Presentation on theme: "2. Community Ecology and Dynamics – Succession and Stability H.J.B. Birks BIO-201 ECOLOGY."— Presentation transcript:
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 basics SuccessionBasic concepts Primary succession on glacial forelands Community changes Ecosystem changes Mechanisms of succession StabilityBasic 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. 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 ft 10,000 ft 5,000 ft Coastal mountain ranges Sierra Nevada Mountain Great American Desert Rocky Mountains Great Plains Mississippi River Valley Appalachian Mountains Coastal chaparral and scrub Desert Coniferous forest Coniferous forest Prairie grassland Deciduous forest Average annual precipitation cm (40-50 in.) cm (30-40 in.) cm (20-30 in.) 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.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 break- up 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/ Neoproterozoic III Neoprotoerozoic IIINeoproterozoic III Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Late Triassic Jurassic Cretaceous Palaeogene Quaternary
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 ice- caps 2.Late Eocene 35 million years ago Cooler, less tropical rain-forest, some ice- caps
3.Oligocene 25 million years ago Cooler, more extensive Antarctic ice-cap. Semi- arid 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 ice- caps, much reduced tropical rain-forest 6.Pleistocene years ago Massive ice-sheets, much tundra and arid vegetation
Temperature changes in the Northern Hemisphere at different time scales Shorter time scales x10 5 years
End of LIA Medieval optimum LIA Holocene 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
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; – 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 p.479)
Primary succession Time Small herbs and shrubs Heath mat Jack pine, black spruce, and aspen Balsam fir, paper birch, and white spruce Exposed rocks Lichens and mosses e.g. New surfaces formed by: Glacier retreat Volcanic eruption Coastal sand-dunes
Secondary succession Time Annual weeds Perennial weeds and grasses Shrubs Young pine forest Mature oak-hickory forest e.g. Disturbance by: Fire Forest cutting Erosion Wind-throw & storms Abandoned fields Large 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 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. GlacierMoraines Age of formation soil pH distance from glacier Age
Mature Alnus forest Mature Betula forest Photo: 1984 Primary Succession after Little Ice Age
Nigardsbreen, Jostedalsbreen 2002
Vegetation changes since ice retreat 20 years 80 years 150 years220 years
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 ( yr)Moraine V ( 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 years - Dryas mats with Alnus crispa, Salix, Populus, and Picea 3.40 years – Alnus forms dense thickets years – Picea and Populus grow above Alnus years – Picea forest with mosses 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
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) 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 pioneers pioneers & 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 Species richness
Succession of plant growth forms at Glacier Bay 3.Changes in plant growth forms in primary succession
Time Annual weeds Perennial weeds and grasses Shrubs Young 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 years Soil 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 populifolia Nyssa sylvatica Acer rubrum Fagus 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 time 1 to time 2 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 pioneers pioneers & '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 herbs and shrubs Heath mat Jack pine, black spruce, and aspen Balsam fir, paper birch, & white spruce climax community Exposed rocks Lichens and mosses Species richness Biomass Time
Secondary succession Annual weeds Perennial weeds and grasses Shrubs Young pine forest Mature oak-hickory forest Species richness Time Biomass
Biomass accumulation model in secondary succession (10 2 – 10 3 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 Science 305: Birks & Birks 2004 Science 305: Six chronosequencesDuration (yrs) Cooloola, AustraliaSand dunes >600,000 Arjeplog, SwedenIslands 6,000 Glacier Bay, AlaskaMoraines 14,000 HawaiiLava flows 4,100,000 Franz Josef, New ZealandMoraines >22,000 Waitutu, New ZealandMarine terraces 600,000
Maximal phase Retrogressive phase Cooloola, Australia Arjeplog, Sweden Glacier Bay, Alaska
Maximal phase Retrogressive phase HawaiiFranz 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 successions 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.
Inhibition and facilitation of spruce at Glacier Bay Primary Succession on Glacial Forelands Evidence for both inhibition and facilitation Net II & FFI 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 ) 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 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 temperature 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 SpeciesLightN 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 artemisiifoliaSI late Fagus grandifoliaVT 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
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 to investigate effects of fertiliser treatments on grasslands. Run for over 150 years. Monitored since 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
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 F c and F d
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 change 3 - 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 Ecol Letters 11: 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 and predicted changes in 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 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
Phases in plant growth with age pioneerbuildingmaturedegenerate age productivity
Phases in growth of Festuca ovina Changes in cover of three species (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 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
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
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 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 years Tree migration patterns in the last years Ordination gradient analysis of many different vegetational and faunal communities Heathland ecology, management, and dynamics in western Norway