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. However there is growing evidence in the scientific literature that such generalities are not consistent. This could be due to a number of reasons including the lack of consistency in the way herbivory is assessed such as different methodologies 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 researchers 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% assessed altitudinal gradients. Insect herbivory was most commonly expressed as percentage leaf damage using point herbivory. Fewer studies measured a range of different types of herbivory (such as sap sucking, leaf mining, galling, and root feeding) as leaves aged. From our synthesis, we hope that future research into insect 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. Measurements of plant traits will also assist in comparing herbivory across habitats, plant species, and within species physiological variation. The true impacts that insects have on plants via herbivory along environmental gradients are still poorly understood.
There is a general assumption in the literature that herbivory increases towards the tropics, but decreases with increasing altitude. However there is growing evidence that such generalities are not consistent across space, time and interacting resource gradients. These inconsistencies may be due to a variety of different methodologies used by researchers, or fundamental differences in leaf damage caused by different types of herbivores. Here we assess the current state of the literature and determine whether the study of insect herbivory along environmental gradients measuring different insect taxa can be compared across space, time and research methodologies. We also examine the biases in the type of herbivory collections that have been undertaken and advocate the use of more consistent methods that can be reproduced in all ecosystems.
Since Coley and Aide published their seminal work finding that herbivory was higher in the tropics compared to the temperate zones [
Another major issue with assessing insect herbivory along environmental gradients is the variation among plant species sampled within the gradient. Different plant species are found across all gradients, especially those covering tropical and temperate zones, lowlands to alpine areas, and coastal to inland desert regions. Along smaller gradients plants have different evolutionary relationships, different interactions with other species, and are under different competitive and symbiotic pressures both within and across species. From a structural point of view, plants have a suite of different aged leaves, with different chemical compositions [
Here we compare literature that has been published on insect herbivory along a range of gradients (including latitudinal, altitudinal, temperature, successional and architectural among others) and assess the methods that they have used to carry out their herbivory measurements.
In this review we assessed how insect herbivory has been measured across a range of studies. We have collected information on study location, the type of insect herbivory measured, plant species, gradient, and how herbivory was measured, either as a single point in time, or measured as a rate (either over leaf lifespan or between two points in time).
A literature search was conducted in Scopus for literature with the keywords: “herbivory and insect”, “herbivory and [each of the insect order names]”. Citations and abstracts were downloaded and assessed. Each article was then assessed to determine if it was appropriate for further assessment, and a reprint was downloaded. It is known that Scopus also lists keywords in addition to the author’s keywords, with many articles not actually referring to herbivory, in any part of their paper. If herbivory (or equivalent term) was not referred to in the paper (abstract or main body) it was not included in the assessment.
Of the 61 studies assessed over 30 years between 1982 and 2012 (
In the 61 studies reviewed, the type of herbivory assessed was also biased. Thirty one studies assessed leaf chewing herbivory. Only nine studies assessed sap sucking herbivory, and a further nine studies assessed a range of herbivory types. Six studies did not assess a specific herbivory type and four were reviews. Other studies assessed a range of herbivory types as well as pit, mining and galling herbivory separately. Even though chewing herbivory is the most obvious and easiest type of herbivory to measure, other herbivory types, such as sap-sucking into phloem and xylem can be more damaging to a plant. For example sap-sucking psyllids on Eucalypts in Australia can reduce sap flow by 50% and reduce leaf mass by 20% from necrotic tissue loss [
Of the studies assessed, most were carried out on a single tree species (n = 14), followed by single native shrub species (n = 11), multiple native tree species (n = 9), single herb/grass species (n = 8), multiple native
shrub species (n = 7) or multiple native tree/shrub species (n = 5). For studies along gradients, restricting the analyses to a single host plant species can be problematic as the gradient length is also constrained by the host plant distribution, but it is the simplest way to constrain host plant variables between species. However, even within host plant species, leaf chemistry and physiological traits can be substantially different. These differences may include variation across plant genotypes and geographical ranges, leaf chemical and physiological variation across different aged leaves, localized soil and microclimatic conditions, and previous herbivore feeding on plant tissue and plant viruses [15-18]. Assessing traits enables direct analysis of herbivory in relation to not only a range of plants, but to leaves at different ages and plants at different ages. For example Moles et al. [
Insect herbivory along latitudinal gradients were the most studied environmental gradient (n = 20) followed by altitudinal gradients (n = 10), with 6 studies along successional gradients, and three along climatic, and host plant architectural gradients. Coley and Aide’s seminal 1991 paper [
Percentage damage of leaf tissue was more widely measured than amount of leaf tissue damage per size of leaf (24 studies versus 19 studies). By using percentage herbivory data, researchers commonly arcsin transform their data to “normalize” it for standard univariate statistical analysis. This analysis of proportional data after arcsine square root transformation should not be used [
Assessing herbivory from leaf emergence to leaf senescence can be difficult to compare changes along gradients. Here we found 27 studies used point herbivory, while 18 used rate of herbivory, and 16 studies were nonspecific in how herbivory was assessed. The importance of using consistent methods can be illustrated by Andrew and Hughes [
Insect herbivory was most commonly assessed as leaf chewing herbivory at a single point in time (
General trends in how insect herbivory may change along biotic or abiotic gradients for different trophic groups is still unclear. For classic gradients, such as those done at different latitudes, the method used to assess herbivory plays a crucial role in the results found. The amount of herbivory has generally been reported as being higher in the tropics than at more temperate latitudes [15,26]. Coley and Aide’s [
Most studies assessing insect herbivory along environmental gradients have concentrated on latitudinal gradients with a bias toward chewing herbivores in the Northern Hemisphere. Future research into insect herbivory along gradients should assess herbivory caused by different types of herbivores rather than concentrate on chewers which are much easier to assess in the field but do not give a complete picture with regard to the impact of total insect herbivory on plant species. Future studies should also assess herbivory as a rate, rather than just a single point in time. This could be done by assessing herbivory across the leaf lifespan, or as a rate of herbivory that can be directly compared between host plant species and sample sites. Ideally knowing leaf life span and measuring herbivory at known leaf ages is also important, particularly for leaves that live for more than a single growing season. Assessing herbivory in relation to plant and leaf trait changes is also crucial as this will allow within and between species comparisons across a variety of environmental gradients. Plant traits (morphological and chemical) can act as defences against herbivores both by reducing herbivore performance directly and attracting natural enemies [