Population Persistence in Rivers and Estuaries

Abstract

A wide variety of organisms inhabit streams, rivers, and estuaries where they are continually subjected to downstream drift. It is well known that when this is the only transport process, extinction is inevitable (the “drift paradox”). Using a series of analytical and numerical models, representing a range of hydrodynamic scenarios, we demonstrate that the action of diffusive dispersal can permit persistence in an advective environment. The mechanism underlying this phenomenon is that diffusive dispersal can allow a proportion of the population to reproduce close to their natal location. For well‐ and poorly mixed non‐tidal systems we establish approximate analytic conditions for diffusion‐mediated persistence both throughout the water column and in a benthic boundary layer. Although tidal forcing results in residual landward flow near the base of the water column, we find that this has little effect on persistence, which is respectably approximated by our analytic results. We apply these analytic results to four hydrodynamically disparate systems: a stream (Broadstone Stream [UK]), a river (Christiana Creek [USA]), a shallow estuary (Ythan [UK]) and a deep fast‐flowing estuary (Saco River [USA]). Using parameters derived from published studies we examine the persistence of a number of real and hypothetical organisms in these systems and identify those for which diffusively mediated persistence is a realistic possibility. We note that such persistence is only likely when advection is low or horizontal dispersal is high.

Population persistence in river networks

“…This theorem extends to river networks the conditions for population persistence seen in interval models (Speirs and Gurney 2001). If r ≤ V 2 4D , no population can persist, regardless of the size of the habitable domain.…”

“…4). For the estimated intrinsic growth rate for stoneflies r = 0.03 day −1 (Speirs and Gurney 2001), the population will persist for D ≈ 1.5 but not for D = 0.6, in either the interval or tree. In the tree case ζ has no discernible effect.…”

Geometric indicators of population persistence in branching continuous-space networks

“…which describes the spatial population dynamics in a network segment e (Speirs and Gurney 2001;Cantrell and Cosner 2003). For the current article we consider e as a finite interval.…”

“…Specifically, we adhere to commonly used boundary conditions: the zero flux condition (organisms do not enter or leave the habitat) at an upstream boundary b and the lethal condition (organisms die or washout of the habitat of interest) at a downstream boundary a (Speirs and Gurney 2001;Lutscher et al 2005):…”

“…However, a unified body of theory that can explain and predict emerging patterns is lacking. Most previous studies have either considered dynamics on a line, representing individual reaches or branches (e.g., Anderson et al 2005;Lutscher et al 2005;Speirs and Gurney 2001) or artificially discretizing an essentially continuous system to a set of interconnected patches (e.g., Auerbach and Poff 2011;Fagan 2002;Goldberg et al 2010;Muneepeerakul et al 2007). These previous studies have shown that dispersal constrained to limited reaches of habitat and/or by network configuration can influence persistence of populations inhabiting the network domain.…”