Author Spotlight: The Need for New Categorizations of Dietary Specialism in Animals

Emilio Pagani-Núñez, Craig A. Barnett, Hao Gu & Eben Goodale

Guangxi University, Nanning, Guangxi Zhuang Autonomous Region, People’s Republic of China

 

Moreover, it is not understood when a species will evolve a particular niche width through the device of being polymorphic with phenotypes which are resource specialists, monomorphic with one generalist phenotype, or some intermediate condition.                                                 J. Rouhgarden (1974) The American Naturalist, 108 (962): 429-442

 

Certain ideas are rapidly and broadly accepted by the scientific community, sometimes even by a whole society.  This is the case of the concepts of ‘generalism’ and ‘specialism’, which have improved our understanding of ecological and evolutionary processes in nature.  However, to some extent, many authors have stressed the need to review and reformulate the existing theory about diet specialization.  This motivated us to write our review article “The need for new categorizations of dietary specialism incorporating spatio-temporal variability of individual diet specialization”, published in the September 2016 issue of the Journal of Zoology.

jzo-september-2016-cover-image_jzo-11-15-r-362r1_cropped
Great tit nestling (Parus major), photo by Emilio Pagani-Nunez

At present, there is no consensus among researchers on a definition of diet specialization. We searched papers defining great and blue tits as ‘generalists’ or ‘specialists’ and we observed a great inconsistency among authors in how they classified the two species.  Traditionally, diet specialization was conceptualized as a continuous gradient from totally generalist species to completely specialized species that were behaviourally and physically adapted to eat specific food types. However, the discovery of individual diet specialization (IDS) challenges the traditional gradient of specialization within and among species. IDS means that, within species and populations, different individuals can specialize in the exploitation of different subsets of the available resources.

Species with broad diets and widespread distributions can provide useful examples of how species theoretically considered as generalists can show a surprising level of specialization at both the population and the individual level. Conversely, irrespective of distribution range, species inhabiting variable environments can evolve strikingly variable levels of specialization among individuals from both spatial and temporal perspective. In this paper, we reviewed the literature on specialization in great tits and blue tits.  Both species are considered caterpillar specialists at the centre of their ranges, but this reliance on caterpillars declines at the periphery of their ranges (e.g. the Mediterranean).  Specifically, we found that great tits show a higher level of IDS than blue tits do, developing new dietary specializations when the most common prey is not available.

Another issue that has not been well explored in the literature is the role of foraging innovations in promoting novel dietary specializations. Research in foraging innovations also has a long history of research (e.g. the opening of milk bottle tops by great and blue tits in United Kingdom). There have been few attempts to link foraging innovations to the capacity to evolve contrasting levels of specialization from the species to the individual level. However, the capacity for foraging innovations may explain why certain ‘generalist’ species show such contrasting levels of specialization between populations and individuals.

Here, we introduce a new categorization of specialization that takes into account all these factors.  We define obligate specialists as species composed of individuals that are primarily adapted to exploiting a single resource (e.g. hummingbirds). Facultative specialists are species in which individuals are adapted to exploit a single food type or niche, but occasionally they can exploit alternative food resources (e.g. birds of prey). Facultative generalists are species composed of individuals that are able to develop novel specializations to preferably exploit alternative resources, (e.g. great tits and sea otters).  Finally, obligate generalists are able to exploit a wide variety of resources (or one if it is very abundant), with a limited capacity to incorporate foraging innovations (e.g. many passerine species).

By publishing our new classification, our aim is to provide clearer and more workable definitions of diet specialization within and among populations and species. We also are interested in initiating a debate among ecologists about the necessity of incorporating IDS as well as spatial and temporal variation into studies of foraging ecology.  This general framework is also attractive because it is applicable to a wide variety of taxa and makes comparison among taxa easier.

 

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