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LETTER LETTER Reply to Hortal et al.: Patterns of bird distribution in Spain support the area–heterogeneity tradeoff Niche theory predicts that species diversity richness at high levels of heterogeneity developed model, the AHTO requires em- should increase with increased environ- (figure 3 in ref. 1). Certainly this result pirical tests. We hope that this correspon- mental heterogeneity. In contrast to this was not limited to “species with very dence will facilitate such tests. prediction, breeding bird diversity in Cat- narrow niches.” alonia shows an overall unimodal hetero- An alternative analysis of bird diversity Omri Allouchea,1, Michael Kalyuzhnya, geneity–diversity relationship (HDR). We in Catalonia performed by Hortal et al. (2) Gregorio Moreno-Ruedab, Manuel attributed this result to a fundamental shows that species richness increases with Pizarrob, and Ronen Kadmona geometric constraint termed the area– habitat diversity (Fig. 1 in ref. 2), but their aDepartment of Ecology, Evolution and heterogeneity tradeoff (AHTO) (1). analysis ignores the potential confounding Behavior, Institute of Life Sciences, Hebrew Hortal et al. (2) reject our conclusion effects of climatic variables, broad-scale University of Jerusalem, Jerusalem 91904, claiming that (i) birds select habitats based trends in species richness and spatial auto- Israel; and bDepartamento de Zoología, on vegetation characteristics, and, therefore, correlation. All of these factors are known Facultad de Ciencias, Universidad de our use of elevation range as a measure of to be important in determining patterns of Granada, 18071 Granada, Spain environmental heterogeneity was unjustified; species diversity in Catalonia (1), suggest- (ii) the observed unimodal HDR was a statis- ing that the interesting pattern reported “ fl tical artifact that merely re ects the well- deserves a fuller analysis. 1 Allouche O, Kalyuzhny M, Moreno-Rueda G, Pizarro M, known hump-shaped relationship between Hortal et al. (2) also claim that the empir- Kadmon R (2012) Area–heterogeneity tradeoff and the diversity of species richness and altitude”;and(iii) ical unimodal patterns found in our metaa- ecological communities. Proc Natl Acad Sci USA 109(43): 17495–17500. species richness decreases with environ- nalysis of published datasets were artifacts of 2 Hortal J, et al. (2013) Species richness can decrease with altitude mental heterogeneity only for species with our linear correction of the correlation be- but not with habitat diversity. Proc Nat Acad Sci USA, 10.1073/ pnas.1301663110. very narrow niches, and such species are tween area and heterogeneity, but the authors 3 Lundholm JT (2009) Plant species diversity and environmental rare in natural environments. do not verify their claim. When the data are heterogeneity: Spatial scale and competing hypotheses. J Veg Sci None of these claims is consistent with corrected using a leveling-off function, as 20(3):377–391. = × 4 Tamme R, Hiiesalu I, Laanisto L, Szava-Kovats R, Partel M the empirical data. First, birds do respond Hortal et al. suggest [heterogeneity a (2010) Environmental heterogeneity, species diversity to elevation gradients at the scales ana- log(area) + b], only 6 of the 43 datasets and co-existence at different spatial scales. JVegSci21(4): lyzed in our study (Fig. 1), indicating that show a significant positive pattern, and 13 796–801. fi 5 Tews J, et al. (2004) Animal species diversity driven by habitat elevation range is an ecologically mean- datasets are signi cantly unimodal. heterogeneity/diversity: the importance of keystone structures. ingful measure of heterogeneity in this When new data contradict a well-estab- J Biogeogr 31(1):79–92. “ ” 6 Muntaner J, Ferrer X, Martínez-Vilalta A (1984) Atles Dels Ocells system. Second, a well-known unimodal lished theory, scientists often tend to reject Nidificants de Catalunya i Andorra [Breeding Birds Atlas of Catalonia response to altitude does not explain the the data, rather then modify the theory. and Andorra] (Ketres, Barcelona). observed shift from negative to positive Nonpositive HDRs contradict the predic- HDR with increasing niche width, the tion of niche theory but have previously observed negative effect of heterogeneity been documented in both plant (3, 4) on species abundance, or the positive ef- and animal (5) communities. To date, the Author contributions: O.A., M.K., G.M.-R., M.P., and R.K. designed fi research; O.A., M.K., G.M.-R., M.P., and R.K. performed research; fect of heterogeneity on extinction proba- AHTO, a simple modi cation of niche the- O.A., M.K., G.M.-R., M.P., and R.K. analyzed data; and O.A. and bility (1). All of these patterns were predicted ory, is the only model capable of explaining R.K. wrote the paper. by the AHTO. Third, 75% of the species both positive and negative HDRs, as well as The authors declare no conflict of interest. in Catalonia (three out of four quartiles) the higher prevalence of negative HDRs at 1To whom correspondence should be addressed. E-mail: omri. showed a significant reduction of species small spatial scales (4). As with any recently [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1305057110 PNAS Early Edition | 1of2 Downloaded by guest on September 25, 2021 A BC D E F Relative frequency Relative GHI Altitude (m) Fig. 1. Response of bird species to altitude gradients in Catalonia. Relative frequency indicates the number of 10 × 10 km grid cells in which the relevant species was documented divided by the total number of cells in the relevant altitude category. Note that species differ from each other in both the position and the width of their niches along the altitude gradient. (A) Acrocephalus arundinaceus.(B) Sylvia melanocephala.(C) Miliaria calandra.(D) Circus cyaneus.(E) Bubo bubo.(F) Aegithalos caudatus. (G) Montifringilla nivalis.(H) Pyrrhocorax graculus.(I) Phoenicurus ochruros. Source: ref. 6. 2of2 | www.pnas.org/cgi/doi/10.1073/pnas.1305057110 Allouche et al. Downloaded by guest on September 25, 2021.

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