The Evolution of Local Co-occurrence in Birds in Relation to Latitude, Degree of Sympatry, and Range Symmetry.

AbstractRecent speciation rates and the degree of range-wide sympatry are usually higher farther from the equator. Is there also a higher degree of secondary syntopy (coexistence in local assemblages in sympatry) at higher latitudes and, subsequently, an increase in local species richness? We studied the evolution of syntopy in passerine birds using worldwide species distribution data. We chose recently diverged species pairs from subclades not older than 5 or 7 million years, range-wide degree of sympatry not lower than 5% or 25%, and three definitions of the breeding season. We related their syntopy to latitude, the degree of sympatry (breeding range overlap), range symmetry, and the age of split. Syntopy was positively related to latitude, but it did not differ between tropical and temperate regions, instead increasing from the Southern to the Northern Hemisphere. Syntopy was also higher in species pairs with a higher degree of sympatry and more symmetric ranges, but it did not predict local species richness. Following speciation, species in the Northern Hemisphere presumably achieve positive local co-occurrence faster than elsewhere, which could facilitate their higher speciation rates. However, this does not seem to be linked to local species richness, which is probably governed by other processes.

Specialization and niche overlap across spatial scales: Revealing ecological factors shaping species richness and coexistence in Australian songbirds.

Ecological specialization enables the partitioning of resources and thus can facilitate the coexistence of species and promote higher species richness. Specialization and niche partitioning are expected to exert a decisive influence on local spatial scales, while species richness at regional scales should be shaped mostly by historical factors and abiotic conditions. Moreover, specialization is expected to be particularly important in communities that are exceptionally species rich for their environmental conditions. Concurrently, niche overlap in these communities should be minimized to enable species coexistence. We tested these hypotheses by studying specialization-richness relationship and niche overlap in assemblages of 298 species of songbirds (Passeriformes) across Australia. We used local (2-6 ha) to regional (bioregions) spatial scales and detailed data on habitat, diet and foraging behaviour (method, substrate and stratum). We expected the richness-specialization relationship to be particularly strong (a) on local spatial scales and (b) in communities exceptionally species rich for given environmental conditions (approximated by moisture and vegetation complexity). We also expected (c) low niche overlap in assemblages with specialized species. Only the third prediction was partly supported. First, while the specialization and species richness were often positively related, the strength and the direction of the relationship changed between traits and across spatial scales. The strength of the specialization-richness relationship was consistently positive only in foraging stratum, and it increased towards smaller spatial scales only in case of habitat and diet. Simultaneously, species in local communities demonstrated high overlap in habitat and diet. Second, we did not find particularly strong specialization-richness relationships in exceptionally species-rich communities. Third, we found the expected negative relationship between specialization and overlap in foraging stratum and substrate (in local communities), suggesting that species partition ecological space locally in terms of where they find food. Our expectations were only weakly supported. Specialization on foraging stratum was probably important in facilitating species coexistence. Conversely, although species were often specialized on habitat and diet, high overlap in these traits did not preclude their local coexistence. Overall, specialization and overlap in foraging traits were more important for species coexistence than habitat or diet.

Disentangling direct and indirect effects of water availability, vegetation, and topography on avian diversity.

Climate is a major driver of species diversity. However, its effect can be either direct due to species physiological tolerances or indirect, whereby wetter climates facilitate more complex vegetation and consequently higher diversity due to greater resource availability. Yet, studies quantifying both direct and indirect effects of climate on multiple dimensions of diversity are rare. We used extensive data on species distributions, morphological and ecological traits, and vegetation across Australia to quantify both direct (water availability) and indirect (habitat diversity and canopy height) effects of climate on the species richness (SR), phylogenetic diversity (PD), and functional diversity (FD) of 536 species of birds. Path analyses revealed that SR increased with wetter climates through both direct and indirect effects, lending support for the influence of both physiological tolerance and vegetation complexity. However, residual PD and residual FD (adjusted for SR by null models) were poorly predicted by environmental conditions. Thus, the FD and PD of Australian birds mostly evolved in concert with SR, with the possible exception of the higher-than-expected accumulation of avian lineages in wetter and more productive areas in northern and eastern Australia (with high residual PD), permitted probably by older biome age.

Smaller beaks for colder winters: Thermoregulation drives beak size evolution in Australasian songbirds.

Birds' beaks play a key role in foraging, and most research on their size and shape has focused on this function. Recent findings suggest that beaks may also be important for thermoregulation, and this may drive morphological evolution as predicted by Allen's rule. However, the role of thermoregulation in the evolution of beak size across species remains largely unexplored. In particular, it remains unclear whether the need for retaining heat in the winter or dissipating heat in the summer plays the greater role in selection for beak size. Comparative studies are needed to evaluate the relative importance of these functions in beak size evolution. We addressed this question in a clade of birds exhibiting wide variation in their climatic niche: the Australasian honeyeaters and allies (Meliphagoidea). Across 158 species, we compared species' climatic conditions extracted from their ranges to beak size measurements in a combined spatial-phylogenetic framework. We found that winter minimum temperature was positively correlated with beak size, while summer maximum temperature was not. This suggests that while diet and foraging behavior may drive evolutionary changes in beak shape, changes in beak size can also be explained by the beak's role in thermoregulation, and winter heat retention in particular.