Date Published: January 18, 2019
Publisher: Public Library of Science
Author(s): Gonzalo A. Ramírez-Cruz, Israel Solano-Zavaleta, Pedro E. Mendoza-Hernández, Marcela Méndez-Janovitz, Monserrat Suárez-Rodríguez, J. Jaime Zúñiga-Vega, Harald Auge.
Exotic species pose a threat to most ecosystems because of their potential to establish negative interactions with native biota. However, exotic species can also offer resources to native species, especially within highly modified environments such as urban ecosystems. We studied 17 exotic-native pairs of species with the potential to compete with one another, or in which one of the species could offer resources to the other, in an urban ecological reserve located within Mexico City. We used two-species occupancy models to analyze the potential association between the presence of the exotic species and the spatial distribution of the native species, as well as to assess if these species tend to avoid each other (negative spatial interaction) or to co-occur more often than expected under the hypothesis of independent occurrences (positive spatial interaction). Our results revealed few cases in which the exotic species influenced occupancy of the native species, and these spatial interactions were mainly positive, indicated by the fact that the occupancy of the native species was usually higher when the exotic species was also present. Seven of the eight observed non-independent patterns of co-occurrence were evident during the dry months of the year, when resources become scarce for most species. Our results also demonstrate that the observed patterns of species co-occurrence depend on the distance to the nearest urban structure and the amount of herb, shrub, and tree cover, indicating that these habitat features influence whether native species avoid or co-occur with exotic species. Our study represents an important contribution to the understanding of temporal dynamics in the co-occurrence between exotic and native species within urban ecological reserves.
Human activity has accelerated the introduction of non-native species in all types of ecosystems [1–4]. Multiple studies have gathered evidence of the negative effects of the presence of exotic species on native populations [5,6]. The introduction of a predator, parasite or competitor may have an evident impact on native animal populations [7–9]. Exotic plants in turn may create a variety of alterations to the local biotic composition, and can also become facilitators for the establishment of other non-native species [10,11]. However, there is also evidence for unexpected benefits brought by the introduction of exotic species . For example, bird populations may benefit from the introduction of plant species that offer nesting sites and additional food resources . Therefore, quantification of the associations between exotic species and native populations is necessary to implement management and conservation strategies [14–16].
Overall, exotic species were detected in a greater proportion of the observation sites and were relatively more abundant than native species, except for the house finch and the tepozan tree (S2 Table). The house finch was detected in a similar proportion of sites and had a relative abundance similar to that of the exotic house sparrow. The tepozan tree was more abundant than any of the exotic trees that we studied (S2 Table).
The evidence of associations between the presence of exotic species and the occupancy of native species in our study area is relatively small. In addition, the few non-independent patterns of species co-occurrence that we detected were evident only in one or two of the six sampling seasons. In other words, the spatial interactions between exotic and native species, as well as the environmental covariates associated with the observed non-independent co-occurrence patterns, were not consistent across years and seasons. This suggests that the underlying ecological interactions between exotic and native species are highly dynamic. We must also emphasize that in four of the six pairs of species in which we detected a spatial interaction, the association between presence of the exotic and occupancy of the native was positive (see Table 3).