The region is its own experimental test site. The foxes were introduced to almost a quarter of the islands, about 100 total. The seabirds that inhabited the islands transported nutrients from the nearby seas to the islands via their guano, or excrements. Rich in phosphorous and nitrogen, researchers estimated that guano was reduced by over an order of magnitude on the fox-infested islands.
The reduced nutrient flow affected plants, spiders, other birds, flies, and slugs.
Nutrient status coincided with strong shifts in the structure of plant communities. Upon further experimentation, the researchers found gramanoids -- or grasses -- out-competed the slower growing dwarf shrubs and forbs. The islands with foxes had more dwarf shrubs and forbs, while those with no foxes were dominated by grasses.
Even when researchers took into account differences in island size and distances from shore, the study still showed significant differences between the fox-free and fox-infested islands.
"By disconnecting the nutritional link from sea to land, the introduction of foxes to the Aleutians has reduced or eliminated the significance of the perimeter-to-area relationship that is so clearly evident on fox-free islands," say the authors.
Previous studies have focused on trophic cascades in marine ecosystems, showing that top predators can indirectly alter plant productivity, with varying effects. Most studies on land-based ecosystems have been smaller in scale. These studies have not revealed the large community-wide indirect effects that are often found in aquatic trophic cascades, or have only shown the effects of predators from the top down to lower levels. This study shows an indirect pat
Source:Ecological Society of America