Species richness, interaction networks, and diversification in bird communities: a synthetic ecological and evolutionary perspective

Abstract

Aquesta tesi examina els mecanismes ecològics i evolutius que mantenen els gradients de riquesa específica en ocells. S'examinen en primer lloc el patrons de diversitat de l'avifauna de Nord Amèrica. (capitol 1). Tot seguit es testen mecanismes a escala regional, tot estudiant el cas de l'avifauna de Catalunya (capitols 2 i 3). Finalment, es testen diferents mecanismes que regulen la diversitat a escala local en una comunitat d'ocells frugívors al Parc nacional de Doñana. El capitol 6 integra tots els anteriors i els relaciona tot fent una revisió bibliogràfica en profunditat.<br/>Paraules clau: biodiversitat, ocells, adaptació, filogènia, switching behaviour, xarxes d'interacció, Nord Amèrica, Catalunya, Doñana, gradients altitudinals, productivitat.

Species richness gradients have been analyzed during many decades and they have progressively emerged as a central topic in community ecology (Darwin 1859, Wallace 1878, Willis 1922, Dobzhansky 1950, Fisher 1960, Hillebrand 2004, Riclkefs 2004, Mittelbach et al. 2007, among others). Historically, species richness gradients have been analyzed from two main points of view: the ecological and the evolutionary perspective (Ricklefs 2004, 2006b). The ecological perspective assumes that populations are evolutionarily fixed and studies species richness gradients as the result of regional colonization and extinction processes, and ecological interactions (MacArthur and Wilson 1967b, Scheiner and Willig 2005). Contrarily, the evolutionary approach states that species richness gradients are the result of geographic differences in the long-term evolutionary processes of speciation and lineage extinction (Rohde 1978, 1992, Mittelbach et al. 2007). For instance, according to the evolutionary view, tropical regions achieve higher species numbers due to increased diversification rates in low latitude areas (i.e. increased speciation and/or reduced extinction rates). Interestingly enough, recent empirical evidence derived from phylogenetic studies suggests that bird diversification rates are effectively higher in the tropics thus providing empirical support for the evolutionary view (Cardillo 1999, Cardillo et al. 2005, Golberg et al. 2005, Ricklefs 2006a, Jablonski et al. 2006, Weir and Schluter 2007). <br/>In contrast with the evolutionary approach, the ecological approach assumes that populations are evolutionary stable units and focus on the effect of regional colonization-extinction dynamics, local conditions and local interactions in determining species richness gradients (Hutchinson 1959, MacArthur and Wilson 1963, 1967ab, MacArthur and Levins 1967, May 1975, Riclkefs 2006b). The ecological approach highlights that species richness gradients can be generated by ecological mechanisms independently of the evolutionary processes of speciation and lineage extinction that conform the regional pool of available species. For instance, an environmental gradient can cause a species richness gradient by limiting the number of successful colonizations in environmentally severe localities without the participation of any speciation process in the regional pool (Wright 1983, Hanski 1997, Boulinier et al. 1998).<br/>These two contrasting and complementary views, the evolutionary and ecological approaches, are progressively being merged in a unified framework (Ricklefs 2006b, Johnson and Stinchcombe 2007). For instance, Johnson and Stinchcombe have recently proposed that two general hypotheses (H1, H2) are underlying a new synthesis between community ecology and evolutionary biology. On one hand (H1), evolutionary processes explain present-day community patterns and the ecological dynamics of species interactions. For instance, extinction and colonization processes that generate species richness gradients might be shaped by species' adaptations, history and phylogenetic relationships (Wiens and Donoghue 2004, Wiens and Graham 2005, Kraft et al. 2007). On the other hand (H2), Johnson and Stinchcombe highlighted that both species interactions and community context strongly influence the direction, rate and outcome of present-day evolutionary processes (Benkman 1999, Thompson 2005). Overall, the emerging synthesis of community ecology and evolutionary ecology highlights that ecological and evolutionary views are profoundly interlinked. <br/>Here we applied an evolutionary and ecological synthetic perspective to the study of the generation and maintenance of species richness in bird communities (Johnson and Stinchcombe 2007). As a first preliminary step, we examined geographical patterns of bird species richness in North America in several functional groups and the associated environmental correlates at the continental scale (Chapter 1). Our results showed that global large-scale patterns of avian diversity in temperate regions were best viewed as the overlayed response of distinct species groups to diverse ecological factors. These results strongly suggested the convenience of choosing specific functional groups in order to examine specific macroecological evolutionary and ecological hypotheses. <br/>Consequently, in Chapter 2, we next chose forest birds as a study group for the analysis of macroecological patterns, because they are the largest group in Nearctic and Palaeoarctic regions among terrestrial birds, and hold strong species-energy relationships (Mönkkönnen et al. 2006). Regional data with colonization and extinction estimates for forest birds were available in Catalonia along an altitudinal gradient (Estrada et al. 2004) providing an exceptional dataset to study the ecological and evolutionary processes behind species richness gradients (Chapters 2 and 3). We examined the altitudinal species richness gradient in Catalonia from an evolutionary and ecological synthetic perspective. First, the role of evolutionary processes in the gradient was assessed studying changes in community phylogenetic structure along the altitudinal gradient. Secondly, the role of ecological processes was examined by studying colonization and extinction dynamics at an ecological time-scale (20 years). These analyses allowed us to examine the role of phylogeny, colonizations, extinctions, community size, productivity, habitat availability, and dispersal limitation in generating bird species richness gradients in Catalonia. However, due to the coarse structure of macroecological data used, the role of local species interactions in such processes remained yet obscure and elusive.<br/>Understanding the role of species interactions in the processes of coexistence and diversification of bird communities thus remained as a big challenge. Since long ago, ecological interactions have been hypothesized to play a role in the maintenance of diversity (Hutchinson 1959, MacArthur and Levins 1967, McPeek 1997, Mittelbach et al. 2007). Therefore, the quantitative study of species interactions was expected to provide insights on the processes that ultimately generate species richness in bird communities (Cattin et al. 2006, Rezende et al. 2007). However, data on species interactions is usually available only for specific local communities. Therefore, a macroecological approach is precluded because the bulk of the studies of community interaction networks are carried out at local scales. Accordingly, we examined the role of species interactions in the generation of species richness in a local Mediterranian bird community for which interaction good-quality data was available (Chapters 4 and 5) (Jordano 1984, 1987). We addressed the two general hypotheses delineated by Johnson and Stinchcombe (2007). First, in chapter 4, we analyzed if evolutionary processes causally affect present-day community interaction patterns and the dynamics of species interactions (H1). Secondly, in chapter five, we assessed if species interactions and community context might influence the direction, rate and outcome of present-day diversification processes that ultimately generate species richness (H2). Overall, our results provide strong support for the two hypotheses examined, thus highlighting the idea that evolutionary and ecological processes are effectively profoundly interlinked. On one hand, we show that long-term evolutionary processes effectively modulate present-day community interaction patterns and dynamics (Webb et al. 2002, Cavender-Bares and Wilczek 2003, Johnson and Stinchcombe 2007, Chapter 4). Likewise, we demonstrate that community context can potentially shape bird morphological diversification processes and drive speciation (Thompson 2005, Abrams 2006, Johnson and Stinchcombe 2007, Chapter 5). Overall, Chapters 4 and 5 provide some new empirical and theoretical insights dealing with the processes that generate and maintain local bird diversity from an integrated evolutionary and ecological perspective.