Applied Soil Ecology 35 (2007) 256–259
www.elsevier.com/locate/apsoil
Short communication
Effects of isolation, area and predators on invasion:
A field experiment with artificial islands
Janne S. Kotiaho a,b,*, Pekka Sulkava a
a
Department of Biological and Environmental Science, P.O. Box 35, 40014 University of Jyväskylä, Finland
b
Natural History Museum, P.O. Box 35, 40014 University of Jyväskylä, Finland
Received 17 January 2006; received in revised form 28 April 2006; accepted 2 May 2006
Abstract
The three most important ecological factors affecting the success of island invasions are the area of the island, isolation of the
island and occurrence of predators on the island. Traditionally, invasion success has been studied on natural islands, which partly
explains the rarity of controlled and replicated experiments. Here we report results from a field experiment investigating the
influence of the above three factors in artificial islands. As an experimental system, we used predatory mites and a nematode
community occurring naturally in boreal coniferous forests. We found that all three factors had an effect on invasion success, but
surprisingly, that there were no interaction effects.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Area effect; Dispersal; Distance effect; Nematode; Predatory mites
1. Introduction
The two most important aspects of invasion ecology
are the effects of invaders on the community being
invaded, and the factors influencing invasion success
itself (MacArthur and Wilson, 1967; Carey et al., 1996).
Of these two, the effect of invading species on the
community has been addressed more often, and several
different effects have been identified (Simberloff,
1981). The most studied are direct effects, such as
predation (Lomolino, 1984; Schoener and Spiller, 1996,
1999), while some evidence for indirect effects, such as
decreased herbivory due to increased predation on
herbivores, also exists (Petren and Case, 1996).
Factors influencing invasion success itself have
attracted less attention. Traditionally, such studies have
been conducted on natural islands, and, perhaps for this
* Corresponding author. Tel.: +358 14 2604221;
fax: +358 14 2602321.
E-mail address: [email protected] (J.S. Kotiaho).
0929-1393/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apsoil.2006.05.003
reason, an experimental approach is rare (Crowell, 1973;
Schoener and Schoener, 1983; Schoener and Spiller,
1995). Existing studies suggest that island size, isolation
and occurrence of predators are all possible factors
affecting invasion success (Simberloff and Wilson, 1969;
Lomolino, 1982; Schoener and Spiller, 1999). However,
no study has examined these factors simultaneously.
Moreover, all of the studies have used an experimental
design where the invaders were introduced onto the
islands. This approach excludes distance effects and
partially also size effects. We adopted an experimental
approach where all three factors were simultaneously
manipulated. Only simultaneous manipulation of all
factors is able to separate their independent effects and
resolve their relative contributions to invasion success.
2. Materials and methods
We created artificial islands of two sizes and two
distances from the mainland with and without predators.
J.S. Kotiaho, P. Sulkava / Applied Soil Ecology 35 (2007) 256–259
Invasion was allowed to take place naturally. Our
experimental system was the soil nematode community
occurring naturally in the boreal coniferous forests. For
the experiment we selected a homogenous area of
7 m 7 m from a Norway spruce (Picea abies) forest
in Central Finland (628070 N; 248120 E).
We refer to sand patches as oceans, which are an
unsuitable habitat for nematodes. Sand is mineral soil
sieved through a 3 mm mesh, washed with tap water and
defaunated by heating to 60 8C for 16 h. Oceans were
constructed by removing the humus layer of the forest
floor to a depth of 100 mm, placing a thin plastic sheet
on the bottom, and filling the hole with the treated sand.
The plastic sheet was used to prevent nematode invasion
from underneath. With this methodology we constructed round oceans of four sizes (diameters 31, 53, 71
and 93 mm).
We refer to coniferous forest humus patches as
islands. Humus was sieved through a 0.5 cm mesh and
defaunated by heating to 60 8C for 8 h. Cylindrical
islands were constructed from plastic mesh baskets
(mesh size 1.0 mm; height 45 mm), filled two thirds
(30 mm) with treated humus, and sealed with micromesh net (mesh size 0.005 mm). With this methodology
two island sizes were created (diameters 11 and
33 mm). The size difference corresponds to a ninefold difference in volume. Accordingly, the mass of the
humus on large islands (12.00 0.01 g) was nine times
greater than that on small islands (1.33 0.01 g).
Before sealing the islands, we added predatory mites
(Parazercon spp.) into half of the islands: 45 mites into
the larger and 5 into the smaller islands resulting in
equal density (1.75 mites per cm3). Mesh size
(0.005 mm) was chosen to be small enough to prevent
the escape and invasion of the predatory mites, but large
enough to allow free movement of the nematodes.
Each island was placed in the centre of one ocean the
surface being flush with the surface of the ocean. By
combining the smaller islands with oceans of 31 and
71 mm in diameter and larger islands with oceans of 53
and 93 mm in diameter we created islands that were
separated from the mainland (undisturbed forest floor)
by 10 or 30 mm. Thus, we had three two-level factors:
small and large islands, near and far distances, and with
and without predators, resulting in eight possible
combinations. A balanced design was created by
assigning 16 replicates to each combination. The total
number of replicates was 128; at the end of the
experiment 3 replicates had been disturbed and were
thus discarded from the analysis. To monitor the number
of predators on the islands, we constructed 10 extra
replicates with mites (5 small and 5 large; 4 near and 6
257
far islands). This was done to allow the estimation of
predator survival in relation to island size and distance
from mainland. The distribution of the replicates within
the experimental area was systematically varied to
approximate an even distribution. Islands were open to
invasion for a period of 62 days starting from the 1 June
1996.
At the end of the experiment, islands were weighed
to estimate the change in moisture. Nematodes were
extracted using wet funnels (Sohlenius, 1979), and
counted, while predators were extracted by the high
gradient method (Macfadyen, 1961) and counted from
the 10 extra replicates.
3. Results
The number of predatory mites was decreased on the
islands, but there was no significant difference (Mann–
Whitney U-test) in the densities on small versus large
islands (mean S.E.: 0.56 0.29 and 0.51 0.17
mites per cm3, respectively), or on near versus far
islands (mean S.E.: 0.45 0.27 and 0.59 0.21
mites per cm3, respectively). No predatory nematodes
(Monochus) had invaded the islands.
Since nematodes prefer moist habitats (Huhta et al.,
1998), the change in the moisture of the islands was
entered into the model as a covariate. To test for the
homogeneity of the slopes between the covariate and
the dependent measure across the cells of the
experimental design, we compared the sums of
squares associated with the error term from the full
model to the sums of squares associated with the error
Table 1
Analysis of variance for number of nematodes per volume
Source
d.f.
MS
F
P
Eta2a
Model
Predation
Size
Distance
Covariate b
Error
Total
4
1
1
1
1
119
124
37.17
29.05
44.50
8.79
31.28
2.04
18.17
14.26
21.84
4.32
15.35
<0.001
<0.001
<0.001
0.040
<0.001
0.38
0.11
0.16
0.04
0.11
Group variances were not homogenous, but tended to increase with the
group means. Cube-root transformation performed best in removing
the heteroscedasticity, after which variances between groups were
equal (Levene’s test F7,116 = 0.87, P = 0.534). There were no significant interaction terms (for all F < 1.0, P > 0.35), and thus the final
model is presented without them.
a
Eta-squared is interpreted as the proportion of the total variability
in the dependent variable that is accounted for by variation in the
independent variable.
b
Change in weight (moisture) of the habitat patch during the
experiment.
258
J.S. Kotiaho, P. Sulkava / Applied Soil Ecology 35 (2007) 256–259
Fig. 1. Effect of distance from the mainland, predation and island size
on invasion success. Bars represent mean (S.E.) untransformed
residual nematode numbers per volume (from regression between
moisture change and number of nematodes per volume). For significance tests see Table 1.
term from the reduced model, which contains only the
covariate and the independent variables, i.e. full
model without the interactions terms involving the
covariate (Hendrix et al., 1982). The F-test was
nonsignificant with an appropriately high a value to
warrant the acceptance of homogeneity of slopes, and
thus the analysis can be carried out as a normal
ANCOVA. There were no significant interactions
between the factors and thus only the main effects
were entered in the final model (Table 1). The whole
model was highly significant explaining 38% of the
total variance. All factors had an effect on the number
of nematodes (Fig. 1); size of the island explained
16%, occurrence of predators 11% and distance from
mainland 4% of the variance (Table 1).
more reliable evidence for island size effects: there was
a sharp threshold island size under which invasion by
lizards predominantly failed (Schoener and Schoener,
1983; Schoener and Spiller, 1999). However, in this
study they did not investigate predation effects.
The effect of isolation on invasion success has been
observed before, although experimental evidence is rare
(Lomolino, 1982; Simberloff and Wilson, 1969). It is
obvious, however, that chance alone can create a distance
effect, and that invasion success is very much influenced
by the relative dispersal ability of different organisms
(MacArthur and Wilson, 1967). Nevertheless, it is
important to manipulate the distance simultaneously
with manipulation of size and predation to determine if
distance has more complicated effects through interactions with other factors. Given that predation may have a
substantial immediate effect, one might expect an
interaction effect between distance and occurrence of
predators. For example, if invasion is rare because of a
high degree of isolation, then the occurrence of predators
on an island may prevent most invasion attempts from
being successful, while most invasion attempts in
predator free islands may be successful. Therefore, it
was surprising that invasion success was not affected by
interactions between the factors.
Acknowledgements
We thank Anne Kotiaho and Anneli Sulkava for their
forbearing assistance during the experiment. We thank
Robert Black, John Hunt, Minna-Liisa Rantalainen,
Dale Roberts, Leigh Simmons and Joseph Tomkins for
comments. J.S.K. was supported by the Academy of
Finland.
4. Discussion
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We found that island size and occurrence of
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and apparently also in the density of predators, so large,
that the power to detect even substantial differences was
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