[Ed. note: This story has been adapted from a multi-part series commemorating the sixth-year anniversary of the Deepwater Horizon disaster. It focuses on conservationists’ efforts to rebuild barrier islands off the coast of Louisiana. Read the full posts on Audubon Louisiana’s website.]
As you look out into the Gulf of Mexico, off Louisiana’s coastline, there lies a string of barrier islands. They’re remnants of former deltas: As the Mississippi River shifted across the southeastern part of state over the last 6,000 years, marshes were created, eventually eroding away and leaving behind these sand berms where the river and sea once met. Today those ancient headland remnants continue to erode, but now the river no longer serves to rebuild them. Sediment that once flowed down the Mississippi River is either dammed upstream or falls off the edge of the continental shelf at the mouth of the river. Louisiana is fighting against nature to keep its barrier islands and prevent its entire coastline from disappearing.
These same barrier islands were significantly impacted by the BP oil disaster, which unfolded six years ago on this day. The spill enveloped them in oil at the height of the avian nesting season and in some cases expedited their rate of disappearance. Louisiana’s barrier islands play a critical role in the life cycle of dozens of migratory shorebirds and breeding seabirds. Since the state is at the base of the Mississippi Flyway and central to the Gulf of Mexico, it supports astoundingly high proportions of regional or global populations of many coastal nesting species of conservation concern. Some of these species, including Brown Pelicans, Tricolored Herons, Sandwich Terns, Royal Terns, and Black Skimmers, largely depend on barrier islands for nesting. Coastal Louisiana also holds important stopover and wintering habitat for a substantial proportion of Great Plains Piping Plovers, Short-billed Dowitchers (hendersoni), and western Willets, as well as many other species of sandpipers and plovers.
Ultimately, these barrier islands play a central role in the population dynamics of many species. This is particularly true for seabirds that nest on the cays, isolated from the mainland and its many mammalian predators. These seabirds live life on the edge of the Earth, and barrier islands are the key to their survival.
So how does the restoration of these landforms benefit the nesting seabirds? Species of terns and gulls, as well as some coastal-nesting shorebirds, like Wilson’s Plovers and American Oystercatchers, lay their eggs on the ground (although a few species, like Brown Pelicans, prefer low shrubs such as mangroves). These nesting birds are not only susceptible to mammalian predators, but storm surges as well. As islands and their dune systems erode, nests are inevitably placed closer and closer to the high tide line, putting them at greater risk, even during small storms. Re-nesting can be possible, but at some point becomes futile. And with fewer and fewer islands available, eventually space runs out, and populations decline. The restoration of these islands increases opportunities and space for nests and helps elevate them to reduce their chances of washing away.
There is another important consideration for barrier island restoration—for seabirds, bigger is not necessarily better. Anyone who has studied ecology might recall “Island Biogeography Theory.” It suggests that the bigger the island and the closer it is to shore, the more species it can support. This sounds great, right? But those additional species can be (and often are) predators. So for a seabird, smaller islands farther from shore are more productive for nesting. Predators like coyotes, raccoons, rats, skunks, foxes, feral cats, fire ants, and nutria have a harder time getting to those islands and surviving there. This becomes important when thinking about barrier island restoration.
The restoration of larger Louisiana barrier islands closer to shore, like Whiskey Island and Scofield Island, raises questions regarding the nesting success of seabirds, if one follows basic tenets of Island Biogeography Theory. An important question that Audubon Louisiana seeks to understand while surveying bird populations is how many more fledglings are produced on a given island after restoration versus before. It’s possible that overall nesting success could decrease after restoration if the island now supports more predators. What do we do if that happens? Audubon is monitoring beach-nesting birds on Grand Isle and the Caminada Headlands to answer some of these questions for Least Terns and Wilson’s Plovers. After protecting certain nesting areas from human disturbance (with vital help from volunteers), we track the nesting success of birds and determine causes of failure, such as storm surge and various predators.
If restored barrier islands act as havens for predators—and not nesting seabirds—what can be done to enhance seabird-nesting success? The removal of predators can be expensive, challenging, and unsustainable. Electric and other kinds of enclosure fencing might be feasible in certain circumstances, but is also relatively expensive, and often requires regular maintenance. A more sustainable approach might instead be to place greater emphasis on the construction of smaller offshore islands, through methods such as dredging sediments, particularly where land-building processes already exist (such as near diversions and naturally accreting deltas). Meanwhile, larger barrier island restoration can play an important role in the protection of other coastal habitats and coastal human communities.
Surely, islands with some predators are better than no islands. Considering how to maximize their efficacy for nesting seabirds will require an island-by-island assessment, regular surveys, and adaptive management. Each of these islands are one hurricane away from losing their predators, so a well-constructed barrier island that withstands one or more storms might suddenly churn out more birds. Most seabirds are long-lived; helping them get back up to healthy levels—to the greatest extent possible—requires a long-term point of view as well.