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Cindy Gao, Emma Helman, Lars Iversen
Integrating complex life cycles into freshwater trait-environment modeling
Numerous freshwater organisms undergo complex life cycles, which involve shifts between discrete aquatic and terrestrial life stages that are often associated with different forms and functions. These environmental transitions are thought to cause selection in one life stage to cascade to non-adaptive stages, generating tradeoffs between life stages and/or conferring a negative impact on overall fitness. Although complex life cycles are ubiquitous in freshwater ecosystems, the trait-trait relationships they induce across life stages are rarely integrated into spatial studies of freshwater biodiversity. Here, we propose three approaches of varying statistical complexity and computational requirements to account for reciprocal trait relationships associated with complex life cycles. We demonstrate these approaches using a dataset of global dragonfly body sizes and climate niches. We show that nymph and adult body size are closely correlated but become increasingly decoupled across certain environmental thresholds. These findings outline the environmental conditions where trait-environment correlations are biased towards a certain life stage and when the causal pathway between traits and the environment at a single life stage collapses. These results provide a framework for building more mechanistic trait-environment correlation models and highlight the importance of accounting for life history components in models of freshwater trait-environment relationships.
