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BIOL
4120
Principles of Ecology
Phil Ganter
320
Harned Hall
963-5782
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The
joshua tree, a member of the lily family, is characteristic of
the Mojave Desert |
Lecture 18 Ecosystems
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Overview - Link
to Course
Objectives
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Limits
to Primary Production
(20.3, 20.4, 20.5, and 20.6)
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- Include the abiotic factors affecting
the total community living in a definable environment
- Notice that here community
refers to all of the species in the environment, not just a subset
of similar species (like the bird community or the grazing community)
- In practice, boundaries of
ecosystems are often hard to define
- Some edges are difficult
to detect (we will deal with this in the next lecture)
- Some species move between
habitats and link ecosystems
- Some ecosystem-level processes
(= emergent properties of ecosystems)
- Trophic structure (covered
in community lecture)
- Energy transfer between species
and trophic levels
- Material cycling (nutrient
recycling) between species and trophic levels
- Ecosystem
measurements preview
- Biomass
- Total standing
crop of any species or trophic level
- standing crop is not
the same as productivity, the new biomass added to a system in a given
time, and large standing crops can be associated with either high
or low productivity
- Energy flow
- Trophic level productivity
(first level is Primary Productivity)
- Does not always correlate
with biomass, as small biomass can still produce large flows if individuals
are productive
- Nutrient flow
- If level of a nutrient controls
the productivity of a trophic level, it is termed a Limiting Nutrient
- Flow rates of limiting nutrients
important to both biomass and energy flow
- Turnover rate important
- Turnover
is the ratio of input/output to total amount present
- High ratio means that
nutrients do not reside in system or at the trophic level for long
- Low ratio means that
nutrients reside in system or at the trophic level for long periods
Some
Physical Constraints on Flow (20.1)
- First Law of Thermodynamics
- Constrains the system to
a complete accounting of all material and energy that enter or leave the
system
- If the system is in equilibrium,
then net input = net output
- Compounds that enter
the system remain in the system until they or compounds made from
them leave the system
- This rule applies
to those materials we intentionally or inadvertently add to systems,
such as nutrients in our household waste or biocides
used to control target organisms
- Second Law of Thermodynamics
- Constrains the system to
a net loss (of heat energy) when energy is transferred between various
components of the system
- 10% Rule of thumb is
that only 10% of the energy that leaves one trophic level is transferred
to the next level and 90% is lost as heat
- Constrains the system
to a net increase in entropy unless sufficient energy enters the system
to counter the tendency for entropy to increase
Primary
Production (20.2)
- Primary production of Terrestrial
Ecosystems is often measured in dry
weight (no water included) of producer tissue added to the
system
- This is often calculated
as the Standing Crop of the system at the end of the growing season minus
the Standing Crop of the system a the beginning of the growing season
- Standing Crop
is just
what is seems to be, the total biomass present at some point in time
- Problem - energy content
of different tissues is not constant
- Better expressed in terms
of energy (calories or joules are most usual)
- One can convert between
biomass and energy through calorimetry
- Calculates energy
in a material by burning it and measuring the temperature change
in the apparatus (usually one called a bomb
calorimeter)
- Of course, this does
not measure total energy, but it does measure total potentially useful
energy, since respiration will oxidize the tissue in any case
- Gross
Primary Production (GPP) is the amount of energy fixed by photosynthesis
and chemosynthesis in an ecosystem
- Net
Primary Production (NPP) is GPP minus the energy lost to respiration
by the producers
- Net
Change in Biomass (B)
is NPP minus the energy lost to herbivory (G) and that lost to plant (algal)
mortality (L)
B
= NPP - G - L or NPP = B
+ G + L
- Primary productivity in Aquatic
Ecosystems often measured as O2
production in a pair of similar bottles, one transparent (Light Bottle) and
the other opaque (Dark Bottle)
- O2
levels are measured at the beginning and end of exposure of bottles to
conditions (light, temperature) of interest and O2
production or consumption is the difference between the final and initial
O2
concentrations
- Light bottle represents NPP as the O2 produced by
photosynthesis is reduced by the amount consumed by respiration
- Dark bottle represents O2 consumed by
respiration since no photosynthesis can take place
- GPP = total O2 produced in
light bottle - O2 consumed in
dark bottle (remember, consumption will be a negative amount, so we are
subtracting a negative here and GPP will be larger than NPP)
- The relationship between productivity
and standing crop
- Primary Productivity and
standing crop (total biomass) are not always positively correlated
- Reefs - most productive
but have a tiny standing crop because turnover is so high
- Total continental standing
crop versus total marine standing crop
- Standing crop of marine
producers is much smaller than terrestrial system
producers with similar productivity, although the
marine system supports lots of herbivores and carnivores
- This means that turnover
is much greater in marine systems
- Production
Efficiency is the proportion of energy fixed to that available
(sunlight for photosynthesis)
- GPP efficiency is never greater
than 4%
- NPP efficiency is usually
1% or less
Limits
to Primary Production (20.3, 20.4, 20.5, and 20.6)
- Terrestrial ecosystems
- In general, terrestrial ecosystem net primary
production increases with increasing:
- moisture availability as measured by
evapotranspiration
- length of growing season
- temperature measured as mean annual temperature
- limiting mineral nutrient availability
(most often soluble organic nitrogen [NO3,
NH4])
- Aquatic Ecosystems
- Aquatic ecosystem net primary production
is affected by
- Water Depth
- photoinhibition occurs near surface
- PAR absorption by the water and by
solutes and suspended particles means that there is a depth at
which PAR is reduced to the level which sustains just enough photosynthesis
to offset the respiration needed for basal metabolism, so NPP
is zero (this is the Compensation Depth)
- Light Penetrates
less in turbid waters
- Temperature
- Important to freshwater
systems (turnover leads to recharging the epilimnion with nutrients)
- Although deep ocean
has constant 4°C temperature, surface temperatures (and photosynthetic
zone) are much closer to air temperature and will vary with the
season in temperate oceans
- Nutrients
- Nitrogen
(NO3, NH4)
- Very important
in open ocean
- Phosphorous
- Important in
freshwater systems
- Eutrophication
of lakes occurred by the addition of phosphate-detergents
- Process of increasing the concentration of
nutrients until unusual photosynthesizers (Cyanobacteria
often) turn the water turbid and often decrease oxygen
concentration
- Iron is often limiting
- Needed to make
many of the electron-accepting members of electron-transport
chains
Question: As
CO2 builds up in the atmosphere, some predict that some dry terrestrial ecosystems
will become more productive and that there will be little gains in aquatic systems.
What is responsible for this? I'll give you a hint - it's not the CO2 concentration,
but something affected by it.
Secondary
Production (20.7, 20.8, 20.9, 20.10, 20.11)
- Biomass produced by producers
(primary production) can only move into two other components
- Detritus feeders
- Majority of plant/algal
biomass dies and is eaten by detritivores
- Herbivores
(and algae feeders)
- Important to plants and
algae, as they eat living material
- The importance of
this pathway is that the size of the herbivore community may affect
the size of the primary producer community
- detritivores only
get what dies and cannot affect the rate of dying, so they cannot
have the same regulatory effect on the primary producer community
as herbivores can
- Terrestrial and Aquatic systems - the
size of the herbivore community increases with primary productivity
- Measuring secondary production
- Gross
assimilation (= Ingestion
or the amount of plant/algal material eaten) is reduced by the amount
of energy (and the compounds needed to supply it) used to maintain the
herbivore/detritivore
- Gross Assimilation = Assimilation
+ Waste (feces)
- Gross Assimilation and Assimilation
differ in that only the portion of ingested materials that crosses
the digestive tract wall is Assimilation
- Net Secondary
Production = Assimilation - Maintenance Respiration
- In endothermic vertebrates,
maintenance is easiest to measure as the basal
metabolic rate
- Net production is
measured as biomass increase, which can be either the
- Growth of individuals
- Addition of
new individuals
- Assimilation
Efficiency = Assimilation/Gross
Assimilation
- Secondary
Production Efficiency is the ratio of Net Secondary Production/Assimilation
- this is a measure
of how much of the assimilated material or energy is used for
production (and not lost to maintenance)
- higher for ectotherms
than for endotherms as the basal metabolic rate is lower in ectotherms
and takes a smaller bite out of the assimilated materials
- Consumption Efficiency
is the ratio of Gross Assimilation of one trophic level/Net
Productivity of the next lower level
- this efficiency measures
the proportion of available productivity that gets eaten by the
next trophic level
- Lindeman
Efficiency is different from the efficiencies discussed
so far
- the ratio of assimilation
between trophic levels
- Maintenance is included
in both the denominator and numerator, and will cancel out if
it is the same proportion for each trophic level
- An alternative to
Consumption Efficiency for comparing different levels of a trophic
pyramid
- Terrestrial
ecosystems
- Often near 10%
for all but decomposers
- Decomposers get
what the herbivores and carnivores do not
- Total energy
flow not much affected by carnivores (who produce 10% of 10%,
or only 1% of primary production)
- Aquatic
Ecosystems
- Higher than for
terrestrial systems, leading to rapid turnover in biomass
- Often as high
as 40 % for aquatic herbivores
- Often as high
as 80% for aquatic carnivores
Nutrient
Cycling (21.1)
- Remember that the input of material
to a system will equal output
- Immature, early succession
systems may accumulate materials
- Agricultural systems lose
more that annual natural inputs can replace, and may lose materials (when
no fertilization takes place)
- Nutrients are cycled internally,
so that they may be retained for a long time within a single trophic level
or may move down the pyramid from consumers to producers
- If you follow the nutrients,
they will cycle within the system, a phenomenon called Nutrient
Cycling, a form of recycling
- Remember that ecosystems are
always open
- New materials arrive
- System always leaks, so materials
leave the system (no matter how slowly)
- Many plants recycle internally (called retranslocation)
by pulling essential nutrients (amino acids, lipids, minerals) out of leaves
before they fall so that they can be re-used in next year's crop of leaves
- store the nutrients in the stem (trunk) or
underground (in roots or tubers)
- smallest roots are also temporary and are
regrown each year (called fine roots) and nutrients are retranslocated
from them as well
Decomposition
(21.2)
- Decomposition
is the complete breakdown of organic molecules into inorganic molecules
- Decomposition is the role of the
Detritivore Community
- Most of the real chemical breakdown, in both
aquatic and terrestrial systems, is done by bacteria and fungi, which secrete
enzymes into the soil or water that can catalyze the breakdown
- The overall process takes a large community
of microbes, not just one species or even a few species
- Hyphal fungi are well suited here
- All fungi are heterotrophs and those with
hyphal growth can penetrate into new habitat as they exhaust the present
habitat's resources
- Decomposition is an example of Succession,
as one set of microbes finishes breaking down their substrates, another group
that can break down the waste from the previous community replaces it
- The first substrates to be consumed are proteins,
nucleic acids, easily digestible lipids, and mono/oligosaccharides (glucose,
disaccharides, etc.)
- The second substrates to be consumed are
the cellulose and hemicellulose components of plant cell walls as well
as less digestible lipids
- The last materials to be consumed are the
Lignins
- Lignins are a class of molecules with
many variations but all are pholyphenolic compounds (a series of phenol
rings bonded in various ways)
- Lignins cross-link cellulose fibers
to harden woody tissue
- Breakdown of lignins yields little
net energy and is done only by Basidiomycete fungi (no bacteria!)
- Much of the above applies to algae (major primary
producers in marine and most fresh water systems so they are the major producers
of detritus) as well, except that:
- There is no lignin
- Some microalgae have mineral cell walls (silicon)
- Cellulose is a major component of some algal
walls but other fibrous carbohydrates (mannanes, xylanes, alginic acids)
are common
- One useful group of fibrous algal carbohydrates
are the Sulfonated Polysaccharides from red algae
- These are chains of galactose with suphonate
groups attached and are a diverse set that includes
- Carrageenan - a
gelling agent in toothpaste, shoe polish, shampoo, and many foods
(sauces, pates, and many ice creams - if your shake or ice cream
does not melt into total soup - you've got carrageenan
- Agar and Agarose
- gels with the property of melting at much higher temperatures
than the gelling temperature, agar is a mix of polysaccharides
and agarose is purified to only one polysaccharide
- Rate of decomposition
(21.4)
- Increases with quality of
litter
- High nitrogen/low lignin is high quality
- Low nitrogen/high lignin is low quality
- Temperature, moisture availability (terrestrial
ecosystems) have their expected effects
- Availability of oxygen increases rate of
decomposition
- Waterlogged soils are Anoxic
and decomposition of cellulose and lignins is very slow
- Net Primary Productivity and decomposition
rates are usually positively related
- High NPP rates result from high nutrient
availability, which also means that the growth rate of the microbial
decomposers is high
- Decomposition
and Mineral Nutrients (including Nitrogen) (21.5)
- Mineralization
is the process of converting organically bound N, P, and metal ions into
inorganic (usually soluble) forms
- Mineralization occurs as waste products
are produced from microbial decomposition
- Their solubility makes them vulnerable
to Leaching from the soil
- Immobilization
is the binding of these nutrients into organic compounds after Uptake
(absorption) by an organism
- Plants immobilize nutrients when they absorb
them and use them to synthesize new biomass
- Microbial Decomposers will also immobilize
nutrients when they produce new biomass
- Because microbes involved in decomposition
will both mineralize and immobilize nutrients, the portion of nutrients
available for uptake by plants is Net Mineralization
(= Microbial mineralization - Microbial immobilization)
- Terrestrial
Ecosystems (21.7)
- Decomposition is a soil process - another
reason to study soils and another reason why soils are so complex
- Involves (for our purposes) two size-based
animal groups:
- Macrofauna
- invertebrate detritivores (insects [springtails, larvae of many
insect groups, termites, roaches and others], crustaceans [isopods
and, in some situations, amphipods], mites, earthworms, snails, slugs,
and millipedes
- Macrofauna have an important role
in the breaking down of detritus particles into smaller and smaller
particles so that the detritus can be attacked by fungi and bacteria
- Remember that the decomposition pathway
supports more than one trophic level as decomposers have predators
and parasites
- Microfauna
- bacteria, protists, small nematodes
- Aquatic
Ecosystems (21.6, 21.8, 21.9,
21.10, 21.11)
- The macrofauna of freshwater
system is similar to those found in soils except that there are no millipedes
- In marine systems, there are no insects,
the annelids are polychaetes, not oligochaetes (earthworms), and many
more crustaceans are present
- When particles size is small enough in aquatic
systems, the particles can become suspended in the water column
- These suspended particles are called
POM (Particulate
Organic Matter) and can be so small that they are
essentially suspended indefinitely (like a solution)
- In aquatic systems, the photic zone
may be separated from the benthos (where most
decomposition goes on)
- In terrestrial systems this is not so,
as plants have roots in the soil, the site of decomposition
- Thus, in aquatic systems, the mineralization
of nutrients may not get to the primary produces immediately
- See the discussion of water column
zonation (in both lakes and oceans) in Lecture 4
- Nutrients are only mixed into the
photic zone when thermocline (or halocline) breaks down and Turnover
of the body of water can occur
- Nutrient Spiraling
- because running waters constantly move nutrients downstream, a nutrient
molecule is not taken up at the same place where it was mineralized,
but downstream from there
- Nutrients in streams and rivers move
downstream in a conceptual spiral of mineralization, then downstream,
uptake, then downstream, mineralization again, then...
- In oceans, mineralization occurs at great
depths and comes to the surface (photic zone) in areas of Upwelling
- Where winds push water away from
continents, the water is replaced by water coming up from deeper
layers
- Upwelling results in increased primary
productivity, which means increased secondary productivity (fish,
in oceanic systems!)
- El
Niño (ENSO) happens when the winds
fail off of the wester coast of South America, the surface
water is not pushed west, and upwelling stalls.
- Primary and secondary productivity
crashes - a disaster for economies dependent on these fisheries
Biogeochemical
Cycles
- Nutrient cycles are the most
thoroughly modeled ecological processes, also called Biogeochemical
cycles, and are the paths of materials as they enter an ecosystem,
pass through various components of the ecosystem, and finally exit the ecosystem
- Local
models - apply to one ecosystem
- Global models
- apply to the biosphere
- Biosphere
-- all earth is a single ecosystem
- Nutrient
pools - nutrients in a species, a trophic level, or in some abiotic
portion of the system (such as the soil or the atmosphere)
- Flux
rates are the rates of flow from pool to pool
- Sources
- a pool that is outside of the system and is often considered to be inexhaustible,
although the flow from the source may be very slow
- Weathering of rock
- Rainfall (local systems)
- Groundwater flow (local systems)
- Atmosphere (local systems)
- Sinks
- where materials go when they leave the system
- Can often be sources too
- Notice that atmosphere can
be a pool in a global cycle, but a source in a local cycle
- Inputs to Terrestrial Ecosystems
- Absorption of gasses
- Weathering
- Dryfall - deposition of particles
by wind
- Wetfall - rain
- Outputs from Terrestrial Ecosystems
- Gas releases (CO2,
H2S,
N2)
- Organic detritus lost from
the system (into a lake or stream or windblown materials)
- Leaching
- Crop Harvests
Global
Carbon Cycle
Sources, Outputs, and flows
Pools
|
Flux pathways |
|
|
Input |
Output |
Producers |
photosynthesis |
respiration, herbivory, death
|
Consumers |
herbivory |
respiration, carnivory (between
trophic levels), death |
Decomposers |
death |
respiration |
Detritus |
plants, animals |
consumption by decomposers
|
Fossil fuels |
anaerobic respiration
|
human-caused combustion
|
Atmosphere/Water |
|
|
- Ocean is generally considered
to be in equilibrium with atmosphere, such that an alteration in one will
inevitably lead to an alteration in the other
- Since Carbon is the primary element
fixed in biosynthesis, carbon cycles and energy flows are positively linked
Global
Phosphorous Cycle
- There is no atmospheric pool
for P
- P at the surface is always part
of a phosphate ion
- Pi,
Po,
Pp
(Inorganic, Organic and Particulate Phosphorous)
- The designation comes not
from any change to the phosphate ion
- Pi
refers to phosphate in rocks or dissolved in water
- Po
refers to phosphate that is part of an organism or found in detritus
- Pp
is phosphate that is part of a particle that can be blown by the wind
and can become part of dryfall
- Input Sources
-
- weathering of some rocks (usually calcium
containing rocks)
- wetfall and dryfall deposition (terrestrial
and aquatic)
- River flow (into lake and marine systems)
- Cycling goes on between uptake of phosphate by
plants or algae, through the trophic pyramid, and it is released from dead
tissue by decomposition
- Output
- River flow (terrestrial systems)
- sediment deposition (aquatic systems)
- wind
- Crop harvesting
Global
Nitrogen Cycle
note that the role of microorganisms
is key
- Fixation
- some nitrogen is "fixed" (changed into a chemical form useful
in living systems) by lightning and some is fixed by bacterial action (including
the blue-green algae, the cyanobacteria)
- nodulation a means of increasing rate of fixation
in soils
- Denitrification
by other bacteria that use NH3 as an energy
source and release N2 back into the atmosphere
Terms
Biomass, Energy flow, Nutrient flow, Turnover, Primary
Production, dry weight, Standing Crop, calorimetry, bomb calorimeter, Gross
Primary Production, Net Primary Production, Net Change in Biomass, Production
Efficiency, Eutrophication, Secondary Production, Gross assimilation (= Ingestion),
Assimilation, Net Secondary Production,
basal metabolic rateAssimilation Efficiency,
Secondary Production Efficiency, Consumption Efficiency, Lindeman Efficiency,
Nutrient Cycling, Decomposition, Detritivore Community, Lignin, Anoxic, Mineralization,
Immobilization, Net Mineralization, Macrofauna, Microfauna, POM (Particulate
Organic Matter), Nutrient Spiraling, Upwelling, Biogeochemical cycle, Local
model, Global model, Biosphere, Nutrient pool, Flux rate, Source, Sink, Fixation,
Denitrification, Pi, Po,
Pp (Inorganic, Organic and Particulate Phosphorous)
Last Updated
March 15, 2007