ABSTRACT
Several authors have described the factors that influence community development on stonework (e.g. Darlington 1981; Woodell 1979). There are two major routes to the colonization of bare surfaces (Lisci et al. 2003). On dry vertical surfaces, the primary colonizers are xerophytes such as lichens which, while particularly frequent on stonework, are often dependant on the substrate type (Figure 19.1). The second occurs where water collects (including on horizontal surfaces) and mosses are often the primary colonizers. Cracks in stonework accumulate dead and decaying biotic matter, which contains nutrients and increases the moisture holding capacity. As organic matter accumulates in cracks, on ledges and where lichens, mosses and climbing plants grow, larger plants including grasses, ferns and flowering plants gain a foothold. These increase the accumulation of nutrient materials and can enlarge cracks enabling larger herbaceous and woody species to establish. Research on
artificial urban sites in Central Europe found clonal plants (self-replicating species) to be nearly twice as common as non-clonal plants (Prach and Pyšek 1994). Early colonizers included many annuals and some clonal species, with the latter spreading more rapidly initially then declining as communities developed. Species with fast germination that exploit sudden rainfall may also be advantaged (Benvenuti 2004). Even large trees can establish on stonework. Jim (1998) found 30 tree species, some over 9 m tall, on stone retaining walls in Hong Kong, and Trocha et al. (2007) recorded Betula pendula (silver birch) on brick walls, often in association with symbiotic ectomycorrhizae (Figure 19.2). Walls can feature unusual communities with colonizers and woody species on the same
site (e.g. Brandes 2004). Paving can also develop rich floras, although species are often at low frequencies. The dormant seeds of annual species often survive freezing in winter better than perennial species (except for those with deep roots). Many of the pavement plants that Chang and Kim (1990) recorded utilized C
low nitrogen and drought conditions in which they were growing). Vegetation on stonework provides habitats for animals. Steiner (1994) collected over 60,000 animals (194 taxa) from 216 moss samples on walls near Zürich. These included nematodes (47 taxa), tardigrades (13 taxa), rotifers and arthropods, of which mites and springtails were the most common (74 taxa) together with larger spiders, woodlice, centipedes, beetles and other insects. The pioneer stages in such communities are dominated by species such as mites, rotifers and snails that inhabit and consume lichens, algae and mosses and tolerate exposed surfaces. Snails are vulnerable to desiccation, sheltering in cracks in dry conditions, and prefer limestone or calcareous mortar (using the calcium for their shells). Some predators occur on bare surfaces; spiders such as Segestridae (jumping spiders) prey on small insects that alight to bask. Butterflies and lizards also bask on bare surfaces and Lepidoptera use crevices for over-wintering, to shelter from rain or wind, and for attaching pupae (Ruszczyk and Silva 1997). Ants explore open surfaces from nests away from the stonework, or in crevices, or under paving stones. Increased vegetation cover provides shelter for larger detritivores (e.g. woodlice and millipedes) and herbivores (e.g. true bugs). Woodlice shelter from dryness and aggregate under paving stones or in cracks on walls. Walls are important for taxa such as woodlice and spiders; over half of native British woodlice are recorded from walls (Harding and Sutton 1985), and Darlington (1981) considered that over 10 per cent of the British spider fauna were mural species. Eventually stonework supports predators such as ground-and rove-beetles, harvestmen, and those orb-web spinning spiders that require established vegetation on which to spin webs. Nectar feeders such as bees and butterflies visit flowering plants. Wasps and bees, including mason wasps, bumblebees and especially mining bees, nest in gaps in stonework. Snakes and lizards hide in crevices and larger holes can form nests for birds and mammals (e.g. small mustelids and rodents). Stonework habitats are important in promoting biodiversity since there is often a paucity of animals such as bees and butterflies in urban areas (Eremeeva and Sushchev 2005).
