Abstract
Our study investigated the influence of bottom-up and top-down drivers on the declining fledging success at a once thriving breeding colony of Caspian terns (Hydroprogne caspia). Situated at the mouth of the Columbia River, OR, East Sand Island (ESI) is home to the largest Caspian tern breeding colony in North America. Since 2001, the decline in fledging success of Caspian terns at ESI has been associated with a significant increase in average river discharge during May and June. During the years 2001–2011, the abundance of forage fish available to terns in the estuary was inversely related to river discharge. This relationship also apparently affected the reliance of nest predators on the tern colony as a food source, resulting in increased disturbance and decreased fledging success at the tern colony in years of higher river discharge. There was a significant longitudinal increase in disturbance rates by bald eagles (Haliaeetus leucocephalus) during June for terns nesting at the ESI colony, and eagle disturbance rates were positively associated with May river discharge. We also found a significant increase in kleptoparasitism rates of terns by hybrid glaucous-winged/western gulls (Larus glaucescens x Larus occidentalis) since 2001, and Caspian tern fledging success at ESI decreased with increasing average annual rates of gull kleptoparasitism. Our results support the hypothesis that the decline in Caspian tern fledging success at this large estuarine colony was primarily driven by the interaction of bottom-up and top-down factors, influencing tern fledging success through the food supply and triggering potential predators to identify the tern breeding colony as an alternative source of prey.
Highlights
In the decades since the top-down versus bottom-up debate began in community ecology, focus has shifted to a more nuanced view: that the unique interactions between predators and producers shape individual ecological communities (Hunt and McKinnell 2006; Suryan et al 2006; Gripenberg and Roslin 2007)
We sought to understand how top-down and bottom-up forces interacted in this system and influenced the fledging success of Caspian terns (Hydroprogne caspia) at East Sand Island in the Columbia River estuary, the largest breeding colony for the species in North America
The negative correlation between June river discharge and fledging success at the Caspian tern colony was significant for the years 2001–2011 (R2 = 0.49, df = 9, P = 0.02)
Summary
In the decades since the top-down versus bottom-up debate began in community ecology, focus has shifted to a more nuanced view: that the unique interactions between predators and producers shape individual ecological communities (Hunt and McKinnell 2006; Suryan et al 2006; Gripenberg and Roslin 2007). Some species of gull (Larus spp.) that may primarily exploit piscine prey can be voracious egg and chick predators, and many raptors will opportunistically take adult colonial seabirds or their eggs and chicks, especially if alternative food sources or prey are scarce (Isaacs and Anthony 2011; Harvey et al 2012). It is in complicated systems such as these that we can potentially see the Bchutes and ladders^ of a dynamic food web proposed by Hunter and Price (1992). We sought to understand how top-down and bottom-up forces interacted in this system and influenced the fledging success of Caspian terns (Hydroprogne caspia) at East Sand Island in the Columbia River estuary, the largest breeding colony for the species in North America
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