Lechowicz Martin J
Department of Biology, McGill University, 1205 Avenue Docteur Penfield, H3A 1B1, Montreal, Quebec, Canada.
Oecologia. 1982 Jan;52(1):22-30. doi: 10.1007/BF00349007.
Electivity indices measure the utilization of food types (r) in relation to their abundance or availability in the environment (p). Foods that constitute a larger proportion of the diet than of the available foods are considered preferred; conversely those proportionately underrepresented in the diet are avoided. A food is eaten at random if its proportion in the diet equals its proportion in the environment. A family of electivity indices stemming from Ivlev's (1961) classic monograph exist and differ only in the particular algorithm used to calculate electivity from r and p.For each available index I graphed the values of electivity as contours for all combinations of r and p. These graphs are compared to illustrate the strengths and weaknesses of each index on the basis of the following criteria: 1) the value of the index when r=p for a food, 2) the symmetry of the electivity value as feeding deviates from random, 3) the possible range of index values, 4) the linearity of changes in electivity over the full range of r and p, 5) the sensitivity of the index to sampling errors, 6) the statistical testability of the electivity, and 7) the stability of the electivity value for a food type that changes relative abundance or occurs in combination with different food types. No one index ideally satisfies all the criteria.The host preferences of gypsy moth, Lymantria dispar, feeding on tree foliage in an undisturbed deciduous forest in southwestern Quebec, Canada were used to compare the available indices: Ivlev's electivity, E; Ivlev's forage ratio, E'; Jacob's modified electivity, D; Jacob's modified forage ratio, log Q; Chesson's alpha; Strauss' linear index, L; and Vanderploeg and Scavia's relativized electivity, E . The electivity values calculated by each index differ one from another; host trees shown as preferred by one index will frequently appear avoided according to an alternative index. The rank order electivities for the 19 available host trees, however, are remarkably similar for all but Strauss' linear index, L. Populus grandidentata, Quercus rubra, Ostrya virginiana, and Amelanchier were the most preferred host trees in the sampled forest; Prunus serotina, Acer pensylvanicum, A. rubrum, Betula lutea, and Fraxinus americana were most avoided. The use of Vanderploeg and Scavia's E index is recommended.
选择性指数衡量食物类型（r）的利用情况与其在环境中的丰度或可获得性（p）之间的关系。在饮食中占比高于可获得食物占比的食物被视为偏好食物；相反，在饮食中占比相对较低的食物则不被选择。如果一种食物在饮食中的比例等于其在环境中的比例，则该食物是被随机摄取的。源自伊夫列夫（1961年）经典专著的一系列选择性指数都存在，只是在用于根据r和p计算选择性的具体算法上有所不同。对于每个可用指数，我将选择性值绘制成r和p所有组合的等值线。通过比较这些图表，根据以下标准来说明每个指数的优缺点：1）当一种食物的r = p时指数的值；2）随着摄食偏离随机状态时选择性值的对称性；3）指数值的可能范围；4）在r和p的整个范围内选择性变化的线性；5）指数对抽样误差的敏感性；6）选择性的统计可检验性；7）对于相对丰度发生变化或与不同食物类型组合出现的食物类型，其选择性值的稳定性。没有一个指数能理想地满足所有标准。以加拿大魁北克西南部一片未受干扰的落叶林中以树叶为食的舞毒蛾（Lymantria dispar）的寄主偏好为例，来比较可用指数：伊夫列夫的选择性指数E；伊夫列夫的饲料比E'；雅各布的修正选择性指数D；雅各布的修正饲料比log Q；切森的α指数；施特劳斯的线性指数L；以及范德普洛格和斯卡维亚的相对化选择性指数E 。每个指数计算出的选择性值各不相同；一种指数显示为偏好的寄主树，根据另一种指数往往会显得是被回避的。然而，除了施特劳斯的线性指数L之外，对于19种可用寄主树的选择性排序非常相似。在采样森林中，大齿杨、红栎（Quercus rubra）、弗吉尼亚铁木（Ostrya virginiana）和唐棣是最受偏好的寄主树；黑樱桃、宾夕法尼亚槭（Acer pensylvanicum）、红花槭（A. rubrum）、黄桦（Betula lutea）和美国白蜡树是最不被选择的。建议使用范德普洛格和斯卡维亚的E 指数。
