**1. Introduction**

Anthropogenic activities have had a significant influence on the geographic distribution, rate of extinction, and endangerment of many of the world's plant species [1]. These activities have led to the fragmentation and destruction of plant habitats, as well as the introduction of invasive competitors and pests [2]. Climate change is also having resonant impacts on plants and wildlife [3–5]. It is predicted that there will be a shift in the distribution of plants towards higher elevations and latitudes to attempt to cope with the changing climate. However, for plants that are rare, endemic, have lower dispersion distances, or persist in fragmented areas, this transition will be difficult, and they will tend instead toward extinction [5,6]. A plant's suitable habitat and distribution is dependent on temperature along with other environmental factors, and with changing temperatures they are expected to expand or restrict [7,8]. Invasive plant species that find higher temperatures favorable are expanding in range and out-competing native species [9], while many endemic plants are projected to lose their suitable habitat and are facing extinction. According to the Intergovernmental Panel on Climate Change (IPCC) global temperatures are projected to increase, with heat waves and heavy precipitation events becoming more frequent [10]. For endemic plants that are already faced with habitat fragmentation and competition by invasive species, climate change could act as a catalyst for extinction [6,9,11–13].

Species distribution models (SDM) are useful tools in conservation planning and management to project the effects that climate change could have on an endangered species' distribution [7,14,15]. As our global awareness on climate change increases, SDM have progressively been used to project the effect of climate change on the distribution of invasive pests, pathogens, and endangered species [14,16–22]. Increasingly, studies have also started assessing the effectiveness of protected areas at conserving endangered species at present and in the future by incorporating climate change SDM [18–23]. One study conducted by Vieilledent et al. (2013) explored the effects of climate change on three endangered species of Madagascar (*Adansonia grandidieri* Baill, *Adansonia perrieri* Capuron and *Adansonia suarezensis* H. Perrier) and how climate change would modify the effectiveness of protected areas in the future. It was found that in the future, as a result of climate change, no protected areas were viable for conserving two of these species, which puts them at risk of future extinction [19].

Walker's Manihot, *Manihot walkerae* Croizat *(Euphorbiaceae*), is a rare plant species that is endemic to the Lower Rio Grande Valley (LRGV) of Texas and northeastern Tamaulipas, Mexico [24–27]. Collectively, they compose part of the Tamaulipan thornscrub ecoregion, a highly biodiverse area that is home to unique endemic species of plants and animals of which nineteen are federally threatened or endangered and nearly 60 are state-protected species [27–29]. Habitat destruction, fragmentation, herbicide application, overgrazing, herbivory by native and introduced wildlife, surface mining of caliche, petroleum and natural gas exploration, urban and residential development, and competition by invasive plant species are risk factors that affect *M. walkerae* [24–26]. With over 95% of the focal region's Tamaulipan thornscrub modified or destroyed, native species of plants and wildlife are faced with the loss of their habitat [27–29]. Part of the Tamaulipan thornscrub ecoregion is found in the southwestern United States, where the greatest temperature increase of any area in the lower 48 states is predicted to occur [11,25]. Additionally, semi-arid areas like the Tamaulipan thornscrub might also experience a decrease in water resources due to climate change. This potential development could have adverse effects on native species by restricting their range and increasing the competitive advantage of invasive species [10,25], such as *Cenchrus ciliaris* buffelgrass [25,30], one of the most detrimental invasive species in the Tamaulipan thornscrub ecoregion and for *M. walkerae* [24–27].

*Manihot walkerae* is a perennial vine-like subshrub that is found in semiarid, shaded shrublands on xeric slopes and uplands, often on overexposed caliche outcrops [24–28]. *M. walkerae* serves an ecological role in the Tamaulipan thornscrub ecoregion and shares species interactions with native wildlife [27]. Additionally, *M. walkerae* is a wild relative of the widely utilized agricultural crop Cassava (*Manihot esculenta*). Cassava is a staple worldwide and serves many roles in food, biofuel, and industrial uses [31–33]. A major problem for the Cassava agricultural industry is post-harvest deterioration, a condition which limits the time that Cassava is viable for consumption after its harvest. Studies have found that hybridizing *M. walkerae* with Cassava has resulted in a tuber that is more resistant to post harvest deterioration [31–33]. Furthermore, *M. walkerae* possesses genes that are resistant to prominent diseases of Cassava, such as Cassava brown streak and Cassava bacterial blight, and it also contains genes for cold resistance [24]. Given the benefits that the genetic constituents of *M. walkerae* provide, it is a crop wild relative (CWR) of great use to improve longevity and disease resistance in Cassava and its extinction could have negative effects on the future of this crop as its genetic diversity would no longer exist [34].

The objectives of this paper were to evaluate the potential effects of climate change on the geographic distribution of *M. walkerae* and assess the usefulness of natural protected areas in future conservation. As *M. walkerae* occurrence data is limited with only a few historical populations documented, we used the maximum entropy algorithm MaxEnt to construct models of its current and future distribution because (1) it uses presence-only data, (2) uses both continuous and categorical data as environmental variables, and (3) its prediction accuracy is reliable even with small sample sizes and gaps [35]. We constructed models of the current and future geographic distribution of *M. walkerae* for the years 2050 and 2070 using three different general circulation models (CM3, CMIP5, and HADGEM) and two climate change scenarios (RCP 4.5 and 8.5). The geographic distribution consensus models were overlapped with polygons of protected areas in Texas and Mexico to assess the effects of climate change on the effectiveness of protected areas in conserving *M. walkerae* in the future. We hypothesize that the most severe emission scenario will lead to a more pronounced reduction of distribution and that climate change could reduce the effectiveness of protected areas at conserving *M. walkerae* in the future. We expect that the results of this modeling exercise can be used to set sound conservation plans for this species.
