Ent. Tidskr. 125 (2004) Exclusion of a native bush-cricket Exclusion of the native bog bush-cricket Metrioptera brachyptera by the currently invading Roesel’s bush- cricket Metrioptera roeseli ÅSA BERGGREN & MATTHEW LOW Berggren, Å. & Low, M.: Exclusion of the native bush-cricket Metrioptera brachyptera by the currently invading bush-cricket Metrioptera roeseli. [Den inhemska ljungvårtbitaren Metrioptera brachyptera trängs undan av den invaderande cikadavårtbitaren Metri- optera roeseli]. – Entomologisk Tidskrift 125 (3): 125-132. Uppsala, Sweden 2004. ISSN 0013-886x. Roesel’s bush cricket Metrioptera roeseli is currently expanding its distribution mainly in southern Sweden. To investigate its impact on a related native species, we censused twenty- five successfully colonised introduction sites of Roesel’s bush-crickets and matched con- trol areas for the presence and density of males of the bog bush-cricket Metrioptera brachyptera. We found a significant difference in the presence and numbers of M. brachyp- tera males between sites with introduced M. roeseli and matched control sites without M. roeseli. Metrioptera brachyptera was absent from 40% of the introduction sites compared to 2% of the control sites and, when present, was significantly more abundant in the control sites. As food appears superabundant, other resources such as acoustic and olfactory space might produce the observed distribution pattern. If the displacement of M. brachyptera as seen in this study is ubiquitous and continues, the two species’ distributions will shift, resulting in M. roeseli dominating grassland habitats and M. brachyptera being forced into drier, less preferred habitat in heath lands and forest edges. Åsa Berggren, Department of Entomology, PO Box 7044, Swedish University of Agricul- tural Sciences, SE-75007 Uppsala, Sweden, email: [email protected] Matthew Low, Ecology group, Massey University, Private bag 11 222, Palmerston North, New Zealand, email: [email protected] Species will naturally colonise new areas, but in son 1989). The resulting impact of this may ma- recent history the rate of colonisation has in- nifest as one or both of the species decreasing in creased with human movement and associated number, or altering their feeding, reproductive or environmental modification (di Castri 1989). other behaviours to escape the pressure of com- While most introductions fail (Simberloff 1981), petition. One consequence of this being an in- when they succeed the new species can have creased likelihood of the local extinction of one profound effects on the community in which it species (Bengtsson 1989). The form of competi- establishes. Although the effects can be positive tion and its impact may be difficult to determine or neutral (Walser et al. 2000), many are negative because of confounding variables. Commonly, and their impacts range from minor changes in the community or species is not studied before an population dynamics to species extinction (At- introduction is undertaken and thus there are no kinson 1989). The negative effects an introduced “control” communities to compare with after- species can have on the community can be multi- wards. ple and include new forms of competition (Byers Interspecific competition has been shown to 2000). This may occur through competition for be an important variable in grasshopper popula- food, reproduction sites and living space (Atkin- tion dynamics and distribution (Belovsky 1986, 125 Åsa Berggren & Matthew Low Ent. Tidskr. 125 (2004) Figure 1. A male of Roesel’s bush cricket Metrioptera roeseli. The body colour of this species is variable, with it ranging from brown to bright green. Photo: Åsa Berggren En hane av ciakdavårtbitaren, Metrioptera roeseli. Individerna är oftast bruna till gröna, men vissa ex- emplar starkt ljusgröna. Beckerman 2000). The underlying mechanisms are not always clear and these studies have been concerned with already existing communities Figure 2. Map of study sites (black squares) in the where the aim has been to understand interspeci- agricultural landscape around Uppsala town in fic relationships already present. While studies southeastern Sweden. into interspecific competition are often concer- Karta över lokalerna i studien (svarta kvadrater) i ned with resources such as food (Ritchie & Til- odlingslandskapet runt Uppsala i sydöstra Sverige man 1993), it is important to understand that other forms of resource limitation may have an impact on species distribution. During stridula- then compared with nearby control sites in the tion, a form of acoustic signalling competition same habitat where M. roeseli has yet to coloni- in orthopterans, one limiting resource is acoustic se. space. Male orthopterans have been found to mask each other’s call and thus make it hard for Materials and Methods competitors to attract females. One result of this The species is that acoustically masked individuals have to Metrioptera roeseli (Fig. 1) is a medium sized move to new sites (Bailey & Morris 1986, Beck- bush-cricket species, 13-26 mm in length and is erman 2000). common in south and central Europe, Finland We investigated the impact of an introduced and Latvia (Marshall & Haes 1988). In Sweden orthopteran on a related native species in the the species is generally restricted to the south- agricultural landscape in south-eastern Sweden. east around Lake Mälaren, with the population In this region Roesel’s bush-cricket Metrioptera core being close to the harbour of Västerås (de roeseli was experimentally introduced into 70 Jong & Kindvall 1991). The combination of the habitat islands previously uninhabited by this location of the core population and the recorded species (Berggren 2001). Twenty-five success- expansion from this point (Pettersson 1996), fully colonised sites were censused for the pre- suggests this species was introduced via cargo sence and density of the bog bush-cricket Metri- ships and is now invading the country. The spe- optera brachyptera, a species previously shown cies’ preferred habitat is moist ungrazed tall- to be displaced by M. roeseli under laboratory grass areas where they feed on grass, grass seeds conditions (McHugh 1971). These sites were and small insects. 126 Ent. Tidskr. 125 (2004) Exclusion of a native bush-cricket Metrioptera brachyptera is also a medium si- zed bush-cricket, 11-21 mm in length. This spe- cies is palaearctic, naturally occurring in Swe- den and also across central and northern Europe from the Pyrenees, through northern Italy, Yugo- slavia and Romania across Russia to Siberia (Marshall & Haes 1988). It is common in diffe- rent types of grass vegetation, both dry and wet. Nymphs of both species hatch in May and pro- gress through six instar phases before becoming adults (Marshall & Haes 1988). Adult males stridulate from July to October and if the weather is warm or sunny they will stridulate al- Figure 3. Road verges, like the ones examined in this most continuously during the day at this time. study, and other linear elements in the landscape faci- Each species’ song is characteristic, making the litate the dispersal and the spread of the species. Pho- males of the species easy to census. A variable to: Åsa Berggren proportion of the population of both species are Dikesrenar, som de som inventerades i denna studie, macropterous (winged), but these numbers are och andra linjära landskapselement underlättar low and in M. roeseli less than 1% (Vickery spridningen av vårtbitare i landet. 1965). ability to establish and expand in the landscape The introduction experiment and censuses (see Berggren 2001 for more details). Propagules of M. roeseli were introduced onto Populations with numbers of M. roeseli of 70 habitat islands, in parts of the country previ- over 25 males in the latest census (1999) were ously uninhabited by the species in a large-scale used in this study, producing a total of 25 intro- experiment in 1994-1995 (Berggren 2001). The duction sites (Fig. 2). Censuses were carried out habitat islands used were situated at a minimum by listening for stridulating males of both spe- distance of 17 km from the edge of the species’ cies during the reproductive seasons of 2001 and current distribution range and were expected to 2002. Because of the grass composition of road be incorporated in the species distribution range verges, these areas are preferred habitats for within 10 years due to the species spreading be- both species and provided the focal point for all haviour. The islands were of ungrazed seminatu- censuses. A 50 m stretch of road verge (Fig. 3) ral grasslands and the minimum distance bet- was searched in each area where M. roeseli had ween the introduction sites was 2 km. The expe- colonised. Within these areas, every individual rimental areas were situated in the agricultural male of the two species was recorded. Control landscapes in the counties of Uppland and sites for the study were located within a few Stockholm, located in south-eastern Sweden. hundred metres from the edge of distribution of Metrioptera roeseli’s current restriction to well- each of these introduced populations, and con- defined areas in the south-east of Sweden, its sisted of the same dimensions of road verge documented range expansion and its conserva- (length, width and depth) and vegetation type. tion status, make it an ideal model species and The control sites were censused on the same oc- an excellent candidate for introduction experi- casion for individuals of M. brachyptera and in ments. Moving it from areas where it occurs the same way as the area in the introduction site. abundantly to areas where it has yet to colonise The censuses were only made during sunny meant that individuals later found at the intro- days, with a temperature over 18°C to ensure duction sites were either the originally introdu- that the bush-crickets were stridulating and ced individuals (if found the same year) or their could be detected. Numbers of individual males descendants. This ensured that there would be of the two species from each introduction and no confusion with already existing individuals control site were then compared. As the distribu- and allowed easy interpretation of the species’ tion of M. roeseli expanded slightly between the 127 Åsa Berggren & Matthew Low Ent. Tidskr. 125 (2004) Table 1. Numbers (mean ± SE) of M. brachyptera and M. roeseli within a 50 m road-verge habitat during the study years at the different sites. Antal (medel ± SE) av M. brachyptera (ljung-) och M. roeseli (cikadavårtbitare) längs 50 m habitat sträcka i de olika typerna av lokaler under de två studieåren. Site Study M. brachypt. M. roeseli year (mean ± SE) (mean ± SE) Introduction site 1 0.72 ± 0.23 8.52 ± 0.70 2 1.96 ± 0.29 10.72 ± 0.74 Control site 1 3.76 ± 0.52 - 2 5.40 ± 0.47 - between the number of M. brachyptera in sites with and without M. roeseli an ANCOVA with years as a covariate was used. A Pearson pro- Figure 4. Mean numbers ± SE of M. brachyptera for duct-moment correlation tested if there was a year one and year two in sites where M. roeseli is pre- correlation between numbers of male M. sent compared to the control sites where M. roeseli brachyptera and M. roeseli. has not yet invaded. Medelantal ± SE av M. brachyptera första och andra Results studie året på lokaler där M. roeseli introducerats There was no interaction effect between year jämfört med kontrollområden där M. roeseli ännu and type of site (introduction sites and control inte finns. sites) (ANOVA, n=100, F=0.39, p=0.53). Metri- seasons the control sites used were not the same optera roeseli was a more abundant species for both seasons. (number of individuals per 50 m) than M. brachyptera, and both species’ abundances Statistical Analyses changed between the years. Both the numbers of The distribution of the variables and residuals M. brachyptera and M. roeseli were higher the were tested for normality by using a Shapiro- second year than the first (Table 1, M. brachyp- Wilk W test and were found not to be normal (all tera: ANCOVA, n=100, F=12.75, p=0.0006; p<0.05). The variables were then transformed M.roeseli: ANOVA, n=50, F=4.67, p=0.036). by usingY©= Y + (Y +1) (Freeman & There was a difference between the introduc- Tukey 1950), which resulted in normal to near tion and control sites in the presence of M. normal distribution of both the variables and the brachyptera. Combining the two years, 20 of the residuals. An ANOVA (analysis of variance) 50 introduction sites (40%) were without a sing- was used to investigate if there was an interac- le male of M. brachyptera while only 2 of the 50 tion effect between years and type of site (intro- control sites (4%) were without a male M. duction sites and control sites) that could influ- brachyptera. Numbers of M. brachyptera were ence analyses combining the two years. An higher in sites without M. roeseli (ANCOVA, ANOVA was also used to examine if there was a n=100, F=65.05, p<0.0001) (Fig. 4). In the in- difference in number of M. roeseli between troduction sites the numbers of M. brachyptera years. An ANCOVA (analysis of covariance) were not correlated with the numbers of M. roe- with the type of site (with or without M. roeseli) seli the first (n=25, r=0.045, p=0.83) or the se- used as a covariate, were used to examine if the- cond year (n=25, r=0.138, p=0.609) (Fig. 5a, b). re were differences between years and the num- ber of M. brachyptera. To compare the presence Discussion of M. brachyptera between sites with and wit- This study showed that 18 more introduction hout introduced M. roeseli a chi-square test was than control sites were without a single M. used. To investigate if there was a relationship brachyptera male, which represents a 90% de- 128 Ent. Tidskr. 125 (2004) Exclusion of a native bush-cricket a b 5 25 4 4 3 3 2 2 1 1 0 0 -1 -1 2 4 6 8 10 12 14 16 18 20 2 4 6 8 10 12 14 16 18 20 Numbers of M. roeseli Numbers of M. roeseli Figure 5. The relationship between numbers of M. brachyptera and M. roeseli study year one (a) and two (b) in sites where M. roeseli had been introduced. Förhållande mellan antal ljungvårtbitare (M. brachyptera) och cikadavårtbitare (M. roeseli) studieår ett (a) och studieår två (b) i områden där cikadavårtbitaren hade introducerats. crease in sites with M. brachyptera. Even if M. such as the type and amount of different lands- brachyptera females were left in these areas, no cape elements, farm practices and local climate. males remained for them to mate with. In intro- Several of the introduction sites are tens of kilo- duction sites where M. brachyptera males were metres apart (Berggren et al. 2001). Therefore, it still present (though fewer), mating opportuni- is reasonable to assume that M. brachyptera ex- ties for females still existed and reproduction isted at a similar density as the matched control was likely to occur. Unfortunately, because of sites in all introduction areas before M. roeseli their smaller effective population size in these was introduced. Because of this, we believe that areas, this would increase the extinction risk as- M. brachyptera has declined in these areas as a sociated with small populations (Richter-Dyn & direct result of the presence of M. roeseli. The Goel 1972). Varying abundances between years mechanism behind this decline is unclear, with did not alter the fact that the presence of M. roe- there being several possibilities such as limita- seli affected the presence of M. brachyptera ma- tion of: food, olfactory space and acoustic spa- les in a negative way. Thirty-six percent of sites ce; these are discussed below. where M. roeseli was introduced and had esta- blished a population, had no M. brachyptera de- Food limitation spite the matched control area containing M. Food resources (grass and grass seeds) in the brachyptera. In sites where M. brachyptera co- study areas appear to be superabundant. The existed with M. roeseli, the density was always road verges are un-cut and ungrazed by during lower than in the control sites. It appears that the the study animals’ life-times, leaving the vegeta- presence rather than the actual number of M. ro- tion high and dense. Due to the habitat structure eseli is the major determining factor affecting there is a moisture gradient within it, which is the presence of M. brachyptera. important for keeping the vegetation growing No data was collected on the abundance of M. throughout the season and making it a good ha- brachyptera males at the introduction sites prior bitat for both species (Berggren et al. 2001). An to M. roeseli establishing. Despite this, there is average road verge width of 1.1 m in this study nothing in the distribution pattern of the sites to produces a density of 0.17 individuals of M. roe- explain the effect seen in this study other than seli per m2 and 0.08 individuals of M. brachyp- the presence or absence of M. roeseli. The 25 tera per m2. These densities are almost two ord- sites differ significantly in landscape characters ers of magnitude lower than the lowest densities 129 Åsa Berggren & Matthew Low Ent. Tidskr. 125 (2004) found to have a food limitation effect (Joern & 9.62 males per 50 m) than M. brachyptera Klucas 1993). Thus, food competition is unlike- (mean 4.58 males per 50 m in control sites). If ly to be responsible for the differences seen in we assume that the equal body size and habitat our study. choice indicate that the area needed for optimal foraging should be similar, then the difference in Limitation of olfactory space density may be an effect of the M. brachyptera A means of communication between some spe- males increasing inter-male distances to reduce cies of orthopterans are chemical cues produced acoustic interference. Where acoustic competi- from hydrocarbons in the individual’s cuticula tion occurs, males will maximise their chances (Nagel & Cade 1982). These pheromones are of being evaluated by females by providing responsible for the well-known aggregation be- enough distance between them and other males haviour seen in locusts (Obeng-Ofori et al. to enable females to discover the individual cal- 1994). For some orthopteran species, pheromo- lers, but also be close enough to other males to nes make it possible for individuals to gain in- attract females by the concentration of numbers formation about other individuals’ age and sex (Arak et al. 1990). Another potential problem (Tregenza & Wedell 1997, Ochieng & Handsson for M. brachyptera is that the exaggerated song 1999). While pheromones have been shown to of M. roeseli may not only mask their call but have an effect on male stridulation in the field also attract M. brachyptera females through a cricket Gryllus bimaculatus (Tregenza & We- combination of conspecific scarcity, mistakes in dell 1997), we do not know of any study investi- mate recognition and supernormal stimuli (Arak gating pheromones in M. roeseli or M. brachyp- et al. 1990, Randler 2002). tera or any related species. Thus we cannot rule- Acoustic competition may cause species to out the possibility that these two species are exhibit temporary or permanent shifts in calling using chemical signalling and are competing for behaviour (frequency or pulse rate) or shifts in olfactory space. habitat use. Because the calls of M. roeseli and M. brachyptera are within the same frequency Limitation of acoustic space range, it is likely that a M. roeseli male calling A large part of the energy expenditure for the close to a M. brachyptera male will mask some males of both species during summer is their al- of its song (Latimer & Broughton 1984). Where most continuous stridulation - calling for fema- the possibility for call variation to minimise les by rubbing their wings together (Hoback & acoustic competition does not exist, a species Wagner 1997). The male’s mating success is at will need to move out of a habitat to prevent be- least partly determined by females hearing his ing out-competed. Active spacing behaviour stridulation. In addition to microclimatic and between males has been seen between P. al- vegetation interference effects, an important bopunctata and M. roeseli (Latimer 1981), bet- factor determining how females perceive a ween M. bicolor and M. roeseli (McHugh 1971) male’s stridulation, is the calling of other males. and between the species Platycleis affinis and P. Males from the same or different species can intermedia (Samways, 1977a). Thus while it is mask each other’s songs, so pieces or larger often assumed that differences in distribution re- parts of the song become inaudible to females flect particular habitat needs, it may simply be a (Bailey & Morris 1986). Masking and modify- result of competitive exclusion (Beckerman ing songs has been observed in several species, 2000). As M. roeseli and M. brachyptera have i.e. Metrioptera bicolor mask the call of Psoro- not co-existed before in Sweden, there has not donotus illyricus (Keuper et al. 1986), Platy- been time for evolution to act to separate perma- cleis albopunctata is inhibited by the M. roeseli nent types of calls and create specific calling ni- song (Latimer 1981) and in the presence of My- ches for the two species. Instead, it may be that galopsis marki the song of Hemisaga denticula- the dominant caller (M. roeseli) is affecting M. ta is suppressed (Römer et al. 1989). brachyptera numbers through the natural acous- In this study, populations of M. roeseli had tic competitive avoidance reaction of the sub- higher population densities in both years (mean ordinate species, producing the pattern seen in the introduction sites. 130 Ent. Tidskr. 125 (2004) Exclusion of a native bush-cricket Implications for conservation Acknowledgements In this study we did not monitor the number of We are thankful to Oskar Kindvall, Mattias Jonsson, females of the two species mainly for methodo- Bengt Gunnarsson and Mats Jonsell for valuable comments on earlier drafts of the manuscript. logical reasons. Females are difficult to accura- tely and consistently locate as they do not stridu- References late and both species are cryptically coloured. Arak, A., Eiriksson, T. & Radesäter, T. 1990. The The drawback of not having data on female adaptive significance of acoustic spacing in male abundance is that we do not know to what fre- bushcrickets Tettigonia viridissima: a perturba- quency females were present in the introduction tion experiment. – Behav. Ecol. Sociobiol. 26: 1- and control sites. However, regardless of female 7. abundance, we can see that males of one orthop- Atkinson, I. 1989. 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On the role of phe- Cikadavårtbitaren Metrioptera roeseli utbred- romones in aggregation formation in camel crick- ningsområde ökar främst i södra Sverige. Det är ets, Ceuthophilus secretus (Orthoptera: Gryllacri- didae). – Can. J. Zool. 61: 95-98. möjligt att detta kan påverka inhemska arter fö- Obeng-Ofori, D., Njagi, P.G.N., Torto, B., Hassanali, rekomst negativt. Vi undersökte hur den närbe- A. & Amiani, H. 1994. Sex differentation studies släktade ljungvårtbitarens (Metrioptera brach- relating to releaser aggregation pheromones of yptera) antal påverkas av cikadavårtbitarens the desert locust, Schistocerca gregaria. – Ent. närvaro. Tjugofem områden som ciakadvårtbi- Exp. Appl. 73: 85-91. taren hade koloniserat jämfördes med tjugofem Ochieng, S.A. & Handsson, B.S. 1999. Responses of matchade kontrollområden där cikadavårtbita- olfactory receptor neurones to behaviourally im- portant odours in gregarious and solitarious des- ren saknades. Vi noterade förekomst och antal ert locus, Schistocerca gregaria. – Physiol. Ento- av hannar av ljungvårtbitare på dessa lokaler. mol. 24: 28-36. Ljungvårtbitare saknades på 40% av områdena Pettersson, B. 1996. Nya fyndplatser för cikadavårt- med cikadavårtbitare jämfört med 2% av kon- bitare (Metrioptera roeseli) norr om Mälaren trollområdena och fanns i statistiskt signifikant (New records of Roesel’s bush-cricket, Metriop- större antal i kontrollområdena. Då förekomst tera roeseli (Orthoptera: Tettigoniidae), north of Lake Mälaren, South-Central Sweden). – Ent. av föda verkar vara riklig, är det antagligen kon- Tidskr. 117: 121-122. kurrens om andra resurser som ljud- och luktut- Randler, C. 2002. Avian hybridization, mixed pairing rymme som leder till det funna mönstret. Om and female choice. – Anim. Behav. 63: 103-119. undanträngandet av ljungvårtbitaren som vi sett Richter-Dyn, N. & Goel, N.S. 1972. On the extinction i denna studie är ett generellt mönster och fort- of a colonizing species. – Theor. Popul. Biol. 3: sätter, kan de två arternas utbredning komma att 406-433. Ritchie, M.E. & Tilman, D. 1993. Predictions of spe- ändras i framtiden. Cikadavårtbitaren kan kom- cies interactions from consumer-resource theory: ma att dominera i gräsmarker medan M. experimental tests with grasshoppers and plants. brachyptera kan bli tillbakaträngd till torrare – Oecologia 94: 516-527. och mindre föredragna habitat såsom hedland- 132 skap och skogskanter.