Tidal and seasonal effects on survival rates of the endangered California clapper rail: does invasive Spartina facilitate greater survival in a dynamic environment?

Abstract

ORIGINAL PAPERTidal and seasonal effects on survival rates of the endangered California clapper rail: does invasive Spartina facilitate greater survival in a dynamic environment? Abstract Invasive species frequently degrade habitats, disturb ecosystem processes, and can increase the likelihood of extinction of imperiled populations. However, novel or enhanced functions provided by invading species may reduce the impact of processes that limit populations. It is important to recognize how invasive species benefit endangered species to determine overall effects on sensitive ecosystems. For example, since the 1990s, hybrid Spartina (Spartina foliosa 9 alterniflora) has expanded throughout South San Francisco Bay, USA, supplanting native vegetation and invading mudflats. The endangered California clapper rail (Rallus longirostris obsoletus) uses the tall, dense hybrid Spartina for cover and nesting, but the effects of hybrid Spartina on clapper rail survival was unknown. We estimated survival rates of 108 radio-marked California clapper rails in South San Francisco Bay from January 2007 to March 2010, a period of extensive hybrid Spartina eradication, with Kaplan-Meier product limit estimators. Clapper rail survival patterns were consistent with hybrid Spartina providing increased refuge cover from predators during tidal extremes which flood native vegetation, particularly during the winter when the vegetation senesces. Model averaged annual survival rates within hybrid Spartina dominated marshes before eradication (Ŝ = 0.466) were greater than the same marshes posttreatment (Ŝ = 0.275) and a marsh dominated by native vegetation (Ŝ = 0.272). However, models with and without marsh treatment as explanatory factor for survival rates had nearly equivalent support in the observed data, lending ambiguity as to whether hybrid Spartina facilitated greater survival rates than native marshland. Conservation actions to aid in recovery of this endangered species should recognize the importance of available of high tide refugia, particularly in light of invasive species eradication programs and projections of future sea-level rise.

Control and consequences of Spartina spp. invasions with focus upon San Francisco Bay

“…The spread of hybrid Spartina coincided with, and could have contributed to, a brief period of increase in Ridgeway's rail (Overton et al 2014). In the early 1990s numbers increased to between 1000 and 1200, which could have been a result of control of red fox (Evens et al 2010).…”

“…Mercury pollution (Schwarzbach et al 2006;Ackerman et al 2012) and predators (Overton et al 2014) further reduced numbers. Only between 4200 and 6000 individuals remained in the 1970s, and the bird was listed as an endangered species.…”

“…The taller stature and denser culms of hybrid Spartina afford better protection during the highest tides than native Spartina. It is during these king tides that the birds suffer greatest mortality, when nests drown and predators kill birds that have been flooded out of shorter vegetation (Overton et al 2014). .…”

“…In the distinctive case of the native, non-migratory, Ridgeway's rail, the effect of non-native Spartina is positive. Predation rates on Ridgeway's rail are high, and the tall dense plants provide cover from predators at high tide (Overton et al 2014). …”

Lessons from the past: isotopes of an endangered rail as indicators of underlying change to tidal marsh habitats

“…However, more sparse communities of rails inhabit pickleweed (Sarcocornia pacifica), the dominant vegetation of the historical and modern northern sections of tidal marsh (Table 1a, De Groot 1927;Moffitt 1941;Garcia 1995). The rail has compact individual home ranges (annual mean 2-4 ha; seasonal mean 1.2-1.8 ha) and limited evidence of long-range dispersal, and thus the rail has been called an Estuary tidal marsh obligate (Albertson 1995;Casazza et al 2008;Rohmer 2010;Overton et al 2014). Rails are considered opportunistic generalists, foraging for invertebrates as secretive marsh birds within the cover of tidal marsh vegetation (Horak 1970;Takekawa et al 2011;Rush et al 2012;Casazza et al 2014).…”