Herbivory and plant survival

mortality: the result of an interaction with another factor?

Generally, it is more usual for herbivores to increase a plant's susceptibility to mortality than to kill it outright. For example, although the flea beetle Altica sublicata reduced the growth rate of the sand-dune willow Salix cordata in both 1990 and 1991 (Figure 9.5), significant mortality as a result of drought stress only occurred in 1991. Then, however, susceptibility was strongly influenced by the herbivore: 80% of plants died in a high herbivory treatment (eight beetles per plant), 40% died at four beetles per plant, but none of the beetle-free control plants died (Bach, 1994).

Repeated defoliation can have an especially drastic effect. Thus, a single repeated defoliation defoliation of oak trees by the gypsy or ring-barking moth (Lymantria dispar) led to only a 5% can kill mortality rate whereas three successive heavy defoliations led to mortality rates of up to 80% (Stephens, 1971). The mortality of established plants, however, is not necessarily associated with massive amounts of defoliation. One of the most extreme cases where the removal of a small amount of plant has a disproportionately profound effect is ring-barking of trees, for example by squirrels or porcupines. The cambial tissues and the phloem are torn away from the woody xylem, and the carbohydrate supply link between the leaves and the roots is broken. Thus, these pests of forestry plantations often kill young trees whilst removing very little tissue. Surface-feeding slugs can also do more damage to newly established grass populations than might be expected from the quantity of material they consume (Harper, 1977). The slugs chew through

I No herbivory Low herbivory High herbivory

I No herbivory Low herbivory High herbivory

Plant Species Richness

Clone number

Figure 9.5 Relative growth rates (changes in height, with standard errors) of a number of different clones of the sand-dune willow, Salix cordata, (a) in 1990 and (b) in 1991, subjected either to no herbivory, low herbivory (four flea beetles per plant) or high herbivory (eight beetles per plant). (After Bach, 1994.)

the young shoots at ground level, leaving the felled leaves uneaten on the soil surface but consuming the meristematic region at the base of shoots from which regrowth would occur. They therefore effectively destroy the plant.

Predation of seeds, not surprisingly, has a predictably harmful effect on individual plants (i.e. the seeds themselves). Davidson et al. (1985) demonstrated dramatic impacts of seed-eating ants and rodents on the composition of seed banks of 'annual' plants in the deserts of southwestern USA and thus on the make up of the plant community.

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