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 demographydemography 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. demographic transition

Age Structure Diagram Demographers can predict future growth using models called age-structure diagrams, or population profiles.age-structure diagrams 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