Predators, shifting subsidies, and regime shifts

The March 25th issue of Science published an interesting article on alternative stable states, ecosystem subsidies and introduced apex predators in Alaska – Introduced Predators Transform Subarctic Islands from Grassland to Tundra by Croll et al in Science, 307(5717) 1959-1961.

Alaskan alt states

From the abstract:

Top predators often have powerful direct effects on prey populations, but whether these direct effects propagate to the base of terrestrial food webs is debated. There are few examples of trophic cascades strong enough to alter the abundance and composition of entire plant communities. We show that the introduction of arctic foxes (Alopex lagopus) to the Aleutian archipelago induced strong shifts in plant productivity and community structure via a previously unknown pathway. By preying on seabirds, foxes reduced nutrient transport from ocean to land, affecting soil fertility and transforming grasslands to dwarf shrub/forb-dominated ecosystems.

The paper not only provides an example of how a trophic cascade can flip the state of an ecosystem, but it also shows that such a flip can occur because a species changes the nature of an ecosystem’s connection to other ecosystems.

The article is a bit of the mirror image of Terborgh et al’s 2001 paper Ecological Meltdown in Predator-Free Forest Fragments that showed the consequences of fragmentation and predator loss on created islands in Venezuela.

The collapse of those island ecosystems is also due to changes in spatial connectivity. Predators formerly connected separate ecosystems, but flooding and the subsequent fragmentation prevented them from doing that.

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