**4. Discussion**

The models produced show that with good predictive ability the potential geographic distribution for *Manihot walkerae* in the years 2050 and 2070 could be slightly reduced as a result of climate change. As a consensus, the future climate change models show a restriction in future distribution for *Manihot walkerae* with the lowest loss of distribution calculated as −2.08% for the year 2050 with an RCP of 8.5, and the highest, −14.37%, for the year 2070 with an RCP of 4.5, whereas for two of the future climate change scenarios at an RCP of 4.5 for the year 2050, it is predicted that there could be a potential increase of approximately 7% in distribution (Table 2). Similarly, another SDM study conducted in the Chihuahan desert found that some endemic plants were shown to be affected by climate change and expanded in distribution [48]. The areas that were shown to be most affected by climate change were those in the northeastern and southernmost portions of the Tamaulipan Thornscrub ecoregion (Figures 3–5). Although there are no documented occurrences of *M. walkerae* in these regions, there are some protected lands within the area that was lost and it is predicted that they will not be suitable for *M. walkerae* in the future (Figures 5 and 6). This potential outcome could limit success in the future for conservation efforts such as reintroduction. Successful reintroduction of *M. walkerae* to increase the number of populations of this species would be best in areas that are predicted to have high potential for geographic distribution. Areas that have a high potential for geographic distribution also have the highest potentially suitable habitat for a said species. Species distribution modeling has been used as a tool for reintroduction of endangered species when models show the areas have potentially suitable habitat for a given species [21,22]. In Texas, there are several protected lands that have high potential for geographic distribution for *M. walkerae* and that are predicted to be unaffected by climate change (Figure 5). These protected lands could be used for future conservation efforts such as the reintroduction of *M. walkerae*. In Mexico, currently there are no protected lands that lie within the areas that are potentially suitable for *M. walkerae*, making the future of this species in Mexico uncertain. In order for successful conservation efforts to be conducted in Mexico, relationships with private landowners that agree to conserve *M. walkerae* on their property would have to be formed.

Some limitations of our study are that we relied solely on bioclimatic variables for our modeling and that we used a small number of occurrences to create our models. Using bioclimatic variables for climate change modeling is common and has been used to model the effects of climate change on the distribution of different species of plants and animals, some of which are endangered and restricted [23,48,49]. Including static topographic variables could have improved the reliability of our models, but in some instances, such as when topographic variables like elevation and bioclimatic variables are highly correlated, they could hinder the statistic reliability of the model [50]. In the case of our study, we obtained AUC and pROC values that were higher than random indicating that even though we used a low number of occurrences and bioclimatic variables, these models could serve as a good reference for future conservation plans for *Manihot walkerae*. Most importantly, these models show that although there are some protected areas that could conserve this species in southern Texas, in Mexico there are no conservation areas that lay within *M. walkerae* historical occurrences or predicted current and future distribution. A probable reason why there are no protected areas for this species can be attributed to a lack of sufficient data on its biotic inventory, species ecological requirements, and species distribution patterns [51]. This study provides valuable information for *M. walkerae's* distribution and can allow for an inference of some of the ecological requirements of this species. The results of the jackknife procedure show that temperature and precipitation are important influencers of *M. walkerae's* distribution.

**Figure 5.** The portion of area lost (blue) as a result of climate change for the CMIP5 RCP 4.5 2070 model that had the highest predicted loss of distribution −14.37%. The known occurrences of *Manihot walkerae* were shown to not be affected, but some U.S. Fish and Wildlife LRGV NWR protected areas (red) in the northeastern portion of the study area are predicted to no longer be suitable for *M. walkerae* in the future. No protected areas in Mexico are shown to overlap with suitable areas of distribution of *M. walkerae* at present and in the future.

Although there is a growing collective awareness for the effects of climate change on the world's species, most of the attention is focused on those that are used in agriculture or provide a direct threat or benefit to humans [52]. There is scarce research done so far that contributes to the conservation of endemic endangered species of the Tamaulipan thornscrub, especially when it applies to rare plant species that are generally unknown. As human populations continue to grow in South Texas and northeastern Mexico, it is probable that there will be a reduction of suitable habitat for *M. walkerae* due to land cover change. As climate change is not predicted to be an imminent threat to *M. walkerae* populations, but could act synergistically with other harmful factors that threaten this species (e.g., loss of genetic diversity), future studies exploring the effects of land cover change on this species would be of great use for conservation efforts. Although most occurrences for *M. walkerae* in Texas show a close distribution to the U.S.–Mexican border, there is one population further north which is isolated from the others. We constructed models where we omitted this record and found that omitting it did not have an effect on *M. walkerae's* predicted distribution, we decided to include it in our study since it is a historical record for this species. Unfortunately, this population exists within private property which restricts our access to this population for potential field studies. Additionally, given that there is a lack of connectivity from this population to other historical occurrences that are located near the U.S.–Mexican border, there is some uncertainty on whether this population is native or could have been introduced. Furthermore, an approaching threat for *M. walkerae* and other native species of the Tamaulipan thornscrub ecoregion is the impending construction of additional border wall segments, which are expected to exasperate fragmentation as well as increase anthropogenic disturbance in the known current distributional range [53]. Collectively, the results of this study show that climate change can potentially have an effect on the geographic distribution of this endangered species and although it is not known if the distribution could expand or restrict, protected areas are essential for conserving *M. walkerae* and we recommend that the geographic distribution of this species be taken into account when designating protected areas in Mexico and southern Texas.

**Figure 6.** Detail of the suitable area lost (blue) in Lower Rio Grande Valley Texas counties of Starr, Hidalgo, Cameron, and Willacy. The protected areas that are shown to be most affected by climate change are U.S. Fish and Wildlife Service LRGV NWR tracts located in Willacy County.
