Long-term decline in white-tailed deer browse supply: can lichens and litterfall act as alternative food sources that preclude density-dependent feedbacks

Abstract

Selective browsing by cervids has persistent impacts on forest ecosystems. On Anticosti Island, Quebec, Canada, introduced white-tailed deer (Odocoileus virginianus (Zimmermann, 1780)) have caused massive changes to the native boreal forest. Despite the apparent stability of the deer population over recent decades, we suspected that they were not at equilibrium with their browse supply and that further degradation of the habitat had occurred. A comparison of two browse surveys conducted 25 years apart showed a strong decline in browse availability. Although balsam fir (Abies balsamea (L.) P. Mill.) remained the most available browse species, it declined or disappeared from most stands (n = 13). Preferred deciduous species that were still available 25 years ago have almost disappeared. The continuous decline of the browse supply confirmed our hypothesis. This situation may be exacerbated by a subsidy from the winter litterfall, a significant and stable alternative food source. The abundance of litterfall from mature trees is independent of browsing over a long time period, which introduces a temporal uncoupling between the impact of deer browsing on balsam fir seedlings and the negative feedback from recruitment failure of mature balsam fir on the deer population. This means that the system is susceptible to being forced into an alternative regime.Résumé : Le broutement sélectif des cervidés produit des impacts persistants sur les écosystèmes forestiers. À l'île d'Anticosti, Québec, Canada, une population introduite de cerf de Virginie (Odocoileus virginianus (Zimmermann, 1780)) a modifié considérablement la communauté végétale de la forêt boréale indigène. Malgré la stabilité apparente de la population de cerfs au cours des dernières décennies, nous soupçonnons l'existence d'un déséquilibre entre le cerf et le brout disponible, ainsi qu'une détérioration persistante de l'habitat. La comparaison de deux inventaires réalisés à 25 ans d'intervalle montre un fort déclin de la disponibilité du brout. Le sapin baumier (Abies balsamea (L.) P. Mill.) demeure la source de nourriture dominante, bien qu'il soit en déclin ou ait disparu de la plupart des peuplements (n = 13). Les espèces décidues préférées du cerf et encore disponibles il y a 25 ans sont pratiquement disparues. La réduc-tion continuelle du brout confirme notre hypothèse. En contrepartie, les lichens et ramilles qui tombent des sapins matures en hiver constituent une source de nourriture de rechange dont l'apport est indépendant de la pression de broutement sur une période prolongée. Cela introduit un découplage temporel entre les impacts du cerf sur les jeunes pousses de sapins et la rétroaction négative associée à l'absence de recrutement des sapinières sur les populations de cerf. Cette situation est susceptible d'entraîner le système vers un régime d'équilibre différent.Tremblay et al. 1096

Mortality in a Predator-free Insular Environment: the Dwarf Deer of Crete

“…In living ruminants, adult mortality is highly density dependent, in contrast to juvenile mortality, with loss attributed at least partly to malnutrition (Hutchinson, 1978;Tremblay et al, 2005, Simard et al, 2008. Interestingly, in less productive environments supporting fewer animals, death rates may actually be lower and the percentage of long-lived individuals higher than in more productive locales (as suggested by Veiberg et al [2007] for roe deer in France).…”

A large herbivore triggers alternative successional trajectories in the boreal forest

“…(Tremblay et al 2006). The resulting relatively open landscape has been typified as spruce savanna (Tremblay et al 2005). …”

Population density of North American elk: effects on plant diversity

“…Indeed, high densities of herbivores, exhibiting density-dependent feedbacks on measures of fitness (Gaillard et al 2000), can drive successional changes leading to dominance of plant species of lower nutritional value, slower rates of nutrient cycling (Pastor and Cohen 1997;Tremblay et al 2005;Stewart et al 2006), and restricted regeneration of palatable shrubs and trees such as black cottonwood (Populus trichocarpa) and bigleaf maple (Acer macrophyllum; Beschta and Ripple 2008). Indeed, trophic cascades resulting from high densities of herbivores following removal of carnivores has been observed in multiple ecosystems (Terborgh et al 2001;Ripple and Beschta 2004;Beschta and Ripple 2008).…”

“…Indeed, changing successional patterns resulting from high levels of herbivory is not uncommon when herbivore densities are close to carrying capacity. Tremblay et al (2005) suggested that because white-tailed deer (Odocoileus virginianus) switched from their primary forage balsam fir (Abies balsamea) to alternatives, such as lichens, which incidentally are higher quality forage than fir (Hodgman and Bowyer 1985;Ditchkoff and Servello 1998;Jenks and Leslie 1989), those populations somehow escaped density-dependent feedbacks. Nevertheless, those authors also reported that the deer population appeared to be stable over 25 years (Tremblay et al 2005), which is a strong indicator of density-dependent feedbacks regulating the population near carrying capacity.…”

“…Tremblay et al (2005) suggested that because white-tailed deer (Odocoileus virginianus) switched from their primary forage balsam fir (Abies balsamea) to alternatives, such as lichens, which incidentally are higher quality forage than fir (Hodgman and Bowyer 1985;Ditchkoff and Servello 1998;Jenks and Leslie 1989), those populations somehow escaped density-dependent feedbacks. Nevertheless, those authors also reported that the deer population appeared to be stable over 25 years (Tremblay et al 2005), which is a strong indicator of density-dependent feedbacks regulating the population near carrying capacity. Carrying capacity is defined as the number of animals at or near equilibrium with their food supply ; however, that definition does not mean that plant communities are static or that primary forage types need to be in equilibrium with the herbivore population.…”