General Impact
Establishment of feral herbivores like sheep (Ovis aries) have had significant ecological impacts on island ecosystems. Island ecosystems are particularly vulnerable to herbivores as insular plants in these ecosystems evolved largely in the absence of large herbivores, therefore lacking in defences against them. Increased bare ground followed by increased erosion are some of the other impacts (Van Vuren and Coblentz, 1987). Van Vuren and Coblentz (1987) in their study on the ecological effects of feral sheep on Santa Cruz Island, California observe that feral sheep are forage generalists when compared to domestic sheep on mainland. Feral sheep diet include annual grasses, forbs and also a substantial quantity of shrubs. The authors summarise the ecological impacts of feral sheep: consumption of endemic species by feral sheep could potentially cause decline in their population levels; loss of vegetation due to trampling while grazing; compaction of the soil and therefore changes in the soil structure; soil erosion due to removal of vegetration and denudation of the soil; removal of hebaceous vegetation caused changes in the grassland community, reduction of litter and a decline in the recruitment of seedlings. Alteration in the plant community led to decrease in species diversity. Grazing and browzing of herbaceous vegetation, and stripping of bark by feral sheep and other introduced mammals (cattle (Bos taurus), Mouflon sheep (Ovis musimon), and feral goats (Capra hircus)) have led to exposure of soil to erosion and degradation of forests on Mauna Kea (Scowcraft and Sakai 1983). Welsh (2002) adds that, "O. aries are extensive and destructive herbivores. They have been found to decrease populations of the mamane (Sophora chrysophylla), an endemic leguminous tree, by stripping the bark off thus facilitating damage from insects and and other disease causing organisms". Results of a study (Scowcroft and Giffen 1983) which evaluated the regeneration of vegetation and forests inside and outside sheep exclosures located in heavily browsed portions of the mamane forest of Mauna Kea, indicated feral sheep browsing suppresses regeneration of mamane and three other endemic species, Hawai‘ian bent, heu-pueo, and aheahea. Liu and Jiang (2004) report that, "The most important food competitor of the critically endangered Przewalski's gazelle (see Procapra przewalskii in IUCN Red List of Threatened Species) is the domestic Tibetan sheep (O. aries) in the steppe and deserts around Qinghai Lake on the Qinghai-Tibet Plateau." Kirby et al. (2004) state that, "The sheep tick Ixodes ricinus (L.) is an ectoparasite of major economic and pathogenic importance in Scotland. Its distribution in the Scottish uplands is assumed to be governed by the abundance and distribution of its definitive hosts (deer and O. aries) and climatic variables such as temperature and rainfall.
Location Specific Impacts:Australia 
Predation: The Australian native Princess parrot (Polytelis alexandrae) is listed as Near Threatened (NT) in the IUCN Red List of Threatened Species. Habitat degradation due to altered fire regimes; herbivory and competition by introduced herbivores, rabbits (Oryctolagus cuniculus); sheep (Ovis aries) and camels (Camelus dromedarius); and predation by cats (Felis catus) and red foxes (Vulpes vulpes) are some of the threats to this species (BirdLife International 2008) Falkland Islands (Malvinas) (sub-Antarctic)
Habitat alteration: Sheep, Ovis aries have been implicated in loss of tussac grass (Parodiochloa flabellata), fachine (Chiliotrichum diffusum) and changes in vegetation composition (McDowall et al., 2001 in Varnham, 2006).
Reduction in native biodiversity: Sheep, Ovis aries may play a possible role in the extinction of some plant species. Kerguelen Is. (sub-Antarctic) (French Southern Territories) 
Reduction in native biodiversity: Sheep undoubtedly contributed to the elimination of two plants Pringlea antiscorbutica and Azorella selago (Chapuis et al., 1994). Socorro (Mexico)
Threat to endangered species: Sheep (Ovis aries) have been reported to destroy nesting sites and habitats of the critically endangered Townsend's shearwater (see Puffinus auricularis in IUCN Redlist of Threatened Species) on Socorro through grazing. Feral cats (Felis catus) have also been implicated in reducing P. auricularis numbers (more than 92% of cat scats above 500m have been reported to contain shearwater remains) (BirdLife International 2006). Ascension Is. (Saint Helena)
Interaction with other invasive species: Ovis aries may disperse seeds of Prosopis in their droppings (Pickup, 1999 in Varnham, 2006). Saint Helena
Reduction in native biodiversity: Sheep, Ovis aries although domesticated, roam freely in some parts of the island and on occasion still seriously damage native vegetation (Cronk, 1989 in Varnham, 2006). Tenerife (Spain)
Predation: Native to the Canary Islands and found on La Palma and Tenerife, Garbancera canaria (Cicer canariense) is listed as Endangered (EN) in the IUCN Red List of Threatened Species. The main threats to the survival of this species is the threat posed by introduced herbivores such as rabbits (Oryctolagus cuniculus); goats (Capra hircus); sheep (Ovis aries); and the game species Ovis musimom and aoudad (Ammotragus lervia) (Guerra & Betancort 2011). Canarias (Spain)
Predation: Native to the Canary Islands and found on La Palma and Tenerife, Garbancera canaria (Cicer canariense) is listed as Endangered (EN) in the IUCN Red List of Threatened Species. The main threats to the survival of this species is the threat posed by introduced herbivores such as rabbits (Oryctolagus cuniculus); goats (Capra hircus); sheep (Ovis aries); and the game species Ovis musimom and aoudad (Ammotragus lervia) (Guerra & Betancort 2011). Scotland (United Kingdom (UK))
Disease transmission: Kirby et al (2004) state that, "The sheep tick Ixodes ricinus (L.) is an ectoparasite of major economic and pathogenic importance in Scotland. Its distribution in the Scottish uplands is assumed to be governed by the abundance and distribution of its definitive hosts (deer and sheep (Ovis aries)) and climatic variables such as temperature and rainfall." Hawaii (United States (USA))
Reduction in native biodiversity: Grazing and browzing of herbaceous vegetation, and stripping of bark by feral sheep (Ovis aries) have led to exposure of soil to erosion and degradation of forests (Scowcroft and Sakai 1983). Damage to vegetaion due to grazing and browzing by feral sheep during the last 150 years on Mauna Kea has been cited as one of the causes for the inclusion of fifteen Hawaiian plant species in the list of threatened and endangered species of the United States (Ripley 1975 in Scowcroft and Giffen 1983).
Threat to endangered species: The subalpine mamane-naio (Sophora chrysophylla-Myoporum sandwicense) woodland is the habitat of the endangered palila (Loxioides bailleui). Grazing and browzing of herbaceous vegetation, and stripping of bark by feral sheep (Ovis aries) and other introduced mammals (Cattle (Bos taurus), Mouflon sheep (Ovis musimon), and feral goats (Capra hircus)) have led to exposure of soil to erosion and degradation of this habitat on Mauna Kea. Feral and domesticated animals including O. aries have altered and degraded the vegetation of Hawaii and led to the decline of threatened species kau silversword (see Argyroxiphium kauense in IUCN Red List of Threatened Species) and Mauna Kea silversword (see Argyroxiphium sandwicense in IUCN Red List of Threatened Species) (IUCN Red List Of Threatened Species, 2005).
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