Species richness and morphological diversity of passerine birds

Abstract

The relationship between species richness and the occupation of niche space can provide insight into the processes that shape patterns of biodiversity. For example, if species interactions constrained coexistence, one might expect tendencies toward even spacing within niche space and positive relationships between diversity and total niche volume. I use morphological diversity of passerine birds as a proxy for diet, foraging maneuvers, and foraging substrates and examine the morphological space occupied by regional and local passerine avifaunas. Although independently diversified regional faunas exhibit convergent morphology, species are clustered rather than evenly distributed, the volume of the morphological space is weakly related to number of species per taxonomic family, and morphological volume is unrelated to number of species within both regional avifaunas and local assemblages. These results seemingly contradict patterns expected when species interactions constrain regional or local diversity, and they suggest a larger role for diversification, extinction, and dispersal limitation in shaping species richness.community | niche packing | principal components analysis A lthough species richness varies predictably with latitude and other environmental gradients (1, 2), the mechanisms responsible for geographic patterns in biodiversity remain poorly understood (3, 4). Classic niche-based hypotheses argue that local species richness is related to the variety of available resources, including escape space from enemies (5, 6), which are partitioned among species to reduce interspecific competition and thereby allow coexistence (7-10). Modifications of nichebased theory incorporating, for example, spatial and temporal variation in the environment (11), relax this prediction but have not been assessed by empirical studies. More recent consumerresource models have addressed conditions for coexistence of similar species (e.g., refs. 12 and 13). These involve small numbers of species in simplified environments, although they do contain many basic elements of more complex natural systems. Alternatively, diversification-based hypotheses posit that elevated rates of species production compared with extinction within a region push up steady-state levels of diversity and pack species more tightly into available ecological space, tending to decouple the relationship between species richness and niche space (14-16). Ecologists have not evaluated these hypotheses adequately, in part because of the difficulty of measuring niche space and its partitioning among species (e.g., refs. 17 and 18), although the idea that diversity generally exists in a long-term, resource-influenced equilibrium has increasing support (19,20).Recent analyses of the phylogenetic structure of communities (21, 22) and the distribution of functional traits in communities (23-25) have attempted to address the roles of ecological specialization and species interactions in the assembly of local communities from regional species pools. These studies ask wh...

Functional traits reveal the expansion and packing of ecological niche space underlying an elevational diversity gradient in passerine birds

“…The increase in morphodensity, as with volume, was no greater than expected under the null model, perhaps implying that trait packing and expansion contribute equally to the gradient in richness. However, across all elevations, most species were concentrated within a central core region of trait space, with the density of species declining rapidly towards the periphery [37]. Thus, increases in trait volume were driven by a relatively small number of species with extreme phenotypes, while most (78%) of the additional diversity in the lowlands was accommodated within a volume of functional trait space equivalent to that occupied in the highlands.…”

“…The use of normalized PC trait axes when quantifying assemblage morphospace assumes that each trait axis is of similar importance in charactering avian niche space [36,37]. To test this assumption, we estimated the contribution of each PC trait in predicting guilds by removing each axis as a term from the model and calculating the drop in classification accuracy.…”

“…Finally, to examine how species are distributed throughout functional trait space, we quantified the density of species within concentric spherical shells spaced at intervals of 1 s.d. from the centroid or morphospace [37].…”

Testing otolith morphology for measuring marine fish biodiversity

“…However, many studies have shown different levels of species packing or grouping, including the presence of empty morphospace zones (Gatz 1979;Goatley et al 2010;Ricklefs 2012). The degree of species packing seems to coincide with the morphologies that are best adapted for the most efficient exploitation of resources in the ecosystem (Schoener 1974;Gatz 1979;Wainwright and Richard 1995;Ricklefs 2012).…”

“…According to the concept of limiting similarity, species have a minimum distance in a one-dimensional niche space (MacArthur and Levins 1967), which should lead to regular spacing of species within morphospace (Ricklefs 2012). However, many studies have shown different levels of species packing or grouping, including the presence of empty morphospace zones (Gatz 1979;Goatley et al 2010;Ricklefs 2012).…”