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Modelling individual life histories and population dynamics of predatory aquatic insects: the role of body size

Final Report Summary - AQUAMOD (Modelling individual life histories and population dynamics of predatory aquatic insects: the role of body size)

Individual body size can explain a broad array of ecological patterns, including relative abundances, spatial distributions, and diversity of species. Body size underlies the patterning of energy fluxes and responses to perturbations in food webs, and plays a crucial role in individual life histories as it affects many ecological processes including competition, predation, intraguild predation, and cannibalism.

This project used a combination of empirical and theoretical approaches to unravel the role of body size in individual life histories, trophic interactions and community structuring in small fishless water bodies. It used predatory aquatic insects as the case study, because they play an important role in biomass transfer and are usually top predators in these habitats. Three main taxonomic and functional groups of predaceous insects in stagnant fishless waters include bugs (Heteroptera: Gerromorpha and Nepomorpha), diving beetles (Coleoptera: Dytiscidae), and larvae of dragonflies (Odonata). Interactions among them and their impact on community structure are insufficiently known. At the same time, these animals are well suited to study the implications of body size for individual life histories and food web interactions because they span a range of body sizes and exhibit varied adaptations to the aquatic environment.

First, the project has demonstrated that body size has important implications for field surveys of aquatic insects and community structuring. In a case study focused on a highly speciose local water beetle community, we showed that survey methods widely used to assess freshwater macroinvertebrate communities differ widely in their ability to record species of various body sizes, and that combinations of two or more methods are thus required to accurately describe size spectra and community structure in species-rich habitats [1]. We also used computer simulations to identify the best sampling protocols, which can be employed in future studies aiming at rapid, cost-effective biodiversity assessment of aquatic insects and other freshwater invertebrates in small standing waters.

Second, the project used laboratory and field experiments to disentangle the role of body size and other individual traits and factors in predator-prey interactions. A study involving multiple predators and their most common types of prey showed that food webs in small fishless water bodies are highly interconnected and that similarly sized predators that live in similar microhabitats share similar diets [8]. This means that these predators should compete intensively for food, both within and between species, which is surprising given the high diversity of predators in many freshwater habitats: other mechanisms such as the presence of temporal and/or spatial niches (i.e. the predators avoid each other in space and time) or high productivity should maintain the commonly high invertebrate diversity in small waters. Data from these experiments also allowed us to show for the first time that current predominately size-based views of trophic interactions and food webs can gain more realism by incorporating other easily measured predator and prey traits, such as microhabitat use, predator morphology and prey defences [9]. This multi-trait approach is very important for the understanding the “who eats whom and how much” questions and can be applied across all taxa and aquatic ecosystems; more holistic approach to studies of food webs is therefore desirable and can be implemented in various application including optimal resource use and conservation [2]. We also investigated how habitat structure modifies the role of body size and other traits in predation strength and community structuring. We found that structurally complex habitats, such as submerged macrophytes or bottom crevices, often modify the strengths of trophic interactions and the results may go beyond the common observation that complex habitat equals prey refuges [5] because some predators can use complex structures to gain better access to prey [10].

Third, we have shown that the method of probabilistic reaction norms, first introduced to study phenotypic plasticity and life history evolution in fishes and to a minor extent applied also to Daphnia, can be with some modifications transferred to aquatic insects and other invertebrates that grow through fairly discontinuous, discrete stages [4]. Detailed data on phenotypic plasticity and individual growth collected during these experiments challenge the theoretical concept of developmental rate isomorphy, an assertion that ectotherms grow faster at higher temperatures but that the relative duration of each instar does not depend on temperature [6].

Last but not least, we could use our empirical results to qualitatively support a suite of models investigating population-dynamical consequences of cannibalism [11], a model of life history implications of the scaling between body size and growth for life history evolution [3], implications of the size-depending scaling of dispersal for community assembly [12]. Data on aquatic insects were also included in a comprehensive overview of currently available empirical and theoretical evidence of individual responses to predation risk [1].

These results are primarily relevant for ecological and evolutionary studies aiming to understand how body size as a characteristic of an individual connects the processes at the individual level to population- and community-level processes and how size-dependent processes shape life histories in aquatic insects. Although most of the results as such are not directly applicable, they can be used to improve our understanding of aquatic ecosystems and help us mitigate the adverse effects of human influence on freshwater habitats.

References:
[1] Beckerman AP, Boukal DS, Childs DZ, Klecka J, Thierry A, Yarlett K (in prep.) Life history and behavioural responses to predation risk: toward a unified framework. Ecology Letters, review proposal accepted in June 2013.
[2] Boukal DS (in review) Trait- and size-based descriptions of trophic links in freshwater food webs: current status and perspectives. Journal of Limnology.
[3] Boukal DS, Dieckmann U, Enberg K, Heino M, Jorgensen C (to be subm.) Life-history implications of the allometric scaling of growth. Journal of Theoretical Biology.
[4] Boukal DS, Ditrich T (in prep.) How to measure moulting and maturation in insects using probabilistic reaction norms: sexual dimorphism in a semiaquatic bug as a case study.
[5] Boukal DS, Peroutka M, Klecka J, Bojkova J (in prep.) Cannibal-victim size ratio and bottom habitat structure drive cannibalism in a benthic dragonfly.
[6] Ditrich T, Sroka P, Papacek M, Salandova P, Boukal DS (to be resubm.) Aquatic insects violate developmental rate isomorphy. European Journal of Entomology.
[7] Klecka J, Boukal DS (2011) Lazy ecologist's guide to water beetle diversity: Which sampling methods are the best? Ecological Indicators 11: 500-508.
[8] Klecka J, Boukal DS (2012) Who eats whom in a pool? A comparative study of prey selectivity by predatory aquatic insects. PLoS ONE 7: e37741.
[9] Klecka J, Boukal DS (2013) Foraging and vulnerability traits modify predator-prey body mass allometry: freshwater macroinvertebrates as a case study. Journal of Animal Ecology 82: 1031-1041.
[10] Klecka J, Boukal DS (in review) Anti-refuge effect of aquatic vegetation for planktonic prey. Oecologia.
[11] Klecka J, Boukal DS (in prep.) Consequences of stage-specific cannibalism for population dynamics.
[12] Klecka J, Boukal DS, Beckerman AP (in prep.) Body mass dependent dispersal and feeding constraints drive food web assembly.

Contact details: David Boukal, Biology Centre AS CR, vvi, Institute of Entomology, Branisovska 31, CZ-37005 Ceske Budejovice, Czech Republic, e-mail boukal@entu.cas.cz http://www.entu.cas.cz/boukal