Chapter 36: Ecological impact of wild boar in natural ecosystems409
that the number of studies in the last 40 years increased, but
without any significant improvement in study length, as one
might have expected. The only positive aspect emerging is the
presence of few upper outliers after 2000. Most of these long-
term studies were performed in the USA with the aim of assess-
ing the impact of this invasive species. Wilcox and Van Vuren
(2009) reported a seven-year study showing the importance of
vertebrate items in wild pig diet in a Mediterranean oak forest
in California. Siemann et al. (2009) investigated the long-term
effects in an oak forest in Texas, showing that feral hogs speed up
the spread of alien plant species and change the above-ground
plant structure. Roemer et al. (2001) showed that feral pigs are
the indirect cause of quasi-extinction of the native island fox
(Urocyon littoralis) in California Channel Islands, mediated by
the golden eagles. Finally, Kuiters and Slim (2002) studied the
impact of wild boar and other ungulates on the regeneration of
a mixed-stand forest with deciduous and coniferous trees in the
Netherlands from 1987 to 1998.
We applied a simple statistical approach to a meta-analysis
of studies on wild boar impact on ecosystems. Our objective and
statistical approach was able to reduce the bias originating from
the large methodological differences among available studies
and to put into light meaningful patterns in the studied biblio-
graphical data set.
From the analysis performed on a large set of published liter-
ature (Appendix 36.1), it may appear that the wild boar is a sort
of environmental nightmare in most terrestrial and freshwater
ecosystems. However, the literature we analysed is not a random
sample of the occurrence of this ungulate and of its effects. Many
studies are concentrated on fragile habitats, such as the islands,
and where the species was introduced and caused evident envi-
ronmental stresses.
Further, there is a clear bias in favour of short-term effects,
while the long-term effects of the species on ecosystems are
almost completely neglected. Most analysed studies, in particu-
lar those conducted within the native range, lasted less than a
year, and thus allowed description of the immediate impact of
wild boar on the environment, but prevented us from under-
standing if this impact has a long-term negative effect. For exam-
ple, the immediate consequence of wild boar rooting is the loss
of above-ground plant cover, but studies conducted in the long
term indicate that this effect might be transient. For instance,Figure 36.3 Correspondence
analysis. We report the observed
impact (Effect) of wild boar on soil
physico-chemical descriptors..03813:02:26