Communities and Ecosystems
The Importance of Biodiversity
The expanding human population threatens biodiversity and the integrity of natural
ecosystems
Only about a quarter of Earth’s land surfaces remain untouched by human
alterations
Healthy ecosystems provide
resources
water, wood, and fish
services
recycling nutrients, preventing erosion and mudslides, controlling agricultural
pests, and pollinating crops
Coastal wetlands
act as a buffer against hurricanes
reduce the impact of flooding
filter pollutants
provide nesting sites and breeding areas for birds and marine turtles and
nurseries for a wide variety of fish and shellfish
Loss of Biodiversity
Biodiversity includes genetic diversity, species diversity, and ecosystem diversity
Thus, the loss of biodiversity encompasses more than just the fate of individual
species
Genetic Diversity
Genetic diversity within a population is the raw material that makes
microevolution and adaptation to the environment possible
If local populations are lost, then the number of individuals in the species
declines, and the genetic resources for that species decline
Many researchers and biotechnology leaders are enthusiastic about the potential
that genetic “bioprospecting” holds for future development of new medicines,
industrial chemicals, and other products
Species Diversity
Ecologists believe that we are pushing species toward extinction at an alarming
rate
The present rate of species loss may be 100x that at any time in the past
100,000 years
Some researchers estimate that at the current rate of destruction, over half
of all currently living plant and animal species will be gone by the end of
this century
The International Union for the Conservation of Nature (IUCN) reports
~13% of the 10,004 bird species and ~25% of the 4,667 mammalian species are
threatened with extinction
>20% of freshwater fishes in the world either have become extinct during human
history
or are seriously threatened
~40% of all assessed amphibian species
are in danger of extinction
Of the approximately 20,000 known plant species in the United States, 200
species have become extinct since dependable records have been kept
More than 10,000 plant species worldwide are in danger of extinction
Ecosystem Diversity
An ecosystem includes the organisms and the abiotic factors in a particular area
Because of the network of interactions among populations of different species
within an ecosystem, the loss of one species can have a negative effect on the
entire ecosystem
The disappearance of natural ecosystems results in the loss of ecosystem
services, functions performed by an ecosystem that directly or indirectly benefit
people, including air and water purification, climate regulation, and erosion
control
Causes of Declining Biodiversity
Ecologists have identified four main factors responsible for the loss of biodiversity:
1. habitat destruction and fragmentation
2. invasive species
3. overexploitation
4. pollution
Habitat Destruction
Biodiversity is threatened by the massive destruction and fragmentation of
habitats by agriculture, urban development, forestry, and mining
According to the IUCN, habitat destruction affects more than 85% of all birds,
mammals, and amphibians that are threatened with extinction
Invasive Species
Invasive species have competed with native species, preyed upon native species,
and parasitized native species …
Overexploitation
Humans can overexploit wildlife by harvesting at rates that exceed the ability of
populations to rebound, that are unsustainable
Excessive commercial harvesting, poaching, and sport hunting have greatly
affected populations of tigers, the American bison, and Galápagos tortoises
Overharvesting also threatens rare trees such as mahogany and rosewood that
produce valuable wood
Pollution
Air and water pollution is contributing to declining populations of hundreds of
species
The global water cycle can transport pollutants from hundreds of miles away
Pollutants emitted into the atmosphere may be carried aloft for thousands of
miles before falling to earth in the form of acid precipitation
Communities
Populations of different species together in a particular ecosystem
Interspecies Interactions
There are four primary types of interaction among community members
Competition
Mutualism
Predation/parasitism
Commensalism
Habitat
A species’ habitat is the physical surroundings in which that species normally can
be found
We can think of it as the species’ address
Niche
A species’ niche is how the species fits into the community, what the organisms do
to obtain the resources it needs
We can think of it as the species’ occupation
Competition
Different species can occupy the same basic habitat
However, if their niches overlap extensively, two species will be in competition
with each other for those resources
One possible outcome of competition is described by the competitive exclusion
principle
When two populations compete for the same limited, vital resource, one
can outcompete the other and thus bring about the latter’s local extinction
Another possible outcome of competition results in resource partitioning
When two similar species use the same kinds of resources from the same
habitat, but over an extended period of time, end up dividing the resources
such that neither of the species undergoes local extinction
Resource partitioning can allow multiple, similar species to coexist within a
particular environment
Mutualism
Mutualism is an interaction between individuals of two species that is beneficial
to both individuals
Predation
Predation is when one organism feeds on parts of or all of a second organism
Predator-prey interactions have spurred the evolution of some crazy physical
modifications in both predator and prey species
Some of the evolutionary adaptations are quite spectacular
So various defenses have evolved in different prey species against predation
Physical defenses
Mechanical
Chemical
Camouflage
Warning coloration
Mimicry
Batesian mimicry - a palatable or harmless species mimics an
unpalatable or harmful one
Müllerian mimicry - two or more unpalatable species resemble one
another
Behavioral defenses have evolved as well
Hiding or escaping
Alarm calling and fighting back
Coevolution
Coevolution is the interdependent evolution of two or more species
Coevolution can be the result of predator-prey interactions
Herbivory
Herbivory is the consumption of plant parts or algae by an animal
Plants have evolved numerous defenses including spines, thorns, and chemical
toxins
Other defensive compounds that are not toxic to humans but may be distasteful
to herbivores are responsible for the familiar flavors of peppermint, cloves, and
cinnamon
Parasitism
Parasitism is a specialized form of predation in which the predator feeds on its
prey but does not kill it immediately and may not ever kill it
Commensalism
Commensalism is an interaction in which an individual from one species benefits
while an individual from another species is neither harmed nor helped
Trophic Structure
Plants and other photosynthesizers are an ecosystem’s producers, while all the other
organisms are the consumers
Every ecosystem has a number of trophic (feeding) levels, with producers forming
the first trophic level, and various consumers forming several additional levels
If we follow the trophic relations, we have a food chain
When we consider all of the different feeding relations, a food web is more accurate
and informative
Food web in the Aleutian Islands (western Alaska)
Orcas (killer whales) eat seals and sea lions
Sea otters eat sea urchins
Sea urchins eat kelp (seaweed)
In the late 1980’s, the seal and sea lion populations crashed (currently about 10% of
normal)
Possibly due to lack of high-nutrition fish because of overfishing, or El Niño
effects, or whaling effects
Orcas now eat otters (population has dropped ~90%)
It is estimated that each killer whale would have to eat >1800 otters/year to
survive
The sea urchin population is increasing (sea urchin population has increased ~10
fold); the amount of kelp is decreasing (~90% reduction)
Possible consequences
“Coastal fish, mussels, marine birds and other predators in the system could
all be impacted … The kelp beds serve as a nursery for small fish and other
animals. A lot of species depend on the kelp beds for their survival.”
James Estes, marine ecologist with U.S. Geological Survey and UCSC
Species Diversity in Communities
The species diversity of a community consists of
1. species richness, the number of differentspecies in the community
2. relative abundance of the different species, the proportional
representation of each species in acommunity
Recent research indicates that greater biodiversity is tied to greater ecosystem
stability and productivity
Keystone species are species whose impact on its community is much larger
than its total mass or abundance indicates
The loss of a keystone species from a community would bring about significant
change in that community
Disturbances in Communities
Since change is normal for our planet, new areas of land or water can arise or ...
Areas of land or water that have been abandoned by humans or devastated by
physical forces will provide new ecological opportunities
Small-scale natural disturbances often have positive effects on community
For example, when a large tree falls in a
windstorm, it creates new
habitats
Ecological Succession
Succession is the change in species composition in a community over time
Three stages
Colonizing community
Intermediate communities
Climax community
Primary succession is seen when new ecological niches form where no life was present
Volcanic islands
Retreat of glaciers
Sand dunes
Ecosystems often can recover from disturbances
Disturbances can be natural
Even climax communities are subject to change
Secondary succession can occur when a climax state of habitat is first disturbed
by some outside force, but life remains, and the soil has nutrients
Even though the disturbance may seem catastrophic, secondary succession can
progress quite rapidly
Ecosystem Ecology
If we consider life in a terrarium, we can see how things work in a miniature
ecosystem
Energy Flow in Ecosystems
The energy-flow model of ecosystems provides ecologists with a powerful analytical
tool; it measures energy as it is used by and transferred among different members of
an ecosystem
All organisms require energy for growth, maintenance, reproduction, and,in many
species, locomotion
Primary Production and the Energy Budgets of Ecosystems
The amount, or mass, of living organic material in an ecosystem is the biomass
The rate at which producers convert solar energy to chemical energy stored in
biomass is primary production; globally, it yields about 165 billion tons of biomass
per year
Different ecosystems vary considerably in their primary production and in their
contribution to the total production of the biosphere
Energy Loss
Very little of the energy that a particular trophic level receives is passed along to the
next trophic level
A useful rule of thumb in ecology is that for each jump up in trophic level, the amount
of available energy drops by 90%
10% rule
This ecological pyramid explains a number of observations
Top-level consumers such as lions and hawks require large geographic
territories
It takes a lot of vegetation to support trophic levels
Most food chains are limited to three to five levels
Chemical Cycling in Ecosystems
Life depends on the recycling of chemicals
While an organism is alive, much of its chemical stock changes continuously, as
nutrients are acquired and waste products are released
Atoms present in the complex molecules of an organism at the time of its death are
returned to the environment by the action of decomposers, replenishing the pool of
inorganic nutrients that plants and other producers use to build new organic matter
Biogeochemical Cycles
Because chemical cycles in an ecosystem involve biotic components (organisms
and nonliving organic material) and abiotic (geologic and atmospheric)
components, they are called biogeochemical cycles
Note that the cycles have an abiotic reservoir where a chemical accumulates or
is stockpiled outside of living organisms
Bioavailability is a measure of how accessible the chemical is while in its abiotic
reservoir
Biogeochemical cycles can be local or global
Three important biogeochemical cycles are
1. carbon
2. phosphorus
3. nitrogen
The Carbon Cycle
Carbon enters the living world through carbon fixation during photosynthesis
Consumers and decomposers obtain their carbon from these photosynthesizers
The carbon cycle is completed when carbon moves back into the atmosphere in the
form of carbon dioxide, which is produced naturally through two processes
the respiration of living things
their decomposition after their death
The Phosphorus Cycle
The phosphorus cycle does not have an atmospheric component
Rocks are the only source of phosphorus for terrestrial ecosystems
Phosphate is a limiting factor in many terrestrial ecosystems
So farmers and gardeners often use phosphate fertilizer, such as crushed
phosphate rock or bone meal, to boost plant growth
The nitrogen cycle
Prior to the twentieth century, nitrogen entered the biotic domain mostly through
the action of certain nitrogen-fixing bacteria that have the ability to convert
atmospheric nitrogen into forms that can be taken up and used by living things
Other denitrifying bacteria have the ability to convert this organic nitrogen back into
atmospheric nitrogen
Now, much of the bioavailable nitrogen is produced by human processes and used as
fertilizer
However, nitrogen and phosphorus runoff from agriculture can be a form of
nutrient pollution that can harm both small and large aquatic ecosystems
... like eutrophication or dead zones
Biodiversity Hot Spots
Conservation efforts are often focused on biodiversity hot spots, relatively
small areas that have a large number of endangered and threatened species
and an exceptional concentration of endemic species, those that are found
nowhere else
Conservation at the Ecosystem Level
Landscape ecology is the application of ecological principles to the study of
land-use patterns
Its goal is to make ecosystem conservation a functional part of the
planning for land use
Some important principles:
Maintain natural edges
Edges between ecosystems are prominent features of landscapes,
whether natural or altered by people, and have their own sets of
physical conditions, such as soil type and surface features
Provide movement corridors
Corridors can promote dispersal, help sustain populations, and are
especially important to species that migrate between different habitats
seasonally
However, a corridor can also spread disease, especially among small
subpopulations in closely situated habitat patches