The environmental challenges

The nature of the physical environment in large measure determines which organisms live in a particular climate or region. Key elements of the environment include:

Temperature. Most organisms are adapted to live within a relatively narrow range of temperatures and will not thrive if temperatures are colder or warmer. The growing season of plants, for example, is importantly influenced by temperature.

Water. All organisms require water. On land, water is often scarce, so patterns of rainfall have a major influence on life.

Sunlight. Almost all ecosystems rely on energy captured by photosynthesis; the availability of sunlight influences the amount of life an ecosystem can support, particularly below the surface in marine communities.

Soil. The physical consistency, pH, and mineral composition of the soil often severely limit terrestrial plant growth, particularly the availability of nitrogen and phosphorus.

An individual encountering environmental variation may maintain a “steady-state” internal environment, a condition known as homeostasis. Many animals and plants actively employ physiological, morphological, or behavioral mechanisms to maintain homeostasis.

Organisms are capable of responding to environmental changes that occur during their lifetime

During the course of a day, a season, or a lifetime, an individual organism must cope with a range of living conditions. They do so through the physiological, morphological, and behavioral abilities they possess. These abilities are a product of natural selection acting in a particular environmental setting over time, which explains why an individual organism that is moved to a different environment may not survive.

Physiological responses

Many organisms are able to respond to environmental change by making physiological adjustments. For example, you sweat when it is hot, increasing evaporative heat loss and thus preventing overheating. Similarly, people who visit high altitudes may initially experience altitude sickness—the symptoms of which include heart palpitations, nausea, fatigue, headache, mental impairment, and in serious cases, pulmonary edema—because of the lower atmospheric pressure and consequent lower oxygen availability in the air. After several days, however, the same people usually feel fine, because a number of physiological changes have increased the delivery of oxygen to their body tissues (table 55.1).

Morphological capabilities

Animals that maintain a constant internal temperature (endotherms) in a cold environment have adaptations that tend to minimize energy expenditure. For example, many mammals grow thicker coats during the winter, their fur acting as insulation to retain body heat. In general, the thicker the fur, the greater the insulation (figure 55.2). Thus, a wolf’s fur is about three times thicker in winter than in summer and insulates more than twice as well.

Behavioral responses

Many animals deal with variation in the environment by moving from one patch of habitat to another, avoiding areas that are unsuitable. The tropical lizard in figure 55.3 manages to maintain a fairly uniform body temperature in an open habitat by basking in patches of sunlight and then retreating to the shade when it becomes too hot. By contrast, in shaded forests, the same lizard does not have the opportunity to regulate its body temperature through behavioral means. Thus, it becomes a conformer and adopts the temperature of its surroundings.
Image details.
[D]
Figure 55.3 Behavioral adaptation. In open habitats, the Puerto Rican crested lizard, Anolis cristatellus, maintains a relatively constant temperature by seeking out and basking in patches of sunlight; as a result, it can maintain a relatively high temperature even when the air is cool. In contrast, in shaded forests, this behavior is not possible, and the lizard’s body temperature conforms to that of its surroundings.

Populations: Groups of a Single Species in One Place

Organisms live as members of a population, groups of individuals that occur together at one place and time. In the rest of this chapter, we consider the properties of populations, focusing on factors that influence whether a population grows or shrinks, and at what rate. The explosive growth of the world’s human population in the last few centuries provides a focus for our inquiry.

Three characteristics of population ecology are particularly important: (1) population range, the area throughout which a population occurs; (2) the pattern of spacing of individuals within that range; and (3) how the population changes in size through time.

Uniform spacing

In animals, uniform spacing often results from behavioral interactions, as described in chapter 54. In many species, individuals of one or both sexes defend a territory from which other individuals are excluded. These territories provide the owner with exclusive access to resources, such as food, water, hiding refuges, or mates, and tend to space individuals evenly across the habitat.

Clumped spacing

Individuals clump into groups or clusters in response to uneven distribution of resources in their immediate environments. Clumped distributions are common in nature because individual animals, plants, and microorganisms tend to occur in habitats defined by soil type, moisture, or other aspects of the environment to which they are best adapted.

Social interactions also can lead to clumped distributions. Many species live and move around in large groups, which go by a variety of names (for example, flock, herd, pride).

Population Demography and Dynamics

Demography is the quantitative study of populations. How the size of a population changes through time can be studied at two levels: as a whole or broken down into parts.

generation time

Generation time is the average interval between the birth of an individual and the birth of its offspring.

Age structure is determined by the numbers of individuals in different age groups

A group of individuals of the same age is referred to as a cohort. In most species, the probability that an individual will reproduce or die varies through its life span. As a result, within a population, every cohort has a characteristic birth rate, or fecundity, defined as the number of offspring produced in a standard time (for example, per year), and death rate, or mortality, the number of individuals that die in that period.

Life tables show probability of survival and reproduction through a cohort’s life span

To assess how populations in nature are changing, ecologists use a life table, which tabulates the fate of a cohort from birth until death, showing the number of offspring produced and the number of individuals that die each time period. Table 55.2 is an examplePage 1172 of a life table analysis from a study of the annual bluegrass.

In table 55.2, the first column indicates the age of the cohort (that is, the number of 3-month intervals from the start of the study). The second and third columns indicate the number of survivors and the proportion of the original cohort still alive at the beginning of that interval. The fifth column presents the mortality rate, the proportion of individuals that started that interval alive but died by the end of it. The seventh column indicates the average number of seeds produced by each surviving individual in that interval, and the last column shows the number of seeds produced relative to the size of the original cohort.

Much can be learned by examining life tables. In the case of the annual bluegrass, we see that both the probability of dying and the number of offspring produced per surviving individual steadily increases with age. By adding up the numbers in the last column, we get the total number of offspring produced per individual in the initial cohort. This number is almost 2, which means that for every original member of the cohort, on average two new individuals have been produced. A value of 1.0 would be the break-even number, the point at which the population was neither growing nor shrinking. In this case, the population appears to be growing rapidly.

Survivorship curves demonstrate how survival probability changes with age

The percentage of an original population that survives to a given age is called its survivorship. One way to express some aspects of the age distribution of populations is through a survivorship curve.

Reproductive events per lifetime represent an additional trade-off

The trade-off between age and fecundity plays a key role in many life histories. Annual plants and most insects focus all their reproductive resources on a single large event and then die. This life history adaptation is called semelparity. Organisms that produce offspring several times over many seasons exhibit a life history adaptation called iteroparity.

Environmental Limits to Population Growth

The simplest model of population growth assumes that a population grows without limits at its maximal rate and also that rates of immigration and emigration are equal. This rate, called the biotic potential, is the rate at which a population of a given species increases when no limits are placed on its rate of growth. In mathematical terms, this is defined by the following formula:

where N is the number of individuals in the population, dN/dt is the rate of change in its numbers over time, and ri is the intrinsic rate of natural increase for that population—its innate capacity for growth.

The biotic potential of any population is exponential (red line in figure 55.16). Even when the rate of increase remains constant, the actual number of individuals accelerates rapidly as the size of the population grows. The result of unchecked exponential growth is a population explosion.

Two models of population growth. The red line illustrates the exponential growth model for a population with an r of 1.0. The blue line illustrates the logistic growth model in a population with r = 1.0 and K = 1000 individuals. At first, logistic growth accelerates exponentially; then, as resources become limited, the death rate increases and growth slows. Growth ceases when the death rate equals the birth rate. The carrying capacity (K) ultimately depends on the resources available in the environment.

In practice, such patterns of unrestrained growth prevail only for short periods, usually when an organism reaches a new habitat with abundant resources. Natural examples of such a short period of unrestrained growth include dandelions arriving in the fields, lawns, and meadows of North America from Europe for the first time; algae colonizing a newly formed pond; or cats introduced to an island with many birds, but previously lacking predators.

Carrying capacity

No matter how rapidly populations grow, they eventually reach a limit imposed by shortages of important environmental factors, such as space, light, water, or nutrients. A population ultimately may stabilize at a certain size, called the carrying capacity of the particular place where it lives. The carrying capacity, symbolized by K, is the maximum number of individuals that the environment can support.

The logistic growth model applies to populations that approach their carrying capacity

As a population approaches its carrying capacity, its rate of growth slows greatly, because fewer resources remain for each new individual to use. The growth curve of such a population, which is always limited by one or more factors in the environment, can be approximated by the following logistic growth equation:
Equation: d N over d t equals r times N times parenthesis K minus N all divided by K parenthesis.

In this model of population growth, the growth rate of the population (dN/dt) is equal to its intrinsic rate of natural increase (r multiplied by N, the number of individuals present at any one time), adjusted for the amount of resources available. The adjustment is made by multiplying rN by the fraction of K, the carrying capacity, still unused [(K − N)/K ]. As N increases, the fraction of resources by which r is multiplied becomes smaller and smaller, and the rate of increase of the population declines.

As the population size then declines toward the carrying capacity, the magnitude of this negative growth rate will decrease until it reaches 0 when N = K.

Factors That Regulate Populations

Some of these factors depend on population size and are therefore termed density-dependent. Other factors, such as natural disasters, affect populations regardless of size; these factors are termed density-independent.

Density-independent effects include environmental disruptions and catastrophes

Growth rates in populations sometimes do not correspond to the logistic growth equation. In many cases, such patterns result because growth is under the control of density-independent effects. In other words, the rate of growth of a population at any instant is limited by something unrelated to the size of the population.

Resource availability affects life history adaptations

When resources are limited, the cost of reproduction often will be very high. Consequently, selection will favor individuals that can compete effectively and utilize resources efficiently. Such adaptations often come at the cost of lowered reproductive rates, as organisms wait longer to reproduce so that they can grow larger and stronger, and produce fewer, larger offspring. Such populations are termed K-selected because they are adapted to thrive when the population is near its carrying capacity (K). Table 55.3 lists some of the typical features of K-selected populations.

By contrast, in populations far below the carrying capacity, resources may be abundant. Costs of reproduction are low, and selection favors those individuals that can produce the maximum number of offspring. Selection here favors individuals with the highest reproductive rates; such populations are termed r-selected. Examples of organisms displaying r-selected life history adaptations include dandelions, aphids, mice, and cockroaches.

Human Population Growth

Humanity’s future growth is uncertain.

The human population is unevenly distributed. Rapid growth in developing countries has resulted in poverty, whereas most resources are utilized by the industrialized world.
The population growth rate has declined.

Even at lower growth rates, the number of individuals on the planet is likely to plateau at 7 to 10 billion.
Consumption in the developed world further depletes resources.

Resource consumption rates in the developed world are very high; a sustainable future requires limits both to population growth and to per capita resource consumption.

Source-sink metapopulations are distinct from other types of metapopulations because

exchange of individuals only occurs in the former.

populations with negative growth rates are a part of the former.

populations never go extinct in the former.

all populations eventually go extinct in the former.

populations with negative growth rates are a part of the former.

The potential for social interactions among individuals should be maximized when individuals

are randomly distributed in their environment.

are uniformly distributed in their environment.

have a clumped distribution in their environment.

have a clumped distribution in their environment.

When ecologists talk about the cost of reproduction they mean

the reduction in future reproductive output as a consequence of current reproduction.

the amount of calories it takes for all the activity used in successful reproduction.

the amount of calories contained in eggs or offspring.

None of the choices is correct.

the reduction in future reproductive output as a consequence of current reproduction

A life history trade-off between clutch size and offspring size

means that as clutch size increases, offspring size increases.

means that as clutch size increases, offspring size decreases.

means that as clutch size increases, adult size increases.

means that as clutch size increases, adult size decreases.

means that as clutch size increases, offspring size decreases.

The difference between exponential and logistic growth rates is

exponential growth depends on birth and death rates and logistic does not.

in logistic growth, emigration and immigration are unimportant.

that both are affected by density, but logistic growth is slower.

that only logistic growth reflects density-dependent effects on births or deaths.

that only logistic growth reflects density-dependent effects on births or deaths.

The logistic population growth model, dN/dt = rN[(K − N)/K], describes a population’s growth when an upper limit to growth is assumed. As N approaches (numerically) the value of K

dN/dt increases rapidly.

dN/dt approaches 0.

dN/dt increases slowly.

the population becomes threatened by extinction.

dN/dt approaches 0.

Which of the following is an example of a density-dependent effect on population growth?

An extremely cold winter

A tornado

An extremely hot summer in which cool burrow retreats are fewer than number of individuals in the population

A drought

An extremely hot summer in which cool burrow retreats are fewer than number of individuals in the population

f the size of a population is reduced due to a natural disaster such as a flood

population growth rates may increase because the population is no longer near its carrying capacity.

population growth rates may decrease because individuals have trouble finding mates.

population rates may remain unchanged if the population was already well below the carrying capacity.

All of the choices are correct.

All of the choices are correct.

In populations subjected to high levels of predation

individuals should invest little in reproduction so as to maximize their survival.

individuals should produce few offspring and invest little in any of them.

individuals should invest greatly in reproduction because their chance of surviving to another breeding season is low.

individuals should stop reproducing altogether.

individuals should invest greatly in reproduction because their chance of surviving to another breeding season is low

In a population in which individuals are uniformly distributed

the population is probably well below its carrying capacity.

natural selection should favor traits that maximize the ability to compete for resources.

immigration from other populations is probably keeping the population from going extinct.

None of the choices is correct.

natural selection should favor traits that maximize the ability to compete for resources.

The elimination of predators by humans

will cause its prey to experience exponential growth until new predators arrive or evolve.

will lead to an increase in the carrying capacity of the environment.

may increase the population size of a prey species if that prey’s population was being regulated by predation from the predator.

will lead to an Allee effect.

may increase the population size of a prey species if that prey’s population was being regulated by predation from the predator.

the temperature range tolerated by most organisms is relatively

narrow

all organisms require which of the following?

water

sunlight is considered to be one the most important factors of an environment because all ecosystems rely on the energy captured by producers in the process of _______

photosynthesis

an individual encountering environmental variation regulates its internal environment to keep within a steady state. This regulation is known as which of the following?

homeostasis.

which of the following best explains why an animal that has evolved in one set of conditions may not thrive in a different set of conditions?

natural selection

which of the following are examples of physiological adjustments made by organisms to adapt to environmental change?

an increase in red blood cells in some one living in san francisco moving to denver, percipitation during a hot day, shivering in the cold.

morphological change

thicker fur during the winter

many mammals that live in cold environments grow thicker fur in the winter. this is a morphological change that minimizes

energy use

one way in which organims can respond to environmental changes is through altering a specific _____ feature.

morphological

spadefoot toads spend most of the year burring below the deserts of north america. how far below the surface can they be found?

one meter

all the organisms that belong to the same species and occur in the same geological location are called which of the following?

a populaiton

the geographic area over which a population is distributed is known as the population's _____

range

a spacing pattern where the organisms are spaced at about equal distance throughout the landscape is called which of the following?

uniform

which of the following factors contribute to clumped spacing patterns?

flocking or herding behaviors, uneven distribution of resources.

demography is the study of which of the following?

birth and death rates, age distribution, and population sizes.

the mean period between birth of females and birth of their offspring is called ______

generation

for a cohort in a population, fecundity is best defined as which of the following?

the number of offspring produced in a standard period of time.

what is a life table?

a life table is a data table of the proportion of individuals surviving and reproducing in different age classes.

a graph of the proportion of surviving individuals at each age group is called a _____ curve.

survivorship

th________ is a reproductive strategy in which organisms reproduce during one single event, whereas, ______ is a reproductive strategy in which organisms reproduce throughout their lifetime.

semelparity, iteoparity.

rapid population growth occurring when per capita growth rate remains above zero is called

exponential growth

unchecked ______ growth of a population can result in a population explosion which would theoretically continue infinitely.

exponential.

which of the following assumptions can be made when modeling exponential growth?

space is available, water is unlimited, food is plentiful.

in which of the following situations would a population exhibit exponential growth?

a population which reaches a new habitat with an abundance of resources.

the carrying capacity is defined as:

the maximum number of individuals that an environment can sustain.

which of the following describe what happens to a population as it approaches its carrying capacity?

as the population approaches carrying capacity, resources are used up and growth date declines

which of the following describes logistic growth?

logistic growth occurs when the population is restricted by one limiting factor.

in the logistic growth model, (K-N)/K represents:

the fraction of the carrying capacity that is still unused.

when a population size does not change over time, the rate is

zero

which of the following accurately compares density depended (DD) factors and density independent factors?

DD: mortality increases with pop size; DI mortality remains unchanged with pop size.

mortality factors whose influence is not affected by changes of individuals per unit are called density ____

independent factors

________ selected species are those with relatively stable populations adapted to existing at or near carrying capacity.

k

____ selected species are those with unstable populations that have high rate of per capita population growth.

r

clumped spacing can be a result of

uneven distribution of resources or social interaction of species

what is exponential growth?

rapid growth with an intrinsic rate of increase and assumes resources are unlimited.

patterns of unrestrained growth in populations typically occur for _____ periods of time, and usually occur when organisms reach a new habitat which has an abundance of ______.

short, resources

what does rN represent in the logistic growth model?

intristic rate of increase.

according to the logistic growth model, when the number (N) of individuals in a population approaches its carrying capacity (K) the populations growth rate should approach ( and eventually become ) which of the following?

zero

which of the following best describe how/when patterns of unrestrained or exponential growth of populations generally occur?

for short periods of time

torder the following organisms according to their geographic range, from very limited (at the top) to worldwide distribution

which of the following accurately describes a network of distinct populations that interact by exchanging members?

metapopulation

natural selection has led to adaptations to prevent dehydration and water loss in frogs living in warm, dry climates. one species developed a method of water-_______ its skin by coating it with a ____ like secreation that greatly reduces water loss.

proofing, wax

density independent factors that influence population size

fire

hard freeze

a population _____ is a bar graph displaying the number of people in several age categories.

pyramid

which type of factors can produce a population growth curve categorized by erratic growth patterns?

density independent factors.

some populations within a metapopulation can become ______ as a result of a disease, a catastrophic fire, or loss of genetic variation.

extinct.

choose the two examples of populations that might exhibit the allele effect

a population of species that relies on large groups to deter predators, a sparsely distributed population, in which individuals have trouble finding mates

when scientists set up experimental plots where they controlled the availability of food for hares and the presence/absence of predators (lynx) they determined that

both food shortage and prey shortage are important factors that are important in determining the cycles of hares and lynx.

choose the two important implications of a metapopulation for the range of species

long term extinction is less likely,

species occupy a larger area than they normally would.

how does the ecological footprint of individuals in developed countries compare to that of individuals in developing countries?

developed countries >>> developing countries.

in a _______ metapopulation structure, dispensers are continually sent from better habitats to the poorer habitats to increase the population there.

source-sink

an individual may alter its physiology, morphology, or behavior as a result of which of the following?

environmental change

the growth rate of a population is greatly influenced by the relative number of individuals in each ____. the number defines the populations's ____ structure.

cohort, age

a plant population may exhibit a clumped dispersion. which of the following contributes to that?

favorable conditions for photosynthesis are arranged in patches, favorable conditions for germination are arranged in patches, certain plants release seeds in groups.

in an attempt to increase the diversity of birds in new york, starlings were introduced in 1896. by 1980 they had _____- their range throughout the united states.

increased.

recently, the gap between the rich and the poor has significantly increased in many developing countries. this is atlas to some extent due to:

a rapid growth of the human population in those countries.

our ecological foot print can help estimate capacity because of which of the following?

it is an indication of whether we are using more productive land than the earth can sustain over time.

the age structure of a population is based on which of the following?

the relative number of individuals in each age group.

select all of the following that would result in random spacing patterns within a population.

abundant resources, lack of aversions among individuals.

which of the following can be affected by density affected factors?

death rates/ and or birth rates.

animals and plants that equilibrate to the conditions of their environment are known as ________. their bodies adapt to temperature, salinity, and other physiological aspects of their surroundings.

conformers.

cohorts are groups of individuals that can be described as which of the following?

the same age.

the mammal community of many distant islands is composed entirely of ______, as they are the only mammals that can colonize such islands.

bats

k selected species

extensive parental care

long life span

many reproductions per life time

r selected species

offspring or eggs typically small

short life span

early age at first reproduction

which of the following are r selected species?

beetles

fleas

dandelions

a ______ comprises distinct populations that may exchange members.

metapopulation

which of the following represents clumped spacing?

a certain species of fungus is found only living on cow patties.

true or false: scientists have estimated that the earth can and will sustain the growth of the human population for atlas another 100 years.

false

the current growth rate of the human population is 1.1 % per year. what does this growth rate mean in terms of the doubling time of the human population?

it will still take relatively little (about 60 years) for the human population to double.

metapopulations prevent the long-term_______of species through the constant colonization of empty patches of habitat.

extinction

how does the ecological foot print of an individual in the USA compare to that of someone in a developing country such as india?

the ecological foot print of someone from the USA is much higher

in certain metapopulations, populations IN better (more suitable) areas constantly send out dispensers that increase the populations in poorer (less suitable) habitats. The better areas are referred to as ____ and the poorer habitats are referred to as ______.

sources and sinks

the relative numbers of individuals of each defined age group is referred to as which of the following?

a population's age structure.

A ______ is defined as a group of individuals that occur together at one place and time, while _____ is defined as a network of distinct populations that interact by exchanging members.

population, metapopulation

choose the two important implications of a metapopulation for the range of species.

long-term extinction is likely, species occupy a larger area then they usually would.

some animals are uniformly spread throughout their environment, which of the following contributes to this spread?

territoriality

competition for resources

in most human populations

the number of old females is disproportianely large compared with the number of older males.

in my bird species, an increase in the reproduction of offspring in one year results in which of the following?

a decrease in the number of offspring produced the following year, an increase in the probability of dying during the same year.

in the song sparrow

reproductive success increases as the population size decreases, mortality rates increase as population size increase

in the average ecological footprint is about 2 hectares and humans exceed that, an ecologist would infer which of the following?

we were in an ecological deficit and we would reach carrying capacity faster.

a ______ compromises distinct populations that may exchange members.

metapopulation.

which of the following organisms are semelparous?

salmon, butterflies, agave.

_____ spacing of individuals within a population occurs when individuals do not strongly interact with each other.

random

populations strongly affected by density independent factors often time exhibit

erratic growth patterns

when the cost of reproducing is high, organisms often

defer reproduction to enhance growth, allocate resources to survival and minimize reproduction.

which of the following is defined as a system of distinct populations that may exchange members?

metapopulations

seeds enclosed in fleshy fruits are generally dispersed by which?

animal transport

which of the following accurately describes how the snowshoe hare population affects the canada lynx population?

as the snowshoe hare population decreases, the candy lynx pop decreases, and both increase as the other increase.

when populations are experiencing logistic growth, they tend to move towards the_______ or K.

carrying capacity

random spacing

lack of aversions among individuals, abundance of resources.