Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Habitat conservation wikipedia , lookup
Occupancy–abundance relationship wikipedia , lookup
Biodiversity action plan wikipedia , lookup
Ecological fitting wikipedia , lookup
Island restoration wikipedia , lookup
Introduced species wikipedia , lookup
Latitudinal gradients in species diversity wikipedia , lookup
Biologzcal Journal of the Linnean SocieQ (1987) 31: 59-74. With 1 figure Plants and insects in early old-field succession: comparison of an English site and an American site VALERIE K. BROWN Department o f Pure and Applied Biology, Imperial College at Silwood Park, Ascot, Berks SL.5 7PT, U.X. STEPHEN D. HENDRIX Department of Botany, University of Iowa, Iowa City, Iowa 52242, U.S.A. AND HUGH DINGLE Department of Entomology, University of California Davis, Calzfornia 95616, U.S.A. Received 3 October 1986, accepted f o r publication 26 January 1987 The plant and insect communities of early, secondary succession beginning with bare ground in an Old World site (southern Britain) and a New World site (Iowa, U.S.A.) shared a number of characteristics. Both sites showed similar temporal patterns of plant species cover and species richness, although overall richness was greater at the Old World site. Annuals dominated at both sites during the first year of succession and were largely replaced by perennials in the second year. Monocotyledons were more abundant at the Old World site, especially in the second year. The two sites differed markedly in the contribution of native and introduced plant species, with the Old World site dominated by natives and the New World site by alien plant species. Insect herbivore load was greater at the Old World site, when expressed in terms of structural complexity of the vegetation, suggesting that there may be major differences in the influence of herbivores on the direction and rate of succession at the two sites. KEY WORDS:-Arthropods vegetation structure - early succession - herbivores - plants ~ transatlantic variation - CONTENTS Introduction . . . Materials and methods. . . Sites.. Site preparation . Sampling. . . 0024-4066/87/050059 . . . . . + 16 $03.00/0 . . . . . . . . . . . . . . . . . . . . . . . . . 59 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 61 61 61 62 0 1987 The Linnean Society of London V. K. BROWN E T A L . 60 Data analysis . . . . . R e s u l t s . . . . . . . Plant cover and species richness . Plant species composition . AIthropod composition . . . . . . . . Discussion. Acknowledgements . . . . Rrfert.ncrs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 63 63 64 70 71 73 73 INTRODUCTION The concept of succession in communities is one of the oldest in ecology, yet the underlying processes are among the least understood. This lack of understanding has been summed up by Horn (1981) with the comment that “the only sweeping generalization that can safely be made about succession is that it shows a bewildering variety of patterns”. I n this paper we report the first of a series of comparative and experimental studies in which we attempt to assess successional trends in two widely separated sites, southern England and Iowa, C.S.A. We ask whether common ecological patterns are discernable in the two sites as MacMahon (1981), for example, suggests, or whether Horn’s “bewildering variety” is found in this case. O u r studies are of the very earliest stages of secondary succession so that we can control the starting point, freshly harrowed bare ground, at each of our sites. There are relatively few experimentally replicated studies of the very early stages of succession. Rather, most have attempted to analyse naturally established successional gradients (e.g. Cowles, 1899; Shelford, 1913; Olson, 1958) or events following abandonment of cropland (Oosting, 1942; Evans & Dahl, 1955; Murdoch, Evans & Peterson, 1972; Bazzaz, 1975). There are also few studies which attempt to integrate changes in and interaction between both the flora and the associated fauna during succession. Where both have been considered, it has usually been at only one stage (Murdoch et al., 1972; Gwynne & Bell, 1968; but see Brown, 1982a, b, 1984, 1985; Brown & Southwood, 1983, in press). Furthermore, there is a general lack of comparative work in different geographical regions because of the obvious difficulties in co-ordinating the necessary efforts. O u r primary aim is to compare general patterns of secondary colonization of experimentally prepared bare ground by vascular plants and arthropods in southern Britain and Iowa, U.S.A. We shall emphasize patterns of plant and arthropod species composition, plant structure, and life-forms rather than the details of‘ specific differences which might well be a consequence of site attributes. Given the differences in climate and soil between the two sites, we expect such differences to occur; one of our objectives is to determine if succession follows similar patterns in spite of such differences. An important aspect of these studies is the inclusion of the above-ground arthropod fauna in terms of total density and guild structure. We consider insects primarily, although other arthropods are mentioned when they occur. An added dimension to our studies is that the two sites differ drastically in the proportions of (native’ and introduced plant species. With European colonization, large numbers of Old World ruderals were introduced into North America and now make up, as we demonstrate, a substantial part of early TRANSATLANTIC VARIATION IN SUCCESSION 61 successional communities. These introductions perforce constitute a large 'natural' experiment, but one of which ecologists seem to have taken little advantage. Here we discuss the differing proportions of natives and introductions in terms of the patterns of early plant succession and arthropod composition, especially as it relates to herbivore load. MATERIAL AND METHODS Sites Iowa, U . S . A . The Iowa site is located in Johnson County, Iowa on the Oakdale Campus of the University of Iowa (41"42'N, 91"36'W, altitude 241 m), about 12 km west of Iowa City. T h e area chosen was a strip of grassland dominated by Bromus inermis Leyss. lying between a single lane gravel road on the east and a small second growth copse on the west dominated by Acer negundo L. and Robinia pseudoacacia L. A short distance from the northern edge is an area of lawn with large oaks (Quercus alba L.) and maples (Acer saccharurn Marsh.). The southern edge is pasture, and the surrounding area is a mixture of pasture, cropland, copse and abandoned fields extending for several kilometres in most directions. Except for the Oakdale Campus, the area has been farmed more or less continuously since the latter part of the 19th century. The soil is predominantly clay and loess with additional components derived from glacial till and underlying Devonian limestone (Pryor, 1976). T h e climate is temperate continental, characterized by dry, cold winters and hot, wet summers; transitions in spring and autumn are relatively rapid. Silwood, U.K. The southern Britain site is located at Imperial College at Silwood Park in Berkshire (51" 21" and 0" 39'W, altitude 91 m ) . The site chosen is part of a long-standing arable area immediately surrounded by mature, grassland on three sides with oak (Quercus robur L.) and birch (Betula pendula Roth. and B. pubescens Ehrh.) scrub on the east side. Nearby vegetation is a mixture of arable crops, acidic grassland and woodland. The soil is derived from Bagshot Sands and gravel of Eocene Bracklesham beds. The climate is temperate maritime with relatively little variation in rainfall throughout the year and far more equable temperatures than in Iowa. Site Preparation In the autumn of 1983 both sites were treated with weed-killer ('Round up' ( = Tumbleweed), Murphy Chemical Limited, Wheathampstead, St. Albans, Hertfordshire, U.K.) to destroy perennial weeds and then shallow ploughed and harrowed in mid-March. T h e vegetation was then left to develop naturally. Each site measured 405 m 2 and was subdivided into 45 subplots, 3 x 3 m, arranged in a 9 x 5 pattern. The sites were 6 weeks 'old' at the time of the first sample in early May. V. K. BROWN E T AL 62 Sump ling Six samples of plants and macro-invertebrates were taken at monthly intervals (at the end of the first week of the month or as close as weather conditions allowed) from May to October in both 1984 and 1985. The vegetation was sampled using pins. Each pin (3 mm diameter) was marked off at intervals of 2, 4,6, 8, 10, and successive 5 cm intervals from soil level to the maximum height of the vegetation. At the Silwood site, 30 pins were positioned at random and lowered vertically through the vegetation in each subplot on each sampling occasion. For 10 pins the number of touches of each species at each height interval was recorded, while for the remaining 20 pins only presencelabsence of plant species touching the pin was noted. At the Iowa site, sampling during the first year was identical to that at Silwood. Because of the increased vegetational complexity the second year, the total number of pins thrown was reduced to 20. In May and June, ten height-interval profile pins and ten presencelabsence pins were thrown, while in the remaining four samples (July-October) five height-interval profile and 15 presence/absence pins were recorded. The arthropods were sampled with a D-vac suction apparatus by identical methods in both sites. I n the first year three samples were taken from each 3 x 3 m subplot, the position of each sample being at random. The apparatus was held in position for 0.5 min and afterwards the sampled area was carefully searched by eye for any large arthropods which had not been sucked up. Samples for three adjacent subplots were combined, thereby giving a total of 15 samples each month. Sampling the second year was reduced to one suction per subplot and samples from five subplots were combined, thereby giving a total of nine samples each month. T h e insects were separated from plant debris in the laboratory, preferably while still alive, and stored in 70% alcohol prior to sorting and identification. Data analysis Plants Importance values for each plant species at each sample date were calculated as follows: Frequency = number of subplots containing a species total number of subplots sampled ( = 45)’ Relative Frequency = Density = frequency for a species sum of the frequencies for all species ’ number of individuals of a species , area sampled ( = 405 m 2 ) Relative Density = density of a species sum of the densities of all species ’ TRANSATLANTIC VARIATION IN SUCCESSION Relative Cover = 63 number of touches of a species from height-profile pins total number of touches of all species from height-profile pins’ Importance Value for a Species = Relative Frequency Relative Cover. + Relative Density + The maximum possible importance value for a species was thus 300. In the calculation of density we make the assumption that when there is more than one touch of a given species for a given height-profile pin, only one individual plant has been touched. Likewise, we assume that the presence of a species at a given presence/absence pin indicates that only one individual has been touched. Our personal observations over two years suggest that these assumptions are realistic. Arthropods Arthropods were sorted into taxa (mainly to order) and then into groups characterized by their feeding habits: for example predators, herbivores, decomposers. Total number of arthropods or number per subcategory were calculated either on a square metre basis, extrapolating from the area of the DVac sampling cone, or on a basis which took into account a measure of plant structure, since this is an important plant community feature affecting arthropod diversity (Lawton, 1983; Stinson & Brown, 1983). As an index of the influence of structure, the mean number of insects per metre square determined for the monthly sample was divided by the number of vegetation height categories (5 cm intervals) on the sampling pins in which vegetation occurred that month. RESULTS Plant couer and species richness Differences in temporal patterns of change in cover at the two sites was evident in the first season (Table 1). I n May of the first year at the Iowa site, the cover was only 10% and reflected the fact that plants were exclusively seedlings, most of which were still in, or were barely past, the cotyledon stage. High levels of cover were not obtained until July. I n contrast, at the Silwood site higher cover values occurred earlier in the season. No difference in cover between sites was apparent in the second year. Species richness was greater a t the Silwood site when seasonal totals are considered (Table 1 ) . The two sites were similar during the height of the growing season in mid- to late summer Uuly-September) in the first year, but, with the exception of October, there were between 11 and 14 more species at the Silwood site in the second year. The temporal pattern of increase in species richness was equivalent at the two sites. By May of the first year, 25% of the season’s species richness was present at the Iowa site and 32% at the Silwood site, while by June 50 and 63% of species richness was present at the Iowa and Silwood sites, respectively. A more rapid autumnal transition at the Iowa site V. K. BROWN E T A L 64 Table 1. Percentage cover and species richness of vegetation during the first 2 years of secondary succession in the U.K. and Iowa, U.S.A. Species richness Percentage cover Year 1 .May June .July August September October Silwood, U.K. Iowa, U.S.A. Silwood, U.K. Iowa, U.S.A. 45.0 94.2 99.6 99.6 99.8 99.4 10.0 75.6 94.9 98.6 94.2 94.2 21 41 40 41 39 50 12 24 38 40 42 36 65 48 46 58 52 56 54 41 35 46 38 44 42 44 78 61 Total for season Year 2 May June July August Septembei Ortober 93.6 100 100 I00 100 100 98.3 100 100 99.7 98.8 98.8 Total for season was also evident, since the number of species present declined 14% from September to October while at the Silwood site there was a 22% increase in species richness during the same period. During the second year, no autumnal decline in species richness occurred a t the Iowa site, while at the Silwood site there was a 31% decline in species number from September to October. Plant species composition In spite of the number of introduced plants in Iowa, there were major differences in the actual plant species composition between the two sites with only ten of the 163 species encountered over 2 years common to both. Probably of more ecological significance, however, are the trends in the major subdivisions of the vegetation. Native and introduced species The sites differed dramatically in the proportion of native plant species (Table 2), with far fewer native species present in the Iowa site than at the Silwood site. At Silwood, native species contributed over 90" of the total seasonal species richness each year, while at the Iowa site native species comprised only 40% of the total seasonal species richness the first year and 60% the second year. O n the basis of the relative contribution of species to community structure, as measured by importance values, the pattern is even more distinct (Table 2). At the Silwood site, native species generally represented in excess of 90% of the total importance value both for any given month and for the season. The slightly lower values for July and August of year 2 were due to the presence of Conyza canadensis (L.) Cronq., the only introduced species at the Silwood site which contributed substantially to vegetational composition (See TRANSATLANTIC VARIATION I N SUCCESSION 65 Table 2. Comparison of the contribution of native species to the early successional flora in the U.K. and Iowa, U.S.A.* Percentage of species Year 1 May June July August September October Total for season Year 2 May June JU ~ Y August September October Total for season Percentage of total importance value (see text) Silwood, U.K. Iowa, U.S.A. Silwood, U.K. Iowa, U.S.A. 81.0 90.2 87.5 87.8 87.2 90.0 33.3 41.7 44.7 47.5 45.2 50.0 94.2 94.6 96.4 94.5 95.4 93.5 5.2 12.8 21.4 26.4 35.4 32.8 92.3 41.7 94.8 22.3 91.3 89.7 92.3 89.3 94.4 92.7 48.2 56.1 54.6 59.0 54.1 56.8 96.0 92.2 89.5 89.3 93.7 98.3 22.7 29.4 32.8 42.7 62.0 59.6 92.3 60.0 94.0 41.5 *Determination of native status for U.K. plant species from Clapham, Tutin & Warburg (1962) and for U.S.A. plants from Shetler & Skog (1978). Table 4). At the Iowa site, native species represented no more than 35% of the total importance value in any given month in the first year; and, even though the contribution of natives increased during the second year, the total for the season was still less than half the Silwood value. The thoroughly mixed nature of the assemblage of species contributing to early succession in the New World is quite evident, as is the predominance of introductions among the ‘pioneer’ species present in the first year. L$e form The pattern of importance values of plants of different life form indicates that at both sites there was a marked decline over time in the importance of annuals (Fig. IA). This is particularly apparent at the Silwood site where in the first year annuals made up 80-100~0of the total, but contributed less than 10% by the end of the second year. The pattern was similar at the Iowa site, but the decline was less marked: from 85 to 32% during the first year with little change during the second year until a decline to 15% in September and October. Concomitant with the decrease in annuals, perennial species increased a t both sites over time (Fig. 1A-C). At the Silwood site, this increase was due to herbaceous perennials which showed a continuous increase the first year and an accelerated increase in the second year until they contributed 90% of the total importance value (Fig. 1B). Neither monocarpic perennials (Fig. 1A) nor woody perennials (Fig. 1C) contributed significantly to the increase in perennials at Silwood. At the Iowa site, all perennials (summed for monocarpic, herbaceous, and woody species) increase steadily through both years of the V. K. BROWN E T A L 66 J 1°/ 0 M < J J A S < Year I (1984) O I M J J A S O I I Year 2 (1985) E'igurc 1. Colonization patterns in plants with different growth forms in the U.K. ( 0 )and Iowa (0) for a ?-year period, based on the monthly total for importance values (see text). A, Annuals (-; and monocarpic perennials (----). B, Herbaceous perennials. C, Woody perennials. study until they contributed 67% of the total importance value. However, the contribution of different types of perennials to this increase at the Iowa site was substantially different from that at the Silwood site. Monocarpic and herbaceous perennials increased the first year but levelled off in the second year (Fig. 1A & B), while woody perennials showed a steady increase in their contribution to total importance value in both years (Fig. IC). This increase was mainly due to Robinia pseudoacacia, a highly invasive species propogating in the Iowa site both by seed and rootshoots. The vegetation can also be partitioned according to the contributions of monocotyledons (primarily grasses) and dicotyledons (Table 3 ) . At the Iowa site, monocotyledons contributed only a small proportion to the total importance value throughout the two years of the study. I n contrast, at the Silwood site the contribution of monocotyledons, while low in the first year (in fact only about one-half that of the Iowa site), increased 6-10 fold in the second TRANSATLANTIC VARIATION IN SUCCESSION 67 Table 3. Proportion of total importance value (see text) attributed to monocotyledons Year 2 Year 1 Silwood, U.K. (%I) May June July August September October Total 1.1 5.2 4.3 4.4 Iowa, U.S.A. (OO) 0.9 7.4 13.5 Silwood, U.K. (%I 43.0 30.8 30.9 43.5 Iowa, U.S.A (%I 4.8 6.1 4.8 7.5 6.6 16.5 18.1 46.6 11.4 9.0 9.7 51.2 14.7 5.1 11.0 41.0 8.2 year. The large change in the contribution of monocotyledons between October of the first year (8.99%) and May of the second year (42.99%) a t the Silwood site was due largely to an early spring flush of Poa annua L. But even after P. annua had disappeared, monocotyledons continued to contribute 30% or more to the total importance value, and by the end of the second year over 50% of the vegetation consisted of monocotyledons (almost exclusively grasses). Shared species The contribution of shared plant species between the two sites was relatively low. Ten species were shared (6.1yo of all species encountered) and these belong to six plant families (Table 4).These shared species contributed more to the vegetation in terms of total importance value at the Iowa site than at Silwood, especially in the second year. The only comparable species, in terms of importance values, is the annual, Chenopodium album L., whose values were very similar in the first year at both sites. (Table 4). By the second year this species had all but disappeared from the Silwood site and was a minor component at the Iowa site. The grass, Poa annua, was only shared by the two sites in the second year, although it was present a t the Silwood site in the first year. By the second year, it became a major species a t the Silwood site, reflecting the generally more important role of grasses (see above). The two -clovers, Trzfolium repens L. and T r i f l i u m pratense L., show an interesting pattern (Table 4). Both were more evident at the Iowa site than at the Silwood site in the first year, but importance values for these species did not change much between years a t the Iowa site. At the Silwood site, both species increased dramatically in the second year but their importance values were the reverse of those at Iowa, possibly reflecting site differences in abiotic factors (e.g. T. repens prefers damper sites and is generally intolerant of drought (Burdon, 1983)). Dominant species The importance values of the ten most commonly occurring (dominant) species at the two sites together with the temporal variations in the rank of these species is given in Table 5. During the first year a t the Silwood site the species V. K. BROWN E T A L . 68 Table 4. Importance values (see text) for shared plant species (total for 6 monthly samples). Plants native to Britain are indicated by an asterisk ~ ~ Year 1 COMPOSITAE *O’irsium aruense (L.) Scop. Cunyza canadensis (L.) Cronq. *Lactuca serriola L. * Taraxacum officianale Weber CHENOPODIACEAE *Chenupodium album L. LEGCMINOSAE * Trifoiium pratense L. * Trifolium repens L. PLAN‘IAGINACEAE *Plantago majur L. POLYGONACEAE *Rumex crispus L. GRAMINEAE *Poa annua L. Total importance value for shared species Number of unshared species ’l’otal importance value for shared species (Y,) Year 2 U.K. Iowa U.K. Iowa 0.52 5.87 0.19 0.42 0.76 18.03 2.29 12.51 7.87 48.43 7.46 15.67 4.10 182.94 57.38 54.27 82.86 79.08 0.31 3.63 4.95 8.60 128.48 40.63 36.85 136.43 127.01 58.95 3.59 60.05 14.69 71.28 2.04 42.7 22.93 14.78 114.47 13.80 20.81 109.0 56 384.5 39 405.1 68 588.1 55 6.0 21.4 22.5 32.7 were exclusively annual herbs, while in Iowa only three annual herbs and one annual grass were among the 10 dominant species. This difference between sites in the life form of the dominants was also reflected in the more general analysis of plant life-forms (Fig. 1). Only the annual herb, Chenopodium album, is among the 10 dominant species a t both sites even though all the Iowa dominants, with the exception of R. pseudoacacia, are aliens presumably introduced from Europe or Eurasia. I n spite of this the distribution of importance values among the ten dominants at each site was similar except that at the Silwood site Spergula aruensis L. made a considerably greater contribution to the overall floral composition than did the first ranking Iowa plant, Abutilon theophrasti Medic. Both species are true ruderals (sensu Salisbury, 1942) and have virtually disappeared by the second growing season. Among the dominants, three general phenological types occur (Table 5). The first, as exemplified by S. aruensis and Polygonum persicaria L. at the Silwood site and A. theophrasti at the Iowa site, involves more of less continuous growth and persistance throughout the first season with consequent little change in either magnitude and/or rank of the importance values. The second type, as exemplified by C. album at both sites, Thlaspi aruense L. at Iowa, and Stellaria media (L.) Vill. and Bilderdykia conuolvulus (L.) Durn. at Silwood, involves rapid growth to maturity early in the season followed by a precipitous decline. However, T. aruense and S. media also show a spring flush of growth in the second year. The phenology of the monocarpic perennials and true perennials at the Iowa site constitute a third pattern characterized by low importance (and in 1RANSATLANTIC VARIATION IN SUCCESSION 69 f the ten dominant plant species based on monthly importance ank in parenthesis; A = annual, M P = monocarpic perennial, P = perennial Year 2 Year I Seasonal total M J J A 498.7 72.5 (1) 33.9 98.7 (1) 30.1 94.1 84.4 (1) (1) 30.8 28.5 (2) 27.1 (3) 25.4 (4) 18.4 (5) 17.9 (6) 8.1 (lo+ 11.0 (1) 171.6 (2) 130.1 (3) 128.6 (4) 115.6 (5) 103.1 (6) 82.9 (7) 81.2 (8) 54.2 (9) 50.2 (10) 360.1 (1) 244.5 (2) 143.2 (3) 129.3 (4) 128.5 (5) 123.0 (6) 79.1 (7) 66.9 (8) 60.1 (9) 43.8 (10) (8) (5) 26.3 7.2 (5) ( l o + ) 8.0 20.7 ( l o + ) (4) 10.4 12.0 (9) (7) 38.8 11.7 (2) (8) 36.3 12.1 (3) (6) 7.0 9.0 (lo+) (lo+) 15.1 21.1 (6) (3) 104.8 (2) 133.9 (1) 2.6 (lo+) ~ 13.0 (4) ~ 17.0 (3) 2.6 (lo+) ~~ ~ 76.3 (1) 75.5 (2) 12.2 (7) 15.8 (5) 26.8 (3) 7.0 (lo+) 21.0 (4) 5.0 (4) 17.4 (5) 30.1 (3) 16.4 (6) 10.7 (8) 12.0 (7) (7) 6.9 9.5 (lo+) (lo+ 2.7 4.3 (lo+) (lo+ 68.7 (1) 15.2 (7) 23.8 (4) 33.1 (2) 20.4 (6) 24.4 (3) 20.5 (5) 6.6 (lo+) (lo+) 6.1 9.7 (lo+) (lo+) 7.3 6.6 (lo+) (lo+) 0 69.1 (1) 24.0 (4) 22.5 (5) 25.3 (2) 17.1 (7) 25.1 (6) (3) 7.0 6.5 (lo+) (lo+) 7.8 2.0 (lo+) (lo+) 8.5 13.3 (lo+) (lo+) 1.4 5.6 (lo+) (lo+) 58.3 38.3 13.6 (1) (1) (8) 15.3 2.3 2.3 (6) ( l o + ) ( l o + ) 30.3 37.2 37.2 (3) (2) (1) 37.1 34.2 9.2 (2) (3) ( l o + ) 18.0 20.9 29.4 (5) (5) (3) 22.3 32.4 37.0 (4) (4) (2) 9.0 6.8 4.7 (lo+) (lo+) (lo+) 11.2 15.2 26.3 (8) (6) (4) 1 1 . 1 15.0 18.1 ( l o + ) ( l o + ) (6) 4.3 3.6 22.0 ( l o + ) ( l o + ) (5) Seasonal total M J 4.3 (lo+) 3.5 -- (lo+) 39.1 8.5 (lo+) (7) 4.6 1.2 (lo+) (lo+) 108.3 17.5 (lo+) (5) 38.0 4.3 (lo+) (lo+) 0.3 (lo+) 1.1 (lo+) 11.6 (5) 1.1 (lo+) 38.0 (21 6.9 (lo+) ~ - ~- ~ 0.2 ‘ko4Tt6, (lo+) 304.8 (1) 1.3 (lo+) 127.0 (6) 155.9 (4) 3.6 (lo+) 165.4 ~ - 17.3 5.3 (7) ( l o + ) 24.0 38.2 (4) (2) ~ - 42.3 29.1 (1) (4) 30.0 39.7 (1) (3) 1.5 -(lo+: 32.2 34.7 year even the total absence of four species) with a steady u t the first year and into the second year. The annual legume, S. F. Gray, at the Silwood site is not typical of any of these it maintains its relative importance in the first two years of 1 the exception of the latter species all the dominant, annual s are considerably reduced or absent in the second year. This It insignificant contribution of annuals during the second year Fig. 1A) was due by and large to new annual colonizers. V. K. BROWN E l A L . 70 Arthropod composition There were substantial differences in the densities of arthropods between the two sites (Table 6). In the first year the number of arthropods per metre squared was generally higher at the Silwood site. We consider the numbers for the Iowa site in June to be atypical, since there was a massive early infestation of aphids on the Melitotus spp. (see below). I n the second year, the number of arthropods per metre squared was generally higher at the Iowa site. This is due to the vast (and spatially variable) numbers of Collembola (Arthropleona) associated with the decomposition litter layer a t this site. However, when vegetation complexity is incorporated into the analysis there are generally fewer arthropods/m2/vegetation height category at the Iowa site throughout both years of the study, except for the June sample in the first year, previously discussed, and the September sample in the second year, which was taken in the middle of a dry spell at the Silwood site when there were very few Collembola. Table 6. Seasonal trends in the abundance of total arthropods and herbivores o n basis of density (numbers/m2; iz s.E .) and vegetational complexity (number/m*/vegetationalheight category-see text) Number/m* Number/m*/vegetationalheight category - Silwood, U.K. Iowa, U.S.A. Silwood, U.K. Iowa, U.S.A. Year I May June July August September O( tobrr 5.5k0.8 559.7 f58.7 852.6k64.7 1310.8k 156.7 2457.5 f233.4 2 142.4+ 337.9 0.03 978.4k233.9 418.0f48.4 696.6k73.2 527.0 f45.5 837.3k146.1 2.8 62.2 60.9 100.8 144.6 164.8 0.03 122.3 27.9 34.8 34.9 31.0 Year 2 May June July August Septembrr October 488.5 f77.5 2750.1 f324.6 4751.6f612.1 6203.8 f 269.7 2569.6k282.3 4985.1 f595.2 655.7 f62.3 1871.0f 119.9 6734.1 f 464.6 9840.4k 1138.5 8006.3 k414.5 6704.9f821.4 122.1 305.6 339.4 477.2 151.2 383.5 35.1 55.0 172.7 240.0 216.4 176.5 rl rthropod~ Herbwows Year 1 May June July August September October Year 2 May June July August September October 2.4k0.6 1 1 1.9 f 9 . 0 1.2 12.4 347.4 _+ 35.0 525.5f60.5 640.0 k 59.1 340.7 f 34.9 909.8k236.5 (7.7 f I .2) 193.4k23.9 484.8 f 60. I 282.3 k28.4 124.6f24.2 24.8 40.4 37.6 26.2 113.7 (2.2) 12.9 24.2 11.3 4.6 98.6f 16.1 604.7 f 102.0 1020.7f 156.4 2134.9f473.5 402.3k55.2 501.4f53.9 27 1.9 k40. I 729.8f 71.1 2028.7 k 140.0 1076.2f 124.3 822.5f67.6 342.2 f42.6 24.7 43.2 48.6 125.6 31.0 41.8 14.3 21.5 52.0 26.3 22.2 9.0 TRANSATLANTIC VARIATION IN SUCCESSION 71 Of particular interest in respect of their potential impact on the vegetation, are the insect herbivores. We have separated, to the best of our ability, the insect herbivores from the remainder of the arthropods. These herbivores include both ‘suckers’ (primarily Homoptera and Heteroptera) and ‘chewers’ (mostly Coleoptera but a few Orthoptera, Lepidoptera and sawfly larvae). The numbers in terms of densities, given in Table 6, are underestimates of the herbivore load, since the D-Vac suction method only samples mobile aboveground insects (mainly nymphal and adult Exopterygota and Apterygota and adult Endopterygota). Thus most endopterygote larvae, leaf miners, and gall formers are not represented in our samples. We have not included the adult leafmining and gall forming species in our estimates of herbivores, since the specimens might have been ‘transients’ from elsewhere. The densities of herbivores for the first year follow the pattern for all arthropods, with higher abundances being associated with the Silwood site. Table 6 also cites the numbers for June of the first year with and without (numbers in parentheses) the aphids associated with the Melilotus spp. I n the second year the herbivore density at the Iowa site was higher for four of the monthly samples. However, the sites differ markedly in the structural complexity of the vegetation, with the Iowa site being much more complex, especially in the second year. When herbivore load is based on number/m2/vegetation height category, values are substantially higher at the Silwood site, with the exception of July in the second year when values for both sites are similar. DISCUSSION Despite the wealth of descriptions and discussions of successional patterns and processes, there have been few, if any, attempts a t comparative studies in different geographical regions. We feel that this approach is both relevant and timely. By using two widely separated sites, one in the New World and another in the Old World, we have attempted to demonstrate how an integrated community study of the very earliest stages of secondary succession may further our knowledge of succession. I n particular, we wish to address the apparent contradiction between MacMahon ( 1981) who suggests that patterns of successional processes are similar throughout the world and Horn (1981) who suggests that generalizations about succession are difficult because of the variety of patterns found. Previous work at one of our sites, Silwood Park, has shown the value of including another trophic level in the study of plant succession (Southwood, Brown & Reader, 1977; Brown & Hyman, 1986; Brown & Southwood, 1983, in press). We have sought to find consistent general patterns in vegetation cover, species richness, turnover, dominance patterns and life form and in faunal relationships. We did not expect to find totally consistent trends in this study, bearing in mind differences in site attributes. However, we have found certain similarities which tend to support the view of MacMahon (1981). Of more interest are certain differences in vegetative characteristics (as suggested by Horn, 1981) and these may be related directly to the faunal load. Having established such differences, the path is clear for manipulative field experimentation which may well contribute to a better understanding of successional processes. 12 V. K. BROWN E T A L Our study to date suggests common general patterns in vegetation cover (Table I ) , since the differences encountered in the first season are primarily a reflection of differences in the length of the growing season in our two sites. Although species richness was higher overall at the Silwood site, the pattern of species accumulation in the two sites was similar. Bearing in mind that only 676 of the plant species occurring in our two site were shared, the general patterns of dominance are consistent (Table 5). For example, there was one dominant species (Abutilon theophrasti in Iowa and S. arvensis at Silwood) at each site and there were three major phenological patterns among the remaining species. T h e relative contribution of the different life forms is of particular interest since there are similarities in general trends but some significant differences (Fig. 1). For example, a t both sites annuals decline and perennials increased in abundance as succession proceeded. Annuals, however, were more abundant at the Silwood site in the first year and declined more rapidly. Even in the first year of colonization, monocarpic perennials and herbaceous perennials were present in the Iowa site throughout the season, and they maintained their level in the second year, whereas a t the Silwood site the decline in annuals was offset by a sudden rise in herbaceous perennials with monocarpic perennials being very sparse. The most significant difference, however, was the rapid invasion of woody perennials at the Iowa site. This was mainly due to one species, R. pseudoacacia. We feel that the high contribution of woody perennials may be atypical at this stage of early succession and the result of specific site characteristics. They may nevertheless have an important bearing on the high structural complexity of the Iowa site. Another major difference between our sites was in the contribution of herbs and monocotyledons, mainly grasses (Table 3). At Silwood, the latter were of much greater importance and this may be reflected in the composition of the insect fauna (Hendrix, Brown & Dingle, in prep.). Perhaps the most significant difference in terms of the composition of the vegetation was in the relative role of native and introduced plant species (‘Table 2 ) . At the Silwood site, true native species or species introduced millenia ago predominated. By contrast, at the Iowa site a high proportion of the plant species and an even higher proportion of the species in terms of importance value were recently introduced aliens. This might have been expected because of the large number of alien ruderals arriving in the Americas since 1492 (e.g. Bard, 1952; Marks, 1983), but our study demonstrates quantitatively the fact that, at least at one site, the entire progress of a major series of ecological events has been radically influenced by the introduced species. A further major difference between our two sites has been in faunal relations. We are currently analysing guild structure in detail, but work reported here on the overall herbivore load indicates that this was generally higher at the Silwood site where native plants abound. This picture is only made clear when the load is expressed in terms of the structural complexity of the vegetation. The more complex Iowa vegetation (as indexed by height profiles) supports fewer insects per unit of plant structure that the ecologically older Silwood community. Current work at Silwood Park (e.g. Brown, 1982a, 1985; Stinson, 1983; Brown et a /., in press) and elsewhere (e.g. Gibson, Brown & Jepsen, in press; McBrien el al., 1983) has now established that insect herbivores can have a considerable impact on the establishment and development of natural plant communities. TRANSATLANTIC VARIATION IN SUCCESSION 73 These differences in herbivore load may therefore constitute a major influence in changing the direction and rate of successtion in our two sites. We are currently exploring this further by manipulative experiments in which herbivore load is reduced in experimental plots by spraying with insecticide. Preliminary results indicate that the impact of herbivorous insects is indeed greater a t the Silwood site where native plants with high herbivore loads predominate. We suggest that the role of herbivorous insects may be one of the factors leading to the “variety of patterns” in plant secondary succession (Horn, 1981). ACKNOWLEDGEMENTS This work was supported by a NATO Grant for International Collaboration in Research, and at Silwood in part by a grant from the Natural Environment Research Council to V.K.B. We are grateful to E. J. Trapp, W. Rhoades, I. F. Sun and R. Schmall in Iowa, and A. Storr, M. Jepsen and I. Evans a t Silwood for assistance with the sampling programme. REFERENCES BARD, G. E., 1952. Secondary succession on the Piedmont of New Jersey. Ecological Monographs, 22: 195-215. BAZZAZ, F. A., 1975. Plant species diversity in old-field successional ecosystems in southern Illinois. Ecology, 56: 4855488. BROWN, V. K., 1982a. The phytophagous insect community and its impact on early successional habitats. In J. H . Visser & A. K. Minks (Eds), Proceedings of the F q t h International Symposium on Insect-Plant Relationships; 205-21 3. Wageningen, Netherlands: Pudoc. BROWN, V. K., 1982b. Size and shape as ecological discriminants in successional communities of Heteroptera. Biological Journal of the Linnean Society, 18: 279-290. BROWN, V. K., 1984. Secondary succession: insect-plant relationships. Bioscience, 34: 7 10-7 16. BROWN, V. K., 1985. Insect herbivores and plant succession. Oikos, 45: 17-25. BROWN, V. K., GANGE, A. C., EVANS, I. M. & STORR, A. L., in press. The effect of insect herbivory on the growth and reproduction of two annual Vicia spp. at different stages in plant succession. Jottnal of Ecoloo. BROWN, V. K. & HYMAN, P., 1986. Successional communities of plants and phytophagous Coleoptera. Journal of Ecology, 74: 963-975. BROWN, V. K. & SOUTHWOOD, T. R . E., 1983. Trophic diversity, niche breadth and generation times of exopterygote insects in a secondary succession. Oecologia, 56: 220-225. BROWN, V. K. & SOUTHWOOD, T. R. E., in press. Secondary succession: patterns and strategies: In M. J . Crawley, P. J . Edwards & A. Gray (Eds), Colonisation, Succession and Stability. Oxford: Blackwell Scientific Publications. BURDON, J. J., 1983. Biological flora of the British Isles. No. 154. Trriflium repem. Journal of Ecology, 71: 307-330. CLAPHAM, A. R., TUTIN, T. G. & WARBURG, E. F., 1962. Flora of the British Isles. Cambridge: University Press. COWLES, H. C., 1899. The ecological relations of the vegetation on the sand dunes of Lake Michigan. Botanical Gazette, 55: 97-1 17, 167-202, 281-308, 361-391. EVANS, F. C. & DAHL, E., 1955. The vegetational structure of an abandoned field in southeastern Michigan and its relation to environmental factors. Ecology, 56: 685-706. GIBSON, C. W. D., BROWN, V. K . & JEPSEN, M., in press. Relationships between the effects of insect herbivory and sheep grazing on seasonal changes in an early successional plant community. Oecologia. GWYNNE, M. D. & BELL, R. H . V., 1968. Selection of vegetation components by grazing ungulates in the Serengeti National Park. Nature, 220: 390-392. HORN, H. S., 1981. Succession: 253-271. I n R. M. May (Ed.), Theoretical Ecology; Principles and Applications, 2nd edition. Oxford: Blackwell Scientific Publications. LAWTON, J. H., 1983. Plant architecture and the diversity of phytophagous insects. Annual Review of Entomology, 28: 23-39. MACMAHON, J . A., 1981, Successional processes: comparisons among biomes with special reference to probable roles of influences on animals. In D. C. West, H. H . Shugart & D. B. Botkin (Edsj, Foresl Succession Concepts and Application: 277-304. New York: Springer Verlag. 74 V. K. BROWN E T A L . MARKS, P. L., 1983. O n the origin of the field plants of the northeastern United States. American Naturalist, 122: 210-228. McBRIEN, H., HARMSEN, R. & CROU’DER, A., 1983. A case of insect grazing affecting plant succession. Ecology, 64; 1035-1039. MURDOCH, W. W., EVANS, F. C. & PETERSON, C. H., 1972. Diversity and pattern in plants and insects. Ecology, 53: 8 19-828. OLSON, J . S., 1958. Rates of succession and soil changes on southern Lake Michigan sand dunes. Botanical Gazette, 119: 125-1 70. OOSTING, H. J . , 1942. An ecological analysis of the plant communities of Piedmont, North Carolina. ‘4merican Midland Naturalist, 28: 1-126. PRYOR, J. C., 1976. A Regional Guide to Iowa Landforms. Educational Series 3. Iowa City: Iowa Geological Survey. SALISBURY, E. J., 1942. The Reproductive Capacity of Plants. London: Bell. SHELFORU, V. E., 1913. Animal Communities in Temperate America. Chicago: University o f Chicago Press. SHETLER, S. G. & SKOG, L. E. (Eds), 1978. A Provisional Checklist of Species for Flora of North America. Vol. 1. Monographs in Systematic Botany. St. Louis: The Missouri Botanical Garden. SOUTHW‘OOD, T. R . E., BROWN, V. K. & READER, P. M., 1979. The relationships of plant and insect diversities in succession. Biological Journal of the Linnean Society, 12: 327-348. STINSON, C. S. A,, 1983. Efects of insect herbivores on early successional habitats. Ph.D. thesis, University of London. STINSON, C. S. A. & BROWN, V. K., 1983. Seasonal changes in the architecture of natural plant rommunities and its relevance to insect herbivores. Oecologia, 56: 67-69.