Ant Community Organization Along Elevational Gradients in a Temperate Ecosystem

Insect. Soc. DOI 10.1007/s00040-014-0374-2 Insectes Sociaux RESEARCH ARTICLE Ant community organization along elevational gradients in a temperate ecosystem A. Bernadou • X. Espadaler • A. Le Goff • V. Fourcassie´ Received: 20 February 2014 / Revised: 27 September 2014 / Accepted: 9 October 2014 Ó International Union for the Study of Social Insects (IUSSI) 2014 Abstract The aim of our study was to characterize the decreased with elevation. A significant nested pattern was factors that shape the pattern of change in ant species observed, indicating that the species found in the poorest richness and community structure along altitudinal gradi- site represented a subset of those found at the richest one. ents in two valleys located on the northern and southern side Ants collected at mid- and high-elevation sites had a wider of the Pyrenees. During three summers, we sampled 20 sites altitudinal range than those collected at low-elevation sites, distributed across two Pyrenean valleys ranging in elevation thus complying with Rapoport’s rule. Our results suggest from 1,009 to 2,339 m using pitfall traps and hand collec- that, although elevation strongly influences the organization tion. We employed diversity index, degree of nestedness of of ant communities, ecological factors such as temperature ant assemblages, ordination method, and multiple regres- and local habitat features (sun exposure, vegetation density) sion analysis to examine the effects of various environ- are the main factors explaining the pattern of ant diversity mental factors on ant species communities. In total, 41 ant along altitudinal gradients. species were found in the two valleys. The number of species was 26 % lower in the valley located on the northern Keywords Ants Á Community ecology Á side than in that located on the southern side. At the valley Elevation gradient Á Andorra Á France Á Pyrenees scale, the number of ant species, as well as the evenness, Introduction Electronic supplementary material The online version of this article (doi:10.1007/s00040-014-0374-2) contains supplementary material, which is available to authorized users. Mountains cover approximately one-quarter of the land surface of the Earth (Ko¨rner, 2007;Ko¨rner et al., 2011) and & A. Bernadou ( ) Á A. Le Goff Á V. Fourcassie´ represent 11.4 % of the protected areas of the Earth surface Universite´ de Toulouse, UPS Centre de Recherches sur la Cognition Animale, 118 Route de Narbonne, 31062 Toulouse (Kollmair et al., 2005). They have generally a high rate of Cedex 9, France endemism and species diversity (Kollmair et al., 2005) and e-mail: [email protected] this probably explains why many biodiversity hotspots are located, at least in part, in highland or mountainous areas A. Bernadou Á A. Le Goff Á V. Fourcassie´ CNRS Centre de Recherches sur la Cognition Animale, 118 (Myers et al., 2000). Mountainous areas with their altitu- Route de Narbonne, 31062 Toulouse Cedex 9, France dinal gradients are characterized by rapid changes in climate, soil, or vegetation over relatively short distances. X. Espadaler They are therefore the ideal place for exploring the eco- Departament de Biologia Animal, de Biologia Vegetal i d’Ecologia, Facultat de Cie`ncies, Universitat Auto`noma de logical mechanisms underlying spatial patterns in species Barcelona, E-08193 Bellaterra, Spain richness (Ko¨rner, 2007). A widespread pattern observed in both plants and ani- Present Address: mals in mountainous areas is a linear and monotonic decline A. Bernadou Evolution, Behaviour and Genetics-Biology I, University of of species richness with elevation (Rahbek, 2005). Other Regensburg, Universita¨tsstraße 31, 93053 Regensburg, Germany studies suggest, however, that a second pattern, with a peak 123 A. Bernadou et al. of species richness at mid elevation, may also be common (Bernard, 1946; Ovazza, 1950; Soulie´, 1962; Espadaler, (Rahbek, 2005 for a review). Both of these patterns are 1979; Sommer and Cagniant, 1988a, b; but see Arnan et al., found in insects (Olson, 1994; Sanders et al., 2010). How- 2009; Bernadou et al., 2013b for more recent studies). ever, the mechanisms and factors responsible for these The aim of our study was to characterize the pattern of patterns still remain poorly understood (Rahbek, 2005; change in ant diversity along altitudinal gradients in the Dunn et al., 2009b). The variation in species richness with Pyrenees and to relate this change to environmental and altitudinal gradients is indeed not easily interpreted as ecological factors. We sampled 20 sites distributed across several factors (e.g., sunlight, temperature, barometric two Pyrenean valleys characterized by contrasted climatic pressure, rainfall, available area) are known to co-vary with conditions: one valley is located in Andorra, on the southern elevation (Ko¨rner, 2007; Sundqvist et al., 2013), which side of the Pyrenees, while the other is located in France, on makes their respective roles difficult to decipher. For the northern side. Our study addresses three interrelated example, climatic factors such as temperature and precipi- questions: (1) Does ant diversity vary between the two tation have been shown to be consistently correlated with valleys and with elevation? (2) Does elevation affect the species richness in several studies (Kaspari et al., 2000; distribution of ant species and the composition of their Sanders et al., 2003, 2007; Dunn et al., 2009a). In general, communities? and (3) Do climatic and/or landscape vari- warmer and more productive sites support more species than ables influence the pattern of species richness along cooler or less productive sites. However, other factors such altitudinal gradients? as spatial heterogeneity or habitat complexity have also been highlighted as relevant for explaining the structure of species assemblages. For example, the distribution of ants Materials and methods and their functional groups can be significantly affected by land-cover variables (Bernadou et al., 2013a), the spatial Study area and study sites heterogeneity generated by fire (Parr and Andersen, 2008) or habitat fragmentation (Vasconcelos et al., 2006). All The Pyrenees extend over the border between France and these factors can interact with each other to drive spatial Spain, along an east–west direction from the Mediterranean variation and shape species richness. Sea to the Atlantic Ocean over a length of approximately Ants are one of the most represented groups of animals in 430 km and a width of 100–140 km. The two sides of the most terrestrial ecosystems (Ho¨lldobler and Wilson, 1990). range present important climatic contrasts. While the Most of the studies on ant assemblage patterns along ele- northern side has an oceanic climate, with rainfall through- vational gradients published in the literature during the last out the year, mild winters and cool summers, the southern two decades have focused on tropical regions (e.g., Peters side, in contrast, has a more continental climate, with very et al., 2014) or on temperate regions of North America (e.g., cold winters and dry summers, high solar radiation and large Sanders et al., 2003, 2007). Studying new biogeographic variations in temperature (Go´mez et al., 2003). regions and thus working with ant assemblages from dif- Two valleys were sampled in this study: the Madriu- ferent species pools, functional or phylogenetic groups and Perafita-Claror, in the Principality of Andorra, and the Pique influenced by different evolutionary histories may help to valley, in France (Fig. 1—distance between the valleys is highlight the mechanisms that could have shaped ant bio- about 80 km). The Madriu-Perafita-Claror valley is a glacial diversity along elevational gradients, and also to investigate valley located in the southeast part of Andorra which has whether these mechanisms are the same across continents. been registered in 2004 as World Heritage for its cultural A case in point is the mountainous areas of Europe which landscape by UNESCO because of the persistence of pas- hitherto have been poorly investigated, in particular the toralism and a strong mountain culture dating back to the Pyrenees. These mountains have always been of great 13th century (http://www.unesco.org, see Madriu-Perafita- interest for naturalists because they are characterized by a Claror valley). It covers an area of 4,247 ha, which repre- relatively high rate of endemism of both animal (Martinez sents nearly 10 % of the territory of the Principality of Rica and Recoder, 1990; for arthropods: Deharveng, 1996; Andorra. The valley is oriented along an east–west axis and Brown et al., 2009) and plant species (Villar and Denda- extends along an altitudinal gradient ranging from 1,055 to letche, 1994; Villar et al., 1997). The Pyrenees present 2,905 m. The Pique valley is a glacial valley, predominantly particular interests for myrmecologists because they are oriented along a north–south axis, extending along an alti- located at the boundary of two climatic zones, the temperate tudinal gradient ranging from 650 to 3,116 m and covering one and the Mediterranean one, in which ants have not yet an area of 8,251 ha. It is part of the Natura 2000 sites (http:// been adequately sampled (Jenkins et al., 2011). Indeed, www.natura2000.fr/-FR7300881). most papers on the Pyrenean ant fauna have been published We sampled ants at 20 sites (9 sites in the Madriu valley before the 80s and consist primarily of lists of species and 11 sites in the Pique valley) in July–August 2005 to 123 Ant community organization Fig. 1 Map showing a the location of the Pyrenees in Europe and b the location of the two valleys in France (Pique valley) and Andorra (Madriu valley) in which ants were sampled 2007 along an altitudinal gradient ranging from 1,300 to immediately and were left in place for 5–8 days (Supple- 2,300 m, and from 1,000 to 2,300 m, for the Madriu and mentary material Appendix A).

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