Environmental Engineering Reference
In-Depth Information
Fig. 9.10 (a) Soil water
content, averaged across
soil depths, during the
4 years of a study
involving plots of
Californian native
perennial grassland
(open squares, ±SE),
exotic annual grassland
(open circle s) and star
thistle-dominated
communities (closed
circles). The star
thistles exploit soil
water and reduce soil
moisture to a greater
extent than their
grassland counterparts.
(b) Soil water content,
averaged across the 4
years, in relation to
depth in the soil profi le.
Because of their deep
taproots, star thistles
can reduce water
content to a greater
depth than the grasses.
Symbols as in (a). (After
Enloe et al., 2004.)
(a)
30
28
26
24
22
20
18
16
14
12
(b)
30
28
26
24
22
20
18
16
14
30
60
90
120
150
Soil depth (cm)
well-known form of exploiter mediated coexistence occurs when parasites exert the
leveling effect. Rhinanthus minor , an annual plant, is capable of its own limited
photosynthesis but is known as a hemiparasite because it typically taps into the
photosynthetic products of other plants by building connections with their roots.
Pywell et al. (2004) reasoned that the presence of the hemiparasite might facilitate
recovery to species-rich grassland via exploiter-mediated coexistence. To test this
hypothesis in a species-poor grassland, they established experimental plots in early
autumn with various densities of Rhinanthus minor . Two years later, after the hemi-
parasite populations had become established, they sowed a seed mixture of ten
native wildfl ower species that had been lost from the grassland as a result of inten-
sive agriculture. After two further years, plant plots where the hemiparasite was
more abundant achieved lower height, because of suppression of growth of the para-
sitized plants (Figure 9.11a). This led, by year 3, to the desired increase in grassland
species richness because competitive exclusion had been circumvented (Figure
9.11b). An understanding of exploiter-mediated coexistence holds promise for future
meadow restoration efforts.
 
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