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Urban ecosystems
Herb robert

Walls

Studying life on a wall is a bit like looking at a natural rock face with its cracks and crevices. There are some differences however, especially when considering urban walls. The photograph shows herb robert (Geranium robertianum) colonising a crevice resulting from the crumbling of mortar in a poorly maintained brick wall.

What environmental factors affect plants and animals on walls?

Tortula muralis

Fieldwork: what could you investigate?

1. Building materials

Less resistant materials will be weathered more rapidly, forming fissures and crevices which can be colonised by pioneer species. The mortar in brick walls is often less durable, and is weathered faster, than the brickwork.

Different species also have preferences for substrates with different acidity and alkalinity. The moss Tortula muralis (see photograph right), will often colonise the alkaline mortar in preference to the more acidic bricks.

Wall top

2. Aspect

The south facing sides of the wall are likely to be lighter, warmer and drier than the north facing sides of a wall. You would therefore expect to see more lichens on south side and more bryophytes on north side (see photograph left of the top of a wall - south is to the left of the photograph).

Where a wall forms a boundary alongside a road remember that other factors (e.g. pollution from traffic splash) may be having an effect.

 

A leaking pipe

The effect of a leaking downpipe

3. Height

A wall can be broadly divided into three zones.

(a) The top of the wall - the shape of the top of the wall is important. A relatively flat surface makes it easier for debris to collect and soil to develop especially where there are cracks and crevices. This results in a more abundant growth of plants and often allows ferns and flowering plants to get a foothold. Where the wall is unshaded this may also be the warmest, driest part of the wall

(b) The middle vertical face of a wall is less easy for larger plants to colonise and where the surface is very smooth it may be dominated by encrusting lichens or where its damp by algal films. Algae as well as mosses are quick to exploit damp areas caused by leaks from gutters and down pipes.

(c) The bottom of the wall is often the dampest area due to ground water seeping up the wall. Mosses and liverworts may be particularly common here. Accumulation of debris and soil at the bottom of the wall will allow nearby flowering plants to colonise and these will also provide shelter for other species colonising the bottom of the wall. Traffic splash may be a significant source of pollution whilst nutrient enrichment is often caused by the activities of dogs.

The following three photographs show how a wall in the shade of a tree has been colonised.

Top of wall (a) The top of the wall where soil has begun to accumulate. Shade has led to an abundant growth of mosses
Middle of wall (b) The vertical face in the middle of the wall, colonised mainly by encrusting lichens
Bottom of wall (c) The bottom of the wall where mosses are taking advantage of dampness creeping up the wall. Flowering plants grow in debris accumulating at the base of the wall.

How are plants adapted to growing on walls?

Many of the plant species found growing on walls are typical of those of the pioneer community in a succession. See here to find out more about their characteristics and adaptations.

Other adaptations may include adaptations to climbing (e.g. in ivy and the Virginia creeper) and special seed dispersal mechanisms (e.g. in ivy-leaved toadflax).

ivy roots Primroses
Ivy (Hedera helix) showing its special climbing roots (haustoria) Ivy-leaved toadflax (Cymbalaria muralis) showing its seed dipersal mechanism

How do animals use walls?

Walls are frequently used as lookout posts by birds or as basking sites by invertebrates such as bees and butterflies. They also provide shelter or permanent habitats for a wide range of invertebrates.

springtail spider's web
A springtail - a tiny animal (1-2 mm long) often found among clumps of moss on walls The web of a spider which lives in crevices in walls, using its sticky web to entangle its prey
millipedes snail trail
Millipedes sheltering in a damp crevice in a brick wall A trail of 'teeth' marks left by a snail feeding on lichens

Looking for a next step?
The FSC offers a range of publications, courses for schools and colleges and courses for adults, families and professionals that relate to the urban environment. Why not find out more about the FSC?

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