Academic journal article The American Midland Naturalist

Modeling the Survival and Population Growth of the Freshwater Mussel, Lampsilis Radiata Luteola

Academic journal article The American Midland Naturalist

Modeling the Survival and Population Growth of the Freshwater Mussel, Lampsilis Radiata Luteola

Article excerpt

ABSTRACT.-Freshwater mussels are a highly imperiled group of organisms. Understanding their demography and population dynamics is central to their conservation. We used markrecapture techniques over a 5-year period to estimate the survival and population growth of two populations of Lampsilis radiata luteola in Ohio Brush Creek, Ohio, USA. Of particular interest was determining temporal variability in survivorship and if there were intermittent pulses in recruitment that could be capable of sustaining populations over the long term. We made 540 captures of 171 individuals at an upstream site and 104 captures of 55 individuals at a second, downstream site. Fitting mark recapture data to survival models indicated that the apparent survival rates of adults was best fit by a temporally constant parameter at both sites. At both sites apparent survival was low in comparison to published values for other freshwater mussels and estimates of survival for L. r. luteola. The yearly survival rate was higher for males (0.576; 95% C.I. 0.483-0.660) than for females (0.4851; 0.391-0.572) at the upstream site, but not at the downstream site (0.570; 0.415-0.699, estimate for sexes combined). Incorporating survival estimates into population growth models indicated that recruitment was low and relatively constant over the 5 y (2005-2009) for both populations. Size distribution data show similar, low recruitment. During this study both populations have been slowly declining in abundance due to high mortality and low recruitment.

INTRODUCTION

Throughout the world, freshwater mussels (Order Unionoida) are a highly imperiled group of organisms. Of the approximately 840 species worldwide (Graf and Cummings, 2007), 31 species are recently extinct and 92 are included in the IUCN red list as critically endangered, endangered, or vulnerable (IUCN, 2009). In the United States over a quarter of species are listed as threatened, endangered, or of special concern (US Fish and Wildlife Service, 2005) . Freshwater mussels face a variety of threats ranging from stream and riparian habitat alteration to point and non-point pollution (Richter et ai, 1997). Loss of host fish required during development and introduced species may also have negative impacts for some species (Ricciardi et ai, 1998).

Determining the population dynamics of freshwater mussels is a key component for their conservation. Particularly important is determining population growth rates and temporal variability in population growth. Extinction risk generally decreases with increasing population size; however, theoretical investigations have shown that the ratio of population growth to variation in growth (r/V) also affects the susceptibility of populations to extinction (Lande, 1993; Hanski, 1998) . Populations with a low ratio, due either to a low mean growth rate or high variation in growth, are more susceptible to extinction due to environmental stochasticity than are populations with a higher ratio. Because they can be long lived (Bauer, 1992; Berg et ai, 2008), temporal variability in survival and recruitment also plays an important role in determining die conservation status of freshwater mussels. A decline in abundance over time is consistent with, but does not always indicate, a declining population. For example, a species that is replacing itself over the long term, but with significant year to year variability in reproduction or survival, may be thought to be declining if randomly sampled at a time of high abundance followed by a time of lower abundance. Understanding temporal variability in survival, recruitment, and population growdi is particularly important for freshwater mussels because several species have been shown to have long periods of low recruitment interspersed by pulses of high recruitment (Negus, 1966; Payne and Miller, 2000; Villella et a?, 2004) while other species have relatively constant rates of population growth, survival, and/or recruitment (Ravera and Sprocati, 1997; Villella et al, 2004). …

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