2. Community Ecology and Dynamics –

Succession and Stability

H.J.B. Birks

BIO-201 ECOLOGY

Community Ecology and Dynamics - Succession and Stability

Some ecological and environmental basicsSuccession Basic concepts

Primary succession on glacial forelandsCommunity changesEcosystem changesMechanisms of succession

Stability Basic conceptsWhat causes resilience?Alternative stable states and regime shiftsMaintenance dynamicsDisturbance and diversity

Community concepts revisitedConclusions 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. McGraw-Hill (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

15,000 ft10,000 ft5,000 ft

Coastalmountainranges

SierraNevadaMountain

GreatAmericanDesert

RockyMountains

GreatPlains

MississippiRiver Valley

AppalachianMountains

Coastal chaparraland scrub

DesertConiferousforest

Coniferousforest

Prairiegrassland

Deciduousforest

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.)

Biomes

Role of climate

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. Triassic220 million years ago

Pangaea continent had its maximum size. Large interior areas, very dry and extensive deserts.

2. Mid-Late Jurassic155 million years ago

Beginnings of the break-up of Pangaea.

3. Late Jurassic149 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 Cretaceous127 million years ago

Break-up of Gondwana.

5. Mid Cretaceous106 million years ago

Europe still a series of islands, North and South America widely separated.

6. Late Cretaceous65 million years ago

Similar to today but for North and South America and India.

CryogenianCryogenianCryogenian/Neoproterozoic IIINeoprotoerozoic IIINeoproterozoic IIICambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicLate TriassicJurassicCretaceousPalaeogeneQuaternary

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. Eocene55 million years ago

Widespread tropical rain-forest and no ice-caps

2. Late Eocene35 million years ago

Cooler, less tropical rain-forest, some ice-caps

3. Oligocene25 million years ago

Cooler, more extensive Antarctic ice-cap. Semi-arid scrub and desert areas, evolution of giant land mammals

4. Miocene3.2 million years ago

Continents almost in today's position, ice-caps at both poles, climate drier, vast grasslands, much mountain uplift

5. Late Pliocene1-2 million years ago

Extensive polar ice-caps, much reduced tropical rain-forest

6. Pleistocene30 000 years ago

Massive ice-sheets, much tundra and arid vegetation

Temperature changes in the Northern Hemisphere at different time scales

Shorter time scales

102

103

104

105

5x105

years years

End of LIA

Medieval optimum LIA

Holocene

11500 years

Last millennium

LIA = Little Ice Age

Past 130 years

Medieval Warm Period

LIA

Millennium scale: warm period 1000 AD and the Little Ice Age

BiosphereBiosphere

Ecosystems

Communities

Populations

Organisms

Biosphere

Biomes

Ecosystems & Landscapes

COMMUNITIES

Species

Populations

Organisms

Today’s Ecological Scale

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), sand-dunes 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

Time

Small herbsand shrubs

Heath mat

Jack pine,black spruce,

and aspen

Balsam fir,paper birch, and

white spruce

Exposedrocks

Lichensand mosses

e.g. New surfaces formed by:

Glacier retreatVolcanic eruptionCoastal sand-dunes

Secondary succession

Time

Annualweeds

Perennialweeds and

grasses

ShrubsYoung pine forest

Mature oak-hickory forest

e.g. Disturbance by:FireForest cuttingErosionWind-throw & storms Abandoned fieldsLarge herbivores e.g. elephants

Differences between primary and secondary succession

Primary succession: no soil, no seed-bank, 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 FægriStorbreen, Jotunheimen - John MatthewsKlutlan Glacier, Yukon - John BirksGlacier 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 Moraines

Age of formation

1930

1890

1850

1750

soil pH

distance from glacier Age

Nigardsbreen 'Little Ice Age' moraine chronology

Photo: Bjørn Wold

Knut Fægri

1750

1770

1815

1912-30

Mature Alnus forest

Mature Betula forestMature

Betula forest

Photo: 1984

Primary Succession after Little Ice Age

Nigardsbreen, Jostedalsbreen

1874 1931

1900 1987

Nigardsbreen, Jostedalsbreen

2002

Vegetation changes since ice retreat

20 years 80 years

150 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)

Dryas drummondii mats (9-25 yr)

Moraine II (10-30 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

2002

Little Ice Age maximum

Little Ice Age in Nepal about 1850

O.R. Vetaas

river

Glacial lake

Lateral moraine stages

Terminal moraine-complex

Little Ice Age maximum (app. 1850)

Neoglacial stages (> 1200 BP)

1988

2001

Glacier fronts

Glacier in 1957

Gangapurna North Nepal stages since 1850 to present

Lateral moraines with trees, Gangapurna, Nepal

Other Primary Successions

1. Coastal fore-dunes

2. Volcanic lava flows

Craters of the Moon, Idaho

Plant colonisation

Over time species invade, then increase, some decrease again and disappear, and some remain as the mature vegetation

pioneerspioneers & late-invaders

Late invaders

Woody & long lived species

TIME

1. Changes in plant abundance and species composition in primary succession

Community Changes During Succession

Late-succession species

K-selected

Fewer, larger offspring

Short dispersed seeds

Later reproductive age

Most offspring survive to reproductive age

Lower population growth rate (r)

Adapted to higher nitrogen and low light (shade)

High ability to compete

Early-succession species

r-selected

Many small offspring

Far dispersed seeds

Early reproductive age

Most offspring die before reaching reproductive age

High population growth rate (r)

Adapted to low nitrogen and high light

Low ability to compete

2. Changes in species richness in primary succession over 1500 years

Over longer time scales (> 2000 yr) richness often declines. Why?

Successional time

Speci

es

rich

ness

Succession of plant growth forms at Glacier Bay

3. Changes in plant growth forms in primary succession

Time

Annualweeds

Perennialweeds and

grasses

ShrubsYoung pine forest

Mature oak-hickory forest

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 yearsSoil and buried seed bank present at the outset

Number of breeding bird species

Woody plant species richness

Number of macroinvertebrate and macroalgae species during secondary succession

Rocky coastal shores: 18 months

Algal species diversity during secondary succession

Rivers after extreme floods: 80 days

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

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.

Betula populifoliaNyssa sylvaticaAcer rubrumFagus grandifolia

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

Time after disturbance: species invade, then increase, some decrease again and disappear, and some remain as part of the mature vegetation

pioneerspioneers & 'late-invaders'

'Late invaders'

Woody & long lived species

TIME

SEED BANK

Secondary Succession

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.

RESPIRATION

NET PRIMARY PRODUCTION

BIOMASS

TIME

Ecosystem Changes During Succession

1. Changes in biomass and production

PRIMARY SUCCESSION

RESPIRATION

NET PRIMARY PRODUCTION

BIOMASS

TIME

SECONDARY SUCCESSION

Primary succession

Small herbsand shrubs

Heath mat

Jack pine,black spruce,

and aspen

Balsam fir,paper birch, &white spruce

climax community

Exposedrocks

Lichensand mosses

Species richness

Biomass

Time

Secondary succession

Annualweeds

Perennialweeds and

grasses

ShrubsYoung pine forest

Mature oak-hickory forest

Species richness

Time

Biomass

Biomass accumulation model in secondary succession (102 – 103 years)

Biomass during stream secondary succession (60 days)

Soil building during primary succession at Glacier Bay

2. Changes in soil during succession

Changes in soil properties during primary succession at Glacier Bay

Changes in soil development nitrogen, pH, cations, organic matter

Nitrogen

pH, cations: Mg & Ca

TIME

Time after fire: secondary succession

Organic matter

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 Total phosphorus &

percentages of total P in weatherable and refractory (unavailable) forms in soils developing on lava flows

P limitation on oldest soils

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 6,000

Glacier Bay, Alaska Moraines 14,000

Hawaii Lava flows 4,100,000

Franz Josef, New Zealand Moraines >22,000

Waitutu, New Zealand Marine terraces 600,000

Maxim

al

ph

ase

Retr

og

ress

ive p

hase

Cooloola, Australia

Arjeplog, Sweden

Glacier Bay, Alaska

Maxim

al

ph

ase

Retr

og

ress

ive p

hase

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.

(1) changes in species composition and diversity

(2) changes in the structure and function of ecosystems.

What mechanisms drive succession?

Primary and secondary succession in a range of environments and time scales produce

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

Inhibition of later successional species

Survivorship of successional species under conditions of low tides in hot afternoons

Intertidal successionsSupport 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.

Inhibition and facilitation of spruce at Glacier Bay

Primary Succession on Glacial Forelands

Evidence for both inhibition and facilitation

Net I I & F F I effect:

Are the facilitation, inhibition, and tolerance models useful?

1. Nature is very complex – three mechanistic models are probably a great over-simplification.

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 dispersalrapid growth rate when resources are abundantpoor growth rate when resources are scarce

Late-successional plants usually have opposite traits

In absence of disturbance, late-successional plants will out- compete early species because they reduce resources (light, water, nutrients) beneath the levels required by early-successional 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)

Population density

Population density

temperature

Feeding resource

Feeding resource

temperature

Some Revision!One- and two-dimensional niches

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 SI

late Fagus grandifolia 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'.

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.

What Causes Resilience?

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

Patterns of species abundance in 60 years

What about individual species?

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

1998

Repeat photography

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 sea-urchin 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'.

Nutrient level

Algae-dominated

Plant-dominated

Macrophyte-dominated system

pre-1960

Use of TBT in boat paints

1960

Plant-dominated

state

Decrease of mollusca (gastropods, etc.)

Increase in algae

Decline of macrophytes

Reduction in grazing of

epiphytic algae

Algae-dominated

state

See Lecture 5 Long-term Ecology for details

1 & 4 - alternate states, 2 - causes of change3 - triggers of resilience and regime shifts

Reduced resilience makes the system vulnerable to a regime shift

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).

(a)

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

Decadal changes in SST 1960-2005 and predicted changes in 2090-2100

Small changes in last 40 years

Beaugrand et al. 2008

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?

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)

Beaugrand et al. 2008

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’ 1947Dr Alex ‘Sandy’ Watt

Phases in plant growth with age

pioneer

building

mature degenerate

age

pro

du

ctiv

ity

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

Time

Calluna spirer

+ urter og gress

forveet calluna

trær

Bjørk og fufu skog ( Eik)

FIRE!BRANN!

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

pioneer

building

mature

degenerate

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

Environmental distribution of populations – individualistic concept

Landscape or spatial distribution of vegetation

types – organismal 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

www.eecrg.uib.no

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