THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
How well do you know your local pig (Sus scrofa)?
For questions or comments regarding this publication contact: Dr. Raoul Boughton, University of Florida, IFAS
Bit of History
If you were asked what has been one of the most problematic and understudied invasive species in Florida, what would you say? Well, I would say wild pigs, also known as wild hogs, feral swine, wild boar or my favorite, razorbacks. I’ll use feral swine for this article. A quick bit of history will inform you that domesticated pigs may have first reached North America during Ponce de Leone expeditions of 1513 or 1521. More definitively, domestic stock was known to be on conquistador Hernando de Soto’s expedition of 1539, which included swine to provision a settlement he established at Charlotte Harbor. Over centuries, explorers and settlers continued to introduce, transport, and distribute domestic pigs and often the preferred animal management practice was open range. Escapes into the Florida wilderness occurred, and pigs were also given and possibly taken off the range by Native Americans, who further distributed the pigs. Overtime, feral populations have become well established throughout Florida, and have been further supplemented through deliberate releases in many areas by private individuals, and at times by government agencies to improve hunting opportunities.
Feral swine are expanding their range in the US (now 35 states) and are at their highest densities in south central Florida. Across the state their total population may be 1 million individuals, although this is just an educated guess and no one knows for sure. In Florida feral swine damage crops, stock, and property, and are reservoirs for many diseases such as leptospirosis, swine brucellosis, and pseudo-rabies. Rooting feral swine also dig up large areas of native vegetation, spread weeds, and disrupt ecological processes such as habitat succession, plant community composition, water retention, and soil health. They are omnivorous and their diet choice is enormous, which includes anything they can grab a hold of and swallow. A high proportion of their diet is vegetative including tubers and fruits but feral swine can also be significant predators. They are also known to eat the eggs and newly hatched young of ground-nesting birds and sea turtles, young tortoises, small mammals, salamanders, frogs, crabs, mussels, and snakes. Though not usually considered a predator of deer, feral swine do occasionally kill and eat newborn white-tailed deer and will scavenge carrion. Indirect impacts upon native wildlife also occur when feral swine compete for food and space with these species. This competition is highest during late summer and fall when seasonal harvest such as hickory nut and acorns become available. Management of this invasive species is complicated by the fact that eradication or control is difficult due to very high reproductive rates, and that it is often not acceptable to local communities that value feral pigs for hunting and food.
Some things we don’t know but should
To help with management options and to better understand the impacts on ranchers we need to know;
- What is the population size?
- How do densities change seasonally and by habitat?
- What habitats do they utilize the most?
- What is the true economic cost of feral swine damage to agriculture? For example how much cattle supplemental feed do they eat? What is the forage loss caused by rooting improved, unimproved and native pasture? How many oranges are harvested by feral swine?
- How large an area do they use?
- How far will they disperse and recolonize?
- What is the economic value of feral swine for hunting and food?
GPS Collar and habitat study
To start to answer a few of these questions a collaborative project among UF, USDA, and MacArthur Agro-ecology Research Center, captured and GPS collared (Fig 1) 10 feral sows and 10 feral boars on the Buck-Island Ranch during the dry season of Jan-May 2013. These collars collected location fixes every 15mins and we used this data to analyze how big of an area they use, also known as their home range (Fig 2). Florida feral swine sows, at least during the dry season when this study was conducted, have on average smaller home ranges than Alabama sows (370 ac versus 988ac, respectively). Whereas boars (not shown) average home range size are more similar for the two studies 790ac versus 1005 ac. This suggests sows also have smaller ranges than boars in Florida (Fig 3). The small area of use by Florida sows indicates high food resource availability in these Florida habitats, possibly assisted by supplemental winter cattle feed. Early indication from a game camera study suggests that 1/5 of all visits to molasses tubs provided to cattle are by feral swine. The small home range of sows may be one reason as to why very high densities of feral swine can be observed in Florida habitats.
From GPS movement data, we can estimate how many sounders (“a herd of sows, usually related”) could be supported by Florida landscapes, but it can also be used in combination with habitat maps to answer the question of what habitats are preferred. To do this, we ask what habitats are available for feral swine and of what proportions (e.g., 50% forest, 30% pasture, 20% wetland), and we then compare the time spent in those habitats (Fig 4). Time spent can be calculated directly from the number of GPS points which were obtained from those habitats. The most obvious outcome and one that is not too surprising is that feral swine spend a lot of time in wetlands (Fig 4). What is interesting about this observation is that it occurred when the wetlands were completely dry (Jan-May). Despite lack of water in wetlands we believe the dense standing vegetation provides perfect cover for feral swine in the dry season, as well as a food source. In a future study during the wet season we may find swine actually using wetlands less due to water levels. In the fall we may expect them to be utilizing hammock where acorn mast is most abundant. On averaged, feral swine spend less time in pasture and semi-improved pasture than the proportion available, possibly because of lack of cover (Fig 4).
Ongoing work using our GPS dataset will look at temporal patterns of activity and response of feral swine behavior to newly burnt pasture. We will be continuing our collaborative studies to estimate the economic costs of the destructive nature of feral swine. We will also track their activities in summer and fall seasons, and are developing a new study to use ear tag GPS units to understand the dispersal of young feral swine. Dispersal is an important parameter to understand how far and how often individuals immigrate into other populations, expand populations, colonize new areas or recolonize controlled areas. These dispersal factors can then be used for estimating how long a control effort should last, how large of an area should be controlled to make a significant impact, and for understanding the movement and transmission of disease.
I will now leave you with a question, and would like to hear your responses, so please have your say.
The Farm Bill of 2014 allocated 20 million dollars a year for feral swine control activities across the US. Was that allocation needed and if so should it have been more?
The study presented here would not have been possible without the support of Dr. Sam Wisely, UF Wildlife, Ecology and Conservation; UF graduate students Andrew Satterlee and Brittany Bankovich; Dr. Tyler Campbell of the East Foundation; Eric Tillman and Mike Millison of the USDA APHIS Wildlife Services; Dr. Elizabeth Boughton and Gene Lollis of MAERC.
Figure 1: Dr. Boughton and M.S. student Brittany Bankovich collaring an anesthetized feral swine.
Figure 2: The home-range of 9 collared feral swine sows from Jan-May 2013 on the Buck Island Ranch (black lines). We used a kernel estimate method that selects the 95% most used areas. Overlapping home ranges are probably due to sows captured from the same sounders (e.g., 115_F and 206_F). Sow 135_F had the smallest home-range of 92 acres and sow 025_F the largest of 911 acres.
Figure 3: Size of area used or home range of feral swine in the dry season in Florida and Alabama. Alabama data reproduced from Gaston et al (2008) Home range and habitat use of feral hogs Sus scrofa on lowndes county WMA, Alabama. In National Conference on Feral Hogs, p. 6