Revisiting the dietary niche: When is a mammalian herbivore a specialist?

Abstract

Understanding dietary specialization in herbivores has theoretical and practical implications in ecology, yet defining niche breadth consistently has been problematic. To increase clarity and communication among ecologists and among disciplines (i.e., chemists, pharmacologists), we propose a specialization key for mammalian herbivores that assigns "obligatory" and "facultative" modifiers to the terms "specialist" and "generalist". These modifiers are assigned based on (1) relative breadth of the animal's realized niche and diet (what it eats), (2) relative breadth of the fundamental niche and available diet (what it could eat), (3) the extent of chemical or physical characteristics, termed "difficulty", that make food items either low in value or unpalatable to most herbivores, and (4) relevant temporal and spatial scales at which diets and niche breadth were measured. Obligatory specialists always have a narrow realized niche consisting of difficult food items, and morphological adaptations and/or the loss of redundant behavioral flexibility that effectively limit their fundamental niches, precluding them from expanding their diet under changed environmental conditions. Facultative specialists have a consistently narrow realized niche for difficult foods during at least one spatial or temporal scale, but have a broad enough fundamental niche to allow them to expand their diet to include less difficult foods when environmental conditions allow. Facultative generalists have the broadest fundamental niche, allowing them to consume a wide variety of foods. However, they may occasionally demonstrate a narrow realized niche, focused on less difficult plants than is the case with specialists. Finally, the obligatory generalists always have a wide realized niche because of a relatively narrow fundamental niche, precluding them from eating much of any difficult plant. We summarize hypothesized characteristics of mammalian herbivores in each of the four categories of specialization. We demonstrate the need for further work on defining the realized and fundamental niches, comparing among herbivores across categories conducted under similar conditions, and understanding the nature of trade-offs required for specialization and generalization for both community and phylogenetically based analyses.

The dilemma of foraging herbivores: dealing with food and fear

“…It recognises a continuum of mammalian herbivores based on feeding niche from browsers, which consume foliage from shrubs and trees with high levels of chemical defence, to grazers, which consume structurally defended grasses (McArthur et al 1991). Importantly, the discipline also differentiates generalists, which consume broad diets, and specialists, which consume narrow diets, because these herbivores seek rewards in different ways (Freeland and Janzen 1974;Shipley et al 2009). In contrast, predator-prey research generally lumps herbivores in the trophic level called "prey", considering prey feeding niche to be unimportant.…”

Laccase detoxification mediates the nutritional alliance between leaf-cutting ants and fungus-garden symbionts

“…Many plant eaters have therefore become specialists, which has allowed them to evolve counteradaptations against specific plant defenses (2). Rather few herbivores have in fact evolved large enough body sizes and population densities to leave substantial ecological footprints as generalist grazers or browsers (2). In vertebrates, the most conspicuous examples are the ungulates and their marsupial analogs, many of whom rely on complex communities of bacterial gut symbionts for making this lifestyle productive (3).…”

A Pharm-Ecological Perspective of Terrestrial and Aquatic Plant-Herbivore Interactions

“…Recent studies suggest that generalist herbivores may be better at regulating PSM dose than specialist herbivores (Torregrossa et al, 2012). Specialist herbivores may not need to regulate intake of PSMs since their biotransformation system has evolved to process high doses of PSMs in a limited diet or because they absorb a lower proportion of PSMs consumed (Marsh et al, 2006b;Shipley et al, 2009;Sorensen et al, 2004). The mechanisms through which regulation of meal size and frequency occurs and why they differ between species are currently unknown.…”

“…Do species that transition between carnivory, omnivory and degree of specialized herbivory during their life span have more generalized or plastic PK-PD mechanisms than species without these transitions? For example, terrestrial vertebrates, aquatic, and particularly marine herbivores, are largely generalists and rarely specialize on particular foods (Poore et al, 2008;Shipley et al, 2009). …”

“…First, the investigator should observe foraging phenotypes that can be compared to other phenotypes (Shipley et al, 2009 there are likely to be mechanisms driving these patterns and more detailed pharmacokinetic and pharmacodynamic approaches such as those outlined below are warranted. We do not provide the details of such studies, but rather provide a general overview of how these pharmacological approaches can be useful in understanding and comparing mechanisms driving variation in foraging phenotypes.…”