FEEDING ECOLOGY
Brent are probably the most coastal of all geese and populations throughout the world are dependent on marine food resources for the majority of the year. Undoubtedly the most important food item are species of inter-tidal Zostera spp..Distribution of the main concentrations of Brent in all seasons except the summer replicate the distribution of inter-tidal Zostera in general terms though due to food depletion many coastal sites which do not contain Zostera are utilised. The reviews by Ward et al. and Ganter (PDFs) provide an overview of the dependence of Brent populations throughout the world on Zostera. Studies by O'Briain (1989), Mathers (19xx), Portig (19xx), Campton (19xx), Stone (19xx), Tinkler (20xx) and Inger (20xx) have all focussed on aspects of feeding ecology primarily in the winter range. Richard Inger's study using stable isotopes overcame many of the previous limitations of traditional approaches to studying habitat use and dietary selection. Fuller details are shown in the section below.
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Zostera is a marine angiosperm and whilst there is some debate about its taxonomy, intertidal Zostera of species Z. marina, angustifolia or noltii are all important food plants for Brent Geese. The most important sites for staging Brent in Iceland at Alftafjordur and Straumfjordur/Myrar have extensive (though largely unquantified) Zostera beds.
In Ireland most of the main wintering sites for Brent have Zostera beds (MAP) including the large beds at Lough Foyle, Strangford Lough, Tralee Bay, and Castlemaine Hbr. A survey of these beds in 2005 (Colhoun et al. in prep.) provided the most detailed and comprehensive assessment baseline survey of inter-tidal Zostera to date in Ireland.
Other food items are utlised in Ireland and Iceland; this includes terrestrial grasslands (incl. golf courses, and parkland), saltmarsh grasslands and other inter-tidal plants including the green algaes Enteromorpha and Ulva.
Use of Stable Isotopes in studies of feeding ecology
Dr Stuart Bearhop and Dr Richard Inger have considerable expertise in the use of stable isotopes to study inter alia feeding ecology and migratory connectivity in a range of species. Richard's 3-year PhD NERC studentship (Univrsity of Glasgow) focussed on the application of this technique for studying habitat switching of Brent Geese between inter-tidal and marine resources. Traditional approaches for studying this would have entailed direct (and time-comsuming) observation of an individuals feeding habits. Easier said than done! This approach presents some significant problems. Firstly these are highly mobile birds, and it’s almost impossible to track individuals all the time, so we can only determine what the birds are eating when we see them. Of course they may be eating other food items when we are not observing them. Also the distinction between habitat use and diet are not always clear. We often suppose that if we see geese in a field that they are feeding on the grass in the field, although this is not always the case.
To overcome the limitations of these traditional methods we have used stable isotope analysis to accurately determine what the birds are eating, and what are their most important food sources. The underlying principle of this approach is that all biologically active elements exist in two or more stable isotopic forms (not to be confused with radioactive isotopes). Different isotopes of an element differ is the number of neutrons found in the nucleus of the atom. By far the most common isotopic form is the ‘light’ isotope, which has the same number of neutrons to protons in the nucleus. For example around 99% of all Carbon found in nature consists of the light isotope, called 12C. Most of the rest of the Carbon is in the heavier form, with an extra neutron called 13C.
The important thing is that the ratio of the two isotopes is altered by various natural processes, such that the isotopic ratios of Carbon and Nitrogen are different in a number of different habitats. The old adage, ‘you are what you eat’ is true, with the atoms that make up the tissues of animals being ultimately derived from the food they eat, and this will be reflected in the isotopic ratios in there tissues. In the case of Brent Geese we were particularly interested in the amount of Zostera spp., green algae (Enteromorpha spp. and Ulva lactuca), and terrestrial grasses that they where eating, and how this changes thoughout the winter.
