Abstract:
The influence of herbivores on plants in tropical forests is enormous. Leaf area loss through herbivory may alter biochemics, physiology and morphology of a plant and thus have a negative influence on the plant‘s fitness. Hence, herbivory is a selective factor for the species composition of plant communities. As a result of the progressive industrialisation on earth an increase in the input of atmospheric nitrogen (N) and phosphorus (P) is expected. Against this background, in this study the influence of N and/ or P fertilization on the herbivory in a tropical montane forest of southern Ecuador was investigated. Here, in a nutrient manipulation experiment (NUMEX), study areas are fertilized with low amounts of N (50 kg/ha/year), P (10 kg/ha/year) as well as N+P (50 kg/ha/year and 10 kg/ha/year respectively). Herbivory was quantified by one-time-measurements of absolute leaf mass loss and percentaged leaf area loss of the four most common tree species of the study area (Alchornea grandiflora, Graffenrieda emarginata, Hieronyma moriztiana, Myrcia sp.) and a sample of randomly chosen trees representing the community. It was investigated whether possibly changed herbivory rates result from increased nutrient concentrations in the leaves or an altered specific leaf area (SLA). The community lost marginally significantly (p<0.1) more leaf area on N fertilized areas than on control areas and herbivory rates were significantly (p<0.05) higher on N fertilized for three of the four species. Combined N+P fertilization had a significant (p<0.05) increasing effect on the herbivory suffered of two species. P fertilization did not show any effect on herbivory. The increased herbivory rates on three of the four most common species with increasing N availability may lead to a decreasing competitive strength of these species in the future and thus to less appearence in the community. Regarding the nutrient content and morphology of the leaves, the most striking feature was a high species-specific variability in the reaction to the different treatments. There was no influence of the different fertilizations on foliar nutrient content or morphology of the community. In one of the four species a connection between increased leaf area loss and elevated leaf N content was found on N fertilized areas. There was no evidence, that increased SLA results in increased herbivory rates. It is to suggest, that within species other parameters, e. g. altered leaf toughness or content of secondary metabolites may exert a stronger influence on the variability of herbivory rates with differing N availability. In conclusion, the study showed that in a tropical montane forest of southern Ecuador soil nitrogen availability had an increasing effect on feeding performance of herbivores.
