Gambian giant pouched rat (also called Gambian pouch rat) is native to parts of western, central and southern Africa (FWC 2011). It was imported as part of the pet trade until 2003, after which a brief ban on imports occurred in response to an outbreak of monkeypox disease. Around 1999, a release led to establishment of a population on Grassy Key (FWC 2011), but eradication efforts may have eliminated the species from the Keys. However occasional suspected sightings still occur (G. Witmer, pers. comm.).
We were interested in exploring whether the CCVI could be applied to non-native species where a primary concern is whether climate change might be expected to make control of these species more difficult or increase their potential to spread. When applied in this non-traditional way, factors that "increase vulnerability" can be thought of as beneficial from a management perspective in that they may limit population growth or range expansion, whereas factors that "decrease vulnerability" may make management of the species more difficult under climate change. In other words, "vulnerability" may be a desired condition when applied to invasive species if it suggests that future climatic conditions and/or factors related to indirect exposure may be less likely to favor increases in abundance or range expansion.
170 Distribution Data
The CCVI utilizes distribution data to calculate estimates of relative exposure for each species. Data considered as part of this assessment included occurrences from the Early Detection and Distribution Mapping System (EDDMapS 2010, Figure A22-1). Recent eradication efforts have greatly reduced, and perhaps eliminated the Gambian giant pouched rat population from Grassy Key. The occurrence on Islamorada is based on a road-killed specimen and surveys have not revealed any other Gambian giant pouched rats in the area (G. Witmer, pers. comm.).
Exposure
We obtained downscaled data from Climate Wizard (Zganjar et al. 2009) for the state of Florida for mid- century projections based on the mean ensemble model under the A1B emissions scenario. Moisture data, in the form of the Hamon AET: PET moisture metric were downloaded from NatureServe, and are derived from Climate
Wizard temperature and precipitation projections for mid-century under the A1B emissions scenario. To use the CCVI, the percentage of the distribution that is exposed to a particular range of projected change in temperature or moisture is calculated in ArcGIS by overlaying the exposure data on the distribution or occurrence data (Tables A22-1 and A22-2). For point data sets, we assigned a single exposure value to each of the points based on the overlay.
Climate Wizard data were only available for the Florida peninsula, so we extrapolated values from the southern tip of the Florida peninsula into the Keys (see the methodology in the main report). Due to the small area occupied by this species, the entire distribution was contained within a single category for both temperature and moisture exposure (Tables A22-1 and A22-2).
Indirect Exposure
Sea level rise (B1). The species expert estimated greater than 90% of the range in Florida would be impacted by a 1-meter sea level rise. This factor was scored as greatly increases vulnerability.
Potential impact of barriers on range shifts. The species expert indicated that the species occupies variable habitats including dry woodland and grass-woodland mix as well as riverine habitats and agricultural fields. They also do well in human-dominated landscapes as evidenced in Grassy Key. Because of the impact of sea level rise (see above), the Florida habitat is vulnerable to climate change. Species occurring in habitats that are considered likely to persist despite climate change would be scored as "neutral" for factors B2a and B2b, which focus on the potential impact of barriers on climate-induced range shifts. Natural barriers (B2a). Gambian giant pouched rat has been introduced only in the Keys. The ocean and large distances between the islands provide a natural barrier to dispersal. The ocean completely surrounds the occupied island with the exception of island- connecting bridges. This factor was scored as increases vulnerability (i.e., natural barriers were considered likely to "greatly impair" distributional shifts).
Anthropogenic barriers (B2b). Bridges between the islands of the Florida Keys are long, narrow, unvegetated and have considerable traffic volume, likely restricting their functionality for passage by Gambian giant pouched rats. This factor was scored as somewhat
Figure A22-1. Distribution inputs considered for the CCVI analysis.
171 increases vulnerability (i.e., anthropogenic barriers were considered likely to "significantly impair" distributional shifts).
Land Use Changes Resulting from Human Responses to Climate Change (B3). The species expert indicated that some of the land uses changes that may occur in response to climate change, including new seawalls and shoreline protection may prevent substantial habitat loss and could benefit this species. This factor was scored as decreases vulnerability (i.e., the species is likely to benefit from land use changes that are "likely or very likely" to occur). We also included a score of somewhat decreases vulnerability to account for the uncertainty regarding whether or not these changes will actually occur.
Sensitivity
Dispersal and movement (C1). The species expert characterized the species as having moderate dispersal ability (100 + meters), corresponding to a score of neutral for this factor.
Historical thermal niche (C2ai). This factor is intended to approximate the species' temperature tolerance at a broad scale by looking at large-scale temperature variation that a species has experienced in the past 50 years within the assessment area, and is calculated as the difference between the highest mean monthly maximum temperature and lowest mean monthly minimum temperature for each cell (Young et al. 2010). Since this species was introduced to Florida, we scored this factor as unknown.
Physiological thermal niche (C2aii). The species expert indicated that the species is associated with "warm" environments. This factor was scored as somewhat decreases vulnerability (the lowest score available for this factor).
Historical hydrologic niche (C2bi). This factor is intended to capture the species' exposure to past variation in precipitation as a proxy for tolerance to large-scale variation in precipitation. The factor is assessed by calculating the range in mean annual precipitation for the period of 1951-2006 observed across the species' distribution in the assessment area. Since this species was introduced to Florida, we scored this factor as unknown.
Physiological hydrologic niche (C2bii). The species expert indicated that the species is believed to need regular access to freshwater for drinking, which is already somewhat limited on the Florida Keys and may become more so with projected climate change. This factor was scored as somewhat increases vulnerability. Impacts of Changes to Specific Disturbance Regimes (C2c). The species expert indicated that the species uses burrows and so is adversely affected by flooding events. This factor was scored as somewhat increases vulnerability.
Dependence on ice, ice-edge, or snow cover habitats (C2d). All species in Florida were scored as neutral for this factor.
Table A22-1. Projected temperature exposure for Gambian giant pouched rat in the assessment area. The percentages are used to calculate the temperature component (ET) of the
exposure metric. See Young et al. (In press) for details.
Data set →
(Distribution) Occurrences EDDSMapS
> 5.5°F warmer 0% 5.1 - 5.5 °F 0% 4.5 - 5.0 °F 0% 3.9 - 4.4 °F 0% < 3.9°F warmer 100% (ET) 0.4
Table A22-2. Projected moisture exposure (based on the Hamon Index) for Gambian giant pouched rat in the assessment area. The percentages are used to calculate the moisture component (EM) of the exposure stress. See Young et
al. (In press) for details.
Data set →
(Distribution) Occurrences EDDSMapS < -0.119 (Driest) 0% -0.119 - -0.097 0% -0.096 - -0.074 100% -0.073 - -0.051 0% -0.050 - -0.028 0% > -0.028 (No change) 0% (EM) 1.3
172 Physical habitat specificity (C3). The species has a broad distribution in native tropical Africa, presumably across variable and diverse geologic features. This factor was scored as decreases vulnerability.
Dependence on other species to generate habitat (C4a). There are no known associations with other species for habitat. This factor was scored as neutral.
Dietary versatility (C4b). Diet was categorized as omnivorous. The species consumes a broad array of plant and invertebrate foods, including agricultural crops and ornamental plants/fruits. This factor was scored as somewhat decreases vulnerability.
Pollinator versatility (C4c). Not applicable.
Dependence on other species for propagule dispersal (C4d). The species disperses on its own. Experts noted that dispersal is facilitated by humans and higher water events. This factor was scored as neutral (the lowest score available for this factor).
Other interspecific interactions (C4e). Additional interspecific interactions that might affect vulner- ability were not identified. This factor was scored as neutral.
Measured genetic variation (C5a). The genetic diversity of the current population is unknown, however the free- ranging population was derived from a release of approximately eight individuals. This information has been captured in the score for factor (C5b).
Occurrence of bottlenecks in recent evolutionary history (C5b). A small founding population and ongoing eradication effort has greatly reduced (and may have eliminated) the existing population. If these populations were to rebound, they would be assumed to have reduced genetic diversity. This factor was scored as increases vulnerability based on this assumption.
Phenological response (C6). The reviewer was not aware of any research specifically assessing the correspond- dence between changes in seasonal dynamics and changes in the timing of phenological events other than shifts in diet when certain types of food (i.e.,
seeds and fruits) become available. This factor was scored as unknown.
Documented or Modeled Response to Climate Change
We did not include these optional factors in the analysis.
Results
The Gambian giant pouched rat ranked as "Moderately Vulnerable" to climate change in Florida (index score: 5.0, range [4.6, 5.3]; Figure A22-2), primarily as a result of its geographical distribution. The majority of factors identified as potentially influencing "vulnerability" for this species are related to the fact that it is currently restricted to the Florida Keys (Table A22-3), suggesting that if it were to be introduced to the Florida peninsula, the species would
Table A22-3. Scores assigned to factors associated with vulnerability to climate change for Gambian giant pouched rat in Florida. Bolded factors were associated with higher levels of uncertainty by the expert reviewers. Not all scores can be assigned to all factors as indicated by dashes.
Vulnerability factor GI I SI N SD D unknown or n/a
Sea level rise • --
Natural barriers • -- --
Anthropogenic barriers • -- --
Human responses to CC -- • •
Dispersal •
Historical thermal niche (GIS) -- •
Physiological thermal niche • --
Historical hydrologic niche (GIS) -- •
Physiological hydrologic niche • --
Disturbance regimes -- •
Ice and snow • -- --
Physical habitat specificity -- •
Biotic habitat dependence • -- --
Dietary versatility -- • --
Biotic dispersal dependence -- • -- --
Other interactions: none -- • -- --
Genetic variation -- • --
173 potentially be able to expand its distribution and would be either unaffected by or even potentially benefit from projected climate change. Three factors were scored as unknown, including historical thermal niche (C2ai) and historical hydrologic niche (C2bi). When factors for which scores were dependent on the current extent of occurrences (i.e., those related to indirect exposure and hydrologic niche) were scored as "neutral," the index score dropped into the "Not Vulnerable/Presumed Stable range (1.5, range [1.2, 1.8]) and the species was flagged as potentially shifting range in the assessment area. Modeling suggests that even under current climate conditions, the potential spread of Gambian giant pouched rats would be very large if they reach mainland Florida due to the adaptability of this species, the thermal and moisture regimes in which they occur in their native range, and the variety of foods and habitat they can use.
The CCVI is intended to be used in combination with conservation status ranks. However, Gambian giant pouched rat is a non-native species and accordingly has no conservation status rank.
Literature Cited
EDDMapS. 2010. Early Detection and Distribution Mapping System. Developed by the Center for Invasive Species and Ecosystem Health, University of Georgia (http://www.eddmaps.org, Accessed November 2010).
Florida Fish and Wildlife Conservation Commission (FWC). 2011. Florida’s Nonnative Wildlife. Species Detail. Gambian Pouch Rat – Cricetomys gambianus [web page]. (http://myfwc.com/wildlifehabitats/ nonnatives/mammals/gambian-pouch-rat,
Accessed November 2011).
Zganjar, C., E. Girvetz, and G. Raber (Developers). 2009. Climate Wizard [web program] (http://www.climatewizard.org).
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domestication and reproductive performance of
the African giant rat Cricetomys gambianus (Waterhouse) in Nigeria. Mammalia 39:343–364. Engeman, R. M., G. W. Witmer, J. B. Bourassa, J. W.
Woolard, B. Constantin, P. T. Hall, S. Hardin, and N. D. Perry. 2007. The path to eradication of the Gambian giant pouched rat in Florida. Pages 305- 311 in Managing Vertebrate Invasive Species: Proceedings of an International Symposium (G. W. Witmer, W. C. Pitt, and K. A. Fagerstone, Eds). USDA National Wildlife Research Center, Fort Collins, CO.
Fiedler, L. A. 1988. Rodent problems in Africa. Pages 35–65 in Rodent Pest Management (I. Prakash, Ed.). CRC Press, Boca Raton, FL.
Figure A22-2. CCVI output for Gambian giant pouched rat in Florida. The index score (black circle) is shown with the range of scores produced by the Monte Carlo simulation. Categorical ranks are coded by color: "Highly Vulnerable" (orange), "Moderately Vulnerable" (yellow), and "Presumed Stable" (green).
174 Kingdon, J. 1974. East African Mammals: Vol. II,
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Perry, N. D., B. Hanson, W. Hobgood, R. L. Lopez, C. R. Okraska, K. Karem, I. K. Damon, and D. S. Carroll. 2006. New invasive species in southern Florida: Gambian rat (Cricetomys gambianus). Journal of Mammalogy 87:262–264.
Peterson, A. T., M. Papes, M. G. Reynolds, N. D. Perry, B. Hanson, R. L. Regnery, C. L. Hutson, B. Muizniek, I. K. Damon, and D. S. Carroll. 2006. Native-range ecology and invasive potential of Cricetomys in North America. Journal of Mammalogy 87:427–432.
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