Agasthya 6.2 Research Highlights

The aim of our visit to the Kalakad – Mundanthurai Tiger Reserve from the 4th to the 11th of May, 2012 was to study plantanimal interactions as part of the elective course at ATREE. Following preliminary reconnaissance and brainstorming, we decided to select one particular interaction - the extent of herbivory on leaves of Macaranga peltata, to conduct field observations. The varying availability of sunlight, nutrients, plant density and species diversity, known to influence resource availability in primary and secondary forest. We set out to test a few hypotheses which were based on the 'resource concentration' and the 'theory of plant apparency'. The resource concentration hypothesis states that the incidence of herbivory is proportional to the availability of the resource (plants). According to the theory of plant apparency, the discovery of a plant by herbivores is dependent upon the microclimatic conditions such as sunlight, location of leaves on a plant and the location of the plant itself in a patch. The theory assumes that plants that are prone to discovery tend to invest more in quantitative defenses such as reduced digestibility. The two theories are not mutually exclusive, and each one is a corollary of the other.

We hypothesized that the extent of herbivory would be higher in secondary growth forests where M. peltata was found in higher density compared to primary growth forests. Furthermore, we also sought to investigate if older trees reported greater herbivory compared to younger ones. At the same time, we sought to understand if the leaves in the upper branches which are exposed to sunlight are more prone to herbivory compared to leaves in the shaded lower branches irrespective of occurrence in primary or secondary forests.

Over the week we established a total of 30 square plots measuring 10 x 10 sq m, 15 each in primary and secondary evergreen forests. Within each plot, we measured the height and diameter of the M. peltata trees. The ratio of non-predated leaves to predated leaves was considered as a measure of herbivory.

Secondary forests carried a higher average number of M. peltata individuals per plot. Primary forest plots had fewer M. peltata trees but the overall tree density was higher than in secondary forests. The M. peltata trees in primary forest plots also displayed a higher mean height as well as girth than those in secondary forest plots.

The findings showed that the incidence of herbivory was marginally higher overall in the primary forests than in secondary forests. This finding was not in accordance with our initial hypothesis. Second, we found that although younger trees suffered herbivory to a greater extent relative to older ones the relationship was not a strong one. Third, regardless of the forest type, herbivores in general preferred the upper branches of trees irrespective of their occurrence in primary or secondary forests. Thus, the findings of this study do not support the resource concentration hypothesis. However, they appear to provide evidence to the theory of plant apparency with respect to M. peltata trees.