Opposing Responses of Bird Functional Diversity to Vegetation Structural Diversity in Wet and Dry Forest

Abstract

Disturbance regimes are changing worldwide, and the consequences for ecosystem function and resilience are largely unknown. Functional diversity (FD) provides a surrogate measure of ecosystem function by capturing the range, abundance and distribution of trait values in a community. Enhanced understanding of the responses of FD to measures of vegetation structure at landscape scales is needed to guide conservation management. To address this knowledge gap, we used a whole-of-landscape sampling approach to examine relationships between bird FD, vegetation diversity and time since fire. We surveyed birds and measured vegetation at 36 landscape sampling units in dry and wet forest in southeast Australia during 2010 and 2011. Four uncorrelated indices of bird FD (richness, evenness, divergence and dispersion) were derived from six bird traits, and we investigated responses of these indices and species richness to both vertical and horizontal vegetation diversity using linear mixed models. We also considered the extent to which the mean and diversity of time since fire were related to vegetation diversity. Results showed opposing responses of FD to vegetation diversity in dry and wet forest. In dry forest, where fire is frequent, species richness and two FD indices (richness and dispersion) were positively related to vertical vegetation diversity, consistent with theory relating to environmental variation and coexistence. However, in wet forest subject to infrequent fire, the same three response variables were negatively associated with vertical diversity. We suggest that competitive dominance by species results in lower FD as vegetation diversity increases in wet forest. The responses of functional evenness were opposite to those of species richness, functional richness and dispersion in both forest types, highlighting the value of examining multiple FD metrics at management-relevant scales. The mean and diversity of time since fire were uncorrelated with vegetation diversity in wet forest, but positively correlated with vegetation diversity in dry forest. We therefore suggest that protection of older vegetation is important, but controlled application of low-severity fire in dry forest may sustain ecosystem function by enhancing different elements of FD.

Improvement of vegetation structure enhances bird functional traits and habitat resilience in an area of ongoing restoration in the Atlantic Forest

“…However, distinct responses have been found in tropical zones. In Australia, FD indices were found to be positively related to vertical vegetation diversity in humid forests, yet inversely related to vertical vegetation in dry forests (Sitters et al 2016). A similar pattern was also found in savannahs in Namibia (Seymour et al 2015).…”

“…Increases in bird functional richness (FRic), functional dispersion (FDis), functional evenness (FEve) and Rao's quadratic entropy (Rao'Q) were found to be related to the vegetation structure gradient, with no effects on bird species richness (SR) and functional divergence (FDiv). Previous studies have also reported positive correlations between functional diversity (FD) measures and vegetation structure for butterflies (Aguirre-Gutiérrez et al 2017), lizards (Berriozabal-Islas et al 2017), birds (Hidasi-Neto et al 2012, Sitters et al 2016) and mammals (Sukma et al 2019); yet there are few data on the responses of FD indexes for birds in Neotropical restored habitats (but see Batisteli et al 2018). The present results add evidence that active restored habitats in the Atlantic Forest need more attention with planning to shade exotic grasses (Parrotta et al 1997, Melo et al 2007, since this may increase vegetation structural complexity and thus favor birds (Munro et al 2011, Becker et al 2013, Batisteli et al 2018.…”

“…Functional traits are individual morphological, physiological and/or phenological attributes (Laliberté & Legendre 2010) that have been used to calculate functional diversity (FD) (Laliberté et al 2014) and to understand how birds interact in an ecosystem (Violle et al 2007). Positive responses of bird FD to vegetation diversity can be modulated by multiple associations between animal traits and vegetation structure (Sitters et al 2016). Likewise, FD metrics are sensitive to vegetation structure of restored sites (Batisteli et al 2018) and have been considered better metrics for evaluating restoration success than taxonomic composition (Brancalion & Holl 2016).…”

Modelling the responses of Australian subtropical rainforest birds to changes in environmental conditions along elevational gradients

“…Further validation of these relationships requires detailed field observations of many species over multiple years (Sitters et al . ), and for similar studies to be conducted along other mountain ranges. Nevertheless, our analyses add to a growing body of literature stressing the importance of temperature in determining the diversity, abundance and distribution of birds along elevational gradients.…”

“…Sitters et al . () found that frugivores and omnivores in southern Australia responded positively to increasing canopy cover and suggested that this interaction was due to food availability. A similar explanation may be drawn from our results, in which large‐bodied species (mainly frugivores and omnivores) responded more strongly to increases in canopy cover than did small‐bodied species (mainly insectivores).…”