Who came first? Wild boar effects in a Mediterranean forest

Cover Photo by Tim Clifton / CC BY

 

I am happy to announce that a new paper I contributed to is now out in Community Ecology © Akadémiai Kiadó, Budapest. This work is the result of couple of years of work of a great team of vegetation scientists and animal ecologists, with whom I had the luck to work for the last 4 years, and to whom I wish all the best.

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Why focusing on wild boar in a blog that talks about forests?

The point is that the populations of wild boar (Sus scrofa) in Europe have grown substantially in recent decades, not to mention Central Italy, where the study is focused. Indeed, this species is able to adapt to different environments, and for sure, it was highly favoured by a combination of reintroduction for hunting purposes, increasing tree mast frequency (climate change?), insufficient hunting pressure, and lack of predators.

Wild boar is a natural component of forest ecosystems. Nevertheless, when the population is over-abundant it may have a profound impact on several components of a forest, besides causing direct economic damage to crops or pastures in adjacent land.

One of the most distinctive signs of wild boar’s activity are the traces of rooting, i.e., the excavation of the surface soil layers in the course of foraging for food, which creates localized disturbance that appears similar to mechanical ploughing. Rooting can affects the soil and the vegetation in areas that range from a few square decimetres to hundreds of hectares, depending on the population density and on local the availability of food resources.

Some of the consequences of rooting may be positive, for instance a disturbed soil may favour the germination of seeds, or may increase the activity of soil fauna. But what happen when this activity becomes pervasive in a forest?

We explored this in the lowland forest of Circeo National Park (central Italy), where the intensity of rooting activity has probably never been as high as it is currently. There are several reasons for that, the widespread land conversion to agriculture following the draining operations in the 1930’s, the consequent isolation of the forest, the local extinction of wild boar natural predators.

Circeo National Park is an incredible place. Established in 1934, Circeo Park was born in order to protect not only a single species but also a rich group of characteristic environments, with a consequent abundance of species. Facing the Thyrrenian sea, it includes a mosaic of several different environments, including more than 20 km of coastal sand dunces, four coastal lakes, the beautiful island of Zannone, Circe’s Cape, and a beautiful lowland forest, the ‘Selva di Circe’. This is biggest remnant of the big wild, impenetrable ‘Selva di Terracina’ the ancient coastal forest which covered more than 11,000 hectares in southern Latium. Most of this forest is now gone, after the draining and the consequent massive conversion to agricultural land.

Sweet memories of this forest remain in Virgil’s Aeneid

 The roars of lions that refuse the chain,
The grunts of bristled boars, and groans of bears,
And herds of howling wolves that stun the sailors' ears.

(“Dryden’s translation”. Classics.mit.edu. Retrieved 2014-03-19)

But how is the current over-population of wild boar expected to affect the composition and richness of the herb-layer in this Mediterranean Forests?
And how is the response of understorey plants modulated by their functional characteristics?

We tried to give an answer to these questions in our research. We compared the herb-layer of areas subjected to high-rotting vs. area less affected by rooting (nowadays there is nothing really untouched by wild boars over there!), and indeed we found a profound impact of wild boar activity on forest vegetation. Contrasting levels of rooting were associated to different understorey species composition, while we observed no significant difference in species richness.

Nevertheless, many of these results were unexpected.

Of course there are some ‘winner’ and ‘loser’ species, i.e. species that are respectively favoured or negatively impacted by rooting. And that to predict who was going to win, it was sufficient to take a look to their functional characteristics. For instance, in heavily rooted sites, we were expecting to find a in increase of species that are able either to avoid or tolerate rooting disturbance or to regenerate after rooting. For instance, species having traits indicating a high investments in ‘defence’ against herbivory, such as spines, or thick, distasteful leafs, or clonal species, i.e. those species that through rhizomes or stolons may quickly resprout after  an intense rooting event.

Well, it came out that it was the other way around!
In contrast with our expectations, sites with lower rooting returned a higher proportion of species characterized by traits related to resistance or response to herbivory.

How to interpret these findings?

In our opinion, this is the classical ‘which came first the chicken or the egg?’ dilemma.
Current vegetation patterns in this lowland forest probably depend on the legacy effect of past rooting disturbance. Out hypothesis is that the areas currently subjected to low rooting intensity have already gone through the process, and that they were intensely rooted in the past. These areas are indeed the most appealing from a wild boar perspective. They are characterized by a lot of pools and ponds, and by a relatively low use of paths and roads by humans, thus making these area the most suitable to wild boar for i) for feeding purposes, since wild boar are known to preferentially feed in moister areas; ii) for wallowing; iii) as a refuge from indirect human disturbance. This is only an hypothesis, of course, since no data about the actual pattern of activity of wild boar over the last years exist. But there are some old vegetation data that seem to support this idea.

This intense rooting disturbance that happened in the near-past is likely to have had important biological legacies on current understorey communities. The most noticeable legacy was the establishment of a dense layer of butcher’s-broom (Ruscus aculeatus) and this had long-lasting consequences on the diversity and evenness of the undestorey. Ruscus aculeatus is terribly spiny and low palatable, really not the first nutritional choice for a wild boar. After its spread the understorey became hardly suitable for further use by wild boar. What would you do if you were a wild boar?  Move to another part of the forest, of course, i.e., to those parts that now display a high rooting level.

However, this sequence of environmental changes is probably not cyclic, since the areas occupied by the dense thickets of Ruscus aculeatus may remain unsuitable to wild boar for years, thus influencing the spatial arrangement of further feeding activities for a long period. What are the long-lasting consequences for the herb-layer of this lowland forest?  And what should the local forest managers do to restore the previous vegetation communities? Reinforcing the effort to control the population density of wild boar, and implementing some eradication measures aimed at lowering the density of these dense thickets of Ruscus aculeatus may be a good start.

Burrascano, S., Copiz, R., Del Vico, E., Fagiani, S., Giarrizzo, E., Mei, M., Mortelliti, A., Sabatini, F.M., Blasi C., 2015. Wild boar rooting intensity determines shifts in understorey composition and functional traits. Community Ecol. 16, 244-253.

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