Hutchings Michael R, Gordon Iain J, Kyriazakis Ilias, Robertson Ewen, Jackson Frank
Animal Nutrition and Health Department, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK.
Macaulay Institute, Craigiebuckler, Aberdeen, UK.
Oecologia. 2002 Aug;132(3):453-460. doi: 10.1007/s00442-002-0971-z. Epub 2002 Aug 1.
We tested the hypothesis that the infra-gastrointestinal parasite population of herbivores affects their grazing behaviour in relation to the supra-parasite population of parasites in the environment. Our first objective was to create a naturally heterogeneous sward structure of gaps and tussocks using a continuous grazing scheme. We then demonstrate that a nutrition vs. parasitism grazing trade-off occurs within that sward structure and that infra-gastrointestinal parasite populations affect the grazing decisions of herbivores faced with the trade-off. A pool of 50 naturally parasitised female Soay sheep and their lambs were used to create a heterogeneous tall, faeces-contaminated tussock/short, non-contaminated gap sward structure in a 1-ha experimental plot. Tussocks offered approximately 1.5 times greater forage intake but contained 5.5 times the number of strongyle parasites compared to the gaps. Following a 10-week period in which the heterogeneous sward structure was created, two 5-day periods of observations of sward structure selection (i.e. gap vs. tussock) were carried out. Twenty female Soay lambs were divided into two groups of ten (balanced for live-weight) immediately prior to the start of the observation period. One of the groups of lambs was treated with an anthelmintic drench before the start of the second observation period creating two levels of parasitism (high and low). On each observation day 5-min focal observations were carried out on each animal at least twice a day, during which time the number of bites taken from gaps and tussocks were recorded along with the number of steps. During the first period of observations, all animals rejected the relatively tall, faeces-contaminated tussocks for grazing to a similar extent and had similar bite and step rates. During the second period of observations all animals showed reduced rejection of the tussocks relative to the first week, however, animals with a reduced parasite population showed a greater reduction in rejection as compare to the highly parasitised animals. We conclude that the infra- and supra-distributions of parasites within herbivore hosts and the environment greatly impact on herbivore grazing behaviour and foraging decisions and thus the structure and heterogeneity of grazed ecosystems.
食草动物胃肠道内寄生虫种群会影响其放牧行为，这与环境中寄生虫的体表寄生虫种群有关。我们的首要目标是通过连续放牧方案创建一个具有自然异质性的草丛结构，包括间隙和草丛。然后我们证明，在这种草丛结构中会出现营养与寄生虫感染之间的放牧权衡，并且胃肠道内寄生虫种群会影响面临这种权衡的食草动物的放牧决策。在一个1公顷的实验地块中，使用50只自然感染寄生虫的雌性索艾羊及其羔羊，创建了一个异质性的高大、粪便污染的草丛/矮小、未受污染的间隙草丛结构。与间隙相比，草丛提供的采食量大约高1.5倍，但包含的圆线虫寄生虫数量是间隙的5.5倍。在创建异质性草丛结构的10周时间后，进行了两个为期5天的观察期，观察对草丛结构的选择（即间隙与草丛）。在观察期开始前，20只雌性索艾羔羊被立即分成两组，每组10只（体重均衡）。在第二个观察期开始前，其中一组羔羊用驱虫药进行了处理，从而产生了两种寄生虫感染水平（高和低）。在每个观察日，每天至少对每只动物进行两次5分钟的焦点观察，在此期间记录从间隙和草丛中采食的口数以及步数。在第一个观察期内，所有动物对相对高大、粪便污染的草丛的拒食程度相似，采食口数和步速也相似。在第二个观察期内，所有动物相对于第一周对草丛的拒食都有所减少，然而，寄生虫数量减少的动物与寄生虫感染严重的动物相比，拒食减少的幅度更大。我们得出结论，食草动物宿主和环境中寄生虫的体内和体表分布极大地影响了食草动物的放牧行为和觅食决策，进而影响了放牧生态系统的结构和异质性。
