Succession in urban ecosystems
Succession is the change in plant species over time. It is brought about by a change in microenvironment by each successive plant communities. Succession is determined by a number of factors:
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the supply of new species
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environmental stress, such as a lack of water or nutrients
competition between species.
The first part of succession is colonisation. This is influenced by:
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aspect - south facing slopes or walls are warmer and drier whereas north facing ones are
colder and damper
porosity - pores hold more rainwater so wood, brick and mortar are colonised more rapidly
than asbestos, stone and metal
slope - horizontal areas and gentle slopes accumulate debris, hence soil may develop and also
rainwater drains away more slowly; steep slopes experience rapid runoff and are hence dry
surface roughness - plants cannot stick to smooth surfaces like metal and glass
levels of pollution - high concentrations of lead may inhibit colonisation.
As the soil deepens and nutrients increase the number of species found also increases. A simple model
of succession on a newly exposed surface in an urban area might have six stages.
1 Lichens colonise the rock and produce acids which weather the rock and begin to form an embryonic
soil.
2 If the material is porous enough to hold water (such as a brick) or when some of the rock has
weathered enough to hold water and soil particles, seeds of more advanced plants may colonise. Moss
is common at this stage.
3 The dead moss and the minerals begin to form a soil. This provides stability, water and nutrients for
the roots of more developed plants, such as groundsel, clover and grass. These plants, known as ruderal
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species, such as Oxford ragwort (Picture 1) and nettles, can tolerate waste land, rubbish and debris.
Plant succession is usually very rapid. The initial colonisers have many of the features of weeds. Plant
cover increases and competition becomes more intense as colonisation proceeds.
4 Rapidly growing annual plants are replaced by perennial grasses. Perennial herbs such as nettles and
dandelions invade. They are longer living.
5 In time dense thickets of bramble, hawthorn and elder develop. Small shrubs become established as
cracks in the rock and the soil gets deeper. Ivy and bramble can outcompete the smaller plants and their
roots can grow into deeper crevices in the rock.
6 Trees can only survive in deep soils, but on a wall trees such as yew can start to grow (although they
tend to remain stunted). Buddleia are common in many urban areas (Picture 2).
Picture 1
Picture 2
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The initial colonisers show characteristics of weeds (Figure 1). A high percentage of bare ground is
present in the early stages, but on older sites this is reduced, due to rapid colonisation by winddispersed seeds, fruits and spores and by vegetative propagation. When the ground is completely
covered by vegetation, competition between plants becomes intense and species diversity may decline.
Shrubs and trees may occur, taking up larger amounts of nutrients, water and light. There is limited
amount of organic matter at the beginning of succession but this increases rapidly as colonisation
proceeds.
Figure 1: Characteristics of weeds
Weeds possess many characteristics. These include:
1 High output of propagules under good conditions – for example groundsel produces more than 1000
fruits per plant.
2 Some manage seed output despite poor conditions.
3 Some seeds are viable before plant maturity – thus if the parent plants dies young, not all the seeds
are lost.
4 Seeds may be produced at an early stage of growth in some species such as meadow grass.
5 In other species, seed production is spread over the growth period.
6 Many seeds are able to germinate after long periods of dormancy.
7 Many seeds grow rapidly and establish as seedlings.
8 Weeds can tolerate wide variations in the physical environment.
9 Weeds have a strong competitive ability which includes efficient uptake of nutrients, rapid growth of
roots, an ability to climb over other plants and a rosette form.
Some faunal species can tolerate a wide variety of conditions e.g. centipedes, millipede, earthworms
and woodlice. Several species of butterflies and the common field grasshopper are also common on
wastelands. Other species are specialists. The larvae of certain moths e.g. the wormwood shark, depend
upon plants living in wastelands.
Urban areas are a good place to look for evidence of succession. There are a number of advantages and
disadvantages of urban environments for plants and animals (Figure 2). Derelict land, abandoned
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works, railway cuttings, parks and open spaces can be found in many places. Succession can be
changed significantly due to human impact. This includes digging gardens, using land for allotments,
house building, commercial developments, retail developments, industrial development, recreation, and
so on.
Figure 2: Advantages and disadvantages of urban living
Advantages
Disadvantages
Warmth - more animals and plants can survive in
winter.
Lacks of soil - large areas are covered with roads and
buildings.
Food - birds, squirrels and small mammals do not die
of starvation or thirst in winter. Fewer birds need to
migrate.
Pollution.
Trees - a greater number of trees give nest sites, food
and shelter to many animals.
Water - drought to the rapid run off and evaporation,
waterlogging because of soil compaction.
Shelter - buildings or bat boxes give safer breeding
places.
Damage due to mowing, trampling and vandalism.
Light - birds breed for longer because of artificial light. Lack of nutrients - removal of debris prevents natural
recycling.
Lack of pesticides, fewer bees, butterflies and other
insects are killed.
Wind in areas where buildings cause excessive
turbulence.
Urban meadows
Picture 3
Increasing awareness of the need to provide more pollen and nectar
sources for the struggling pollinator population, combined with some
austerity measures and nostalgia for wildflowers, have contributed to the
growth of the urban meadow – everywhere from the tiniest window box
to the acres of flowers in London's Olympic Park (Picture 3). Urban
meadows are an ‘easy win’ for councils faced with budget cuts. They are
attractive, they are cheaper than mowing grass or growing bedding plants
– and they are more sustainable. It also has a good psychological effect,
for example, in Birmingham when the riots took place, none of the areas
planted as meadows were touched.
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At St Andrews Park in Bristol, mini-meadows in petal shapes, spirals and wavy lines have been sown
to gently discourage antisocial behavior by stopping large groups of people gathering on the grass.
In the past, these grassy areas typically became jam-packed with people, which was often intimidating
for older people and children. Now, no one wants to sit on the flowers, dispersing people and making
the environment a much friendlier place.
Visit Urban pollinators project at:
www.urbanpollinators.com - USA version
www.urbanpollinators.co.uk - UK version
Have a look around your school and see if you can see any evidence of colonization and succession
(look at walls and derelict or abandoned land)
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