Ecological Assemblages in a Warming Climate: Addressing Knowledge Gaps in the Role of Thermal Heterogeneity and Realised Niches at a Global Scale

Abstract

Ecological responses to Anthropogenic climate warming are occurring across the globe. The aim of this thesis is to critically examine knowledge gaps in: i) how assemblages respond to multidimensional temperature change; ii) patterns of realised niches across species’ assemblages. I investigate these gaps to build the requisite knowledge to describe and predict assemblage scale
responses to climatic warming.
Chapter 2 develops a conceptual framework relating the processes of individual movements and population dynamics to the spatial and temporal dimensions of temperature change. I find that most studies do not consider the dimensionality of temperature change when quantifying assemblage dynamics.
Chapter 3 finds that, on average, the abundance of reef fish species across their thermal ranges supports the ‘abundant-niche centre’ hypothesis. I also find a systematic pattern in the skew of realised thermal niches amongst species, which relates to latitude and biogeographic habitat variation.
Chapter 4 quantifies spatial patterns in the diversity of species’ responses to heatwave events in five functional groups of reef fishes on the Great Barrier Reef and western Coral Sea. Browsing herbivores, scraping herbivores and corallivores have spatially homogenous patterns of response diversity indicating resilience at a regional scale. Further, all functional groups positively respond to temperature warming but only corallivores and excavators negatively respond to coral loss, and no functional groups strongly respond to algae loss.
Chapter 5 critically examines the ‘wide-ranged winners’ paradigm for terrestrial ectotherms (insects) across land-use types that represent a microclimatic gradient. I find that species’ niche metrics consistently outperform geographic range size in predicting species occurrence. Species with warmer and drier affinities increased in occupancy in agricultural land uses which matched the
expected warmer and dryer conditions.
Overall, realised niches are structured by abundance, exhibit diversity within local assemblages and regional species pools, and can predict occupancy at local scales even in thermally heterogeneous terrestrial systems. This thesis therefore further establishes the thermal niche as a core concept to quantify the dynamics of assemblages in a warming world. Chapter 6 critically evaluates the benefits and limitations of this niche perspective for quantifying biodiversity change and suggests future research avenues.

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