The Prevalence of Ecological Drift in the Heath Vegetation of South America 1, 2 3 3 J

Community assembly in harsh environments: the prevalence of ecological drift in the heath vegetation of South America 1, 2 3 3 J. L. A. SILVA, A. F. SOUZA, J. G. JARDIM, AND B. T. GOTO 1Programa de Po´s-Graduaçaõ em Ecologia, Universidade Federal do Rio Grande do Norte, Natal, Rio Grande do Norte 59072-970 Brazil 2Departamento de Ecologia, Universidade Federal do Rio Grande do Norte, Natal, Rio Grande do Norte 59072-970 Brazil 3Departamento de Botaˆnica e Zoologia, Universidade Federal do Rio Grande do Norte, Natal, Rio Grande do Norte 59072-970 Brazil Citation: Silva, J. L. A., A. F. Souza, J. G. Jardim, and B. T. Goto. 2015. Community assembly in harsh environments: the prevalence of ecological drift in the heath vegetation of South America. Ecosphere 6(7):111. http://dx.doi.org/10.1890/ ES14-00548.1 Abstract. Metacommunity dynamics is marked by a gradient ranging from pure ecological determinism to pure stochasticity. This gradient encompasses compositional turnover that is governed by ecological drift, selection and dispersal. Here we estimate the influences of selection, dispersal limitation acting in concert with drift, drift acting alone and homogenizing dispersal on the structure of tropical restinga heath vegetation growing under stressful conditions in north-eastern South America. We hypothesize that if abiotic heterogeneity is strong enough, it could select distinct sets of colonizing species from neighbor ecosystems, with stress sensitive species occupying refuges created by abiotic heterogeneity and stress tolerators dominating the more exposed areas. In this case selection would occur at both biogeographical and local scales. Under its prevalence, we expect selection to have a major signature in the woody plant community structure in a phylogenetic null model. Alternatively, if abiotic heterogeneity is not strong enough to impose significant selection, the environment would be homogeneously stressful for the majority of species, and would be dominated species selected at biogeographical scale only. Under its prevalence, we expect drift to have a major signature in the phylogenetic null model. We used an analytical framework based on phylogenetic and community structure null models to assess the relative importance of ecological processes. We also aimed to characterize the ecosystem features that impose selection and dispersal limitation. We found that 95.1% of turnover in composition is attributable to drift, 2.4% to homogenizing dispersal, 2.1% to selection, and just 0.40% to dispersal limitation, thus confirming our Neutrality hypothesis. As expected, both soil and topographic variables influenced metacommunity structure. However, contrary to our prediction, light availability and vegetation structure were also important. The predominance of coarse spatial patterns correlated to topographic and soil properties suggests that coarse differences in wind exposure and associated vegetation and soil factors represent the main selective forces acting on the studied vegetation. The dominance of drift in the assembly of restinga heath vegetation is likely to result from homogeneously stressful environmental conditions and also from the ongoing colonization process that is taking place in the restinga by immigrants from species-rich neighboring ecosystems. Key words: Brazil; dune ecosystems; neutral dynamics; niche; soil nutrients. Received 29 December 2014; revised 22 February 2015; accepted 4 March 2015; final version received 31 March 2015; published 15 July 2015. Corresponding Editor: D. P. C. Peters. Copyright: Ó 2015 Silva et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. http://creativecommons.org/licenses/by/3.0/ E-mail: [email protected] v www.esajournals.org 1 July 2015 v Volume 6(7) v Article 111 SILVA ET AL. INTRODUCTION sented by Vellend’s (2010) conceptual framework, which proposes that community assembly is A major goal of community ecology is to based on selection, drift, dispersal, and specia- understand the processes that determine local tion. Selection is the result of deterministic fitness species abundances and the resulting community differences among species due to biotic and structure. Recent developments in this area have abiotic factors; dispersal dictates the intensity of been marked by the attempt to disentangle the individual movement among communities; drift relative importance of deterministic niche-assem- represents stochastic changes in species abun- bly and stochastic processes (Cottenie 2005, dance; and speciation may produce differences in Chase and Myers 2011, Vellend et al. 2014). species richness among communities that do not Variation partitioning analyses of community exchange individuals through dispersal (Vellend structure between environmental and spatial 2010). Speciation is expected to have little components based on Redundancy Analysis has influence within a set of communities connected been associated with deterministic and stochastic through dispersal, known as a ‘metacommunity’ processes, respectively (Borcard et al. 1992, 2011). (Leibold et al. 2004). Species composition turn- Based on this approach, Cottenie (2005) found a over in within a metacommunity is therefore gradient in the distribution of different commu- governed by the combined effects of selection, nity types, with communities structured by dispersal and drift (Stegen et al. 2013). environmental processes and communities with In spite of advances in the characterization of an important independent spatial component at selection through null models (e.g., Chase et al. opposite ends of the same spectrum. Hence, the 2011), a methodological framework for the position of a community along this axis would identification of the relative importance of indicate the relative importance of deterministic multiple processes acting together in ecological and stochastic processes in the assembly of its systems has only very recently been proposed species (Cottenie 2005, Chase and Myers 2011, (Stegen et al. 2013), and has remained so far Logue et al. 2011). Given the recognition of such restricted to microbial communities. Here we gradient, researchers have begun to seek the estimate for the first time the influences of mechanisms able to influence the proportion of selection, dispersal limitation acting in concert determinism and stochasticity in ecological com- with drift, drift acting alone and homogenizing munities (Chase and Myers 2011). Different dispersal on the structure of a community of mechanisms seem to act for distinct species macroscopic organisms. We studied the tropical groups at distinct scales (Driscoll and Linden- restinga heath vegetation growing under stress- mayer 2009). Examples are suggestions that ful conditions in northeastern South America. ecological determinism increases with organism Restinga refers to the heath vegetation that size (Farjalla et al. 2012) and age of ecological covers the South American sandy plains mostly succession (Vellend et al. 2007) and decreases along the approximately 7,000 km of the Brazil- with the size of the regional species pool (Chase ian coast, forming vegetation complexes includ- and Myers 2011). Furthermore, ecological deter- ing open scrubland and short and dense forests minism has been proposed to (Brown et al. 2011) on flat terrain or on dune fields (Oliveira-Filho and indeed found to increase with habitat 2009). Plants colonizing these habitats have heterogeneity and the consequent increase in reduced productivity (Pires et al. 2006) due to available niches (Laliberte´ et al. 2009, Legendre et nutrient-poor soils with poor water retention al. 2009, Baldeck et al. 2012, De Caćeres et al. (Brancalion et al. 2012), face strong edaphic 2012). gradients and, in northeastern South America, Methodological (Økland 1999, Gilbert and seasonal water shortage. Heath vegetation is very Bennett 2010, Smith and Lundholm 2010) and young in the South American Atlantic coast, interpretational (Vellend et al. 2014) flaws in colonizing sand deposits mainly in the last 7,000 traditional variance partition analyses have, years (Scarano 2002). The restingas present few however, been identified and cast doubt on the endemics, with most of its species also occurring aforementioned findings. An alternative and in neighbor species-rich ecosystems (Scarano theoretically sounder approach has been repre- 2002). Even though many species have physio- v www.esajournals.org 2 July 2015 v Volume 6(7) v Article 111 SILVA ET AL. logical features to deal with water shortages, have a major signature in the woody plant high light levels and lack of specialized pollina- community structure. tors seem to constrain the growth and reproduc- We used a combination of phylogenetic and tion of many species (Scarano et al. 2001, Farias et abundance-based null models (Stegen et al. 2013) al. 2006). to test which of the two competing hypotheses Several studies have shown reduced levels of outlined above prevail in the assembly of South both a- and b-diversity among localities that American heath vegetation plant community. experience relatively ‘harsh’ environmental con- Furthermore, we aimed to characterize the ditions, such as higher disturbance or lower ecosystem features that impose selection and productivity (review in Chase 2010). From these dispersal limitation. Since it is a limiting factor in

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