Masters Thesis

The effects of bird and bat arthropod predation on sapling black cottonwoods in the context of restoration

Phytophagous arthropod consumption by birds and bats has been shown to initiate top-down cascading effects that reduce plant damage or accelerate plant growth in terrestrial ecosystems. Birds and bats attracted to ecological restoration sites can potentially contribute to ecosystem functioning by reducing herbivore-caused plant damage and indirectly facilitating plant growth. I examined tri-trophic interactions between birds and bats, arthropods, and sapling black cottonwoods (Populus balsamifera ssp. trichocarpa) along two recovering tributary streams of Mono Lake, California, USA. I used mesh exclosures over saplings to remove the direct effects of bird and bat predation on arthropods dwelling on black cottonwoods, and I compared arthropod density and biomass, herbivore-caused foliage damage, and shoot growth between predator-free saplings and paired control saplings to which birds and bats had foraging access. Avian foraging surveys conducted in the riparian vegetation along the streams revealed that 49 percent of foraging attacks were made in black cottonwoods compared to other substrates but that only four percent of black cottonwood foraging attacks occurred in saplings comparable in height to experimental saplings. Contrary to predictions, neither total arthropod biomass or density, nor any arthropod foraging functional group (i.e., herbivores, predators and parasites, leaf chewers) differed between exclosure saplings and controls. It followed that bird and bat exclusion did not significantly influence herbivore-caused leaf damage or sapling shoot growth. Arthropod densities on sapling black cottonwoods were lower than has been reported by researchers who found bird-initiated species-level trophic cascades on other plants in the Salicaceae family. Further, I found no evidence that birds and bats preferentially consumed predaceous or herbivorous arthropods, or that intratrophic arthropod predation attenuated vertebrate predation effects. It appears that birds and bats do not indirectly reduce foliage damage or facilitate black cottonwood sapling growth in the context of riparian ecosystem restoration when herbivore loads are low.

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