Human Population Growth

Essential Biology with Physiology Fifth Edition

Chapter 19 Population Ecology

• Lionfish, with their graceful, flowing fins, bold stripes, and eye-catching array of spines, are striking members of tropical reef communities. • But lionfish – have venomous spines, – are merciless predators, and – grow quickly from 2 to 18 inches.

• Apparently, some aquarium owners who regretted their purchase released their lionfish into the wild. • Now lionfish populations have spread up the East Coast of the United States throughout the Atlantic and Caribbean regions; they are now swarming into the Gulf of Mexico.

• Lionfish consume prodigious numbers of fish, including – species that are key to maintaining the legendary diversity of reef communities and – juveniles of economically important fishes such as grouper and snapper. • The National Oceanic and Atmospheric Administration (NOAA) has launched an “Eat Lionfish” campaign to encourage human predation on the tasty fish.

• A population is a group of individuals of a single species that occupy the same general area at the same time. – Individuals in a population ▪ rely on the same resources, ▪ are influenced by the same environmental factors, and ▪ are likely to interact and breed with one another.

• Population ecology is concerned with changes in population size and the factors that regulate populations over time. • A population ecologist might describe a population in terms of its – size (number of individuals), – age structure (proportion of individuals of different ages), or – density (number of individuals per unit area or volume).

• Population ecologists also study population dynamics, the interactions between biotic and abiotic factors that cause variation in population size. • One important aspect of population dynamics—and a major topic for this chapter—is population growth.

• Population ecology also plays a key role in applied research. – It provides critical information for conservation and restoration projects, is being used to develop sustainable fisheries throughout the world, and is used to manage wildlife populations. – Studying the population ecology of pests and pathogens provides insight into controlling how they spread.

• Population density is the number of individuals of a species per unit area or volume of the habitat. • Examples include the number of – largemouth bass per cubic kilometer (km3 ) of a lake, – oak trees per square kilometer (km2 ) in a forest, and – nematodes per cubic meter (m3 ) in a forest’s soil.

• How do we measure population density? – In most cases, it is impractical or impossible to count all individuals in a population. – Instead, ecologists use a variety of sampling techniques to estimate population density. – Population densities may also be estimated by indicators such as number of bird nests or rodent burrows rather than by actual counts of organisms.

• The age structure of a population is the distribution of individuals in different age-groups. • The age structure of a population provides insight into – the history of a population’s survival, – reproductive success, and – how the population relates to environmental factors. • Figure 19.3 shows the age structure of males in a population of cactus finches on the Galápagos Islands in 1987.

Copyright © 2016, 2013, 2010 Pearson Education, Inc. All Rights Reserved Figure 19.3 Age Structure for the Males in a Population of Large Cactus Finches (Inset) on One of the GaláPagos Islands in 1987 (1 of 2)

• Life tables – track survivorship (the chance of an individual in a given population surviving to various ages) and – help to determine the most vulnerable stages of the life cycle.

Table 19.1 Life Table for the U.S. Population in 2008

blank Number Living at Number Dying Chance of Start of Age Interval During Interval Surviving Interval Age Interval (N) (D) 1 − (D/N) 0–10 100,000 833 0.992 10–20 99,167 363 0.996 20–30 98,804 941 0.990 30–40 97,863 1,224 0.987 40–50 96,639 2,640 0.973 50–60 93,999 5,643 0.940 60–70 88,356 11,203 0.873 70–80 77,153 21,591 0.720 80–90 55,562 33,215 0.402 90+ 22,347 22,347 0.000

• Survivorship curves plot the number of individuals still alive at each age in the maximum life span. • By using a percentage scale instead of actual ages on the x-axis, we can compare species with widely varying life spans. • Species that exhibit a Type I curve usually produce few offspring but give them good care, increasing the likelihood that they will survive to maturity.

– Species that exhibit a Type III curve have low survivorship for the very young, followed by a period when survivorship is high for those few individuals who live to a certain age. – Species that exhibit a Type II curve (black) are intermediate, with survivorship constant over the life span.

• An organism’s life history is the set of traits that affect the organism’s schedule of – reproduction and – survival.

• A population’s pattern of survivorship is an important feature of its life history. • Key life history traits include the – age at first reproduction, – frequency of reproduction, – number of offspring, and – amount of parental care given.

• Reproductive success is key to evolutionary success. • The combination of life history traits represents trade- offs that balance the demands of reproduction and survival. • Because selective pressures vary, life histories are very diverse.

• Organisms with an opportunistic life history – take immediate advantage of favorable conditions and – typically exhibit a type III survivorship curve.

• Organisms with an equilibrial life history – develop and reach sexual maturity slowly, – produce few, well-cared-for offspring, – are typically larger-bodied and longer-lived, and – typically exhibit a type I survivorship curve. • Table 19.2 compares key traits of opportunistic and equilibrial life history patterns.

Table 19.2 Some Life History Characteristics of Opportunistic and Equilibrial Populations

Characteristic Opportunistic Populations Equilibrial Populations (such as many wildflowers) (such as many large mammals) Climate Relatively unpredictable Relatively predictable Maturation time Short Long Life span Short Long Death rate Often high Usually low Number of offspring per Many Few reproductive episode Number of reproductions per Usually one Often several lifetime Timing of first reproduction Early in life Later in life Size of offspring or eggs Small Large Parental care Little or none Often extensive

– Population size fluctuates as new individuals are born or move into an area and others die or move out of an area. – Population ecologists use idealized models to investigate how the size of a particular population may change over time under different conditions.

• Exponential population growth describes the expansion of a population in an ideal and unlimited environment. • In this model, the population size of each new generation is calculated by multiplying the current population size by a constant factor that represents the birth rate minus the death rate.

• The increasing speed of population growth produces a J-shaped curve that is typical of exponential growth. The slope of the curve shows how rapidly the population is growing. • Exponential population growth is common in certain situations, such as following a disturbance, such as a fire, flood, hurricane, drought, or cold snap, that may suddenly reduce the size of a population.

• Most natural environments do not have an unlimited supply of the resources needed to sustain population growth. • Limiting factors – are environmental factors that restrict population growth and – ultimately control the number of individuals that can occupy a habitat.

• The carrying capacity is the maximum population size that a particular environment can sustain. • Logistic population growth occurs when the growth rate decreases as the population size approaches carrying capacity. – When the population is at carrying capacity, the growth rate is zero.

• The carrying capacity for a population varies, depending on the – species and – resources available in the habitat. • Ecologists hypothesize that selection for organisms exhibiting equilibrial life history patterns occurs in environments where the population size is at or near carrying capacity.

• Figure 19.7 compares logistic growth with exponential growth. • Both the logistic model and the exponential model are theoretical ideals of population growth. No natural population fits either one perfectly.

• Density-dependent factors – are limiting factors whose intensity is related to population density and – can limit growth in natural populations. • The most obvious example is intraspecific competition, competition between individuals of the same species for the same limited resources.

• As a limited food supply is divided among more and more individuals, birth rates may decline as individuals have less energy available for reproduction. • Density-dependent factors may also depress a population’s growth by increasing the death rate.

• A limited resource may be something other than food or nutrients. For example, – young kelp perch hide from predators in “forests” of the large seaweed known as kelp and – the number of nesting sites on rocky islands may limit the population size of oceanic birds such as gannets, which maintain breeding territories.

• Density-dependent factors may include – competition for resources, – increased disease transmission under crowded conditions, and – accumulation of toxic waste products.

• In many natural populations, abiotic factors such as weather may limit or reduce population size well before other limiting factors become important. • A population-limiting factor whose intensity is unrelated to population density is called a density- independent factor. These factors include – seasonal changes in the weather and – environmental disturbances, such as fire, floods, and storms.

• Over the long term, most populations are probably regulated by a complex interaction of density- dependent and density-independent factors.

• Some populations of insects, birds, and mammals undergo dramatic fluctuations in density with remarkable regularity. – “Booms” characterized by rapid exponential growth are followed by “busts,” during which the population falls back to a minimal level.

• Examples of boom-and-bust cycles include 1. lemmings, small rodents that live in the tundra, and 2. snowshoe hares and one of the main predators of the snowshoe hare, the lynx.

• The hare and lynx cycles may be caused by – winter food shortages that result from overgrazing, – predator-prey interactions, or – a combination of food resource limitation and excessive predation.

• To a great extent, we humans have converted Earth’s natural ecosystems to ecosystems that produce goods and services for our own benefit. • Population ecology is used to – increase populations of organisms we wish to harvest, – decrease populations of pests, and – save populations of organisms threatened with extinction.

• The U.S. Endangered Species Act defines – an endangered species as one that is “in danger of extinction throughout all or a significant portion of its range” and – a threatened species as one that is likely to become endangered in the foreseeable future. • The challenge for conservationists is to determine the circumstances that threaten a species with extinction and try to remedy the situation.

• The red-cockaded woodpecker requires longleaf pine forests, where it drills its nest holes in mature, living pine trees. – The numbers of red-cockaded woodpeckers declined as suitable habitats were lost to ▪ logging, ▪ agriculture, and ▪ suppressing the fires that are a natural occurrence in these ecosystems.

• Research revealed that breeding birds tend to abandon nests when vegetation among the pines is thick and higher than about 4.5 m (15 feet). – Apparently, the birds require a clear flight path between their home trees and the neighboring feeding grounds. – Wildlife managers protected critical habitat and began a maintenance program that included controlled fires to reduce forest undergrowth. – As a result of such measures, populations of red- cockaded woodpeckers are beginning to recover.

• According to the logistic growth model, the fastest growth rate occurs when the population size is at roughly half the carrying capacity of the habitat. • Theoretically, a resource manager should achieve the best results by harvesting the population down to this level. • However, the logistic model assumes that growth rate and carrying capacity are stable over time.

• Fish, the only wild animals still hunted on a large scale, are particularly vulnerable to overharvesting. • In the northern Atlantic cod fishery, estimates of cod stocks were too high, and the practice of discarding young cod (not of legal size) at sea caused a higher mortality rate than was predicted. • The fishery collapsed in 1992 and has not recovered.

• Sustainable catch rates can’t be estimated without knowing these essential life history traits for the target species. • In addition, knowledge of population ecology alone is not sufficient; sustainable fisheries also require knowledge of community and ecosystem characteristics.

• Organisms that are introduced into non-native habitats can have a devastating effect on the ecosystem. • An invasive species – is a non-native species that has spread far beyond the original point of introduction and – causes environmental or economic damage by colonizing and dominating suitable habitats.

• In the United States alone, there are hundreds of invasive species, including plants, mammals, birds, fishes, arthropods, and molluscs, with an estimated cost of $137 billion per year in the United States.

• Not every organism that is introduced to a new habitat is successful, and not every species that survives in its new habitat becomes invasive. • There is no single explanation for why any non-native species turns into a damaging pest, but invasive species typically exhibit an opportunistic life history pattern.

• Cheatgrass – is an invasive plant of the arid western United States, – covers millions of acres of rangeland that was formerly dominated by native grasses and sagebrush, – produces seeds earlier and in greater abundance than its competitors, and – matures in early summer, becoming extremely dry and flammable and creating abundant fuel that is easily ignited by lightning or a stray spark.

– Cheatgrass fires are more intense and occur much more frequently than the fires that native plants have evolved to tolerate. – After a few fire cycles, the native plants are gone, robbing more than 150 species of birds and mammals of the food and shelter they derive from sagebrush. – Global climate change is also hastening the transition of rangeland into fields of cheatgrass. – Studies have shown that cheatgrass responds to

increased CO2 levels by growing faster and accumulating more tissue, which in turn becomes more fuel for the fires that extend its domain.

• Burmese pythons – are another invasive species, – were set loose in South Florida, either accidentally or deliberately, and – are now abundant in South Florida, eating native species of ▪ birds, ▪ mammals, ▪ reptiles, and ▪ amphibians.

• Invasive species may benefit from the absence of – pathogens, – predators, or – herbivores. • Biological control – is the intentional release of a natural enemy to attack a pest population and – is used to control insects, weeds, and other organisms that reduce crop yield.

• Biological control has been effective in numerous instances, especially with invasive insects and plants. • For example, beetles were brought in to combat St. John’s wort, a perennial (long-lived) European weed that invaded the western United States and had overgrown millions of acres of rangeland and pasture, leaving few edible plants for grazing livestock.

– Researchers imported leaf beetles from the plant’s native region that feed exclusively on St. John’s wort. – The shiny, pea-sized insects reduced the weed to less than 5 percent of its former abundance, restoring the land’s value to ranchers.

• One potential pitfall of biological control is the danger that an imported control agent may be as invasive as its target. • One cautionary tale comes from introducing the mongoose to control rats. – Cane planters imported the small Indian mongoose, a fierce little carnivore, to deal with the problem. – Mongooses were introduced to dozens of natural habitats, including all of the largest Caribbean and Hawaiian islands—and became invasive themselves.

• On island after island, populations of reptiles, amphibians, and ground-nesting birds have declined or vanished as mongoose populations have grown and spread.

• Kudzu – is an invasive vine, – covers an estimated 31,000 square kilometers, and – has a range limited by cold winters.

• Kudzu does have natural enemies in the United States, but it easily outgrows the damage they inflict. • A fungal pathogen called Myrothecium verrucaria appears to be a promising candidate for biological control.

• Observation: The fungus Myrothecium verrucaria causes severe disease in other weeds belonging to the same family as kudzu. • Question: Will M. verrucaria treatment work on an established stand of kudzu in a natural setting? • Hypothesis: M. verrucaria treatment that was most effective in small outdoor plantings would also be most effective in a natural setting.

• Prediction: The treatment that sprayed the highest concentration of spores in combination with a wetting agent would produce the highest death rate. • Results: The hypothesis was supported by the data, as indicated in the following figure.

• Agricultural operations create their own highly managed ecosystems that – have genetically similar individuals (a monoculture), – are planted in close proximity to each other, and – function as a banquet for plant-eating animals and pathogenic bacteria, viruses and fungi.

• Like invasive species, most crop pests have an opportunistic life history pattern that enables them to rapidly take advantage of a favorable habitat. • The history of agriculture abounds with examples of devastating pest outbreaks. – For example, the boll weevil is an insect that feeds on cotton plants both as larvae and as adults. – Its unstoppable spread across the southern United States in the early 1900s severely damaged local economies and had a lasting impact on the region.

• Pesticides may – result in populations that are not affected by a pesticide as a result of natural selection, – kill the pest and their natural predators, and – kill pollinators that are essential for both agricultural and natural ecosystems.

• Integrated pest management (IPM) uses a combination of biological, chemical, and cultural methods for sustainable control of agricultural pests.

• Integrated pest management (IPM) advocates – tolerating a low level of pests instead of total eradication and – lowering the habitat’s carrying capacity for the pest population by using pest-resistant varieties of crops, mixed-species plantings, and crop rotation to deprive the pest of a dependable food source.

• Biological control is also used when possible. • For example, many gardeners release ladybird beetles to control aphid infestations.

• Now that we have examined the regulation of population growth in other organisms, what about our own species? • Let’s begin by looking at the history of the human population and then consider some current and future trends in population growth.

• In the few seconds it takes you to read this sentence, approximately 21 babies will be born somewhere in the world and 9 people will die.

• An imbalance between births and deaths is the cause of population growth (or decline). • The human population is expected to continue increasing for at least the next several decades. • But the number of people added to the population each year has been declining since the 1980s.

• Throughout most of human history parents had many children, but the death rate was also high, resulting in a rate of increase only slightly higher than 0.

• As economic development in Europe and the United States led to advances in nutrition and sanitation and, later, medical care, people took control of their population’s growth rate. – At first, the death rate decreased while the birth rate remained the same. – The net rate of increase rose, and population growth began to pick up by the beginning of the 1900s. – By midcentury, improvements in nutrition, sanitation, and health care had spread to the developing world, spurring growth at a breakneck pace as birth rates far outstripped death rates.

• As the world population skyrocketed from 2 billion in 1927 to 3 billion just 33 years later, some scientists became alarmed. ▪ But the overall growth rate peaked in 1962. ▪ In the more developed nations, advanced medical care continued to improve survivorship, but effective contraceptives held down the birth rate. ▪ As a result, the overall growth rate of the world’s population began a downward trend as the difference between birth rate and death rate decreased.

Table 19.3 Population Trends in 2012

Population Birth Rate Death Rate Growth per 1,000 per 1,000 Rate (%) World 19.1 7.9 1.1

More developed 11.2 10.1 0.3 countries Less developed 20.8 7.4 1.3 countries

• Age structures help predict a population’s future growth. • Figure 19.22 shows the estimated and projected age structures of Mexico’s population in 1989, 2012, and 2035.

• Population momentum is – the continued growth that occurs after a population’s high fertility rate has been reduced to replacement rate and – results from girls in the 0–14 age group reaching their childbearing years.

• Age structure diagrams may also indicate social conditions. An expanding population has increasing needs for – schools, – employment, and – infrastructure. • Figure 19.23 reveals important patterns in the age structure of the United States from 1989 to 2035.

• The U.S. Census Bureau projects a global human population of 8 billion within the next 10 years and 9.6 billion by 2050. • Do we have sufficient resources to sustain 8 or 9 billion people?

• World food production must increase dramatically to – accommodate all the people expected to live on our planet in the coming decades and – improve the diets of those who are currently malnourished or undernourished.

• But agricultural lands are already under pressure. – Overgrazing by the world’s growing herds of livestock is turning vast areas of grassland into desert, – water use has risen sixfold over the past 70 years, – changes in precipitation patterns due to global warming are already causing food shortages in some regions of the world, and – because so much open space will be needed to support the expanding human population, many other species are expected to become extinct.

• An ecological footprint is an estimate of the area of land and water required to provide the resources an individual or a nation consumes, including – food, fuel, and housing and – the ability to absorb the waste a nation generates, of which carbon emissions are a major component. • Comparing our demand for resources with Earth’s capacity to renew these resources, or biocapacity, gives us a broad view of the sustainability of human activities.

• According to the World Wildlife Fund, in 2008 (the most recent year for which data are available), the average ecological footprint for the world’s population was roughly 1.5 times the planet’s biocapacity per person. – By overshooting Earth’s biocapacity, we are depleting our resources.

• Figure 19.24 compares the ecological footprints of several countries to the world average footprint and Earth’s biocapacity.

• Affluent nations such as the United States and Australia consume a disproportionate amount of resources. – So the problem is not just overpopulation, but overconsumption. – The world’s richest countries, with 15% of the global population, account for 36% of humanity’s total footprint. – Some researchers estimate that providing everyone with the same standard of living as the United States would require the resources of more than four planet Earths.

• Pronghorn antelope – roamed the open plains and shrub deserts of North America millions of years ago, – have a top speed of 97 km/h (60 mph), and – are easily the fastest mammal on the continent. • Pronghorn speed is likely an adaptation to outrun American cheetahs, which went extinct about 10,000 years ago.

• About 10,000 years ago, human invasion, combined with climate change at the end of the last ice age, coincided with the extinction of many large North American mammals, including ▪ American cheetahs, lions, jaguars, and saber-toothed cats, ▪ towering short-faced bears, ▪ massive ground sloths, and ▪ elephant-like mammoths.

• Taken together, changes in the biotic and abiotic environments happened too rapidly for an evolutionary response by these large mammals. • Like other invasive species, we change the environment of the other organisms that share our habitats. • As the scope and speed of human-induced environmental changes increase, extinctions are occurring at an accelerating pace.