Exotic species are fairly common within cities. However, urban areas also contain isolated fragments of original ecosystems, which may act as small ecological reserves. Unfortunately, these natural areas that are completely surrounded by urban structures are more vulnerable to human-induced disturbance and to the presence of nonnative species. Given the importance of these remnants of original ecosystems embedded within cities, efforts must be made to know if, once a nonnative species enters an urban reserve, it occupies the majority of the area or becomes restricted to some specific patches. This knowledge is important to determine the invasiveness and success of nonnative species, as well as the level of threat to the native biota. Additionally, it is important to understand which microhabitat conditions promote the presence and establishment of such nonnative species. Here, we used occupancy models to estimate the proportion of the total area that is occupied by exotic species of plants and animals within a natural reserve completely surrounded by urban areas. We also examined which environmental factors are associated with the presence of these exotic plants and animals. Our results revealed that occupancy of nonnative species is relatively high (>40%) within this urban reserve. However, we found an overall preference of nonnatives for specific patches of the reserve where human-made structures are present and where gardening activities take place. Some of the species that we studied deserve special attention because of their potential negative effects on native species.