Chapter 15: Ecosystems and

Communities

Organisms and their environmentsby Mark Manteuffel, St. Louis Community College

What are ecosystems? A community of biological organisms plus

the non-living components with which the organisms interact

Take-home message An ecosystem is all of the living organisms

in a habitat as well as the physical environment.

Ecosystems are found not just in obvious places such as ponds, deserts, and tropical rainforests but also in some unexpected places, like the digestive tracts of organisms or the shell of a beetle.

Take-home message

A variety of biomes occur around the world, eachdetermined by temperature and rainfal l.

Biomes What is the average temperature? What is the average rainfall (or other

precipitation)? Is the temperature relatively constant or

does it vary seasonally? Is the rainfall relatively constant or does it

vary seasonally?

Biomes Temperature and precipitation dictate:

• Primary productivity levels the amount of organic matter produced

The numbers and types of primary producer:• are the chief determinants of the amount and

breadth of other l i fe in the region.

Take-home message Biomes are the major ecological

communities of earth, characterized mostly by the vegetation present.

Different biomes result from differences in temperature and precipitation, and the extent to which they vary from season to season.

Global air circulation patterns create deserts and rainforests.

Take-home message Global patterns of weather are largely

determined by the earth’s round shape.

Solar energy hits the equator at a more direct angle than at the poles, leading to warmer temperatures at lower latitudes.

This temperature gradient generates atmospheric circulation patterns that result in heavy rain at the equator and many deserts at 30° latitude.

Take-home message

Local topography influences the weather.

Is i t warmer or cooler in urban areas relative to nearby rural areas?

“Urban Heat Islands”

Why is it so windy on streets with tal l buildings?

Take-home message Local features of topography influence the

weather.

With higher altitude, the temperature drops.

On the windward side of mountains, rainfall is high; on the backside, descending air reduces rainfall, causing rain shadow deserts.

Urban development increases the absorption of solar energy, leading to higher temperatures, and creates wind near the bottom of tall buildings.

Take-home message

Ocean currents affect the weather.

Global Patterns of Circulation in the Oceans

Why do beach communities have milder weather than more inland communities?

Water’s Capacity to Absorb and Hold Heat

Take-home message Oceans have global circulation patterns.

Disruptions in these patterns occur every few years and can cause extreme climate disruptions around the world.

Energy flows from producers to consumers.

First Stop: Primary Producers

Second Stop: Primary Consumers—the Herbivores

Third Stop: Secondary Consumers—the Carnivores

Fourth Stop: Tertiary Consumers—the “Top” Carnivores

Chains or Webs? Food chain

• Pathway from photosynthetic producers through the various levels of animals

Food web• Involve harvesting energy from multiple stops

in the food chain

Energy Flows Losses at every “step” in a food chain

Inefficiency of energy transfers

Take-home message Energy from the sun passes through an

ecosystem in several steps.

First, it is converted to chemical energy in photosynthesis.

Take-home message Herbivores then consume the primary

producers, the herbivores are consumed by carnivores, and the carnivores, in turn, may be consumed by top carnivores.

Take-home message Detritivores and decomposers extract

energy from organic waste and the remains of organisms that have died.

At each step in a food chain, some usable energy is lost as heat.

Energy pyramids reveal the ineff iciency of food chains.

10% Efficiency

Biomass 10% rule

Where does the rest go?

Expended in cellular respiration or lost as feces

Why are big, f ierce animal species so rare in the world?

Take-home message Energy from the sun passes through an

ecosystem in several steps known as trophic levels.

Energy pyramids reveal that the biomass of primary producers in an ecosystem tends to be far greater than the biomass of herbivores.

Take-home message The biomass transferred at each step

along the food chain tends to be only about 10% of the biomass of the organisms being consumed, due to energy lost in cellular respiration.

As a consequence of this inefficiency, food chains rarely exceed four levels.

Essential chemicals cycle through ecosystems.

The Recycling of Molecules

Chemical Reservoirs Each chemical is stored in a non-living part of

the environment.

Organisms acquire the chemical from the reservoir.

The chemical cycles through the food chain.

Eventually, the chemical is returned to the reservoir.

The Three Most Important Chemical Cycles

1) Carbon2) Nitrogen3) Phosphorus

Why are global CO 2 levels rising?

Fossil Fuels

Global CO2 levels are rising in general, but they also exhibit a sharp rise and fall within each year.

Why?

Nitrogen is l ike a bott leneck l imit ing plant growth.

Fertilizers

Take-home message Chemicals essential to life—including carbon,

nitrogen, and phosphorus—cycle through ecosystems.

Take-home messageThey are usually captured from the

atmosphere, soil, or water by growing organisms; passed from one trophic level to the next as organisms eat other organisms; and returned to the environment through respiration, decomposition, and erosion.

Take-home message These cycles can be disrupted as human

activities significantly increase the amounts of the chemicals utilized or released to the environment.

Interacting species evolve together.

Natural Selection Causes organisms to become better

adapted to their environment

Does not distinguish between biotic and abiotic resources as selective forces

Take-home message In producing organisms better adapted to

their environment, natural selection does not distinguish between biotic and abiotic resources as selective forces.

Each species’ role in a community is defined by its niche.

More than just a place for living, a niche is a complete way of living.

“You can’t always get what you want.”

Fundamental niche• the full range of environmental conditions

under which a species can live

Realized niche• where and how a species is actually living

Take-home message A population of organisms in a community

fills a unique niche, defined by the manner in which they utilize the resources in their environment.

Organisms do not always completely fill their niche; competition with species that have overlapping niches can reduce their range.

Competit ion can be hard to see, but it sti l l influences community structure.

Take-home message Populations with completely overlapping

niches cannot coexist forever.

Competition for resources occurs until one or both species evolve in ways that reduce the competition, through character displacement, or until one becomes extinct in that location.

Predation produces adaptation in both predators and their prey.

Predation One of the most important forces shaping

the composition and abundance of species in a community

Why do exotic species often f lourish when released into novel habitats, even though natural selection has not adapted them to this new environment?

Prey Adaptations for Reducing Predation

There are two broad categories of defenses against predators:

• Physical

• Behavioral

Physical Defenses

Include Mechanical, Chemical, Warning Coloration, and Camouflage Mechanisms

1) Mechanical Defenses

2) Chemical Defenses

3) Warning Coloration

4) Camouflage

Behavioral Defenses

Include both seemingly passive and active behaviors: hiding or escaping, or alarm calling or

fighting back

Predator adaptations for enhancing predation

Take-home message Predators and their prey are in an

evolutionary arms race.

As physical and behavioral features evolve in prey species to reduce their predation risk, predators develop more effective and efficient methods of predation.

Take-home message

The coevolutionary process can result in brightly colored organisms, alarm calling, and many types of mimicry.

Parasit ism is a form of predation.

Parasite Predators Parasites have some unique features and

face some unusual challenges:

• The parasite generally is much smaller than its host and stays in contact with the host for extended periods of time.

• Complicated life cycles as means of getting from host to host.

Case 1: Parasites can induce foolish, fearless behavior in their hosts.

Toxoplasma

Rats and cats

Case 2: Parasites can induce inappropriate aggression in their hosts.

Case 3: Parasites can induce bizarre and risky behavior in their hosts.

The lancet fluke

Take-home message Parasitism is a symbiotic relationship in

which one organism benefits while the other is harmed.

Parasites face some unusual challenges relative to other predators, particularly how to get from one individual host to another, and some complex parasite life cycles have evolved.

Not al l species interactions are negative: mutualism and commensalism.

Take-home message Not all species interactions are combative.

Evolution produces beneficial species interactions as well, including mutualism, in which both species benefit from the interaction, and commensalism, in which one species benefits and the other is neither harmed nor helped.

Many communities change over t ime.

Secondary Succession

Much faster than primary succession• life and soil are already present

Take-home message Succession is the change in the species

composition of a community over time, following a disturbance.

In primary succession, the process begins in an area with no life present.

Take-home message In secondary succession, the process

occurs in an area where life is already present.

But in both types, the process usually takes place in a predictable sequence.

Some species are more important than others within a community.

Take-home message Keystone species have a relatively large

influence on which other species are present in a community and which are not.

Take-home message

• Unlike other species, when a keystone species is removed from the community, the species mix changes dramatically.

• For this reason, preserving keystone species is an important strategy in maximizing the preservation of biodiversity.