How to measure top–down vs bottom–up effects: a new population metric and its calibration on Daphnia

Abstract

Research on the role of top–down (predation) and bottom–up (food) effects in food webs has led to the understanding that the variability of these effects in space and time is a fundamental feature of natural systems. Consequently, our measurement tools must allow us to evaluate the effects from a dynamical perspective. A population‐dynamics approach may be appropriate to the task. More specifically, because food and predators both affect birth rate, birth rate dynamics may be a key to understanding their impact on the population of interest. Based on the Edmondson–Paloheimo model for birth rate, we propose a new population metric to assess the relative strength of top–down vs bottom–up effects. The metric is the ratio of contributions of changes in proportion of adults and fecundity to change in birth rate. Proportion of adults reflects a top–down effect (predators are assumed to be size‐selective), fecundity reflects a bottom–up effect, and birth rate appears as a common currency with which to compare the former and the latter. Using microcosm experiments and computer simulations on the cladoceran Daphnia, we calibrate the metric and show that, in both types of tests, the ratio of contributions is typically 0.5–0.7 under a strong bottom–up effect and 2.0–2.2 under a strong top–down effect. This provides experimental evidence that the ratio of contributions may allow one to distinguish a strong top–down effect from a strong bottom–up effect.

Transient top‐down and bottom‐up effects of resources pulsed to multiple trophic levels

“…For example, aquatic insects moving from water to land can subsidize diverse communities of terrestrial predators such as spiders (Power et al 2004) and birds (Nakano and Murakami 2001), enhancing top‐down control on local herbivores and thereby increasing plant productivity. Resource pulses potentially exert multiple direct and indirect effects on recipient systems, making them a valuable context in which to study temporal variation in the strength of bottom‐up and top‐down control, a topic of recent and general ecological interest (Meserve et al 2003, Fath et al 2004, Polishchuk et al 2013, Leroux and Loreau 2015, Vidal and Murphy 2018, Piovia‐Scott et al 2019).…”

Theoretical perspectives on bottom-up and top-down interactions across ecosystems

“…For many years, ecologists have focused on demonstrating the primacy of their favorite hypothesis (White, 1978;McQueen et al, 1986). However, recent empirical results from a wide range of ecosystems, many of which are reviewed in this volume, provide unequivocal evidence that both resources and consumers interact to shape natural populations, communities, and ecosystems (e.g., Hunter and Price, 1992;Brett and Goldman, 1996;Hassell et al, 1998;Polis, 1999;Fath, 2004;Borer et al, 2006;Gruner et al, 2008;Polishchuk et al, 2013;Whalen et al, 2013). Ecological theory (e.g., Hairston et al, 1960;Oksanen et al, 1981) has been at the forefront of integrating our empirical knowledge of the interdependence of resource and consumer impacts on food webs and ecosystems (Table 1.1).…”

A genetic component of extinction risk in mammals

“…, the strength of the effect of a variable depends on the slope coefficient associated with the variable and on a characteristic shift in that variable (Kremer and Putko , p. 90). The product of the slope coefficient, which is a change in the model function in response to a unit change in the variable, and the characteristic shift in the variable is, therefore, a convenient measure of the effect of that variable (see Polishchuk et al for how this approach, called contribution analysis, has been used in various, typically nonlinear contexts) A one‐standard‐deviation increase in our variables, σ ln W and σ ln( K a/ K s) , is used to represent their characteristic shifts. Combining the slope coefficients from Eq.…”