(Ger-many: June–September; Wolff 2002). – Collect-ing period: 1819–1912. – Number of specimens and collecting events: 6, 4. – Distribution: This
species has not been collected in Denmark since 1912. Prior to 1912 it was found on Bornholm, North Zealand and Lolland (Fig. 58). – Red List category: DD.
Fig. 58
(Appendix 1). Furthermore, Asilidae can be iden-tified as such under field conditions so that tar-geted collecting by nonspecialists is feasible. The robber flies are thus one of the few Diptera groups that could be promoted for the kind of non-specialist collecting that is required for covering the large area of a country like Denmark. Further-more, robber flies are not only easily recognized and attractive, but at the same time vulnerable to habitat disturbance. Being relatively large and predaceous as larvae and adults, populations tend to be small. Moreover, the adults of many species favor warm and dry habitats like dry pastures, heathlands, and dune landscapes. These habitats are either quickly disappearing from many tem-perate countries and/or are subject to much dis-turbance. It is thus not surprising that a decline in asilid populations has been observed in many European countries (e.g., van Aartsen & van der Goot 1981, Verlinden 1982).
The use of specimen databases from collec-tions has recently received much attention (e.g., Fagan & Kareiva 1997, Kress et al. 1998, León-Cortés et al. 1998, Funk et al. 1999, Soberón et al.
2000). They contain the “most comprehensive, reliable source of knowledge” (Ponder et al.
2001) about the distribution of invertebrate groups and the data are of great value in biodiversity and conservation biology studies.
However, the information should not be used uncritically. As Ponder et al. (2001) aptly summa-rized “shortcomings of the data include the ad-hoc nature of the collections, presence-only data, biased sampling, and large collecting gaps in time and space...” The same problems plague our data and they can thus mostly serve as a baseline for future work. A more complete picture of the species diversity and ranges will only emerge after additional and systematic collecting cover-ing poorly sampled regions (see Smith 2000).
However, studies like the one presented here will
varying number of collecting events across the different 20-km grid cells in Denmark (Fig. 28).
Normally one would attempt to correct for un-even sampling and some techniques have been suggested (e.g., Fagan & Kareira 1997, Ponder et al. 2001). However, none can be used here and the two main problems for our data remain non-standardized collecting and the fact that the number of unsuccessful collecting attempts in the various grids is unknown. The latter is particu-larly damaging, because the uneven distribution of successful collecting events documented in Fig. 28 can be equally well explained by either the uneven sampling, uneven abundance of Asilidae in the different grids, or a combination of both.
However, it appears likely that uneven sampling is particularly important.
(1) There are relatively few collecting events in large regions such as Northwest- and South Zealand, Lolland, Falster and Møn, and South Jutland (Table 3) and it is inconceivable that these regions are genuinely as species-poor as our sam-ple suggests. It is more likely that they have simply been poorly collected. It is interesting that the same areas are also poorly sampled for the focal taxon of the only other atlas study of Danish Diptera, the Syrphidae (Torp 1994).
(2) Further evidence for unequal sampling comes from the regional bias created by impor-tant collectors. A typical case is J. G. Worm-Hansen (Tuxen 1961), who, between 1940 and 1959, collected 268 specimens (6% of our sam-ple) thus contributing 130 collecting events to our data set (7%). Of these collecting events, 89 (68%) took place in his two favorite UTM grids (MG92: 62 events; MJ72: 27 events), which have since only been resampled 16 times. A similar bias was created by the extensive sampling scheme on Thy (UTM MJ72) undertaken by the Entomological Department of the Copenhagen Museum between 1953 and 1961 (Tuxen 1961–
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1965). There are 68 collecting events during this short period, while Thy has only been revisited twice afterwards. Such patchy sampling can lead to seriously distorted distribution maps and state-ments about the relative abundance of species over time. For example, when J. G. Worm-Hansen stopped collecting it may appear as if the asilid diversity in his favorite UTM grids had dropped dramatically, while at least for some rare species, ceasing collecting more likely had the opposite effect.
Another mixed blessing of collection data is the large proportion of old specimens. Our data set spans from 1818 to 2001 and includes 384 (9%) specimens from the 19th century. Obviously the old specimens are of great importance in documenting that a species has been present in a particularly locality. Indeed, without the old mu-seum specimens several species that were only collected before 1960 would not even be known to have been part of the Danish fauna. Old speci-mens can also be important for documenting that a population has continuously occupied a particu-lar grid. However, for present-day conservation decisions they are only of limited value. It would, for example, not be advisable to protect the only two known collecting sites of Tolmerus cingu-latus just because the species was collected there in 1871 and 1908. Instead, the old specimens suggest that the locality should be surveyed again for the presence of the species and conservation action should only be taken if its continued pres-ence is confirmed.
Minimum flying period
Based on the collection material, the flying peri-ods of Asilidae in Denmark range from mid-March to the end of October. However, no spe-cies is found throughout the entire period and for most, adults are only found 2–4 months per year.
The longest activity periods are the approxi-mately five months of Dioctria hyalipennis, Dioctria linearis and Tolmerus atricapillus.
These are all abundant species and one therefore wonders whether rare species indeed have shorter flying periods or whether for these species our sample is simply too small for documenting the entire period. One quarter of the species in our sample are known from fewer than 25 records and
for all of these the observed short flying periods could either be due to poor sampling and/or due to a genuinely short adult life.
Reliable flying periods for such species can only be obtained via continuous sampling in the areas with a known presence and we are not confident that such sampling has been carried out.
The minimum flying periods for Denmark that we are here reporting are nevertheless of consider-able value for local collectors who would like to confirm the presence of a particular species in a particular grid or who aspire to collect this spe-cies in another part of Denmark. As further infor-mation on flying periods we are also listing the known periods for Germany (Wolff 2002). They are based on larger samples, but also come from a climatically rather different country.
Species ranges in space and time
The following discussions of species ranges are obviously tentative given the uneven sampling of Denmark for Asilidae. It appears that most of the 30 species can be roughly assigned to five differ-ent groups:
(1) “Ubiquitous species”. About eight species are found throughout all regions in Denmark (e.g., Choerades gilvus, Dioctria atricapilla, D.
hyalipennis, Lasiopogon cinctus, Leptogaster cylindrica, Asilus crabroniformis, Philonicus albiceps, Rhadiurgus variabilis).
(2) “Widespread species”. A second group of 10 species is found throughout Denmark except for West Jutland or Northwest Jutland. They are:
Choerades marginatus, Dioctria oelandica, D.
rufipes, Didysmachus picipes, Eutolmus rufi-barbis, Laphria ephippium, Leptogaster gutti-ventris, Tolmerus atricapillus, Neoitamus cyan-urus and Pamponerus germanicus.
(3) “Eastern species”. Seven species have a predominantly eastern distribution and are most-ly known from Funen, Zealand, and Bornholm.
They are: Antipalus varipes, Cyrtopogon latera-lis, Dioctria linearis, Leptarthrus brevirostris, Tolmerus cingulatus, Molobratia teutonus, Neo-itamus cothurnatus.
(4) “Western species”. Relatively rare are typi-cal “western” species mostly known from Jut-land. They are: Dioctria cothurnata, Dysmachus trigonus, Machimus arthriticus.
heaths habitats. Machimus gonatistes has been taken on five occasions between 1934 and 1974.
It has never been collected in the rest of Denmark and it is apparently also absent in Sweden and Great Britain (Soós & Papp 1988, Lyneborg 1968). One can only wonder how it managed to establish a population on Anholt. Within Europe it is widespread and mostly found in countries south of Denmark (except for records on the Gulf of Finland; Lyneborg 1968). Within Germany it is only known from the states of Brandenburg and Berlin (Wolff 2002).
In order to be able to study distribution changes over time we followed Damgaard (1997) and Torp (1994) in indicating the collecting peri-ods for the various species on our distribution maps. Based on our data it appears that the popu-lations of many of the “eastern” species (mostly known from Zealand and Bornholm) are declin-ing. Indeed, three, Antipalus varipes, Tolmerus cingulatus and Molobratia teutonus, have not been found in Denmark since 1960. Another two are Leptarthrus brevirostris and Neoitamus co-thurnatus, and although they have been found after 1960, no records exist for the majority of their old localities. One might surmise that the decline could be the result of reduced collecting, but collecting activity has here clearly increased (Table 3). Furthermore, one exclusively “east-ern” species, Dioctria linearis, and many of the species with wide geographical distribution throughout Denmark (“ubiquitous” and “wide-spread” species) have a stable abundance in the East.
When the predominantly “western” species are considered, one species, Dysmachus tri-gonus, has a stable abundance, whereas the other two, Dioctria cothurnata and Machimus arthri-ticus, have disappeared from several sites. In contrast to the “eastern” species the decline of the
“western” asilid populations also affects the
ubiq-Pamponerus germanicus, Neoitamus cyanurus, Laphria ephippium, Machimus arthriticus, Tol-merus atricapillus and Didysmachus picipes.
However, collecting activity might partly explain this “decline”, because sampling decreased in Northwest and South Jutland (Table 3). Only one region in Denmark seems to have become more species-rich after 1960. This is North East Jutland where Dioctria atricapilla, Dioctria hyalipennis, Dioctria rufipes, Lasiopogon cinctus, Lepto-gaster cylindrica, and Rhadiurgus variabilis have mostly been found after 1960. However, the observed increase in diversity is probably at least partly related to the increased collecting activity in this region (Table 3), and most “new” species belong to the ubiquitous type.
Red List for Danish Asilidae
One obvious question is whether our data can be used to propose a Red List for the Danish Asilidae. We can apply the Version 3.1: IUCN (2001) Red-List criteria tentatively, but would have to argue that for most species our informa-tion is at least close to being “Data Deficient”. On the other hand, IUCN discourages the liberal use of the “Data Deficient” category (IUCN 2001: 9) and favors evaluating the available data for as good an assessment as can be made at any point in time (see also Gärdenfors 2001). We are applying the criteria in the modified form for regional Red Lists (Gärdenfors et al. 2001, Gärdenfors 2001), which involves two steps (Gärdenfors et al.
2001). First, the IUCN criteria are applied to the regional populations of each species. Afterwards, the status of the populations for the same species is assessed in the surrounding regions. The ration-ale behind the second step is that for species with good dispersal capabilities, a healthy population in an adjacent region reduces the threat to the local population under assessment. However, for
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two reasons we largely omit this second step in our analysis. Denmark is a very isolated country with only a small terrestrial border to the sur-rounding countries, which makes non-Danish populations of Asilidae of little interest for the conservation status of asilids in the country (the only exception could be the “southern” species Laphria flava). Second, although Asilidae are considered strong fliers they do not appear to be good dispersers (see Verlinden 1982).
Of the criteria suggested by IUCN for catego-rising species, we believe the “geographic-range criterion” (criterion B, and rarely category D) can most realistically be used for insects. It requires that it is documented that the “extent of occur-rence” or “area of occupancy” (see IUCN 2001) is smaller than a certain defined size, whereby different threat categories have different size-thresholds. It is furthermore necessary to docu-ment that two of the following three phenomena apply to the species in question: (1) severely fragmented distribution or existence at few loca-tions, (2) population decline (observed, inferred, or projected), and (3) extreme fluctuations in number of populations or range. The latter condi-tion can probably not be used for most insect groups, because there are not enough data to document such fluctuations. However, the first two criteria can be applied if a large data set is available although the serious problem of uneven sampling over space and time remains. The fact that a species has become rarer in the collection or disappeared from a locality is meaningless unless it can be demonstrated that this is probably not due to lack of recent collecting within its range.
That is why we chose to compare two time peri-ods of the same length (1920–1960 and 1961–
2001) with regard to area of occurrence, number of collecting events, and number of specimens collected. We also take collecting activity into account by considering the overall sampling ac-tivity in the various grids during the two different time periods (see Materials & Methods). Of course, only successful collecting events are documented so that this measure of collecting effort is conservative in the sense that those occa-sions on which an entomologist was sampling in an area and did not find any robber flies are not counted.
The inclusion of a species on the regional Red
List also poses the question of whether the species might be threatened at a global scale (IUCN 2001). Red Lists including information on Asi-lidae are unfortunately lacking for most countries surrounding Denmark. However, the Palaearctic Catalogue of Asilidae (Soós & Papp 1988) pro-vides information on the full geographic range of the Danish species. We counted the number of European countries in which the different “Dan-ish” species occur (countries as listed in cata-logue: Soós & Papp 1988). Two interesting points emerge. First, all Danish species of Asilidae are quite widespread and are known from many countries (average: 16 countries; low-est number: 9). Second, 50% of the Danish spe-cies have to be considered rare, and these may be
“Threatened” “Near Threatened” or “Least Con-cern”. The distribution of these species is on average about as wide as the distribution of a species that is common (16 versus 18 countries).
Furthermore, in several cases species rare in Den-mark have been reported to be also rare in other northern European countries. All these countries are near to the northern edge of the species’
distribution and it seems likely that the species’
rarity can be explained by a geographic-range edge effect.
A more direct assessment on the global threat for any species on the Danish Red List should ideally be based on an assessment of its entire range. Asilidae are only included on few Red Lists such as the ones for Sweden and Finland. In several cases species threatened or extinct in Sweden and Finland also meet the Red List crite-ria in Denmark. But there are also two cases where species that are on the Swedish list are apparently doing well in Denmark. All cases are mentioned below and we also provide some infor-mation on the status of the Danish species in Germany (Wolff 2002).
Regionally Extinct (EX) and Extinct in the wild (EW)
Molobratia teutonus. – There is only one locality known for this species. This conspicuously large and black species which is likely to be collected by entomologists (Tomasovic & Libert 1999) has only been collected twice before 1910 in North Zealand. The grid in which it had been found has
(Tomasovic & Libert 1999). The number of specimens collected has also been declining in Germany, where the species is not known from any of the northern states (Wolff 2002).
Critically Endangered (CR)
Antipalus varipes. – IUCN criteria B1ab (i, ii, iv) + 2ab (i, ii, iv): extent of occurrence and area of occupancy are probably less than 100 km2 and 10 km2, respectively; a: the species is now only known to exist at a single locality (Tisvilde); b:
there is a continued decline in number of sub-populations, extent of occurrence and area of occupancy.
Evidence. – Between 1900 and 1936 this spe-cies had been collected 27 times in seven different localities within six 10 km UTM grids. For these grids we have 114 additional collecting events after 1950 and the species was only found twice (1952, 1959). The figures in Table 1 also substan-tiate the decline.
Status in surrounding countries. – Antipalus varipes is listed as Vulnerable on the Swedish Red List based on criteria B1+2c (Gärdenfors 2000). It is a widespread species occurring in 17 European countries (Soós & Papp 1988). There is no decline in German records (Wolff 2002).
Machimus gonatistes. – IUCN criteria B1ab (iii):
extent of occurrence less than 100 km2; a: this is the only locality known for this species; b: the quality of the habitat is endangered.
Evidence. – This species is only known from the very small island of Anholt (22 km2), which has many habitats that are in danger of being overgrown by pines (Pinus mugo Turra). In the 1970s Rald (1978) still regarded the species as
“common” on Anholt, but the last seven collect-ing events yielded only one additional specimen.
Status in surrounding countries. – The species
number of populations.
Evidence. – The species has only been col-lected three times in two 10 km2 UTM grids.
Unfortunately these grids have only been revis-ited six times after 1960. On one occasion a single specimen was collected (1977). Revisiting the old localities is thus urgently needed.
Status in surrounding countries. – Cyrtopo-gon lateralis is widespread and occurs in 17 European countries. There is no decline in Ger-man records, but only very few records come from northern Germany (Wolff 2002).
Machimus arthriticus. – IUCN criteria B1ab (iv) + 2 ab (iv): extent of occurrence and area of occupancy are probably less than 100 km2 and 10 km2, respectively; a: there is only one post-1950 locality known; b: there is apparently a decline in the number of populations.
Evidence. – The species has been collected 5 times in 2 grids between 1920 and 1960. After-wards, it has only been taken once and the figures inTable 1 indicate a decline in UTMs, collecting events, and number of specimens. However, it must be pointed out that collecting in the grids with known occurrence has dramatically de-creased after 1960 (1920–1960: 80 events; post-1960: 12). One might thus be inclined to consider the decline as sampling artefact, but between 1900 and 1920 alone the species was known from 4 additional grids. These grids have 20 additional collecting events without any evidence for M.
arthriticus. Collecting at the old localities is nev-ertheless urgently needed.
Status in surrounding countries. – The species is widespread and occurs in 17 European coun-tries. However, it is listed on the Swedish Red List as Endangered (criteria B1+2c; Gärdenfors 2000) and is extremely rare in Belgium (Ver-linden 1982) and the United Kingdom (Oldroyd 1969). There is no decline in German records, but
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the species is only known from widely scattered
the species is only known from widely scattered