Larvae leave latecomer’s leaves
It is often said that ‘there's no such thing as a free lunch’. Equally we may say that there is no such thing as a free protection from being something else’s lunch. Defence of plants against herbivory has an associated cost and in the ‘cost–benefit analysis’ imposed by natural selection that cost may sometimes be too high. There is an indication of this in the work of Xiang and Chen (Mengla, China, pp. 377–384). They have looked at resistance to insectivorous herbivory in different species of Ficus that occupy different time-zones within an ecological succession. Ficus hispida is a pioneer, F. auriculata and F. racemosa are intermediate species and F. altissima is a late successional species. Vulnerability to herbivory was assayed by estimating damage sustained by seedlings in the field and by feeding leaves to caterpillars of the lepidopteran species Asota caricae (these caterpillars are natural predators of Ficus species). It has been suggested that pioneer species are often poorly defended because they allocate more resources to rapid growth and the authors’ results support this view. The pioneer species F. hispida was indeed the most vulnerable and its leaves were highly palatable to the predator, while the late successional species F. altissima was the least vulnerable. The authors have used principal component analysis to examine the correlation between herbivory and particular plant characteristics. Vulnerability to herbivory was positively correlated with high leaf contents of N, Ca and P, with low leaf toughness, shorter leaf life-times and lower C : N ratios, all of which are characters of F. hispida. Strangely, leaf pubescence in this species had no deterrent effect on the caterpillars. At the other end of the scale, not only was F. altissima less vulnerable to predators, but if its leaves were attacked, synthesis of the anti-feedant Ca oxalate was induced and the leaves became tougher. This species commits a lot to defence!
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
