Aloe nectar, birds and stable isotopes: opportunities for quantifying trophic interactions

A common issue associated with studies of animal diets and resource use is the difficulty of estimating the contribution of a dietary source to the nutrient budget of individual animals. In southern Africa, the tree-like succulent Aloe marlothii makes available to the animal community its abundant energy and water resources in the form of floral nectar in exchange for pollination services. The copious production of floral nectar (50–100 L ⁄ha, yielding 100 000–200 000 kJ ⁄ha in energy) and the observation that at least 73 species of birds have been observed visiting aloe flowers suggests that this plant may be of significant energetic (and ⁄or osmoregulatory) importance to the bird community. In this issue, Symes et al. (2011) attempt to quantify these interactions using carbon stable isotopes to track aloe nectar use by the bird community. Given a suitable isoscape and a few caveats, stable isotope methods can provide direct estimates of resource use by consumers. Here, we provide an overview of the opportunities and constraints that stable isotope methods offer, using the aloe nectar ⁄bird community system as an example. In many ecosystems, stable isotope approaches can provide powerful insights into the depth and breadth of species interactions and the movement of energy and materials through individuals and foodwebs. In the past, estimating the contribution of a certain food source to the nutritional ecology of a particular species or community has been time and labour intensive, and has been accomplished through observations of feeding activities (e.g. Cecere et al. 2010), stomach sampling (e.g. Tsipoura & Burger 1999) and fecal analysis (e.g. Durst et al. 2008). Although these approaches provide snapshot insights into the use of specific dietary items, they do not provide quantitative information on the resources that are actually assimilated. Stable isotope analysis focuses on the nutrients assimilated by animals and relies on the old adage that ‘you are what you eat’, which is both isotopically and literally true. The isotopic composition of an animal’s tissues generally reflects that of its diet, with some predictable offset due to biochemical and physical processes that tend to favour the lighter and more abundant isotopes of a given element. Thus, for carbon isotopes (C and C), animals and their tissues tend to be enriched in the C isotope compared with their dietary sources. These differences are represented using the delta notation (d) and reported on a partsper-thousand or per mil (&) basis compared with a standard reference material for a given element (i.e. dC – & VPDB (Vienna Pee Dee Belemnite)). If an animal’s dietary sources differ appreciably in their carbon isotope ratios (dC), then estimates of carbon incorporation can be made from measurements of tissue isotope ratios (dC). Stable isotope approaches do not provide a magic bullet for divining the sources of nutrients flowing into any consumer. For the stable isotope approach to be useful, the dietary source or sources of interest must differ isotopically (by a few & or more) from the background isotopic landscape. The Suikerbosrand Nature Reserve in South Africa, where Symes et al. conducted their study, is a savannah ecosystem and provides a particularly informative example because plants using all three common photosynthetic pathways occur there. How does photosynthetic pathway come into play? In arid ecosystems, plants use C3, C4 or CAM photosynthesis and the photosynthetic chemistry of these differing pathways leads to differences in the isotopic composition of the tissues they produce. Globally, C3 plants represent the bulk of plant biodiversity and terrestrial biomass, accounting for approximately 95% of all species and about 70% of global plant biomass. They include most flowering trees, shrubs and annual plants found in temperate and tropical climates. Isotopically, their tissues tend to be greatly depleted compared with plants using C4 or CAM photosynthesis (e.g. at Suikerbosrand values averaged: C3 d C = )27.2& VPDB; C4 d C = )14.7& VPDB and CAM dC = )12.6& VPDB). Globally, C4 species account for approximately 3% of total plant biodiversity and a surprising 25% of global plant biomass. C4 plants are dominated by warm season tropical and sub-tropical grasses and are greatly enriched isotopically compared with C3 plants (see above). The third photosynthetic pathway type found on the Suikerbosrand reserve are the CAM plants (crassulassian acid metabolism), which include A. marlothii (Asphodelaceae) and represent a diverse group (26 families), many of which are succulents. Well-known families include the Cactaceae, Crassulaceae, Euphorbiaceae, Orchidaceae, *Corresponding author. Email: [email protected]