Opposing effects of floral visitors and soil conditions on the determinants of competitive outcomes maintain species diversity in heterogeneous landscapes.

Jose B Lanuza,Ignasi Bartomeus,Oscar Godoy,José María Gómez

doi:10.1111/ele.12954

Abstract

Theory argues that both soil conditions and aboveground trophic interactions have equivalent potential to limit or promote plant diversity. However, it remains unexplored how they jointly modify the niche differences stabilising species coexistence and the average fitness differences driving competitive dominance. We conducted a field study in Mediterranean annual grasslands to parameterise population models of six competing plant species. Spatially explicit floral visitor assemblages and soil salinity variation were characterised for each species. Both floral visitors and soil salinity modified species population dynamics via direct changes in seed production and indirect changes in competitive responses. Although the magnitude and sign of these changes were species-specific, floral visitors promoted coexistence at neighbourhood scales, while soil salinity did so over larger scales by changing the superior competitors' identity. Our results show how below and aboveground interactions maintain diversity in heterogeneous landscapes through their opposing effects on the determinants of competitive outcomes.

Highlights

One central aim in ecology is understanding how plant species diversity is maintained
We focus on three questions: (1) How do soil salinity and floral visitors modify species' population dynamics via direct changes in per capita seed production and indirect changes in species' responses to competitive interactions?
Our ability to combine coexistence theory advances with plant population models and spatially explicit observations provides direct evidence that variation in soil salinity content and floral visitor frequency modify the likelihood of plant coexistence, yet they do so in opposite directions

Summary

Introduction

One central aim in ecology is understanding how plant species diversity is maintained. Extensive empirical work has documented that variation in soil conditions and multitrophic interactions modulate key processes of plant population dynamics. Plant offspring and the strength of competition depend on the combined species' ability to deplete shared limiting soil resources (Tilman 1982) and to cope with stressful soil conditions such as the amount of salt (Bertness & Shumway 1993; Crain et al 2004). It is obvious that outcrossing plants directly depend on their mutualistic floral visitors to maximize their reproductive success (Morris et al 2010; Ollerton et al 2011). Subtler is the fact that floral visitors indirectly mediate competition among plants through a wide variety of density-dependent processes including variation in the number and diversity of floral visitors as well as heterospecific pollen deposition (Moeller 2004; Arceo-Gómez & Ashman 2011; Runquist & Stanton 2013)

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Results

Conclusion