Download Plants and insects in early oldfield succession

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.
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V. K. BROWN E T A L .
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Data analysis .
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R e s u l t s . .
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Plant cover and species richness
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Plant species composition .
AIthropod composition
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Discussion.
Acknowledgements
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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.
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