Insect herbivory along environmental gradients

Abstract

There is a general assumption in the literature that insect herbivory increases towards the tropics, but decreases with increasing altitude. Similar generalities have been identified along other environmental gradients, such as resource, temperature, climatic and biotic gradients. How-ever there is growing evidence in the scientific literature that such generalities are not consis-tent. This could be due to a number of reasons including the lack of consistency in the way herbivory is assessed such as different metho-dologies used by researchers, or fundamental differences in leaf damage caused by different types of insect herbivores. Here we assess 61 publications researching insect herbivory along a range of environmental gradients (both biotic and abiotic) and review the methods that re-searchers have used to collected their data. We found leaf chewing from samples collected in North America dominated the field and most studies assessed herbivory on a single host plant species. Thirty three percent of the studies assessed latitudinal gradients, while 10% as-sessed altitudinal gradients. Insect herbivory was most commonly expressed as percentage leaf damage using point herbivory. Fewer stu-dies measured a range of different types of her-bivory (such as sap sucking, leaf mining, galling, and root feeding) as leaves aged. From our synthesis, we hope that future research into in-sect herbivory along environmental gradients will take into account herbivory other than just leaf chewing, such as sap sucking, which may cause more damage to plants. Future research should also assess herbivory as a rate, rather than just a single point in time as damage to a young leaf may be more costly to a plant than damage to a mature or senescing leaf. Meas-urements of plant traits will also assist in com-paring herbivory across habitats, plant species, and within species physiological variation. The true impacts that insects have on plants via her-bivory along environmental gradients are still poorly understood.