School of Biological Sciences, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK.
J Anim Ecol. 2013 Jul;82(4):739-48. doi: 10.1111/1365-2656.12049. Epub 2013 Jan 30.
The optimal foraging theory, the nutrient balance hypothesis, and the plant association theories predict that foraging decisions and resulting tree damage by large mammalian browsers may be influenced by the species richness and species composition of forest stands. This may lead to either associational susceptibility (increased damage on a focal plant in a mixed stand) or associational resistance (reduced damage in a mixed stand). Better understanding of the mechanisms and the relative importance of tree species richness and composition effects on foraging by mammalian browsers is needed to support sustainable management of forests and mammal populations. However, existing knowledge of forest diversity effects on foraging by large mammalian browsers comes largely from observational studies while experimental evidence is limited. We analysed winter browsing by moose (Alces alces L.) in a long-term, large-scale experiment in Finland, which represents a tree species richness gradient from monocultures to 2-, 3- and 5-species mixtures composed of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies L.), Siberian larch (Larix sibirica Ledeb.), silver birch (Betula pendula Roth.) and black alder (Alnus glutinosa L.). The intensity of browsing per plot increased with tree species richness while browsing selectivity decreased with tree species being targeted more equally in species-rich mixtures. Tree species composition of a plot was also an important determinant of intensity of browsing. The greatest browsing occurred in plots containing preferred species (pine and birch) while intermediate preference species (larch and alder) experienced associational susceptibility when growing with pine and birch compared with their monocultures or mixtures without pine and birch. In contrast, we found no evidence of associational resistance; the presence of a least preferred species (spruce) in a mixture had no significant effect on moose browsing on other tree species. We demonstrate that the presence of alternative forage species allows moose to spend longer opportunistically foraging in a plot, resulting in increased level of damage in species-rich stands and stands containing preferred tree species. Our results highlight the limitations of the optimal foraging theory in predicting browsing patterns and demonstrate the importance of associational effects within mixed stands.
最优觅食理论、养分平衡假说和植物联会理论预测，觅食决策以及大型食草哺乳动物对树木造成的损害可能会受到森林林分物种丰富度和物种组成的影响。这可能导致联会易感性（在混交林中对焦点植物的损害增加）或联会抗性（在混交林中损害减少）。为了支持森林和哺乳动物种群的可持续管理，需要更好地了解物种丰富度和组成效应对哺乳动物食草动物觅食的机制和相对重要性。然而，关于森林多样性对大型食草哺乳动物觅食影响的现有知识主要来自观察性研究，而实验证据有限。我们分析了在芬兰进行的一项长期、大规模实验中驼鹿（Alces alces L.）在冬季的觅食情况，该实验代表了一个从单一栽培到由 2、3 和 5 种树种组成的混交林的树种丰富度梯度，这些树种包括欧洲赤松（Pinus sylvestris L.）、挪威云杉（Picea abies L.）、西伯利亚落叶松（Larix sibirica Ledeb.）、银桦（Betula pendula Roth.）和黑桤木（Alnus glutinosa L.）。每个样地的觅食强度随树种丰富度的增加而增加，而选择性觅食则随目标树种在丰富的混交林中更加均匀地分布而降低。样地的树种组成也是觅食强度的一个重要决定因素。在含有首选树种（松树和桦树）的样地中，觅食最为严重，而在与松树和桦树一起生长时，中间偏好树种（落叶松和桤木）与它们的单一栽培或不含松树和桦树的混交林中相比，表现出联会易感性。相比之下，我们没有发现联会抗性的证据；在一个混交林中存在最不受欢迎的物种（云杉）对驼鹿在其他树种上的觅食没有显著影响。我们证明，替代饲料物种的存在使驼鹿有机会在一个样地中更长时间地机会性觅食，从而导致丰富树种林分和含有首选树种的林分中的损害水平增加。我们的研究结果突出了最优觅食理论在预测觅食模式方面的局限性，并证明了混交林中联会效应的重要性。
