To be effective as a control measure, the culling of a pest must exceed its capacity to regenerate. In North Australia, where feral donkeys are a serious pest of rangelands, a culling policy is being applied. Population studies have shown that 23 percent of the donkeys need to be removed each year to control density. Also, this figure must be exceeded if the population is to be reduced.

D. Choquenot, "Rate of increase for populations of feral donkeys in northern Australia," Journal of Mammalogy, 71, 151-5, July 1990. (Canberra College of Advanced Education, Australia)

Rapid nutrient turnover in tropical rain forests contributes to their high productivity. Lichen epiphytes, which often represent a significant proportion of the canopy biomass, are commonly dislodged by storms. Their decomposition, determined by litter bag experiments, has now been shown to be rapid: Between 60 and 90 percent of their biomass is lost in the course...

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