植食性哺乳动物觅食功能反应模型机制的检验

Tests of a model mechanism of functional response in mammalian herbivores foraging

植食性哺乳动物在食物密集斑块的觅食为 型功能反应。在新鲜苜蓿叶片构成的食物密集斑块上 ,以高原鼠兔作为实验动物 ,检验植食性哺乳动物的觅食功能反应及其模型机制。食物大小可调节高原鼠兔的口量 ,尽而控制其瞬时摄入率 ;高原鼠兔觅食叶片的口量 S与瞬时摄入率 I存在渐近的函数关系 ,为 型功能反应 ;高原鼠兔的食物收获率 B随口量 S的增加呈非线性递减 ;最大处理速率 Rmax的测定值与模型的预测值极为近似 ;瞬时摄入率 I的测定值与模型的预测值线性回归显著 ( P<0 .0 1 )。研究结果充分验证了提出的假设 :植食性哺乳动物 型功能反应模型能有效地预测其摄入率的动态 ;

TypeⅡfunctional responses are frequently observed in mammalian herbivores feeding in patches where plants are concentrated in space We tested a mechanistic model of regulation of intake rate of herbivores foraging in food concentrated patches that accounts for asymptotic, typeⅡresponses. The model is based on the hypothesis that competition between cropping andchewing regulates instantaneous intake rate in response to changes in the size of bites obtained by the forager. We tested this hypothesis and examined the ability of the model to account for observations of intake rate of plateau pikas ( Ochotona curzoniae ). We devised the patches 200×61×50cm 3 thick for pikas, in which fresh alfalfa (Medicago satival) leaves were uniformly spaced 4 5 cm apart to scale leaves distribution, and differences in bite size were imposed by varying the size of leaf offered to animals in patches. Because we were interested in the maximum instantaneous intake rate achievable at a given bite size, trials were kept brief, typically lasting≈2 min. We varied leaf size on the patches. In so doing, we were able to force a 24 fold difference in mass between the largest and smallest bites taken by pikas. We measured dry matter intake rates of 10 individuals, females 5 and males 5, feeding in artificial patches during foraging trails. As the animal was foraging, we counted the number of bites cropped and recorded the elapsed time of active foraging. We used video tapes of trails replayed in slow motion to enhance the accuracy of our counts of cropping bites. We estimated total dry matter intake during a trial as the difference between the dry mass of leaves offered at the beginning of the trial and the dry mass remaining on the patche at the end of the trial. The average bite size consumed during each trial was determined by dividing total amount of leaves removed by the number of cropping bites observed during that trial. All calculations were corrected for dry matter. We designed three model tests. First, we examined whether the model could provide a reasonable statistic fit to the data. Second, we examined whether the intake rates were controlled by competition between cropping and chewing, and the asymptotic functional relationship between cropping rate and bite size. Finally, we compared observed intake rates of the pikas to predicted by the mechanistic model of mammalian herbivores feeding in plant concentrated space. We observed an asymptotic relationship between leaf sizes and intake rates of pikas. Leaf size proved to be an accurate predictor of intake rate ( P <0 01). Using bite size as an independent variable could improve the fit of the model. As leaf size was small (1～12mg), there was an isometric relationship between leaf size and bite size consumed for pikas. However, as leaf size increased (>12mg), we failed to detected a clearly demarcated upper limit to bite size for pikas. We observed changes in food intake rate that occurred in response to changes in bite size of the animals, and measured short term intake rates of these animals feeding in the patches of leaves. Asymptotes ( R max ) estimated from nonlinear regression were within the measured range of observed asymptotic intake rates for pikas( P >0 01). Leaf size could control the intake rates of the animals through regulating their bite size. There was an asymptotic functional relationship between bite size of animals and their intake rates, which belonged to a TypeⅡfunctional response. Predictions of maximum intake rate closely resembled observations of processing capacity, and the regression of the observed and predicted intake rates was significant ( P <0 01), demonstrating that processing rather than cropping sets upper limit on short term intake. Tests of model mechanisms provided strong support for the hypothesis that competition between cropping and chewing regulates is responsible for the typeⅡfunctional response seen in herbivores feeding in food concentrated patches. These results indicated that plant size regulating