Responses of odonate assemblages to climate: importance of body size, dispersal and phylogeny
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Philipps-Universität Marburg
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The global decline in insect populations and biodiversity due to temperature changes highlights the need for a deeper understanding of functional traits and evolutionary adaptations of ectothermic species, which is crucial for accurate predictions of species’ responses to climate change and assessing their survival strategies under changing environmental conditions.
The objective of my PhD thesis is to understand the role of functional traits in the contemporary distribution and diversification of odonates, including dragonflies (Anisoptera) and damselflies (Zygoptera), offering a global perspective on the relationships between trait variation, dispersal abilities and ecological adaptations. I used odonates as model organisms due to their relatively small number of species and their popularity. Odonates are among the best-documented insect orders in terms of distribution and taxonomy, with only about 14% of species yet to be described. As ectothermic organisms, odonates depend on ambient conditions and compared to endothermic taxa, these physiological differences can lead to distinct biogeographical patterns and differing responses to changing environmental conditions. Research on global biodiversity patterns has primarily focused on mostly endothermic species, but the mechanisms and causes underpinning the diversity of ectotherms, particularly in insects, remain poorly understood.
The first chapter investigates the ecological and evolutionary functions of body size in insects. This chapter goes beyond examining latitudinal gradients, such as those proposed by Bergmann’s rule, by testing how temperature, productivity and thermal niche conservatism influence body size variation of dragon- and damselflies on a global scale. The results show strong ecogeographical gradients in body size, reflecting the interplay of thermoregulation and resource -driven growth constraints. Data compiled for 43% of all described odonate species demonstrates that body size increases with latitude and temperature, with dragonflies exhibiting a stronger temperature-size relationship than damselflies. The results further highlight a strong phylogenetic signal in the thermoregulatory function of body size, which appears to have facilitated the diversification of Anisoptera, particularly enabling their dominance in higher latitude and cooler environments. This study reconciles previous smaller-scaled research on the thermoregulatory importance of body size and highlights an overlooked environmental driver significant in tropical regions.
The second chapter assesses environmental drivers of species richness and the proportion of lentic species, which are assumed to be better dispersers compared to lotic species, extending the global trait and distribution dataset from the first chapter with information on species’ habitat affiliation. Testing the metabolic theory of ecology, which posits temperature as the main driver of species richness (more precisely the expected slope = –0.65), the study finds that the proportion of lentic to lotic species increases with decreasing temperature and productivity. This provides the first global support for this pattern among freshwater organisms, highlighting the importance of habitat stability and dispersal abilities in shaping species distribution patterns. Additionally, lentic species demonstrate greater dispersal abilities and deviate from MTE predictions, more than lotic species. These findings suggest that integrating biogeographical factors, particularly seasonality as reflection of climatic stability into the predictions of the MTE, could enhance predictions of species richness patterns. The likely divergent effects of future temperature warming on lentic and lotic communities underlines the need for further research into these habitat specific responses, emphasizing the need for conservation strategies tailored to the different vulnerabilities of lentic and lotic species. Evolutionary conservatism significantly affects current distribution and species richness patterns, emphasizing the need to consider historical factors in biodiversity conservation.
The third chapter investigates how intrinsic drivers (such as body size, dispersal ability and habitat preference) and extrinsic environmental drivers (including temperature, elevation, productivity, and latitude) interact to influence vulnerability in odonates. Since the IUCN traditionally assesses species extinction risks based on range size and comprehensive range size data is often lacking, this chapter of 2,220 dragon- and damselflies examines through phylogenetic generalized linear regressions and structural equation models how the interplay of intrinsic and extrinsic drivers shape range size to influence extinction risk. Key findings show that environmental drivers, especially elevation and latitude are the main predictors of range size. Intrinsic dirvers, while important, have rather indirect effects that get modulated by their respective environment. In particular, the findings reveal that species adapated to lentic habitats and larger-bodied species in colder environments are less threatened, due to their thermoregulatory advantage compared to smaller species. Conversely, larger-bodied species often face higher extinction risks in warmer, more productive environments due to their greater energetic demands. Dispersal ability generally correlates positively with broader range sizes and lower extinction risks. Analyses confirm this, as species adapted to lotic habitats have a limited dispersal ability and thus increased risk of extinction. This suggests that habitat stability has a significant influence in determining species’ vulnerability. By considering the interplay of both types of drivers, this provides a more accurate understanding of species extinction risk, particularly for species experiencing varied ecological pressures compared to analyses that do not account for environmental conditions.
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This item has been published with the following license: In Copyright