Density-independent factors can also limit populations, but they seldom regulate populations because they act irregularly, regardless of the population’s density. Populations limited by density-independent factors seldom reach carrying capacity. Weather is a good example. Agaves (Century Plants) reproduce once at the end of a long lifespan (Figure below). The average lifespan is about 25 years rather than a full century, but an individual’s lifespan depends at least in part on erratic rainfall. Agaves will reproduce only after rainfall allows sufficient growth – however long that takes. Eventually, a wet season will bring about a single episode of flowering and the production of a huge number of seeds. Their growth and eventual reproduction will, in turn, depend on erratic rainfall. The density-independent factor rainfall limits birth rate, which in turn limits growth rate, but because of its unpredictability, it cannot regulate Agave populations.
Figure 17.25
Each Century Plant reproduces only once during its long lifespan. This strategy allows it to gather sufficient water over a number of years in an environment where rainfall is scarce and unpredictable. Then, during an especially wet season, the plant produces a huge number of seeds and dies. Does the Century Plants pattern remind you of the salmons life cycle?
Other density-independent limiting factors include human activities:
Pesticides and herbicides: For example, DDT thinned the eggshells of Peregrine Falcons, reducing their birthrates and leading to their extirpation from the eastern half of North America.
Habitat destruction: Conversion of prairies and grasslands worldwide drastically reduced populations of Burrowing Owls in North America and Giant Pandas in China.
To conclude our discussion of population dynamics, let’s look at two sets of adaptations related to the logistic growth curve which describe the growth of most populations. These should remind you of the survival patterns we discussed earlier in this lesson. Recall that for logistic growth, r is the growth rate of the population, and K is the carrying capacity.
Scientists have found that species adapted to unstable or unpredictable environments are usually limited by density-independent factors to population densities considerably lower than carrying capacity. Such environments favor adaptations which maximize growth rates: early maturity, small size, high numbers of small offspring, single episodes of reproduction, short life expectancy, and the ability to disperse widely. Because populations are usually far below carrying capacity, crowding is minimal, so these species invest little energy in competitive adaptations. Survivorship curves (Figure above) are Type III, with high early death rates. Such species are said to be r-selected – that is, selected for rapid growth. Weed species are often r-selected for colonization and rapid population of disturbed or newly created habitats such as roadsides, abandoned fields, mudslides, or lava flows. Jack pine trees are r-selected species which “pioneer” clear areas immediately after forest fires. They grow quickly in hot, dry soils and release seeds from cones which are opened only by fire – reproducing and dispersing seeds at just the right, if unpredictable, time (Figure below).
Figure 17.26
Jack pines show r-selected adaptations to an unpredictable (density-independent) limiting factor: fire. Cones (bottom image) open to release many tiny seeds only at high temperatures. The trees (top image) grow quickly in the open, bare areas left by forest fires, so are often called pioneer species.
Whereas density-independent factors limit r-selected species in unpredictable environments, K-selected species are adapted to stable environments and regulated by density-dependent factors. Stable environments support K-selected populations at or near carrying capacity, at which point crowding leads to significant intraspecific competition. Such environments favor adaptations for efficient resource utilization which confer competitive ability. K-selected individuals often grow slowly to large size, live long, and delay but repeat reproduction of fewer offspring. They may provide extensive parental care because they can count on environmental stability and survival of these relatively few offspring. Survivorship curves resemble the Type I pattern: long life expectancy and relatively low death rates in the stable environment. Maple trees are K-selected “climax” species which grow slowly in their own shade and reproduce relatively large seeds over a number of years throughout their relatively long lifespan (Figure below).
Figure 17.27
Maple trees show K-selected adaptations to a predictable shade environment they help to create. Maples release relatively large seeds annually, and offspring grow slowly but steadily in the shaded, rich soil of their parents. Maples experience significant intraspecific competition, and their populations tend to be limited by density-dependent factors. Because maple forests tend to persist for long periods because they can grow in their own shade, they are often called climax species.
Characteristics of r-selected and K-selected species are compared in Table below.
r- Selected Species K-Selected Species
Environment Unstable Stable
Type of Regulating Factors Density-independent Density-dependent
Organism Size Small Large
Maturity Early Late
Number of Offspring Many Few
Energy used to make each Individual Low High
Average Life Expectancy Short Long
Number of Reproductive Events per Individual Once Many times
Survivorship Type III: only a few individuals live long lives
Type I or II: most individuals live long lives
In conclusion, all populations eventually reach limits, at or below carrying capacities for the ecosystems in which they live. Some have adaptations for rapid growth, but the unpredictable environments in which they live inflict high death rates. Others live in stable environments where death rates are relatively low, but their populations are high, so individuals must spend energy on costly competitive strategies in order to gather scarce sunlight, nutrients, or water - or fight disease or predation. Many species live between these extremes, but all populations have limits.
Lesson Summary
The ways in which populations change are called population dynamics.
Populations have the potential to grow exponentially, at least under ideal conditions.
Exponential growth begins with slow growth, but as population increases, growth rate increases.
J-curves depict the pattern of exponential population growth.
Malthus first described exponential growth for the human population and predicted that humans would outgrow their food resources, leading to widespread famine or war.
If birth rate (plus immigration) exceeds death rate (plus emigration), a population grows. If death rate exceeds birth rate, the population declines. And if birth rate and death rate are in equilibrium, growth rate is zero and the population remains stable.
In a stable population, each individual (on the average) produces one offspring which survives long enough to reproduce itself.
Altricial species have a few undeveloped offspring but invest a great deal of energy in parental care. Precocial species invest energy in a large number of well-developed offspring, but little in parental care.
The earlier species begin to reproduce, the faster their population grows, with all other factors being equal.
Dispersal moves offspring away from parents, reducing intraspecific competition.
Migration, seasonal movement of populations, can affect all four components of population growth rate.
Regular wandering behavior (nomadism) adapts specific populations to fluctuating food supplies.
Irruption, range expansion, and colonization have irregular, unpredictable effects on population growth.
Few populations in nature grow exponentially. No population can continue such growth indefinitely.
The logistic (S-curve) model best describes the growth of many populations in nature.
In the logistic model, growth rate depends on both population size and availability of resources. Growth is s
low at first, but as size increases, growth accelerates. At higher densities, limited resources cause growth rate to decline, and populations stabilize at carrying capacity.
A limiting factor is a property of a population’s environment which restricts population growth.
Density-dependent limiting factors lower birth rates or increase death/emigration rates via increased intraspecific competition at higher population densities.
Many natural populations are kept at or below carrying capacity by one or a complex interaction among several density-dependent limiting factors, such as competition, predation, or disease.
Density-independent factors, such as rainfall, drought, or pollution, can also limit populations, but they seldom regulate populations because they act irregularly, regardless of the population’s density.
Cycles of growth and decline limit some predator and prey populations.
Density-independent factors limit r-selected species in unpredictable environments, while K-selected species are adapted to stable environments and regulated by density-dependent factors.
Review Questions
Explain Malthus’ ideas about population growth and their significance to evolutionary theory.
Compare exponential(J-curve)growth to logistic(S-curve)growth, and explain the conditions under which each occurs in nature.
Summarize the equation for population growth rate, and explain each factor.
Compare survival and reproduction in altricial species to the same factors for precocial species.
How might delaying age of childbirth prevent the need to limit family size, as China has done?
Give examples of dispersal and migration, and how they affect populations.
Define carrying capacity and explain its importance to population growth.
Compare and contrast density-dependent and density-independent limiting factors.
Relate predator-prey cycles to density-dependent population control.
Compare and contrast the adaptations and environmental characteristics typical of r-selected species to those of K-selected species.
Further Reading / Supplemental Links
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookpopecol.html
http://www.geography.learnontheinternet.co.uk/topics/popn1.html
http://curriculum.calstatela.edu/courses/builders/lessons/less/biomes/breeding.html
http://www.bestfootforward.com/
http://www.footprintnetwork.org/gfn_sub.php?content=footprint_overview
http://www.panda.org/news_facts/publications/living_planet_report/index.cfm
http://www.worldchanging.com/archives/006904.html
http://lca.jrc.ec.europa.eu/lcainfohub/introduction.vm
http://www.ilea.org/leaf/richard2002.html
Vocabulary
altricial
Refers to a pattern of growth and development in organisms which are incapable of moving around on their own soon after hatching or being born.
birth rate (b)
Number of births within a population or subgroup per unit time; in human demography, the number of childbirths per 1000 people per year.
carrying capacity (k)
The maximum population size that a particular environment can support without habitat degradation.
colonization
Movement of a population into a newly created or newly found area.
death rate (d)
Number of deaths within a population or subgroup per unit time; in human demography, the number of deaths per 1000 people per year.
density-dependent factor
Factor which has the potential to control population size because its effects are proportional to population density.
density-independent factor
Factor which may affect population size or density but cannot control it.
dispersal
Movement of offspring away from parents, resulting in reduced competition within the population and more effective colonization of suitable habitat.
emigration (e)
Movement of individuals out of a population’s range.
exponential model (geometric or J-curve)
A model of population growth which assumes that growth rate increases as population size increases.
immigration (i)
Movement of individuals into a population’s range from other areas.
intraspecific competition
Competition between members of the same population for the same resource.
irruption (invasion)
Irregular movements, often caused by food source failures.
K-selected species
A species which has adaptations which maximize efficient utilization of resources, conferring competitive strength near or at carrying capacity.
limiting factor
A property of a population’s environment – living or nonliving – which controls the process of population growth.
logistic (S-curve)
A model of population growth which assumes that the rate of growth is proportional to both population size and availability of resources.
migration
The direct, often seasonal movement of a species or population.
nomadism
Regular, wide-ranging wandering behavior, which allows some species to compensate for fluctuating food supplies.
population
A group of organisms of a single species living within a certain area.
population dynamics
Changes in population size and structure.
population growth rate (r)
The change in population size per member of the population per unit time.
precocial
Refers to species in which the young are relatively mature and mobile from the moment of birth or hatching.
predator-prey cycle
Regular, repeating increases and decreases in a prey population followed by corresponding changes in its predator’s population.
r-selected species
Species which has adaptations which maximize growth rate, r.
range expansion
The gradual extension of a population beyond its original boundaries.
Points to Consider
Why do you think Malthus’ predictions of widespread famine and war have not (yet?) been realized? Do you think his ideas make sense for the future?
Are humans altricial or precocial? Why?
In your opinion, could delaying age of first childbirth help solve human population problems?
How important do you think dispersal, range expansion, or immigration are for human populations?
Do you think humans have more r-selected adaptations, or K-selected adaptations?
Do you think Earth has a carrying capacity for humans? If so, what kinds of limiting factors determine that carrying capacity?
Lesson 17.3: Human Population Growth: Doomsday, Cornucopia, or Somewhere in Between?
Lesson Objectives
Contrast the Neo-Malthusian or “limits to growth” and cornucopian or “technological fix” views of human population growth.
Compare the overall pattern of human population growth to the J-curve (exponential) and S-curve (logistic) models.
Analyze the factors which have influenced human population growth from our beginnings 200,000 years ago to 1804, when we first reached the one billion mark.
Describe the four stages of human population growth as outlined by the demographic transition model.
Evaluate the demographic transition model as it applies to European population growth in the late 18th and 19th centuries.
Evaluate the demographic transition model as it applies to less developed countries.
Apply the demographic transition model to recent changes in developed countries.
Using age-sex structures, contrast population growth in developed countries to growth in undeveloped countries.
Explain the concept of replacement fertility rate.
Discuss the implications of Stage 5 population dynamics.
Know and understand predictions for future worldwide human population growth.
Analyze limiting factors and technological advances which may contribute to a carrying capacity of Earth for the human population.
Explore the concept of sustainability as a goal for economic, social, and environmental decision-making.
Explain the tool of ecological footprint analysis as a means of evaluating the sustainability of lifestyles for individuals, countries and the world.
Calculate your ecological footprint and compare it to averages for your country and the world.
Recognize our human potential to make decisions which could direct future population growth.
Explore some options for social, political and cultural change, and environmental conservation which could help to balance population dynamics and resource utilization.
Introduction
Hundreds of stone figures measuring up to 10 meters tall and weighing up to 87 tons overlook a low-diversity grassland on Easter Island in the Pacific Ocean (Figure below). The food sources, woody trees, and rope-yielding plants which helped to build and transport these statues over five hundred years ago are gone.
CK-12 Biology I - Honors Page 77