Succession and management - Ecological Applications: Toward a Sustainable World - page 222

When sand mining ceases, the surface substrate is defi cient not only in seeds but
also in soil organic matter and plant nutrients. An additional problem in the sub-
tropical Bongil Peninsula in New South Wales, Australia, has been invasion by the
aggressive perennial grass Imperata cylindrica . Cummings et al. (2005) devised
experiments to identify the barriers limiting restoration of these invader-dominated
sand-mined sites. They fi rst tested the hypothesis that the barrier to regeneration
was poor native seedling establishment because of competition by I. cylindrica , but
burning of the grassland and weed control, coupled with native seed planting, did
not result in regeneration of native woody cover. So competition was not the most
important factor. A second experiment involved the addition to the soil of organic
material (mulch) and this signifi cantly improved survival and growth of planted
native species. These results support the idea that the absence of appropriate abiotic
conditions was inhibiting native succession. In other words, when appropriate niche
conditions prevailed, the succession moved forward.
The South African sand mine restoration, discussed in Section 8.2.2, provides a
further illustration of the role of changing niche conditions, this time for succes-
sional patterns in the dung beetle community. Africa has a rich diversity of dung
beetles, whose ecosystem role involves the consumption and burying of dung from
a variety of vertebrates. Dung beetles were sampled in eight stands of regenerating
vegetation at different stages of succession after cessation of mining, and the com-
munities were compared with those found in unmined, mature sand dune forest
sites. Profound abiotic changes occur during this succession, as in other forest suc-
cessions, with decreasing air temperatures and increasing humidities associated
with the shade offered by more mature vegetation (Figure 8.6a, b). Insects vary in
their ability to operate at high and low temperatures and in humid or dry conditions,
and the changing abiotic conditions are refl ected in the representation of different
dung beetles during the succession (Figure 8.6c-f).

8.2.5 Invoking
facilitation theory

Facilitation is a factor that can induce species switches during many successions
(Box 8.1). One species may benefi t another by enhancing local abiotic conditions,
by improving the supply of plant nutrients or by enhancing the probability of repro-
duction. I consider examples of each of these in restoration projects.
After thousands of years of forest clearance for agriculture, timber production
and urbanization, most forests in the Mediterranean area of Europe have disap-
peared. Restoration projects in such degraded shrub-dominated habitats have gener-
ally begun by removing these shrubs, on the assumption that they compete with
newly planted tree seedlings. But Gómez-Aparicio et al. (2004) challenged this
idea, arguing that because Mediterranean environments are prone to stress from
high temperatures and low water availability, the pioneer shrubs might have a posi-
tive effect on the establishment of seedlings of desirable mid-successional woody
shrubs and of trees. If the pioneer shrubs act as facilitators of succession to woody
vegetation they can be referred to as nurse plants , which it would be foolish to
remove.
Between 1997 and 2001 in the Sierra Nevada area of southeast Spain, more than
18,000 seedlings of 11 woody species were planted in a series of experiments: (i)
under a variety of nurse shrubs (16 species); (ii) at high or low altitude; and (iii) on
sunny-dry or shady-wet slopes. Overall, in a remarkably high 75% of cases the pres-
ence of shrubs increased the probability of seedling survival.