Community ecology and dynamics

Report
BIO-201
ECOLOGY
2. Community Ecology and
Dynamics –
Succession and Stability
H.J.B. Birks
Community Ecology and Dynamics Succession and Stability
Some ecological and environmental basics
Succession Basic concepts
Primary succession on glacial forelands
Community changes
Ecosystem changes
Mechanisms of succession
Stability
Basic concepts
What causes resilience?
Alternative stable states and regime shifts
Maintenance dynamics
Disturbance and diversity
Community concepts revisited
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 16, 1 in part)
Bush, M. (2003) Ecology of a Changing Planet. Prentice Hall
(Chapters 15, 16)
Krebs, C.J. (2001) Ecology. Benjamin Cummings (Chapter 21)
Miller, G.T. (2004) Living in the Environment. Thomson (Chapter
8)
Molles, M.C. (2007) Ecology Concepts and Applications. McGrawHill (Chapter 20)
Ricklefs, R.E. & Miller, G.L. (2000) Ecology. W.H. Freeman
(Chapter 28)
Smith, R.L. & Smith, T.M. (2007) Ecology and Field Biology.
Benjamin Cummings (Chapters 21, 22)
Townsend, C.R. et al. (2008) Essentials of Ecology. Blackwell
(Chapters 9, 10)
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!
Some Ecological and Environmental
Basics
Environment varies continuously in
SPACE at all spatial scales (geology, soils,
climate, altitude, slope, etc.) and varies at
all TIME scales (days, months, seasons,
years, decades, centuries, millennia, etc.)
Broad spatial scale
Coastal chaparral
and scrub
Coniferous
forest
Desert
Coniferous
forest
Prairie
grassland
Deciduous
forest
Biomes
Role of
climate
Coastal
mountain
ranges
15,000 ft
10,000 ft
5,000 ft
Sierra
Nevada
Mountain
Great
American
Desert
Rocky
Mountains
Mississippi
Great
River Valley
Plains
Appalachian
Mountains
Average annual precipitation
100-125 cm (40-50 in.)
75-100 cm (30-40 in.)
50-75 cm (20-30 in.)
25-50 cm (10-20 in.)
below 25 cm (0-10 in.)
Long time scales
a) Change in temperature in the
North Sea over the past 65
million years (M yr).
b)The ancient continent of
Gondwanaland began to
break up about 150 M yr ago.
c) ~50 M yr ago distinctive
bands of vegetation had
developed.
d)By 32 M yr these are more
sharply defined.
e) By 10 M yr ago much of the
present geography of the
continents was established
but with different climates
and vegetation from today:
position of Antarctic ice cap is
schematic.
Changing continental positions in last 220 million years
Tectonic plates in constant motion. Environment on earth changes
accordingly.
1. Triassic
220 million years ago
Pangaea continent had
its maximum size. Large
interior areas, very dry
and extensive deserts.
2. Mid-Late Jurassic
155 million years ago
Beginnings of the breakup of Pangaea.
3. Late Jurassic
149 million years ago
Break-up of Pangaea,
large (100 m) rise in
sea-level, Siberia and
China now island
continents, Europe a
series of islands.
4. Early Cretaceous
127 million years ago
Break-up of
Gondwana.
5. Mid Cretaceous
106 million years ago
Europe still a series of
islands, North and
South America widely
separated.
6. Late Cretaceous
65 million years ago
Similar to today but
for North and South
America and India.
Cryogenian/
Cryogenian
Neoprotoerozoic
Cretaceous
Late
Silurian
Cambrian
Triassic
Devonian
Jurassic
Palaeogene
Triassic
Carboniferous
Neoproterozoic
IIIIII
Quaternary
Permian
Ordovician
At the same time, major changes in plant
evolution and hence in earth vegetation
Major evolutionary developments in last 500 million
years
Global ecological changes in the last 55 million years
1. Eocene
55 million years ago
Widespread tropical
rain-forest and no icecaps
2. Late Eocene
35 million years ago
Cooler, less tropical
rain-forest, some icecaps
3. Oligocene
25 million years ago
Cooler, more extensive
Antarctic ice-cap. Semiarid scrub and desert
areas, evolution of giant
land mammals
4. Miocene
3.2 million years ago
Continents almost in
today's position, ice-caps
at both poles, climate
drier, vast grasslands,
much mountain uplift
5. Late Pliocene
1-2 million years ago
Extensive polar icecaps, much reduced
tropical rain-forest
6. Pleistocene
30 000 years ago
Massive ice-sheets,
much tundra and arid
vegetation
Shorter time scales
years
102
Temperature changes in the Northern Hemisphere
at different time scales
years
105
103
5x105
104
Holocene
11500 years
Medieval optimum
LIA
Last millennium
LIA = Little Ice
Age
End of LIA
Past 130 years
Millennium scale: warm period 1000 AD and the
Little Ice Age
Medieval Warm
Period
LIA
Today’s Ecological
Scale
Biosphere
Biosphere
Biosphere
Biomes
Ecosystems
Ecosystems & Landscapes
COMMUNITIES
Communities
Species
Populations
Populations
Organisms
Organisms
Succession – Basic Concepts
1.Changing plant and animal communities,
ecosystems, and landscapes through time
following the creation of new substrates or
following disturbance, usually directional changes.
2.Primary succession – occurs on newly formed
surfaces such as volcanic lava flows, areas
recently deglaciated (glacial forelands), sanddunes along coast, etc.
3.Secondary succession – occurs where
disturbance destroys a community without
destroying the soil. Occurs after agricultural areas
are abandoned, after forest fires, forest clearance,
erosion, etc.
4. Successional change is usually directed towards the
undisturbed surrounding vegetation and fauna.
5. Succession generally ends with a mature
community whose populations are relatively stable.
'Climax vegetation'.
6.Environment is changing at a range of scales in
time and space, so communities are always in a
state of flux and change.
7. Successional time scales – can be short or long.
Few years; 250 years after the Little Ice Age; 10000
– 11500 years since the last glaciation.
8. Ecological succession “non-seasonal, directional, and
continuous pattern of colonisation and extinction on
a site by species populations” (Begon et al. 2006 p.479)
Primary succession
e.g. New surfaces formed by:
Glacier retreat
Volcanic eruption
Coastal sand-dunes
Exposed
rocks
Lichens
and mosses
Small herbs
and shrubs
Heath mat
Time
Jack pine,
black spruce,
and aspen
Balsam fir,
paper birch, and
white spruce
Secondary succession
e.g. Disturbance by:
Fire
Forest cutting
Erosion
Wind-throw & storms
Abandoned fields
Large herbivores e.g.
elephants
Mature oak-hickory forest
Annual
weeds
Perennial
weeds and
grasses
Young pine forest
Shrubs
Time
Differences between primary and
secondary succession
Primary succession: no soil, no seedbank, no organic matter
Secondary succession: soil is present but
disturbed, seed bank present, organic
matter present
Secondary succession is very common
within landscapes, primary succession is
less common
Primary Succession and Glacial
Forelands
Little Ice Age at about 1750 AD caused rapid advance of
glaciers in, for example, Jostedal and Jotunheimen.
As ice subsequently retreated, deposited glacial moraines
(silt, sand, gravel) on which primary succession could
begin.
Some classic studies mentioned in this lecture:
Nigardsbreen, Jostedalsbreen
- Knut Fægri
Storbreen, Jotunheimen
- John Matthews
Klutlan Glacier, Yukon
- John Birks
Glacier Bay, Alaska
- W. Cooper et al.
Surface ages determined by historical observations, from
the size of lichen (lav) thalli on rocks on the surface
('lichenometry'), and from annual growth rings of shrubs
and trees. Surfaces of different ages form a
CHRONOSEQUENCE.
Chronosequences – series of sites (e.g. glacier moraine
forelands, volcanic lava flows, sand dunes, recently formed
islands) of different but known age.
Study vegetation and soils today on surfaces of different but
known age.
Substitute space today for time – "space-for-time" substitution.
Glacier
Age of formation
Moraines
1930
1890
1850
soil pH
distance from glacier   Age 
1750
Nigardsbreen 'Little Ice
Age' moraine chronology
Knut Fægri
Photo: Bjørn Wold
Primary Succession after
Little Ice Age
Photo:
1984
1912-30
1815
Mature 1770
Betula
forest
1750
Mature
Betula
forest
Mature Alnus forest
Nigardsbreen, Jostedalsbreen
1874
1900
1931
1987
Nigardsbreen, Jostedalsbreen
2002
Vegetation changes since ice retreat
20 years
150 years
80 years
220 years
Styggedalsbreen, Jotunheimen
Distribution of selected species on Storbreen moraines
‘Pioneer’
r-selected
species
‘Late stage’
K-selected
species
Klutlan Glacier, Yukon
Moraines of
different
ages at the
terminus of
the Klutlan
Glacier
Pioneer plants on Moraine II
(2-5 yr) (Crepis nana)
Moraine II (10-30 yr)
Dryas drummondii mats (9-25 yr)
Moraine III (30-60 yr)
Moraine IV
(60-80 yr)
Moraine IV (95-180 yr) Moraine V (180-240 yr) Harris Creek (>250 yr)
Species
abundance
change with time
Changes in major
plant-growth
forms with time
Glacier Bay, Alaska
Phases
1. Pioneer phase – 20 years – Epilobium latifolium,
Dryas drummondii, Salix spp.
2. 30 years - Dryas mats with Alnus crispa, Salix,
Populus, and Picea
3. 40 years – Alnus forms dense thickets
4. 50-70 years – Picea and Populus grow above Alnus
5. 75-100 years – Picea forest with mosses
6. 200 years – Tsuga heterophylla & T. mertensiana
forest
7. >300 years – more open forest with areas of bogs
and tundra meadows
Some Glacier Bay pioneer species
Dryas
drummondii
Epilobium
latifolium
William S.
Cooper
1957
Little Ice Age
in Nepal
about 1850
2002
Little Ice Age
maximum
O.R. Vetaas
Terminal morainecomplex
Neoglacial stages
(> 1200 BP)
Little Ice Age
maximum
(app. 1850)
Glacier in
1957
1988
Glacier
fronts
2001
river
Glacial
lake
Gangapurna
North Nepal
stages since
1850 to
present
Lateral
moraine
stages
Lateral moraines with trees, Gangapurna, Nepal
Other Primary Successions
1. Coastal fore-dunes
2. Volcanic lava flows
Craters of the Moon, Idaho
Plant colonisation
Community Changes During Succession
1. Changes in plant abundance and species
composition in primary succession
Late invaders
Woody & long
lived species
pioneers
pioneers & late-invaders
TIME
Over time species invade, then increase, some decrease again
and disappear, and some remain as the mature vegetation
Early-succession
species
Late-succession
species
r-selected
K-selected
Many small offspring
Fewer, larger offspring
Far dispersed seeds
Short dispersed seeds
Early reproductive age
Later reproductive age
Most offspring die before
reaching reproductive age
Most offspring survive to
reproductive age
High population growth
rate (r)
Lower population growth
rate (r)
Adapted to low nitrogen
and high light
Adapted to higher nitrogen
and low light (shade)
Low ability to compete
High ability to compete
2. Changes in species richness in primary
succession over 1500 years
Species richness
Over longer time scales (> 2000 yr) richness
often declines. Why?
Successional time
3. Changes in plant growth forms in primary succession
Succession of plant growth forms at Glacier Bay
4. Changes in species richness in secondary
succession from 80 days to 200 years
Eastern N America –
abandoned fields, tree
colonisation and forest
development 200
years
Soil and buried seed
bank present at
the outset
Mature oak-hickory forest
Annual
weeds
Perennial
weeds and
grasses
Young pine forest
Shrubs
Time
Woody plant
species richness
Number of breeding
bird species
Rocky coastal shores: 18 months
Number of macroinvertebrate and macroalgae
species during secondary succession
Rivers after extreme floods: 80 days
Algal species diversity during secondary succession
5. Species replacement during secondary succession
Henry Horn – predictive model for changes in tree
composition given
(1) for each tree species, probability that within a
particular time, an individual would be replaced by
another of the same species or by a different species
(2) an assumed initial species composition
Horn argued that the proportional representation of
various series of saplings established beneath an
adult tree reflects the probability of that tree’s
replacement by the species represented by the
saplings.
Using this, Horn estimated probability after 50 years
of a site occupied by a given species will be replaced
by another species or will still be occupied by same
species in a forest in New Jersey, USA
Betula populifolia
Nyssa sylvatica
Acer rubrum
Fagus grandifolia
A 50-year tree-by-tree transition matrix from Horn (1981),
showing the probability of replacement of one individual by
another of the same or different species 50 years hence.
Using so-called Markov chain model, predicted
compositional change over 200 years (and to ∞!)
See initial Betula, then Acer rubrum, then Fagus
dominance.
Assumes that transition probabilities from time1 to
time2 are constant in space and time and not affected
by historical factors such as initial biotic conditions
and arrival of species
Secondary Succession
SEED BANK
'Late invaders'
Woody & long
lived species
pioneers & 'late-invaders'
pioneers
TIME
Time after disturbance: species invade, then increase, some
decrease again and disappear, and some remain as part of the
mature vegetation
In secondary succession after disturbance, two very
different kinds of response according to the
competitive relationships shown by the species
involved.
Founder-controlled – occurs if large number of
species are approximately equivalent in their ability
to colonise an opening following disturbance, are
equally well fitted to the abiotic environment, and
can hold their space until they die. Result of
disturbance is essentially a LOTTERY. Winner is
species that happens to reach and establish itself
first.
Dominance-controlled – occurs when some
species are competitively superior (e.g. grow taller,
grow faster) to others so that the initial colonisers
of an opening do not necessarily maintain their
presence there. Result is a reasonably
PREDICTIVE SEQUENCE of species because
different species have different strategies for
exploiting resources. r-selected species are good
colonisers and fast growers, whereas later species
can tolerate lower resource levels and grow to
maturity in presence of early pioneer species and
eventually out-compete them.
Secondary succession tends to be a mixture of
both kinds of response.
Ecosystem Changes During Succession
1. Changes in biomass and production
PRIMARY SUCCESSION
BIOMASS
NET PRIMARY PRODUCTION
RESPIRATION
TIME
SECONDARY SUCCESSION
BIOMASS
NET PRIMARY PRODUCTION
RESPIRATION
TIME
Primary succession
Species richness
Biomass
Exposed
Lichens
rocks
and mosses
Small herbs
and shrubs
Heath mat
Jack pine,
black spruce,
and aspen
Time
Balsam fir,
paper birch, &
white spruce
climax community
Secondary succession
Species richness
Biomass
Mature oak-hickory forest
Annual
weeds
Perennial
weeds and
grasses
Young pine forest
Shrubs
Time
Biomass accumulation model in secondary
succession (102 – 103 years)
Biomass during stream secondary
succession (60 days)
2. Changes in soil during succession
Soil building
during primary
succession at
Glacier Bay
Changes in soil properties during primary
succession at Glacier Bay
Changes in soil development nitrogen, pH,
cations, organic matter
Time after fire:
secondary succession
Organic
matter
Nitrogen
pH, cations: Mg & Ca
TIME
3. Changes in biomass and soil over very long
time scales
Hawaiian Islands – volcanic lava flows of
different ages extending back to 4.1 million
years.
Studied vegetation succession and soil
changes, especially soil nitrogen and soil
phosphorus.
Organic carbon and total
nitrogen content of soils
developing on lava flows
P limitation on oldest soils
Total phosphorus & percentages
of total P in weatherable and
refractory (unavailable) forms
in soils developing on lava flows
Nitrogen and phosphorus loss rates
from soils developing on lava flows
Biomass changes
Why?
Primary succession
Recent study on six long chronosequences to
investigate reasons for decline in biomass over
long time periods.
Wardle et al. 2004 Science 305: 509-513
Birks & Birks 2004 Science 305: 484-485
Six chronosequences
Duration (yrs)
Cooloola, Australia
Sand dunes
>600,000
Arjeplog, Sweden
Islands
Glacier Bay, Alaska
Moraines
Hawaii
Lava flows
Franz Josef, New Zealand
Moraines
>22,000
Waitutu, New Zealand
Marine terraces
600,000
6,000
14,000
4,100,000
Maximal phase
Retrogressive phase
Cooloola,
Australia
Arjeplog,
Sweden
Glacier Bay,
Alaska
Maximal phase
Retrogressive phase
Hawaii
Franz Josef,
New Zealand
Waitutu,
New Zealand
Tree basal area
– unimodal or
decreasing
response with
age
Measured C:N, C:P, and N:P ratios for humus and litter
Significant increases in N:P and C:P ratios with age and
forest retrogression
Soil changes:
In the transition from the maximal forest
biomass phase to the retrogressive phase, P
becomes more limiting relative to N and P
concentrations decline in the litter.
N is biologically renewable but P is not, as P
is leached and bound in weathered soils.
Over time, P becomes depleted and less
available, relative to N.
Other ecosystem properties:
Also reduced rates of litter decomposition and release
of P from litter and decreased activity of microbial
decomposers.
Proportion of fungi relative to bacteria increases.
Fungal-based food webs retain nutrients better than
bacterial-based food webs.
Nutrient cycling thus becomes more closed & essential
nutrients, especially P, become less available.
Summary: Long-term decline in biomass is
accompanied by increasing P limitation relative to N,
reduced rates of P release from decomposing litter,
and reductions in litter decomposition, soil respiration,
microbial biomass, and ratio of bacterial to fungal
biomass.
Primary and secondary succession in a range
of environments and time scales produce
(1) changes in species composition and
diversity
(2) changes in the structure and function of
ecosystems.
What mechanisms drive succession?
Mechanisms of Succession
Three mechanistic models – Connell & Slatyer (1977)
1. Facilitation – pioneer species modify environment
with time, becomes less suitable for them, and
new species invade.
2. Tolerance – initial colonisation by all species,
those tolerant of initial conditions become
abundant, then species tolerant of new conditions
become abundant.
3. Inhibition – initial colonisation by all species, but
some species make the environment less suitable
for other species, i.e. early arrivals inhibit
colonisation by later arrivals.
Alternative successional mechanisms
Intertidal successions
Inhibition of later
successional species
Survivorship of successional
species under conditions of
low tides in hot afternoons
Support for inhibition
by Ulva
Facilitation by algae of colonisation in intertidal
succession of surfgrass, Phyllospadix scouleri
Mt St Helens, Washington. Erupted 1980,
created vast new volcanic lava fields.
Common pioneer plants
1. Anaphalis margaritacea,
Epilobium angustifolium
– many wind-dispersed
small seeds
2. Lupinus lepidus –
few large seeds,
fixes atmospheric
nitrogen
Lupinus lepidus
Experiments provide evidence for both
inhibition and facilitation models
Lessons from the 25 years of ecological
change at Mount St. Helens
1. Succession is very complex, occurring at different rates along
different pathways with periodic setbacks through secondary
disturbances (e.g. landslides, mudflows).
2. No single over-arching model of succession provides an
adequate framework to explain the observed changes.
3. Chance factors (e.g. timing of the disturbance at various
spatial and temporal scales) have strongly influenced survival
and successional patterns and pathways.
4. Lakes & most streams largely returned to their pre-1980 state.
5. In contrast, terrestrial vegetation still a mosaic of open areas
on steep slopes and eroding sites and well-vegetated areas
with shrubs and surviving trees on stable sites.
6. Almost all small mammals have returned but birds have not,
possibly because of the lack of extensive forest with vertical
structure (niches).
7. Rate of change determined by a complex of factors – position
in the landscape, local topography, climate, biotic factors,
human factors, and chance.
Primary Succession on Glacial Forelands
Inhibition and facilitation of spruce at Glacier Bay
Net
effect:
I
I&F
F
I
Evidence for both inhibition and facilitation
Are the facilitation,
inhibition, and
tolerance models
useful?
1. Nature is very
complex – three
mechanistic models
are probably a
great oversimplification.
2. Real-life situation
probably more
complex.
3. General models may
not be appropriate for
a major ecological
process such as
succession that
consists of a large
number of different
ecological process –
seed arrival, seed
bank, competition,
herbivory, chance, etc.
C = colonisation
M = maturation
S = senescence
Despite this undoubted complexity of succession, further
mechanisms underlying succession have been proposed
Begon et al. (2006) Chapter 16, pp.483-487
1) Competition-colonisation trade-off and
successional niche mechanisms
Early-successional plants have several correlated traits
high fecundity
effective dispersal
rapid growth rate when resources are abundant
poor growth rate when resources are scarce
Late-successional plants usually have opposite traits
In absence of disturbance, late-successional plants will outcompete early species because they reduce resources (light,
water, nutrients) beneath the levels required by earlysuccessional species
Early species persist because
(1) their dispersal ability and high fecundity
permit colonisation and establishment in
recently disturbed sites
(2) their rapid growth under resource-rich
conditions allows them to out-compete
temporarily late-successional species even if
they arrive at same time
(1) = competition-colonisation trade-off
(2) = successional niche (early conditions favour
early species because of their niche
requirements)
Some Revision!
One- and two-dimensional niches
Population
density
Population
density
temperature
Feeding resource
Feeding resource
temperature
In reality, niche is multi-dimensional
Realised versus fundamental niche
Fundamental niche
= only environment
Realised niche
Biotic control
Broad and narrow niches
Generalist species
Specialist species
2) Resource-ratio hypothesis – David Tilman
Rate of changing relative competitive abilities of
plant species as conditions slowly change with time.
Species dominance in any point in succession
strongly influenced by the relative ability to capture
two resources – LIGHT and available SOIL
NITROGEN.
Early in succession, the habitat has low N but high
light. Nitrogen availability increases with time but
light availability decreases with time as biomass
increases with time.
Requirements
Species
Light
N
A
+++
(+)
B
+++
+
C
++
++
D
+
+++
E
+
+++
Tilman’s resource-ratio hypothesis of succession
3) Vital attributes (Noble & Slatyer 1981)
Vital attributes relate to
(1) recovery after disturbance (V = vegetative
spread; S = seedling from abundant seedbank in
soil; D = dispersal; N = no special dispersal
and/or small seedbank)
(2) ability to reproduce in face of competition (T =
high tolerance; I = intolerance)
Species then classified on basis of vital attributes
e.g. pioneer Ambrosia artemisiifolia
late Fagus grandifolia
SI
VT or NT
4) r and K-selection
Certain attributes are likely to occur together more
often than by chance, as expected from an
evolutionary perspective.
Two alternatives that increase fitness of a species in
a succession
(1) avoids competition, high reproduction, good
dispersal, r-selection
(2) tolerant of competition or highly competitive,
low reproduction, poor dispersal, K-selection
r-selection
K-selection
Concept of ‘climax’
Do successions come to an end?
Frederic Clements (1916) single dominant climax in a
given climatic region – Monoclimax view
Arthur Tansley (1939) local climax governed by soil,
climate, topography, land-use, history, fire –
Polyclimax view
Robert Whittaker (1953) - climax-pattern view.
Continuum of climax types varying along
environmental gradients, not necessarily separable into
discrete climaxes.
However, environment is constantly varying at all
spatial and temporal scales, so idealised climax is
probably never reached in nature, nor is it attainable.
Community and Ecosystem
Stability - Basics
1. Stability – absence of change. May be stable for
several reasons (e.g. absence of disturbance,
constant environment).
2. In reality, communities and ecosystems are always
changing because of changing environment and biotic
interactions that may change as organisms age.
3. Stability – ability of community or ecosystem to
maintain structure and/or function in the face of
potential disturbance.
4. Stability may result from the ability of a community
to return to its original state after a disturbance –
'resilience'.
What Causes Resilience?
Succession is the basis for resilience.
Some systems change more quickly than others.
Depends on many factors – climate, soils,
available species pool, severity of disturbance,
etc.
Require long-term direct observations to study
stability and resilience. These are very rare.
Chronosequence is not the same because in the
substitution of space for time we assume that the
environment has not changed with time.
Park Grass Experiment,
Rothamsted Experimental Station
Started 1856-1872 to investigate effects
of fertiliser treatments on grasslands.
Run for over 150 years.
Monitored since 1862.
Shows virtually no new species colonised
since 1862.
1910 – 1948
Three
treatments
Proportions
changed from
year to year
(annual rainfall)
but relatively
stable
proportions in
the three
treatments
What about individual species?
Patterns of
species
abundance in
60 years
Are the Park Grass plots stable or not?
1. Yes, at a very coarse scale – started as a
grassland and stayed as a grassland with no
new species.
2. Yes, at a less coarse scale of grasses,
legumes, and other species but some
variation from year to year.
3. No, at the scale of individual species.
Are there stable natural communities?
Answer dependent on the scale of interest
Environment is changing constantly at a range of scales
Temperature changes in
the Northern Hemisphere
at different time scales
Sonoran Desert, Mexico
Saguaro cactus
1959
1984
Repeat photography
1998
Changes in populations of creosote bushes and
saguaro cactus due to major drought in 1960s
Alternative Stable States and
Regime Shifts
Common idea in ecology is of populations and
communities fluctuating around some trend or stable
average.
Can be an abrupt shift to a dramatically different
regime.
Norfolk Broads, England – shallow freshwater lakes
showing a rapid regime shift from dominance of
aquatic macrophyte plants to a dominance of
phytoplankton algae. Regime shift is a result of the
use of TBT paint on boats and its toxic effects on
gastropod mollusca that graze algae on aquatic
plants. (See Lecture 5 Long-term Ecology)
Saharan desert – gradually declining trend
of vegetation cover from 9000 to 5500 years
ago, then a sudden collapse into desert.
Changes in sand and silt content in a
sediment core near the west African coast
Coral reefs –
very high
biodiversity
Caribbean coral reefs – sudden dramatic shift of
reefs into an algal encrusted state.
Increased nutrient loading as a result of changing
land-use promoted algal growth, but this effect
did not show as long as herbivorous fish
suppressed the algae.
Intensive fishing reduced the fish population and
in response the sea-urchin Diadema antilliarum
became dominant and became the key herbivore.
When a pathogen killed the dense Diadema seaurchin population, algae were released from
herbivore control, and the coral reefs became
overgrown rapidly.
Different grazers at different spatial scales
Other examples of dramatic regime shifts:
1. Savannah that is rapidly encroached by shrubs
2. Lakes that shift from clear water to turbid
water
3. Standing waters that can suddenly be
overgrown by floating plants
4. Different populations in open ocean suddenly
change to different abundances synchronously
Alternate stable states – How can they occur?
Although plants compete for resources, this
competition can be overruled by facilitation
because the vegetation ameliorates certain critical
conditions.
Terrestrial vegetation in dry regions can enhance
soil moisture and microclimatic conditions.
Leads to positive
feedback between
vegetation and
moisture
1. Precipitation in absence of vegetation is determined
by climate
2. Vegetation has a positive feedback on local rainfall
3. No vegetation when precipitation falls below critical
level
Actual precipitation can be drawn as two
different functions of global climate; one
without vegetation, one with vegetation.
Above the critical level, vegetation is present.
Below the critical level, vegetation is absent.
If general climate gets wetter, only the plant
regime exists. If very dry, regime of no
vegetation.
Over a range of climatic conditions, two
alternative stable states or regimes can
exist.
Instability between Fc and Fd
Shallow freshwater lakes and two
alternative stable states
Stability landscapes
showing resilience
of equilibria
Ball (state of ecosystem) tends to settle in
'valleys' = stable regime state.
'Hill' between the 'valleys' is barrier
between two alternative states or regimes.
Changes in external conditions can change
the stability landscape by changing the
depth of the 'valleys' and the height of the
'hill'.
Plantdominated
state
Macrophyte-dominated
system pre-1960
Use of TBT in
boat paints 1960
Decrease of mollusca (gastropods, etc.)
Increase in algae
Algaedominated
state
Reduction in grazing
of epiphytic algae
Decline of macrophytes
Algae-dominated
See Lecture 5
Long-term Ecology
for details
Plant-dominated
Nutrient level
1 & 4 - alternate states,
2 - causes of change
3 - triggers of resilience and regime shifts
Reduced resilience makes the system vulnerable to a
regime shift
(a)
Resilience of the low P input state is high as the
likelihood of crossing the threshold from one state to
another is low (big distance between the two states).
Resilience of the high P input system is low as the
likelihood of crossing the threshold from one state to
another is high (low distance between the two
states).
Evidence from field data
(a) Pacific Ocean
(b) Dutch ditches
(c) Shade in shallow lakes
 = dominated by cyanobacteria
 = dominated by other algae
Alternative stable states – can they be
predicted?
Beaugrand et al. 2008 Ecol Letters 11: 1157-1168
North Atlantic – critical thermal boundary where a
small increase in temperature triggers abrupt
ecosystem shifts across multiple trophic levels.
Boundary is located
where abrupt shifts
occur.
All closely related to
annual sea-surface
temperature (SST).
Critical at 9-10°C,
establishment of
Westerly winds marine
system.
Beaugrand et al. 2008
Beaugrand
et al. 2008
Decadal changes in SST 1960-2005 and
predicted changes in 2090-2100
Small changes in last 40 years
Ecosystem state shifts between 1986 and 1988, preceded by a
period of high ecosystem variability
Pre-1981, 72% of cells have SST of 9-10°C; post-1988, 20%
Major shift in SST affecting many aspects of ecosystem. Shift
predicted by increasing variance in biological systems
What of the future?
Beaugrand
et al. 2008
Two future climate scenarios: progressive shift northwards
from 2000 to 2090
Climate changes in SST will alter biodiversity and carrying
capacity of ecosystems.
Changes will precipitate major reduction in stocks of Atlantic
cod, already severely impacted by exploitation from fishing.
Relatively small climatic change may ‘tip the balance’ in an
already over-exploited ecosystem (reduced resilience)
Summary of Alternative Stable States
1. System has alternative states if there can be
more than one 'stable state' for the same
external variable (e.g. nutrients in lakes).
2. Stable states are really dynamic regimes.
Show slow trends, natural population
fluctuations due to climate and internal
population dynamics.
3.Multiple causes are the rule in regime shifts.
4. Patterns depend on spatial scale. May have a
mosaic of alternative stable states. May remain
unaltered until an extreme event triggers a shift
in the patterns.
5. External conditions should really be external
and independent and not an interactive part
of the system.
6. External conditions may be affected by the
system if the change in external conditions is
very slow relative to the natural rates of
change in the system.
Collapse of vegetation in the Sahara occurred
over 100-200 years but this is fast compared
with the forcing function, namely gradual
changes in the Earth's orbit.
7. In some systems, fast and slow components
can affect each other mutually and this leads
to population cycles (e.g. recurrent pest
outbreaks).
8.Resilience is necessary to sustain desirable
ecosystem states in variable environments and
uncertain futures.
9. Humanity has drastically altered the capacity of
ecosystems to withstand or 'buffer' disturbance.
Cannot assume that there will be a sustained flow of
ecosystem 'services' or functions to our well-being.
10.Biological diversity appears to enhance the
resilience of ecosystem states
11."Nature is not fragile … what is fragile are the
ecosystem 'services' on which humans depend"
Simon Levin (1999)
What causes natural population fluctuations, the
fluctuations around some mean in one 'stable state'?
Maintenance Dynamics
Even if the environment is stable (which it never is!),
there are factors INTERNAL to the community that
cause change, so-called 'cyclic' succession.
Cycle of events replicated many times over the whole
of the community as a series of PHASES. Provides a
mosaic of phases within community. PATCH
DYNAMICS
Succession is a directional change
Cyclic changes or maintenance dynamics
or patch dynamics are fluctuations
about a mean value.
A.S. Watt ‘Pattern and Process’ 1947
Dr Alex ‘Sandy’ Watt
productivity
Phases in plant growth with age
age
pioneer building
mature
degenerate
Phases in growth of Festuca ovina
Changes in cover of three species 1936-1973
(F. = Festuca, H. = Hieracium, T. = Thymus)
Important factors in maintenance or patch dynamics
1. Disturbance (or ageing)  gaps
2. Dispersal  recruitment  growth
3. Frequency of gap formation
4. Size and shape of gaps
View landscape as patchy with disturbance and
recolonisation by individuals of different species
Critical roles for disturbance (and ageing) as a
RESET mechanism, for dispersal and establishment
between habitat patches, and competition between
species concerned
Community dynamics need a landscape-scale
perspective to be understandable
Fire: control of secondary succession in west
Norwegian coastal heathlands
FIRE!
BRANN!
Bjørk og fufu skog ( Eik)
Calluna
spirer
+ urter og
gress
forveet
calluna
trær
Time
Fire also important in community maintenance
dynamics – fine-scale burning
Burnt versus unburnt heath
Mosaic of burning phases
Maintenance dynamics of Calluna (røsslyng) in
coastal heathlands involving fire
Traditional heathland cycle
Dereliction
degenerate
mature
building
pioneer
Combination of controlled burning, mowing, & grazing
'Cultural landscape'
Disturbance and Diversity
Disturbance resets the clock in any succession. Elimination of
existing populations, allows colonisation by early successional
species - frequency of disturbance critical.
a)high frequency of
disturbance, pioneers only
b)intermediate disturbance,
pioneers plus later
species, giving maximum
diversity
c)low disturbance, late
species only
Result is hump-backed curve of diversity in relation to disturbance
'intermediate disturbance hypothesis'
Hypothesis formulated in relation to successional responses after
disturbance.
Community Concepts Revisited
Palaeoecology – study of the distribution & abundance
of organisms (plants and animals) in the past.
Pollen analysis – major technique.
Last glaciation
about 18000 years
ago and
subsequent
deglaciation at
about 11000 years
ago were a major,
broad-scale primary
succession.
Extent of glacial ice at 18000 and 8000 years ago
Large number of sites where pollen analysis has been
done. Can determine when a particular tree arrived
and expanded at a site and then map the times of tree
arrival to detect tree migration patterns since the last
deglaciation.
Each tree genus
has its own
individualistic
history. Did not
move as forest
communities.
Same in the British Isles – strongly individualistic
behaviour of forest trees
Bjørk
Hassel
Eik
Alm
Furu
Lind
Organismal concept – F.E. Clements
Individualistic concept – H.A. Gleason
In fact these two concepts refer to different
scales and biological concepts – no real
conflict!
Organismal concept is a spatial concept
Individualistic concept is a population
concept
4 species populations along an environmental gradient (vertical
plot)
4 species along a geographical or spatial gradient (horizontal
gradient)
Can recognise several communities along spatial gradient – A,
A+B, B, C, D, and transitions B+C and C+D
Great Smoky Mountains, Eastern USA
Robert H. Whittaker
Landscape distribution of
vegetation types
Spatial arrangement of
vegetation types
Landscape or spatial
distribution of vegetation
types – organismal concept
Environmental distribution of
populations – individualistic
concept
Community structure is thus the product of a
complex interaction of pattern and process in
space and time.
Each species responds to a wide range of
environmental factors that vary continuously in
space and time across the landscape.
Interactions between organisms influence the
nature of these responses.
The end result is a dynamic mosaic of
communities within the landscape.
Study of this mosaic at the landscape scale is
landscape ecology (see Lecture 7 on Landscape and
Geographical Ecology).
Conclusions and Summary
1. Succession is the gradual, directional change in
plant and animal communities in an area following
the creation of new substrates (primary succession)
or disturbance (secondary succession).
2. Succession generally ends with a mature community
that is similar to the surrounding vegetation and
fauna and that has relatively stable populations
('climax' vegetation).
3. Environment varies at a wide range of temporal and
spatial scales.
4. Primary succession has been studied in detail on
glacial forelands in western North America and
Norway. Moraines of different but known ages
provide a chronosequence.
5. Plant abundance, species composition, and species
richness change over time. Richness increases and
then often declines with time.
6. Ecosystem changes during succession include
increases in biomass, primary production, soil
composition, and nutrient retention. Phosphorus
limitation becomes more important in 'old' systems.
7. Mechanisms to explain succession include
facilitation, tolerance, and inhibition.
8. Field evidence provides support for facilitation,
inhibition, or a combination of the two.
9. Nature is more complex than 3 mechanistic models.
Succession is a combination of many different
ecological processes –germination, herbivory,
competition, chance, etc.
10. Community stability may be due to a lack of
disturbance or community resistance ('resilience')
to disturbance.
11. Communities are both stable and unstable,
depending on scales of study. Alternative states
can exist and catastrophic regime shifts can occur.
12. Within-community maintenance dynamics or patch
dynamics ('cyclic' changes) are what makes a
community maintain itself.
13. Human activity can prevent secondary succession
and can influence maintenance dynamics, to create
so-called cultural landscapes.
14. Succession occurs over a wide range of time scales
ranging from days, months, centuries, to millions
of years. Basis of ecological change.
15. Palaeoecological data indicate that forest trees
showed individualistic behaviour in their
migration patterns after the last deglaciation.
16. The community is a spatial concept. The
individualistic continuum is a population concept.
17. The real world lies between the organismal and
individualistic concepts, depending on our spatial
and temporal scales of study and on our choice
of gradient (spatial, environmental).
18. Vegetation at the landscape scale is a mosaic
depending on topography, environment, primary
succession, secondary succession, and
maintenance dynamics.
EECRG Research Topics in this Lecture
Primary succession on glacial forelands in Norway,
Nepal, and Tibet
Alternative stable states in Norwegian forest
vegetation
Natural climatic variability in NW Europe in the last
15000 years
Tree migration patterns in the last 12000 years
Ordination gradient analysis of many different
vegetational and faunal communities
Heathland ecology, management, and dynamics in
western Norway
www.eecrg.uib.no

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