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Biodivers Conserv DOI 10.1007/s10531-009-9697-0 ERRATUM Ecological impacts of feral pigs in the Hawaiian Islands Sérgio L. G. Nogueira-Filho Æ Selene S. C. Nogueira Æ José M. V. Fragoso ! Springer Science+Business Media B.V. 2009 Erratum to: Biodivers Conserv DOI 10.1007/s10531-009-9680-9 The Author would like to add the following paragraph on page 4 after the sentence ‘‘…. Proper controls should consider animal behavior and spatial components such as pig home range size, movements, and plant distribution patterns. ‘‘Another potentially confounding factor is that large grazing and/or browsing ducks and geese where once common to the islands but are now extinct or greatly reduced in population size (Paxinos et al. 2002). One of these geese species was four times the size of a Canada goose (Branta canadensis) to which they were closely related. The nene (Branta sandvicensis) is extant but populations and range have been severely reduced. These were not wetland dwellers; instead they inhabited a variety of habitat types (Paxinos et al. 2002). Canada geese can strongly affect native plant community composition and reduce the abundance of native species (Haramis and Kearns 2007). The extinction of the Hawaiian species probably severely altered Hawaiian plant communities and populations, possibly in a manner analogous to what was described by Dirzo and Miranda (1990) for tropical plant communities when native mammalian grazers and browsers are extirpated. They described a significant reduction in plant species diversity in areas missing large vertebrate browsers and grazers and a shift to increased numerical dominance by a few species. Understanding the ecological significance of pig impacts on ‘‘native’’ biotic communities may thus be further confounded by the impacts generated by the extinction of native flightless geese and ducks.’’ The online version of the original article can be found under doi:10.1007/s10531-009-9680-9. S. L. G. Nogueira-Filho (&) ! S. S. C. Nogueira Applied Ethology Laboratory, Universidade Estadual de Santa Cruz, Rod. Ilhéus Itabuna km 16, Ilhéus, BA 45662-000, Brazil e-mail: [email protected] S. L. G. Nogueira-Filho ! S. S. C. Nogueira ! J. M. V. Fragoso Department of Botany, University of Hawaii at Manoa, 3190 Maile Way, St. John Lab 101, Honolulu, HI 96822, USA 123 Biodivers Conserv In the Conclusion part on page no. 5 the Author would like to replace the following sentence ‘‘Despite the many potential negative impacts to native biota and ecosystems generated by pig activities, eliminating the pig from Hawaiian Islands remains difficult if not impossible, mostly because many Hawaiians further value it for its cultural, and religious significance (Stone 1985).’’ with ‘‘Despite the many potential negative impacts to native biota and ecosystems generated by pig activities, eliminating the pig from Hawaiian Islands remains difficult if not impossible, mostly because many Hawaiians value the pig for its recreational value (Stone 1985), while indigenous Hawaiians further value it for its cultural and religious significance (Mueller-Dombois and Wirawan 2005).’’ Also the following references are to be added in the References: 1. Dirzo R, Miranda A (1990) Contemporary Neotropical defaunation and forest structure, function, and diversity—a sequel to John Terborgh. Conserv Biol 4:444–447 2. Haramis GM, Kearns GD (2007) Herbivory by resident geese: the loss and recovery of wild rice along the tidal patuxent river. J Wildl Manag 71:788–794 3. Mueller-Dombois D, Wirawan N (2005) The Kahana Valley Ahupua‘a, a PABITRA study site on O‘ahu, Hawaiian Islands. Pac Sci 59:293–314 4. Paxinos EE, James HF, Olson SL, Sorenson MD, Jackson J, Fleischer RC (2002) mtDNA from fossils reveals a radiation of Hawaiian geese recently derived from the Canada goose (Branta canadensis). Proc Natl Acad Sci USA 99:1399–1404 123 Biodivers Conserv DOI 10.1007/s10531-009-9680-9 REVIEW PAPER Ecological impacts of feral pigs in the Hawaiian Islands Sérgio L. G. Nogueira-Filho Æ Selene S. C. Nogueira Æ José M. V. Fragoso Received: 13 October 2008 / Accepted: 18 June 2009 ! Springer Science+Business Media B.V. 2009 Abstract The foraging habits of exotic ungulate species can directly and indirectly affect native plant and animal distribution and abundance patterns. Most of the studies on feral pig interactions with other biota in the Hawaiian Islands have been published as difficult to access reports to governmental and nongovernmental organizations, graduate student theses, and a few in peer reviewed journals. In this paper we discuss the origins of pig introductions to Hawaii, their feralization process, population expansion, and interactions with native and non-native biota. We also consider the environmental effects triggered by pigs on local ecosystems and biotic communities. Feral pig activities can reduce the abundance of native plant species, enhance conditions for the establishment of invasive nonindigenous plants, and perhaps indirectly negatively impact native forest bird species. Pig foraging and traveling patterns may also lead to physical alteration of ecosystems by increasing soil erosion that may lead to watershed degradation. However, much remains to be learned about the strength and significance of aforementioned interactions and their longterm effects on Hawaiian biota and ecosystems due to some confounding events. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions. Keywords Habitat disturbance ! Invasive species ! Plant–animal interactions Introduction Humans were the first large-terrestrial mammals to populate Hawaii. This occurred when the Polynesians colonized the islands during prehistoric times (Kirch 1982). These early S. L. G. Nogueira-Filho (&) ! S. S. C. Nogueira Applied Ethology Laboratory, Universidade Estadual de Santa Cruz, Rod. Ilhéus Itabuna km 16, Ilhéus, BA 45662-000, Brazil e-mail: [email protected] S. L. G. Nogueira-Filho ! S. S. C. Nogueira ! J. M. V. Fragoso Department of Botany, University of Hawaii at Manoa, 3190 Maile Way, St. John Lab 101, Honolulu, HI 96822, USA 123 Biodivers Conserv Polynesian settlers (indigenous Hawaiians) introduced the Polynesian breed of pig, probably derived from Sus scrofa vittatus, to the Hawaiian Islands as an important source of food (Kirch 1979; Diong 1982; Tomich 1986). The direct effects of people and their introduced animals led to major changes in the ecology of islands (Kirch 1982). The rate and extent of environmental change probably increased dramatically after the first European explorers arrived on the Hawaiian Islands. The Europeans released more pigs (Sus scrofa scrofa) and additional ungulate species, such as goats (Capra hircus), for the purpose of establishing a readily available food source to support subsequent voyages (Tomich 1986). Later introductions included more pigs as well as domestic cattle (Bos taurus) and sheep (Ovis aries) (Stone and Pratt 1994). More recently, ‘‘game animals’’, such as mouflon sheep (Ovis musimon) and axis deer (Axis axis) were introduced to the islands to establish populations for sports hunting (Giffin 1978b; Gagne 1988). The freeroaming populations of these exotic ungulate species have affected the landscape of Hawaii (Nichols 1962; Tomich 1986). Most information on potential mechanisms of pig-induced changes in community composition and ecosystem processes have been published as reports to governmental and nongovernmental organizations, with a few published as graduate student theses, and a few in peer reviewed journals, limiting their accessibility to the research and management community. We review this literature and provide additional commentary on pig interactions with other biota and ecosystems pointing out the weaknesses and the strengths of the available studies on the ecological consequences of feral pig habitation of the Hawaiian Islands. Pig introductions to Hawaii and their feralization process At the time of European contact pigs were probably present throughout most of the islands (McEldowney 1979). Polynesian pigs were maintained around early Hawaiian houses in the lowlands and were usually kept close by the household (Tomich 1986). These animals were smaller in size than European pigs (Diong 1982; Tomich 1986). Barret and Stone (1983) proposed that the reportedly small size of Polynesian pigs might have been caused by a low protein diet since a diet rich in protein is required for maximum growth of pigs. Diong (1982), Barret and Stone (1983), and Stone (1985) also posit that feral pig populations in Hawaiian rainforests before European contact occurred at low densities. They report that these were frequently hunted (indicating the presence of feral populations) and therefore hypothesized that they did not cause great modification to the interior rainforest vegetation. Captain Cook brought European pigs on his first voyage to Hawaii and gave these to indigenous Hawaiian leaders (Diong 1982; Tomich 1986). Many other importations followed and some point more animals became feral and dispersed throughout the islands; the general consensus has been that afterward the Asiatic form of pig was absorbed or replaced by European breeds (e.g., Tomich 1986). However, Diong (1982) considered the Hawaiian feral pigs to be closely related to the original Polynesian pigs. In a recent study on feral pig phylogeography using mitochondrial DNA, Larson et al. (2005) found that modern Hawaiian feral pigs form a monophyletic group with pigs from New Guinea, Vanuatu, and Halmahera (the ‘‘Pacific clade’’) within the large Eastern Eurasian cluster which is well separated from the other groups, confirming Diong’s (1982) proposition that feral populations already existed before European contact. Land use changes associated with the post-contact era probably contributed to and accelerated the 123 Biodivers Conserv feralization process. This included forest clearing, introduction of western agricultural practices, ranching, and the uphill recession of the forest line (Diong 1982, p. 74). Barret and Stone (1983) suggested that the factor that most contributed to the posited low density of feral pig populations before European contact, was the absence of an abundant animal protein source such as the earthworms that were introduced after European colonization. Some studies support the hypothesis that earthworms are the most important source of animal protein for Hawaii feral pig populations (Giffin 1978a; Diong 1982; Anderson 1994). Additionally, Diong (1982) proposed that the introduction of agriculture along with earthworms contributed to the expansion of feral pig ranges. However, in conflict with this hypothesis is the observation that pre-European times pigs would have had accesses to protein sources such as sea turtle eggs and sea birds in breeding colonies. The establishment of beef cattle industry may also have contributed to feral pig range expansion in historical times, through the clearing of native forest to establish pastures (Diong 1982), and the introduction of another protein source, dung beetles which were introduced to aid in the decomposition of cattle feces (Howarth 1985). Giffin (1978a) detected dung beetle adults and larvae in pig stomachs. Dung beetles are known to enhance soil nutrient cycling (Howarth 1985), therefore we suggest that such increment would increase earthworm population and positively affecting feral pig population. Interactions with native and non-native biota Some hypothesize that no Hawaiian endemic plant is evolutionary adapted for coexistence with herbivorous mammals, making them very vulnerable to damage from pig foraging habits (e.g., Spatz and Mueller-Dombois 1975). Evidence presented in support of this hypothesis is that some plant species have lost ancestral traits such as thorns and secondary leaf compounds that may have served to reduce or prevent herbivory. Plants belonging to this group, such as the Haleakala greensword (Argyroxiphium virescens), of the bogs on Maui can be severely damaged by pig foraging (Loope 1983). Evaluating pig impacts on native plants is further complicated by the reduction in the range and abundance of plants generated by direct human activity. Teasing apart the impact of these multiple factors can be difficult. When pigs forage on rare plant species they represent an additional threat towards a group that may be on the brink of extinction due to human activities. They may also threaten the structure and dynamics of native rainforests by altering the dominance pattern of plant species (Tomich 1986). In one rainforest they foraged on at least 40 plant species, 75% of which were native plants (e.g., tree ferns Cibotium chamissoi and C. splendens; Barret and Stone 1983). The introductions of fruit-producing non-indigenous plant species, such as strawberry guava (Psidium cattleianum), may also have contributed to pig population expansion (Barret and Stone 1983). Pigs are generalist feeders and will eat what is available. Diong (1982) observed that feral pigs in the Kipahulu Valley, Maui, shifted from a diet composed primarily of hapu’u tree ferns (Citobium spp.) to one comprised mostly of strawberry guava fruits according to their availability. A similar shift was observed by Giffin (1978a) who observed that Citobium spp. was the main food item in diets but this shifted almost exclusively to banana poka (Passiflora mollissima) fruit when these became available. Pigs in Hawaii compound and intensify the problem of non-indigenous plant invasions by facilitating their introduction by carrying seeds in their guts (e.g., strawberry guava) and on their coats (e.g., Carex alligata, Paspalum conjugatum) (Diong 1982). As early as 1936, 123 Biodivers Conserv Judd reported that native Hawaiian forests where being replaced by stands of non-indigenous guava established by feral pigs. Diong (1982) posits that feral pigs are the primary modifiers of remaining native rainforests because of their removal of native tree ferns (eaten) and their dispersal of guava seeds. Their feeding decreases the populations of native tree ferns and sub-canopy cover and increases the abundance of guava. It is through this interactive process that feral pigs facilitate the displacement of native species with a nonindigenous invader (Diong 1982). Pigs can also accelerate damage to native plant populations by trampling individuals of native species, and by increasing soil fertility (Stone 1985). Apparently many native plants are adapted to poor soils and increasing soil fertility after pig rooting works against their reestablishment and favor the establishment of some non-indigenous plants (Stone 1985). Aplet et al. (1991) also reported pigs dispersing Myrica faya seeds into rainforests. This non-indigenous species is a nitrogen-fixing tree that can generate several effects through communities. For example, earthworms increased in abundance in areas inhabited by M. faya, probably due to a response to the increased soil nitrogen generated by M. faya (Aplet 1990). We suggest that an increased abundance of earthworms could lead to an increase in pig foraging near such trees and, consequently, dispersing more Myrica faya seeds. Exclosure studies have been used on the Hawaiian Islands to evaluate and demonstrate impacts of feral pigs upon native vegetation and to assess vegetation recovery from feral pig damage. To obtain these data, the researchers usually build a pig-proof exclosure, sampling a proportion of the exclosure interior as well as a corresponding area on the outside. Loope and Scowcroft (1985) reviewed the exclosure studies and found more than 50 studies using small exclosures established to demonstrate and evaluate vegetation response after ungulate removal in Hawaii. Only a few studies, however, documented plant succession through several years and using large fenced areas (e.g., Loope et al. 1991; Medeiros et al. 1991; Pratt et al. 1999). In exclosure studies, a variety of methods and sampling intervals have been used for assessing vegetation change. For example, Jacobi (1981), Katahira (1980), and MuellerDombois (1981) used a point frequency sampling method for estimating percentage ground cover and species composition. Scowcroft and Giffin (1983) used the line-intercept method to estimate cover in exclosures located in sparse sub alpine forest on Mauna Kea. Additionally, the percentage of litter, exposed soil, the potential area rooted by feral pigs, the percentage of the area actually dug, and the degree of root exposure were also examined (e.g., Higashino and Stone 1982; Stone and Taylor 1984). Sampling has typically been done annually up to 5 years after the exclosure was built (Loope and Scowcroft 1985). However, statistical analyses of cover, abundance, and other exclosure data have been barely used in these studies (Loope and Scowcroft 1985). Another potential problem in many of these studies is that control areas were actually located close to or very near the fence. This type of placement may generate a confounding problem. The fence impedes regular movements and dispersal events and this would tend to concentrate activities and animals in areas along and adjacent to the fence. This outcome would magnify pig impacts to some distance beyond the fence because the fence restrains the pig’s usual movement pattern leading to overus around the control areas. Proper controls should consider animal behavior and spatial components such as pig home range size, movements, and plant distribution patterns. The exclosure studies showed that removal of feral pigs can result in the recovery of native vegetation (e.g., Jacobi 1976; Katahira 1980; Higashino and Stone 1982; Stone et al. 1992). This activity, however, has been a negligible impact on non-indigenous plant populations within fences (Aplet et al. 1991). Therefore, the feral pigs affected native but 123 Biodivers Conserv not non-indigenous plant species. Thus, removing pigs does not always alter plant population and community dynamics, which suggests that in these areas other factors may be driving plant population dynamics. Stone (1985) published a review on other consequences of pig introduction on the biota of Hawaii, and highlighted their indirect negative effects on native forest birds. For example, the foraging behavior of pigs may reduce the amount of nectar produced by understory plants, such as Rubus hawaiiensis (Stone 1985). Less nectar has been posited to negatively impact native nectar feeding birds, such as Oreomystis mana, Loxops coccineus, Chasiempis sandwichensis ibidis, Myadestes myadestinus, and Moho braccatus (van Riper and Scott 2001). Pigs may also increase breeding habitat for exotic mosquitoes (Culex quinquefasciatus), and these are the vector of avian poxvirus (Poxvirus avium) and avian malaria (Plasmodium relictum). They do this by increasing water pool availability when they eat the inner core of tree fern trunks (Baker 1979). Diong (1982, pp. 166–167) does not support this hypothesis, arguing that there is an unlimited number of naturally occurring breeding sites for mosquitoes, such as in tree cavities, trunk axils, forest floor and at bases of fallen trees. However, the decrease of Culex sp. adults captured inside pig exclosure areas when compared to places where feral pig activity was high provides some evidence for a relationship between pig activities and the abundance and distribution of the disease vector (Lease et al. 1996). More recently, LaPointe (2006) corroborated the proposition that feral pigs may be pivotally important to the avian diseases system, at least in remote Hawaiian rain forests. The author observed an increment of rain-filled cavities in tree fern trunks eaten by pigs and verified that such cavities are the most abundant and productive habitat for larval mosquitoes. Conclusions Direct consumption of native plant species by feral pigs is the most common impact reported in the literature. Foraging and trampling by pigs can also cause severe erosion and lead to the degradation of watersheds (Cuddihy and Stone 1993). However, perhaps the most dramatic ecological consequence of feral pig introduction to the Hawaiian Islands may be the environmental effects they trigger in biotic communities. They do this, along with many other animal species, by influencing the population dynamics of native species and promoting the dispersal and establishment of non-indigenous species, thus altering the composition and structure of communities (Diong 1982; Cuddihy and Stone 1993). The feeding habits of pigs and consequent habitat alteration may also indirectly negatively impact native forest birds (van Riper and Scott 2001; LaPointe 2006). Despite the many potential negative impacts to native biota and ecosystems generated by pig activities, eliminating the pig from Hawaiian Islands remains difficult if not impossible, mostly because many Hawaiians further value it for its cultural, and religious significance (Stone 1985). There are needed improvements in assessing the adequacy of sample sizes, experimental study designs, issues of scale, spatial perspectives and statistical analyses so that we can better assess the role of pigs in Hawaii ecosystems. In other words, methods have to be appropriate to the question asked. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions. An example of this problem is found with studies that seek to obtain pig home range area and movement pattern data. Many if not all studies of pig movements in Hawaii occurred during pig eradication efforts, when the 123 Biodivers Conserv animals were being chased and killed (e.g., Diong 1982). Data obtained in this manner are useful in developing methods to eradicate feral pigs more efficiently. However, this approach is unlikely to produce representative measures of pig home range area and movement patterns without hunting pressure, which are also crucial information to establish appropriate control measures. Therefore, much remains to be learned about the strength of the aforementioned interactions and their long-term effects on Hawaiian biota and ecosystems. Elucidating the dynamics and long-term ecological effects generated by pigs is a crucial step towards increasing our understanding of and more effectively managing biotic interactions. Acknowledgments We thank Edwin D. Johnson, Statewide Hunting Coordinator, for giving us access to DLNR/DOFAW files, David Duffy, for giving us access to PCSU files, and Biodiversity and Conservation anonymous reviewers. SSCN and SLGNF were supported by CNPq (Proc. 200335/2005-7) and CAPES (Proc. 0597-05-8) Brazilian Educational Agencies provided funding through their grants while in Hawaii. We thank Carter Miller, Janisete Gomes da Silva Miller, Lynn Schnurr and Sean Giery for providing helpful comments to improve the final manuscript. This research was supported by the Ko’olau Mountains Watershed Partnership. References Anderson SP (1994) Some environmental indicators related to feral pig activity in a Hawaiian rain forest. M.Sc. thesis, University of Hawaii, Honolulu Aplet GG (1990) Alteration of earthworm community biomass by the alien Myrica faya in Hawaii. 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