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= Effects of urbanization on Sciurus carolinensis (Eastern Grey Squirrel) and importance of Population Control = Sciurus carolinensis, commonly known as the grey squirrel is one of the most familiar mammals of the eastern United States because it is one of the most frequently encountered small mammals in urban and suburban areas (Bonnington et al 2014). Ecologically it plays an important role as predator and prey of many species. In recent centuries its population and distribution has increased significantly and its behavior has impacted as well as changed around the presence of urbanization and human presence. While an important component in their native ranges, these squirrels should be controlled and not spread to other continents due to the fact that they become pest-like and quickly invasive. Ranging from 9 to 17 ounces, this squirrel can reach lengths of 16 to 21 inches fully grown which is smaller and skinnier for its relative the fox squirrel. Half of their body length is usually comprised of their long bushy tail which they use for balance in climbing as well as predatory warning and mate attraction. They also have long toes and claws they use for climbing. Other adaptations include large eyes for good sight which proves advantageous for predation avoidance and running or jumping through branches and trees. Overall, they are grey in color though their coats can be varied in color from reds to blacks in both dark and light morphs due to the salt and pepper mix of black, white and brown in the fur. There is even an albino morph (Webster et al 1985). Color morphs have not displayed any sexual dimorphism (Goldstein et al 2015). Their diet includes mostly plant material though it can vary seasonally, usually including berries and seeds in the summer and nuts in the fall and winter. They have been known to be predators to insects and bird eggs as well as scavengers to human trash (Webster et al 1985). One study observed that the most frequently cached foods were acorns followed second by longleaf pine seeds and then loblolly pine seeds (Spritzer 2002). Grey squirrels are mainly arboreal found in mature hardwood deciduous forests occasionally with a mix of conifers. They have also become common in parks and residential areas due to their compatibility and adaptation to urbanization (Webster et al 1985). The native range of the grey squirrel is on North America from southern Ontario to Florida in the east and North Dakota to eastern Texas in the west (Spritzer 2002). They adapt to a range of temperatures but are less common in colder temperatures and higher elevations (Webster et al 1985). I suspect this is due to competition and a lack of food.

Urbanization has had impact on many species and while some are not able to coexist with humans in urban areas, grey squirrels have been able to adapt to urbanization. Populations have been able persist and have started to display urban wildlife syndrome as well as elevated levels of intraspecific aggression. Urbanization changes the interactions of humans and wildlife and impacts how wildlife responds to stimuli such as predation threats. Urban grey squirrels are more likely than rural grey squirrels to confront an introduced predator, thus they experience more contact with humans and exist at a higher population density (Sarno et al 2015). Some studies have shown that grey squirrels were not present in areas with less than 20 percent green space (Bonnington et al 2014). While I believe that more rural areas with more tree cover and “green space” positively influence squirrel populations, I think these rodents can persist in any habitat they can find a steady source of food and shelter in including urbanized areas with little green space. This is partially why they have been able to become invasive in other areas around the world. Urbanization has changed how animals react to stimuli. Scanning and foraging animals encounter a survival tradeoff when looking for food. A foraging animal cannot effectively scan and similarly scanning limits foraging. It has been suggested that animals will select for an optimal rate of vigilance time in respect to their environment (Sarno et al 2015). It is expected that grey squirrels who were accustom to the presence of many humans would be less vigilant than rural grey squirrels who were not as frequently exposed to the human stimulus but more frequently exposed to predatory threats. Sampling periods of ten minutes were conducted in rural and urban areas and time being vigilant was recorded in seconds. Vigilance increased from rural to urban habitats and squirrel density had a positive effect on vigilance time. For every one unit of increase squirrel density, vigilance time increased by nearly four seconds which is a weak predictor of vigilance due to the Pearson correlation coefficient being low. Squirrels in urban areas proved to be more vigilant than those in rural areas. This contrasted with a study done on the campus of Texas A&M compared to a rural environment with fox squirrels but it is hypothesized that campuses may be a safer habitat with less human harassment than city or county parks. The differentiation between urban and rural classification of land would have to be investigated and possibly quantified by amount of green space, landscapes or structures to obtain more accurate results (Sarno et al 2015). An additional study was conducted on the influence of humans on grey squirrel behavior. Natural habitats become fragmented and reduced with human development. In presence of humans some animals alter their behavior. Human presence has been equated to predator risk with most animals so the presence of humans should cause increased vigilance but some animals who live in urbanized areas highly populated by humans alter their antipredation response. Most frequent responses in the study by Cooper were run responses but more squirrels in the high human activity areas responded with running rather than freezing. This proved to be conclusive with other studies that also proved urban mammals typically allowed closer approaches of humans. It can be concluded from this study that mammals are able to habituate their responses to threat levels and recognize humans as relatively low threat (Cooper et al 2008).

Reproductive habits and adaptations of these squirrels also had led to population success and their invasive ability. These polygamous males compete for reproductive access to many mates. This behavior as well as having 2 reproductive seasons per year leads to high rates of reproduction. They mate in midwinter and June and litters consist of 2 to 5 blind young. By 8 weeks they are independent and by 10 to 12 months they are sexually mature. These factors are crucial in their ability for a few individuals to populate an area which make control efforts crucial to rehabilitating an invaded environment and limiting the spread of this species (Goldstein et al 2015). These rodents ecologically, are an important component of the ecosystems they are part of so they should not be treated as pests in their natural range. They are typically preyed upon by snakes, birds of prey such as hawks, larger mammals and even have proven to be a crucial part of the lifecycle of the parasitic bot fly. The bot fly will bury into the squirrels’ skin and deposit its larvae which will develop in the squirrels’ skin which although makes visible warbles in the skin, has no significant effect on the squirrel. This species was threatened in the early 1900s but has since made in impressive comeback and now serves as one of the most important game species with over 40 million hunted yearly (Webster et al 1985). These rodents play an important role in the forest ecosystems that they inhabit by having an impact on the trees that they forage from (Spritzer 2002). Their food caching habits as scatter hoarders help with the regeneration of mast-producing trees. As scatter hoarders, grey squirrels bury caches of nuts and fruits with the intention of returning to them later. These caches are not defended and often forgotten about. These mast-producing trees that make the nuts that the squirrels are stashing require the habits of the caching mammals such as the grey squirrels for reproductive success. The buried caches provide favorable conditions for germination and growth of seedlings. Evidence has been presented for the coevolutionary ties of caching squirrels and mast-producing trees and it has been concluded that grey squirrels are partially responsible for the regeneration of some deciduous forest areas. When other species which compete with the scatter hoarding of the squirrels are introduced or prevalent, success of future forest regeneration can be negatively impacted. For example, Sciurus vulgaris, commonly known as the red squirrel is a caching relative of the grey squirrel but they cache their food in hollowed out areas of trees thus not giving the tree the same chance at reproductive success. In this study the caching habits of 12 red squirrels and 13 grey squirrels were documented. The red squirrels would unearth the caches of the grey squirrels. It was concluded that the tree caching habits of the red squirrels negatively impacted the trees reproduction by competing with and disrupting scatter caches made by grey squirrels (Goheen and Swihart 2003). Grey squirrels have positive impacts on their native environment and may be considered for methods to promote forest regeneration in some areas.

While they are positive influences in their native ranges, grey squirrels have also become pests and invasive in other areas across the globe and should therefore be monitored and regarded as a potential threat to ecosystems they are introduced to. They are classified as Rodentia, or gnawing mammals and they have been known to display the gnawing behaviors in human dwellings and structures by chewing through electrical wires and insulation. This is not only a threat to the squirrels which are notorious for electrocuting themselves to death, but it is also a nuisance to the humans whose homes they destroy and cause damage to (Bonnington et al 2014). Invasions threaten biodiversity, ecological process, agricultural and transport systems (Goldstein et al 2016). These squirrels although only native to the United States, have been introduced all over the world in continents like Europe, Africa and Australia. They have invaded Great Britain, Italy, Ireland and South Africa and unspecified areas of Australia. Introduction in these foreign areas has led to the decline of native species. Grey squirrels have been known to be the host of squirrel viruses such as a pox virus known to cause high mortality in the red squirrel. Along with disease they also threaten other species by being competitors for the same food sources (Bertoling et al 2014). Introduction of these animals is a direct result of deliberate release or escapes from pet and collection trade. They are extremely effective invaders for three main reasons: they have nonselective diets, they thrive in both urban and rural habitats and they have a high breeding rate which allows them to establish from a low number of individuals. Many studies have been done to examine the population dynamics of this species and understanding this as well as invasive range expansion can inform design of management plans(Goldstein et al 2015) Complete eradication is not always economically probable in invaded areas but control strategies are being investigated. Models for control must have an understanding of species abundance, spatial distribution and population viability and these help create options useful for future scenarios for endangered or exploited species and problems caused by invasive species. High cost and ethical resistance often lead to the failure of eradication efforts but control methods continue to be investigated. One study was done in the UK because the damage to local trees and the commercial foresting had amounted to around 14.5 million euros. Predictions were generated using SEPMs which concluded that high intensity culls proved to delay invasion of uninvaded areas by 15 to 25 years while they delayed invasion of buffer areas by only 8 to 13 years. Decrease in abundance from immunocontraceptive efforts did not last as long as those from culls. After 25 years overall abundance decreased by 8 to 11% with vaccination scenarios compared to 26 to 43% with high intensity culls both at a regional scale. Five-year intensive regionwide culls would be effective in significantly reducing squirrel population (Goldtstein et al 2016). This model can hopefully be useful for other areas impacted by invasion events.

Grey squirrels are familiar neighbors to anyone along the eastern cost of North America. While they serve a crucial role here as seed dispersers and food resources for other species, they have proven to be quite invasive and negative influences in other continents such as Europe, Australia and Africa. Control efforts have been investigated and methods of population control are being tested to stop the rapid spread of this animal. In all of the areas that it has become invasive, this squirrel would have never naturally dispersed to the presence of large oceans that separate these areas. It is a direct result of human-mediated dispersal that has led to the invasion of these animals to non-native territory, so it is also the responsibility of humans to control the negative impact they have sparked. Through population studies and control efforts the spread of this species can be monitored and limited in order to ensure the conservation of species native to the areas it has invaded.

Literature Cited

Bertolino S, Di Montezemolo NC, Preatoni DG, Wauters LA, Martinoli A. 2014. A grey future for europe: Sciurus carolinensis is replacing native red squirrels in italy. Biol Invasions. 16(1):53-62.

Bonnington C, Gaston KJ, Evans KL. 2014. Squirrels in suburbia: Influence of urbanisation on the occurrence and distribution of a common exotic mammal. Urban Ecosystems. 17(2):533-46.

Cooper CA, Neff AJ, Poon DP, Smith GR. 2008. Behavioral responses of eastern gray squirrels in suburban habitats differing in human activity levels. Northeast Nat. 15(4):619-25.

Goheen JR, Swihart RK. 2003. Food-hoarding behavior of gray squirrels and north american red squirrels in the central hardwoods region: Implications for forest regeneration. Can J Zool. 81(9):1636-9.

Goldstein EA, Butler F, Lawton C. 2015. Frontier population dynamics of an invasive squirrel species: Do introduced populations function differently than those in the native range? Biol Invasions. 17(4):1181-97.

Goldstein EA, Butler F, Lawton C. 2016. Modeling future range expansion and management strategies for an invasive squirrel species. Biol Invasions. 18(5):1431-50.

Sarno RJ, Parsons M, Ferris A. 2015. Differing vigilance among gray squirrels (sciuridae carolinensis) along an urban-rural gradient on long island. Urban Ecosystems. 18(2):517-23.

Spritzer MD. 2002. Diet, microhabitat use and seasonal activity patterns of gray squirrels (sciurus carolinensis) in hammock and upland pine forest. The American Midland Naturalist. 148(2):271-81.

Webster D, Parnell J, Biggs W. 1985. Mammals of the Carolinas, Virginia and Maryland. Chappell Hill (NC): University of North Carolina Press. P. 121-124.