Ecosystems
Reading: Freeman Chapter 54
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An ecosystem is the unit composed of all the living things in
a single place at a given time, in addition to, the important
non-living components of the system.
– Nonliving components include sunlight, rainfall, silica and
clay particles in the soil, the air, the water in the soil, etc.
– Thus, an ecosystem encompasses all aspects of a
biological community, in addition to factors such as rates
of CO2 uptake, rates of nitrogen fixation from the
atmosphere, precipitation, seasonal flooding and its effects
on nutrients, etc.
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Ecosystems vary in size. Like communities, small
ecosystems are “stacked” within larger ones, and the
boundaries are sometimes diffuse.
The biosphere the largest and most encompassing
ecosystem we know-it encompasses all the plants and
animals on Earth.
Energy and Biomass
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Much of ecosystems ecology concerns itself with the flow of
energy and biomass.
– Nutrient cycling and energy flow are common to all
biological communities.
– These phenomena are both a consequence, and a function
of biological communities.
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The complex matrix of interactions among members of
a community expends energy, as well as passing it from
one member to the next through trophic interactions.
Likewise, biomass is constantly recycled through
production, predation, herbivory, and decomposition.
Energy
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The sun is the ultimate energy source for almost every
ecosystem on earth.
 Hydothermal vent communities are a partial exception(they rely on geothermal energy, but still depend upon
oxygen fixed by photosynthetic organisms).
– Energy enters ecosystems via photosynthesis (or, in a few
exotic excosystems, chemosynthesis).
– Organisms that bring energy into an ecosystem are called
producers.
– Producers include green plants, algae, cyanobacteria,
etc..anything that can make its own energy from nonliving
components of the environment.
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Organisms continuously use energy.
– All metabolic processes consume
energy in some way, and in each
reaction, much of it is effectively
“wasted”…
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..this is one reason why rapid metabolism
makes us homeothermic-the waste heat
from metabolic processes, mostly as
molecular motion, warms our bodies.
Ultimately, all biological energy radiates
into the environment as infrared light (a
by-product of respiration).
Much energy is lost every time it passes
from one trophic level to the next.
Energy does not recycle.
– it must be continually replenished
from the sun.
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Autotrophs fix their own energy from
inorganic sources.
– Autotrophs are the producers in an ecosystem.
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Heterotrophs depend upon energy and
carbon fixed by some other organism
– they are consumers, detritivores, or
decomposers.
(A mixotroph is gets its energy from inorganic
sources, but relies of organic sources of carbon.)
A
food web is a schematic diagram that
describes the patterns of energy flow in
an ecosystem
Every instance of predation, herbivory, and
parasitism is a trophic interaction that moves
energy from one organism to another.
 Decomposition is also a trophic interaction that
uses up the energy left over in dead bodies of
organisms.
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– A food chain is one path through a food
web, from bottom to top.
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Because energy is lost at each step, food chains have a
limited number of links.
Matter
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Unlike energy, matter recycles through
ecosystems.
– Atoms of every biologically important element
constantly recycle through ecosystems, into the
abiotic component of the biosphere, and back into
living systems.
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Elements are passed from one organism to another via
trophic interactions, or are taken directly from the
environment.
Via the process of decomposition, each element
ultimately becomes nonliving, and has the potential to
re-enter the biosphere again.
Thus, each element has its own biogeochemical cyclethese can take days, years, or eons, depending upon the
element and the circumstances.
Biomass
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Biomass can be defined as the weight of living
matter (usually measured in dry weight per unit
area).
A pyramid of biomass is a figure that quantifies the
relative amounts of living biomass found at each
trophic level.
In most ecosystems, the amount of biomass found in
each trophic level decreases progressively as one
moves from the bottom to the top of the food chain.
Pyramid of biomass for a pond. (Source: Data from Whittaker,
R.H. 1961. Experiments with radiophosphorus tracer in
aquarium microcosms. Ecological Monographs 31:157-188).
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Primary consumers eat producers.
They generally possess significantly less biomass than
producers.
– Plants have evolved numerous mechanisms to protect their
tissues from consumption by herbivores and pathogens
– In most ecosystems only a small amount of producer
biomass is eaten.
– Significant losses of biomass occur because of digestive
inefficiencies, and return of CO2 to the atmosphere via
respiration.
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Assimilation efficiencies for most terrestrial herbivores range from 20
to 60 percent. Some invertebrates do better than that..some do not.
A very large proportion of the assimilated biomass is lost through the
process of respiration, so only a small amount of the biomass is
available to the next level.
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Secondary consumers consume primary consumers.
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Tertiary consumers consume secondary consumers, and so forth.
– Not all organisms at one level are eaten, because of
defensive mechanisms-and predation is only one way to
die.
– Defensive adaptations include the ability to fly and run,
body armor, quills and protective spines, and camouflage.
 In general, carnivores have higher assimilation
efficiencies than herbivores. These range from 50 to 90
percent.
 Only a small fraction of the assimilated energy
becomes carnivore biomass because of the metabolic
energy needs of body maintenance, growth,
reproduction, and locomotion.
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Most food chains have at most
four or five trophic levels.
– The amount of biomass found
at each trophic level is small
relative to amount found at the
next lowest level.
– This is because less energy is
available to successive
consumers.
http://www.bioquip.com
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Decomposers, scavengers, saprophytes, and
detritivores are organisms that eat dead organic matter.
– Detritivores eat the dead bodies of living things, such
as carrion, leaf litter, etc..
 “Scavenger”s are animals that eat dead animals.
– Decomposers are microscopic organisms that break
down organic compounds into nonliving, inorganic
precursors.
 Saprophytes are organisms that feed on dead
organic matter, this term is usually applied to fungi
or bacteria, but there are plant saprophytes as well
Primary Productivity
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Primary productivity is the amount of biomass
produced through photosynthesis per unit area and time
by producers.
– It is usually expressed in units of energy (e.g., joules
/m2 day) or in units of dry organic matter (e.g., kg /m2
year).
Globally, primary production amounts to 243 billion
metric tons of dry plant biomass per year.
The total energy fixed by plants in a community through
photosynthesis is referred to as gross primary
productivity (GPP).
Net vs. Gross Primary Productivity
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Most gross primary productivity is used via respiration by the
producers themselves.
Subtracting respiration from gross primary production gives
net primary productivity (NPP)
NPP represents the rate of production of biomass that is
available for consumption (herbivory) by heterotrophic
organisms (bacteria, fungi, and animals). It is also easier to
measure, because it tends to accumulate over time.
Problem:
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A plot of Panicum sp. grass has a
gross primary productivity of 10,700
kcal/m2year. The grass respire
approximately 9,100 kcal/m2year.
 What is the net primary productivity?
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Answer:
 10,700kcal/m2year - 9,100
kcal/m2year=1600kcal/m2year.
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Problem:
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The field is 10m x 10m. Over the
course of one year, what is the total net
primary productivity for the field?
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Answer:
 100m2 x
1600kcal/m2year=1.6x105kcal/year.
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Problem:
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If Panicum grass has an energy
value of 6kcal/gram, and all of the
primary productivity were to accumulate
as biomass, how much biomass
(expressed as dry weight) will have
accumulated in the field over the course
of 1 year?
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Answer:
 (1.6x105kcal/year x 1
year)/(6kcal/gram)=2.67x104 grams or
267kilograms.
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Problem:
 Suppose herbivores (wild mules) eat ALL this
biomass, and assimilate 10%. The respiration
of the mule is 15kcal/kilogram day.
 Would this field be sufficient to support a 150
kilogram mule?
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Answer:
 The mule would assimilate
(1.6x105kcal/year x
10%)=1.6x104kcal/year.
 Over the course of the year, the mule
would require 15kcal/kilogram day x 365
days x 150 kilograms=8.21x105kcal.
 The field is not nearly enough. This is why
large herbivores move around so much.
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Communities Differ in their Productivity
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Globally, patterns of primary productivity vary both spatially
and temporally.
– The least productive ecosystems are limited by heat
energy, nutrients and water like the deserts and the polar
tundra.
The most productive ecosystems have high temperatures,
plenty of water and lots of available soil nitrogen.
Productivity is high in areas of oceanic upwellingoceanic producers, which include diatoms,
dinoflagellates, cryptomonads, and other algaerequire nutrients
Nutrient Cycling
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Each biologically important element has nutrient
cycle.
A nutrient cycle is the path of an element from one
organism to another, and from organisms into the
nonliving part of the biosphere and back.
– Nutrient cycles are sometimes referred to as
biogeochemical cycles, reflecting the fact that chemicals
are cycled between biological organisms, and between
organisms and the geologic (physical) environment.
C, H, O, N
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Carbon, hydrogen, oxygen, and nitrogen make up
most of the biological molecules found in living
organisms. These elements are passed from
organism to organism by chemical conversion
processes, which occur in food webs.
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They are also converted from non-living forms to
living forms by photosynthesis and nitrogen fixation,
and from living forms to non-living forms through
cellular respiration.
Reservoirs
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The non-living forms of carbon, hydrogen,
oxygen, and nitrogen form huge reservoirs in
the physical environment. For instance,
nitrogen makes up 78% of the atmosphere as
N2, and hydrogen comes from water.
– In ecosystems ecology, a reservoir is a supply of a
biologically meaningful element that is not easily
obtainable by living organisms.
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Elements can have multiple reservoirs
Carbon
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Most of the material substances that make up living
organisms consist of organic compounds of carbon. In
contrast, carbon is relatively scarce in the nonliving part of
the Earth.
– Carbon exists in the non-living environment as carbon
dioxide in the atmosphere, dissolved carbon dioxide
(HCO3-, etc.) in the ocean, and as carbonates in the Earth’s
crust.
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It is also locked in fossil deposits, and embedded in the
ocean floor as deposits of methane anhydride.
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Carbon cycles between the
living and nonliving
components of the
biosphere.
– The most important reservoir
for carbon is the atmosphere:
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Although CO2 makes up less
than one percent of the
atmosphere, it is very important
to the biosphere.
– Much of the carbon in your
body was part of the
atmosphere, some of it
relatively recently.
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When you decompose, it will
return to the atmosphere.
Carbon Fixation
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Fixation, in this sense, means capture and conversion
to a biologically useful form.
– Eg., water does not need to be “fixed”, neither does
sodium, but carbon and nitrogen do.
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CO2 is fixed by plants during photosynthesis.
Photosynthesis converts atmospheric CO2 into
organic carbohydrates by combining them with
water, also from the nonliving part of the biosphere.
– This process requires the input of specific light photons,
which plants capture with the pigment chlorophyll.
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Once fixed by plants, CO2 is passed up the food
chain by trophic interactions such as herbivory and
predation.
Respiration
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Most organisms, including plants, respire.
– Respiration liberates carbon back into the atmosphere and
provides energy to the organism.
– CO2 enters the atmospheric reservoir.
– If it is not eaten and respired, or decomposed, organic
carbon may become buried and enter a carbon reservoir
in the soil, or ultimately fossilize.
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Carbon that is "fixed" can also return to
the atmosphere if the plant material is
burned, either naturally, or through
human activities.
– Even ancient plant and animal
material that contains carbon that
was fixed millions of years ago can
be returned to the atmosphere by
burning fossil fuels.
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Carbon can also be recycled back into
the atmosphere through volcanic
activity.
– As a tectonic plate goes underneath
a continent, superheated oceanic
material upgasses through
geological vents and reenters the
atmosphere.
Carbon, Global Warming,
Anthropogenic Climate Change
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CO2 has a crucial role in the climate of the Earth because
it is quite transparent to light at the visible wavelengths,
and relatively opaque to infrared light.
Gasses with this property are called greenhouse gasses,
because they tend to trap heat, forcing a higher
equilibrium temperature.
– Methane, and CFC’s are also greenhouse gasses, but CO2 is the
most important because it occurs at higher concentrations.
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Geological periods of low CO2 concentration (such as the
present) are strongly correlated with low global
temperatures, higher CO2 is strongly correlated with
higher global temperatures.
– Additionally, sudden increases in CO2 can be linked to
a sudden warming of the climate.
– Such an event occurred in the Miocene, 15 million
years ago.
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There is very solid evidence that CO2 concentrations
have increased significantly over the course of the last
150 years.
– This is partially due to the burning of fossil fuels, and
partially due to deforestation.
– By cutting and burning of forests, the carbon that
once was locked in the trees is released into the
atmosphere.
Huge stores of fossilized carbon are present within the
Earth’s crust, much of it buried and fossilized during the
Carboniferous period, 200million years ago.
– Liberation of these stores into the atmosphere has the
potential to dramatically change the climate of the
Earth.
Evidence is mounting that these higher CO2 levels have
already affected the climate of the Earth.
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Some possible effects:
– Higher temps, especially in the high latitudes
– Drier continental interiors
– More unpredictable weather patterns, with more
extreme storms, and extreme heat events
– The potential for tropical diseases to enter
higher latitudes and higher elevations
– The potential for currently farmable areas to
become too dry to farm
– The potential to interfere with oceanic
thermohaline circulation, and cause conditions
in Europe and Eastern North America to
become very cold.
– The potential to interfere with oceanic
productivity through changes in Ph
– The potential for increases in sea level.
N
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N is one of the most common elements that
form biological molecules.
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It is a major component of amino acids, also
a primary constituent of nucleic acids.
The major reservoir for nitrogen is the atmosphere
– N2 makes up 78% of the Earth's atmosphere.
– The majority of living organisms are not able to use it in
that form.
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N2 contains a triple bond between the atoms, it is a
very stable molecule and therefore, biologically inert.
A large amount of energy is required to break the
triple bond.
– lightning is responsible for converting some of the
atmospheric nitrogen into forms that organisms can
use.
– The process of converting atmospheric nitrogen
into forms that organisms can use is called
nitrogen fixation.
– Although most organisms are not able to
convert nitrogen, there are a few that are able
to "fix" atmospheric nitrogen.
– Some free-living soil bacteria as well as some
blue-green bacteria have the ability to convert
nitrogen into ammonia.
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Nitrogen is also fixed by symbiotic bacteria
that live in and among the root cells of
several types of plants, most notably, the
legume plants such as beans, peanuts, and
peas. Other plants such as alfalfa, locust,
and alders also have root nodules.
– There are a few that are able to "fix"
atmospheric nitrogen.
 These include bacteria in the genus
Rhizobium and Bradyrhyzobium, and also
some cyanobacteria, such as Anabaena
and Nostoc,
– This process, which is energetically
expensive, converts nitrogen into ammonia.
 Other bacteria convert ammonia to
nitrates through nitrification.
 Most plants use nitrogen in the form of
nitrates, though ammonia is also useful.
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Nitrogen fixing bacteria frequently live in mutualistic symbiosis with plants,
notably legumes.
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Thus, legumes can be disproportionately important to
the ecology of a plant community.
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Once nitrogen is absorbed by plants and built into
the plant molecules, the nitrogen can be passed to
consumers and to decomposer organisms through
the food chain.
 Nitrogen can be mineralized and converted to
organic compounds that enter the soil or
water upon their death, or enter as waste
through their digestive tracts.
– These decomposed nitrogen compounds ammonia, nitrite, and nitrates, then become
available for other plants to absorb and recycle.
This process is called ammonification.
– Alternatively, other bacteria, known as
"denitrifiers," convert nitrites and nitrates in the
soil to N2O and N2, which returns to the
reservoir in the atmosphere. This process, which
completes the nitrogen cycle, is called
denitrification.
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Certain bacteria convert ammonia to
nitrates through nitrification. Most plants
use nitrogen in the form of nitrates.
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Once nitrogen is absorbed by plants and
built into the plant molecules, the nitrogen
can be passed to consumers and to
decomposer organisms through the food
chain.
Water
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The water cycle is one of the most important processes to
living organisms on Earth.
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Water that has evaporated into the atmosphere condenses
and falls as precipitation.
– This precipitation will either run off as surface water and collect as
streams or rivers, or it can seep into the ground and collect in huge
underground rock formations called aquifers, that act much like
sponges.
– The water eventually flows from lakes or streams down into the oceans,
where it can reside for long periods of time, or get evaporated back
up into the atmosphere as water vapor, which collects as clouds.
– A portion of the water absorbed into the ground
is taken up by plants, which use the water to
transport minerals internally as well as to take
part in the photosynthetic process.
– Some of this water is transferred to animals that
feed on plants; from there, water can cycle within
the food web of an ecosystem.
– Water can be given off to the atmosphere by
plant leaves through transpiration, or by
animals through respiration, perspiration, or
excretion.