When assessing changes in populations of species it is essential that the methods used to collect data have some level of precision and preferably also good accuracy. One commonly used method to collect pollinators is colour pan-traps, but this method has been suggested to be biased by the abundance of surrounding flowers. The present study evaluated the relationship between pan-trap catches and the frequency of flowers on small (25 m2) and large (2-6 ha) spatial scales. If pan-traps work well, one should assume a positive relationship, i.e. more insects caught when they have more food. However, in contrast, we found that catches in pan-traps were often negatively affected by flower frequency. Among the six taxa evaluated, the negative bias was largest in Vespoidea and Lepturinae, while there was no bias in solitary Apoidea (Cetoniidae, Syrphidae and social Apoidea were intermediate). Furthermore, red flowers seemed to contribute most to the negative bias. There was also a tendency that the negative bias differed within the flight season and that is was higher when considering the large spatial scale compared to the small one. To conclude, pan-trap catches may suffer from a negative bias due to surrounding flower frequency and colour. The occurrence and magnitude of the negative bias was context and taxon dependent, and therefore difficult to adjust for. Thus, pan-traps seems less suited to evaluate differences between sites and the effect of restoration, when gradients in flower density is large. Instead, it seems better suited to monitor population changes within sites, and when gradients are small.

Methods 

STUDY SITES: Data were collected in 2015 in the province of Östergötland, southern Sweden. The landscape in the study area consists mainly of coniferous forest, but there are also bogs, lakes, small patches of seminatural grasslands and arable fields. Twelve clear-cuts were selected (2-6 ha and logged 4-6 years prior). Six of them had been marked as coniferous forests on maps from the 1870s when the other six were marked as meadows. Clear-cuts on former meadows have higher amounts of herbs than clear-cuts which were formerly forests. Hence, our site selection strategy covered the wide range of flower abundances that occur on clear-cuts in the study area. 

PAN-TRAPS: The pans used to collect pollinators were painted in one of the following colours: blue, white and yellow with UV-reflecting-colour (Soppec, Sylva mark fluo marker, Nersac, France). The pans had a diameter of 8.7 cm, a volume of 0.5 L and were filled with non-toxic propylene glycol (40% concentration), to conserve the pollinators and to decrease the surface tension. A small opening (4 mm in diameter) at the top of each bowl was made to ensure that rainwater could drain. One set of pan-trap consisted of three pans, one in each colour, placed on a steel stick.

Four sets of pan-traps were placed in each clear-cut, in the same height as the vegetation and in places that were considered representative for the clear-cut. During the main flight period of most pollinating insects, three sampling periods were conducted: in the beginnings of June, beginning of July and beginning of August. Each period lasted for one week and had at least some days with more than 17°C and wind velocity less than four on the Beaufort scale. The pans were covered with caps between collecting periods. When a new collecting period started, some sets of pan-traps were moved - at most 30 cm - or some of the vegetation was removed to prevent overgrowth.

In total there were 48 set of pan-traps collecting during each period, but a few sets of pan-traps had been knocked down by animals and were therefore excluded (1, 1 and 2 during the first, second and third period, respectively). In addition, a single blue pan went missing during the second period. 

Four taxonomic groups dominated the catches – Aculeata, Lepturinae, Cetoniidae and Syrphidae – and they were identified to species-level. Aculeata was subdivided as solitary Apoidea, social Apoidea (Bombus spp. Apis mellifera), and Vespoidea (including one species of Chrysidoidea). Other insects caught, that are not identified, were mainly small Coleoptera, small Lepidoptera and Symphyta.

FLOWER FREQUENCY: The clear-cuts were photographed in conjunction with each collecting period to estimate flower frequency. A 1 m2 square was placed on the ground and photographed from above (from c 160 cm height). Around each pan-trap, at least 25 such 1 m2 squares were photographed reflecting small-scale flower abundance (“trap scale”). An additional at least 100 pictures were systematically distributed along transects over the whole clear-cut reflecting large-scale (“clear-cut scale”) flower abundance. Photos were taken during each collecting period, or at most 5 days before or after. 

All 8048 photos taken were inspected to see if they held flowers within the 1-m2 square and if so of which colours (red, yellow, blue and white). The frequencies of colours (i) around set of pan-traps, and (ii) on clear-cuts were expressed as the odds for the colour occurring in a square metre plot: (0.5+p)/(0.5+(1-p)), where p=frequency of photos with the colour. Also, we calculated the odds for flower of any colour occurring in a plot.

With steadily shrinking areas of species-rich semi-natural grasslands in Europe, the management of theremaining fragments becomes ever more crucial for the preservation of key elements of biodiversity inthe agricultural landscape. Detailed knowledge about species-wise, as well as group-wise, relationshipscan provide guidelines for conservation management and a basis for predictions about differentmanagement scenarios. In the present study, we related the occurrence of species of bumblebees (N = 12),butterflies (31) and day-flying moths (4) and their total richness in 424 sites in southern Sweden to threegrass sward attributes, (i)flower abundance, (ii) height of the grass sward and (iii) within-site variation inheight of vegetation. The abundance of nectar-bearingflowers proved overall the best predictor ofrichness and in most of the occurrence-based species-wise models. However, both high grass swardheight and high variation in grass sward height contributed significantly to species richness andoccurrence of individual species. There was a tendency for bumblebees to respond less positively toflower abundance and vegetation height than butterflies and moths. One expectation was that grasslandspecialists, red-listed species, or species decreasing in other part of Europe, would be more responsive totall vegetation orflower abundance but there was no support for this in the data. Hence, managementstrategies that promote common species will also benefit all, or most of, the rare ones as well.

Butterfly conservation in Europe is mainly focused on well-defined grassland habitat patches. Such anapproach ignores the impact of the surrounding landscape, which may contain complementary resourcesand facilitate dispersal. Here, we investigated butterfly species richness and abundance in a habitatnormally regarded as unsuitable matrix: production forestry clear-cuts. Butterflies were recorded in 48clear-cuts in southern Sweden differing with regards to the time since clear-cutting and land-use history(meadow or forest based on historical maps from the 1870s). All clear-cuts had been managed as productionforests for at least 80e120 years. A total of 39 species were found in clear-cuts of both land-usehistories, but clear-cuts with a history as meadow had on average 34% higher species richness and 19%higher abundance than did clear-cuts with a history as forest. No effect of the time since clear-cuttingwas found, irrespective of land-use history, which was likely due to the narrow timespan sampled (<8years). The absence of temporal effect suggests that clear-cuts may provide butterflies with valuableresources for 10e15 years. Assuming a 100 year forest rotational cycle, this means that 10e15% of thetotal forested area are made up by clear-cuts valuable to butterflies, which corresponds to an area aboutfour times as large as that of species-rich semi-natural grasslands. The study illustrates the importance ofconsidering land-use legacies in ecological research and question the landscape-ecological view thatclear-cuts make up an unsuitable matrix for butterflies. Moreover, forest conservation management withspecial attention to land-use history may increase the quality of the landscape, thus facilitating butterflymetapopulation persistence. Given their large area and assets of nectar and host plant resources, clearcutsmust be considered as a butterfly habitat in its own right. Being a man-made environment withshort history, we might call it a neo-habitat.

Light trapping is an ideal method for surveying nocturnal moths, but in the absence of standardised survey methods effects of confounding factors may impede interpretation of the acquired data. We explored the influence of weather, time of year, and light source on nightly catches of macro moths in light traps, and compared four strategies for sampling by estimating observed species richness using rarefaction. We operated two traps with different light sources for 225 consecutive nights from mid-March to the end of October in eastern Germany in 2011. In total, 49 472 individuals of 372 species were recorded. Species richness and abundance per night were mainly influenced by night temperature, humidity and lamp type. With a limited sample size (less than10 nights) it was slightly better to concentrate sampling on the warmest summer nights, but with more sampling nights it was slightly better to sample during the warmest nights in each month (March to October). By exploiting the higher moth activity during warm nights and an understanding of the species phenology, it is possible to increase the number of species caught and reduce effects of confounding abiotic factors.

Plant species richness in central and northern European seminatural grasslandsis often more closely linked to past than present habitat configuration, which isindicative of an extinction debt. In this study, we investigate whether signs ofhistorical grassland management can be found in clear-cuts after at least80 years as coniferous production forest by comparing floras between clear-cutswith a history as meadow and as forest in the 1870s in Sweden. Study sites wereselected using old land-use maps and data on present-day clear-cuts. Speciestraits reflecting high capacities for dispersal and persistence were used toexplain any possible links between the plants and the historical land use. Clearcutsthat were formerly meadow had, on average, 36% higher species richnessand 35% higher richness of grassland indicator species, as well as a larger overallseed mass and lower anemochory, compared to clear-cuts with history asforest. We suggest that the plants in former meadows never disappeared afterafforestation but survived as remnant populations. Many contemporary forestsin Sweden were managed as grasslands in the 1800s. As conservation of remaininggrassland fragments will not be enough to reduce the existing extinctiondebts of the flora, these young forests offer opportunities for grassland restorationat large scales. Our study supports the concept of remnant populationsand highlights the importance of considering historical land use for understandingthe distribution of grassland plant species in fragmented landscapes, aswell as for policy-making and conservation.