Transcript Ecological Efficiency - DAVIS-DAIS

Ecological Efficiency
• The percentage of
energy transferred from
one trophic level to the
next varies between 5%
and 20% and is called
the ecological
efficiency.
Plant material
consumed by
caterpillar
200 J
An average figure of
10% is often used.
This ten percent law
states that the total
energy content of a
trophic level in an
ecosystem is only
about one-tenth that
of the preceding level.
100 J
Feces
33 J
Growth
67 J
Cellular
respiration
Energy Flow in Ecosystems
‣ Energy flow into and out of each trophic level in a food chain can be
represented on a diagram using arrows of different sizes to represent the
different amounts of energy lost from particular levels.
• The energy available to each trophic level will always equal the amount
entering that trophic level, minus total losses to that level.
•
Energy Flow Diagrams
The diagram illustrates energy flow through a hypothetical ecosystem.
Ecological Pyramids 1
• Trophic levels can be compared by determining the number, biomass, or
energy content of individuals at each level.
• This information can be presented as an ecological pyramid.
• The base of each pyramid represents the producers and the subsequent
trophic levels are added on top in their ‘feeding sequence’.
Ecological Pyramids 2
‣ Various types of pyramid are used
to describe different aspects of an
ecosystem’s trophic structure:
Pyramids of numbers: In which
the size of each tier is
proportional to the number of
individuals present at each
trophic level.
Pyramids of biomass: Each tier
represents the total dry weight of
organisms at each trophic level.
Pyramid of numbers
Pyramid of biomass
Pyramids of energy (production):
The size of each tier is
proportional to the production
(e.g. in kJ) of each trophic level.
Pyramid of energy
•
Pyramids of Numbers
In a typical pyramid of numbers, the number of individuals supported
by the ecosystem at successive trophic levels declines progressively.
•
This reflects the fact that the smaller biomass of top level consumers
tends to be concentrated in a relatively small number of large animals.
•
There are some exceptions. In some forests a few producers (of a very
large size) may support a larger number of consumers, and the
pyramid is inverted. This also occurs in plant/parasite food webs.
Forest
Grassland
•
Pyramids of Biomass
In pyramids of biomass, dry weight is usually
used as the measure of mass because the water content of organisms
varies.
•
Organism size is taken into account so meaningful comparisons of different
trophic levels are possible.
•
Biomass pyramids may be inverted in some systems (e.g. in some plankton
communities) because the algal (producer) biomass at any one time is low,
but the algae are reproducing rapidly and have a high productivity.
A Florida bog community
The English Channel
Pyramids of Energy
•
Pyramids of energy (or production) are
often very similar in appearance to
pyramids of biomass.
•
The energy content at each trophic level
is generally comparable to the biomass
because similar amounts of dry biomass
tend to have about the same energy
content.
•
This example illustrates the similarity
between pyramids of biomass (gm-2)
and energy (kJ) in a freshwater lake
community. During warm months, when
algal turnover time is short, pyramids of
energy and biomass may be inverted.
Zooplankton (C1)
‣
Processes
in
Carbon
Cycling
Carbon cycles between the
Burning fossil fuels
living (biotic) and non-living
(abiotic) environments.
Gaseous carbon is fixed in the
process of photosynthesis and
returned to the atmosphere in
respiration.
Carbon may remain locked up
in biotic or abiotic systems for
long periods of time, e.g. in
the wood of trees or in fossil
fuels such as coal or oil.
Humans have disturbed the
balance of the carbon cycle
through activities such as
combustion and deforestation.
Petroleum
The Carbon Cycle
‣
Nitrogen
in
the
Environment
Nitrogen cycles between the biotic
and abiotic environments. Bacteria
play an important role in this
transfer.
Nitrogen-fixing bacteria are able to
fix atmospheric nitrogen.
Nitrifying bacteria convert ammonia
to nitrite, and nitrite to nitrate.
Denitrifying bacteria return fixed
nitrogen to the atmosphere.
• Atmospheric fixation also occurs as a
result of lightning discharges.
‣ Humans intervene in the nitrogen
cycle by producing and applying
nitrogen fertilizers.
Nitrogen Transformations
‣ The ability of some bacterial species to fix
atmospheric nitrogen or convert it between
states is important to agriculture.
Nitrogen-fixing species include Rhizobium,
which lives in a root symbiosis with
leguminous plants. Legumes, such as clover,
beans, and peas, are commonly planted as
part of crop rotation to restore soil nitrogen.
Nitrifying bacteria include Nitrosomonas and
Nitrobacter. These bacteria convert ammonia
to forms of nitrogen available to plants.
NH3
NO2Nitrosomonas
NO3Nitrobacter
Root nodules
in Acacia
Nodule
close-up
Nitrogen Cycle
Phosphorus Cycling
• Phosphorus cycling is very slow
and tends to be local; in aquatic
and terrestrial ecosystems, it cycles
through food webs.
Deposition as guano…
Phosphorous is lost from
ecosystems through run-off,
precipitation, and sedimentation.
A very small amount of
phosphorus returns to the land as
guano (manure, typically of fisheating birds). Weathering and
phosphatizing bacteria return
phosphorus to the soil.
Loss via sedimentation…
Human activity can result in excess
phosphorus entering water ways
and is a major contributor to
eutrophication.
Fertilizer production
The Phosphorus Cycle
Guano
deposits
Sulfur Cycling
• Sulfur is an essential component of proteins
and is important in determining the acidity of
precipitation, surface water, and soil.
• Sulfur circulates through the biosphere as:
Sulfur in petrol
hydrogen sulfide (H2S)
sulfur dioxide (SO2)
sulfate (SO42-)
elemental sulfur (S)
Molecular bridges in proteins
• Human activity releases large quantities of
sulfur through:
combustion of sulfur-containing coal and oil,
refining petroleum,
smelting, and other industrial processes
Elemental sulfur
The Sulfur Cycle
SO2 from
combustible
fossil fuels
Sulfates in the atmosphere (SO42-)
Acid
precipitation
SO2 and sulfates
from volcanoes,
hot springs and
biogenic activity
Sulfur in living organisms
Decomposition and
other processing
Mining
Uplifting in
groundwater
and and
weathering
Sulfates in
soil(SO42-)
Reduced sulfur
(H2S)
Microorganisms
Inorganic
sulfur
Sulfur in fossil
fuels
Iron sulfides in deep
soil and sediments
Sulfates in
water (SO42-)
Uptake by
plants
Sedimentation of
sulfides and sulfates
Organic deposition
Water Transformations
• The hydrological (water) cycle,
collects, purifies, and distributes
the Earth’s water.
Over the oceans, evaporation
exceeds precipitation. This
results in a net movement of
water vapor over the land.
On land, precipitation exceeds
evaporation. Some precipitation
becomes locked up in snow and
ice for varying lengths of time.
Precipitation
Most water forms surface and
groundwater systems that flow
back to the sea.
Rivers and streams
Transport overland: net movement of water vapor by wind
The Water Cycle
Condensationconversion of
gaseous water vapor into liquid
water
Precipitation
(rain, sleet, hail, snow, fog)
Rain clouds
Evaporation
from inland
lakes and rivers
Precipitatio
n to land
Transpiration
Evaporatio
n from the
land
Precipitation
Precipitation
over the
ocean
Surface
runoff
(rapid)
Transpiration
from plants
Evaporation
Evaporation
from the
ocean
Rivers
Water locked up
in snow and ice
Ocean storage
97% of total water
Lakes
Infiltration:
movement of water
into soil
Percolation: downward
flow of water
Aquifers:
groundwater storage
areas
Groundwater movement (slow)
The Demand
for Water
• Humans intervene in the water cycle by
utilizing the resource for their own needs.
Hydroelectric power generation…
• Water is used for consumption, municipal
use, in agriculture, in power generation,
and for industrial manufacturing.
• Industry is the greatest withdrawer of
water but some of this is returned.
Agriculture is the greatest water consumer.
Irrigation…
• Using water often results in its
contamination. The supply of potable
(drinkable) water is one of the most
pressing of the world’s problems.
Washing, drinking, bathing…
Ecosystem Stability
‣ An ecosystem’s stability refers
to its apparently unchanging
nature over time.
• Components of ecosystem
stability include inertia (the
ability to resist disturbance)
and resilience (the ability to
recover from external
disturbance).
The diversity of ecosystems at low latitudes
(nearer the equator) is generally higher than
at higher latitudes (nearer the poles). This
photograph shows a forest in Hawaii.
Environmental Change
Supercell
thunderstorms
Medium-term
Long-term
Very longterm
Mountain
building
Communities
104
103
102
Drainage basins,
soil landscapes
Area (km2)
Medium-scale
Short-term
10
1
104
Hill slopes, flood plains,
glacial moraines and
alluvial fans
Local communities and
populations
103
102
10
1
Individual organisms
0.1
Billion
years
Million
years
Millennium
Century
Decade
Year
Month
Day
Time (years log10 scale)
Area (m2)
Small-scale
105
Environmental Change
Short-term
Medium-term
Long-term
Very longterm
Size of Earth
Phylum
Global circulation
Very large-scale
108
Class
Cyclones and
anticyclones
Order
Zonobiomes
Family
Genus
107
Large tectonic plate
movements
Species
Large-scale
106
Hurricanes/
cyclones
105
Squall lines
Communities
Major
tectonic
movements
Billion
years
Million
years
Millennium
Century
Decade
Year
Month
Day
Time (years log10 scale)
104
Area (km2)
Biomes
Fronts
Tectonic
plates form
Stability and Species Loss
‣ Ecological theory suggests that
all species in an ecosystem
contribute to ecosystem
function.
• Species loss past a certain point
is likely to be detrimental to the
functioning of the ecosystem
and on its ability to resist change
(its stability).
• Ecosystem stability is closely
linked with biodiversity but it is
not clear what level of
biodiversity is required to guard
against ecosystem dysfunction.
Species play different roles in ecosystems
Key Species
• Species whose influences on ecological
communities are greater than would be
expected on the basis
of their abundance are called key
(keystone) species.
They are more influential in ecosystem
stability than other species because of
their pivotal role in some ecosystem
function such as nutrient cycling.
Elephants knock down and consume
taller trees and shrubs...
• Elephants are a key species, and can
alter the entire structure of the
vegetation in those areas into which
they migrate.
Their pattern of grazing on taller plant
species promotes a predominance of
lower growing grasses with small
leaves.
… allowing lower growing species to
predominate
Invertebrates as Key Species
• Termites are key species. They are
among the few larger soil organisms
able to break down plant cellulose.
They shift large quantities of soil and
plant matter and have a profound
effect
on the rates of nutrient processing in
tropical environments.
• Pisaster ochraceous, the ochre star, is
also a key species. It feeds on mussels
along the coasts of North America.
Termite hill, Australia
If it is removed, mussels dominate,
crowding out most algal species and
leading to a decrease in the number
of herbivore species.
The ochre star, California
Environmental Change
• In models of ecosystem function, higher species diversity
increases the stability of ecosystem functions such as
productivity and nutrient cycling.
A low diversity system varies more consistently with
environmental variation.
A high diversity system is buffered against major fluctuations.
Low Diversity Systems
• Monocultures (single species
crops), are low species diversity
systems. They are vulnerable to
disease, pests, and disturbance.
• In contrast, natural ecosystems
may appear homogeneous, e.g.
grasslands, but contain many
species which vary in their
predominance seasonally.
Monoculture
Although natural grasslands may
be
easily disturbed, e.g. by burning,
they are very resilient and usually
recover quickly from disturbance.
Savanna
High Diversity Systems
• The biodiversity of ecosystems at
low latitudes is generally higher
than that at high latitudes,
where climates are harsher,
niches are broader, and systems
may be dependent on a small
number of keystone species.
• Tropical rainforests are amongst
the highest diversity ecosystems
on Earth. They are generally
quite resistant to disturbance,
but once once degraded they
have little ability to recover.
Deforestation of tropical rainforest
Diversity Indices
• One of the best ways to determine the health of an ecosystem is
to measure the variety of organisms living in it.
• Diversity indices attempt to quantify the degree of diversity and
identify indicators for environmental stress or degradation.
Natural, unaltered headwater streams are
generally high in diversity
Artificially managed and channelled rivers are
generally low in diversity
Calculating Diversity Indices
• Most indices of diversity are easy to
use and widely used in ecology.
• Diversity indices include Simpson’s
Index for finite populations and the
complement of Simpson’s Index for
an infinite population (below).
The index ranges from 0 to almost 1
High diversity stream community
DI = 1 –  pi2
Photos; Stephen Moore
pi2 = Ni/N: the proportion of species i in
the population
Ni = the number of individuals of each
species in the sample
N = the total number of individuals of all
species in the sample
Low diversity stream community
Ecological Succession
‣ Ecological succession is the process by which
communities in a particular area change over time.
• Succession takes place as a result of complex
interactions of biotic and abiotic factors.
Community composition changes with time
Past
community
Present
community
Future
community
Some species in the
past community were
out-competed or did not
tolerate altered abiotic
conditions.
The present community
modifies such abiotic factors as:
Changing conditions in the
present community will
allow new species to
become established.
These will make up the
future community.
• Light intensity and quality
• Wind speed and direction
• Air temperature and humidity
• Soil composition and water content
Early Successional
Communities
Pioneer community, Hawaii
• A succession (or sere) proceeds in
seral stages, until the formation of
a climax community, which is
stable until further disturbance.
• Early successional (or pioneer)
communities are characterized by:
Simple structure, with a small
number of species interactions
Broad niches
Low species diversity
Broad niches
Climax Communities
• In contrast to early successional
communities, climax
communities typically show:
Complex structure, with a
large number of species
interactions
Climax community, Hawaii
Narrow niches
High species diversity
Large number of species interactions
Primary Succession
‣ Primary succession refers to
colonization of a region where
there is no pre-existing community.
Examples include:
Newly emerged coral atolls,
volcanic islands
Newly formed glacial moraines
Islands where the previous
community has been extinguished
by a volcanic eruption
Hawaii: Local plants are able to rapidly
recolonize barren areas
Primary Succession
‣
A classical sequence of colonization
begins with lichens, mosses, and
liverworts, progresses to ferns, grasses,
shrubs, and culminates in a climax
community of mature forest.
In reality, this scenario is rare.
Mosses and
liverworts
Bare rock
and lichens
Grasses and
herbaceous
plants
Shrubs and
fast growing
trees
Mature, slow
growing trees
Secondary
Succession
• Secondary succession occurs
where an existing community
has been cleared by a
disturbance that does not
involve complete soil loss.
Cyclone
• Such disturbance events
include cyclone damage,
forest fires and hillside slips.
Forest fire
Secondary Succession
• Because there is still soil present, the
ecosystem recovery tends to be more rapid
than primary succession, although the
time scale depends on the species
involved and on climatic and
edaphic (soil) factors.
Mature forest
Bare land
Grasses and
herbaceous plants
Pioneer community
(annual grasses)
Shrubs and
small trees
Young fast
growing trees
Deflected Successions
• Humans may deflect the natural course of succession, e.g.
through controlled burning, mowing, or grazing livestock.
The resulting climax community will differ from the natural
(pre-existing) community.
• A relatively stable plant community arising from a deflected
(or arrested) succession is called a plagioclimax.
Grassland and heathland in lowland Britain are plagioclimaxes.
Gap Regeneration
‣ The reduced sunlight beneath
large canopy trees impedes the
growth of the saplings below.
When a large tree falls, a crucial
hole opens in the canopy,
allowing sunlight to reach the
saplings below.
‣ The forest regeneration following
the loss of a predominant canopy
tree is called gap regeneration.
‣ Gap regeneration is an example
of secondary succession.
Gap Regeneration Cycle
‣ Gap regeneration is an important
process in established forests in
temperate and tropical regions.
‣ Gaps are the sites of greatest
understorey regeneration and
species recruitment.
‣ The creation of a gap allows more
light to penetrate the canopy and
alters other factors that affect
regeneration, exposing mineral
soils and altering nutrient and
moisture regimes.
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