How do Ecosystems Work?



All the organisms that live in a particular area

Abiotic environment in which they live & interact

Chemical Cycles Within Ecosystems

Within an Ecosystem

Energy can enter the ecosystem - sun or other sources - converted into chemical energy by photosynthesis or other mechanisms - energy is usually passed on to other organisms in organic compounds or dissipated as heat

Chemical elements move through ecosystems in biogeochemical cycles

On a global scale - most of these substances are in non-living reservoirs that include the atmosphere, water and rocks

How does Energy Flow Through communities?

About 1% of the suns energy that reaches earth is available to photosynthetic organisms.  These organisms capture about only 3%or less

Thus all life on the planet is supported by less than 0.03 % of the energy reaching earth from the sun

During photosynthesis pigments such as chlorophyll capture the suns energy, store it in chemical bonds and then produce sugars and other high energy molecules

Photosynthetic organism are called autotrophs or producers

Those organism that cannot photosynthesize are called heterotrophs or consumers

How much life an ecosystem can support depends upon how much energy the producers capture

Net primary productivity - amount of energy captured and available to other members of the community

Ecosystem Productivities Compared








Trophic Levels

The amount of energy available to an organism depends upon the number of transformations which have occurred before the energy reaches the organism

The position of an organism in the sequence of transformations is called its trophic level

First trophic level - photoautotrophs &chemoautotrophs

Algae, trees& nitrifying bacteria

Trophic Levels

Second trophic level -primary consumers - herbivores,

Moth larvae & deer

Third trophic level -secondary consumers - primary carnivores

Spiders, mice and beetles

Fourth trophic level -tertiary consumers - secondary carnivores and parasites of carnivores

Owls, weasels &parasitic flies

Decomposers & Detritivores - use all trophic levels

Bacteria, fungi, earthworms and crabs

Simplified Grassland Food Web







Food Webs

Food chain - straight line sequence of how eats whom

Food web - when several food chains intertwine - more accurately describes the actual feeding relationships in a given community

Omnivores - organisms that, at different times act as primary, secondary and even tertiary consumers

Raccoons, bears, rats and humans

Energy Transfer Through Trophic Levels

Energy use is never completely efficient (in automobile engines lose 75% of the energy of the gasoline in heat)

Energy transfer between trophic levels is also inefficient

An insect eating the leaves of a plant only gets some off the energy originally captured by the plant from the sun

Energy Pyramids & Energy Transfer Between Trophic Levels

The net transfer of energy between trophic levels is roughly 10% efficient

Thus the energy stored in primary consumer is only about 10% of the energy stored in the bodies of the producers

In turn, the bodies of secondary consumes possess about 10% of the energy stored in the primary consumers

Thus an energy pyramid exists

Biomass (dry weight of organic material in a ecosystem) - can be a measure of the energy stored at each trophic level

Thus a biomass pyramid often has the same shape as an energy pyramid for a given community

Food Chains








How Do Nutrients Move Within and among Ecosystems

Nutrients are elements and small molecules that form the chemical building blocks of life

These include, water, carbon, hydrogen, oxygen, nitrogen, phosphorous, sulfur and calcium

Micronutrients are required in trace quantities and include zinc, molybdenum, iron, selenium, and iodine

Nutrient cycles or biogeochemical cycles describe the pathways these substances follow as they move from communities to nonliving portions of an ecosystem and back again to communities

Biogeochemical Cycles

Occur at all levels from molecular (within an organism) to planetary (within the atmosphere or lithosphere (outer part of the earth))

Occurs also at all time scales (seconds (biochemical reactions) to millennia (weathering of rocks and sedimentation)

Furthermore most occur simultaneously

The Hydrologic (Water) Cycle

H2O - Most organism consist mainly of water, source hydrogen ions whose movements generate ATP

Path of Free Water

Oceans cover three fourths of the earth

H2Oevaporates into the atmosphere - via the sun

Over land - 90% of the water that reaches the atmosphere comes from evaporation from the surface of plants (transpiration)

Most precipitation lands in the oceans, some on land where it passes into surface and subsurface bodies of H2O

Only about 2% of all H2Ois captured in any form - frozen, held in the soil or incorporated in to organisms - all the rest is free H2O - circulating between the atmosphere, the land and the oceans

The Hydrologic Cycle










Ground Water

H2O that occurs in aquifers - permeable, saturated, underground layers of rock, sand and gravel

It is 96% of all freshwater in the United States

Upper unconfined portion- water table - flows into streams & partly accessible to plants

Lower, confined layers -out of reach - can be mined by humans

Water table recharged byH2O that percolates through the soil from precipitation &seepage from ponds, lakes and streams

Deep aquifers -recharged very slowly from the water table

The Ogallala Aquifer of the Great Plains - depleted faster that it is being naturally replaced


Pollution is a growing problem - 2% of US ground water already polluted

Pesticides, herbicides& fertilizers are a growing threat

In the US 200,000surface pits, ponds and lagoons that are actively used for chemical waste disposal - adding to the problem

It is virtually impossible to remove pollutants from aquifers

Breaking the Water cycle

In the tropical rainforests - 90% of the moisture is taken up plants and transpired back into the air

Because of the high density of plants actively transpiring - most of the rainfall comes from the plants themselves

If the forests are cut down the water cycle is broken - moisture is not returned to the atmosphere - H2Odrains to the oceans instead of rising to the clouds

This has happened in Madagascar - from lush tropical rain forest of this island is now a semiarid desert

Carbon Cycle

CO2 makes up about 0.03% of the atmosphere - 700 billion metric tons

Synthesis of organic compounds from CO2 & H2O through photosynthesis utilizes only about 10% of the CO2 in the atmosphere

1 trillion metric tons of CO2 are dissolved in the oceans - about half is in the upper layers of the ocean where photosynthesis occurs

Fossils fuels add about5 trillion metric tons of carbon

Between 600 million and one trillion metric tons of carbon are locked up in living organism at any onetime

Carbon Cycle







The Nitrogen cycle

Nitrogen is essential inorganic compounds such as proteins and amino acids

A few prokaryotes can convert or “fix” atmospheric nitrogen into forms that can be used by other living organisms

N2 + 3H2   2NH3 - requires energy in the form of ATP

Nitrogen in decomposing organisms, excretions and feces can be used by soil decomposers like bacteria and fungi and excess nitrogen is released into the soil as ammonium ions(nitrification) that can be converted by other organisms into nitrites and nitrates that can be used by plants

Nitrates are also broken down to nitrogen gas and nitrous oxide - denitrification (loss from the soil)

The Nitrogen Cycle







The Phosphorus Cycle

Phosphorus -representative of other mineral cycles

Phosphorus is a component of ATP, phospholipids and nucleic acids

Typically scarce -phosphates, in the form of phosphorous anions exists in very small amounts in the soil

These can weather out of the soil & eventually end in sediments in the ocean - these may brought to the surface by upwelling ocean currents

The Phosphorous Cycle

Seabirds can deposit guano (feces) rich in phosphorous along certain coasts

Millions of tons of phosphate are added every year to agricultural lands - unfortunately most of the phosphate is not being used by the crops - plants apparently only use so much of the phosphorous

The Phosphorus Cycle








Recycling in Forest Ecosystem

The Hubbard Brook Experimental Forest in New Hampshire has been used to study overall recycling pattern of nutrients in an ecosystem

Hubbard Brook is the central stream in a large watershed that drains a certain region of deciduous forest

Measurements were made of all the water that flows out of the valley to determine nutrient content

Undisturbed forests are very efficient at retaining nutrients

Small net loss of calcium (0.3%) and net gains of nitrogen & potassium

In a disturbed watershed (trees cut & herbicides applied)

40% increase in water runoff

Loss of Calcium was 10times higher & also large increase in other nutrients in the run off.

The loss of 120kilograms per hectare (nitrogen was gained at a rate of 2 kilograms/hectare in undisturbed forest)

Hubbard Brook Experiment