Effects of ferric soils on arthropod abundance and herbivory on Tibouchina heteromalla (Melastomataceae): is fluctuating asymmetry a good indicator of environmental stress?

Abstract

High concentration of heavy metals in the soils represents an important factor of physiological stress that influences the normal functioning of plants through oxidation processes, and negatively affects insect performance and leaf consumption by herbivorous insects. One useful indicator to evaluate environmental stress in plants by heavy metals and herbivory is the fluctuating asymmetry, which describes the random differences in size or shape between the two sides of a bilateral character in organisms and it is a widely used measure of developmental instability in plants. We evaluated under natural conditions, the effects of variation of heavy metals in the soils on herbivore patterns, fluctuating asymmetry and arthropod abundance in Tibouchina heteromalla in rupestrian grasslands along the Espinhaco chain in Brazil. We selected two study areas, the first characterized by the presence of soils with low concentration of heavy metals (quartzite soils). In the second area, the soils are characterized by the presence of high concentration of heavy metals such as iron (ferric soils). We found that leaf thickness was higher in ferric soils than in quartzite soils. Conversely, total leaf area was greater in quartzite soils in comparison to ferric soils. Plants in soils with heavy metals had both lower herbivory levels and arthropod abundance than plants in soils with low concentrations of heavy metals. Fluctuating asymmetry levels were significantly greater in individuals from quartzite soils compared to individuals from ferric soils. Herbivory was positively related with individual fluctuating asymmetry in quartzite soils. Our results suggest that T. heteromalla presents tolerance to soils with heavy metals suggesting an acclimatization to these environmental conditions, and therefore, ferric soils may not represent a factor of environmental stress.