and RUSSEL-SMITH, 1971; SENSTAO et al., 1977; SEO- ... At each site, three emergence traps and two ... ficantly different from site to site; 65% of the imag-.
NETHERLANDSJOURNALOFAQUATICECOLOGY26(2-4) 269-271 (1992)
TERRESTRIALCHIRONOMIDAE:CONTRIBUTIONOF LOCALEMERGENCE TO GLOBALAERIALFLOW IN A HETEROGENEOUSENVIRONMENT
Y.R. DELETTRE
KEYWORDS:Chironomidae; emergence; dispersal; colonization; life history tactics; landscape heterogeneity.
ABSTRACT Terrestrial Chironomidae were studied for one year in four closely related habitats (pondbanks, woodland, grassland, heathland). Community structure and emergence phenology are analysed and compared with the aerial flow composition and timing. The contribution of local emergence to aerial flow is estimated using Principal Component Analysis with Instrumental Variables (PCAIV). Results are discussed, taking in account larval capabilities, adults behaviour and landscape heterogeneity.
INTRODUCTION Terrestrial Chironomidae have always been understudied. Chironomidologists have focused their studies on aquatic habitats where most species occur. On the other hand, the small size of terrestrial species and taxonomic difficulties has deterred soil ecologists from undertaking detailed studies. Thus, except the work of STENZKE(1950), very few ecological studies are available (HEALEY and RUSSEL-SMITH,1971; SENSTAOet al., 1977; SEODON,1986). A previous study (OELE'n'RE,1984) on terrestrial midges of heathlands in Brittany (France) showed that only three species formed the core of the community, viz. Smittia sp. 1, S. pratorum G. and Pseudosmittia /ongicrus (K.). But, through time, it became obvious that other species attempted to colonize heathlands on several occasions (Limnophyes minimus (Mg.), Gymnometriocnemus (Rhaphidocladius) bruma/is Edw., Bryophaenoc/adius subverna/is Edw.). Depending on sites and. years, they either failed or succeeded in establishing new populations on heathlands. This fact induced the present study, which investigates the dispersal strategies of species and their interactions with habitat heterogeneity. Some preliminary results dealing with 269
Chironomidae and Empididae have already been published (DELETTRE eta/., 1992). Due to space limitations, the present paper must be considered as an extended summary of the lecture given in the XIth International Symposium on Chironomidae rather than a detailed publication. STUDY SITESAND METHODS The selected area consists of four different biotopes bordering an acid, oligotrophic pond in central Brittany, viz. the pondbanks, a woodland, a small grassland and a dry heathland. All these biotopes are in close proximity and can be considered as part of a single landscape. The maximum distance between the pondbanks and the dry heathland does not exceed 50 meters. At each site, three emergence traps and two yellow water traps were used for two years. Emergence traps give a reliable estimate of the emergence flow of species really living in the soils. Yellow water traps attract flying adults. Their efficiency was tested by BAILLIOTand TREHEN(1974) who reported that chironomid midges are always trapped when flying, whatever the behavior involved (swarming or dispersal). The comparison of the relative abundance of
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species in the two sets of traps provides an estimate of the part played by local emergence in the global aerial flow. This indirect technique was chosen because previous attempts to mark adults with fluorescent micronized dust failed, due to increased predation on marked individuals and their high dilution in natural (unmarked) populations. RESULTS Only the first year of trapping will be considered below. 67 species were caught, of which 52 were aquatic and only 15 species were terrestrial. Among them, only 6 species were abundant: B. subvernalis, G. brumalis, L. minimus, Pseudosmittia angusta (Edw.), $. pratorum and Smittia sp. 1. Emergenceflow During the first year of trapping, the number of emerging adults of terrestrial species was significantly different from site to site; 65% of the imagines were caught on the pondbanks, 18% on the heathland, 11% in the woodland, and only 4% in the emergence traps of the dry heathland. Principal Component Analysis (PCA) performed on total unweighted emergence data showed three highly contrasting communities (pondbanks, woodland, heathland) while the species composition on the grassland was intermediate between those of the pondbanks and heathland. Differences in the structure of these communities were pronounced. For instance, P. angusta occurred only on the pondbanks, G. brumalis was mainly found in the woodland but also (in small numbers) in the three other sites. The case was similar for L. minimus. Smittia sp. 1 emerged from the dry heathland only. Aerial flow Yellow trapping provided very different results. Flying individuals of terrestrial species, considered as a whole, were quite evenly distributed above the four sites (26%, 25%, 22% and 27% on the pondbanks, woodland, grassland and heathland, respectively) and a strong intermingling of midges seemed to occur. But, in fact, the contribution of each species to the aerial flow was very different. P. angusta did not fly away from the pondbanks. Flying adults of Smittia sp. 1 were only found above the dry heathland. On the contrary, G. brumalis and L. minimus were caught flying above
the whole transect, while S. pratorum flew mainly above its two larval habitats, viz. the heathland and the grassland. It is not possible to compare absolute numbers between emergence traps and yellow traps because their efficiencies are different. However, the relative abundance of each species in the two kinds of traps provided interesting differences. Considering all traps in all habitats, G. brumalis, which accounted for 75% of the aerial flow, did not exceed 15% in the emergence traps. On the contrary, P. angusta accounted for 40% of the emergence but only for 5% in the yellow traps. The same result was obtained for Smittia sp. 1 while the relative abundances of L. minimus and S. pratorum were of the same magnitude in the emergence traps and the yellow traps. The contribution of local emergence to global aerial flow was estimated, using PCA with instrumental variables (SABATIERet al., 1989). Emergence from the four sites accounted for 84% of the total variance in aerial flow (spatial variance among habitats). As expected, midges emerging from the woodland contributed the greatest part; 76% of the total aerial flow variance could be explained by the emergence from this site. This result was induced by G. brumalis, the most abundant species in the aerial flow, which mainly emerged from the woodland. When this species was neglected, the part of aerial flow variance explained by local emergence decreased to 41%. CONCLUSIONS It is not yet possible to distinguish between effective colonization and transit of adults. The real impact of dispersal will be more accurately estimated by examination of the second year of trapping and comparison with the observed dispersal on the first year. However, the above mentioned results suggest different strategies. (a) G. brumalis seems an invading species which is able to colonize numerous habitats from a single site. (b) On the contrary, Smi~'a sp. 1 and P. angusta exhibit a 9very limited dispersal. This behaviour increases the isolation of the different populations. (c) The larvae of L. minimus live in wet soils and cannot withstand dryness. Adults are widespread at the landscape level. This species can switch from one habitat to another one, depending on mesologic fluctuations (DELETTRE,1986). Thus, the impact of habitat heterogeneity on populations dynamics appears to be different for species, depending on their abilities to fly and on
Terrestrial Chironomidae in heterogeneous environment their behaviour. Species do not respond to landscape heterogeneity in the same way. In any case, it is necessary to have a good knowledge of their life-history tactics to define the impact of habitat fragmentation on their dynamics. Furthermore, chironomid communities cannot be studied at a site without taking into account the role of dispersal and the surrounding habitats; community structure in a given site can be affected simultaneously by the fluctuations of mesologic conditions at that site and by the heterogeneity of these factors at the landscape level.
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ACKNOWLEDGEMENTS We are indebted to Dr. P. Vernon and to reviewers for fruitful comments on the manuscript. This research is part of a Landscape Ecology program financially supported by the French 'Minist~re de I'Environnement', SRETIE grant nr. 87357.
REFERENCES BAILLIOT, S. and P. TREHEN, 1974. Variations de I'attractivitd des pi~ges colorGs de Moericke en fonction de la Iocalisation spatiotemporelle de I'~mergence, des comportements sexuels et des phases de dispersion de quelques esp~ces de Dipt~res. Ann. Zool. EcoL Anita., 6: 575-584. OELETTRE,Y.R., 1984. Recherchessur les Chironomides (Diptera) ~ larves ~daphiques: biologie, ~cologie, m~Jcanismesadaptatifs. Th~se Doctorat d'Etat. Universit~ de Rennes I. DELETTRE, Y.R., 1986. La colonisation de biotopes multiples, une alternative ~ la r~sistance in situ aux conditions mGsologiques d~favorables. Cas de Umnophyesminimus (Mg.), Oipt~re Chironomide ~Jlarves 6daphiques des landes armoricaines. (Rev. Ecol. Biol. Sol, 23: 29-38. DELETrRE, Y.R., P. TREHEN and P. GROOTAERT,1992. Space heterogeneity, space use and short dispersal in Diptara.-A case study. Landscape Ecology 6:175-181. HEALEY, I.N. and A. RUSSEL-SMITH, 1971. Abundance and feeding preferences of fly larvae in two woodland soils. Proc. Vth Colloquium Pedobiologiae, Dijon (France), pp. 177-193. SABATIER, R., J.D. LEBRETONand O. CHESSEL, 1989. Principal Component Analysis with instrumental variables as a tool for modelling composition data. In: R. Coppi and S. Bolasco, ed., Multiway data analysis. Elsevier Science PubL, North Holland. SEDDON, A.M., 1986. Abundance and life-history of four species of terrestrial Chironomidae (Diptera) from deciduous woodland soil in South East England. (Ent. month. Mag., 22: 219-228. SENSTAD, E., O. SOLEM and K. AAGAARD, 1977. Studies of terrestrial Chironomidae (Diptera) from Spitzbergen. Norw. J. Ent., 24: 91-98. STENZKE, K., 1950. Systematik, Morphologie und Okologie der terrestrischen Chironomiden. Arch. Hydrobiol. Suppl., 18: 204-414.
Address of the author.
Rennes I University, Paimpont Biological Station, Soil Ecology and Populations Biology Laboratory (U.A. 696, C.N.R.S.), 35380 PIElan-le Grand, France.
