Effects of bird predation on arthropod abundance and tree growth across an elevational gradient

Abstract

Considerable uncertainty surrounds the conditions under which birds can cause trophic cascades. In a three-year experiment, we studied the direct and indirect effects of insectivorous birds on arthropod abundance, herbivory, and growth of striped maple Acer pensylvanicum saplings in a northern hardwood forest of central New Hampshire, USA. We manipulated bird predation by erecting exclosures around saplings and directly manipulated herbivory by removing herbivores. We also examined how climate modifies these interactions by replicating the experiment at three locations along an elevational gradient. Effects of bird predation were variable. Overall, mean arthropod biomass was 20% greater on saplings within bird exclosures than on controls (p<0.05). The mean biomass of leaf-chewing herbivores, primarily Lepidoptera larvae, was 25% greater within exclosures but not statistically different from controls. To a lesser degree, mean herbivore damage to foliage within exclosures exceeded that of controls but differences were not significant. We also did not detect significant treatment effects on sapling shoot growth. The high understory vegetation density relative to bird abundance, and low rate of herbivory during the study (mean 5% leaf area removed, controls), may have limited the ability of birds to affect sapling growth. Climate effects operated at multiple scales, resulting in a complex interplay of interactions within the food web. Regional synchrony of climatic conditions resulted in annual fluctuations in herbivore abundance and tree growth that were shared across elevations. At the same time, local environmental variation resulted in site differences in the plant, herbivore, and bird communities. These patterns resulted in a mosaic of top-down strengths across time and space, suggesting an overall pattern of limited effects of birds on plant growth, possibly interspersed with hotspots of trophic cascades.

Effects of forest heterogeneity on the efficiency of caterpillar control service provided by birds in temperate oak forests

“…We hypothesized that high predation level lead to decreasing caterpillar abundance, which was supported by the results of numerous cage experiments (e.g. Tanhuanpää et al, 2001;Schwenk et al, 2010). Contrary to our hypothesis, we were not able to directly demonstrate the negative effect of bird predation on caterpillar abundance.…”

Tree cover mediates the effect on rapeseed leaf damage of excluding predatory arthropods, but in an unexpected way

“…Some studies found higher damage on crops (e.g., barley and cabbage) from which ground arthropods were excluded (Caballero-López et al, 2012;Martin et al, 2013), while other studies noted a greater importance of birds for top-down pest control than predatory arthropods (e.g., spiders) (Hooks et al, 2003). However, there are also studies that, in line with our findings, did not find a strong impact of birds on leaf herbivores (Tremblay et al, 2001;Low and Connor 2003;Schwenk et al, 2010). The contradictory findings in the relative importance of bird and arthropod predators for pest control may arise from the variation in environmental factors, abundance and functional diversity of predator organisms over the study regions, but also due to differences in study design.…”

“…obs.). However, it is not easy to associate the abundance of birds with their pest control efficiency as the effects vary between seasons and years, but only additional studies will verify this possibility (Schwenk et al, 2010). Several previous studies have found that habitat complexity tend to enhance the top-down control provided by birds and predatory arthropods (Johnson et al, 2010;Rusch et al, 2013;Bianchi et al, 2013), even though other studies did not show such an effect (Kellermann et al, 2008;Martin et al, 2013;Barbaro et al, 2014: Michel et al, 2014.…”

Herbivore damage increases avian and ant predation of caterpillars on trees along a complete elevational forest gradient in Papua New Guinea

“…Schwenk et al () did not find any effect of elevation on bird predation of arthropods between 290 and 780 m a.s.l. in White Mountains of New Hampshire, while we found higher predation rate at 1700 m than at 200 m a.s.l.…”