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Abstract
To clarify the modification of physical defences of Glochidion obovatum Sieb. et Zucc., we compared the length of spine-like branches and frequency of lateral branches of G. obovatum between Kashima Island, which supports a high density of sika deer (Cervus nippon), and its neighbouring areas. The length of spine-like branches of G. obovatum in Kashima Island was not significantly longer than that of the neighbouring areas, but the frequency of lateral branches on the island was higher than that of the neighbouring areas. Since the branches of G. obovatum are heavily foraged by the sika deer, the shrub form of the species has evolved into the bonsai form, suggesting increased frequency of occurrence of lateral branches, which could develop as shoot-like branches. Thus, the observed features are facultative defence mechanisms of G. obovatum against heavy browsing in a small-sized island with very high density of sika deer.
Highlights
Thorns, prickles, spines, and trichomes have often been considered as a defence mechanism against herbivory [1] [2], and plants have developed physical and chemical defences against herbivores
For the category of below 1.5 m in height, the length of spine-like branches of G. obovatum in Kashima Island was not significantly longer than those of the other areas (Figure 3, Table 2), suggesting that it does not need to increase the length of its spine-like branches to prevent grazing pressure of sika deer in Kashima Island
Why did G. obovatum differ in the number of lateral branches between Kashima Island and its neighbouring areas? One of the answers is that the rise of the number of lateral branch leads to increases in spine-like branches
Summary
Prickles, spines, and trichomes have often been considered as a defence mechanism against herbivory [1] [2], and plants have developed physical and chemical defences against herbivores. In Japan, comparative studies of physical defence between different browsing pressures by sika deer (Cervus nippon) have been conducted. Lév., has recently increased in this island [12] [13] These plant species may have increased physical defence by changing of prickles under the high browsing pressure by sika deer because they have many prickles on leaves and stems, and the differences between individuals in the spinescence induced by browsing sika deer indicate phenotypic variation. Hashigoe (1995) [14] described some plants without many prickles on a leaf and stem in the flora of Kashima Island. How have these been adapted for Kashima Island?
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