Acoustic activity and conservation of the endangered heath bush-cricket Gampsocleis glabra (Orthopetera, Tettigonioidea) on xerothermic habitats in southeastern Poland
Dr. Emilia Grzędzicka
Foundation for Silesia Park
The project began on 1st May 2018, with the preparation of a voice bank of 18 insect species belonging to the Tettigonioidea group, that live in the research area (S-E Poland). Files found using the Internet were used to learn how to distinguish species by their voices in the field. Since mid-June, 10 sites have been visited with xerothermic grasslands in the Nida Valley, where are positions of protected grasshoppers, heath bush-cricket Gampsocleis glabra, found in earlier studies by other authors. On every site with protected grasslands, 3-5 research plots – each of which has an area of 1000 m2 – were designated for recording concerts of singing grasshoppers and describing plant vegetation. Patches of xerothermic grasslands in some places are degraded (see: photographs 1, 2), which confirms the necessity to carry out the research. The first adult heath bush-crickets were found in the last week of June. Until 30 June, 10 grasshoppers' concerts were recorded using the digital recorder Song Meter SM4, which is 25% of concerts planned to be recorded in the first dates of the field inspection (which is until 10th July). So far, the Pińczów Mountains proved to be the richest position with numerous heath bush-cricket (see: photograph 3). It seems that it would be possible to record even a dozen different individuals only in the Pińczów Mountains. In the next dates, it is planned to record at least another 100-110 concerts of insects for analyses in the Kaleidoscope program. The year 2018 is dry in Poland, and the air temperature is often very high (above 25°C, even above 30°C), which positively affects the voice activity of Tettigonioidea, which do not sing below 17°C. Both the large habitat diversity shown at research sites, as well as the favorable weather conditions have so far promoted the feasibility and success of the project.
Since the submission of the previous grant report, field studies planned under the project have been completed. The number of singing males of the heath bush-cricket Gampsocleis glabra found in 2018 in Poland can be estimated at around 100, which means that the population size is around 200 insects assuming that every singing male lured the female, or even less if we assume that males were more numerous than females. The average density calculated based on all controls was 1 specimen/ 1000 m2. These data clearly indicate that the population in Poland needs urgent and thoughtful conservation strategy.
In the case of steppe animals, the best way of their conservation is to describe accurately their habitat preferences and plan such treatments (e.g. mowing and grazing) at locations that would increase the area of the most optimal habitat, which was the main goal of the project. Habitat of G. glabra was described on small squares 1×1 m around singing males. On each square, one phytosociological reléve was taken consisting in listing all plant species present there with the scale of coverage of each plant species per square. For comparison, squares without heath bush-crickets were also designated, where the habitat was described similarly, as the example not preferred by protected insects. Since the plant censuses were updated during every control date (20.VI - 10.VII; 15.VII - 10.VIII, 1-20.IX), phytosociological reléves provide a complete picture of the vegetation. A strong preference for the steppe xerothermic grasslands from the Festuco-Brometea class was demonstrated for the heath bush-cricket. A total of 112 plant species were listed on the example 28 sites with G. glabra and 25 reléves without it, from one location. 49 plant species were xerothermic ones, and 63 were typical meadow plant species from the Molinio-Arrhenatheretea class. On the sites occupied by the heath bush-crickets, on average 55.8% were xerothermic plants, which was significantly more than only 21.8% of xerothermic plants per 1m2 observed on squares without the researched insect (Mann-Whitney test: Z = -6.23, P <0.0001, df = 1). This means that the phenomenon of expansion of meadow plants in the area inhabited by the heath bush-cricket is unfavorable for this species.
It has been shown that G. glabra lived in places where the vegetation below 10 cm had an average vegetation coverage of 98.7%, and at a height of 40 cm only 35.4%. The value in the second case was significantly lower than the average vegetation coverage of 60% in squares without the protected species (Mann-Whitney test: Z = 6.05, P <0.0001, df = 1). This result suggests the heath bush-crickets' preferences for dense vegetation near the ground (potential shelter, for example: from the sun) and rarer vegetation a bit higher, e.g. to find a well-exposed blade of grass useful for good voice communication (without sound attenuation).
Meadows (which are not the optimal hath bush-crickets' habitat) should be mown every year, whereas xerothermic grasslands should be mown better in a mosaic of patches mowing alternatively in different years. The gradual grazing from June to September is not recommended for the safety of insects that could fall prey to animals. The project also allowed to suggest the date of mowing – preferably after 25th August. The last day when G. glabra was heard was 29th August 2018, and only 3 specimens were found in one location. Although the recordings of various Tettigonidae were made at the September dates, same as habitat description, heath bush-crickets were no longer heard in September, although it could have been the effect of an exceptionally warm year that speeded up the phenology in nature.
The preliminary analysis of the heath bush-crickets' sound recordings has shown the dependence of their voice activity on the temperature noted on each day of the study. It turned out that as soon as the sun was hiding behind the clouds or the temperature dropped below 20°C, the voice activity of insects was reduced. The phase duration (PD) relationship with habitat quality was also shown for 4-5 min. recordings of G. glabra voices (after cutting out fragments, when voice activity decreased due to the weather conditions). The higher coverage of xerothermic plant species, the longer the PD (Spearman correlation coefficient: rS = 0.39, P = 0.036), so in more xerothermic habitat communication between insects was more effective.
The results of the research carried out during the project were presented in the oral presentation of the author, at the II. International Orthopterological Symposium in Smolenice in Slovakia (September 19-21, 2018). The speech met with great interest and the results were discussed with specialists. The next stage of the project will be the in-depth analysis of collected recordings of insects' voices, and the preparation of an article.
In the third reporting stage of the project, a more detailed analysis was prepared using W4V sound files, which were recorded with the digital Song Meter SM4. First, the author listened to the recordings to know exactly where the unnecessary disturbances were recorded, e.g. wind, voices of people or cars. Only carefully selected 4-5 minutes fragments were analyzed. In the Kaleidoscope Pro 4.5.5 program, the following parameters of the heath bush-crickets’ voices were measured: intensity in dB: minimal, maximum, medium; frequency in kHz: min. (lowest frequency LF), max. (highest frequency HF), mean frequency (MF) and the average frequency of “peaks”, which was defined as a dominant frequency (DF).
Seven GLM models were designed in the JMP 8 statistical program on N = 80 voice files with Gampsocleis glabra recordings from different dates, taking into account “recording date” as a factor (as the habitat changes dynamically throughout the season); the other two explanatory variables were: the type of habitat (1 – meadow, 2 – xerothermic) and distance to the edge of the crop (1 – far; 2 – near). No significances in voice frequency-related GLMs have been found, which means that the type of habitat (and therefore its smaller or larger degradation of xerothermic grassland towards the meadow) did not affect the frequency of male voices. The average singing intensity also did not depend on habitat, but only on its edge: it was lower at the border of steppes with cultivation (-17.57 dB) than in the middle of habitat (7.03 dB). This can be explained by the lack of a competitor on the outskirts of inhabited patches. Max. volume of voice was determined by the type of habitat – higher in the meadow (1.24 dB) than in the xerothermic grassland (-0.29 dB).
Recorded and listened concerts of the heath bush-crickets with males of other species belonging to the Tettigoniidae family in the background did not show interspecies competition in singing. It has not been found that G.glabra males were regularly interrupted when other species were singing. Other species of grasshoppers shown in the vicinity of the heath bush-crickets were mainly: Roesel’s bush-cricket Metrioptera roeselii, Metrioptera bicolor, dark bush-cricket Pholidoptera griseoaptera, wart-biter Decticus verrucivorus, Tettigonia caudata. Some of them do not display such habitat specialization as G.glabra for xerothermic grasslands; for example: M.roeselii with a very similar voice is a species typical for dry meadows but with small habitat requirements, D.verrucivorus prefers low vegetation and P.griseoaptera prefers meadows overgrown with shrubs – in contrast to the tested protected insect. There may be no voice competition between species, because they separate in space while living in other habitats. The only exception is M.roeselii, who occupied habitat patches similar to those in which G.glabra lives, in locations without the second species. The progressive degradation of xerothermic grasslands may, therefore, decline the heath bush-cricket through the expansion of less demanding habitat grasshoppers, but not through the voice competition between them.
The most interesting and surprising result obtained within the framework of the project is the lack of a clear link between the presence of the studied G.glabra with bunchgrasses Stipa spp., which presence largely qualifies the given habitat as a steppe. It turned out that the coverage of Stipa capillata was significantly higher in patches inhabited by the heath bush-crickets than in the others (Mann-Whitney test: Z = -3.25; P = 0.001), but on the other hand – Stipa sp. grew only in 36 % of the research areas inhabited by G.glabra. The key species of grass in sites of the examined insect was the tor grass Brachypodium pinnatum – typical for xerothermic grasslands from the Festuco-Brometea class, which grew in 93 % of patches inhabited by G.glabra and only in 16 % of the other examined areas. Plant communities with this grass resemble the physiognomy of the steppes with tall, rather dense stems, freely moving in the wind, denser closer to the ground than above.
Furthermore, a negative correlation was found between the number of G.glabra vocal episodes per recording and the mean coverage of B.pinnatum in the patch (Spearman correlation: rs = -0.57, P = 0.002, N = 53), as well as a significant positive relationship between the total length of male singing in the recording (phase duration, PD) and the coverage of the tor grass (Spearman correlation: rs = 0.57, P = 0.001, N = 53), which suggests a large energy expenditure for singing in habitat dominated by B.pinnatum. This may result from the presence of competitors from the same species in xerothermic habitat with a high abundance of tor grass, which stimulates males to intensify their singing.
The main goal of the project was, however, to investigate the impact of a degradation level of steppe habitats on the presence, distribution and voice communication of G.glabra in Europe. The research was carried out in ten locations and on most of them xerothermic grassland from the Festuco-Brometea class is now degraded. Not only the patches of Stipa spp. have decreased, as compared to the published data from 20-30 years ago, but also the physiognomy of these habitats is different. The high grassland typical for the steppes is disappearing, and thin, xerothermic grasslands with a clumping structure develops instead. Instead of heath bush-crickets, other species of grasshoppers exist in such places. Since there was no regular voice competition between different species within the project, it can be concluded that disappearance of vegetation with the physiognomy of high grassland steppes and strict specialization of G.glabra contribute to the decline of this species not only in Poland, but also throughout Central Europe.
The results of the project were discussed in detail during seminar at the Institute of Systematics and Evolution of Animals of the Polish Academy of Sciences in Krakow on December 11, 2018. The results will be published soon and used in practice in future.