Environmental changes in biodiversity hotspot ecosystems of
South Ecuador: RESPonse and feedback effECTs (FOR2730)

Resumen:

Climate change and forest degradation are major threats to forest ecosystems. The climatic changes can affect the mineralization rate in soils and therefore change the amount of available nitrogen components for plants. Thus these changes can influence foliar C/N-ratio. Previous studies used an elevation gradient to test the influence of climatic changes on ecosystems, and showed that foliar C/N-ratio is positively correlated with elevation. Changes in foliar C/N-ratio may affect the feeding behavior of insect herbivores and therefore influence leaf area loss (LAL). LAL is used as an indicator of herbivory and therefore may consequently also be related to ant abundance because ants prey on insect herbivores. Ants in return, are also affected by the abiotic changes with increasing elevation and therefore sensitive to climate change. However, forest degradation is currently the greater threat to ecosystems. Degradation can lead to changes in the nutrient uptake of plants and therefore to changes in foliar C/N-ratio. Moreover it can reduce ant abundance as ants are sensitive to the conversion of forests. We nalyzed the causal effects of abiotic factors (elevation and degradation) on biotic factors (foliar C/N-ratio and ant activityabundance) and the relationships between the biotic factors and LAL with linear mixed-effect models in a path model. We found a significant positive correlation between foliar C/N-ratio and elevation, and a significant negative correlation between ant activity-abundance and elevation. Foliar carbon-content had a significant negative correlation with degradation. Our results revealed no significant relationships with LAL, but LAL correlated negatively with foliar C/N-ratio in the shrub layer. The results of our study point out that foliar C/N-ratio and ant activityabundance change with elevation and thus respond to climatic changes. Moreover our results indicate that foliar C/N-ratio can be connected to LAL, but that influences such as the abundance and distribution of insect herbivores along elevation should be examined closely. Our finding that foliar C/N-ratio, ant activity-abundance and LAL were unaffected by degradation indicates that the studied degraded forests are still suitable habitats for insect herbivores and ants.

Resumen:

Ant assemblages are sensitive to abiotic changes in the environment, therefore they are widely used as indicators of environmental changes. Previous studies demonstrated that abiotic changes with elevation and increased anthropogenic disturbance not only reduce species richness of ant assemblages, but also modify their trophic composition and nutrient use. In tropical ecosystems where nutrient availability may vary between dry and wet seasons, seasonal or interactive effects might play an important but still neglected role in shaping these patterns. Here I used standardized bait experiments in natural and disturbed sites along an elevation gradient in a tropical montane rainforest during the wet and dry season. In order to analyze the single and interactive effects of elevation, disturbance and season on species richness, nutrient use and trophic composition of ants, I used linear mixed effect models. Additionally, I used principal component analysis (PCA) to assess whether morphological traits of ants are linked to their nutrient use. Species richness decreased monotonically along the elevation gradient, with a stronger decline in the dry season. Forest disturbance had no significant effect on species richness. The relative use of most nutrients decreased with increasing elevation. Forest disturbance only affected the relative use of lipids by decreasing it compared to natural forests. However, my results revealed complex interactive effects of elevation, disturbance and season on species richness and the use of nutrients by ant assemblages. Furthermore, I found a shift from predominantly omnivore species to more predatory species with increasing elevation. PCA revealed a preference of lipid baits by species with morphological traits associated with predatory taxa. My findings highlight the importance of seasonality and mixed effects on the composition of ant assemblages and their nutrient use in a tropical montane forest. Additionally, these results highlight the value of disturbed forests within my study area, since they support similar species richness and trophic composition of ant assemblages compared to natural forests.