Competitive exclusion, beta diversity, and deterministic vs

Hila Segre,1Ronen Ron,1

Niv De Malach,1Zalmen Henkin,2Micha Mandel 3and Ronen Kadmon 1*

Abstract

Species diversity has two components –number of species and spatial turnover in species composi-tion (beta-diversity).Using a ?eld experiment focusing on a system of Mediterranean grasslands,we show that interspeci?c competition may in?uence the two components in the same direction or in opposite directions,depending on whether competitive exclusions are deterministic or stochastic.Deterministic exclusions reduce both patch-scale richness and beta-diversity,thereby homogenising the community.Stochastic extinctions reduce richness at the patch scale,but increase the differ-ences in species composition among patches.These results indicate that studies of competitive effects on beta diversity may help to distinguish between deterministic and stochastic components of competitive exclusion.Such distinction is crucial for understanding the causal relationship between competition and species diversity,one of the oldest and most fundamental questions in ecology.

Keywords

Annual plants,community ecology,functional groups,habitat productivity,Mediterranean grass-lands,niche vs.neutral processes,plant competition,removal experiment,species diversity.

Ecology Letters (2014)17:1400–1408

INTRODUCTION

Classical competition theory assumes that interspeci?c com-petition is deterministic in the sense that species differ in their competitive ability and these differences determine the outcome of local competitive interactions (Grime 1973;Ked-dy &Shipley 1989).Within a spatially uniform habitat,deterministic competitive exclusion is expected to reduce the degree of spatial turnover in species composition (beta diver-sity),by limiting the composition of the overall community to a subset of competitively superior species (Kunstler et al.2012).Nevertheless,in spite of an increasing interest in pat-terns and mechanisms of beta diversity (Gering &Crist 2002;Anderson et al.2011;Kraft et al.2011;Carvalho et al.2012),almost nothing is known about the effect of competi-tive exclusion on beta diversity.Clearly,if competitive inter-actions affect beta diversity,quantifying the magnitude,patterns and scales of such effects is crucial for understand-ing observed patterns of species diversity and their causal mechanisms.

Moreover,in a recent article,Chase et al.(2011)proposed that beta diversity can be used as a yardstick for discrimi-nating between deterministic and stochastic drivers of species diversity (see also Chase 2007,2010;Chase &Myers 2011).This approach is based on a null model that generates ran-dom species assemblages from a set of sites representing the relevant species pool.For any pair of sites,beta diversity is calculated in ?ve steps:(1)random assignment of species

from the relevant pool to each site until the number of spe-cies assigned to the site equals the observed number,(2)cal-culation of a presence –absence measure of compositional similarity between the assemblages of species assigned to the two sites,(3)repeating this procedure a large number of times to obtain a ‘null’distribution of compositional similar-ity between the two sites,(4)determination of the fraction of null values of compositional similarity that are equal to or larger than the observed value,and (5)rescaling of the resulting value to range from à1to 1.Importantly,step (4)converts the measure of similarity into a measure of dissimi-larity and step (5)rescales this measure to have a null expec-tation of zero.Values signi?cantly lower than zero are interpreted as evidence for deterministic drivers of species diversity.

In this study,we apply the approach proposed by Chase et al.(2011)to investigate the effect of competitive interac-tions on the number of species in a local community.Our study is based on the premise that competition affects the number of species in a community via two distinct mecha-nisms:(1)deterministic competitive exclusion at the scale at which individuals directly compete with each other (the ‘com-petitive neighbourhood’sensu Stoll &Weiner 2000),and (2)reduction in differences in species composition among differ-ent neighbourhoods.The advantage of the null-model approach proposed by Chase et al.(2011)is that it allows one to statistically separate the two mechanisms,that is,to test the effect of competitive exclusion on beta diversity while con-1

Department of Ecology,Evolution and Behavior,The Hebrew University of Jerusalem,Givat Ram,Jerusalem,91904,Israel

2

Beef Cattle Section,Newe-Ya’ar Research Center,Department of Natural

Resources,Agricultural Research Organization,P.O.Box 1021,Ramat Yishay,30095,Israel

3

Department of Statistics,The Hebrew University of Jerusalem,Mount Scopus,

Jerusalem,91905,Israel

*Correspondence:E-mail:kadmon@mail.huji.ac.il

?2014John Wiley &Sons Ltd/CNRS

Ecology Letters ,(2014)17:1400–1408doi:10.1111/ele.12343