Feral horses may also reduce the richness of plant species. Exposure of soil caused by trampling and vegetation removal via grazing, combined with increased nutrients being recycled by horse dung, favour weed species, which then invade the region and overtake native species, diminishing their diversity. The dispersal of weeds is aided by the attachment of seeds to the horses' manes and tails, and are also transferred via horse dung after consumption of weeds in one location and excrement in another. Although the effects of the weeds that actually germinate after transfer via dung is debated, the fact that a large number of weed species are dispersed via this method is of concern to those interested in the survival of native plant species in Australia. The effect on plants and plant habitats are more pronounced during droughts, when horses travel greater distances to find food and water. They consume the already threatened and limited vegetation, and their negative influences are more widespread. Feral horses may also chew the bark of trees, which may leave some trees vulnerable to external threats. This has occurred during drought, among eucalyptus species on the Red Range plateau. It appears as though feral horses may prefer these species.