Diverse plant communities are predicted to have higher abundance of predators as compared to species-poor ones. This might be due to higher abundance and species richness of herbivores, more favourable microclimate and shelter as well as availability of alternative feeding sources (e.g. nectar and pollen) in diverse plant communities. Higher abundance and activity of predators is one of the proposed mechanisms explaining lower herbivore densities in mixed stands (the enemies hypothesis). However, evidence supporting this hypothesis in forest ecosystems is very limited.
We have tested the enemies hypothesis in the Satakunta forest diversity experiment using a variety of methods including pitfall traps, predator exclusion experiments, camera traps, and plasticine caterpillars.
Wood ants are one of the most important arthropod predators in boreal forest ecosystems. While sampling ground-dwelling arthropods in the Satakunta tree species diversity experiment by means of pitfall traps revealed no effects of tree diversity on ants, number of wood ants foraging on trees was higher in birch-pine mixtures than in corresponding monocultures. This was due to the abundance in our study areas of ant-tended pine aphid Cinara pinea colonies on pines. As a result, the survival of autumnal moth (Epirrita autumnata) larvae and pupae and the European pine sawfly (Neodiprion sertifer) larvae was lower on birch and pine trees in pine-birch mixtures as compared to monocultures.
Birds are important control agents of insect pests in forest, but only a handful of studies have examined effects of tree diversity on avian predation in forest ecosystems. We studied bird predation in pure and mixed stands in the Satakunta tree species diversity experiment using artificial larvae made of plasticine and installed on birch, alder and pine trees.
Effects of tree species diversity and identity on bird predation were tested at two different scales: between plots and within the neighbourhood around focal trees. At the neighbourhood level, birds preferentially foraged on trees surrounded by heterospecific neighbours. However, predation rates did not increase with tree species richness at the plot level and were instead negatively affected by tree height variation within the plot. The highest probability of predation was observed on pine, and rates of predation increased with the density of pine regardless of scale. Footage from camera traps revealed that great tits (Parus major) were responsible for predation on artificial larvae on pine.