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it also likely restricts the number of co-introduced species, particularly natural
enemies (i.e., parasites, pathogens, herbivores or predators that have an evolu-
tionary history with the invader). Keane and Crawley [72] formalized the ERH
as a rapid increase in the distribution and abundance of a plant species (i.e.,
successful proliferation, stage IVb or stage V (Fig. 1)) in their introduced
range owing to a reduction in the number of natural enemies including herbi-
vores. However, the idea that NIS benefit from a reduction of enemies has a
long history as a largely untested 'rule of thumb'. Enemies can be specialists
- defined as a species that is specialized to attack one or a small number of
hosts, or generalists - enemies that attack a wide range of hosts. Keane and
Crawley [72] proposed that the ERH could account for the successful prolif-
eration of NIS if specialist enemies are absent from the introduced range of
their host and if generalist enemies present in that range have a greater impact
on native resident species. In support of this hypothesis, Mitchell and Power
[73] and Torchin et al. [74] found that introduced populations of a variety of
plant and animal hosts, respectively, had fewer enemies than populations from
the native range [75]. Prati and Bossdorf [76] found that Senecio inaequidens
was larger in size and had comparatively fewer insect parasites on them in the
invaded range than in the native range. Callaway et al. [77] reported that soil
biota could promote exotic invasion of Centaurea maculosa , which had high-
er biomass in soil collected from its introduced range. Soil biota in C. macu-
losa 's native Eurasia inhibits its growth, while soil microbes from N. America
promoted its growth. Van der Putten [78] suggested two biotic benefits for
invaders in the invaded range. These are: (1) invaders escape from enemies
such as soil pathogens and do not encounter new species-specific enemies in
the naturalized range, while (2) root symbionts are available to the invader to
help it to establish in the invaded range. DeWalt et al. [79] tested the ERH with
the neotropical shrub Clidemia hirta (Melastomataceae), a noxious invader in
tropical forest in its introduced range of several islands in the Pacific and
Indian oceans. In its native range, i.e., Central and South America and
Caribbean, C. hirta is found in disturbed, relatively open areas. The absence of
C. hirta from the forest understory in its native land was due to presence of
natural enemies (fungal pathogens and herbivores) and these enemies were
absent from Hawaii, where C. hirta is invading the forest understory. Silene
latifolia , a native of Europe, is a serious invader in the USA, where it is con-
sidered a problematic weed of disturbed habitat and agricultural land. Wolfe
[80] found that specialist enemies (e.g., a seed predator and an anther smut
fungus) were absent or present in very low numbers in N. America. The suc-
cess of S. latifolia as an invader in N. America was explained by its release
from specialist enemies in its naturalized range.
Contrary to the ERH however, Agrawal and Kotanen [81] found that the
level of attack on introduced herbivores was significantly higher than attack on
native residents. Colautti et al. [82] reviewed 25 studies that tested the ERH
and found that biogeographical studies, which compare enemies (i.e., their
effects, abundances or species richness) in native and introduced populations
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