northwestern Switzerland, in a mosaic landscape of mixed intense agriculture ..... example of mutual influence is the carabid Pterostichus melanarius in Fig. 4.
Biological Conservation 54 (1990) 193-207
~
Population Movements of Arthropods between Natural and Cultivated Areas Peter Duelli Swiss Federal Institute of Forest, Snow and Landscape Research, Department of Landscape Ecology, CH-8903 Birmensdorf, Switzerland
Michel Studer, Iris Marchand & Simone Jakob Zoological Institute, University of Basel, Rheinsprung 9, CH-4051 Basel, Switzerland (Received 27 February 1989; revised version received 24 January 1990:
accepted 8 February 1990) ABSTRACT 'Edge permeability' between habitat patches in a mosaic landscape of mixed intense agriculture and semi-natural areas was investigated with directional trap devices along field borders and in a 300-m long transect through crop .fields. pasture, wetland and a dry meadow. Almost all identified arthropod species perjbrmed population exchanges over the field borders. Species abundances clearly depend on habitat quality rather than area or distance to related habitat islands'. Most insect species collected in stick), traps over crop fields were never or rarely encountered in the vegetation layer. Surface-dwelling .flightless species showed gradual transitions over the field borders. 'Hard edge'-species tended to be specialists for undisturbed perennial habitats, while 'soft edge'-species with a d(ffuse distribution were mainly associated with annual crops. It is concluded that in cultivated areas a mosaic landscape of smalLsized crop fields and semi-natural habitats maximizes arthropod diversity and decreases the probability for overall extinction even of rare species.
INTRODUCTION F r a g m e n t a t i o n and insularization of natural habitats in heavily populated and cultivated landscapes create increasing concern among ecologists
193 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier SciencePublishers Ltd, England. Printed in Great Britain
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involved in conservation biology. The possibilities for genetic exchange between subpopulations in insular habitats are of vital importance to the survival of local populations. While there is a vast number of investigations on the temporal aspects of population dynamics, we largely ignore its spatial aspects, the population movements of faunal communities in cultivated areas. Among the arthropods, the best-documented species are those considered as pests or beneficials in agriculture and medicine (cf. Johnson, 1969). These are usually highly mobile species, well-adapted to living in the temporary habitats of our 'cultural steppe' (Rabb & Kennedy, 1979). However, nature conservation is mainly concerned with the less-adapted forms, the large number of species which have become rare due to the drastic changes of land use in recent times. As part of an ongoing project with the aim to assess the influence of natural and semi-natural habitats on the fauna in cultivated areas, we here report on local population exchanges of arthropods over the field borders, where the 'edge permeability' (Stamps et al., 1987) of habitat patches largely determines the extent of gene flow among local populations.
MATERIAL A N D METHODS The study site is located in the northwestern Switzerland, in a agriculture, industry, settlements exchanges over the field borders devices as follows.
Rhine valley 30km east of Basel, in mosaic landscape of mixed intense and semi-natural areas. Population were monitored with directional trap
(1) Sticky grids, consisting of wire-mesh squares ( l m 2) mounted at different heights on 7-m high wooden poles. One pole with a catching area of 7 m 2 w a s placed in the middle of each side of a rectangular maize field (see Fig. 3). The traps were smeared with a water-resistant insect glue (Tanglefoot Co., Michigan, USA) on both sides and replaced weekly (Kokubu & Duelli, 1986). The traps were used in 1983 and 1984 from mid-May to the end of September; (2) Directional pitfall traps for surface-dwelling arthropods, consisting of two parallel troughs of 150 cm length (Fig. 1). From early May to the end of September, four such traps were placed close to the sticky traps along the field borders of a maize field, with two additional traps being used as controls in the centre of that field. 'Edge permeability' was recorded with a transect consisting of 35 traps arranged in a line perpendicular to the field borders. The pitfall traps used in
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RESULTS
Directional traps along the field edges Transport by air covers large distances per unit time and thus contributes most to the population exchange on a regional or local scale. In 1983, directional sticky traps and directional pitfall traps were placed on the four sides o f a maize field o f 1.6 ha. An overview on the 'aerial plankton' flying over the maize field up to a height of 7 m is given by K o k u b u (1986). Rates o f immigration and emigration were calculated for selected species. An example is shown in Fig. 2, where the net emigration of the most abundant coccinellid beetle in maize peaks shortly after the m a x i m u m emergence o f adults in the field. F r o m the n u m b e r o f pupae monitored in the field by weekly sampling of 100 maize plants, a production of up to 6000 adult Propylaea 14-punctata/ha/day was estimated, based on an estimated 80 000 maize plants/ha and a period of pupal development o f 6 days at 20°C. F r o m the results o f the sticky grid catches, which cannot be given here in detail, the following conclusions were drawn. Most insect species flying over
Peter Duelli et al.
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the maize field up to a height of 7 m are never encountered within the level of the crop plants. Even among the well-known aphidophagous families (Syrphidae, Coccinellidae, Chrysopidae), where a total of 42 species were collected in flight over maize, only 6 species were found to develop in the maize field. In most insect species capable of flight, the number of individuals flying over one hectare of maize per day is larger than the number of adults present in the crop at any one moment. The results of the directional pitfall traps are shown here with the data for carabids and spiders, two arthropod groups which predominantly disperse on the surface (Fig. 3). While population movements of carabid beetles (dashed wider arrows) consistently exhibited a net immigration (shown as
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Fig. 3. Numbers of carabids (hatched wider arrows) and spiders (smaller white arrows) collected in directional pitfall traps along the border and in the centre of a maize field (160m × 100m, up is North). The black portion of the arrows indicates net population movement in the dominant direction, i.e. net immigration.
black portion of arrow) in the range of 6-28%, the migration of spiders (smaller white arrows) seems to have a consistent tendency of similar magnitude to move from east to west. This movement follows the nightly winds in the region, but is opposite to the prevalent westerly winds during daytime. For the period May-September 1983, 57 species ofcarabids and 78 species of spiders have been identified. The net immigration rate of about 20% in carabids is only due to the medium-sized and smaller species, while the migration rates of species from the larger genera Carabus and Pterostichus are more balanced. In several years of observation, the numbers of carabids in wheat and barley have always been higher than in maize. This may explain the strong influx from the east. A detailed analysis of the seasonal migration phenology of the most abundant species"shows that the heavy immigration especially from the east is not caused by the wheat harvest in late July. A significant barrier effect of the two types of roads in Fig. 3 is visible: the 3-m wide dirt road to the south of the maize field reduced population exchanges by 47% for spiders and 49% for carabids. The 6-m wide tarred road in the north had an even stronger effect, with a reduction of 57% for spiders and 60% for carabids. On the other hand, the influence of a broad grassy field border in the west, between two maize fields, was inconsistent. The number of immigrant carabids was 41% higher than in the control traps in the field centre, but there was no significant increase in the spider influx.
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Transect perpendicular to the field borders In 1985, a transect based on different collecting methods was laid through a wheat field (the former barley field o f Fig. 3) and some adjacent semi-natural habitats. The focus here is on the results from funnel pitfall traps for carabids, staphylinids and spiders. The underlying idea for a 300-m long row of traps through crop fields and their surroundings is to visualize the effect of edge permeability on both sides of a biotope border. Sixty species ofcarabids, 79 species of staphylinids (without the subfamily of the Aleocharinae) and 89 species of spiders were collected from 2 M a y to 15 August. O f these, 113 (50%) were abundant enough (more than 20 specimens) to allow for a specific distribution analysis along the transect from an area of wetland through a pasture and the wheat field into a dry meadow (Fig. 4). The distribution patterns, especially of flightless species, often show gradual changes between the preferred habitat and the 0
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neighbouring areas. In Fig. 4 the two species to the left seem to prefer the intensely cultivated wheat field, with a limited dispersal into the less disturbed biotopes, whereas the two species to the right clearly prefer the perennial vegetation. The carabid beetle Harpalus azureus is a 'grass ecotone species' with immigration both into wheat and the semi-natural areas. The lycosid spider Pardosa pullata, as opposed to the closely related Pardosa agrestis, is mainly a species of natural or semi-natural habitats, with occasional excursions into all other biotope types. In most cases the patch sizes were too small to allow for a graphical estimation of the spatial extent of mutual influences (see + d and - d in Fig. 5). Most examples of very limited dispersal (less than 50 m) stem from spiders, while many carabids and staphylinids with the possibility to fly show inconsistent or diffuse distribution patterns. In 12 out of the 15 most abundant flying carabid species there was no correlation between numbers of specimens caught in pitfall traps and those caught in flight on sticky traps in the same habitat. Wherever possible, each species was assigned to one of six types of border transitions defined in Fig. 5. 'Hard edge' means that there is no measurable result of population exchange between neighbouring patches. On the other hand, there are several possibilities for 'soft edges' between natural areas and i~/ h a b i t a t b o r d e r preferred habitat
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Peter Duelli et al.
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cultivated fields. Category 2 in Fig. 5 contains species which usually do not leave their preferred habitat and even avoid the border areas. Quite the contrary is true for the category 3 species. They thrive in their preferred habitat and invade the adjacent areas to various degrees. In many cases both these effects happen at the same time, depicted as category 4 in Fig. 5. An example of mutual influence is the carabid Pterostichus melanarius in Fig. 4. Only a few cases of an 'ecotone' effect were observed, where the border areas between two biotope types seem to be the preferred habitat (see the carabid H. azureus in Fig. 4). 'No edge' (category 6) stands for a more or less even distribution in most habitats, which can either be the result of a highly mobile (e.g. flying) species, or an ubiquitous species which can develop everywhere. For all 113 species with a total of more than 20 specimens collected, an index of 'naturalness' was calculated on the basis of catches from an equal number of traps in semi-natural (perennial) habitats and annual crop fields. Species with more than two-thirds of their individuals collected in more or less undisturbed perennial habitats (wetland, pasture, grassy slope and dry meadow) are considered 'natural' (n in Fig. 5), species with more than twothirds in annual crop fields (wheat, rape, maize) as 'agricultural' (c in Fig. 5). It was not possible to assign 16 of the 113 species ofcarabids, staphylinids and spiders to one of the categories for edge permeability. Of the remaining 97 species almost a third were considered to belong to category 6. On the right hand side of Fig. 5 the last column displays the total number of species O c
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Arthropod populations in natural and cultivated areas
found in each category. These figures are further divided into species numbers for 'natural' (n), 'agricultural' (c) and 'intermediate' (i) species. The most obvious differences turned up between categories 1 and 6. 'Hard edges', as a sign of almost no population movement across the habitat borders, tend to be a characteristic of species with an index of'naturalness' of 96%, with two exceptions--a staphylinid and a carabid beetle with equally extreme indices as low as 2 and 15%. An almost unlimited permeability, however, appears to be the privilege of 'agricultural' and 'intermediate' species. In the categories with negative, positive or mutual influences there are no obvious correlations with habitat preference. There is, however, a predominance of both 'natural' or 'agricultural' species compared to 'intermediate' ones. In order to visualize the local dimensions of mutual influences between neighbouring habitats, the data for either 'natural' or 'agricultural' species were summed (Fig. 6). The gradual decrease of 'naturalness' from both sides into the wheat-field is visible especially in spiders, but also the decreasing negative effect of the crop field on 'natural' species into the semi-natural areas. Thirty of the 42 most abundant spider species were collected in the wheat field. F r o m their specific distribution patterns we can conclude that 18 (60%) would most probably not have been present there without neighbouring semi-natural habitats. One example is Pardosa pullata shown in Fig. 4. The situation in carabids and staphylinids is slightly different. In carabids (Fig. 7) the distribution o f ' n a t u r a r species shows two maxima in the ecotone O. O
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seem to owe their presence in the wheat field to the neighbourhood of seminatural habitats. The staphylinid beetles, although totalling 79 species in the whole transect, only contribute 22 species with more than 20 specimens. Of these, 18 were collected in wheat, and for four (22%) the distribution in the transect allows the assumption that their presence in wheat was due to immigration from semi-natural areas. A large number of rare species, especially staphylinids, may depend at least temporarily on undisturbed habitats, but their sporadic occurrence in the transect is not sufficient for a firm statement. Yet another method to visualize the effect of local faunal exchange between semi-natural habitats and cultivated fields is shown in Fig. 8. For the four major habitat types (wetland, pasture, wheat, dry meadow) the data of their central trap are compared with all other traps in the transect with the modified SBrensen index of similarity. Additionally, rows of five traps in a rape and a maize field at distances of 150 and 200 m to the transect area are included in the analysis. The higher the bars in Fig. 8, the more similar is the fauna to that of the trap with index 1"0. The different mobility in the three arthropod groups is reflected in their similarity distributions along the transect. The spiders show the highest homogeneity within each single habitat, but stronger dissimilarities between habitats. In the staphylinids, with the largest percentage of species capable of flight, the average index of similarity is highest, but also the variability within single habitats.
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Similarity indices along the transect over the field borders can either change gradually or stepwise. A gradual change indicates local movement, stepwise transitions are the result of a combination o f ' h a r d edges' in some species and a diffuse distribution in others. Comparing the wetland fauna (top transect for each family in Fig. 8) and the fauna in the dry meadow (bottom transect) with the rest of the traps in the transect produces a majority of stepwise transitions. On the other hand, comparing the transect data with the centre traps in pasture or wheat results in a majority of gradual decreases towards the neighbouring biotopes. Gradual decreases on either side of the habitat border are an illustration of mutual influences. Theoretically, in a very long transect, the spatial extent of local faunal exchange of non-flying species can be localized where the similarity indices are levelling off. Most organisms, however, disperse too far and too fast to affect the transitions in a transect as short as 300m. Comparing the transect data with those of rape and maize reveals some astonishing differences in similarities. The Staphylinidae show a high similarity between wetland and maize, which is not at all the case in spiders and carabids. In spiders, on the other hand, the species composition in wheat and rape is extremely similar, which again is not the case in the two other groups. By far the most special fauna is that of the dry meadow. No sign of lateral influences is visible, although on the species level there are several examples of beetles and spiders which are clearly xerophilous immigrants into the wheat field. The grassy areas (pasture and slope) have an obvious local influence on neighbouring biotopes, but the dominant influence clearly emanates from the wheat field. In the two beetle families with some highly mobile species this influence extends over the grassy areas into the semi-natural biotopes.
DISCUSSION Agricultural areas offer a diversity of mostly temporary and patchy habitats. Population survival of arthropods thus depends very much on dispersal and colonization. The dominating theoretical concept dealing with gene flow between subpopulations in habitat islands is the equilibrium theory of insular biogeography (MacArthur & Wilson, 1967). But evidence is growing that species numbers and abundances in cultivated areas are mainly the result of other factors than patch size and distance from the nearest habitat island (Simberloff & Abele, 1976; Mader, 1983). Habitat heterogeneity was proposed as the determinant factor for diversity by Lack as early as 1969. To date we still do not have enough empirical data to assess the significance of the theories of insular biogeography for the fauna of a cultivated landscape.
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Our sticky trap catches give evidence that the fauna of the 'aerial plankton' above crop fields and semi-natural areas consists of a multiple of the species numbers encountered within the vegetation or on the ground. Almost all flying species were rather evenly distributed over the various habitat types, and even in rare fliers such as the carabids there was no correlation between the numbers of specimens collected in a particular pitfall trap on the ground and in the sticky traps at the same location. Moreover, large numbers of forest insects were collected in the sticky traps positioned several kilometers away from the nearest forest (Huber & Duelli, 1987). In aphidophagous coccinellids, syrphids and chrysopids, the number of daily immigrants and emigrants were higher than the number of adults present in the aphid-infested maize field shown in Fig. 2 (Kokubu & Duelli, unpublished data). Our field data suggest that in any of the habitat patches the turnover rate in most flying species is extremely high (Duelli, 1988). Thus, immigration is not a limiting factor in colonization. In diversified agricultural areas, however, colonization depends on habitat suitability rather than the size or distance from other habitat islands. Even in surface-dwelling arthropods which are unable to fly or rarely do so, population exchanges over field borders seem to be a c o m m o n event, as suggested by the data from directional pitfall traps along the field borders, and from the 'soft edge' transitions in the transect. 'Edge permeability' is highest between crop fields, but strongly impeded by roads (Fig. 3). While the dirt road reduced population movements by almost 50%, the barrier effect of the tarred road was close to 60%. A difference between the influences of dirt or tarred roads was found by Mader et al. (1988). The presence of undisturbed habitats such as wetland or dry meadow has a significant influence on the spider fauna in cultivated land. These seminatural areas contribute at least 60% of the spider fauna found in the wheat field. Carabids and staphylinids, on the other hand, are mainly represented in crop fields and the surrounding grass strips. Population exchanges among cultivated fields seem to be of more importance than the influx from natural areas. For both beetle families the grassy strips around field crops are a source of diversity which influences both annual crops and semi-natural areas. Their importance as overwintering sites for surface-dwelling arthropods has been stressed by Sotherton (1985), Wallin (1985) and others. While it appears that the concept of island biogeography is of limited value for the interpretation of the faunal composition in cultivated areas, it may be applicable to extremely specialized and rather sedentary species, for which their habitat patch is a 'true island in an otherwise hostile environment'. Indeed, 14 of the 16 'hard edge' species found in the transect are exclusive specialists of natural habitats. But they constitute only 6% of the surface-dwelling arthropods identified so far, and this percentage will
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continue to diminish drastically as soon as we consider the whole arthropod fauna including all flying species. Compared to other cultivated areas in Central Europe, the fauna of the crop fields in our experimental compound turned out to be very rich. In five months of sampling, the maize field shown in Fig. 3 yielded 57 species of carabids and 78 species of spiders. One possible explanation is the influx of species from adjacent semi-natural areas. Another explanation is the mosaic structure of diversified land use typical for most of the densely populated regions in Switzerland. On a regional scale, a mosaic of various biotope patches provides a high degree of habitat heterogeneity. For the large majority of moderately to highly mobile organisms, this is an optimal prerequisite for regional survival. Even spatially limited population exchanges contribute to an increase in species richness per patch. But even for rare and rather sedentary species a mosaic landscape may enhance the possibilities for regional survival. Quinn & Hastings (1987) found theoretical and empirical evidence that subdivision into a number of subpopulations may decrease the probability of overall extinction for rare species. This view contradicts the notion that survival is positively correlated with population (and habitat) size (MacArthur & Wilson, 1967; Den Boer, 1981) unless a limited amount of genetic exchange is possible. Our pitfall trap experiments suggest that this is indeed the case even in the vast majority of the flightless surface-dwelling species. The conclusion is that in intensively cultivated areas a mosaic landscape consisting of small habitat patches can yield maximum levels of species richness. Consequently, the conservation or creation of even the smallest patches or strips of natural vegetation can contribute to the protection of genetic resources. We have to keep in mind, however, that local species richness is not always the prime concern of nature conservation. Remnants of endangered habitats such as peat bogs or heathland may subsist in otherwise cultivated areas and contain highly endangered specialized species which cannot cope with an influx of large numbers of ubiquitous species. Fragmentation of such areas may well increase species diversity at the expense of a few, but particularly interesting, species (Mader, 1983; Webb & Hopkins, 1984).
ACKNOWLEDGEMENTS We appreciate the financial support of the Swiss National Science Foundation (grant No. 3.242-0.82) and of the Swiss Federal Office for Forestry and Landscape Protection. We are grateful to Roche AG Sisseln for the permission to work on their properties and for continuous support in all possible ways. We are obliged to Dr A. H/inggi and F. Wittwer for the
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identification of spiders and staphylinids, and to E. Katz, D. Pichler, M. Sebek and E. StSckli for the preparation of figures.
REFERENCES Den Boer, P. J. (1981). On the survival of populations in a heterogeneous and variable environment. Oecologia, Berl., 50, 39-53. Duelli, P. (1988). Aphidophaga and the concepts of island biogeography in agricultural areas. In Ecology and Effectiveness of Aphidophaga, ed. E. Niemczyk & A. F. G. Dixon. SPB Academic Publishing, The Hague, pp. 89-93. Huber, M. & Duelli, P. (1987). Vergleich der flugaktiven Coleopterenfauna 5ber naturnahen Biotopen und lntensivkulturen. Revue Suisse Zool., 94, 525--32. Johnson, C. G. (1969). Migration and Dispersal of Insects by Flight. Methuen, London. Kokubu, H. (1986). Migration rates, in situ reproduction and flight characteristics of aphidophagous insects in maize fields. Doctoral thesis, University of Basel. Kokubu, H. & Duelli, P. (1986). Aerial population movement and vertical distribution of aphidophagous insects in maize fields. In Ecology of Aphidophaga, I!, ed. I. Hodek. Academia, Prague & Junk, Dordrecht, pp. 279-84. Lack, D. (1969). The number of bird species on islands. Bird Stud)', 16, 193 209. MacArthur, R. H. & Wilson, E. O. (1967). The Theoo' of Island Biogeography. Princeton University Press, Princeton. Mader, H.-J. (1983). Warum haben kleine Inselbiotope hohe Artenzahlen? Namr Landschaft, 58(10), 367-70. Mader, H.-J., Schell, C. & Kornacker, P. (1988). Feldwege-Lebensraum und Barriere. Natur. Landschaft, 63(6), 251-6. Quinn, J. F. & Hastings, A. (1987). Extinction in subdivided habitats. Conserv. Biol., I, 198-208. Rabb, R. L. & Kennedy, J. S. (1979). Movement of Highly Mobile Insects: Concepts and Methodology in Research. NCS University Press, Raleigh. Simberloff, D. S. & Abele, L. G. (1976). Island biogeography theory and conservation practice. Science, NY, 191,285-6. Sotherton, N. W. (1985). The distribution and abundance of predatory arthropods overwintering in field boundaries. Ann. AppL Biol., 106, 17 21. Stamps, J. A., Buechner, M. & Krishnan, V. V. (1987). The effects of edge permeability and habitat geometry on emigration from patches of habitat. Amer. Nat., 129, 533-52. Wallin, H. (1985). Spatial and temporal distribution of some abundant beetles in cereal fields and adjacent habitats. Pedobiologia, 28, 19-34. Webb, N. R. & Hopkins, P. J. (1984). Invertebrate diversity on fragmented Calluna heathland. J. Appl. Ecol., 21, 41 6.
