Sus scrofa-one of the World´s worst invadors?

by Eva Andersson,vet med stud 1st year, 2003/2004

  

 

Sus scrofa(Linnaeus 1758),Wild boar.

Common names:kuhukuhu(Maori),kune-kune(Maori-New Zealand)petapeta (Maori),pig(English),poretere(Maori),te poaka(Maori)Wildswein(German)

 

Due to it´s rapidly increasing population size and feeding habits which lead to largely damaged  both agricultural and natural vegetation and spreading of weeds,it has been nominated  by ISSG(Invasive Stationpecies Specialist Group)as among 100 of the "World´s worst" invadors.Anderson and Stone(1993)regarded pigs as"currently the most pervasive and disruptive alien influence on the unique native forests of the Hawaiian islands"

On the other hand a research project in Sweden finansed by WWF pointed out that the wild boars increases the biological  diversity by contributing to the spreading of plant species.

Before getting deeper into the subject of pigs as fearless destroyer or useful soil cultivator I will make a short presentation of Sus scrofa.

The wild boar has a strong and compact,laterally flattened body conformation and bears most of the body´s weight on the frontpart.It has a thick neck,shoulders are upright, the back is sloping and terminates in a short and straight tail.The head is wedge-shaped,with powerful jaws and big visible mandibular canines.Since feedinghabits consists of rooting up ground the snout is strong and mobile.The skin is thick and covered by a brownish to black hairy coat.

Pigs feed on a mainly high-fibre(>25%),low protein diet(grasses.legumes,herbs and roots),but also on crops,seeds and other animals such as snails,earthworms,insects,eggs and young of nesting birds.The stomach is of the single type which is suitable to the various diet.

A mature pig belonging to Sus scrofa scrofa, found in northern,west and central Europe is about one meter high and 1.5 meter long.The male´s weight can reach 150 kg´s,and the somewhat smaller female´s around 80-90.In eastern and southeast of Europe Sus scrofa attila can be found which is much larger and can reach the double weight.In the mediterranean countries there are some smaller variants.

Pigs are very powerful omnivorous mammals that lives in disturbed areas,planted and natural forests,coastlands,agricultural areas,wetlands,scrublands and urban areas.

They are native to Eurasia and North Africa but by early seafarers introduced them to new areas,and they are now extinct over a larger range including Britain,Southern Scandinavia and  the Nile valley

In Sweden the first wild boars appeared about 8-10 000 years ago,(approx same time as Homo sapiens),but became common when real forest developed 2-3000 years later.Due to hunting and domestication the wild boars dissappeared in the 17th century and were absent for 200 years, but were in the 70´s reintroduced  in parks.After escaping and rapid spreading they are now present again concentrated to the forests of south east and south regions of the country.For the moment there is about 20 000 wild boars in Sweden.A poulation can even when hunted increase rapidly.Sows has good reproduction functions giving birth to up to eight piglets,and the pig are relatively resistant to climate changes.Mortality is highest at young age.

The sow gets mature at the age of 11/2 years and are receptable for the male during several months.Oestrus that is regulated by the length of daylight and resources of food often occurs in all sows in a group at the same time.The length of gestation is 115 days.Close to giving birth the sow builds a nest by digging a hole in the soil and then filling it up with vegetation parts forming a protective roof.

The piglets grow rapidly,already after a few weeks they follow the sow on smaller excursions and after 3-4 months they are weaned.The sow normally gives birth to one litter per year and repells the piglets,now weighing about 60kg`s when it is time for next offspring.

Wild boars are seldom seen in daylight,they are very shy animals having their active time during the night.Although traces left by their activity in feeding, nestbuilding,scratching and even bathing are easy to recognise.The soils gets largely "ploughed" with ripped vegetation,grooves and holes.The stem of trees gets damaged from scratching,roots are digged up and eaten and during hot summerperiods in wet areas big holes can be seen from mud bathing.

Pigs are normally rather slow animals but can make fast short moves if frightened or threathened.They are mobile animals not fixed in locality and can move 20 km in 48 hours when suffering from food shortage.During their journey large damages can occur which in the case of agricultural areas can be of severe economic loss for the farmers.Oat and wheat fields are especially  favourable to the pigs,but also potatoes and peas can be eaten by hungry pigs.Planted trees can also be digged up for the sake of tasty roots.

As mentioned before there are two sides of viewing the wild boars role in nature,destroyer or cultivator.

Of course the damage of growing crops and spreading of weeds can be considered  as a big problem,especially if the population grows unlimited.On the other hand in natural forests the positive effects of the pigs is spreading of plant species.Due to research made in Sweden the number of plant species was 30% higher in areas "disturbed" by pigs compared to similar areas whithout pigs.Especially in biotopes consisting of wet areas and damp forests,older pine forests and in grasslands mosses, fruit trees and  species with small seeds showed increased spreading.There seem to be two ways of pigs increasing the floral diversity in their environment; spreading seeds and spores by faeces or transporting in hair coat and cultivating the soil.

What is the best way to protect cultivated areas and sensitive forest from the pig´s damaging effects?

In Sweden positive effects has been seen from getting pigs attention away from sensitive crops by feeding of left-over fruit and seeds,also by planting oat and wheat in wild fields close to forests rich in pigs.

Electric fences are also an effective way to keep the pigs away from attractive areas.

To keep the populations down hunting with dogs and poisoning are standard methods.In Hawaii snaring has been used to control the pigs but that should not be regarded as an acceptable method.

In my opinion wild boars seem to play an important role in the ecosystem but most of course be controlled from unlimited spreading and thereby severe damaging of their environment.Another thought that goes through my mind after getting a closer insight in how pigs live and behave;since the pig seems to be a extremely active and mobile animal,maybe the methods of keeping our domesticated pigs are not the best....Many improvements could easily be done in simple and not too expensive manners.

Sources:

Liv och död bland vildsvinen,article in Svensk Jakt 7:2000 by Jonas Lemel

ISSG Global Invasive species database, www.issg.appfa.auckland.ac.nz/database/species/ecology

Vilt och vetande,www.jagareforbundet.se

Lär känna vildsvinet,Sv jägareförbundets förlag,1987,Görgen Göransson