Download Chapter 7 Community Structure and Species Diversity Biological

Chapter 7
Community Structure and Species Diversity
 Biological communities differ in their structure and physical appearance
 Physical appearance: relative sizes, stratification, and distribution of the populations and
species in each community
 Physical structure within a particular type of community or ecosystem can also vary
 Appearance Matters
 Community structure varies around its edges where one type of community makes a
transition are between to a different type of community
 Increasing the edge area through habitat fragmentation makes any species more
vulnerable to stresses such as predators and fire
o Also creates barriers that can prevent some species from colonizing new areas and
finding food and mates
Species Diversity and Niche Structure
 Different species play different roles
 Biological communities differ in the types and numbers of species they contain and the
ecological roles those species play
 Biological communities are shaped by the individual species that live in them, by
interactions among these species, and by how the species interact with their physical
environment
 Species Diversity: the number of different species contains (species richness) combined
with the abundance of individuals within each of those species (species evenness)
 Niche structure: how many potential ecological niches occur, how they resemble or differ
from one another, and how the species occupying different niches interact
 Geographic location: species diversity is highest in the tropics and declines as we move
from the equator toward the poles
o This is because in the tropics have a fairly constant daily climate and a more
reliable supply of food sources
 Tropical species tend to be specialists with narrow niches and live in
microhabitats
o Species that live at high latitudes where weather is cold and variable ten to be
generalist species with wide niches
 They have adaptations that enable them to thrive in a wide range of
environments and occur over large expanses of territory
 The most species-rich environment are tropical rain forests, coral reefs, the deep sea, and
large tropical lakes
o Have high species richness and generally low species evenness
Types of Species
Types of Species in Communities
 Native, nonnative, indicator, keystone, or foundation species describe the major niches
filled by various species in communities
 Native species: those species that normally live and thrive in a particular community
 Nonnative species: species that migrate into or are deliberately or accidentally
introduced into a community (a.k.a. invasive species or alien species)
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Most of these nonnative species are beneficial to us
Sometimes, nonnative species can reduce some or most of the community’s native
species and cause unintended and unexpected consequences
o The killer bees introduced in Brazil
Indicator Species:
 Biological smoke alarms
 These species can alert us to harmful environmental changes taking place in biological
communities
 Indicator Species: species that serve as early warnings of damage to a community or an
ecosystem
o Trout, butterflies, and birds
 The plight of some amphibian indicator species us a warning signal. They may not need
us, but we and other species need them
Keystone Species: Major Players
 Indicator Species: remove this species and the community collapses
 Eliminating this species may dramatically alter the structure and function of a community
 Many roles
o Pollination of flowering plants (bees, butterflies, humming birds, bats)
o Top predator: feed on and help regulate the populations of other species (wolf,
lion, alligator, leopard, and some shark species)
o Clean up species rapidly remove, burry, and recycle dung as a source of food for
their newly hatched larvae
 Dung beetle
 These beetles also churn and aerate the soil, making it more
suitable for plant life
 Without them, in many places we would be up to our eyeballs in
animal wastes and many plants would be starved for nutrients
 The loss of keystone species can lead to population crashes and extinctions of other
species in a community that depends on it for certain services
 Super important to protect these species
Foundation Species: Other Major Players
 Foundation Species: play a major role in shaping communities by creating and
enhancing their habitats in ways that benefit other species
 Expansion of keystone species category
Species Interactions: Competition and Predation
How Do Species Interact?
 Species can interact through competition, predation, parasitism, mutualism, and
commensalism
 When species in a community have activities or resource needs in common, they may
interact with one another
 Five basic interactions
o Interspecific Competition
o Predation
o Parasitism
o Mutualism
o Commensalism
 Predation and competition help limit population size
 Interactions serve as agents of natural selection
o Species interactions that help control population sizes illustrate one of the four
scientific principles of sustainability
 The most common interaction is competition for shared or limited resources such as food
and space
o Interspecific Competition: attempts by two or more organisms of a single
species to use the same limited resources in an ecosystem
o Instead of fighting for the resources, most competition involves the ability of one
species to become more efficient in acquiring food or other resources
 This cannot go on for long periods of time
 When it gets intense the one of the competing species must migrate (if
possible) to another area, shift its feeding habits or behavior through
natural selection, suffer a sharp population decline, or become extinct in
that area
Reducing or Avoiding Competition: Sharing Resources
 Some species evolve adaptation that allow them to reduce or avoid competition for
resources with other species
 Resource Partitioning: when species competing for similar scarce resources evolve
more specialized traits that allow them to use shared resources at different time, in
different ways or in different places
 Niches become separated to avoid competition for resources.
Predators and Prey: Eating and Being Eaten
 Species called predators feed on other species called prey
 Predation: members of one species (the predator) feed directly on all or part of a living
organisms of another species (the prey)
 Predator-prey relationship: the interaction between the two organisms of different
species in which one is the predator and feeds on the prey
 Most predation occurs at the microscopic level in soils and in the sediments of aquatic
systems
 Does important role in controlling some other species
 Also, predators plat a role in evolution by natural selection
o Because they tend to kill the sick, weak, aged, and least fit members of the
population because they are the easiest to catch
o This leaves behind individuals with better defenses against predation
 These individuals tend to survive longer and leave more offspring with
adaptations that help them avoid predation
Sense the Environment to Find Food and Mates
 Organisms use their senses their senses to locate objects and prey and to attract
pollinators and mates
 Animals with ears sense sound—pressure waves created in the air or water by objects that
vibrate, move, or collide with other objects
o Sound detection is enhance by ear shapes and the ability of organisms to move
their ears in different directions to pinpoint where sounds are coming from
 Some predators locate prey by smelling volatile chemicals their prey give off
 Bates use a sonar system to find their way around and locate prey
How Do Predators Increase Their Chances of Getting a Meal?
 Some predators are fast enough to catch their prey, some hide and lie in wait, and some
inject chemicals to paralyze their prey
 Carnivores feeding on mobile prey have two main options
o Pursuit
o Ambush
 Other predators use camouflage to in plain sight and ambush their prey
 Some predators use chemical warfare to attack their prey
o Paralyze their prey
How Do Prey Defend Against or Avoid Predators?
 Some prey escape their predators or have outer protection, some are camouflaged, and
some use chemicals to repel predators
 Prey species have evolved many ways to avoid predators, including abilities to run, swim
or fly fast, and a highly developed sense of sight or smell that alerts them to the presence
of predators
 Some have protective shells, thick bark, and thorns
 Other prey species use the camouflage of certain shapes or colors or the ability to change
color
 Chemical warfare is another common strategy
o Some prey species discourage predators with chemicals that are poisonous,
irritating, foul smelling, or bad tasting
o Plants also emit chemicals
 Herbivore poisons and herbivore repellents
 Many of these prey species have evolved warning coloration, brightly colored advertising
that enables experienced predators to recognize and avoid them
o If it is small and strikingly beautiful, it is probably poisonous
o If it is strikingly beautiful and easy to catch it is probably deadly
 Mimicry—when a species will copy another to get the same protective benefits as the
other species
 Behavioral Strategies—behaviors that protect the prey from predators
o Living in large groups
o Puffing up to scare of the predator
o Spreading wings
o Mimicking the predator
o The prey making itself look bigger than it actually is
Species Interactions: Parasitism, Mutualism, and Commensalism
Parasites: Sponging Off Others
 Although parasites can harm their host organisms, they can also promote community
biodiversity
 Parasitism: occurs when one species (the parasite) fees on part of another organism (the
host), usually by living on or in the host
o The parasite benefits and the host is harmed
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A parasite usually is smaller than its host (prey) and rarely kills its host
Most parasites remain closely associated with, draw nourishment from, and may
gradually weaken their hosts over time
 Some parasites can live inside or outside of their hosts
 For the hosts view parasites are harmful
 Parasites promote biodiversity and control populations by helping keep some species
from becoming so plentiful that they eliminated other species
Mutualism: Win-Win Relationships
 Species can interact in ways that benefit both of them
 Mutualism: two species or a network of species interact in a way that benefits both
o Help with reproduction (seed dispersal), being supplied with food, or receiving
protection
 Each species benefits by exploiting the other
Commensalism: Using without Harming
 Some species interact in a way that helps one species but has little or no effect on the
other
 Commensalism: is an interaction that benefits one species but has little, if any, effect on
other species
Ecological Succession: Communities in Transition
Ecological Succession: How Communities Change Over Time
 New environmental conditions allow one group of species in a community to replace
other groups
 All communities change their structure and composition in response to changing
environmental conditions
 Ecological Succession: the gradual change in species composition of a given area
o Colonizing or pioneer species arrive first
o As the environmental conditions change, they are replaced by others, and later
these species may be replaced by another set of species
 Two types of succession:
o Primary succession: the gradual establishment of biotic communities in lifeless
areas where there is no soil or sediment.
o Secondary succession: series of communities develop in places containing soil or
sediment.
 More common than primary succession
 Facilitation: when species in one community may modify the environment (such as by
producing soil), makes it easier for other species to movie in
 Also, some species in earlier and later communities can coexist because they are not in
direct competition for resources
Primary Succession: Starting from Scratch
 Over long periods, a series of communities with different species can develop in lifeless
areas where these is no soil or bottom sediment
 Begins with an essentially lifeless area where these is no soil in a terrestrial ecosystem or
no bottom sediment in an aquatic ecosystem
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Usually takes a long time because there is no fertile soul to provide the nutrients needed
to establish a plant community
 The slow process of soil formation begins when pioneer or early successional species
arrive and attach themselves to inhospitable patches of bare rock
 These tough species start the soil formation process by trapping wind-blown soil particles
and tiny pieces of detritus, producing tiny bits of organic matter, secreting mild acids that
slowly fragment and break down the rock, and adding their own wastes and dead bodies
 The chemical breakdown or weathering is hastened by physical weathering such as the
fragmentation of rock that occurs when water freezes in cracks and expands
 After hundreds or even thousands of years, the soil may be deep and fertile enough to
store the moisture and nutrients needs to support or mid-successional plant species
 Soon this area is replaces with late successional plant species, it soon become a complex
forest unless it is destroyed
 Can also take place in newly created small ponds as a result of an influx of sediments and
nutrients in runoff from the surrounding land
 Over time this process can transform the pond first into a marsh and eventually to dry
land
Secondary Succession: Starting Over with Some Help
 A series of communities with different species can develop in places containing some soil
or bottom sediment
 Begins in an area where the natural community of organisms has been distributed,
remove, or destroyed, but some soil or bottom sediment remains
o This can include abandoned farmland, burned or cuts forests, heavily polluted
streams, and land that has been flooded
 New vegetation usually can germinate within a few weeks from seed in the soil and those
brought in from nearby plants by wind or by birds and other animals
 Vegetation changes and the numbers and types of animals and decomposers also change
 Disturbances can convert a particular stage of succession to an earlier stage
o Create new conditions that encourage some species and discourage or eliminate
others
 Doing so by releasing nutrients and creating unfilled niches
 Disturbances can be beneficial for the species diversity of some communities and
ecosystems
 Intermediate Disturbance Hypothesis: fairly frequent but moderate disturbances lead to
the greatest species diversity
Can We Predict the Path of Succession, and is nature in Balance?
 Scientists cannot predict the course of succession or view it as preordained progress
towards a stable climax community that is in balance with its environment
 The equilibrium model of succession is what ecologists once meant when they talked
about the balance of nature
 Old view on balance-of-nature: a large terrestrial community undergoing succession
eventually become covered with an expected type of climax vegetation such as a mature
forest
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New view on balance-of-nature: that mature late successional communities are not in
state permanent equilibrium. Instead, they are in a state of continual disturbances and
change
Succession reflects the ongoing struggle by different species for enough light, nutrients,
food, and space
o This competition allow them to survive and gain reproductive advantages over
other species
Ecological Stability and Sustainability
Stability of Living Systems: Surviving by Changing
 Living systems maintain some degree of stability through constant change in response to
changing environmental conditions
 Everything is always changing
 These systems contain complex networks of negative and positive feedback loops that
interact to provide some degree of stability over each system’s expected life span
 This stability is maintained only by constant change in response to changing
environmental conditions or disturbances
 Three aspects of stability or sustainability in living systems:
o Inertia or persistence: the ability of a living system to resist being disturbed or
altered
o Constancy: the ability of a living system such as a population of plants to keep
its numbers within the limits imposed by available resources
o Resilience: the ability of a living system to bounce back and repair damage after a
disturbance that is not too drastic
Community Productivity and Sustainability
 Having many different species appears to increase the sustainability of many
communities
 Communities with more species tend to have a higher net primary productivity (NPP) and
can be more resilient than simpler ones
o In diverse communities each species is able to exploit a different portion of the
available resources
 A complex community with a diversity of species and feeding paths has more ways to
respond to most environmental stresses because it does not have “all its eggs in one
basket”
 How much biodiversity is needed to help sustain various communities remains uncertain
Why Should We Bother to Protect Natural Systems?
 Human activities are disrupting ecosystem services that support and sustain all life and all
economics
 We need to use great caution in making potentially harmful changes to communities and
ecosystems