Ecology and management of feral pigs (Sus scrofa) in rainforests
Mitchell, James Leonard (2002) Ecology and management of feral pigs (Sus scrofa) in rainforests. PhD thesis, James Cook University.
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Abstract
The World Heritage Area (WHA) rainforests of the wet tropics region of northeast Queensland, is regarded as natural heritage of outstanding universal value and one of the most significant regional ecosystems in the world. The introduced feral pig (Sus scrofa) have now become established and widely distributed throughout the WHA. Feral pigs are believed to have a severe negative impact on the conservation values of the WHA, however, very little quantitative information on their ecological impacts or ecology is available for this region. This study aims to obtain information on selected aspects of feral pig ecology and their ecological impacts and to utilise this information to assist in developing management strategies for feral pig control within the WHA.
The study was conducted near Cardwell, north Queensland, Australia (180 16' S, 1460 2' E) from 1995 to 1999. Three broad macrohabitat areas were identified within the study site: highland rainforests, rainforests / cropping ecotone and the coastal lowlands. Within each of these areas, key microhabitats were selected to establish spatial and temporal patterns of pig diggings. Fenced exclosures were also established within the highland area to quantify the ecological impacts associated with pig diggings. Seedling survival, the biomass of roots, leaf litter and earthworms and soil moisture levels were used as ecological indicators of impact. Radio tracking was used to determine seasonal migration patterns, and seasonal home range sizes. Aspects of pig biology including reproductive parameters, population dynamics, population density and morphological models were derived from a sample of captured feral pigs.
Feral pig diggings were found to have spatial and temporal patterns; rainfall (soil moisture)appeared to be the major influence on digging patterns. Pigs preferred to dig in specific microhabitats and these digging patterns varied significantly due to seasonal influences. Most diggings occurred in the early dry season and predominantly in moist (swamp and creek) microhabitats where seasonally suitable soil moisture levels and associated earthworm populations were present. A significant relationship between diggings and rainfall was found with the majority of diggings occurring 3 to 4 months after the peak of the rainfall. The majority of pig diggings were concentrated in only a small proportion of the total WHA, and only minimal pig diggings were found throughout the general forest floor. The small area microhabitats that were preferred by pigs experienced intense digging impacts, especially as the soil began to dry out after the end of the wet season. The spatial pattern of diggings appear to be correlated with the availability of suitable soil moisture levels for earthworm populations to exist. The overall mean amount of ground disturbance by pig diggings for each day was 0.09% of the surface area. Highland swamps recorded the most pig diggings with over 80% of the swamp area dug up by pigs at some time during the 2 year study period. The frequency of diggings occurring on transects was 23%.
The overall ecological effects of feral pig diggings were difficult to quantify. No significant effects of pig diggings were detected on leaf litter, root and earthworm biomass or on soil moisture levels. Significant correlations between earthworm biomass, seedling survival and soil moisture levels were observed. However, there was a general trend that more seedlings survived when protected from pig diggings. In total 5852 seedlings were monitored over a two year study period. On average, 31% more seedlings survived within the protected exclosures, compared to the unprotected controls. Nine of the twelve exclosures had more seedlings within the protected exclosures than within the unprotected controls. A statistically significant impact of diggings influencing the survival of seedling was only demonstrated in the drier microhabitats, and could not be quantified in the moist microhabitats.
The exclosures were established specifically to examine the recovery of the ecological variables after protection from further pig diggings. A clear trend of recovery of seedling numbers was demonstrated when protected from pig diggings. Over the 2 year study, the mean number of seedlings within the protected exclosures increased 7%, while the number in the controls decreased 37%. The difference in seedling numbers between the exclosures and controls were influenced by the recovery time. This was pronounced in the dry microhabitat where a significant interaction (treatment x time) effect was found. Significantly more seedlings survived inside exclosures during the last 8 months of the study. It was concluded that seedling numbers will recover when protected from pig diggings.
No evidence of the hypothesised large-scale seasonal migration was found in this study. Pigs in the lowlands and the highlands were sedentary and stayed within their defined home range throughout the 4 year study period. The mean distance that any pig moved from the centre of their calculated home range was 1.03 km. Pigs on the rainforest/crop ecotone have established home ranges that vary in size due to seasonal influences. Males tended to have a slightly larger mean home range size (7.9 km²) then females (7.3 km²) and both have a significantly larger mean home range size in the dry season (7.7 km²) compared to the wet season (2.9 km²). The mean home range size calculated for all studied pigs was 5.5 km². No significant difference in home range size was detected between the sexes.
The absence of seasonal migration movements was contrary to general community perceptions. Most landholders within the region believed feral pigs migrated from the highlands in the dry season to the coastal lowlands to forage on the ripening sugar cane and banana crops, returning to the highlands in the wet season when the sugar cane is harvested. No evidence was found in this study area to support this migration model. Rather, the home range study suggested that pigs moved greater distances and foraged further when food and water become scarce in the dry season, thus increasing their interaction with humans. During the wet season, feral pigs were more sedentary, thus human/pig interaction was lower. This leads to the perception of higher pig populations in the dry season, interpreted as being due to migration from the highland rainforests. The seasonal fluctuations of pig home range size on the rainforest/crop boundary, coupled with the fluctuations in pig/human encounter rates is the cause of the community perception of a seasonal migration pattern.
A sample of 336 pigs was trapped during the 5 year trapping project. Most of the captured sample (56%) were less then 12 months of age (mean age 15.3 months); less then 5% of the population was calculated to be older than 5 years of age. Female pigs have an all year round breeding pattern with a birth peak in January, the start of the wet season. The prevalence of pregnancy was 41% with 1.64 pregnancies per year; litter size in utero was 6.4. The mortality rate in the first year of life was 51%. Growth rates and morphological information suggest feral pigs within this rainforest region have faster growth rates and are on average heavier then feral pigs in dry tropical regions. Morphometric models were developed that found significant relationships between body measurements with body weight and age variables. These models may be of value in future feral pig ecological studies. The studied pig population had the general characteristics of a young healthy, fecund population, with a capacity to expand rapidly. Reproductive potential is high, however high juvenile mortality counter - balances the potential population growth. Population density in the lowland area was approximately 3.1 pigs km².
Feral pigs have been identified as a major issue facing the management of the wet tropics WHA. The implementation of a feral pig management strategy must rely on a clear understanding of the severity of ecological impacts of pig diggings on WHA values and also the level of population control required to protect these values. The management of feral pigs within the WHA is a complex issue. A range of factors have to be considered in developing a strategy that takes into account the bio-physical, economic and social issues of pig management within the WHA region.
The economic impact caused by feral pigs to the agricultural industries on the WHA boundary are due mainly to pigs that permanently reside in the rainforest/crop ecotone, and not by pigs migrating down from the highlands. The timing and location of control strategies to reduce this economic impact should therefore concentrate on the WHA boundary during the dry season when pig home ranges expand into the maturing cropping systems. Environmental impacts caused by feral pigs need to be managed on a priority basis. Management of pigs in the inaccessible highland areas is impractical. Logistic and economic problems are insunnountable with present control technology. Resources for pig management need to be targeted in areas identified as having high environmental, social or economic values. The ecology and management of feral pigs within the WHA rainforests encompasses a range of issues. This study has highlighted and quantified some aspects of feral pig ecology and management issues. Hopefully this thesis can be used as the basic stepping stone for future research into developing effective management strategies.
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