Optimal Planting Distance in a Simple Model of Habitat Restoration With an Allee Effect

Liv Hammann,Brian Silliman,Bernd Blasius

doi:10.3389/fmars.2020.610412

Abstract

Ecological restoration is emerging as an important strategy to improve the recovery of degraded lands and to combat habitat and biodiversity loss worldwide. One central unresolved question revolves around the optimal spatial design for outplanted propagules that maximizes restoration success. Essentially, two contrasting paradigms exist: the first aims to plant propagules in dispersed arrangements to minimize competitive interactions. In contrast, ecological theory and recent field experiments emphasize the importance of positive species interactions, suggesting instead clumped planting configurations. However, planting too many propagules too closely is likely to waste restoration resources as larger clumps have less edges and have relatively lower spread rates. Thus, given the constraint of limited restoration efforts, there should be an optimal planting distance that both is able to harness positive species interactions but at the same time maximizes spread in the treated area. To explore these ideas, here we propose a simple mathematical model that tests the influence of positive species interactions on the optimal design of restoration efforts. We model the growth and spatial spread of a population starting from different initial conditions that represent either clumped or dispersed configurations of planted habitat patches in bare substrate. We measure the spatio-temporal development of the population, its relative and absolute growth rates as well as the time-discounted population size and its dependence on the presence of an Allee effect. Finally, we assess whether clumped or dispersed configurations perform better in our models and qualitatively compare the simulation outcomes with a recent wetland restoration experiment in a coastal wetland. Our study shows that intermediate clumping is likely to maximize plant spread under medium and high stress conditions (high occurrence of positive interactions) while dispersed designs maximize growth under low stress conditions where competitive interactions dominate. These results highlight the value of mathematical modeling for optimizing the efficiency of restoration efforts and call for integration of this theory into practice.

Highlights

Over the past century, many ecosystems worldwide and the valuable services they provide have been lost and degraded as a result of anthropogenic stressors, such has habitat loss, over-exploitation, and climate-change (Leemans and De Groot, 2003)
We study the spatial coverage of the recovering ecosystem from the different initial conditions and investigate how it is influenced by positive species interactions, which are incorporated into the model in form of a weak or strong Allee effect (Courchamp et al, 1999)
In this study we proposed a simple mathematical model that predicts the success of a plant restoration based on the planting configuration

Summary

Introduction

Many ecosystems worldwide and the valuable services they provide have been lost and degraded as a result of anthropogenic stressors, such has habitat loss, over-exploitation, and climate-change (Leemans and De Groot, 2003). The magnitude of ecosystem degradation and the associated loss of biodiversity and ecosystem functions, such as the protection of shorelines from flooding and storm events in coastal systems (Barbier et al, 2011), generated a pressing need for conservation strategies that actively combat this decline. Ecological restoration is one conservation intervention used to combat habitat loss. It aims to repair or otherwise enhance the structure and function of an ecosystem that has been impacted by disturbance or environmental change. Restoration has emerged as an important conservation tool for improving the recovery of degraded lands and to counteract habitat and biodiversity loss (Jordan et al, 1990; Dobson et al, 1997; Young, 2000; Young et al, 2005; Suding, 2011). As restoration resources are economically limited, it is of utmost importance to guarantee the efficiency of ecological restoration (Aronson et al, 2006; Suding, 2011; Zhang et al, 2018)

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