Do we expect a latitudinal gradient in herbivory?

Well, actually, you can make a good argument for a positive, negative or quadratic relationship between herbivory and latitude, or even for no relationship. Personally (ATM), I think the latter is the most likely. The alternative arguments are set out below.

1) The case for higher levels of herbivory in the tropics.
It has often been suggested that because tropical climates are more continuously favourable than those farther from the equator (not proven), species in the tropics are less likely to be limited by abiotic factors. This release from abiotic limitation, along with the long evolutionary history of the tropics (which have escaped the intermittent glaciation to which much of the world has been subjected) is thought to have allowed species interactions to intensify (See box below for Dobzhansky's thoughts on this topic). This idea, applied directly to plant-animal interactions would predict higher levels of herbivory in the tropics. This hypothesis is supported by the existing (sparse) data on herbivory (e.g. Coley and Barone 1996).
 
"The process of adaptation for life in temperate and especially in cold zones consists, for man as well as for other organisms, primarily in coping with the physical environment and in securing food. Not so in the tropics. Here little protection against winter cold or inclement weather is needed. In the rainforests, the amount of moisture is sufficient at any time to prevent the inhabitants from suffering from dessication. Relatively little effort is necessary for man to secure food, and it seems that the amount of food is less often a limiting factor for the growth of populations of tropical animals than it is in the extratropical zones. But the biological environment is likely to be harsh and exacting."

"Where physical conditions are easy, interrelationships between competing and symbiotic species become the paramount adaptive problem".

from: Dobzhansky, T. (1950) Evolution in the tropics. American Scientist 38: 209-221.
Theodosius Dobzhansky

2) The case for lower levels of herbivory in the tropics.
Hairston et al. (1960). argued that since “the world is green” (mostly), the third trophic level of predators and parasitoids must be suppressing herbivores more than herbivores suppress plants. Hairston et al. (1960), taken together with the idea of more intense biological interactions in the tropics (Dobzhansky 1950) leads to the prediction that animal impacts on plants will actually be less in the tropics.

3) The case for a mid-latitude peak in herbivory.
It has been suggested that the slow-growing and unpalatable plants characteristic of systems with very low primary productivity do not provide sufficient resources to sustain populations of herbivores, and therefore have low levels of herbivory.  At intermediate levels of productivity, there is sufficient plant material available to sustain healthy populations of herbivores, but not enough herbivores to sustain  populations of carnivores. Thus, the herbivores are relatively unsupressed, and the plants sustain a lot of damage. At very high levels of productivity viable populations of carnivores can be maintained, which keeps the herbivores in check, and leads to low levels of herbivory. Fraser and Grime's (1997) data from derbyshire dales supports this hypothesis. Combining this hypothesis with the latitudinal gradient in productivity leads to the prediction of a mid-latitude peak in herbivory.

4) The case for no relationship between latitude and herbivory.
We would predict no relationship between latitude and herbivory if levels of plant productivity were matched by levels of herbivore abundance and activity. This might happen if herbivore abundance was determined primarily by the mass of available food. In support of this idea, Moran and Southwood (1982) found high consistency in the proportion of species of different feeding guilds on several tree species in two ecosystems.

 

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