Article

Stable Light Isotopes and Diet: A Broader Scale for Feeding Studies

Daryl Codron and Jacqui Codron, Quaternary Research Centre, University of Cape Town

Effective conservation of Africa's remaining wilderness areas requires a complete understanding of the resource needs of species, the interactions between various components of the ecosystem, and the means to monitor local conditions. For herbivorous mammals, plant foods are a key resource. Our knowledge of dietary preferences provides critical insight into the interactions between two major components of an ecosystem: plants and mammals. This is why an unprecedented amount of research focuses on the feeding ecology of mammals.

The most common techniques for studying animal diets are by direct observations, and physical and chemical analyses of digestive tract contents, both methods having provided an enormous amount of information to wildlife and nutritional ecologists. However, traditional methods for studying diet are intrusive, and are immensely time-consuming, limiting the scope of studies in space and time, and in the reproducibility of data. Because animal diets can change considerably with time, it is difficult to appreciate the full extent of variability amongst plant-mammal interactions.

Stable carbon isotope ecology is a natural-abundance tool for assessing diet, based on distributions in which the ratios of the stable isotopes of carbon (¹³C/¹²C) occur in biological materials. In herbivorous mammals of the African savannas, ¹³C/¹²C ratios ( ¹³C values) distinguish between browsers and grazers. This is because the carbon isotopic compositions of the plants they feed on are bimodaly distributed; C₃-photosynthesizing plants (dicotyledonous trees, shrubs, and forbs) have ¹³Cvalues consistently lower than do C₄-photosynthesizers (grasses). The tissues and excreta of herbivores faithfully record the isotopic composition of the diet, thereby qualifying and quantifying relative proportions of browse (C₃) to grass (C₄) intake.

The benefit of employing this technique is that numerous samples can be analyzed relatively quickly to gain accurate and reliable information, thus increasing the spatio-temporal scales that can be addressed. For instance, many studies based on observational methods have been restricted to particular family groups or herds, but ¹³C analysis offers a means to examine entire populations. The diets of geographically distant animals can be studied simultaneously without researchers necessarily being present in the field! Rapid analyses also means that one can examine numerous species to reveal broader community feeding ecology patterns, rather than being limited to one or two species of interest. Importantly, carbon isotope ecology can be used to track changes in diets through time - analysis of different materials offers the chance to explore diet over various time scales.

Faeces record dietary carbon isotopic compositions over a few days, hairs accumulate and aggregate dietary information over several months as they grow, and teeth and bones archive diets for a period of several years or even decades. Furthermore, faecal collections require no interference with the animals, and high-resolution seasonal information can be obtained according to the timing of sampling. Incrementally growing tissues such as hair and teeth offer the potential for serial analysis to trace feeding variations within an individual. With dietary data stored in hard tissues, it is also possible to reconstruct the diets of animals long since dead, potentially yielding invaluable historical records.

Stable carbon isotopic studies of diet cannot replace traditional methods; they do not provide important information about aspects such as species utilized, preferred foraging areas, height classes of grasses eaten, etc. But, analysis of ¹³C does broaden the scale in which feeding ecology can be explored. The method has often been applied to archaeological research and for reconstructing palaeo-diets of extinct taxa. Conservation and management has shifted from a static to an adaptive approach based on a dynamic, 'nature-evolving', paradigm. This approach requires methods that can answer research questions, which conform to a scale that accounts for natural ecological flux. Autecological single-species studies can seldom reveal the information necessary for holistic biodiversity management, and short-term data assumes that ecosystems are static.

Scientists at the Quarternary Research Centre at the University of Cape Town are currently employing analysis of ¹³Cto monitor spatio-temporal changes in browse/grass consumption of herbivores living in the Kruger National Park, South Africa. Thus far, they have already shown gross dietary differences amongst mixed-feeders such as elephants (Loxodonta africana) and impala (Aepyceros melampus) in different parts of the Park, as well as seasonal shifts. These scientists are also examining how C₃/C₄ consumption by different species has changed during the last century, following changes in climate, vegetation, and management regimes. This will all provide the Park with knowledge of areas where herbivory is more likely to impact heavily on woody vegetation, and will improve our understanding of how different species respond to different environmental conditions. Furthermore, the study will address the dietary ecology of locally rare species such as sable antelope (Hippotragus niger), roan antelope (Hippotragus equinus), and tsessebe (Damaliscus lunatus) in comparison with more abundant, sympatric species.

Stable light isotope ecology is not limited to ¹³Calone. Although less well-understood than carbon isotopes, stable nitrogen (¹⁵N/¹⁴N) and oxygen (¹⁸O/¹⁶O) isotopic ratios also provide invaluable insight into herbivore ecology. Used in conjunction with ¹³C, these variables are informative regarding trophic behaviour, water and nutrient stress, seasonality, and water sources. With these variables, the team are aiming to construct a clear picture of herbivore feeding ecology in KNP, which will reveal dietary variability over numerous spatial and temporal scales that have hitherto been unattainable. Documenting and understanding short- and long-term flux in plant-mammal interactions will go a long way towards resolving concepts of dietary variability, so often neglected and seldom appreciated.

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