Avian functional responses to landscape recovery

Abstract

Restoring native vegetation in agricultural landscapes can reverse biodiversity declines via species gains. Depending on whether the traits of colonizers are complementary or redundant to the assemblage, species gains can increase the efficiency or stability of ecological functions, yet detecting these processes is not straightforward. We propose a new conceptual model to identify potential changes to complementarity and redundancy in response to landscape change via relative changes in taxonomic and functional richness. We applied our model to a 14-year study of birds across an extensive agricultural region. We found compelling evidence that high levels of landscape-scale tree cover and patch-scale restoration were significant determinants of functional change in the overall bird assemblage. This was true for every one of the six traits investigated individually, indicating increased trait-specific functional complementarity and redundancy in the assemblage. Applying our conceptual model to species diversity data provided new insights into how the return of vertebrates to restored landscapes may affect ecological function.

Keywords: agricultural restoration; functional complementarity; functional redundancy; functional richness; species diversity; woodland birds.

Figure 1.

Predicted responses of taxonomic and functional richness to landscape change, and the consequences for ecological function. The relative change in the easily calculated diversity metrics, taxonomic and functional richness, underpins our new conceptual model. Where both taxonomic richness and functional richness increase with a landscape attribute (e.g. vegetation cover), species gained contribute complementary functions to the assemblage [–31]. Functional complementarity is related to the efficiency in the performance of ecological function [32]. Where only taxonomic richness increases, species gained contribute redundant functions [27,28]. Functional redundancy is related to stability in the performance of ecological function [32]. Where only functional richness increases, functional turnover (from species with redundant traits to species with complementary traits) is occurring. Where both are unchanged, the assemblage is stable. It is also possible to have a loss of functional richness with increasing taxonomic richness (i.e. turnover from complementary to redundant traits) [33]. (Online version in colour.)

Figure 2.

Relationship between taxonomic richness and functional richness. Solid line shows model prediction, dashed lines show 95% prediction intervals. Points show raw data. (Online version in colour.)

Figure 3.

Relationships between taxonomic and functional richness and two aspects of landscape recovery: patch type (old growth, regrowth and planting) and landscape-scale vegetation cover. Solid line shows model prediction, dashed lines show 95% prediction intervals. Arrows indicate whether taxonomic and functional richness was significantly related (p < 0.05; ‘sig’) or not (ns) to the explanatory variables. (Online version in colour.)

Figure 4.

Standardized explanatory variable coefficients for individual trait functional richness (shaded) and guild richness (not shaded). Error bars are 95% confidence intervals, and black points indicate that these confidence intervals did not cross zero, i.e. that functional or guild richness was significantly related (p < 0.05) to the explanatory variable. Functional richness of the feeding and nesting aggregation traits was not modelled because nearly all sites had both guilds present. (Online version in colour.)