Herbivory

Herbivory can be tested using real herbivores or by simulating herbivory by manually cutting or removing plant tissue. The type of manipulation selected will depend on the type of plants and herbivores examined. For example, the effect of elephant raids on crop yield cannot be simulated easily in a greenhouse experiment. Tiffin and Inouye (2000) have reviewed the advantages and disadvantages of using natural vs. imposed herbivory.

Herbivore addition and removal

To test for the effect of a herbivore on vegetation, it is possible to either increase or decrease herbivore density. Herbivore removal is often easier but can necessitate killing the organisms. Herbivore additions are difficult, because organisms must be caught and then added and maintained in a new habitat. This may cause problems if animal behaviour varies with density or if organisms are territorial. Thus the biology of the animal must be understood before density changes are imposed. For example, if animals become more aggressive and eat less when in high density, you will not get

Table 10.5. Mean (se) above-ground biomass and species richness of grasses and forbs with and without sawdust and tillage in a coastal Californian grassland 2 years after treatment (adapted from Alpert and Maron, 2000).

Tillage + sawdust

Tillage

Control

Biomass (g dry above-ground biomass m-2)

Grasses

Native

1 (1)

18 (18)

0 (0)

Non-native

253 (40)

466 (95)

386 (56)

Broadleaves

Native

44 (12)

23 (6)

23 (10)

Non-native

102 (26)

106 (17)

90 (16)

Species richness (number per 0.25 m2)

Grasses

Native

1.8 (0.1)

2.0 (0)

1.6 (0.2)

Non-native

2.4 (0.2)

2.5 (0.3)

2.4 (0.3)

Broadleaves

Native

4.4 (0.3)

3.4 (0.6)

3.0 (0.8)

Non-native

1.8 (0.3)

1.1 (0.3)

0.7 (0.3)

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