Not just the birds

Introduced foxes throw a wrench in the food web

Indirect effects of predators

In an extensive study, researchers from the University of Montana, University of California – Santa Cruz, and the University of California – Davis have shown that a top predator strongly affected plants and animals at the bottom of an island food web by eating organisms that transport nutrients between ecosystems. “An introduced predator alters Aleutian island plant communities by thwarting nutrient subsidies,” is published in the February issue of Ecological Monographs.

As any biologist can attest, cells, organelles, and organisms maintain specific surface to area ratios to ensure life. Similar relations have been observed in regard to an island’s size and nutrient deposition. In many cases, a small island with a large perimeter touching the sea receives more nutrients from marine ecosystems than a large island due to the differences in surface-to-area ratios. In a new study by John Maron, James Estes, Donald Croll, Eric Danner, Sarah Elmendorf, and Stacey Bucklelew, scientists have discovered that the introduction of a top predator has even affected this system.

“Our results show that the ecological effects of fox introductions extended well beyond the direct reductions of bird populations. We have clear evidence that foxes influence the terrestrial plant community and ecosystem dynamics through one particular route,” state the researchers in their study.

The experiment

The Aleutian archipelago has over 450 islands in the highly productive North Pacific Ocean and Bering Sea. In the harsh northern climate, the islands are covered in maritime tundra and nutrient-impoverished soil. With no native mammals, vast populations of seabirds (29 species currently) use the area for nesting. However, over the last 100-150 years, the introduction of arctic foxes and Norway rats (Rattus norvegicus) reduced the number of birds nesting on the islands.

Arctic foxes were introduced in the late 1800’s to maintain the North Pacific fur trade, and later removed in an attempt to return the islands to their more natural states. Most of the islands are now fox-free, but the effects of their presence are still visible; birds have been slow to return to the islands to nest.

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 pathway in which a predator affects a food web.

According to the researchers, the results “bolster a growing body of work indicating that island food webs are strongly subsidized by the movement of nutrients from adjacent productive waters onto less productive land.”