Herbivores, plant parasites, and plant diversity

The small size of plant parasites, coupled with their relatively subtle per-capita impact, has meant that visible signs of their feeding are often overlooked. Because most consequences of parasite attack (eg reduced plant vigour) could be readily attributed to other, more conspicuous environmental factors, parasite attack has rarely been invoked as accounting for differences in competitive ability between plants. Plant parasites are likely to influence plant diversity if there is a correlation between susceptibility to parasite attack (palatability) and competitive ability. When a plant's competitive ability is positively correlated with its palatability to herbivores, then plant species richness will be increased, because competitive dominance will be prevented. Less palatable and less competitive plants will obtain an advantage in this type of grazed community. A negative correlation between palatability and competitive ability will typically mean that herbivore feeding leads to reduced species richness, because the most competitive plants suffer relatively low rates of attack. If there is no correlation between palatability and plant competitive ability, herbivores could still influence diversity, so long as different plant species suffered differential rates of performance loss under the same degree of herbivory. The indirect activity of larger herbivores may affect plant diversity through altering the availability of recruitment microsites. Increasing the availability of microsites for relatively uncompetitive plants would tend to increase diversity and vice versa. -from Authors