Dogmatic is problematic: Interpreting evidence for latitudinal gradients in herbivory and defense

Abstract

It has long been believed that plant-animal interactions, including herbivory, are more intense toward the tropics. The more intense interactions in the tropics are thought to have selected for higher levels of defenses against herbivores. These ideas are fundamental to our understanding of global patterns in diversity, and for our understanding of plant-animal interactions. However, recent analyses have tended not to support the traditional hypothesis of higher herbivory and defenses at lower latitudes. Despite mounting empirical evidence, many ecologists have been slow to reassess their beliefs. I show clear evidence for citation bias, with papers that support the traditional idea being cited over six times as often as papers that show higher herbivory at higher latitudes and over four times as often as papers showing higher defense at higher latitudes. I also highlight examples where interpretations that are counter to the available empirical evidence have been published in high profile journals. I suggest that providing rigorous empirical tests for ideas that have become widely established without appropriate testing should be a priority for ecologists. We need to make sure the objectivity of peer-reviewed science stands out from the mass of unchecked opinion available on the web. Herbivory and plant defenses have enormous fitness consequences for both the plant and the animal populations involved (affecting plant ecological strategy, community structure, and biodiversity). Traditional theory suggested that interactions between species (including herbivory) would be more intense in the tropics , because herbivore populations are not knocked back by harsh winters, and because plants and animals in low latitude ecosystems have had longer to coevolve than have species in areas that experience glaciation (Schemske et al. 2009). The higher level of herbivory in low-latitude ecosystems is generally thought to have selected for higher levels of defense against herbivores (Coley and Aide 1991). These theories were very widely believed. However, recent syntheses of empirical data have not supported the idea that either herbivory, or plant defenses against herbivores are higher at lower latitudes (Moles et al. 2011, Poore et al. 2012). Unfortunately , it is our nature as humans to have trouble letting go of long-held theories, even when these are not well supported by empirical data. In this paper, I argue that belief has been shading our interpretation of the evidence regarding the latitudinal gradient in herbivory and defense. I began by asking whether there was evidence for citation bias in studies of herbivory and chemical defense. I compiled data on citation rates for all of the papers that met the criteria for inclusion in Moles et al.'s (2011) meta-analyses. I did not complement these results with more recently published papers, to avoid problems with estimating citation rates for papers that have been available for citing for a very short time. To account for the differences in the length of time that different papers have been available for citing, I used citations per year as the response variable. Papers were grouped in to those showing a significant (P < 0.05) result in the expected direction (or the expected result