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Introductions of non-native species can pose serious threats to native populations and ecosystems. However, the impact of introduced species depends on intrinsic characteristics, local habitat conditions, and the interaction with native species. Case-specific management strategies may therefore be required. Using phenotypic characters and molecular markers for species identification, we provide insights into an artificial hybrid zone between two closely related newt species, the native Triturus cristatus and the introduced T. carnifex, near Tübingen, south-west Germany. Our analyses revealed a central Italian origin of the non-native T. carnifex and suggested their sustained presence in the study area for at least six years, probably much longer. In some ponds, extensive hybridization with native T. cristatus was detected. However, we found no evidence for a displacement of the native species by its non-native congener. The gradient from pure T. carnifex to pure T. cristatus currently extends over 7 km. A future expansion of the hybrid zone and swamping of a neighboring T. cristatus meta-population appears unlikely under the local configuration of breeding ponds. We propose to monitor the hybrid zone using genetic markers for evaluating the direction and speed of gene flow, complemented by capture-recapture studies to reveal trends in species-specific population sizes. To protect the native T. cristatus, we recommend practitioners to maintain their habitats, for example, by preventing illegal release of gold fish, by counteracting early drying of the breeding ponds, and by regularly cutting back trees and shrubs along the shoreline.

For many elusive insect species, which are difficult to cover by standard monitoring schemes, innovative survey methods are needed to gain robust data on abundance and population trends. We suggest a monitoring of overwintering larvae for the endangered nymphalid butterfly Limenitis reducta . We tested different removal and capture‐mark‐recapture (CMR) approaches in a field study in the “Alb‐Donau” region, Germany. Classical removal and CMR studies require movement of the organisms under study, but in our approach, we replaced movement of the study organisms by random movement of multiple different surveyors. We tested the validity of the approach by comparing detection frequencies from our field data with simulated detections. Our results indicate that multi‐surveyor removal/CMR techniques are suitable for estimating abundance of overwintering L. reducta larvae. Depending on surveyor experience, the average detection probability ranged between 16% for novices and 35% for experts. The uncertainty of population estimates increased with a decrease in personnel expenditure. Estimated larval densities on a spruce clear‐cut varied between one and three individuals per 100 m 2 , probably related to habitat conditions. We suggest a CMR approach with three to four trained surveyors for the monitoring of L. reducta populations in the overwintering stage. Compared with previous sampling methods, our approach is a powerful tool with clear advantages: long survey period, estimates of the absolute population size accompanied by uncertainty measures, and estimates of overwinter mortality. The proposed method can be adapted and used for several different butterfly species, other insect taxa with specific immobile life stages, and some sessile organisms, for example, elusive plants, fungi, or corals.

1. For many elusive insect species, which are difficult to cover by standard monitoring schemes, innovative monitoring methods are needed to gain robust data on population trends. We suggest a monitoring of overwintering larvae for the endangered nymphalid butterfly Limenitis reducta. 2. We tested one removal and three detection-mark-redetection (DMR) approaches in a field study in the “Alb-Donau” region, Germany. We replaced movement of the study organisms by random movement of multiple different surveyors, and we examined the model assumption of equal detectability using simulations. 3. Our results indicate that multi-surveyor removal/DMR techniques are suitable for estimating abundance of overwintering L. reducta larvae. Detection probabilities varied with surveyor experience and the uncertainty of population estimates increased with a decrease in personnel expenditure. Estimated larval densities on a spruce clear-cut ranged between one and three individuals per 100 m². 4. We suggest a detection-mark-redetection (DMR) approach with three trained surveyors for the monitoring of L. reducta populations in the pre-imaginal stage. Besides L. reducta, the proposed method is likely to be suitable for other insect taxa with specific immobile life-stages and some sessile organisms, e.g. corals, elusive plants, or fungi.

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