Kenward R E, Holm J L
Institute of Terrestrial Ecology, Furzebrook Research Station, Wareham, Dorset, U.K.
Proc Biol Sci. 1993 Mar 22;251(1332):187-94. doi: 10.1098/rspb.1993.0028.
Diffusion modelling has shown that conservative demographic traits combined with feeding competition could explain red squirrel replacement by grey squirrels. We used field data from seven separate red and grey squirrel populations, in oak-hazel woods and Scots pines, to reject the hypothesis that red squirrel density and breeding is intrinsically poorer than that of grey squirrels. In oak-hazel woods, grey squirrel foraging, density and productivity were related to oak and acorn abundance. In contrast, red squirrels foraged where hazels were abundant; their relatively low density and breeding success were related to the abundance of hazel nuts. Red squirrels failed to exploit good acorn crops, although acorns were more abundant than hazels, but in Scots pines had densities and breeding success as high as grey squirrels in deciduous woods. Captive grey squirrels thrived on a diet of acorns, but red squirrels had a comparative digestive efficiency of only 59%, apparently because they were much less able than grey squirrels to neutralize acorn polyphenols. A model with simple competition for the autumn hazel crop, which was eaten by grey squirrels before the acorn crop, shows that red squirrels are unlikely to persist with grey squirrels in woods with more than 14% oak canopy. With oaks in most British deciduous woods giving grey squirrels a food refuge which red squirrels fail to exploit, replacement of red squirrels can be explained by feeding competition alone, exacerbated by the post-war decline in coppiced hazel.
扩散模型表明，保守的种群统计学特征与觅食竞争相结合，可以解释灰松鼠取代红松鼠的现象。我们利用来自七个独立的红松鼠和灰松鼠种群的实地数据，这些种群分布在橡树林、榛树林和苏格兰松林中，以驳斥红松鼠的种群密度和繁殖能力本质上比灰松鼠差的假设。在橡树林和榛树林中，灰松鼠的觅食、种群密度和繁殖力与橡树和橡子的丰度有关。相比之下，红松鼠在榛树丰富的地方觅食；它们相对较低的种群密度和繁殖成功率与榛子的丰度有关。尽管橡子比榛子更丰富，但红松鼠未能利用好橡子丰收的机会，但在苏格兰松林中，它们的种群密度和繁殖成功率与落叶林中的灰松鼠一样高。圈养的灰松鼠以橡子为食时茁壮成长，但红松鼠的相对消化效率仅为59%，显然是因为它们中和橡子多酚的能力远不如灰松鼠。一个简单的模型显示，对于秋季榛子作物存在竞争，灰松鼠在橡子作物之前就吃掉了榛子，这表明在橡树树冠覆盖率超过14%的树林中，红松鼠不太可能与灰松鼠共存。由于大多数英国落叶林中的橡树为灰松鼠提供了红松鼠无法利用的食物避难所，红松鼠被取代仅靠觅食竞争就能解释，战后矮林榛树的减少加剧了这种情况。
