No link to ecosystem collapse.

Much confusion has arisen from controversies concerning the possible irreversibility of land degradation processes. One source of confusion relates to the - often implicit - assumption that some state of the ecosystem is the 'natural' (Sprugel, 1991) stable state, and that a disturbance, be it a drought or human intervention, would move the system away from this state. If the disturbance were large enough, and the system sufficiently vulnerable, the system would not be able to return to the equilibrium state, and the change would be termed irreversible. The validity of this idea of ecosystem behaviour has been questioned, and semi-arid areas may alternatively be descnbed as eventdriven' systems, constantly undergoing change (Ellis & Swift, 1988 ; Scoones, 1993 ; Behnke & Scoones, 1993). However true it may be that variability is an important feature of dryland ecosystems, it should be remembered that pollen analysis of sediment cores from lakes in the Sahel-Sudan shows a relatively constant floral composition over the last three thousand years (Andres et al., 1996). This indicates that whereas productivity and relative abundance of species may be characterized by variability, ecosystem structure and species composition may be relatively stable. However, much more research is required to resolve this question.