Spring 2007
Two Decades of Change in the Distribution of Exotic Plants at the Desert Laboratory, Tucson, Arizona
Janice E. Bowers, U.S. Geological Survey Travis M. Bean, University of Arizona Raymond M. Turner, U.S. Geological Survey
In the spring of 1983, we undertook a survey of exotic plants on the Desert Laboratory grounds by walking regular gridlines and recording Cartesian coordinates of exotic species (Burgess et al. 1991). In the spring of 2005, native and exotic annuals were abundant after a relatively wet winter (precipitation = 151 mm), making it worthwhile to repeat the initial survey, which was undertaken after an unusually wet winter (precipitation = 250 mm). Fifty-two exotic species were encountered in all, 34 in the first survey and 44 in the second. The proportion of ornamental exotics doubled over time, reaching 50% of the exotic flora in 2005. Following the terminology of Richardson et al. 2000*, we found that mean residence time increased significantly from casual to naturalized to invasive species, suggesting that the longer a species was present, the more likely it was to surmount barriers to naturalization and invasion. In both 1983 and 2005, casual, naturalized, and invasive species differed in mean frequency by an order of magnitude, with casual species having the lowest frequencies and invasive species the highest. Between surveys, the frequency of 18 species decreased; most other species increased in frequency. The recent surge in ornamental exotics, combined with the temporal trend in invasiveness, indicates that the proportion of invasive species in the flora will increase with time.
At least three aspects of this research are particularly relevant to resource management. First, we found that the proportion of ornamentals in the exotic flora nearly doubled in 22 years on Tumamoc Hill, likely as a result of spread from nearby housing developments. Ornamental species like fountain grass (Pennisetum setaceum), African sumac (Rhus lancea), African daisy (Castalis tragus), bird of paradise (Caesalpinia gillesii), lantana (Lantana urticoides), Aloes (Aloe spp.), Mesquite hybrids (Prosopis spp.), European olive (Olea europa), agaves (Agave americana), and several species of prickly pear (Opuntia basilaris, O. engelmannii var. linguiformis, O. ficus-indica, O. microdasys, and O. santa-rita, none of which are native to the Tucson Mountain region) have either recently appeared or increased in frequency since the 1983 survey (Figure 1, Tables 1 & 2). This suggests that natural areas are becoming increasingly vulnerable to urban horticulture as development further encroaches into the surrounding desert and suggests that more caution should be used when selecting plants for landscaping in developments that border natural areas.
Fig. 1. Distribution of all ornamental species at the Desert Laboratory,
Tucson, Arizona, in 1983 and 2005.
Table 1. Frequency of casual exotic plants at the Desert Laboratory, Tucson,
Arizona.
Table 2. Frequency of naturalized plants at the Desert Laboratory, Tucson,
Arizona.
Second, we found that the longer a species has been present, the more likely it will become invasive. Invasive species are defined as self-replacing species that are ubiquitous and abundant in undisturbed habitats. Of the 52 encountered in both surveys, seven qualify as invasive on Tumamoc Hill (Table 3). Buffelgrass (Pennisetum ciliare), a species first introduced in Tucson in 1938 and first documented at the Desert Laboratory in 1968, underwent the most dramatic expansion (Figure 2) from 1983 to 2005, illustrating the rapidity with which the transition to invasiveness can occur. Exotic species that we view to be "safe" at the present time may very well become invasive in the future, and we should pay particular attention to those species that have transitioned from casual to naturalized.
Table 3. Frequency of invasive plants at the Desert Laboratory, Tucson,
Arizona.
Fig. 2. Distribution of Pennisetum ciliare at the Desert Laboratory,
Tucson, Arizona, in 1983 and 2005.
Finally, this paper provides one example of the kinds of baseline data that are needed to evaluate and manage exotic species in the state of Arizona. Without baseline mapping data, it is impossible to estimate rates or patterns of spread for a particular species, budget for control measures, or monitor the success of control efforts. This study represents a simple method for assessing changes in exotic species distributions over time and for assisting in the prioritization of control efforts.
*We modified the terms of Richardson et al. (2000) slightly to fit the spatial and temporal scales of our study. We defined exotic as non-native to the vicinity of Tucson, casual exotic plants as species that have not yet formed self-replacing populations but rather depend on repeated introductions for their persistence, naturalized exotics as those species that reproduce consistently and sustain populations over many lifecycles, typically recruiting close to the parent plants, and invasive exotics as those species that produce large numbers of offspring at considerable distances from the parents and thus can spread over wide areas in relatively short times.
Bowers, J.E., T.M. Bean, and R.M. Turner. 2006. Two decades of change in distribution of exotic plants at the Desert Laboratory, Tucson, Arizona. Madroño 53:254-265.
Burgess, T.L., J.E. Bowers, and R.M. Turner. 1991. Exotic plants at the Desert Laboratory, Tucson, Arizona. Madroño 38:96-114.
Richardson, D.M., P. Pysek, M. Rejmánek, M.G. Barbour, F.D. Panetta, and C.J. West. 2000. Naturalization and invasion of alien plants: concepts and definitions. Diversity and Distributions 6: 93-107.
For more information contact:
Travis Bean (bean@email.arizona.edu)