In a recent “My Word” piece, Mr. Uri Driscoll expressed a wide range of opinions and observations regarding the current management practices directed at recovering the threatened Snowy Plover. In his essay, he focused especially on dune restoration because he sees it as unnecessarily costly, damaging to wetland habitats, and altering the coastal dunes that protect the county's infrastructure. His rambling essay was filled with phantom opinions attributed to experts, disarticulated bits of information, and poorly explained “facts” derived from a cursory understanding of the ecology of coastal dunes and plovers in particular. We wish to clarify and explain the real situation involving the status of the plover. In doing so, we'll rely on scientific evidence collected and published in peer-reviewed scientific journals rather than presenting anecdotes derived from hearsay.
The snowy plover is widely distributed in North America, where an estimated total population of 1,747 breeds along the West Coast. It is this population, breeding within 50 miles of the Pacific, that was listed as threatened under the U.S. Endangered Species Act (ESA) in 1993. The listed population continues to be protected because numbers remain below that necessary for recovery under the ESA. Over the past 10 years, plover numbers in Northern California have varied between 19 and 71, which is well below the number (150 locally and 3,000 along that Pacific coast) that the USFWS determined necessary for the population to be recovered.
Ecologists understand that two important things influence annual variation in the size of animal populations: adult survival and reproduction. Simply put, a population must reproduce enough young to replace those adults that die each year for the population to remain stable. When reproduction exceeds mortality, the population grows; when it does not, numbers decline. Recent evidence shows conclusively that the population of plovers in our region routinely has such poor reproduction that numbers have declined to a low of 19 adults in 2009; the number increased to 31 this past summer. The only thing that sustains the local population is immigration of plovers from elsewhere along the Pacific coast.
In the species' recovery plan, finalized by the US Fish and Wildlife Service in 2007, biologists identified three main factors that contribute to poor reproductive success of plovers. They are: 1) predation of eggs and young by mammals and birds, especially the common raven; 2) human-caused mortality of eggs and chicks stemming from encounters with people, dogs and vehicles, as well as the subtle effects of disturbance to breeding birds; and 3) degradation and loss of breeding habitat, in large part due to the spread of introduced plant species, principally European beach grass.
In our area, predators are the leading cause of poor reproduction, based on direct and indirect evidence. For example, we have shown that plover eggs do not survive long on Clam Beach and Mad River Beach where ravens are particularly abundant (probably because garbage left by humans attracts scavenging ravens). Recently, we used video cameras to monitor 25 plover nests on Clam Beach because we wanted to know the cause of nest failure with greater certainty. Our recordings showed that ravens were the culprits at 70 percent of the 20 nests where eggs disappeared. We also showed that humans caused some failures because they took eggs from nests, their dogs trampled eggs, or vehicles ran over them. So, humans have played a direct and indirect role (via garbage) in contributing to reproductive failure of plovers on Clam Beach. Early on, we attempted to increase plover reproduction by building cages around nests to keep ravens from eating plover eggs. But ravens and other predators learned what was inside and ate newly hatched chicks and adults as they left the cages, which caused us to end this practice. We currently do very little to manage predators, which pose a serious challenge to the species' recovery.
Each year, humans cause some nest failures, as shown by cameras. We have tried to ameliorate these negative effects. For instance, our survey data suggested that plovers favored the area between the two parking lots on Clam Beach, an area where human activity is also often high. Accordingly, in each of the past six years we have worked with Humboldt County staff to erect a temporary fence in this area to create a refuge for breeding plovers in an effort to limit direct mortality of eggs and chicks and minimize disturbance from humans during the sensitive period when plovers breed. This approach has been successful elsewhere along the Pacific coast. When we compared the first three years without the fence with the subsequent four years with protection, we found that plover chicks hatched inside the fenced area often restricted their movements to that area, and young reared inside the fence were more likely to reach maturity. This last point is debatable because the number of young reared on the south end of Clam Beach, where there was no fence, also increased. But the south stretch of beach also has less human activity than the north end of the beach. Unfortunately, over the past several years ravens have eaten nearly all plover eggs on Clam Beach so that few young hatch. This limits our ability to evaluate the fenced area for reducing direct impacts from humans, and it underscores the overarching importance of raven predation on reproduction for this species.
Habitat loss was a third factor identified as contributing to the plover's decline. In Humboldt County, a main cause of habitat loss is a reduction in open, sparsely vegetated beaches favored by breeding plovers. European beach grass is an invasive species that has spread rapidly to cover dunes in dense stands. Efforts to restore habitats have removed beach grass (and a few other species) with the aim of restoring the native dune biota, of which plovers are one species. Plovers preferentially nest in the open and rely on early detection of ravens (and humans) to evade predation of eggs. Thus, it is not uncommon to observe plovers nesting in newly restored areas. This happened on Clam Beach in 2008, after Caltrans removed beach grass from a large area below the Vista Point. In response, plovers initiated almost half of the 25 nests on Clam Beach that year in this restored area. This past summer, a similar pattern unfolded within the newly restored habitats managed by California State Parks on Little River State Beach. Unfortunately, predators readily consumed eggs in all but one of these nests. So restoration does attract breeding plovers, but these restored habitats will require additional management of predators and people to ensure that nests and chicks survive well.
In his recent essay, Driscoll attempts to surprise the reader with an ironic suggestion that removing invasive non-native beach grass won't help the plover recover. What he failed to recognize is that we are way ahead of him. We already know that removing beach grass -- by itself -- will not safeguard the threatened snowy plover. That's only a third of the story. Biological research and the USFWS's recovery plan, published more than three years ago, clearly identified three inter-related factors: predators such as ravens, human-caused egg and chick losses, and the loss of breeding habitat from invasive non-native beach grass and other causes. Those factors operate together, so plover conservation must address all three. Enjoying coastal habitats, dunes, and the species that have lived amongst them for millennia will require that people recognize our role as simply one piece of this dynamic ecosystem. We urge everyone to behave in ways that honor the other pieces. Even if one of those pieces is as small, and as beautiful, as a plover.
Mark Colwell and Matt Johnson are professors in the Wildlife Department at Humboldt State University.