Succession and environmental change

What succession is, and the model of what drives it

Succession is directional change in the composition of a community over time. In the facilitation model, each community alters the abiotic conditions in ways that suit other species better than itself.

Succession

The change over time in the species occupying an area, in which each community alters the environment and makes it more suitable for the community that follows.

Pioneer species

The first species to colonise a bare surface, adapted to survive there when almost nothing else can.

Seral stage

One identifiable community in the sequence of a succession; the whole sequence is a sere.

Climax community

The relatively stable community a succession reaches under the local climate, which persists until something disturbs it.

The species already present change the conditions, and those changed conditions determine which species can establish next. Lichens on bare rock trap dust, hold water and secrete acids that break the rock down, and when they die their remains add organic matter. The result is a thin soil, which a lichen does not require and a moss does, so the moss can then establish where it previously could not.

The mechanism at each handover is competition, usually for light: a taller species shades a shorter one, which then photosynthesises below its compensation point, grows less and is lost from the community. That is why the sequence runs small to large rather than in any other order.

Facilitation, in which each stage makes conditions more suitable for the next, is the model the specifications require, and it fits the classic rock-to-woodland seres. It is one model among several. Succession can also proceed by inhibition, in which early arrivals occupy the ground until disturbance removes them, or by tolerance, in which later species establish under the existing conditions without requiring change by the pioneers; repeated disturbance can redirect a sere. Not every replacement is determined by light; roots compete for water and nutrients as well. Give the facilitation account and state these as its limitations.

Primary succession, stage by stage

Primary succession starts on a surface where no soil and no living thing has been: a new lava flow, a cliff face exposed by a landslide, ground uncovered by a retreating glacier, a sand dune building above the strandline, bare rock exposed by quarrying.

Describe the drawing

Five stages of primary succession animated over the same ground. It begins as bare rock with no soil layer at all. Lichens and mosses appear as a scattering of small discs and a thin band of soil forms beneath them. Grasses and herbs follow, rooted in a soil band several times deeper. Shrubs and small trees replace them, taller and fewer, over deeper soil again. The last stage is oak woodland: five large trees over the deepest soil band of all. A measured arrow at the right and a figure under the ground give the soil depth at each stage, rising from nothing to sixty centimetres.

The pioneers. On bare rock these are lichens, a fungus and an alga together, in which the alga photosynthesises while the fungus grips the rock and holds water, tolerating temperature extremes, drought and near-total absence of nutrients. On a sand dune the pioneer is sea rocket, tolerant of salt spray and burial, and then marram grass, which grows up through accumulating sand and binds it. The functions of a pioneer species, rather than its identity, are what a question requires: stabilising the surface, holding water, and adding the first organic matter on death.

The first soil. Dead pioneers are broken down into humus, which holds water and mineral ions and gives roots something to grip, so mosses and shallow-rooted herbs can follow. Nitrogen is the scarce element on new ground, which is why alders and legumes, both carrying nitrogen-fixing bacteria, turn up early on a glacier foreland or a dune.

Grasses, herbs, then shrubs and small trees. With several centimetres of soil there is water and nutrient enough for larger plants with deeper roots, which add far more organic matter when they die and shade out the mosses that made them possible. Woody plants follow on deeper soil: they cast heavier shade, so the grasses beneath decline, and their bulkier litter changes the soil chemistry again.

The climax. The tallest species that the climate and soil allow come to dominate, and the composition of the community then changes more slowly. It does not become fixed: trees die and gaps open, species turn over, and the climate changes, so describe the climax as relatively stable rather than final. Which community forms depends on climate, soil, disturbance regime and herbivory. In lowland Britain the climax is usually deciduous woodland; in the far north it is coniferous forest or tundra; under low rainfall it may be grassland or scrub, which further time will not convert to forest. Heavy grazing or frequent fire holds a community below the climax the climate would otherwise permit. A climax community is the end point of a particular sere under particular conditions, not a universal end point.

Secondary succession, and why it is quicker

Secondary succession starts on land that has been cleared but still has soil: an abandoned field, a burnt heath, a wood felled by a storm, a garden left alone.

Describe the drawing

Three curves of community development against time in years. The primary curve starts at nearly nothing, stays low for decades and only then climbs to a dashed line marking the climax community. The secondary curve starts well above zero and reaches the same dashed line in under half the time. The third curve, dashed, follows the secondary one at first and then flattens off well short of the climax, held there by a row of upward arrows marking repeated grazing, mowing or burning.

Secondary succession is faster for three reasons. The soil already exists, so no time is spent on a century of soil formation; it already holds nutrients, water and decomposers; and it holds a seed bank of seeds, roots and rhizomes that survived the clearance, so the plants of middle seral stages appear within a year or two instead of waiting to arrive as windblown seeds or spores. A ploughed field left alone in southern England is scrub within about fifteen years and young woodland within fifty; the same endpoint on bare rock takes centuries.

The end point is often the same, because the climate and the underlying geology are unchanged. There is an exception: if the soil itself was destroyed or the seed bank exhausted, whether by severe erosion, by decades of intensive cultivation or by a change in the climate in the meantime, the community that returns may not be the one that was there before.

What changes as a sere runs

Four quantities change through a sere. Three rise throughout; species richness does not.

Describe the drawing

A line graph with five seral stages along the bottom and three lines above them, each scaled against its own largest value. Soil depth and biomass both climb at every stage and are still climbing steeply at the last one. The number of plant species climbs too, but reaches its highest point at the scrub stage and falls back at the woodland stage. Underneath, the five stages are listed with the soil depth in centimetres, the biomass in kilograms per square metre and the species count each one carries.

Soil depth and organic content rise. Every generation of plants adds litter, and every generation of decomposers turns some of it into humus. The soil also holds more water and more mineral ions as it deepens, so the abiotic environment becomes progressively less hostile.

Biomass rises, and so does productivity. A lichen crust holds a few tens of grams of organic matter per square metre; a mature oak wood holds tens of kilograms. Net primary productivity, meaning gross productivity minus what the producers respire, as set out in the photosynthesis unit, is low in a pioneer community, rises steeply through the middle stages, then falls back somewhat in the climax, because a mature wood has an enormous mass of non-photosynthetic wood to keep alive.

Food webs become more complex. More plant species, more architecture and more litter mean more niches, so more herbivores, predators and decomposers, and a food web with many links can absorb the loss of one species where a two-species chain cannot. That is what makes a climax community more stable.

Species diversity rises, and then often falls. Diversity climbs steeply through the early and middle stages as conditions become tolerable to more species. But a closed canopy casts deep shade and a few dominant trees take most of the light, so many of the light-demanding herbs of the earlier stages are lost and richness at ground level can fall. The community with the most species is frequently not the climax but the stage just before it.

Deflected succession and the plagioclimax

If something stops a sere before it reaches its climax, and keeps stopping it, the community stabilises at an earlier stage. That arrested community is a plagioclimax, and the process is deflected succession.

Plagioclimax

A community held at a stage short of its climax by a repeated interference, usually human or from grazing animals.

Deflected succession

Succession that has been diverted from its natural course by an ongoing external factor.

The interference is nearly always grazing, mowing, burning or ploughing. All four remove the tall woody seedlings that would otherwise shade everything else out, and all four must be repeated indefinitely, because the sere resumes the moment they stop.

Britain is largely a country of plagioclimaxes, and conservation exists to protect several of them. Chalk downland grazed by sheep and rabbits can hold several dozen plant species in a square metre, with the orchids and butterflies that depend on them; ungrazed, it becomes hawthorn scrub within a couple of decades and beech woodland after that, and most of those species go. Lowland heath is maintained by grazing and periodic burning, a hay meadow by an annual cut, and coppiced woodland by cutting on a rotation of seven to twenty years, which keeps a patchwork of open and shaded ground that a closed canopy does not offer.

Managing a reserve therefore often means reproducing the interference that created the habitat rather than leaving the site unmanaged. Introducing grazing or reinstating coppicing holds succession at a chosen stage, because the climax that would replace the plagioclimax does not support the species the reserve was designated for. A conservation answer has four parts: name the current stage, the community it would develop into, the species that would be lost, and the management that prevents the change.

Studying succession in the field

Succession runs over decades and centuries and the practical is an afternoon, so ecologists rarely watch it. There are three ways round the problem and each gains something at a cost.

Space for time: the chronosequence. Find a set of sites that differ in age but are otherwise alike, and read the sequence off them. A dune system read as a belt transect inland from the strandline is the classic school version; the ground uncovered by a retreating glacier, where the date of exposure of each band is known, is the classic research one. The cost is the assumption already met above, that the sites really do differ only in age.

Permanent quadrats resurveyed. Mark a fixed area, record it, and come back. Repeated permanent quadrats are one direct way to measure change through time; repeated transects, mapped surveys, photographs and remote sensing can also provide direct temporal evidence. What limits this one is how long somebody has been willing to keep it up. The Park Grass plots at Rothamsted have been cut and recorded since 1856, and a fenced corner of the same farm has been left to succeed since 1882 and is now woodland. Datasets like that are rare, and irreplaceable precisely because they cannot be started retrospectively.

Documentary and stratigraphic evidence. Old maps, estate records and aerial photographs date a clearance. Pollen preserved in peat, counted layer by layer and dated by radiocarbon, records which plants dominated over thousands of years, which is how the post-glacial succession of Britain is known at all.

The field methods belong to the sampling practical and are worth listing because questions ask for them. A belt transect is the right tool where there is a gradient, with quadrats at regular intervals along a line; record percentage cover for plants that grow as a mat and frequency or density where individuals are distinct. Take abiotic measurements at every station too: soil depth with a probe, moisture and organic content by drying and burning a weighed sample, pH, light intensity. A succession answer needs the abiotic change as well as the change in species. Where the question is about a general pattern rather than a gradient, place quadrats at randomly generated coordinates and say how they were generated.

Types of quadrat, and the ACFOR scale

The Edexcel B specification asks for the types of quadrat to be known separately, along with the ACFOR scale.

Frame quadrat. A square frame, commonly 0.5 m × 0.5 m or 1 m × 1 m, in which individuals are counted where they are distinct and percentage cover is estimated where plants grow as a mat.

Gridded quadrat. A frame divided by strings into a grid, usually of 100 small squares, which makes estimates less subjective. Percentage cover is the number of squares more than half filled by a species. Frequency is the number of squares in which the species occurs at all, so a species in 37 of the 100 squares has a local frequency of 37 per cent.

Point quadrat. A frame holding a row of pins, usually ten, lowered one at a time vertically through the vegetation; every species a pin touches counts as a hit. Percentage cover is hits ÷ pins lowered × 100. It is the most objective measure of cover in dense, low vegetation, and because one pin can touch several layers of leaves, the covers of all the species can add up to more than 100 per cent. Small or rare species are often missed unless many pins are dropped.

The ACFOR scale is a quick, subjective estimate of abundance in five steps: Abundant, Common, Frequent, Occasional, Rare. The recorder gives each species a letter for the quadrat or the site. It needs no counting, so it suits a first survey of a large area or organisms such as seaweeds and lichens that are hard to count. But two recorders can give the same quadrat different letters, and categories cannot go into a statistical test that needs measured numbers. Tying each letter to a band of percentage cover before the survey makes it more consistent.

Point quadrat

A frame holding a row of pins lowered vertically through vegetation; each species a pin touches is recorded, and hits per pin give percentage cover.

Gridded quadrat

A frame quadrat divided into a grid of small squares, usually 100, used to estimate percentage cover or frequency by counting squares.

ACFOR scale

A subjective five-point scale of abundance: Abundant, Common, Frequent, Occasional, Rare.

What a long series can and cannot show

Many ecological changes occur over periods longer than a typical research project, so a three-year study can miss a trend or record a run of wet summers as one. Long-term data sets address that.

A long series can establish four things. It can show that a change happened and measure how fast. It can set a baseline, so a later disturbance is measured against something rather than an impression. It can falsify a prediction outright: if a model says a community will recover in twenty years and forty years of data say it has not, the model is wrong. And where the series includes a deliberately untreated plot, it has a control and can do more than describe.

What it cannot do on its own is establish a cause. Two series that move together over fifty years are correlated, and correlation between two things measured over the same period is weak evidence of causation, because almost everything else changed over the same period as well. If nitrogen deposition rose while species richness fell, average temperature and the intensity of grazing changed too, and any of the three could be responsible. Attributing the change requires a mechanism you can state and, ideally, a manipulation: plots that were treated and plots that were not, assigned in advance.

Three further limitations apply. A chronosequence substitutes space for time and inherits every difference between the sites along with their difference in age. A series beginning after a disturbance can make recovery look like a trend, and if each generation treats the state it inherited as normal, the baseline itself drifts. And whether monitoring is representative depends on how the sites were selected, and the selection method is often not reported.

State the claim precisely rather than discounting the data: this community changed in this direction at this rate over this period, this is the proposed mechanism, and this is the evidence that distinguishes it from the alternatives. An evaluation should name the evidence that would discriminate between the competing explanations.

Succession on a body, and the time of death

A forensic pathologist asked when someone died is estimating the post-mortem interval. The Edexcel A specification names five lines of evidence: body temperature, the degree of muscle contraction, the extent of decomposition, forensic entomology and the stage of succession. Each is useful over a different stretch of time and all run faster when it is warm, so the estimate is a range, more reliable where several methods agree.

Body temperature. Heat from respiration holds the body's core at about 37 °C. After death the body cools towards the temperature of its surroundings. The core temperature, taken in the rectum or liver, often holds for a plateau of up to a few hours, then falls steeply at first and more slowly as it nears the surroundings. Rules of thumb of about 1 °C an hour are quoted, but the rate depends on body size and fat, since a large body has a smaller surface area to volume ratio; on clothing; on air temperature and movement; on whether the body lies in water, which cools it much faster than air; and on the temperature at death, which fever or hypothermia would have changed. Pathologists use a published chart that corrects for these. Once the body reaches the surrounding temperature, usually within a day or so, the method has nothing more to tell.

Describe the drawing

A graph of core body temperature against hours since death, from nought to thirty-six hours. Both curves start level at 37 degrees for about an hour, then fall, steeply at first and more slowly as they approach the temperature of the room, marked by a dashed line. The cyan curve, for a room at 5 degrees, falls faster and further than the amber curve, for a room at 20 degrees, which levels off just above 20. A note at the foot says that a reading of 25 degrees corresponds to about nine hours in the cold room and about twenty-one in the warm one.

Degree of muscle contraction: rigor mortis. ATP is needed for relaxation: it detaches myosin heads from actin, and it powers the pumps that return calcium ions to the sarcoplasmic reticulum. After death no oxygen reaches the cells, ATP is made briefly by anaerobic respiration, and then it runs out. Calcium ions leak from the sarcoplasmic reticulum and are not pumped back, so binding sites on actin stay exposed and myosin heads attach; with no ATP to detach them the cross-bridges stay locked and the muscle sets rigid. Stiffness appears first in small muscles such as those of the jaw within a few hours, spreads to the limbs, is fully developed by roughly twelve hours, and passes off over the next day or two as enzymes and then bacteria break down the proteins. It develops faster when warm and after strenuous exertion just before death, which had already used up ATP, and more slowly in the cold.

Extent of decomposition. Decomposition begins with autolysis: enzymes released from the cells' own lysosomes digest them. Bacteria from the gut, many of them anaerobic, then spread through the tissues. They produce a recognisable sequence: greenish discolouration of the abdomen, usually within the first few days in temperate conditions; swelling, called bloat, as bacterial gases build up; blistering and loosening of the skin; then collapse and liquefaction of the soft tissues, leaving bone, hair and dried skin. The order is consistent and the timing is not. These are enzyme-catalysed reactions, so they are much faster when warm, and they are slowed by burial, cold water and dry air, and speeded by insects and wounds.

Forensic entomology is the use of insects to estimate time since death. Blowflies can find a body within minutes to hours and lay eggs in the moist openings of the face and in wounds. The larvae (maggots) feed and grow through three stages called instars, each ended by a moult, then leave the body to pupate in the soil, and adults emerge. The duration of each stage depends on temperature and has been measured in the laboratory for the species that matter. The entomologist identifies the species, finds the oldest stage present, and uses temperatures from the scene and the nearest weather station to work out how long that stage took to reach. One method uses accumulated degree hours: below a base temperature the species does not develop, and each stage needs a fixed total of degrees above the base multiplied by hours. A species needing 1500 degree hours above 10 °C would take, at a steady 16 °C, 1500 ÷ 6 = 250 hours, about ten days; those numbers are for the arithmetic only.

Entomology gives a minimum time since death, because it dates the eggs rather than the death. Blowflies rarely lay at night or in heavy rain, and a body indoors, wrapped or buried may not be reached for days.

Stage of succession. A body is a habitat that changes as it decays, and its community changes in a predictable order: succession. Blowflies are the pioneers on fresh and bloated remains, with flesh flies soon after. Predatory rove beetles and parasitoid wasps follow to feed on the fly larvae. As the remains dry, the flies leave and beetles that eat dried skin and ligament, such as hide beetles, take over, followed by mites and by moths whose larvae eat hair. The groups present show the stage reached over weeks to months, after temperature and rigor have nothing more to say. The sequence varies with region, season and whether the body lies in the open, indoors or buried, so it is read against reference studies made in comparable conditions.

Post-mortem interval

The time that has passed since death.

Rigor mortis

The stiffening of muscles after death, caused by myosin heads staying bound to actin once ATP has run out.

Autolysis

The digestion of cells by their own enzymes, released from lysosomes after death.

Forensic entomology

The use of insects found on a body, their species and stage of development, to estimate the time since death.

Instar

One of the larval stages of an insect, separated from the next by a moult.