Transcript Ecosystem ecology - energy flux

BIO-201
ECOLOGY
3. Ecosystem Ecology Energy Flux
H.J.B. Birks
Ecosystem Ecology - Energy Flux
Introduction
Some definitions
Global primary production
Patterns of terrestrial primary production
Patterns of aquatic primary production
Role of consumers on rates of primary production
Productivity – standing crop biomass relationships
Trophic levels
Interactions across ecosystems
Conclusions and summary
Pensum
The lecture, of course,
and
the PowerPoint handouts of this lecture
on the BIO-201 Student Portal
Also ‘Topics to Think About’ on the
Student Portal filed under projects
Topics to Think About
On the Bio-201 Student Portal filed under
Projects, there are several topics to think about
for each lecture. These topics are designed to
help you check that you have understood the
lecture and to identify important topics for
discussion in the Bio-201 colloquia.
In addition, there are two or three more
demanding questions at the sort of level you can
expect in the examination question based on my
10 lectures. These can also be discussed in the
colloquia.
Background Information
There is now a wealth of good or very good ecology
textbooks but perhaps no excellent, complete, or
perfect textbook of ecology.
Not surprising, given just how diverse a subject
ecology is in space and time and all their scales.
This lecture draws on primary research sources, my
own knowledge, experience, observations, and
studies, and several textbooks.
Textbooks that provide useful background
material for this lecture
Begon, M. et al. (2006) Ecology. Blackwell (Chapter 17)
Bush, M. (2003) Ecology of a Changing Planet. Prentice Hall
(Chapter 5)
Krebs, C.J. (2001) Ecology. Benjamin Cummings (Chapters
25, 26)
Miller, G.T. (2004) Living in the Environment. Thomson
(Chapters 3, 4)
Molles, M.C. (2007) Ecology Concepts and Applications.
McGraw-Hill (Chapter 18)
Ricklefs, R.E. & Miller, G.L. (2000) Ecology. W.H. Freeman
(Chapters 9, 10)
Smith, R.L. & Smith, T.M. (2007) Ecology and Field Biology.
Benjamin Cummings (Chapter 24)
Townsend, C.R. et al. (2008) Essentials of Ecology.
Blackwell (Chapter 11)
A Reminder
If you try to read Begon, Townsend, and Harper
(2006) Ecology – From Individuals to Ecosystems,
there is a 17-page glossary of the very large (too
large!) number of technical words used in the book
on the Bio-201 Student Portal. It can be
downloaded from the File Storage folder.
Good luck!
Today’s Ecological Scale
Biosphere
Biosphere
Biosphere
Biomes
Ecosystems
ECOSYSTEMS & Landscapes
Communities
Communities
Species
Populations
Populations
Organisms
Organisms
Introduction
Consider ecosystems and associated
communities as functional units
Organisms and their environment
are linked by energy flux,
transformation of energy, and flux of
matter (nutrients, etc.)
The source of energy for all life
Solar
radiation
Energy in = Energy out
Reflected by
atmosphere (34%)
Radiated by
atmosphere
as heat (66%)
UV radiation
Absorbed
by ozone
Lower Stratosphere
Visible (ozone layer)
Greenhouse
light
Troposphere
effect
Heat
Absorbed
by the earth
Heat radiated
by the earth
Earth
Solar energy – some is reflected, some is
converted to heat energy, some is absorbed by
chlorophyll in plants.
Infra-red radiation – absorbed by molecules in
organisms, soil, and water, increasing their kinetic
state, raising temperatures.
Community temperature affects the rate of
biochemical reactions and rate of water loss by
transpiration from vegetation.
Sustaining life on earth
Water & sun
+ chlorophyll
+ minerals
+ CO2
 Primary Production
Gives oxygen to
Carbon
plants and animals
cycle
Plants are the reason
why Planet Earth is
the only planet in the
solar system with an
oxygen-rich atmosphere
Biosphere
Phosphorus Nitrogen
cycle
cycle
Water
cycle
Heat in the environment
Heat
Heat
Heat
Oxygen
cycle
Plants use photosynthetically active radiation (PAR)
to synthesize carbohydrate sugars.
Some of this fixed energy is used to meet plant's
energy needs. Some goes into plant growth. Some
is stored as non-structural carbohydrates which act
as energy sources in roots, seeds, and fruits.
Photosynthesis increases plant biomass.
Some of this fixed energy is consumed by
herbivores, some by detritivores living on detritus,
some ends up as soil organic matter.
Energy fixed by plants powers animal motion, bird
flight, etc.
Plants are PRIMARY PRODUCERS
(PHOTOAUTOTROPHS)
Vegetation is thus a system that absorbs,
transforms, and stores solar energy.
In this, physical, chemical, and biological
structures and processes cannot be
separated.
Comprise ECOSYSTEM – a biological
community (or several communities) plus
all the abiotic factors influencing the
community or communities.
Term ecosystem coined in 1935 by Arthur
Tansley when he realised the importance of
considering organisms and their environment
as an integrated system.
"Though the organisms may claim our primary
interest … we cannot separate them from their
special environment with which they form one
physical system. It is the system so formed,
the ecosystem, which, from the point of view
of the ecologist, are the basic units
of nature on the face of the earth"
Tansley (1935)
Sir Arthur Tansley
Ecosystem ecologists study the flows of
energy, water, and nutrients in ecosystems.
Physical and chemical processes as well as
biological aspects.
Primary production and energy flux – this
lecture
Nutrient cycling – Lecture 4 on ‘Ecosystem
Ecology –Flux of Matter’
Ecosystem is, in some ways, an unsatisfactory
term. System studied by ecologists, a biotic
community and its abiotic environment but very
difficult to define or delimit.
“Interaction or energy transformation between
abiotic (climate, geology, soils), biotic factors
(primary and secondary production), and humans”
Ecosystem includes primary producers (plants),
decomposers, and detritivores, a pool of dead
organic matter, herbivores, carnivores, and
parasites plus the physiochemical environment that
provides living conditions and acts as both a
SOURCE and a SINK for energy and matter.
Use of the term ecosystem
COMMON USE
SAVANNA ECOSYSTEM
FOREST ECOSYSTEM
OR SPECIFIC: “San Francisco Bay ecosystem”
Ecosystem as a unit for system analysis
We cannot see ecosystems, only landscapes.
Ecosystems are always open systems
We choose the interactive variables, and have to
set arbitrary or practical boundaries in order to
be able to study an ecosystem
Ecosystem ecology in practice involves the holistic
approach and is usually very difficult because of
numerous synergistic effects and interactions. Inevitably
'broad-brush' 'black-box' approach.
Holistic approach
Aluminium (Al)
Some Definitions
Standing crop – living organisms within a unit area
Biomass – mass of living matter per unit area or unit
volume of water (e.g. t ha-1, g cm-2)
Primary production – fixation of energy by autotrophs in
an ecosystem
Rate of primary production – amount of energy fixed over
some interval of time
Gross primary production (GPP) – measure of total
amount of dry matter produced by autotrophs by
photosynthesis in an ecosystem. Units of dry weight
per unit area per unit time (e.g. kg ha-1 yr-1)
But all organisms respire and some of the GPP is
converted back into CO2 and water by respiratory heat
(RA – autotrophic respiration)
Net primary production (NPP) – overall gain of dry weight
after autotrophic respiration. Same units as GPP. The
amount of energy available to consumers in an ecosystem
GPP = Respiration (RA) + NPP
NPP = GPP – Respiration (RA)
Terrestrial GPP  2.7 x NPP
Ocean GPP  1.5 x NPP
GPP or NPP measured as rate of carbon uptake or by the
amount of biomass produced or oxygen produced
Secondary productivity – rate of biomass production by
heterotrophs (bacteria, fungi, and animals)
Net ecosystem productivity (NEP: units as for NPP and GPP)
– recognises that C fixed in GPP can leave the system as
inorganic C (e.g. CO2) via autotrophic respiration (RA) or
consumption by heterotrophic respiration (RH)
Total ecosystem respiration (RE) = RA + RH
 NEP = GPP - RE
= GPP - RA - RH
If GPP > RE, ecosystem is fixing carbon faster than it is
being released, and the system is thus a carbon sink
If RE > GPP, carbon is released faster than it is fixed and
the system is a net carbon source
How can RE exceed GPP? Ecosystem can receive organic
matter other than by its own photosynthesis, e.g. import
of dead organic matter produced elsewhere
Autochthonous – organic matter produced by
photosynthesis within ecosystem’s boundaries
Allochthonous – organic matter imported from outside
ecosystem’s boundaries
Trophic structure in an ecosystem – species arranged into
trophic levels based on the major source of their nutrition
Trophic level – position on a food web determined by the
number of transfers of energy from primary producers to
that level
Trophic Levels
Fourth trophic level (large carnivores)
Third trophic level (carnivores)
Primary consumers (herbivores)
Primary producers (plants)
Inorganic energy (solar energy) plus water, CO2, etc.
Global Primary Production
NPP of Earth about 105 petagrams of carbon per year
(1 petagram (Pg) = 1015 g)
Terrestrial ecosystems 56.4 Pg C yr-1
Aquatic ecosystems
48.3 Pg C yr-1
Although oceans 72% of Earth’s surface, account
for 46% of Earth’s total NPP
On land, tropical rainforest and savannas account
for 60% of terrestrial NPP and 32% of Earth’s
total NPP
Forest biomes show trend of increasing GPP with
decreasing latitude
Grassland biomes show
similar trend in aboveground NPP (ANPP) and
below-ground NPP
(BNPP)
In aquatic systems, similar latitudinal gradient in
GPP of lakes.
Not in oceans where GPP more limited by nutrients
and is high where there are upwellings of nutrientrich waters, even at high latitudes and hence low
temperatures.
Terrestrial trends suggest that radiation (a resource)
and hence temperature (a condition) may limit
productivity. But other factors frequently constrain
productivity, even within narrow limits.
Patterns of Terrestrial Primary Production
Mainly determined by temp and moisture at biome scale.
Terrestrial biomes in World Vegetation Biomes (Lecture 6)
differ in their NPP
Biome
NPP (g m-2 yr-1)
Mean
Range
Tropical forest
2000
Savanna
Area
World NPP
(106 km2)
(109 dry ton yr-1)
1000-3500
25
50
900
200-2000
15
14
Desert
40
0-250
42
2
Temperate forest
1250
600-2500
12
15
Temperate woodland
& shrubland
700
250-1200
9
6
Temperate grassland
600
200-1500
9
5
Boreal forest
800
400-2000
12
10
Tundra
140
10-400
8
1
Cultivated land
650
100-3500
14
9
Can we combine temperature and moisture into a
robust predictor of NPP?
Actual evapotranspiration (AET) is total amount of
water that evaporates and transpires off a
landscape in a year. Units are mm water yr-1.
AET influenced by
both temperature
and moisture.
Good predictor of
NPP at biome
scale.
Evapotranspiration
and above-ground
primary production
Important to note that AET has not been measured.
It is estimated from latitude, mean monthly
temperature, and mean monthly precipitation.
Also AET is bound to be greater the more plant leaf
area there is, as large leaves increase chances of
water being transpired or evaporated rather than
reaching the soil.
In addition, NPP will be greatest in communities with
large leaf areas.
NPP and AET are not strictly independent variables
– both are functions of leaf area, in particular leaf
area index – number of leaves a typical vertical ray of
sun-light passes through before hitting soil (range
from <1 to >9 globally).
What determines variation in primary production
within similar ecosystems?
Temperate grasslands in central US
Major influence of annual precipitation
Well known to farmers that productivity can be
increased by adding fertilizers to soil.
Liebig (1840) nutrients limited plant growth.
At fine scale, soil fertility can influence
productivity.
Addition of N, P, and
K to Alaskan arctic
tundra
2-4 years, NPP
increased by 23300% at all sites
Adding fertilizer to arctic tundra
Colorado alpine tundra
Added P, N, or N + P to
dry and wet meadow
tundra
Large effects (relative
to control) in dry
meadows
N may be limiting in
dry meadow
N + P may be limiting
in wet meadow.
Effect of fertilizers on wet and
dry alpine tundra environments
How important is nitrogen-limitation on terrestrial
net primary production?
Le Bauer and Treseder 2008 Ecology 89: 371-379
126 Nitrogen addition terrestrial experiments worldwide. Looked at effects on NPP ('meta-analysis')
Used response ratio
R = ANPPN/ANPPCTRL
where ANPP = above-ground NPP,
N = nitrogen addition, and CTRL = control
R = 1.29 for all experiments
R
Temperate forests
Tropical forests
Temperate grasslands
1.19
1.60*
1.53
Tropical grasslands
Wetlands
Tundra
1.26
1.16
1.35
Deserts
*Tropical forests
1.11 not significant
Young volcanic
soils (Hawaii)
2.13
'Old' soils
1.20 (very similar to
temperate forests)
Are there any relationships between R (measure of
N limitation) and latitude or climate?
All
Wetlands
Forests (all)
Forests (old soils only)
Grasslands
Tundra
Latitude
Mean
annual T
Mean
annual pptn
+
+
+
-
+
+
+
+
-
(+ significant at p < 0.05)
Suggests that global N and C cycles interact strongly
and that geography can influence ecosystem
response to N within certain biome types (e.g.
tundra, wetlands, grasslands).
Seasonal and annual trends in terrestrial
primary production (GPP)
Can vary in an ecosystem by a factor of 2, presumably
due to annual fluctuations in temperature and rainfall
Within a year, GPP depends on seasonal variations in
conditions, especially temperature and the length of
the growing season
Period of high
GPP longer in
temperate
forests than
boreal forests
Summary of factors limiting primary
production in terrestrial ecosystems
Resources: Solar radiation, CO2, Water, Soil nutrients
Condition: Temperature
Remember that
1. terrestrial systems use radiation inefficiently (self-shading)
2. photosynthetically active radiation may be limiting in some
systems (e.g. C4 plants)
3. shortage of water is often a critical limiting factor
4. temperature and precipitation interact (‘energy-water’
model)
5. soil texture can influence productivity (dry vs. mesic)
6. length of growing season can be very important (tropics
vs. boreal)
7. nutrient availability can also be limiting (N, P)
See Begon et al. (2006) pp. 505-511 for further details
Patterns of Aquatic Primary Production
72% world is covered by water, but only 46% of
world's NPP is from aquatic systems.
Type
NPP (g m-2 yr-1)
Mean
Range
Area
World NPP
(106 km2)
(109 dry ton yr-1)
Swamps
2000
800-3500
2
4
Algal beds &
coral reefs
1800
500-2000
2
4
Continental shelf
360
200-1000
27
10
Lakes & streams
250
100-1500
2
1
Oceans
125
2-400
332
42
Why lower NPP in aquatic systems?
Cannot be a shortage of water!
Some systems have high NPP (algal beds, swamps,
coral reefs); ocean has low NPP (less than tundra!)
Why is the sea blue?
Physicist's answer – blue light with its short
wavelength is less likely to be absorbed than red or
green light. Light reflected back to our eyes is most
likely to be blue.
Biologist's answer – it is blue because it is not green
with plants!
So why are terrestrial systems mainly green and
aquatic systems mainly blue?
Most aquatic systems are starved of nutrients.
What determines algal biomass?
Phosphorus
concentration and algal
biomass in lakes
What is the relation between algal biomass
and rate of primary production?
Test ideas on aquatic productivity by
whole-lake ecosystem manipulations.
Experimental Lakes Area, Ontario, Canada.
Lake 226 divided into sub-basins
C added to one basin
C, N, and P to other basin
1973-1980.
Measured phytoplankton biomass before,
during, and after fertilization and in control
lake.
Effect of nutrient additions
Clear that nutrient availability,
especially phosphorus and nitrogen,
controls rate of primary production in
freshwater ecosystems.
What about marine ecosystems?
Much more difficult to study!
Main depth zones in
the ocean – continental
shelf, continental
slope, abyssal plain
Main vertical life zones in the ocean. Almost all
life depends on the phytoplankton growth in the
epipelagic zone (0 - 150 m depth)
Marine primary production
Phytoplankton concentration in the Atlantic as shown in
a satellite image. Note concentrations in northern half.
Highest production along continental shelves and in areas of
upwelling.
Nutrients renewed by run-off from land and sediment
redistribution.
Up-welling brings nutrient-rich water to surface.
These areas are not a clear blue but a murky green due to the
high phytoplankton density. Start the aquatic food web. Great
fishing areas of the world.
Main ocean has low nutrient availability and low rate of
primary production.
Iron is limiting nutrient in about 30% of open ocean. Very
insoluble in sea-water. Main input is wind-blown particulate
material.
Nutrient renewal in open ocean is by vertical mixing. Such
mixing is often blocked in open tropical oceans by a permanent
thermocline.
Open tropical oceans thus have low nutrient concentrations and
hence lowest rates of primary productivity (< arctic tundra!).
Some marine primary producers in phytoplankton
– epipelagic zone
Diatom
Coccolithophorid
(Coccolithus pelagicus)
Dinoflagellate
Zooplankton – epipelagic zone
Copepod
Squid
(Chiroteuthis)
Arrow worms (Chaetognatha)
Jellyfish
(Pentachogon sp.)
Krill (Euphausia superba)
Mesopelagic animals
Deep red shrimp
(Acanthephyra sp.)
Deep sea jellyfish (Atolla sp.)
Shrimp (Systellaspis debilis)
Mesopelagic and Bathypelagic animals
Lantern fish
(Myctophum punctatum)
Deep-sea squid
(Watsenia scintillans)
Hatchet fish (Argyropelecus
aculeatus)
Is there any experimental evidence for
nutrient limitation in marine systems?
Difficult to do an Experimental Lake Areatype experiment with an ocean!
Edna Graneli (1990) Baltic Sea.
Collected filtered sea-water from 5 sites
looked at algal growth in control
with P added
and with N added.
Nitrate control
of primary
production
In contrast to freshwater systems, N limitation
appears to be most important in some coastal
and marine systems.
Himmelfjärden, Sweden – brackish inlet of Baltic
Sea with area of 195 km2
N limitation can shift to P limitation by altering
N:P ratios.
Adding P reinforces N limitation
Decreasing P and adding N leads to P limitation.
Nature is never simple!
Global survey of estuaries and coastal marine
phytoplankton in relation to nitrogen and
phosphorus – Val Smith (2006) L & O 51: 377-384
Total N and
coastal systems
Chlorophyll a and
annual mean
total N
concentrations in
coastal marine
systems
log10 Chl a = -3.71 + 4.26 log10TN – 0.88 (log10 TN)2
r2 = 0.84
Total P and
coastal systems
Chlorophyll a and
annual mean
total P
concentrations in
coastal marine
systems
log10 Chl a = 0.39 + 0.71 log10 TP
r2 = 0.60
Which is more important - total N or total P – in
coastal systems?
Redfield ratio – particulate N:P in marine organisms
= 16:1 by moles
Total N and total
P in coastal
systems
Data almost
divided into 2 by
Redfield ratio
log10 TN = 1.38 + 0.41 log10 TP
r2 = 0.55
Cumulative
frequencies of TN:TP
ratios
See 50% of sites
show evidence for
phosphorus
limitation, with
TN:TP ratio >16
Pristine marine environments (annual TP < 0.8
mmol P l-1) tend to have TN:TP ratios > Redfield ratio of
16 and thus are likely to be consistently P limited
Heavily nutrient-enriched environments (annual TP
> 8 mmol P l-1) have TN:TP < Redfield ratio and thus
are likely to be consistently N limited
Both N-limited and
P-limited marine
sites
P-limited sites: log Chl a = 0.99 log TP + 0.11
r2 = 0.74
N-limited sites: log Chl a = 1.48 log TP + 0.61
r2 = 0.69
Comparisons between lakes □ and coastal  sites
A: All sites
B: TN:TP < 20, only N
limited sites
C: TN:TP > 20 sites,
only P limited sites
Similarity in physiological response of freshwater
and marine phytoplankton to nutrients probably a
result of the shared evolutionary histories of
freshwater and marine phytoplankton.
Nature is both complex (total N, total P, and N:P
ratios) and simple (freshwater and marine
phytoplankton have shared histories)!
What else can determine algal biomass?
Scatter of points
around relationship
between P
concentrations and
algal biomass.
Residual variation in
algal biomass not
explained by P
concentrations.
Phosphorus concentration
and algal biomass in lakes
What other factors
can influence algal
biomass?
Role of Consumers on Rates of
Primary Production
Influence of physical and chemical abiotic factors –
BOTTOM-UP CONTROLS
Influence of consumers –
TOP-DOWN CONTROLS
Influences of evapotranspiration, precipitation, or
nutrients on productivity are all bottom-up
controls.
What about top-down controls?
Trophic-Cascade Hypothesis
A change in one of the trophic levels will have
a cascading effect on the rest of the food web.
Similar to the keystone species hypothesis in
community ecology except that the trophiccascade hypothesis is concerned with
ecosystem processes such as productivity
rather than species diversity.
Important to distinguish between species
cascades and ecosystem trophic cascades.
Trophic-cascade is now an ambiguous concept.
Trophic-cascade
hypothesis
1. Large fish feed on smaller
fish and invertebrates
2. Large fish therefore
influence zooplankton and
large zooplankton (eaten
by planktivorous fish)
dominate the zooplankton
3. This reduces phytoplankton biomass and rate
of primary production
Trophic cascade consistent
with negative correlation
between zooplankton body
size and primary production
Predicted effect of piscivores
Experimental manipulations of fish in two small
lakes plus a third control lake.
Lake with large-mouth bass (piscivorous)
Lake with no bass but abundant minnows
(planktivorous)
Removed 90% of large-mouth bass and put them
in the other lake
Removed 90% of minnows and put them in the
other lake
Experimental manipulation of lakes and their responses
As predicted, reducing the planktivorous fish
led to reduced primary production, as large
zooplankton increased.
Adding planktivorous minnow fish led to
increased primary production. By adding
minnows, food for remaining bass, 50-fold
increase in bass population that preyed on
zooplankton, and phytoplankton increased.
Support for top-down controls in lakes.
What about marine systems?
Are long-term sustainable fish catches from the
continental margin in marine ecosystems controlled by
(1) primary production rates (bottom-up controls)
or
(2) predator-prey interactions at higher trophic levels
(top-down controls)?
Satellite derived estimates of chlorophyll-a
concentrations to test hypothesis that fish production
along the western North American continental shelf is
mainly controlled by phytoplankton production
(bottom-up controls)
Chlorophyll-a – a proxy measure for phytoplankton
concentrations
Ware & Thompson 2005 Science 308: 1280-1284
Looked at two spatial scales (67,157 & 18,830 km2)
High correlation (r2 = 0.87) between fish
yield and mean annual chlorophyll-a
concentrations at broad scale
Why are there spatial variations in the
chlorophyll-a concentrations?
1. Negative correlation between chlorophyll-a
and offshore winds and upwelling.
2. High correlation with year-round freshwater
flux. Provides micronutrients and water
column stability. Mainly in Washington –
southern British Columbia regions, from
Columbia and Fraser rivers.
At the finer, regional scale, correlation between fish
yield and chlorophyll-a concentrations is high (r2 =
0.76)
Also high correlation (r2 = 0.85) between
zooplankton productivity and chlorophyll-a
concentrations
High correlation also between fish yield and
zooplankton productivity (r2 = 0.79)
All consistent with bottom-up trophic control
Primary production by phytoplankton moved
through the pelagic and benthic food-webs to
the fish community.
Controlled by terrestrial nutrient supply and
water column stability as a result of freshwater
input from the major rivers.
Clear bottom-up trophic control and processes.
Is nature that simple?
Frank et al. 2005 Science 308: 1621-1623
Top-down trophic cascades (predator dominated)
in a formerly cod-dominated ecosystem off Nova
Scotia, Canada. 1963-2004
1. Collapse of benthic fish community (major
predators) in mid 1980s. Mainly cod but also 10
other fish species.
2. Abundance of small pelagic fish and benthic
macro-invertebrates increased markedly after
benthic fish collapse. Correlations between
benthic fish biomass and pelagic fish or macroinvertebrates all negative (r = -0.61 to -0.76)
3. With the removal of the top predator (cod and
other large benthic fish) conspicuous indirect
effects.
(a) herbivorous zooplankton declined as
benthic fish declined (r = 0.45) and small fish
increased (r = 0.52). Change from large to
small size, resulting from size-selective
predation on zooplankton by pelagic fish that
increased with decline of cod.
(b) phytoplankton increased as benthic fish
declined (r = -0.72) as less herbivorous
zooplankton.
More small predators
Increase of
small pelagic
fish & benthic
macroinvertebrates
Cod
decline in
1980s
Less large predators
More nutrient
utilisation
Increase of
phytoplankton
as less
herbivorous
zooplankton
Decline of
herbivorous
zooplankton &
size change;
large - small
Less herbivory
Decrease of
nitrate, major
limiting factor,
due to increased
phytoplankton
Top-down cascading
effects before and
after the collapse of
cod and other large
predators on the
Nova Scotia shelf in
mid 1980s
Scheffer et al. 2005
Trophic cascade here involves four trophic levels and
nutrients. Driven by changes in the abundance of top
predator (primarily cod).
Top-down control with indirect effects and multiple
links.
Change from benthic fish commercial fishery
>100 x 103 metric tons (kt) to pelagic fish –
macroinvertebrate system with poor benthic fishery
of <50 kt. Attempts to reverse the trend have failed
so far.
Shows strong non-linearities in community behaviour
and regime shifts induced by over-exploitation.
Nature is never simple!
Are there any top-down trophic cascades
on land or are 'top-down' trophic cascades
all wet?
Grazing by Large Mammals
Serengeti-Mara grasslands (25 000 km2) on
Tanzania-Kenya border
One of the last ecosystems where huge numbers
of large mammals still roam
Photo: John Grimshaw
Grazers consume 66% of annual primary production
Complex interaction of abiotic and biotic factors
Soil fertility and rainfall stimulate plant growth,
hence grazing mammals
Grazing mammals also affect water balance, soil
fertility, and plant production
Compensatory plant growth in area grazed by
wildebeest. Grazing increases the growth rate of
many grass species. Possible mechanisms include
lower rates of respiration due to lower plant
biomass, reduced shelf-shading, and improved water
balance due to reduced leaf area.
Growth response of grasses grazed by wildebeest
Compensatory growth highest at intermediate
grazing intensities
"African ecosystems cannot be understood
without close consideration of the large
mammals. These animals interact with their
habitats in complex and powerful patterns,
influencing ecosystems for long periods."
Serengeti system is a terrestrial trophic
cascade of top-down controls, with wildebeest
influencing primary production.
Are trophic cascades widespread? What factors
determine the occurrence of trophic cascades?
Factors and processes that may influence the
occurrence of trophic cascades (based on 41 studies)
Factor
Self-regulation
Cannibalism
Interference competition
Territoriality
Inter-guild predation
Regulation across
trophic levels
Omnivory
Intra-guild predation
Predation-mediated
coexistence
Behavioural responses
Positive interactions
Consumer age-structure
Food-web complexity
Net
Factor
effect
Landscape features
Disturbance patterns
Spatial variation
Refugia
Resource availability
and quality
+ or - Low resource quality
+ or - High resource quality
Resource dominated by
few species
+ or - Temporal heterogeneity
+ or - Rapid nutrient cycling
+ or -
Net
effect
+ or + or -
+
+
+ or +
Not all ecosystems will show trophic cascades.
Effects of different factors are
negative (8)
positive (3)
either negative or positive (8) depending on
other factors
Trophic cascades appear to be common in
freshwater and marine systems but rare in
terrestrial systems
Why are aquatic and terrestrial systems different?
Aquatic
Terrestrial
Discrete, homogeneous
habitats
Prey populations dynamics
rapid relative to predator
dynamics (e.g. rapid algal
turnover)
Common prey are
uniformly edible
Systems simple and
trophically stratified
Fuzzy, heterogeneous
habitats
Variable prey population
dynamics
Common prey not
uniformly edible
Systems complex and
reticulate and species
interactions weak and
diffuse
But terrestrial agricultural systems may be the
exception that proves the rule. Such systems
tend to have the features of aquatic systems.
Suggests that more diverse systems are tied
together by multiple trophic influences among
species. Less likelihood of trophic cascades in
diverse systems, including many natural or
semi-natural terrestrial systems.
Nature is never simple!
What is the Relationship Between
Productivity and Standing-Crop
Biomass?
Can think of standing crop as the biomass sustained
by the productivity (capital resource sustained by
earnings)
Can think of productivity as a function of the standing
crop that produces it (interest rate on the capital)
Total biomass –
land
800 Pg
oceans
2 Pg
freshwater <0.1 Pg
Allowing for area – land
0.2 – 200 kg m-2
oceans
<0.001 – 6 kg m-2
freshwater <0.1 kg m-2
Relationship between average NPP and average
standing-crop biomass for a range of ecosystems.
1. Higher NPP produced by a smaller biomass (B) in non-forest
systems, and in aquatic systems
2. NPP:Biomass (kg dry matter produced per year per kg of
standing crop) is
0.042 forests
0.29 other terrestrial systems
17
aquatic systems
3. Reason is that much of forest biomass is dead (and has been
for some time) and much of supporting tissue (wood) is not
photosynthetic
4. In grasslands, higher fraction of biomass is alive and is
photosynthetic, but 50% of biomass may be roots
5. In aquatic systems, especially where NPP is dominated by
phytoplankton, there is no support tissue, no roots, no
accumulation of dead cells, and the photosynthetic output
per kg of biomass is thus very high
6. High NPP:B ratios in aquatic systems also due to rapid
turnover of biomass (aquatic systems 0.02-0.06 years
turnover time of biomass, terrestrial systems 1-20 years)
In succession, NPP:B tends to decrease
1. Early successional pioneers (rapidly growing
herbs) have little support tissue so NPP:B is high
2. Late successional plants (shrubs and trees) have
much support tissue and NPP:B is low
3. Within trees, common pattern is for above-ground
NPP to peak early in succession and then
gradually decline by as much as 76%, with a
mean reduction of 34% later in the succession
Shift from primarily photosynthetic to respiring
tissues, along with nutrient limitation (N, then P)
Trees of different successional stages show different
patterns of NPP with stand age
 Early successional
Pinus albicaulis
(whitebark pine) –
peak NPP about
250 yrs
 Late successional
Abies lasiocarpa
(subalpine fir) –
peak NPP after 400
yrs
200% more biomass in fir foliage than in pine, maintains
high photosynthesis:respiration ratio to greater age than
pine
Trophic Levels
Idea of energy
transfer from one
level to another –
primary producers
primary consumers
carnivores
etc.
Chapman & Reiss
In natural ecosystems, usually only
2-5 trophic levels.
What limits the number of trophic
levels?
Raymond E. Lindeman,
University of Minnesota.
Died aged 27 in 1942
Cedar Bog Lake.
Proposed concept of trophic dynamics – transfer of
energy from one part of an ecosystem to another.
Trophic levels -
primary producers
primary consumers
secondary consumers
etc.
Each level feeds on the one immediately below it.
Energy enters the ecosystem by photosynthesis
by primary producers, converting solar energy
into biomass.
As energy is transferred from one trophic level
to another, energy is lost due to limited
assimilation, respiration by consumers, and
heat production.
Forms pyramid-shaped distribution
of energy between trophic-levels.
'Eltonian pyramids' named after
Charles Elton, the great Oxford
animal ecologist.
Annual production by trophic level in two lakes
Ecosystems thus consist of
1. Autotrophs - photoautotrophs or primary producers
- chemoautotrophs fixing atmospheric N
2. Decomposers
3. Herbivores and carnivores
4. Omnivores
5. Organic sink
6. Atmosphere, minerals, and water
7. Solar energy
Amount of dead organic matter in biomes
Tropical forest
2 tons ha-1
Temperate forest
15-30 tons ha-1
Boreal forest
30-45 tons ha-1
Tundra
85 tons ha-1
Major organic sinks in northern areas
Energy Flow in a Temperate Forest
Hubbard Brook Experimental Forest, Eastern
USA
Quantified energy flow as kilocalories (k cal)
per square metre per year
Gene Likens
Herbert Borman
Energy budget for a temperate deciduous forest (%)
Biomass
k cal m-2
Dead organic matter
122,442
(88,120 soil)
(34,322 plant litter)
Total living-plant biomass
71,420
(59,696 above ground)
(11,724 below-ground)
Energy Budget
Solar energy
15% reflected, 41% converted
into heat, 42% absorbed in
evapotranspiration
Only 2.2% fixed by plants as
gross primary production
Plant respiration
1.2%
 NPP
1.0%
Net primary production ca.
4800 k cal m-2
Plant growth
1199 k cal m-2
Herbivores
Litter fall
Detritus, root exudates, etc.
41 k cal m-2
3037 k cal m-2
437 k cal m-2
Net primary production 1%
96% of this available to consumers is lost
by consumer respiration.
Little energy left for a third trophic level.
It is the losses with each transfer of energy
in a food chain that limits the number of
trophic levels.
As these losses between levels accumulate,
insufficient energy to support viable
populations at a higher trophic level.
Inverted Trophic Pyramids
Oceans where phytoplankton may support a
larger mass of zooplankton
Chapman
& Reiss
Energy Flow Through Different Ecosystems
Three transfer efficiencies needed for quantitative
comparison between systems
1. Consumption Efficiency (CE)
CE = In / Pn-1 x 100
Percentage of total productivity available at one
trophic level (Pn-1) that is consumed by a trophic
level one up (In)
5% in forests
25% in grasslands
50% in phytoplankton-dominated systems
2. Assimilation Efficiency (AE)
AE = An / In x 100
Percentages of food energy taken into guts of
consumers at a trophic level (In) that is
assimilated across the gut wall (An) and
becomes available for growth, etc.
20-50% for herbivores
80% for carnivores
60-70% for seeds and fruits
50% for leaves
15% for wood
3. Production Efficiency (PE)
PE = Pn / An x 100
Percentage of assimilated energy (An) that is
incorporated into new biomass (Pn). Rest is
lost by respiration
10% vertebrate ectotherms
1-2% vertebrate endotherms
Overall trophic level transfer efficiency from one
level to next is CE x AE x PE
Great variation in transfer efficiency
mean =
10.13%
SE = 0.49
No complete NPP, CE, AE, and PE data for any systems
Some general patterns for forests, grasslands, planktondominated systems, and freshwater streams
1. Decomposer
system responsible
for main ecosystem
respiration (RE,
mainly RH)
2. Grazers most
important in
plankton-system,
least important in
terrestrial and
stream systems
DOM = dead organic matter;
GS = grazer system
3. Deep-ocean benthic system most like stream system,
with low NPP, high dead organic carbon sinking from the
euphotic zone above
NPP for 9 systems (a) and estimates of how much NPP is
consumed by herbivores (b), how much becomes detritus
(c), how much persists as detritus (d), and how much is
exported (e). See differences in herbivory, detritus
production, long-lasting detritus production, and export.
Do Trophic Levels Exist?
1. How to decide to which level a particular
species belongs?
2. Dead organisms, urine, and faeces are
usually ignored but may represent a
large amount of biomass.
3. Symbiotic bacteria in ruminant guts –
are they herbivores or saprotrophs?
Can we use stable isotopes to identify energy
sources?
Ribbed mussel Geukinsia demissa living in coastal
salt-marshes on eastern coast of USA.
Primarily eats fine detritus as filter-feeder.
Detritus consists of:
1. Upland plant material brought in by rivers
2. Spartina salt-marsh grass
3. Phytoplankton
Using stable-isotope analysis of 13C, 15N, and 34S,
could establish the isotope signature of potential
food sources for the mussel.
Isotopic content of food sources for the ribbed mussel,
Geukinisia demissa, in a New England salt-marsh
Upland C3
plants most
depleted in 13C
Spartina (a C4
plant) least
depleted in 13C
Plankton has
highest
concentrations
of 34S
Variation in isotopic composition of ribbed mussels by
distance inland
Gets most of its energy from plankton and Spartina.
Mainly Spartina in centre of salt-marsh; mainly
plankton near mouth of marsh.
Shows (a) how stable isotopes can help identify
energy sources and (b) how energy sources can be
different in different places within a habitat.
Trophic levels are generally a useful tool in
understanding ecosystem functioning. Stable
isotope techniques may be needed to identify
energy sources.
Why is the World Green?
Current estimates are that for every one plant
species, there are at least five species of animal
herbivore.
Why have grazing animals not consumed all the
vegetation and reduced the Earth’s land to dust?
Hairston et al. (1960) hypothesise that herbivore
numbers are controlled by predation by carnivores
and by keeping herbivores in check, carnivores
keep the world green and allow plant primary
production to continue.
Difficult to test.
Terborough et al. 2006 J. Ecology 94: 253-263
Lago Guri, Venezuela formed by flooding of a broad
valley for hydroelectric development
Lake is 4300 km2 and contains many islands of
different sizes
Islands < 20,000 m2 (2 ha) lost most of their
vertebrates in a few years after island created. Higher
numbers of invertebrates (e.g. leaf-cutting ants), also
some vertebrates such as tortoise, monkey, and iguana
Islands > 75 ha retained more vertebrate grazers
(deer, peccary, primates) but also predators (snakes,
jaguar, puma, eagles).
Hairston et al. green-world hypothesis proposes:
1. small islands with no predators and high densities
of grazers should show vegetation decline
2. medium-sized islands (< 15 ha) should show less
vegetation decline
3. large islands should show no change
Vegetation changes (sapling density relative to large
islands)
Pre-lake
1997
2002
Small islands
100%
37% 
25% 
Medium islands
100%
12%
16%
Large islands
100%
2%
3%
Tree mortality 46% on small islands, 32% on medium
islands, 12% on large islands
Loss of predators on grazers and leaf-cutting ants
on small islands resulted in a trophic cascade that
destabilised food web.
Difficult to show in terrestrial systems (cf.
Serengeti example above). Loss of wolf predators
has resulted in increased grazers and overgrazing.
Will overgrazing on small islands in Lake Guri result
in the total destruction of vegetation on these
islands? Would this lead to an extinction of grazing
herbivores? Would there then be re-invasion by
different plant species and associated herbivores?
Importance of long-term ecological observations
and regular monitoring to understand ecological
processes.
Interactions Across Ecosystems
A Complex Ecosystem with Marine-Terrestrial Links
Aleutian Islands – 1900 km chain of islands west of
the Alaska peninsula
Islands support large
colonies of sea birds
(29 species, >10
million birds). Deliver
nutrient-rich guano
from productive
ocean waters to the
nutrient limited plant
communities on the
islands.
Following collapse of the fur-hunting trade in late
19th and early 20th century, foxes were introduced
to some but not all the islands as an extra source
of fur.
Introduced foxes severely reduced the sea bird
colonies on the islands where they were introduced.
Some islands remained free of foxes.
Large-scale natural experiment on the effects of top
predators on sea bird numbers and hence on soil
and plant nutrients, plant abundance, composition,
and productivity, and nutrient flow.
Croll et al. 2005 Science 307: 1959-1961.
9 islands with foxes
9 islands without foxes
Sea bird density much greater (x2)
on fox-free islands
Soil phosphorus x3 on fox-free islands
Grass biomass x3 on fox-free islands
Shrub biomass x10 LESS on fox-free
islands
Nitrogen content on plants greater on
fox-free islands
Vegetation on fox-free islands
dominated by grasses
Vegetation on islands with foxes had
more shrubs
Examined δ15N isotope values in soils, plants, and
herbivores. Greater in soils on fox-free islands.
Indicates that nutrients on fox-free islands are
enriched by marine supply coming from sea-bird
guano.
Introduction of foxes onto some islands has
transformed islands from grasslands to shrubtundra. Fox predation has reduced sea bird
populations, thereby reducing nutrient transport
from sea to land.
Trophic cascades are a series of strictly top-down
interactions. Predators affect herbivore populations
and alter the intensity of herbivory and hence the
primary plant production at the base of the food web.
Aleutian Islands show that it is more complex because
the predators have powerful indirect effects on the
ecosystem by reducing the transport of nutrients
between marine and terrestrial systems.
By preying on sea birds, foxes have reduced nutrient
transport form ocean to land and have thus affected
soil fertility and changed grasslands to dwarf-shrub
dominated ecosystems.
Striking example of the complexity of nature and the
interaction of different components in the ecosystem.
Nature is never simple!
Trophic Cascades Across Ecosystems
Knight et al. 2005 Nature 437: 880-883
Freshwater fish indirectly influence terrestrial
plant reproduction through cascading trophic
interactions across ecosystem boundaries.
Interaction web
showing pathway
by which fish
facilitate plant
reproduction
TOP-DOWN trophic cascades across ecosystems
8 ponds in northern Florida, 4 with fish, 4
without fish
Examined fish densities, larval and adult
dragonfly densities and size, and plant pollinator
visits by Diptera, Lepidoptera, and Hymenoptera
insects.
Dragonflies were more abundant in and around
ponds without fish than at ponds with fish.
Predation of larvae by fish.
Large- and medium-sized dragonflies dominate by
fish-free ponds; small-sized dragonflies at ponds
with fish. Size-selection predation.
Pollinator visits much greater on Hypericum shrub
by ponds with fish than by fish-free ponds. Ponds
with fish mainly Hymenoptera (bees), ponds
without fish mainly Diptera (flies).
Plant reproduction by fish-free ponds greatly
limited by pollination compared to ponds with fish.
Fish presence leads to low dragonfly larval and adult
abundances. Dragonfly abundance influences
pollinating insects, so there are fewer pollinators by
ponds without fish as a result of high dragonfly
populations. Dragonflies predate on pollinating
insects directly and pollinating insects avoid foraging
near adult dragonflies.
Dragonfly consuming
a pollinating bee
Knight et al. 2005
Clear example of interactions across ecosystem
types. Many implications for conservation.
1. Deliberate fish introductions might have cascading
effects on terrestrial systems, leading to increased
reproductive success of insect-pollinated plants.
2. Could change competitive relationships between
terrestrial plants, as insect-pollinated plants would be
at an advantage.
3. Destruction of wetlands decrease dragonfly populations,
with effects on terrestrial plants and their pollinators.
4. Decline in fish abundance (e.g. due to pollution) would
affect dragonflies, and hence pollinating insects and
plant reproduction.
Shows how predation in one system can affect another
system and how local interactions can have communityand landscape-level effects.
Nature is never simple!
Conclusions and Summary
1. An ecosystem is a biological community (or several
communities) plus all the abiotic factors influencing
the community or communities.
2. Life on earth depends on primary production by
plants.
3. Gross primary production (GPP) is the total amount of
energy fixed by all the autotrophs in the ecosystem.
4. Net primary production (NPP) is the amount of
energy left over after all autotrophs have respired
and met their energy needs.
5. Ecosystems contain a small number of trophic levels
and have a trophic structure.
6. GPP in terrestrial systems is about 2.7 x NPP, GPP in
oceans is about 1.5 x NPP.
7. Tropical forest has the highest global NPP.
8. Terrestrial primary production is generally limited
by temperature and moisture. Actual
evapotranspiration (AET) is positively correlated
with NPP in terrestrial systems. At finer scales
within biomes, soil fertility can be important. NPP
can vary from year to year.
9. Aquatic productivity is generally limited by nutrient
availability.
10. Oceans have low NPP.
11. Phosphorus concentrations usually limit rates of
primary production in freshwaters, whereas
nitrogen concentrations usually limit rates in
marine systems. Both systems can be limited
either by N or P. Depends on N:P ratio.
12. Consumers can influence rates of primary
production in aquatic and terrestrial systems.
13. Evidence for both bottom-up controls (abiotic) and
top-down controls (biotic).
14. Trophic-cascade hypothesis proposes that a
change in one trophic level may have a cascading
effect on the rest of the trophic structure.
15. There is evidence for trophic-cascade hypothesis
from both freshwater and terrestrial systems.
Marine systems appear to be bottom-up or topdown systems. Trophic cascades are a result of
many factors and interactions between factors.
16. There are some relationships between
NPP:Biomass ratios within major systems (forests,
other terrestrial systems, aquatic systems).
17. The number of trophic levels in an ecosystem is
determined by the energy loss from one trophic
level to the next level.
18. Trophic levels are a useful tool in understanding
ecosystem functioning, but like guilds, plant lifeforms, or plant functional-types, they are often
abstractions.
19. Different ecosystems differ in their transfer
efficiency between trophic levels.
20. Different ecosystems differ in the magnitude of
herbivores, decomposers, and organic export.
21. Carnivores play an important role in influencing
herbivore densities and hence grazing intensity
and plant NPP.
22. Nature is never simple, only demanding and
surprisingly complex!
EECRG Research Topics in this Lecture
Plant-animal interactions and their impact on
growth in alpine environments
Primary productivity changes at tree-line in the
Himalaya
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