Chapter 54

Ecosystems

Ecosystems

Ecosystems are all of the organisms living in a community along with the abiotic factors with which they interact.

Ecosystems can be thought of as transformers of energy and processors of matter.

Ecosystems

Grouping species into trophic levels allows us to follow the transformation of energy and the movements of chemical elements through the community.

Autotrophs Autotrophs ultimately support all organisms and they

are called primary producers. Plants, algae and photosynthetic prokaryotes are

the biosphere’s main primary producers. They synthesize sugars and other organic

compounds from sunlight.

Primary Production

Primary production the amount of light energy converted to chemical energy during a given time.

Huge amounts of solar energy hit the earth each day.

Only a small proportion of this hits a primary producer.

Primary Production

Of the light that hits a primary producer, only about 1% of it is converted into chemical energy.

170 billion tons of organic material is created each year.

Primary Production

Gross Primary Production (GPP) is the total primary production in an ecosystem.

All of the light energy that is converted to chemical energy by photosynthesis in a given time period.

Primary Production

Net Primary Production (NPP) is the gross primary production minus the energy used by the primary producers for respiration.

NPP = GPP - RNPP is important because it represents

the storage of chemical energy available to consumers.

NPP

It is always the total new biomass added in a given time.

Always calculated as dry weight.Standing crop is the total biomass of

an ecosystem.Rainforests, estuaries, and coral reefs

have a high net primary production.

Primary Production

Factors which control primary production:

1. Light limitations2. Nutrient limitations

Primary Production

1. The depth of light penetration controls production.– More than half of the total solar energy is

absorbed in the first meter of water.– Light isn’t the only factor which controls

primary production.– The lack of a production gradient between

the equator and the poles demonstrates this.

– Something else must be occurring.

Primary Production

2. Nutrient limitations must be overcome in order for a lake or ocean to increase its production.

Nitrogen and phosphorous are the 2 elements that most limit marine production.

Primary Production

Nutrient limitations are also common in freshwater lakes.

This was first noticed and studied by David Schindler.

David Schindler

He noticed that runoff from farms and yards added a lot of nutrients to lakes.

He also documented the idea of eutrophication--the increased numbers of cyanobacteria.

EutrophicationAs a result of eutrophication, many fish

die due to decreased levels of O2.

David Schindler

He conducted a lot of research on eutrophication.

Discovered that phosphorous was the main limiter of cyanobacteria growth.

He is the reason why phosphorous was removed from detergents and other water reform measures were adopted.

Heterotrophs

The trophic level above the primary producer. Feeds on autotrophs.

Herbivores are primary consumers.Carnivores eat herbivores and are

secondary consumers.Carnivores that eat other carnivores

are tertiary consumers.

Detritivores

An important group of heterotrophs.Feed on and break down dead organic

matter.The prokaryotes, fungi, and animals

that feed as detritivores are a major link between primary producers and consumers in an ecosystem.

Detritivores

Make chemical elements available to other organisms.

Global ProductivityWhen looking at productivity on a

global scale, many areas are very unproductive:– Tundra, deserts, polar regions.– Equatorial regions are very productive.– Midlatitude temperate regions are

moderately productive.

Global Productivity

To assess productivity levels, we look at evapotranspiration rates.

The more rainfall in a given area, the higher the amounts of evapotranspiration.

There is a positive relationship between evapotranspiration and NPP.

Secondary Production

The amount of energy in a consumer’s food source that gets converted into biomass.

Most primary production is not consumed because consumers can only eat so much.

Trophic Efficiency

This is the percentage of production transferred from one trophic level to the next.

To see how much production is actually lost, we can look at trophic efficiency.

Trophic Efficiency

Trophic efficiencies are usually between 5-20%.

So, they are 80-95% unproductive.Because of this, most food webs only

have 4 or 5 trophic levels.This contributes to pyramids of

numbers.

Trophic EfficiencyPyramids of numbers represent the

number of individuals present at each trophic level.

Trophic Efficiency

Pyramids of numbers represent the number of individuals present at each trophic level.

Recycling of Nutrients

Life on earth depends on the recycling of nutrients.

The assimilation of nutrients and their removal from the body through waste is a way in which many nutrients are recycled.

When organisms die, detritivores recycle many of their nutrients.

Biogeochemical Cycles

The nutrient recycling which involves both biotic and abiotic components.

There are 2 general categories:1. Global2. Local

Biogeochemical Cycle

Gaseous forms of C, N, O, S are found in the atmosphere--globally.– These elements are more mobile.

K, P, Ca are found in different concentrations on a more local scale in the soil.– They are less mobile.

Model for Nutrient Cycling

We have to look at:The main reservoirs for elements.Processes that transfer the elements

between reservoirs.

Model for Nutrient CyclingEach reservoir has two characteristics:Does it contain organic or inorganic

materials?Are the materials directly available for

use by organisms?

Water Cycle

Rates of 1-2% production depend on the water cycle as well as decomposition.

The liquid phase is the important phase available to most organisms.

97% of H2O is in the oceans.2% is in glaciers and polar ice caps.1% is in lakes and streams.

Water Cycle

Evaporation by solar energy and wind are the main processes driving the water cycle.

Condensation into clouds and precipitation make up the rest of the water cycle.

Carbon Cycle

Carbon is important because it forms the framework for organic molecules essential to all life.

Photosynthesis makes use of CO2 by converting it into forms used by consumers.

Carbon Cycle

Carbon is found in great proportions in fossil fuels, sediments of aquatic ecosystems, the oceans, plants and animal biomass, and the atmosphere.

Carbon Cycle

The largest carbon reservoir is found in sedimentary rocks like limestone.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Carbon Cycle

Key processes concerning the carbon cycle:

Fixing of CO2 by plantsThe return of CO2 to the atmosphere

through cell respiration.

Carbon Cycle

Volcanoes contribute a lot of CO2 to the atmosphere over geologic time.

The burning of fossil fuels is also adding a lot of CO2 to the atmosphere.

Nitrogen Cycle

Nitrogen is important to the building of amino acids, proteins, and nucleic acids.

It is the crucial and limiting nutrient to plants. – Plants and algae use NH4

+ and NO3-

– Bacteria use NH4+, NO3

-, and NO2-

– Animals can only use organic forms of nitrogen--aa’s and protein.

Nitrogen Cycle

The atmosphere is the main reservoir of nitrogen ~80%.

It is also found dissolved in the various water sources on the planet and locked in living organisms.

Phosphorous Cycle

Phosphorous is a major constituent of nucleic acids, phospholipids, and ATP.

It is important to living organisms for bone and teeth too.

PO43- is the only biologically important

form of phosphorous.Plants use it to synthesize organic

compounds.

Phosphorous Cycle

Sedimentary rock of marine origin is the largest source of PO4

3-. It is also found in the soil.

The weathering of rock ads phosphorous to the soil in the form of phosphate.

Phosphorous Cycle

Phosphate taken up by producers can get distributed throughout the ecosystem when organisms excrete it or they die and decompose.

Only small amounts of phosphorous are found in the atmosphere because there are no significant phosphate gases.

Nutrient Cycles

These are generally faster in the tropics where temperatures are warmer and more moisture falls.– ~10% of nutrients are available in the

detritus of tropical forests.It is slower in temperate regions.

– ~50% of nutrients are available in temperate forests.

Nutrient Cycles

In aquatic ecosystems, nutrient cycling is generally much slower and only occurs during the turnover.

Generally due to the lack of oxygen in the water.

Nutrient Cycles

Human activities have greatly disrupted the nutrient flows within ecosystems.

One example:Normally, organisms grow and die in

the “same” spot and their nutrients are cycled there.

Now, many crops are grown and shipped all across the globe.

Nutrient Cycles

To combat this problem, we apply fertilizers.

One problem with this, however, is adding too much.

Nitrogen has become a large problem.It contaminates ground water, water

ways, and freshwater and marine ecosystems.

Nutrient Cycles

High concentrations of nitrogen containing compounds--NH4

+, NO3- and

other biologically usable nitrogen containing compounds accumulate.

The problem is that aquatic ecosystems get overgrown and use up all of the dissolved O2.

This chokes out many organisms.

Nutrient Cycles

Recall what we discussed about lakes:– Oligotrophic lakes--have a low primary

production.– Eutrophic lakes--have a high primary

production.Humans have contributed to cultural

eutrophication due to runoff.

Nutrient Cycles

The burning of fossil fuels has increased the amount of acid producing compounds in the atmosphere (SO4

2-, NO3-, etc.).

Acid Precipitation

This becomes the problem.– Normal pH of rain is 5.6 due to dissolved

CO2 in the water.– Any precipitation lower than 5.6 is

considered to be acidicSoil pH decreases as a result, and

calcium and other nutrients get leached from the soil in an attempt to buffer the soil pH.

This limits plant growth.

Acid Precipitation

Many plants and freshwater ecosystems have been altered as well.

The acidity damages the plants, and lowers the pH of the water of lakes, streams and rivers.

Human Activities

The activities of humans is producing a wide variety of changes to the global environment.

As a result, there are a variety of consequences.

Biological Magnification

Biological magnification is a process by which toxic chemicals accumulate in higher concentrations the higher up we go in the trophic ladder.

The top level feeds on the level below it, so you can see the problem.

Biological Magnification

PCB’s and the Great Lakes.Herring Gulls are the top level

carnivores and they feed mainly on fish.

The concentration of PCB’s in these gulls is about 5000x higher than at the base of the food chain.

Biological Magnification

DDT is also a problem.It was used as a pesticide in the 1940’s

and 50’s.Many bird populations began to decline

shortly after its use began.This is because DDT interferes with

calcium deposition in egg shells.Egg shells were weaker and the weight

of the parents crushed the eggs.

Biological Magnification

Mercury is another big problem.Its main source in the atmosphere is

from coal fired power plants and plastic production.

It finds its way into lakes, rivers and streams.

Bottom level decomposers break it down into a compound called methyl mercury which is very toxic.

Biological Magnification

Accumulation within the food chain contributes to a variety of problems.

Mainly neurological problems.

Human Activities

Human activities have contributed to a wide variety of gases being released into the atmosphere.

CO2 is a main gas.Comes from the burning of fossil fuels

and deforestation.

Human Activities

Elevated CO2 levels contribute to global warming.

Higher CO2 levels trap radiant energy from the sun which would normally escape back into space.

Global Warming

Global warming does do some good. The temperature of earth would be about 60°F cooler if it were not for greenhouse gases.

Global Warming

The environmental implications of increased CO2 levels are dramatic.

A 1.3°C increase in average surface temperature will mean that the earth will be warmer than at any time during the past 100,000 years.

Global Warming

Worst case scenario is that the polar ice caps will melt and coastal regions will become inundated with water.

Global Warming

Some of the early signs of global warming is that the ice shelf in the Antarctica is melting and collapsing.

Ozone Layer

The ozone layer is like the earth’s sunscreen. It acts to protect us from harmful UV rays. An accumulation of CFC’s in the atmosphere

is the main contributor to the destruction of the ozone.

Ozone Layer

Fluctuations in the size of the hole in the ozone layer is somewhat seasonal.

Examination of the fluctuations in the hole in the ozone layer shows that the hole is gradually getting larger.

Ozone LayerThis allows more UV in which causes a

variety of problems. – Increased rate of skin cancer– Increased rate of cataracts– Increased problems with phytoplankton

and crop production.The Montreal Protocol has been signed

by a number of nations to stop the production of chemicals that deplete the ozone layer.

Ozone Layer

There is enough chlorine in the atmosphere to affect the ozone layer for another 100 years.