OSCIENCE Population variability is lower in diverse rock pools when the obscuring effects of local processes are removed 1 Tamara N. ROMANUK*, Dkpartement des sciences biologiques, Universite du Quebec a Montreal, CP 8888, Succ. Centre Ville, Monthl, Quebec H3C 3P8, Canada, and Pacific Ecoinformatics and Computational Ecology Laboratory, 1604 MgGee Ave, Berkeley, California 94709, USA, e-mail: romanukel78@rogers.com Jurek KOLASA, Department of Biology, McMaster University, Hamilton, Ontario L8S 4K1, Canada. Abstract: Ecological theory predicts that species richness should impact population variability. In contrast, most empirical evidence suggests no or only a weak positive relationship between species richness and population variability. We investigated the hypothesis that the obscuring noise of local processes at small scales such as differences in environmental conditions and species composition may mask the effects of species richness on population variability. Using long-term data on invertebrate populations in rock pools, we considered species richness-population variability relationships using three analytic resolutions in which data for the two key variables, species richness and population variability, were averaged for each population at decreasing levels of resolution to successively remove more noise arising from local processes. Of these levels the resolution most useful in making predictions about the effect of species richness on population variability removed the most noise in population responses arising from local processes. Our results show that populations are less variable in species-rich environments, a finding that reiterates the importance of species richness not only for aggregate properties such as biomass stability, but also for individual species abundances. Comparing results at different resolutions also provides a methodology to identify relevant detail in richness-population variability relationships. Keywords: diversity, environmental noise, resolution, rock pools, stability. Risumt : En theorie, la richesse en esp6ces devrait avoir un effet sur la variabiliti des populations. En realid, les donnees empiriques sugg6rent au contraire qu'il n'existe aucun lien, ou seulement une faible relation positive entre la richesse et la variabilite. Nous avons verifie I'hypothtse selon laquelle le bruit des processus locaux a fine echelle, tels que des diff6rences au niveau des conditions enviromementales ou de la composition en esp&ces, masque les effets de la richesse sur la variabilig des populations. Pour ce faire, nous avons utilise des donnees sur des populations d'invertkbres qui colonisent des mares pksentes sur des rochers. Nous avons ttudie les relations qui existent entre la richesse et la variabilitt en utilisant trois methodes de ksolution analytique. Nous avons calcule les moyennes de variabilitk et de richesse a diffirents niveaux decroissants de resolution pour enlever de fa~on successive un peu du bruit provenant des processus locaux. De ces niveaux de resolution, celui qui est le plus utile pour pridire les effets de la richesse sur la variabiliti est celui qui enleve le plus de bruit provenant des processus locaux. Nos resultats montrent que les populations sont moins variables dans les enviromements riches en eseces, ce qui kit8re I'importance de la richesse en esptces, non seulement pour des propriktes cornme la stabilite de la biomasse, mais aussi pour favoriser I'abondance des esptces individuelles. La comparaison de resultats a differents niveaux de ksolution constitue egalernent une methodologie permettant d'identifier des details pertinents ayant trait aux relations entre la richesse et la variabilitk des populations. Mots-clb : bruit environnemental, diversite, mares de rochers, resolution, stabilite. Nomenclature : Merrjtt & Commins, 1996; Therriault & Kolasa, 1999a; Thorp & Covich, 2001 Introduction Species richness appears to affect variability in com- munity abundance or biomass differently than population variability (May, 1973; Peterson, 1975; Tilman, 1996; Hughes & Roughgarden, 1998; McGrady-Steed & Morin, 2000; Romanuk & Kolasa, 2002; Gonzalez & Descamps- Julien, 2004). For communities, greater richness reduces temporal variability in aggregate community abundance (reviews McCann, 2000; Cottingham, Brown & Lemon, 2001; Loreau et al., 2001) through biological and statisti- cal processes, including overyielding (Tilman, 1999), complementarity (Hooper, 1998; Tilman, 1999; Norberg, -- - - - Rcc. 200441-29; acc. 2004- 08- 17. .miate Editor: Don McQueen. :Author for correspondence. 2000; Cardinale, Palmer & Collins, 2002), insurance effects and asynchronous population fluctuations (Petchey et al., 1999; Yachi & Loreau, 1999), weak interaction effects (McCann, Hastings & Huxel, 1998), statistical averaging (Doak et al., 1998), mean-variance relation- ships (Cottingham, Brown & Lennon, 2001), and sam- pling effects (Huston, 1997). For populations, the effect of richness on variability in abundance or biomass is unclear (McCann, 2000; Cottingham, Brown & Lemon, 2001). Empirical studies generally show non-existent or only very weak effects of richness on population variability (Tilman, 1996; Wardle et al., 1999; McGrady-Steed & Morin, 2000; Romanuk & Kolasa, 2002). Exceptions include Valone and Hoffman (2003a), who found that plant species in more diverse