Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 254

into account when planning restoration. Thus, for example, burning may reduce soil
nitrogen stocks to a greater extent than mechanical removal of the invader plants.
In other cases it is soil water balance that is affected by an invader. Californian
grasslands were once dominated by perennial grasses such as wheatgrass ( Pseud o-
roegnaria sp.). Then, after European settlement, the area became dominated by
exotic annual grasses such as hare barley ( Hordeum murinum ) and exotic annual
forbs (nongrasses) such as tumble mustard ( Sisymbrium altissimum ). Most recent of
all has been invasion by the deeply rooted and late-maturing forb, the yellow star
thistle ( Centaurea solstitialis ), an invader whose notoriety has already been noted
(Section 1.2.5). In an attempt to understand the star thistle's success, Enloe et al.
(2004) created experimental plots with three communities that were dominated,
respectively, by perennial native grass, annual exotic grasses and star thistle.
Soil moisture measurements were made regularly for 4 years at depths down to
1.5 m in the three community types. The exotic grassland maintained the highest
water contents whereas star thistle dramatically reduced soil moisture (Figure
9.10a). The perennial grassland showed an intermediate pattern. It seems that the
competitive status of star thistle is due, at least in part, to its ability to reduce soil
water to levels at which other plants fail. The star thistle has an exceptionally long
life cycle for an annual plant. The seeds germinate after the fi rst autumn rain, then
during winter the seedling forms a rosette that develops a deep taproot by spring.
This enables star thistles to exploit soil water at greater depths than their competi-
tors (Figure 9.10b). The plants bolt to a height of 1 m or more in late spring and
fl ower and seed during summer and early autumn, well after annual grasses have
died back. Attempts to restore perennial grassland could include the introduction
of deeply rooted summer forbs or shrubs to help suppress star thistle by extracting
water from further down the soil profi le. On the other hand, if star thistle is targeted
for removal, and annual grassland is regained, you might expect increases in soil
water content and, consequently, of water runoff to streams. Intermittent streams
might even be turned into permanent ones. The ecosystem effects of some invasive
plants can extend very far indeed.

9.6 Ecosystem
approaches to
restoration - fi rst
aid by parasites and
sawdust

Ecological restoration can also benefi t from knowledge of food web interactions or
of ecosystem processes such as nutrient dynamics.
Species-rich meadows are now uncommon in agricultural landscapes in Europe
because decades of fertilizer application and intensive mowing or grazing have
allowed a few species to competitively exclude others. You have already seen that
one limitation to meadow recovery is slow colonization of restored sites by seeds of
many of the species, something that can be helped by sowing appropriate mixtures
(Section 3.2.1). A more insidious problem, especially where high soil nutrient con-
centrations persist, is for competitively superior species to exclude less competitive
species that would be represented if only nutrient concentrations were lower. A
possible solution is to manipulate the food web so that the competitive status of the
species is evened out.
One way to enhance coexistence of more species is to cut and/or graze at key
moments to open space and allow seeds of less competitive species to fi nd a place
in the sun. It is well established that grazers can reduce biomass of plant species to
such an extent that the most competitive do not become abundant enough to exclude
their weaker brethren - a process known as exploiter mediated coexistence . A less