W

Resources high

Disturbance high

Fig. 3.2.13. Schematic presentation of the relationship between species diversity and resource availability, and between species diversity and habitat disturbance. Species diversity reaches a maximum at average ecosystem disturbance. (Hobbie et al. 1994)

Disturbance high

Fig. 3.2.13. Schematic presentation of the relationship between species diversity and resource availability, and between species diversity and habitat disturbance. Species diversity reaches a maximum at average ecosystem disturbance. (Hobbie et al. 1994)

disturbance intensities (intermediate disturbance hypothesis; Fig 3.2.13; Hobbie et al. 1994; see also Chap. 4.2). If the frequency of disturbance increases, only a few specialists remain. The number of species also decreases in the absence of external disturbance. Either only a few very competitive species dominate or there is competition for other resources, and thus biodiversity increases further (see also Fig. 3.2.5). Two examples demonstrate this effect:

1. Invasion and water use: For individual plants the limitation of water use was discussed in the context of adaptation to stress and optimisation of water use (see Chap. 2.4), but at the level of a plant stand the situation is different. If a plant "saves water", this may have the consequence that another species or individual plants of the same species uses this water. Opportunists which do not save water have an advantage, inasfar as they manage to sustain the species, e.g. by early seed production. The "saver of water" could dry out under these conditions. This occurs particularly at sites where new species invade existing vegetation (invasion, Fig. 3.2.14). Invasion of Mediterranean Bromus species into the North American Artemisia tridentata prairie led to a marked reduction of Artemesia (West and Young 2000). Bromus species use the water supply without limitation at the beginning of

120-1 Manitoba, Canada

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