Presentation on theme: "2. Community Ecology and Dynamics – Succession and Stability"— Presentation transcript:
12. Community Ecology and Dynamics – Succession and Stability BIO-201 ECOLOGY2. Community Ecology and Dynamics – Succession and StabilityH.J.B. Birks
2Community Ecology and Dynamics - Succession and Stability Some ecological and environmental basicsSuccession Basic conceptsPrimary succession on glacial forelandsCommunity changesEcosystem changesMechanisms of successionStability Basic conceptsWhat causes resilience?Alternative stable states and regime shiftsMaintenance dynamicsDisturbance and diversityCommunity concepts revisitedConclusions and Summary
3the PowerPoint handouts of this lecture on the BIO-201 Student Portal PensumThe lecture, of course,andthe PowerPoint handouts of this lecture on the BIO-201 Student PortalAlso ‘Topics to Think About’ on the Student Portal filed under projects
4Topics to Think AboutOn 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.
5Background 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.
6Textbooks 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)
7A ReminderIf 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!
8Some 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.)
9Average annual precipitation Broad spatial scale15,000 ft10,000 ft5,000 ftCoastalmountainrangesSierraNevadaMountainGreatAmericanDesertRockyMountainsPlainsMississippiRiver ValleyAppalachianCoastal chaparraland scrubConiferousforestPrairiegrasslandDeciduousAverage annual precipitationcm (40-50 in.)cm (30-40 in.)50-75 cm (20-30 in.)25-50 cm (10-20 in.)below 25 cm (0-10 in.)BiomesRole of climate
10Long time scalesChange in temperature in the North Sea over the past 65 million years (M yr).The ancient continent of Gondwanaland began to break up about 150 M yr ago.~50 M yr ago distinctive bands of vegetation had developed.By 32 M yr these are more sharply defined.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.
11Changing continental positions in last 220 million years Tectonic plates in constant motion. Environment on earth changes accordingly.Triassic 220 million years agoPangaea continent had its maximum size. Large interior areas, very dry and extensive deserts.Mid-Late Jurassic 155 million years agoBeginnings of the break-up of Pangaea.
123. 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 agoBreak-up of Gondwana.
135. Mid Cretaceous 106 million years ago Europe still a series of islands, North and South America widely separated.6. Late Cretaceous 65 million years agoSimilar to today but for North and South America and India.
15At the same time, major changes in plant evolution and hence in earth vegetation
16Major evolutionary developments in last 500 million years
17Global ecological changes in the last 55 million years Eocene 55 million years agoWidespread tropical rain-forest and no ice-capsLate Eocene 35 million years agoCooler, less tropical rain-forest, some ice-caps
183. Oligocene 25 million years ago Cooler, more extensive Antarctic ice-cap. Semi-arid scrub and desert areas, evolution of giant land mammals4. Miocene 3.2 million years agoContinents almost in today's position, ice-caps at both poles, climate drier, vast grasslands, much mountain uplift
195. Late Pliocene 1-2 million years ago Extensive polar ice-caps, much reduced tropical rain-forest6. Pleistocene years agoMassive ice-sheets, much tundra and arid vegetation
20Shorter time scalesTemperature changes in the Northern Hemisphere at different time scalesyearsyears1021051035x105104
21Holocene 11500 years Last millennium LIA = Little Ice Age Medieval optimumLast millenniumLIA = Little Ice AgeLIAEnd of LIAPast 130 years
22Millennium scale: warm period 1000 AD and the Little Ice Age Medieval Warm PeriodLIA
24Succession – Basic Concepts Changing plant and animal communities, ecosystems, and landscapes through time following the creation of new substrates or following disturbance, usually directional changes.Primary succession – occurs on newly formed surfaces such as volcanic lava flows, areas recently deglaciated (glacial forelands), sand-dunes along coast, etc.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.
25Successional change is usually directed towards the undisturbed surrounding vegetation and fauna. Succession generally ends with a mature community whose populations are relatively stable. 'Climax vegetation'.Environment is changing at a range of scales in time and space, so communities are always in a state of flux and change.Successional time scales – can be short or long. Few years; 250 years after the Little Ice Age; – years since the last glaciation.Ecological succession “non-seasonal, directional, and continuous pattern of colonisation and extinction on a site by species populations” (Begon et al p.479)
27Mature oak-hickory forest Secondary successione.g. Disturbance by: Fire Forest cutting ErosionWind-throw & storms Abandoned fieldsLarge herbivores e.g. elephantsTimeAnnualweedsPerennialweeds andgrassesShrubsYoung pine forestMature oak-hickory forest
28Differences between primary and secondary succession Primary succession: no soil, no seed- bank, no organic matterSecondary succession: soil is present but disturbed, seed bank present, organic matter presentSecondary succession is very common within landscapes, primary succession is less common
29Primary 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.
30distance from glacier Age 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.GlacierMorainesAge of formation1930189018501750soil pHdistance from glacier Age
44Glacier Bay, Alaska Phases Pioneer phase – 20 years – Epilobium latifolium, Dryas drummondii, Salix spp.30 years - Dryas mats with Alnus crispa, Salix, Populus, and Picea40 years – Alnus forms dense thickets50-70 years – Picea and Populus grow above Alnusyears – Picea forest with mosses200 years – Tsuga heterophylla & T. mertensiana forest>300 years – more open forest with areas of bogs and tundra meadows
46Some Glacier Bay pioneer species Dryas drummondiiEpilobium latifoliumWilliam S. Cooper
47Little Ice Age in Nepal about 1850 1957Little Ice Age in Nepal about 18502002Little Ice Age maximumO.R. Vetaas
48Gangapurna North Nepal stages since 1850 to present Terminal moraine-complexNeoglacial stages (> 1200 BP)riverLittle Ice Age maximum (app. 1850)Glacial lakeGangapurna North Nepal stages since 1850 to presentGlacier in 19571988Lateral moraine stagesGlacier fronts2001
51Craters of the Moon, Idaho 2. Volcanic lava flowsCraters of the Moon, IdahoPlant colonisation
52Community Changes During Succession Changes in plant abundance and species composition in primary successionLate invadersWoody & long lived speciespioneers & late-invaderspioneersTIMEOver time species invade, then increase, some decrease again and disappear, and some remain as the mature vegetation
53Early-succession species Late-succession species r-selectedMany small offspringFar dispersed seedsEarly reproductive ageMost offspring die before reaching reproductive ageHigh population growth rate (r)Adapted to low nitrogen and high lightLow ability to competeLate-succession speciesK-selectedFewer, larger offspringShort dispersed seedsLater reproductive ageMost offspring survive to reproductive ageLower population growth rate (r)Adapted to higher nitrogen and low light (shade)High ability to compete
542. Changes in species richness in primary succession over 1500 years
55Over longer time scales (> 2000 yr) richness often declines. Why? Species richnessSuccessional time
56Succession of plant growth forms at Glacier Bay Changes in plant growth forms in primary successionSuccession of plant growth forms at Glacier Bay
57Mature oak-hickory forest 4. Changes in species richness in secondary succession from 80 days to 200 yearsEastern N America – abandoned fields, tree colonisation and forest development 200 yearsSoil and buried seed bank present at the outsetMature oak-hickory forestYoung pine forestPerennialweeds andgrassesShrubsAnnualweedsTime
58Woody plant species richness Number of breeding bird species
59Rocky coastal shores: 18 months Number of macroinvertebrate and macroalgae species during secondary succession
60Rivers after extreme floods: 80 days Algal species diversity during secondary succession
615. Species replacement during secondary succession Henry Horn – predictive model for changes in tree composition givenfor each tree species, probability that within a particular time, an individual would be replaced by another of the same species or by a different speciesan assumed initial species compositionHorn 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.
62Using 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, USABetula populifoliaNyssa sylvaticaAcer rubrumFagus grandifoliaA 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.
63Using 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
64Secondary SuccessionSEED BANK'Late invaders'Woody & long lived speciespioneers & 'late-invaders'pioneersTIMETime after disturbance: species invade, then increase, some decrease again and disappear, and some remain as part of the mature vegetation
65In 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.
66Dominance-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.
67Ecosystem Changes During Succession Changes in biomass and productionPRIMARY SUCCESSIONBIOMASSNET PRIMARY PRODUCTIONRESPIRATIONTIME
71Biomass accumulation model in secondary succession (102 – 103 years)
72Biomass during stream secondary succession (60 days)
732. Changes in soil during succession Soil building during primary succession at Glacier Bay
74Changes in soil properties during primary succession at Glacier Bay
75Changes in soil development nitrogen, pH, cations, organic matter pH, cations: Mg & CaTIMETime after fire: secondary successionOrganic matter
76Changes 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.
77P limitation on oldest soils Organic carbon and total nitrogen content of soils developing on lava flowsTotal phosphorus & percentages of total P in weatherable and refractory (unavailable) forms in soils developing on lava flowsP limitation on oldest soils
78Nitrogen and phosphorus loss rates from soils developing on lava flows
80Recent study on six long chronosequences to investigate reasons for decline in biomass over long time periods.Wardle et al Science 305:Birks & Birks 2004 Science 305:Six chronosequences Duration (yrs)Cooloola, Australia Sand dunes >600,000Arjeplog, Sweden Islands ,000Glacier Bay, Alaska Moraines ,000Hawaii Lava flows 4,100,000Franz Josef, New Zealand Moraines >22,000Waitutu, New Zealand Marine terraces 600,000
81Maximal phaseRetrogressive phaseCooloola, AustraliaArjeplog, SwedenGlacier Bay, Alaska
82Franz Josef, New Zealand Maximal phaseRetrogressive phaseHawaiiFranz Josef, New ZealandWaitutu, New Zealand
83Tree basal area – unimodal or decreasing response with age
84Measured 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
85Soil 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.
86Other 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.
87Primary and secondary succession in a range of environments and time scales produce changes in species composition and diversitychanges in the structure and function of ecosystems.What mechanisms drive succession?
88Mechanisms 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.
90Support for inhibition by Ulva Intertidal successionsInhibition of later successional speciesSurvivorship of successional species under conditions of low tides in hot afternoons
91Facilitation by algae of colonisation in intertidal succession of surfgrass, Phyllospadix scouleri
92Mt St Helens, Washington Mt St Helens, Washington. Erupted 1980, created vast new volcanic lava fields.
93Lupinus lepidus – few large seeds, fixes atmospheric nitrogen Common pioneer plantsAnaphalis margaritacea, Epilobium angustifolium – many wind-dispersed small seedsLupinus lepidus – few large seeds, fixes atmospheric nitrogenLupinus lepidus
94Experiments provide evidence for both inhibition and facilitation models
95Lessons from the 25 years of ecological change at Mount St. Helens
96Succession is very complex, occurring at different rates along different pathways with periodic setbacks through secondary disturbances (e.g. landslides, mudflows).No single over-arching model of succession provides an adequate framework to explain the observed changes.Chance factors (e.g. timing of the disturbance at various spatial and temporal scales) have strongly influenced survival and successional patterns and pathways.Lakes & most streams largely returned to their pre-1980 state.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.Almost all small mammals have returned but birds have not, possibly because of the lack of extensive forest with vertical structure (niches).Rate of change determined by a complex of factors – position in the landscape, local topography, climate, biotic factors, human factors, and chance.
97Primary Succession on Glacial Forelands Inhibition and facilitation of spruce at Glacier BayNet I I & F F I effect:Evidence for both inhibition and facilitation
98Are the facilitation, inhibition, and tolerance models useful? Nature is very complex – three mechanistic models are probably a great over-simplification.Real-life situation probably more complex.
993. 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 = colonisationM = maturationS = senescence
100Despite this undoubted complexity of succession, further mechanisms underlying succession have been proposedBegon et al. (2006) Chapter 16, pp1) Competition-colonisation trade-off and successional niche mechanismsEarly-successional plants have several correlated traits high fecundityeffective dispersalrapid growth rate when resources are abundantpoor growth rate when resources are scarceLate-successional plants usually have opposite traitsIn 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
101Early species persist because their dispersal ability and high fecundity permit colonisation and establishment in recently disturbed sitestheir rapid growth under resource-rich conditions allows them to out-compete temporarily late-successional species even if they arrive at same time= competition-colonisation trade-off= successional niche (early conditions favour early species because of their niche requirements)
102Some Revision! One- and two-dimensional niches Population densitytemperatureFeeding resourceIn reality, niche is multi-dimensional
103Realised versus fundamental niche Fundamental niche = only environmentRealised nicheBiotic control
104Broad and narrow niches Generalist speciesSpecialist species
1052) 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.
106Tilman’s resource-ratio hypothesis of succession RequirementsSpeciesLightNA+++(+)B+C++DETilman’s resource-ratio hypothesis of succession
1073) Vital attributes (Noble & Slatyer 1981) Vital attributes relate torecovery after disturbance (V = vegetative spread; S = seedling from abundant seedbank in soil; D = dispersal; N = no special dispersal and/or small seedbank)ability to reproduce in face of competition (T = high tolerance; I = intolerance)Species then classified on basis of vital attributese.g. pioneer Ambrosia artemisiifolia SIlate Fagus grandifolia VT or NT
1084) r and K-selectionCertain 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 successionavoids competition, high reproduction, good dispersal, r-selectiontolerant of competition or highly competitive, low reproduction, poor dispersal, K-selection
110Concept of ‘climax’Do successions come to an end?Frederic Clements (1916) single dominant climax in a given climatic region – Monoclimax viewArthur Tansley (1939) local climax governed by soil, climate, topography, land-use, history, fire – Polyclimax viewRobert 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.
111Community and Ecosystem Stability - Basics Stability – absence of change. May be stable for several reasons (e.g. absence of disturbance, constant environment).In reality, communities and ecosystems are always changing because of changing environment and biotic interactions that may change as organisms age.Stability – ability of community or ecosystem to maintain structure and/or function in the face of potential disturbance.Stability may result from the ability of a community to return to its original state after a disturbance – 'resilience'.
112What 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.
113Park Grass Experiment, Rothamsted Experimental Station Started to investigate effects of fertiliser treatments on grasslands. Run for over 150 years.Monitored since 1862.Shows virtually no new species colonised since 1862.
1141910 – 1948Three treatmentsProportions changed from year to year (annual rainfall) but relatively stable proportions in the three treatments
115What about individual species? Patterns of species abundance in 60 years
116Are the Park Grass plots stable or not? Yes, at a very coarse scale – started as a grassland and stayed as a grassland with no new species.Yes, at a less coarse scale of grasses, legumes, and other species but some variation from year to year.No, at the scale of individual species.
117Are there stable natural communities? Answer dependent on the scale of interestEnvironment is changing constantly at a range of scalesTemperature changes in the Northern Hemisphere at different time scales
119Changes in populations of creosote bushes and saguaro cactus due to major drought in 1960s
120Alternative 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)
121Saharan 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
123Caribbean 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.
125Other examples of dramatic regime shifts: Savannah that is rapidly encroached by shrubsLakes that shift from clear water to turbid waterStanding waters that can suddenly be overgrown by floating plantsDifferent populations in open ocean suddenly change to different abundances synchronously
126Alternate 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
127Precipitation in absence of vegetation is determined by climate Vegetation has a positive feedback on local rainfallNo vegetation when precipitation falls below critical level
128Actual 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
129Shallow freshwater lakes and two alternative stable states
130Stability landscapes showing resilience of equilibria
131Ball (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'.
132Plant-dominated state Macrophyte-dominated system pre-1960 Use of TBT in boat paints 1960Decrease of mollusca (gastropods, etc.)Reduction in grazing of epiphytic algaeIncrease in algaeAlgae-dominated stateDecline of macrophytesAlgae-dominatedSee Lecture 5 Long-term Ecology for detailsPlant-dominatedNutrient level
1331 & 4 - alternate states, 2 - causes of change 3 - triggers of resilience and regime shifts
135Reduced 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).
136Evidence from field data Pacific OceanDutch ditchesShade in shallow lakes = dominated by cyanobacteria = dominated by other algae
137Alternative stable states – can they be predicted? Beaugrand et al Ecol Letters 11:North Atlantic – critical thermal boundary where a small increase in temperature triggers abrupt ecosystem shifts across multiple trophic levels.
138Boundary 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
139Decadal changes in SST 1960-2005 and predicted changes in 2090-2100 Beaugrand et al. 2008Decadal changes in SST and predicted changes inSmall changes in last 40 years
140Ecosystem 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
141What of the future?Beaugrand et al. 2008Two future climate scenarios: progressive shift northwards from 2000 to 2090Climate 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)
142Summary of Alternative Stable States System has alternative states if there can be more than one 'stable state' for the same external variable (e.g. nutrients in lakes).Stable states are really dynamic regimes. Show slow trends, natural population fluctuations due to climate and internal population dynamics.Multiple causes are the rule in regime shifts.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.
143External conditions should really be external and independent and not an interactive part of the system.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 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).
144Resilience is necessary to sustain desirable ecosystem states in variable environments and uncertain futures.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.Biological diversity appears to enhance the resilience of ecosystem states"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'?
145Maintenance DynamicsEven 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 DYNAMICSSuccession is a directional changeCyclic changes or maintenance dynamics or patch dynamics are fluctuations about a mean value.A.S. Watt ‘Pattern and Process’ 1947Dr Alex ‘Sandy’ Watt
146Phases in plant growth with age productivityagepioneerbuildingmaturedegenerate
147Phases in growth of Festuca ovina Changes in cover of three species (F. = Festuca, H. = Hieracium, T. = Thymus)
148Important factors in maintenance or patch dynamics Disturbance (or ageing) gapsDispersal recruitment growthFrequency of gap formationSize and shape of gapsView landscape as patchy with disturbance and recolonisation by individuals of different speciesCritical roles for disturbance (and ageing) as a RESET mechanism, for dispersal and establishment between habitat patches, and competition between species concernedCommunity dynamics need a landscape-scale perspective to be understandable
149Fire: control of secondary succession in west Norwegian coastal heathlands
150Bjørk og fufu skog ( Eik) FIRE!BRANN!Bjørk og fufu skog ( Eik)trærforveet calluna+ urter oggressCalluna spirerTime
151Fire also important in community maintenance dynamics – fine-scale burning
154Maintenance dynamics of Calluna (røsslyng) in coastal heathlands involving fire Traditional heathland cycleDerelictiondegeneratematurebuildingpioneer
155Combination of controlled burning, mowing, & grazing 'Cultural landscape'
156Disturbance and Diversity Disturbance resets the clock in any succession. Elimination of existing populations, allows colonisation by early successional species - frequency of disturbance critical.high frequency of disturbance, pioneers onlyintermediate disturbance, pioneers plus later species, giving maximum diversitylow disturbance, late species onlyResult is hump-backed curve of diversity in relation to disturbance 'intermediate disturbance hypothesis'Hypothesis formulated in relation to successional responses after disturbance.
157Community Concepts Revisited Palaeoecology – study of the distribution & abundance of organisms (plants and animals) in the past.Pollen analysis – major technique.Last glaciation about years ago and subsequent deglaciation at about years ago were a major, broad-scale primary succession.Extent of glacial ice at and 8000 years ago
158Large number of sites where pollen analysis has been done 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.
159Each tree genus has its own individualistic history Each tree genus has its own individualistic history. Did not move as forest communities.
160Same in the British Isles – strongly individualistic behaviour of forest trees HasselBjørk
163Organismal concept – F.E. Clements Individualistic concept – H.A. GleasonIn fact these two concepts refer to different scales and biological concepts – no real conflict!Organismal concept is a spatial conceptIndividualistic concept is a population concept
1644 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
165Great Smoky Mountains, Eastern USA Robert H. WhittakerLandscape distribution of vegetation typesSpatial arrangement of vegetation types
166Landscape or spatial distribution of vegetation types – organismal concept Environmental distribution of populations – individualistic concept
167Community 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).
168Conclusions and Summary 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).Succession generally ends with a mature community that is similar to the surrounding vegetation and fauna and that has relatively stable populations ('climax' vegetation).Environment varies at a wide range of temporal and spatial scales.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.
1695. 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.Field evidence provides support for facilitation, inhibition, or a combination of the two.Nature is more complex than 3 mechanistic models. Succession is a combination of many different ecological processes –germination, herbivory, competition, chance, etc.
17010. 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.
171Palaeoecological data indicate that forest trees showed individualistic behaviour in their migration patterns after the last deglaciation.The community is a spatial concept. The individualistic continuum is a population concept.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).Vegetation at the landscape scale is a mosaic depending on topography, environment, primary succession, secondary succession, and maintenance dynamics.
172EECRG Research Topics in this Lecture Primary succession on glacial forelands in Norway, Nepal, and TibetAlternative stable states in Norwegian forest vegetationNatural climatic variability in NW Europe in the last yearsTree migration patterns in the last yearsOrdination gradient analysis of many different vegetational and faunal communitiesHeathland ecology, management, and dynamics in western Norway