ISSN 10674136, Russian Journal of Ecology, 2010, Vol. 41, No. 3, pp. 229–236. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.V. Akatov, T.V. Akatova, 2010, published in Ekologiya, 2010, No. 3, pp. 191–198.
Communities where the number of species is
smaller than the species carrying capacity of the envi
ronment are termed unsaturated (Rabotnov, 1983).
It is currently believed that communities may be
unsaturated with species because they are young (in
terms of succession or evolution) or isolated, or they
were damaged in the historical past (MacArthur and
Wilson, 1963; Ricklefs and Schluter, 1993, Davis et al.,
2005). More than 20 years ago, Rabotnov (1983) sup
posed that, since such communities are characterized
by a lower intensity of interspecific interactions than
saturated communities, they should be less resistant to
invasion of exotic (alien) species. However, it was not
until recent years that the problem of invasibility
became connected with the discussion on the degree
of species saturation/unsaturation of currently exist
ing biotas (Ricklefs and Schluter, 1993; Sax and
Brown, 2000; Moore et al., 2001; Brown and Peet,
2003; Davis et al., 2005; Gilbert and Lechowicz, 2005;
Sax et al., 2005; Akatov et al., 2007). Comparisons of
the degrees of adventity of insular and continental flo
ras (Sax and Brown, 2000; Mirkin and Naumova,
2002; Sax et al., 2005) and evidence that most inva
sions occur without replacing native species (Ricklefs
and Schluter, 1993; Sax and Gaines, 2003; Sax et al.,
2005) served as arguments to support a high invasibil
ity of unsaturated communities. In addition, theoreti
cal analysis showed that unsaturation of many com
munities invaded by alien species is indirectly con
firmed by a positive correlation between the numbers
of invading and native species (Moore et al., 2001).
Although this line of research is obviously promis
ing, it has not been paid sufficient attention thus far
(Gilbert and Lechowicz, 2005). In particular, it
remains unclear whether the degree of species satura
tion may affect the richness of alien species in non
interactive communities, i.e., the communities where
local species extinction results more from abiotic dis
turbance than from competition for space and
resources (Cornell and Lawton, 1992; Cornell, 1993).
The purpose of our study was to fill this gap. Open
plant assemblages in shoals of western Caucasian
mountain rivers containing different proportions of
alien species were used as objects of analysis.
MATERIALS AND METHODS
Estimation of species saturation.
If the species satu
ration of communities (plant groups) actually affects
their resistance to invasion of alien species, this
parameters should be expected to be significantly neg
Saturation and Invasion Resistance
of NonInteractive Plant Communities
V. V. Akatov
and T. V. Akatova
Maikop State Technological University,
ul. Pervomaiskaya 191, Maikop, 385000 Russia
Caucasian State Biosphere Reserve,
ul. Sovetskaya 187, Maikop, 385000 Russia
Received March 27, 2008
—Open plant assemblages in shoals of western Caucasian rivers were used as examples to analyze the
relationship between the species saturation and the number and total abundance of alien species in noninter
active communities. Invasion of exotic species into highly saturated communities has been demonstrated to
be, on average, less probable than their invasion into unsaturated communities. A hypothesis explaining the
relationship between these parameters has been put forward. According to the hypothesis, the number of alien
species in a specific locality in a community is determined by their ratio to the number of native species in the
species pools of these communities; and their mean abundance, by the ratio of the total number of species to
the number of individuals in the localities. Both ratios are smaller in saturated biocenoses, which determines
a relatively small admixture of alien species in them.
: plant assemblages, river shoals, species pool, species richness, species saturation, invading species,
invasibility, western Caucasus.