Herbivory defoliation and plant growth

Despite a plethora of defensive structures and chemicals, herbivores still eat plants. Herbivory can stop plant growth, it can have a negligible effect on growth rate, and it can do just about anything in between. Plant compensation may be a general response to herbivory or may be specific to particular herbivores. Gavloski and Lamb (2000b) tested these alternative hypotheses by measuring the biomass of two cruciferous plants Brassica napus and Sinapis alba in response to 0, 25 and 75% defoliation of seedling plants by three herbivore species with biting and chewing mouthparts - adult flea beetles Phyllotreta cruciferae and larvae of the moths Plutella xylostella and Mamestra configurata. Not surprisingly, both plant species compensated better for 25% than 75% defoliation. However, although defoliated to the same extent, both plants tended to compensate best for defoliation by the moth M. configurata and least for the beetle P. cruciferae (Figure 9.3). Herbivore-specific compensation may reflect plant responses to slightly different patterns of defoliation or different chemicals in saliva that suppress growth in contrasting ways (Gavloski & Lamb, 2000b).

7 14 21 28 Days after defoliation

Phyllotreta cruciferae Plutella xylostella Mamestra configurata

7 14 21 28 Days after defoliation timing of herbivory is crucial

Figure 9.3 Compensation of leaf biomass (mean ± SE: (loge biomass defoliated plant) - (loge of mean for control plants)) of Brassica napus and Sinapis alba seedlings with 25 or 75% defoliation by three species of insect (see key) in a controlled environment. On the vertical axis, zero equates to perfect compensation, negative values to undercompensation and positive values to overcompensation. Mean biomasses of defoliated plants that differ significantly from corresponding controls are indicated by an asterisk. (After Gavloski & Lamb, 2000b.)

Figure 9.4 The survivorship of leaves on waterlily plants grazed by the waterlily leaf beetle was much lower than that on ungrazed plants. Effectively, all leaves had disappeared at the end of 17 days, despite the fact that 'snapshot' estimates of loss rates to grazing on grazed plants during this period suggested only around a 13% loss. (After Wallace & O'Hop, 1985.)

Figure 9.4 The survivorship of leaves on waterlily plants grazed by the waterlily leaf beetle was much lower than that on ungrazed plants. Effectively, all leaves had disappeared at the end of 17 days, despite the fact that 'snapshot' estimates of loss rates to grazing on grazed plants during this period suggested only around a 13% loss. (After Wallace & O'Hop, 1985.)

In the example above, compensation, which was generally complete by 21 days after defoliation, was associated with changes in root biomass consistent with the maintenance of a constant shoot: root ratio. Many plants compensate for herbivory in this way by altering the distribution of photosynthate in different parts of the plant. Thus, for example, Kosola et al. (2002) found that the concentration of soluble sugars in the young (white) fine roots of poplars (Populus canadensis) defoliated by gypsy moth caterpillars (Lymantria dispar) was much lower than in undefoliated trees. Older roots (>1 month in age), on the other hand, showed no significant effect of defoliation.

Often, there is considerable difficulty in assessing the real extent of defoliation, refoliation and hence net growth. Close monitoring of waterlily leaf beetles (Pyrrhalta nymphaeae) grazing on waterlilies (Nuphar luteum) revealed that leaves were rapidly removed, but that new leaves were also rapidly produced. More than 90% of marked leaves on grazed plants had disappeared within 17 days, while marked leaves on ungrazed plants were still completely intact (Figure 9.4). However, simple counts of leaves on grazed and ungrazed plants only indicated a 13% loss of leaves to the beetles, because of new leaf production on grazed plants.

The plants that seem most tolerant grasses are of grazing, especially vertebrate grazing, particularly tolerant are the grasses. In most species, the of grazing meristem is almost at ground level amongst the basal leaf sheaths, and this major point of growth (and regrowth) is therefore usually protected from grazers' bites. Following defoliation, new leaves are produced using either stored carbohydrates or the photosyn-thate of surviving leaves, and new tillers are also often produced.

Grasses do not benefit directly from their grazers' attentions. But it is likely that they are helped by grazers in their competitive interactions with other plants (which are more strongly affected by the grazers), accounting for the predominance of grasses in so many natural habitats that suffer intense vertebrate grazing. This is an example of the most widespread reason for herbivory having a more drastic effect on grazing-intolerant species than is initially apparent - the interaction between herbivory and plant competition (the range of possible consequences of which are discussed by Pacala & Crawley, 1992; see also Hendon & Briske, 2002). Note also that herbivores can have severe nonconsumptive effects on plants when they act as vectors for plant pathogens (bacteria, fungi and especially viruses) - what the herbivores take from the plant is far less important than what they give it! For instance, scolytid beetles feeding on the growing twigs of elm trees act as vectors for the fungus that causes Dutch elm disease. This killed vast numbers of elms in northeastern USA in the 1960s, and virtually eradicated them in southern England in the 1970s and early 1980s.

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