Invasive vs. endangered

Resolving conflicting ecosystem management goals—such as maintaining fisheries while conserving marine species or harvesting timber while preserving habitat—is a widely recognized challenge. Even more challenging may be conflicts between two conservation goals that are typically considered complementary. Here, we model a case where eradication of an invasive plant, hybrid Spartina, threatens the recovery of an endangered bird that uses Spartina for nesting.

Efforts to eradicate invasive species increasingly occur side by side with programs focused on recovery of endangered ones. But what should resource managers do when the eradication of an invasive species threatens an endangered species?

The California Clapper Rail (Rallus longirostris obsoletus) is a bird found only in the San Francisco Bay. It has developed an interest in an invasive salt marsh cordgrass called hybrid Spartina and came to depend on it for nesting habitat. Its native habitat had slowly vanished over decades, largely due to urban development and invasion by Spartina. Spartina alterniflora was introduced to the Bay in the mid-1970s by the Army Corps of Engineers as a method to reclaim marshland. It hybridized with native Spartina and invaded roughly 800 acres. Eradication of hybrid Spartina began in 2005, and about 92 percent of it has already been removed from the Bay. 

In 1998 two researchers stated that the San Francisco Bay is perhaps the most invaded estuary in the world. They identified a total of 234 exotic species established in the ecosystem, including plants, protists, invertebrates, and vertebrates. Most of those were transported in ballast water that was purged in the bay making the estuary a very good example for the extend of introductions of new species.

In a new study researchers at the University of California, Davis examined the described conundrum now taking place in the San Francisco Bay. Their results showed that, rather than moving as fast as possible with eradication and restoration, the best approach is to slow down the eradication of the invasive species until restoration or natural recovery of the system provides appropriate habitat for the endangered species:

We show that the optimal management entails less intensive treatment over longer time scales to fit with the time scale of natural processes. In contrast, both eradication and restoration, when considered separately, would optimally proceed as fast as possible. Thus, managers should simultaneously consider multiple, potentially conflicting goals, which may require flexibility in the timing of expenditures.

Given how much the eradication has already progressed these results should be put into action rather swiftly.