American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Spatial scales of population synchrony in predator-prey systems”

Posted on

Javier Jarillo, Bernt-Erik Sæther, Steinar Engen, and Francisco Javier Cao García (Feb 2020)

Predator population has larger spatial scale of synchrony than the prey and the environmental noise

Read the Article (Just Accepted)

Predators have a higher risk of large regional extinction

<i>a</i>, 1D environmental fluctuations affecting the prey population dynamics (green), population fluctuations of the prey (blue) and of the predator (red). Environmental fluctuations affecting the predator are assumed to be negligible. <i>b</i>, Spatial autocorrelation functions of the environmental fluctuations affecting the prey (green solid line), the population fluctuations of the prey in the absence of the predator (grey dashed lines), and the population fluctuations of the prey (blue dot-dashed line) and of the predator (red dotted line) provided both species are present. <br />(Credit: Jarillo et al., &copy;&nbsp;2020&nbsp;The&nbsp;University of Chicago)
a, 1D environmental fluctuations affecting the prey population dynamics (green), population fluctuations of the prey (blue) and of the predator (red). Environmental fluctuations affecting the predator are assumed to be negligible. b, Spatial autocorrelation functions of the environmental fluctuations affecting the prey (green solid line), the population fluctuations of the prey in the absence of the predator (grey dashed lines), and the population fluctuations of the prey (blue dot-dashed line) and of the predator (red dotted line) provided both species are present.
(Credit: Jarillo et al., © 2020 The University of Chicago)

Predators often have changes in population size correlated over larger distances than their prey species, and therefore face a higher risk of regional extinction. Environmental conditions in an ecosystem are not constant; they fluctuate around their mean value, but they are usually similar in locations close to each other. These environmental fluctuations change the population growth of the different species, influencing their population size. However, there is a tendency for environmental fluctuations to have a particularly strong effect at the lower levels of the food web, and then for these effects to propagate bottom-up in the trophic chain.

A recent study concluded that bottom-up propagation of the environmental fluctuations in a predator-prey ecosystem leads to higher spatial scale of population synchrony for the predator than for the prey. This is likely to imply a larger size of the regional extinctions for the predator.

The study also supported the conclusion that harvesting also affects the spatial scale of population synchrony of the unharvested species. This implies that human perturbations of ecosystems through exploitation or modifying dispersal processes can affect food web structures and trophic interactions over large geographical areas. Norwegian (CBD, NTNU) and Spanish (UCM) researchers conducted this study supported by the Norwegian SUSTAIN and Abel projects and by the Spanish Ministry of Science.


Abstract

Many species show synchronous fluctuations in population size over large geographical areas, which are likely to increase their regional extinction risk. Here we examine how the degree of spatial synchrony in population dynamics is affected by trophic interactions using a two-species predator-prey model with spatially correlated environmental noise. We show that the predator has a larger spatial scale of population synchrony than the prey, if the population fluctuations of both species are mainly determined by the direct effect of stochastic environmental variations in the prey. This result implies that, in bottom-up regulated ecosystems, the spatial scale of synchrony of the predator population increases beyond the scale of the spatial autocorrelation in the environmental noise and in the prey fluctuations. Harvesting the prey increases the spatial scale of population synchrony of the predator, while harvesting the predator reduces the spatial scale of the population fluctuations of its prey. Hence, the development of sustainable harvesting strategies should consider the impact also on unharvested species at other trophic levels; and the human perturbations of ecosystems, through exploitation or through an effect on dispersal processes, which can affect food web structures and trophic interactions over large geographical areas.

Escalas espaciales de sincronía poblacional en sistemas depredador-presa

Muchas especies presentan fluctuaciones sincronizadas en su tamaño poblacional en grandes áreas geográficas, lo que probablemente aumenta su riesgo de extinción regional. Aquí examinamos cómo el grado de sincronía espacial en la dinámica poblacional se ve afectado por las interacciones tróficas utilizando un modelo depredador-presa con ruido ambiental espacialmente correlacionado. Mostramos que el depredador tiene una mayor escala espacial de sincronía poblacional que la presa, si las fluctuaciones poblacionales de ambas especies están determinadas principalmente por el efecto directo de las variaciones ambientales estocásticas en la presa. Esto implica que, en ecosistemas regulados de abajo hacia arriba (“bottom-up”), la escala espacial de sincronía de la población del depredador supera a las escalas de autocorrelación espacial del ruido ambiental y de las fluctuaciones de la presa. La extracción de presas del ecosistema aumenta la escala espacial de sincronía poblacional del depredador, mientras que la extracción de depredadores reduce la escala espacial de las fluctuaciones poblacionales de su presa. Por lo tanto, el desarrollo de estrategias de explotación sostenible debería considerar el impacto en las especies no explotadas de otros niveles tróficos; y las perturbaciones humanas sobre los ecosistemas, a través de la explotación o de cambios provocados en los procesos de dispersión, que pueden afectar a las estructuras de las cadenas alimenticias y a las interacciones tróficas en grandes áreas geográficas.