Functional diversity of avian communities increases with canopy height: From individual behavior to continental‐scale patterns

Abstract

Vegetation complexity is an important predictor of animal species diversity. Specifically, taller vegetation should provide more potential ecological niches and thus harbor communities with higher species richness and functional diversity (FD). Resource use behavior is an especially important functional trait because it links species to their resource base with direct relevance to niche partitioning. However, it is unclear how exactly the diversity of resource use behavior changes with vegetation complexity. To address this question, we studied avian FD in relation to vegetation complexity along a continental‐scale vegetation gradient. We quantified foraging behavior of passerine birds in terms of foraging method and substrate use at 21 sites (63 transects) spanning 3,000 km of woodlands and forests in Australia. We also quantified vegetation structure on 630 sampling points at the same sites. Additionally, we measured morphological traits for all 111 observed species in museum collections. We calculated individual‐based, abundance‐weighted FD in morphology and foraging behavior and related it to species richness and vegetation complexity (indexed by canopy height) using structural equation modeling, rarefaction analyses, and distance‐based metrics. FD of morphology and foraging methods was best predicted by species richness. However, FD of substrate use was best predicted by canopy height (ranging 10–30 m), but only when substrates were categorized with fine resolution (17 categories), not when categorized coarsely (8 categories). These results suggest that, first, FD might increase with vegetation complexity independently of species richness, but whether it does so depends on the studied functional trait. Second, patterns found might be shaped by how finely we categorize functional traits. More complex vegetation provided larger "ecological space" with more resources, allowing the coexistence of more species with disproportionately more diverse foraging substrate use. We suggest that the latter pattern was driven by nonrandom accumulation of functionally distinct species with increasing canopy height.

Vegetation complexity and pool size predict species richness of forest birds

“…High local species richness in tall forests might be a consequence of (i) a higher number of individuals (Storch et al, 2018) present due to higher productivity and more abundant resources (Sam et al, 2019), (ii) more available niches due to structurally complex vegetation (MacArthur and MacArthur, 1961;Remeš et al, 2021b), or (iii) a combination of these two mechanisms (Hurlbert, 2004;Mönkkönen et al, 2006). Indeed, the total number of individuals recorded during standardized censuses increased with canopy height.…”

“…We calculated several measures of vegetation complexity based on our field measurements and confirmed by a principal component analysis (PCA) analysis that canopy height (varying from 9.4 to 31.7 m) was a good proxy for how complex the vegetation was in terms of the number of vegetation layers and the amount of foliage available for birds to forage on (see Supplementary Table 2 and Supplementary Figures 2, 3). We thus used it as a predictor in all our analyses, in accordance with previous studies (e.g., Gouveia et al, 2014;Coops et al, 2018;Remeš and Harmáčková, 2018;Feng et al, 2020;Remešová et al, 2020;Remeš et al, 2021b).…”

Resource use divergence facilitates the evolution of secondary syntopy in a continental radiation of songbirds (Meliphagoidea): insights from unbiased co‐occurrence analyses

“…These observations suggest that both niche conservatism (Laiolo et al 2017) and the partitioning among species of available resources play a critical role in the build‐up of local diversity. Conditioning species co‐occurrence on common dietary preferences makes sense in Meliphagoidea, because this clade includes species with diverse beak morphologies (Friedman et al 2019) linked to feeding preferences ranging from complete insectivory to almost complete nectarivory (Miller et al 2017, Remešová et al 2020, Remeš et al 2021a, b). Then, horizontal patchiness of resources might lead to locally clustered occurrence of species with similar dietary preferences, unlike if all species had similar food preferences as in dietarily more conservative clades.…”

“…The traits and their categories were as follows: habitat (ten categories: rainforest, forest, woodland, shrub, grassland, heaths, marshes, marine mangrove, bare ground and human settlements), diet (eight categories: leaves, fruit, nectar and pollen, seeds, insects, other invertebrates, vertebrates and carrion), foraging method (nine categories: gleaning, hang‐gleaning, snatching, hover‐snatching, probing, manipulating, pouncing, flycatching and flush chasing; see Remešová et al 2020 for definitions), foraging stratum (four categories: ground, shrub, subcanopy and canopy) and foraging substrate (eight categories: ground, bark, leaves, buds, fruit, flowers, air and other). Previous work showed that resource partitioning in terms of foraging substrates and methods is important for species co‐existence on a local scale in Australian passerines (Harmáčková et al 2019, Remešová et al 2020, Remeš et al 2021a, b), and in passerines in general (MacArthur 1958, Holmes and Recher 1986, Terborgh and Robinson 1986, Korňan et al 2013). We calculated distance matrices using the Bray–Curtis metric to express species differences along several niche dimensions: habitats, diet, foraging method, stratum, substrate and overall resource use (i.e.…”

“…3). Avian species can partition resources, for example, in terms of foraging behavior, substrates or vegetation layers (Holmes and Recher 1986, Terborgh and Robinson 1986, Korňan et al 2013, Remeš et al 2021a, b). The hypothesis of ecological isolation thus assumes niche divergence during speciation and post‐speciation processes (Germain et al 2021).…”