Community Ecology, Population Ecology, and the Human Population 4 Community Ecology, Population Ecology, and the Human Population

4-1 What Roles Do Species Play in Ecosystems? Each species plays a role in its ecosystem Ecological niche (niche) Species’ way of life in a community: everything that affects survival and reproduction, e.g., sunlight, water, space, food, predators, and temperature Generalist species Broad niches: wide range of tolerance Specialist species Narrow niches: narrow range of tolerance

Herring gull is a tireless scavenger Brown pelican dives for fish, which it locates from the air Ruddy turnstone searches under shells and pebbles for small invertebrates Avocet sweeps bill through mud and surface water in search of small crustaceans, insects, and seeds Dowitcher probes deeply into mud in search of snails, marine worms, and small crustaceans Black skimmer seizes small fish at water surface Figure 4-1 Various bird species in a coastal wetland occupy specialized feeding niches. This specialization reduces competition and allows sharing of limited resources. Flamingo feeds on minute organisms in mud Scaup and other diving ducks feed on mollusks, crustaceans, and aquatic vegetation Louisiana heron wades into water to seize small fish Oystercatcher feeds on clams, mussels, and other shellfish into which it pries its narrow beak Knot (sandpiper) picks up worms and small crustaceans left by receding tide Piping plover feeds on insects and tiny crustaceans on sandy beaches Stepped Art Fig. 4-1

Case Study: The Giant Panda—A Highly Endangered Specialist 1,600 to 3,000 giant pandas in the wild Most in China Specialist species Requires bamboo stalks and leaves as food Low reproductive rate Threats Limited range of bamboo forests Poaching

Species Can Play Four Major Roles within Ecosystems Native species Nonnative species (invasive, alien, and exotic species) Indicator species Keystone species

Indicator Species Serve as Biological Smoke Alarms Provide early warning of damage to a community Widespread Affected quickly by environmental changes Examples: Birds Butterflies Some amphibians

Science Focus: Why Are Amphibians Vanishing? Natural and human-caused factors Parasites Viral and fungal diseases Habitat loss and fragmentation Prolonged drought Increases in UV radiation Pollution Overhunting Nonnative predators and competitors

Keystone and Foundation Species Play Critical Roles in Their Ecosystems Keystone species Roles have a large effect on the types and abundances of other species Pollinators Top predators Vulnerable to extinction Loss of a keystone species can lead to population crashes and other species’ extinctions

Case Study: Why Should We Protect Sharks? Keystone species Shark studies could help save human lives Sharks rarely get cancer Highly effective immune system Three largest species Plant-eaters 100 million sharks killed every year Among most vulnerable and least protected

4-2 How Do Species Interact? Species interact in five major ways Interspecific competition Resource partitioning Predation Parasitism Mutualism Commensalism

Black-throated Green Warbler Yellow-rumped Warbler Blackburnian Warbler Black-throated Green Warbler Cape May Warbler Bay-breasted Warbler Yellow-rumped Warbler Figure 4-2 Resource partitioning is a strategy used by five species of insect-eating warblers in the spruce forests of the U.S. state of Maine. Each species minimizes competition with the others for food by spending at least half its feeding time in a distinct portion (shaded areas) of the spruce trees, and by consuming somewhat different insect species. (After R. H. MacArthur, “Population Ecology of Some Warblers in Northeastern Coniferous Forests,” Ecology 36 (1958): 533–536.) Resource partitioning Fig. 4-2

Most Consumer Species Feed on Organisms of Other Species Predation Predator–prey relationships Plays a role in evolution by natural selection Predators play an important ecological role

Some Species Feed off Other Species by Living on or in Them Parasitism Parasite is usually much smaller than the host Parasite rarely kills the host In some cases, parasites help to keep the hosts’ populations in check + -

In Some Interactions, Both Species Benefit Mutualism Nutrition and protection relationship Gut inhabitant mutualism Mutual exploitation Example: clownfish and sea anemones + +

In Some Interactions, One Species Benefits and the Other Is Not Harmed Commensalism Example: epiphytes attach to the trunks of large trees in tropical forests + 0

4-3 How Do Communities and Ecosystems Respond to Changing Conditions? Communities and ecosystems change over time: ecological succession Primary succession Secondary succession Primary succession and secondary succession Tend to increase biodiversity and interactions among species

paper birch, and white spruce forest community Jack pine, Balsam fir, paper birch, and white spruce forest community Figure 4-4 Primary ecological succession: Over almost a thousand years, plant communities developed, starting on bare rock exposed by a retreating glacier on Isle Royal, Michigan, in northern Lake Superior. The details of this process vary from one site to another. Question: What are two ways in which lichens, mosses, and plants might get started growing on bare rock? Jack pine, black spruce, and aspen Heath mat Small herbs and shrubs Lichens and mosses Exposed rocks Time Fig. 4-4

Mature oak and hickory forest Figure 4-5 Secondary ecological succession: This diagram shows the undisturbed secondary ecological succession of plant communities on an abandoned farm field in the U.S. state of North Carolina. It took 150 to 200 years after the farmland was abandoned for the area to become covered with a mature oak and hickory forest. Questions: Do you think the annual weeds (left) would continue to thrive in the mature forest (right)? Why or why not? Mature oak and hickory forest Young pine forest with developing understory of oak and hickory trees Shrubs and small pine seedlings Perennial weeds and grasses Annual weeds Time Stepped Art Fig. 4-5

4-4 What Limits the Growth of Populations? Populations can grow, shrink, or remain stable Population change = (births + immigration) – (deaths + emigration) Biotic potential Intrinsic rate of increase Population growth rate with unlimited resources

Populations Can Grow, Shrink, or Remain Stable Limiting factors Physical or chemical environmental conditions Environmental resistance All factors that act to limit the growth of a population Carrying capacity Maximum population a given habitat can sustain

Population overshoots carrying capacity Environmental resistance 2.0 1.5 Population recovers and stabilizes Population runs out of resources and crashes 1.0 Number of sheep (millions) Exponential growth .5 Figure 4-6 Growth of a sheep population on the island of Tasmania between 1800 and 1925. After sheep were introduced in 1800, their population grew exponentially, thanks to an ample food supply and few predators. By 1855, they had overshot the island’s carrying capacity. Their numbers then stabilized and fluctuated around a carrying capacity of about 1.6 million sheep. 1800 1825 1850 1875 1900 1925 Year Fig. 4-6

When a Population Exceeds Its Habitat’s Carrying Capacity It Can Crash A population exceeds the area’s carrying capacity Population crash (dieback) Damage may reduce area’s carrying capacity Area’s carrying capacity Can rise or decline seasonally and from year to year

Population overshoots carrying capacity 2,000 1,500 Population crashes Number of reindeer 1,000 Carrying capacity 500 Figure 5.18 Exponential growth, overshoot, and population crash of reindeer introduced onto the small Bering Sea island of St. Paul in 1910. 1910 1920 1930 1940 1950 Year Fig. 4-7

Species Have Different Reproductive Patterns Opportunist species Many, usually small, offspring Little or no parental care Massive deaths of offspring Examples: algae, bacteria, and insects

Species Have Different Reproductive Patterns Competitor species Reproduce later in life Small number of offspring with long life spans Young offspring develop inside their mothers Long time to maturity Protected by parents and potentially groups Examples: elephants, whales, and humans

4-5 What Factors Influence the Size of the Human Population? Human population growth continues but it is unevenly distributed Reasons for human population increase Movement into new habitats and climate zones Emergence of early and modern agriculture methods Control of infectious diseases through sanitation systems, antibiotics, and vaccines

Human Population Growth Continues but It Is Unevenly Distributed World population is growing exponentially Rate of 1.2 percent a year Projected population by 2050: 9.6 billion More-developed countries (2012): 0.1 percent yearly growth rate Less-developed countries (2012): 1.5 percent yearly growth rate Least equipped to deal with rapid growth

World population (in billions) Population in less-developed countries Figure 4-9 Most of the world’s population growth since 1950 has taken place in the world’s less-developed countries. This gap is projected to increase by 2050. (Compiled by the authors using data from United Nations Population Division, The 2008 Revision, and Population Reference Bureau, 2010 World Population Data Sheet) Population in more-developed countries Year Fig. 4-9 Fig. 6-3, p. 123

Women Having Fewer Babies but Not Few Enough to Stabilize the World’s Population Fertility rate Replacement-level fertility rate Rate is higher in less-developed countries Total fertility rate (TFR) Plays a key role in determining population size How many children does a woman need to have to keep the population the same?

Case Study: The U.S. Population— Third-Largest and Growing Population still growing 76 million in 1900 310 million in 2010 Drop in TFR in U.S. Population growth rate has slowed The U.S. has the world’s largest total and per capita ecological footprint

Baby boom (1946–64) Replacement level Births per woman Baby boom (1946–64) Replacement level Figure 4-11 Total fertility rates for the United States between 1917 and 2012. Question: While the U.S. fertility rate has declined and remained at or below replacement levels since 1972, why do you think the population of the United States is still increasing? (Compiled by the authors using data from the Population Reference Bureau and the U.S. Census Bureau) Fig. 4-11

Married women working outside the home 8% 81% 77 years Life expectancy 47 years 1900 2000 Married women working outside the home 8% 81% High school graduates 15% 83% Homes with flush toilets 10% 98% Homes with electricity 2% 99% Living in suburbs 10% 52% Figure 4-12 Some major changes that took place in the United States between 1900 and 2000. Question: Which two of these changes do you think were the most important? (Compiled by the authors using data from the U.S. Census Bureau and Department of Commerce) Hourly manufacturing job wage (adjusted for inflation) $3 $15 Homicides per 100,000 people 1.2 5.8 Stepped Art Fig. 4-12

Several Factors Affect Birth Rates and Fertility Rates Children as part of the labor force Cost of raising and educating children Availability of private and public pension Urbanization Educational and employment opportunities for women

Several Factors Affect Birth Rates and Fertility Rates Availability of legal abortions Availability of reliable birth control methods Religious beliefs, traditions, and cultural norms

Several Factors Affect Death Rates Life expectancy U.S. ranks 32nd among nations Infant mortality rate Based on live births that die in first year Impacted by undernutrition and malnutrition U.S. ranks 44th among nations due to Inadequate health care for poor women during pregnancy and for infants after birth Drug addiction among pregnant women

Migration Affects an Area’s Population Size Immigration and emigration Reasons for movement Economic Religious Ethnic Political Wars Environmental

Science Focus: Projecting Population Change UN high-fertility variant (2008 revision) U.S. Census Bureau (2008 update) UN medium-fertility variant (2008 revision) IIASA (2007 update) UN low-fertility variant (2008 revision) World population (in billions) Figure 4-B This graph shows world population projections to 2050 from three different organizations: the UN, the U.S. Census Bureau, and IIASA. Note that the upper-most, middle, and lower-most curves of these five projections are all from the UN, each assuming a different level of fertility. (Compiled by the authors using data from the United Nations, U.S. Census Bureau, and International Institute for Applied Systems Analysis) Year Fig. 4-B

Population Age Structure Can Affect Growth or Decline Age structure categories Prereproductive ages (0-14) Reproductive ages (15-44) Postreproductive ages (45 and older) Seniors (ages 65+) Fastest-growing age group in the U.S.

Expanding Rapidly Guatemala Nigeria Saudi Arabia Male Female Male Female Male Female Male Female Expanding Rapidly Guatemala Nigeria Saudi Arabia Expanding Slowly United States Australia China Stable Japan Italy Greece Declining Germany Bulgaria Russia Figure 4-13 Generalized population age-structure diagrams for countries with rapid (1.5%–3%), slow (0.3%–1.4%), zero (0%–0.2%), and negative (declining) population growth rates. A population with a large proportion of its people in the prereproductive age group (far left). Question: Which of these diagrams best represents the country where you live? (Compiled by the authors using data from Population Reference Bureau) Prereproductive ages 0–14 Reproductive ages 15–44 Postreproductive ages 45–85+ Fig. 4-13 Fig. 6-11, p. 131

Case Study: The American Baby Boom Growing senior population: the graying of America Animated Figure 4-14 You can track the baby-boom generation in the United States on these charts that show the U.S. population age structure for 1955, 1985, 2015 (projected), and 2035 (projected). (Compiled by the authors using data from the U.S. Census Bureau) Fig. 4-14

Populations Made Up of Mostly Older People Can Decline Rapidly Severe effects Fewer adults working and paying taxes to support an increasing elderly population Countries facing rapid decline Japan, Russia, Germany, Bulgaria, Hungary, Ukraine, Serbia, Greece, Portugal, and Italy

Some Problems with Rapid Population Decline Can threaten economic growth Labor shortages Less government revenues with fewer workers Less entrepreneurship and new business formation Less likelihood for new technology development Figure 4-15 Rapid population decline can cause several problems. Question: Which two of these problems do you think are the most threatening? Increasing public deficits to fund higher pension and health-care costs Pensions may be cut and retirement age increased Fig. 4-15

4-6 How Can We Slow Human Population Growth? Is the earth overpopulated? An important controversy Can the world provide an adequate standard of living for a projected 2.5 billion more people by 2050 without causing widespread environmental damage? Population regulation Opponents and proponents

Birth rate and death rate Population grows very slowly because of a high birth rate (to compensate for high infant mortality) and a high death rate Stage 1 Preindustrial Growth rate over time 80 70 60 50 40 30 20 10 (number per 1,000 per year) Birth rate and death rate Low Death rate Total population Birth rate Population grows rapidly because birth rates are high and death rates drop because of improved food production and health Decreasing Stage 2 Transitional Increasing Very high Population growth slows as both birth and death rates drop because of improved food production, health, and education Stage 3 Industrial Low Population growth levels off and then declines as birth rates equal and then fall below death rates Stage 4 Postindustrial Negative Zero Figure 4-17 The demographic transition, which a country can experience as it becomes industrialized and more economically developed, can take place in four stages. Unless otherwise noted, all art on this page is © Cengage Learning 2015. Stepped Art Fig. 4-17

Empowering Women Helps to Slow Population Growth Factors that decrease total fertility rates Education Paying jobs Rights supported Women’s roles Do most of the domestic work and child care Provide unpaid health care within families Bottom-up change in taking charge

Family Planning Can Provide Several Benefits Educational and clinical services Responsible for a 56 percent drop in TFRs in less-developed countries (1960-2012) Issues in less-developed countries 42 percent of all pregnancies unplanned, 26 percent end with abortion Many couples lack access to family planning

Is It Possible to Reduce Population Growth? United Nations’ Conference on Population and Development goals by 2015 Provide universal access to family planning and reproductive health care Improve health care for infants, children, and pregnant women Implement national population policies Improve the status of women

Is It Possible to Reduce Population Growth? Goals by 2015 Increase the involvement of men in child-rearing Sharply reduce poverty Sharply reduce unsustainable patterns of production and consumption

Case Study: India’s Attempts to Slow Its Population Growth Projected 1.69 billion people by 2015 Most populous country Problems: poverty, malnutrition, environmental degradation Factors for larger families Poor couples want many children Work and care for parents in old age Bias toward having male children

Case Study ─ Slowing Population Growth in China: the One-Child Policy Projected 1.4 billion people by 2026 Promotes one-child families Free sterilization, contraceptives, and abortion Fast-growing economy since 1980 Rapidly growing middle class Need for more environmentally sustainable economic development Concerns regarding the graying of China