Long‐term variability and density dependence in Hudson River Dreissena populations

Abstract

We used a 27-year record of Dreissena populations in the freshwater tidal HudsonRiver to describe interannual variation in population density, body size, and body condition; estimate long-term variation in recruitment, survivorship, and shell growth; and assess possible controls on the populations. 2.Dreissena populations in the Hudson have been highly variable, with interannual ranges of c. 100-fold in abundance and biomass, and 7-fold in mean body mass. This large interannual variation arises from both long-term trends and 2-5-year cycles.3. Long-term trends include the 2008 appearance of the quagga mussel (Dreissena rostriformis), which still forms a small part (<10%) of the dreissenid community, and a decline in zebra mussel body size. The decline in body size was caused by a longterm decline in adult survivorship rather than a decline in rates of shell growth. We could detect no long-term trends in adult abundance or spread of Dreissena onto soft sediments in the Hudson. 4. We observed persistent, strong cycles in adult abundance and body size. These were driven by the appearance and decay of eight dominant year classes over the 27 years of our study, and were a result of temporal variation in recruitment rather than temporal variation in survivorship. The observed strongly irregular recruitment appears to arise from strong adult-larval interactions, and is consistent with previous simulation model results showing that interactions between adults and larvae can drive persistent cycling. 5. We found evidence that negative density dependence affects recruitment, somatic growth, and body condition of Dreissena in the Hudson. Warm summers may also cause high adult mortality. 6. We put our results into the context of a conceptual model of Dreissena population dynamics, and argue that neither the dynamics nor the controls of populations of these important invaders is known satisfactorily. K E Y W O R D Sbiological invasions, cycling, invasive species, population dynamics, zebra mussel | 475 STRAYER ET Al.

Accounting for sources of uncertainty when forecasting population responses to climate change

“…For many sessile species, such as mussels, competition for both space and food can lead to decreased growth and survival rates (Fréchette et al., 1992; Strayer et al. 2020). Although many retrospective analyses have explored density‐dependent processes, few studies have accounted for uncertainty about the strength and form of density dependence in population forecasts (but see Colchero et al., 2009; Reed et al., 2013) or assessed how interactions between population density and climate could affect future population trajectories.…”

“…Density-dependent processes are known to be important mechanisms driving rates of population change when species depend on one or more limited resources (Turchin, 1995). For many sessile species, such as mussels, competition for both space and food can lead to decreased growth and survival rates (Fréchette et al, 1992;Strayer et al 2020). Although many retrospective anal- This exploration of the different ways in which density-dependent processes could affect population forecasts of blue mussels provided insights that would have been overlooked had analyses simply relied on the best-supported model for population forecasts.…”

Zebra or quagga mussel dominance depends on trade-offs between growth and defense—Field support from Onondaga Lake, NY

“…None of the 42 longer-term (>10 year) data series on adult dreissenid mussels from Europe and North America analyzed by Strayer et al [15] showed a differential decline of quagga mussels, and there was no general decline in the combined dreissenid mussels with time since invasion. But only four of these 42 data sets included more than 10 years of annual data on adult mussels from systems with both quagga and zebra mussels (Oneida Lake [12], Hudson River [75], Lake Balaton [54], and Onondaga Lake-this study). Interestingly, in the Hudson River, quagga mussels have remained subdominant for decades, perhaps due to higher predation rates in the river [75].…”

“…But only four of these 42 data sets included more than 10 years of annual data on adult mussels from systems with both quagga and zebra mussels (Oneida Lake [12], Hudson River [75], Lake Balaton [54], and Onondaga Lake-this study). Interestingly, in the Hudson River, quagga mussels have remained subdominant for decades, perhaps due to higher predation rates in the river [75]. There are also studies that were not included in the Strayer et al data set that suggest a link between predator abundance and mussel species dominance.…”

Long‐term increases in shell thickness in Elliptio complanata (Bivalvia: Unionidae) in the freshwater tidal Hudson River

“…Although the water in the study area is fresh, rarely with even a trace of sea salt, the entire study area is strongly tidal, with tidal amplitudes of 0.8–1.6 m, and daily tidal flows usually are much greater than net downriver flows. Zebra mussels first appeared in the Hudson in 1991, and the population has been large but variable since autumn 1992 (Strayer et al., 2020).…”