Chapter 36: Ecological impact of wild boar in natural ecosystems405
but in doing so treatments were not randomized. Because of
these biases in study design and because the metrics describ-
ing impact were often crude and highly variable among studies,
we decided to pool impacts in three broad categories (positive,
negative, and null). In a few cases, control was in time and not
in space. Thus, we can have negative effects on a bird species, a
null effect on soil, or a positive effect on exotic species. For sev-
eral papers it was also possible to derive the impact (using the
same three categories) of wild boar on biodiversity, according
to experts’ opinion. When possible, we used the opinion of the
authors; elsewhere, we decided on the basis of the experimental
data. In the case where wild boar appears to favour exotic plant
species, we coded as negative the impact on biodiversity.
For each paper we defined the following data columns: refer-
ence, WB metrics (abundance, presence, rooting, nesting, zoo-
chory, foraging), the country of the study area, the habitat (hill,
plain, mountains, marshes, island), the origin of the population
(native or introduced), climate (e.g. Mediterranean, tropical),
while the general argument refers to the main items of study
(e.g. soil, animals).
In relation to the impact of rooting on the chemical prop-
erties of soil, the physical/chemical parameters used in the dif-
ferent studies were quite heterogeneous. Because of the large
number of different analyses performed, we selected the follow-
ing metrics: cations, carbon, CO 2 , decomposition rate, (poten-
tially dangerous) pathogens, microbial biomass, mineralization
rate, moisture, soil acidity (pH).
The heterogeneity of such materials is so high that it is very
difficult to conduct a quantitative meta-analysis. We decided
not to use a ‘weight’ based on sample size or study duration.
However, the most relevant papers are quoted in the comments.
We used multiple correspondence analysis, MCA (PROC
CORRESP of SAS 9.4) to reveal patterns in the published data.
The method allowed us to display a multivariate data set in a
two-dimensional graphical form. In MCA, the values close to
the origin show no associations, or, in other words, χ^2 residuals
are close to zero. Note that to compare categories of different
variables one has to use the orientation of the line connecting
the point to the origin; categories with similar orientations are
positively associated and categories with angular differences
around π are negatively associated. Angles around π/2 between
two categories denote that the observed frequencies are those
expected under independence.
As noted above, associations among the categories of differ-
ent variables are characterized by a similar angular coefficient
of the line connecting the category to the origin. We built up
95 per cent confidence ellipses containing at least four categories
with the most similar angular coefficients. If the set was too scat-
tered we reduced the confidence value for better visualization.
When a 95 per cent ellipse contained the origin, we considered
the correlation among categories to be non-significant.
Results
After a first selection, we retained 119 papers (listed in
Appendix 36.1) and excluded eight papers because it was not
possible to find the article, the article was too descriptive and
lacked quantified information, because wild boar only played a
limited part in the study, or we were unable to understand the
language. Thirty-two countries are represented in the database;
many papers concerned studies performed in the European
Union (n = 42) and USA (n = 25). The structure of the database is
summarized in Table 36.1, which draws attention to the sparse-
ness of available information. Introduced (n = 55) and native
(n = 64) populations are almost equally represented. In terms of
habitat, islands appear to be over-represented in case of intro-
duced populations. The presence of feral pigs on islands has
raised much concern because of the potentially severe impact
on native fauna and flora. The case has been widely documented
in the California Channel Islands, for instance, by Roemer
et al. (2001). The presence of feral pigs induced an increased
presence of golden eagles (Aquila chrysaetos), which in turn
prey upon island foxes (Urocyon littoralis) and island spotted
skunk (Spilogale gracilis), a process of apparent competition
(Melstrom 2014). Eradication of feral pigs from Santiago Island
(Galapagos) has reduced the conservation threats to several
endangered species such as the Galapagos tortoise (Geochelone
elephantopus), the lava lizard (Microlophus albemarlensis), and
the Galapagos petrel (Pterodroma phaeopygia). In the same
vein, Cole and Litton (2014) showed that the removal of feral
pigs from enclosures in Hawaii improved plant recruitment.
Another emergent area of investigation is the impact of wild
boar on wetlands. Doupé et al. (2009) showed cascading effects
of feral pigs in tropical Australian marshes through foraging and
increased water turbidity, affecting many ecological parameters
of the ecosystem. Engeman et al. (2007) investigated the role of
feral pigs in conservation of natural habitats of seepage slopes
in Florida. These studies are paralleled by some investigations
about the impact of wild boar on water quality (Kaller & Kelso
2003; Dunkell et al. 2011). Despite the small number of stud-
ies, it seems that the negative impact of wild boar on wetlands,
including impacts on nesting birds, can be significant.
Mediterranean habitats are well represented in the database
as well as tropical environments, even if most studies were car-
ried out under temperate climatic conditions. In terms of meas-
ured wild boar parameters, rooting (n = 64) is to a large extent
the most used parameter sampled. Several studies (n = 19)
are based on presence/absence data, some are interested in
zoochory (n = 8), while the population abundance of wild boar
(n = 2) is almost never estimated (spotlight transects, Carpio
et al. 2014a) and sometimes relative index of abundance of signs
of presence are used (Roda 2014). Finally, in terms of object of
interest, most studies are focused on impact on plants (23 per cent)
and soil (17 per cent).
There is widespread concern for the impact of introduced
populations of wild boars on ecosystems, indicated by the large
number of studies performed on introduced populations. For
instance, performing well-designed experiments, Sweitzer and
Van Vuren (2002) evaluated the severe impact of wild pigs on the
regeneration of oak woods in California, while Barrios-Garcia
et al. (2014) investigated the impact of pig disturbance on soil
properties, plant biomass and structure in Patagonia.
In Figure 36.1 we report the impact of wild boar on differ-
ent ecological parameters classified as general arguments of.03813:02:26