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Larval Development, Metabolism and Diet Are Possible Key Factors Explaining the Decline of the Threatened Dytiscus Latissimus

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Larval Development, Metabolism and Diet Are Possible Key Factors Explaining the Decline of the Threatened Dytiscus Latissimus Insect Conservation and Diversity (2018) doi: 10.1111/icad.12294 Larval development, metabolism and diet are possible key factors explaining the decline of the threatened Dytiscus latissimus ILSE SCHOLTEN,1 HEIN H. VAN KLEEF,1,2 GIJS VAN DIJK,3,4 JULIAN 1 2,4 BROUWER and WILCO C.E.P. VERBERK 1Bargerveen Foundation, Nijmegen, The Netherlands, 2Department of Animal Ecology and Ecophysiology, Radboud University, Nijmegen, The Netherlands, 3B-WARE Research Centre, Radboud University, Nijmegen, The Netherlands and 4Institute of Water and Wetland Research, Radboud University, Nijmegen, The Netherlands Abstract. 1. In Western Europe, the diving beetle Dytiscus latissimus (Coleop- tera: Dytiscidae) has become rare and went extinct in several countries during the last century. This study investigated whether larval development rate, meta- bolism and feeding ecology differ between D. latissimus and the congeneric D. lapponicus to explore factors explaining its decline. 2. During instar I and II, D. latissimus larvae developed faster and gained more weight than D. lapponicus larvae. In accordance, D. latissimus larvae had higher oxygen consumption rates than D. lapponicus larvae, which signifies a greater energy expenditure. 3. Food preference tests showed that D. latissimus larvae strongly prefer cad- disfly larvae (Trichoptera: Limnephilidae) with early instars being obligatory dependent on caddisfly larvae for their development. Only instar III larvae readily fed on alternative prey items. In contrast, D. lapponicus larvae had a broader diet and even rejected caddisfly larvae. 4. Based on field observations, availability of caddisfly larvae strongly declined before the end of the larval development of D. latissimus, suggesting that time constraints on food availability limit completion of larval develop- ment. 5. Our results suggest that food limitation during (early) larval stages is a possible bottle-neck for this species, potentially explaining its disappearance from former localities. Promoting caddisfly larvae in the vicinity of D. latissimus oviposition sites, may possibly safeguard the present distribution of D. latis- simus and support the species recovery. Although more research is needed, pro- moting leaf litter in shores may be beneficial to the shredding caddisfly larvae and in turn for their predator D. latissimus. Key words. Feeding ecology, habitat directive, phenology, respiration physiol- ogy, Trichoptera. Introduction Worldwide scientists have expressed their concern for the current loss of biodiversity (Barnosky et al., 2011; Cebal- Correspondence: Hein H. van Kleef, Bargerveen Foundation, los et al., 2015). Research has suggested that the extinc- P.O. Box 9010, 6500 GL Nijmegen, The Netherlands. E-mail: tion rates of insects exceed those of birds and vascular [email protected] plants (Thomas et al., 2004). For successful recovery of Ó 2018 The Authors. Insect Conservation and Diversity published by John Wiley & Sons Ltd 1 on behalf of Royal Entomological Society This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. 2 Ilse Scholten et al. threatened insect species, it is paramount to understand Previous studies furthermore indicate that D. latissimus their life history and what habitat conditions they require larvae have a strong preference for caddisfly larvae (fam- (e.g. Anthes et al., 2003; Thomas et al., 2009). ily: Limnephilidae) above other prey species like tadpoles, One of those insects for which that valuable knowl- isopods (family: Asellidae) and mayfly nymphs (family: edge is required but largely missing is Dytiscus latissimus Siphlonuridae; Blunck, 1923; Johansson & Nilsson, 1992; (Coleoptera: Dytiscidae), Europe’s largest predacious div- Vahrusevs, 2009). Being dependent on one or a few food ing beetle. Its geographical range includes Central- and sources, oligophagous species require higher densities of North-Europe and the west of Siberia (Nilsson & Hol- suitable food sources which likely results in a more strin- men, 1995). During the twentieth century, its population gent demand on the quality of the habitat for their pre- numbers in Central and Western Europe have declined ferred prey. Moreover, oligophagous species are more drastically (Hendrich & Balke, 2000; Holmen, 1993). In sensitive to decreasing prey availability since they cannot seven countries, D. latissimus has not been observed for switch to other food sources. the past 50 years and here the species is considered to Finally, the type of ponds where D. latissimus is found be extinct (e.g. Gerend, 2003; Hajek, 2004; Queney, in the Netherlands is rich in other (large) carnivorous div- 2004; Cuppen et al., 2006; Scheers, 2015). In the Nether- ing beetles like C. lateralimarginalis and D. lapponicus lands, D. latissimus was also believed to be extinct (Hui- (Cuppen et al., 2006; Reemer et al., 2008). If the larvae of jbregts, 2003) until it was rediscovered in 2005 (Van these other species compete with the larvae of D. latis- Dijk, 2006). simus for the same prey items, this could further increase In Europe, D. latissimus inhabits a wide range of water food limitation in D. latissimus. types, including fish ponds, non-polluted clear or dys- A fast development, possibly resulting in a high metabo- trophic lakes, old river oxbows and water bodies formed lism and high food demand, in combination with a special- by damming or peat and turf excavation (Holmen, 1993; ist diet of the larvae may play an important role in habitat Hendrich & Balke, 2000, 2005; Cuppen et al., 2006; Vah- selection and population dynamics of D. latissimus.In rusevs & Kalninßs, 2013). These habitats show a large vari- order to test this hypothesis, the growth and moult of ation in depth, transparency and hydrochemical aspects. D. latissimus larvae was followed in the laboratory The presence of plants that are suitable for egg deposition throughout the larval stages when subjected to a diet with is considered a key factor for its reproduction and and without Limnephilidae larvae. Furthermore, the meta- includes marsh-marigold (Caltha palustris), acute sedge bolic rate and thermal sensitivity of metabolism of larvae (Carex acuta), cyperus sedge (Carex pseudocyperus), bottle kept under natural feeding conditions was determined by sedge (Carex rostrata) and bogbean (Menyanthes trifoli- measuring their oxygen consumption from both air and ata; Holmen, 1993; Hendrich and Balke, 2000, 2005). water at different temperatures. Finally, the diet of D. latis- However, these factors by themselves are inadequate to simus larvae was studied with food preference tests. All the explain its current rarity and scattered distribution. experiments were repeated in larvae of D. lapponicus to be Thus, it seems likely that additional factors limit the able to compare development, metabolism and diet between population size and distribution of D. latissimus (Cuppen the two species. The reason for choosing D. lapponicus as a et al., 2006; Reemer et al., 2008). There is an urgent need comparison is that this species is common and frequently to identify these factors as these will enable the develop- co-occurs with D. latissimus in reasonable numbers in the ment of conservation strategies for existing habitat and Netherlands (Cuppen et al., 2006). A comparison of life- the development of new habitat, and hereby realize the history traits between the successful D. lapponicus and less international protection of this species [Council of Europe successful D. latissimus in the same environment may give 1979, enforced by the European Habitats Directive of an indication why the latter is rare. The laboratory data 1992 (annex II)]. were extrapolated to the field situation with the help of the Several characteristics of D. latissimus suggest that the phenology and densities of potential prey species and the species, especially its larva, is sensitive to food shortage. phenology of D. latissimus larvae monitored in the field. First of all, the larvae of D. latissimus take about 2 months to develop from an egg into an adult (Vah- rusevs, 2009), which is about the same amount of time Materials and Methods required by larvae of other Dytiscidae like Dytiscus lap- ponicus and Cybister lateralimarginalis. The adults of Laboratory study D. latissimus are however larger than those of any other species of diving beetles found in Europe with an average Collection of the animals. To minimise the impact on size of 3.9–4.4 cm, and adults weighing 2.8 and 2.2 times the Dutch population, larvae of D. latissimus used in the more than adults of respectively D. lapponicus and C. lat- experiments were acquired from eggs in stems of C. acuta eralimarginalis (Nilsson & Holmen, 1995; own data). that were collected from fish ponds in Ruģelßi, Latvia Completing larval development in the mentioned time (55°52033.1″N, 26°35021.2″E), where the species is much span may thus be more challenging for D. latissimus than more common. Here, its eggs are easier to find and col- for smaller dytiscid species and is expected to require a lecting them does not negatively affect the population higher metabolic rate and food intake. (Vahrusevs & Kalninßs, 2013). The larvae of D. lapponicus Ó 2018 The Authors. Insect Conservation and Diversity published by John Wiley & Sons Ltd on behalf of Royal Entomological Society Explaining the decline of D. latissimus 3 were bred in the laboratory from two pregnant females taken regularly. To measure its length, the larva was caught in March 2016 in Langeven (51°47000.0″N, taken out of the water and put on a narrow v-shaped sur- 5°48009.7″E), the Netherlands. Both females deposited face coated with a thin film of water. Most of the time their eggs in captivity in the stems of C. rostrata and this immobilised the larva long enough to measure its M. trifoliata. In total 83 D. latissimus and 36 D. lapponi- length. In other cases a pair of tweezers was used to care- cus larvae were used in the experiments.
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