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Chapter 5
To understand the current extinction crisis of species, one must under- , stand ecology, which requires an
understanding of evolution. New species are formed through the process of speciation. Separation of populations, which
prevents mixing of genes, is the key to speciation. Over time, populations are no longer able to interbreed, and new
species have formed, either through allopatric or sympatric speciation. The evolution of life involved complex structures
evolving from simpler ones, and larger sizes evolving from smaller sizes. Species with small populations or specialized
requirements, or that live in only a few areas are vulnerable to extinction, the permanent disappearance of a species.
Unlike previous extinction events, the current massive wave of extinction is caused by human population growth,
resource use, and pollution. Ecology is the study of the interactions among organisms and between the organisms and
their environment. Ecologists study interactions at several levels, including the organismal, population, community, and
ecosystem levels. The population size of a species is limited by the environment's carrying capacity, environmental
resistance, limiting factors, and the species' biotic potential. One of the most important factors in a community is the
transfer of energy and matter via food chains and webs, and other interactions between species. Although community
changes, such as succession, are natural, invasive species can rapidly interfere with a community's normal
functioning. Human development, resource use, and population are in- creasing the types and rates of community
changes. Economic and social factors, such as ecotourism, can help prevent extinction and protect diversity, and
restoration ecology can help restore damaged communities
Chapter Outline
Striking Gold in a Costa Rican Cloud Forest
Monteverde, a village on the slopes of a mountain in Costa Rica, is surrounded by lush, lower montane rainforests.
These forests are also known as cloud forests because clouds bring in much of the moisture the rainforests require.
In 1964, Dr. Jay Savage discovered patches of brightly colored toads on
the forest floor; 200 golden toads lived within a 5-meter area. ,
Within 25 years, due to drying out of the rainforest, the toad was extinct.
Evolution as the Wellspring of Earth's Biodiversity
Some people say that the extinction of species is the biggest environmental problem humans face.
To put current extinction trends in context, the processes of speciation and extinction must be examined in detail.
Biological diversity is the product of evolution
Biological diversity (biodiversity for short) is the total of all organisms in an area, including species diversity, genetic
diversity, population diversity, and community diversity.
A species is a population whose members share characteristics and can freely breed with each other and produce
fertile offspring.
There may be up to 100 million species worldwide, but only between 1.5 million and 1.8 million species have been
described.
Speciation is the process that produces new types of organisms
Speciation is the process by which new species are created.
As individuals in a population breed, their genes mix, which enables a trait to spread through a population.
Separation of a population prevents mixing of genes; this separation is the key to speciation.
Over time, each population accumulates its own set of genes, and the two populations become so different that they
can no longer interbreed.
Genetic divergence can be accelerated by natural selection if environmental conditions are different for the two
populations.
Populations can be separated in many ways
Allopatric speciation occurs when a population is physically separated over a geographic distance.
Populations may become geographically separated by melting glaciers, rivers, mountain ranges, and changing climate
patterns.
To undergo speciation, populations must remain isolated for thousands of generations.
The moment of truth for speciation is when isolated populations meet again
If the populations have not changed enough, they may be able to breed if they become reunited.
If the two populations have changed enough, they will not interbreed and two species will have been formed. Not all speciation is allopatric
Other species may arise through sympatric speciation, for which geo- graphic isolation is not necessary.
Insects are the most likely group to undergo this type of speciation, because they easily become specialized in their feeding and mating requirements, which separates them from other
populations.
Life's diversification results from numerous speciation events I
Repeated bouts of speciation generate complicated patterns of biodiversity above the species level.
Evolutionary biologists examine how the major groups of organisms arose and their distinguishing characteristics
evolved.
Phylogenetic trees represent the history of divergence
Phylogenetic trees show how divergence of species took place and the relationships between species, groups of
species, and populations.
Individual traits can also be placed in phylogenetic trees to determine how they evolved.
Life has diversified in amazing ways
The evolution of life generally involved complex structures evolving from
simpler ones, and larger sizes evolving from smaller sizes.
Animals, plants, and fungi have evolved exquisite adaptations for survival.
Extinction and speciation together determine Earth's biodiversity
Extinction is the disappearance of an entire species from Earth.
The average life span of a species is 1-10 million years.
Natural extinction is gradual and occurs one species at a time. I
Organisms can adapt to environmental changes, but not if the changes are too rapid.
Specialized species and small populations are vulnerable to extinction.
Endemic species, those found in only a few places, are also vulnerable to extinction because all their members belong
to a single population.
Earth has seen several episodes of mass extinction
Earth has experienced five mass extinction events in its history, which killed off enormous numbers of species at once.
The K- T event, caused by a massive extraterrestrial impact near Mexico 65 million years ago, killed off the dinosaurs.
An even bigger extinction event occurred 250 million years ago and may have been caused by volcanic eruptions.
The sixth mass extinction is upon us
Biologists believe humans are the cause of this sixth mass-extinction event.
Human population growth, resource use, and development have already caused species extinction.
Specific human causes of extinction include destruction of habitat, hunting and harvesting of species, and the
introduction of invasive species.
Units of Ecological Organization
Ecology is the study of the interactions among organisms and between the organisms and their environment.
Life occurs on a multitude of levels: organismal, population, community, and ecosystem.
At the organismal level, ecologists describe the relationships between organisms and their environment.
At the population level, ecologists describe how individuals within a species interact.
At the community level, ecologists focus on the interactions between species.
At the ecosystem level, ecologists study patterns in nature, including energy and nutrient flows.
Ecology on the Organismal Level
A major relationship between a species and its environment is its habitat, the place where it lives.
A habitat can be tiny or huge and includes both abiotic and biotic components.
Habitat selection describes how a species selects its habitat according to its needs.
An organism's survival depends on the availability of quality habitat.
A species' niche is its functional role in a community (what it does), including its use of habitats, what it eats, and its
interactions with other organisms.
Population Ecology
A population consists of individuals of the same species living in a particular area.
Populations exhibit characteristics that can help predict their future dynamics Ecologists can understand how a
population grows or declines by studying density, distribution, sex ratios, age structures, and birth and death rates.
Predicting population growth and decline is very important for threatened and endangered species
Population size, the number of individuals of a species present at a given time, can change over time.
Zero population growth describes a population that stays the same size over time, neither growing nor declining.
Even superabundant species, such as the passenger pigeon, which numbered in the billions, can show population
declines and become extinct.
Population density describes the number of individuals within a population per unit area.
Larger organisms generally have lower densities because they need more resources.
High population density can make it easier to find mates, but can also in- crease competition, predation risk, and
disease.
Low population density can allow more food and space, but can make it hard to find mates.
Population distribution, or population dispersion, describes the location of organisms within an area.
In a random distribution, organisms are located haphazardly, with no
particular pattern; this occurs when resources are located throughout an area.
Evenly spaced individuals have a uniform distribution; this happens when individuals compete for space.
In a clumped distribution, organisms arrange themselves around an area that provides a needed resource.
Animals that show habitat selection usually have a clumped distribution. Population distributions can make a species
more or less vulnerable to extinction.
Age distribution, or age structure, describes the relative numbers of organ- isms of each age within a population.
There are generally three age classes: (1) pre-reproductive individuals,
(2) individuals of reproductive age, and (3) post-reproductive individuals.
A population with many pre-reproductive individuals will increase in the future, while one with many post-reproductive
individuals will decrease.
Age structure diagrams, also called age pyramids, show the age structure of a population, with each horizontal bar
showing the size of each age class.
The sex ratio of a population is the proportion of males to females.
The sex ratio can also help determine whether a population increases or decreases; if there are lots of breeding-age
females, the population will likely increase.
Populations may grow, shrink, or remain relatively stable
Population growth and decline is determined by four factors: (1.) births, (2) deaths, (3) immigration, and (4) emigration.
Immigration is the arrival of individuals from outside the population. Emigration is the departure of individuals from a
population.
The crude birth rate is the number of births per I,OQO individuals.
The crude death rate is the number of deaths per 1,000 individuals.
The population's growth rate, which tells us the change in a population's size, equals the (crude birth rate + immigration
rate)-(crude death rate + emigration rate).
The growth rate is expressed as a percentage, so that a growth rate of 6/1,000 equals 0.6%.
Unregulated populations tend to increase by exponential growth
Exponential growth is an increase by a fixed percent each year, so that a
fixed percentage of a small number is a small increase, but 'as the number gets larger, so does the increase.
Population growth curves visually show changes in a population's size.
Exponential growth in nature occurs when a population enters a new environment, the population size is small, and
resources are abundant.
Limiting factors restrain population growth
Exponential growth is stopped by limiting factors, which are physical (temperature, sunlight, moisture, etc.), chemical, or
biological (food, space, resources, shelter, etc.) characteristics that restrain population growth.
The carrying capacity is the maximum population size an area can maintain. The logistic growth curve rises quickly at
first (the exponential growth phase), but finally comes to a standstill because of limiting factors.
The total limiting factors to growth are called environmental resistance, which stabilizes population growth at or below
carrying capacity.
Carrying capacities can change
Carrying capacity can change when environmental conditions change.
Humans can change the carrying capacity for ourselves by changing the environment to reduce environmental
resistance.
The influence some factors have on population depends on population density
Density-dependent factors are limiting factors that affect a population's -. Size, but their effects depend on the density of
the population; environmental resistance increases with a denser population.
Density-independent factors are limiting factors whose effects are constant,
regardless of the population's density; examples are temperature, floods, and fires.
Biotic potential and reproductive strategies vary from species to species. The biotic potential is the innate reproductive
capacity of a species.
The interaction between an organism's biotic potential and environmental resistance to its population growth determines
the fate of that population.
Species with low biotic potential are called K-strategists.
K-strategists have few offspring but devote lots of time and energy to caring for and protecting their offspring.
R-strategists have lots of offspring in a short time, but do not care for their young.
K-strategists' populations are limited by density-dependent factors such as disease, predation, and food.
R-strategists' populations are limited by density-independent factors, and can experience large swings in population
size.
It can be difficult for scientists to know whether rapid population declines are part of natural cycles or a sign of serious
trouble.
Beyond Populations to Communities
A community is a group of populations of organisms living in the same area at the same time and interacting with each
other.
How cohesive are communities?
Frederick Clements believed that communities were cohesive, and many organisms shared similar limiting factors and
evolutionary histories.
Henry Gleason believed that communities were not cohesive units, and that each species responded to its own limiting
factors and could move into and out of a community.
Under Gleason's theory, when one species disappears from a community, another species can become the dominant
one, which is supported by field research.
A food web is a conceptual record of feeding relationships and energy flow Some of the most important interactions
within a community involve who eats whom.
Food chains are conceptual, visual representations of community feeding interactions.
Food webs are a more complicated representation of feeding interactions
because they involve more relationships.
Trophic levels are the ranks in the feeding hierarchy to which organisms belong.
Producers: Green plants are located at the first trophic level; they do not consume other organisms, and they are
autotrophs (they produce their own food).
Producers are the foundation of all energy and matter exchange in the community.
Primary consumers are organisms that consume producers and include grazing animals.
Herbivores are primary consumers, and occupy the second trophic level. \
The third trophic level consists of secondary consumers, such as wolves, I that eat herbivorous, animals.
Some secondary consumers are carnivores, animals that eat other animals. Omnivores are animals that eat both plants
and animals.
Tertiary consumers are animals that eat at higher trophic levels, such as hawks and owls.
Decomposers, also called detritivores, are bacteria and' fungi that consume' the waste products or dead bodies of
organisms.
Decomposers are the community's recyclers, making nutrients from organic matter available for reuse.
Some organisms play bigger roles in communities than others.
Keystone species in communities, such as beavers, have a greater impact on other species because of what they eat or
how they physically change the community.
Many keystone species, such as wolves and jaguars, are at the top of the food chain. '
Removal of a keystone species has major effects on the community, such as when beavers are killed and the beaver
dam is removed.
Species interact in several fundamental ways
Species interaction is at the heart of community ecology, and includes predation, competition, parasitism, and
mutualism.
Symbioses are relations in which two organisms live in close physical con-, tact with each other.
Predation describes one organism eating another for food; the predator hunts and kills the prey.
Predation rates help determine the ratios of predator to prey, and affects community composition.
Predation can also drive population dynamics: When too many prey are eaten, food decreases, resulting in fewer
predators, which results in more prey, which increases the number of predators.
Evolutionarily, the best predators survive and reproduce, so their offspring have the adaptations that make them better
hunters.
Prey organisms have also evolved better defenses against predation, such as being able to run fast
Herbivory, when animals eat plants, has caused plants to evolve elaborate defenses, such as thorns and spines.
Competition involves conflict between two organisms for the same resource.
Competition between two members of the same species is termed
intraspecific competition; interspecific competition is between two members of different species.
Most competition is subtle and indirect and does not involve outright fighting
Competitive exclusion occurs when one species prevents another from using a resource.
When species live side by side, species coexistence occurs, which brings about an equilibrium in which the population
size of both species remains stable.
To minimize competition, species adjust so as to use only some of the resources, and do not fulfill their entire ecological
role (niche).
Thus, species do not use their full (fundamental) niche, but use only a part of their niche, the realized niche.
Over time, species evolve to use resources in different ways through resource partitioning, which decreases competition
by dividing up a resource.
Resource partitioning may result in character displacement, where competing species evolve physical characteristics
that make them more different from each other, which decreases competition between them.
In parasitism, one organism (the parasite) harms another (the host) without killing it.
Endoparasites, such as tapeworms, live inside their hosts; ectoparasites, 1 such as ticks, live on the outside of the host.
Other parasites, such as cowbirds, only parasitize other species during certain times of the year.
Parasites and hosts are involved in an evolutionary arms race, which involves both sides evolving new responses to
either attack or defend themselves.
Mutualism involves a relationship in which two species both benefit from their interaction.
Generally, in mutualism one species provides shelter or protection, while the other provides food.
Although most mutualistic relationships involve the organisms living together, some, such as pollination of flowers by
bees, involve the species encountering each other periodically.
Amensalism is a relationship in which one organism is harmed, but the other is unaffected.
In commensalism, one species is benefited while the other is unharmed, such as when a plant creates a favorable
condition for another plant species to grow.
Succession occurs after a disturbance or after a new substrate emerges
Communities change over time in their structure and composition due to the effects of the species present. '
Succession is the series of regular, predictable, and quantifiable changes in a community.
Primary succession begins with bare rock, sand, or sediment.
Pioneer species arrive first, and are often lichens (mutualistic aggregates of fungi and algae).
Lichens secrete acids that break down the rock, forming soil, which allows other organisms to move in, which provides
nutrients and habitat for other future organisms.
The climax community is the final community in succession and will remain until some environmental change alters or
displaces it. Primary succession also occurs in aquatic systems.
Secondary succession occurs when some form of disturbance (flood, fire, farming) changes an existing community. '
Because secondary succession does not begin with bare rock, some
organisms will remain and provide building blocks for the successionary process.
Invasive species pose a new threat to community structure
Although community changes such as succession are natural, invasive species can rapidly interfere with a community's
normal functioning.
Invasive species spread widely and become dominant in a community, and are usually non-native species introduced
from other parts of the world.
Invasive species are so successful because their natural limiting factors, such as predators and competitors, are left
behind.
The United States is full of invasive species, from Dutch Elm disease to the common house sparrow.
Humans and the Conservation of Biodiversity
Human development, resource use, and population are increasing the, types and rates of community changes.
Millions of people worldwide are taking action to protect endangered species and habitats and to safeguard Earth's
biodiversity.
Social and economic factors affect species and communities
Previous generations have seen natural areas simply as vast pools of resources, ready to be extracted.
Costa Rica has taken steps to protect its environment
Countries such as Costa Rica experienced rapid population growth and subsequent loss of their forests.