
Age Structure: Young Populations Can Grow Fast Ø How fast a population grows or declines depends on its age structure. l Prereproductive age: not mature enough to reproduce. l Reproductive age: those capable of reproduction. l Postreproductive age: those too old to reproduce. Limits on Population Growth: Biotic Potential vs. Environmental Resistance Ø No population can increase its size indefinitely. l The intrinsic rate of increase (r) is the rate at which a population would grow if it had unlimited resources. l Carrying capacity (K): the maximum population of a given species that a particular habitat can sustain indefinitely without degrading the habitat. 4
Exponential and Logistic Population Growth: J-Curves and S-Curves Ø Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4 Exponential and Logistic Population Growth: J-Curves and S-Curves Ø As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4 5
Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Ø Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6 Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size Ø Over time species may increase their carrying capacity by developing adaptations. Ø Some species maintain their carrying capacity by migrating to other areas. Ø So far, technological, social, and other cultural changes have extended the earth s carrying capacity for humans. 6
How Would You Vote? To conduct an instant in-class survey using a classroom response system, access JoinIn Clicker Content from the PowerLecture main menu for Living in the Environment. Ø Can we continue to expand the earth's carrying capacity for humans? l a. No. Unless humans voluntarily control their population and conserve resources, nature will do it for us. l b. Yes. New technologies and strategies will allow us to further delay exceeding the earth's carrying capacity. Population Density and Population Change: Effects of Crowding Ø Population density: the number of individuals in a population found in a particular area or volume. l A population s density can affect how rapidly it can grow or decline. e.g. biotic factors like disease l Some population control factors are not affected by population density. e.g. abiotic factors like weather 7
Types of Population Change Curves in Nature Ø Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. l Stable: fluctuates slightly above and below carrying capacity. l Irruptive: populations explode and then crash to a more stable level. l Cyclic: populations fluctuate and regular cyclic or boom-and-bust cycles. l Irregular: erratic changes possibly due to chaos or drastic change. Types of Population Change Curves in Nature Ø Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources. Figure 8-7 8
Case Study: Exploding White-Tailed Deer Populations in the United States Ø Since the 1930s the white-tailed deer population has exploded in the United States. l Nearly extinct prior to their protection in 1920 s. Ø Today 25-30 million white-tailed deer in U.S. pose human interaction problems. l Deer-vehicle collisions (1.5 million per year). l Transmit disease (Lyme disease in deer ticks). REPRODUCTIVE PATTERNS Ø Some species reproduce without having sex (asexual). l Offspring are exact genetic copies (clones). Ø Others reproduce by having sex (sexual). l Genetic material is mixture of two individuals. l Disadvantages: males do not give birth, increase chance of genetic errors and defects, courtship and mating rituals can be costly. l Major advantages: genetic diversity, offspring protection. 9
Sexual Reproduction: Courtship Ø Courtship rituals consume time and energy, can transmit disease, and can inflict injury on males of some species as they compete for sexual partners. Figure 8-8 Reproductive Patterns: Opportunists and Competitors Ø Large number of smaller offspring with little parental care (rselected species). Ø Fewer, larger offspring with higher invested parental care (K-selected species). Figure 8-9 10
Reproductive Patterns Ø r-selected species tend to be opportunists while K-selected species tend to be competitors. Figure 8-10 11
Survivorship Curves: Short to Long Lives Ø The way to represent the age structure of a population is with a survivorship curve. l Late loss population live to an old age. l Constant loss population die at all ages. l Most members of early loss population, die at young ages. Survivorship Curves: Short to Long Lives Ø The populations of different species vary in how long individual members typically live. Figure 8-11 12