Integrated assessment of ecosystem connectivity and functioning: coastal forest avifauna of northeast Australia

Abstract

The extraordinary diversity of species-environment relationships that occur across space and time can engender a deep curiosity of their mechanistic underpinnings. Moreover, the rapid rate of ecosystem change associated with anthropogenic and climatic pressures makes information regarding species' landscape and resource use ever more important. Without this information, we will be unable to effectively protect landscapes and their constituent species. The coastal ecosystem mosaic of northeast Australia, which is comprised of a high diversity of habitat types, provides a suitable region for investigating how species respond to heterogeneity in habitat and resource availability. The present thesis examined ecosystem functioning in heterogeneous coastal landscapes of northeast Australia for forest avifauna. An array of analytical approaches were employed to establish a comprehensive understanding: 1) spatial assessment to determine relationships between regional landscape connectivity and coastal forest bird assemblages, 2) isotopic assessment to evaluate the local foraging ecology of mangrove bird assemblages, and 3) nutrient assessment of cross-ecosystem connectivity provided by a migratory coastal forest bird species (i.e. the Pied Imperial-Pigeon (Ducula bicolor)). Within the coastal ecosystem mosaic, mangrove forests sit at the land-sea interface. Therefore, to effectively 'set the scene' I review how mangrove birds require and facilitate connectivity through their use of the broader coastal landscape. Next, to specifically assess regional landscape patterns and processes influencing northeast Australia's coastal forest avifauna, I surveyed the composition of bird assemblages in four of the major coastal forest types occurring throughout the region (i.e. Eucalypt, Melaleuca, mangrove, and rainforest). Following this, spatial patterns of habitat configuration within the coastal landscape (i.e. structural connectivity) were quantified to understand broad relationships between coastal forest bird assemblage composition and landscape heterogeneity at multiple spatial scales. Most bird species in coastal northeast Australia occurred in multiple forest types. Spatial assessment suggested that Melaleuca woodlands are a keystone structure that supports use of the entire coastal landscape mosaic by coastal forest generalist species. However, the species composition of mangrove bird assemblages was distinct relative to other coastal forest types. Therefore, to provide more detailed information regarding the response of coastal forest generalists and mangrove specialists to specific forest attributes, functionally connected forest networks were developed to assess the relative importance of forest area, availability, and connectivity to their compositional turnover. This revealed that mangrove specialists and coastal generalists differ in the forest attributes they require (i.e. area vs. availability) to maintain regional beta diversity. Understanding landscape pattern-process relationships that drive bird assemblage composition and turnover can inform the prioritization of regional-scale landscape features for protection. However, species' responses to local-scale spatiotemporal variability in resource availability may also play a role in these relationships. I used isotopic analysis to better understand the foraging ecology of coastal forest birds in a highly dynamic mangrove forest environment. This demonstrated that flexible and opportunistic foraging strategies were prevalent among coastal forest generalist species. However, specialized foraging strategies were employed by some species, primarily for resources that were uniquely available in mangrove forests (i.e. estuarine fish and crabs). Mobile species not only respond to landscape patterns and processes, but can also facilitate connectivity processes through their movement (e.g. nutrient transfer, pollination, genetic linking, etc.). To determine the implications of avian mobility for ecosystem functioning in northeast Australia, I focused on a migratory coastal forest bird species, the Pied Imperial-Pigeon (Ducula bicolor). Nutrient measurements demonstrated that Pied Imperial-Pigeons provide mainland-derived nutrient subsidies to island forests, highlighting their important role as an avian mobile-link species. The integrated analytical approach used in this thesis has provided insight to the complexity of coastal landscapes and their use by forest avifauna. This has broadened our understanding of coastal ecosystem functioning to include a hierarchy of ecosystem components that exist at local and regional scales. The ecosystem properties that emerge from interactions across coastal ecosystem components include: vegetative connectivity, compositional turnover, avian foraging strategy, and nutrient transfer. Results from this thesis can inform the holistic conservation and management strategies that are required to maintain coastal ecosystem functioning in regional northeast Australia.