Food supplementation experiments revisited: verifying that supplemental food is used by its intended recipients

Abstract

Field-based food supplementation experiments can determine whether populations are limited by natural food availability. However, they may yield spurious results if added food is hoarded by dominants, exploited by immigrants, or delivered ineffectively. Surprisingly, population-level approaches accounting for these potential sources of bias have not been established. We explored responses of free-ranging snowshoe hares (Lepus americanus Erxleben, 1777) to food supplementation by contrasting per capita browsing rates on three study sites with added food with those on three control sites during two winters. Food augmentation reduced foraging pressure by hares; this reduction was significant when browse species of high dietary importance were considered. By implication, hares on manipulated sites switched from favored natural foods to supplemental food, meaning that the extra food found its mark and was not heavily exploited by immigrants or hoarded by dominants. Demographic responses to food addition were not detected on the manipulated sites, indicating that the study population was not food limited. Given the success with which we detected the signal of supplementation and eliminated potential confounding factors, we suggest that future studies using food addition in the context of herbivore population dynamics would benefit from a more mechanistic approach; this may involve measurement of browsing rates on manipulated and control sites.Résumé : Des expériences en nature avec des suppléments alimentaires peuvent déterminer si des populations sont limitées par la disponibilité de la nourriture naturelle. Cependant, elles peuvent produire de faux résultats si la nourriture ajoutée est accumulée par les dominants, utilisée par les immigrants ou distribuée de manière inefficace. Il est surprenant qu'on n'ait pas établi de méthodologies pour tenir compte de ces sources potentielles d'erreur au niveau de la population. Nous examinons les réactions de lièvres d'Amérique (Lepus americanus Erxleben, 1777), libres en nature, à l'addition de suppléments de nourriture en comparant les taux de broutage par animal dans trois sites d'étude avec suppléments de nourriture et dans trois sites témoins au cours de deux hivers. L'addition de nourriture diminue la pression de recherche de nourriture chez les lièvres; cette réduction devient significative lorsqu'on tient compte des espèces de brout de grande importance dans le régime alimentaire. Les lièvres dans les sites manipulés ont donc implicitement passé de leur nourriture naturelle favorite à la nourriture ajoutée en supplément, ce qui indique que la nourriture supplémentaire a atteint ses destinataires et qu'elle n'a pas été fortement exploitée par les immigrants ou accumulée par les dominants. Nous ne décelons aucune conséquence démographique de l'addition de nourriture dans les sites manipulés, ce qui indique que la population d'étude n'est pas limitée par la nourriture. Compte tenu de notre capacité de détecter les signaux de l'addition de nourriture et d'éliminer les facte...

Ecological and evolutionary implications of food subsidies from humans

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Subalpine Bumble Bee Foraging Distances and Densities in Relation to Flower Availability

“…Although it is difÞcult to experimentally test whether populations of highly mobile consumers are food limited, examining consumer behaviors and patterns of abundance can provide insights into whether consumers have food shortages or surpluses (Petersen et al 2006, Wirsing andMurray 2007). If food is scarce, mobile consumers should have to spend more time foraging and may travel farther to collect sufÞcient resources when resources are exhausted near the nest (Suzuki et al 2007).…”

Population and individual level effects of over‐winter supplementary feeding mountain hares

“…Critically, however, analysing the data at the individual level, by using PIT tag data to identify which individuals actually used supplementary feed, suggested that food may have had a significant effect at the individual level. The use of supplementary feeding studies to assess the effect of food on population dynamics has been questioned -in particular doubts exist over whether supplementary feed is actually used by the intended recipients, and assessing both individual and population level changes in response to supplementary feeding is a challenge (Boutin, 1990;Wirsing & Murray, 2007). Direct and indirect evidence suggests that not all animals present on fed areas actually use supplementary feed stations (Kenward et al, 2005;Newey et al, 2009).…”

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“…treatment or block). In response to concerns over whether supplementary food actually reaches the target population, and how the postulated benefits of supplementary feed translate to changes in life history parameters (Boutin, 1990;Wirsing & Murray, 2007), we developed a passive induced transponder (PIT) tag system which enabled us to identify which individuals used supplementary feed and how long individuals spent at feeding stations . This allowed us to assess the effect of experimentally manipulating food availability on survival at both the individual and population level.…”