Limits to Growth 5-2 Limiting Factors • Remember, primary productivity of an ecosystem can be reduced by limiting nutrients • In the context of populations, limiting factors can cause population size to decrease • A resource base that is limited can also affect the long-term survival of a species • Ex: pandas and bamboo Density-Dependant Factors • Limiting factors that depend on population size • Density-dependent factors become limiting only when population density reaches a certain level • Do not affect small, scattered populations • Examples of density-dependent limiting factors: – Competition – Predation – Parasitism – Disease Competition • Occurs when populations become crowded • All organisms in a population need sunlight, food, water, space, etc. • Competition among members of the same species is density dependent • The more members in the population, the faster the resources are used up • Competition also happens between members of different species – Major force behind evolutionary change • When 2 species compete for same resource, they are under pressure to change – Over time, evolve to occupy diff. niches Predation • Populations in nature are controlled by predation – Known as predator-prey relationship • Ex: – sea otters and urchins – Sea otters and whales Isle Royale • Well known example of predator-prey relationship is the moose/wolf populations on Isle Royale in Lake Superior – When moose numbers are high, wolf #’s increase – As wolves feed on moose, moose #’s decrease, causing a decrease in wolves – With fewer wolves, moose #’s can increase again Parasitism & Disease • Parasites can also limit population growth • Parasites are similar to predators – weaken and kill hosts • Large populations can be under stress making them more susceptible to diseases • More members = easier to pass “things” around Density-Independent Factors • Factors that affect populations regardless of their size • Examples: – Unusual weather – Season cycles – Human activities (damming rivers, clear cutting) • Populations usually respond to such factors with a crash in population size • After crash, numbers may increase quickly, or stay low for awhile • Storms and hurricanes can wipe out populations of insects • Extreme cold or hot weather can also take a toll on populations • Droughts can affect entire populations of vegetation, which can affect populations of consumers • Environments are always changing • Most populations can adapt to a certain amount of change (grow or shrink) • Major upsets in ecosystems can lead to long-term decline in certain populations • Human activities have caused some of these upsets 5-3 Human Population Growth • Like the populations of many other living organisms, the size of the human population tends to increase with time. • For most of human existence, the population grew slowly • Until fairly recently, only half the children in the world survived to adulthood. • About 500 years ago, the human population began growing more rapidly. • Agriculture and industry made life easier and safer. • The world's food supply became more reliable, and essential goods could be shipped around the globe. • Improved sanitation, medicine, and health care dramatically reduced the death rate and increased longevity. • With these advances, the human population experienced exponential growth, as shown in the figure at right. Patterns of Population Growth • English economist Thomas Malthus observed that human populations were growing rapidly. • Malthus predicted that such growth would not continue indefinitely. • Instead, according to Malthus, war, famine, and disease would limit human population growth. • Scientists have identified a variety of other social and economic factors that can affect human populations. • The scientific study of human populations is called demography • Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly. Demographic Transition • Over the past century, population growth in the United States, Japan, and much of Europe has slowed dramatically. • Demographers have developed a hypothesis to explain this shift. • According to this hypothesis, these countries have completed the demographic transition, a dramatic change in birth and death rates. Age Structure Diagram • Demographers can predict future growth using models called age-structure diagrams, or population profiles. • Age-structure diagrams show the population of a country broken down by gender and age group. • Each bar in the age-structure diagram represents individuals within a 5-year group. Percentages of males are to the left of the center line and females to the right in each group. Future Population Growth • To predict how the world's human population will grow, demographers must consider many factors including: – the age structure of each country – prevalence of life-threatening diseases