Desert ecosystems are increasingly affected by the human-driven
environmental pressures currently changing global landscapes and climates. Plant species are especially susceptible due to their limited ability to move and a frequent reproductive dependence on pollinators that could themselves be displaced. Rising temperatures, aridity and development have already led to decreasing vegetation cover and increasing isolation of desert plant populations (1). It is essential that we identify the limitations climate and landscape already exert on species in these zones, as well as the ability of damaged habitats to recover from disturbance. Using this information we can inform future efforts to protect these fragile ecosystems as damage continues. In my dissertation I address these issues through three parts: First, I document the recovery of an area ravaged by prolonged desertification in the Sahara to determine if recent greening represents true reestablishment of lost habitat, or simply growth within refugial patches that were never lost during drought. Second, I examine reproductive limits in a representative shrub species in the Mojave, Acacia greggii A Gray. In A. greggii I determine if variation in pollinator availability or climate currently influences seed production across The Mojave National Preserve. Third, I identify movement of this species to determine if current climate or landscape features limit dispersal. I have shown that decreased environmental pressure in the Sahara has not guaranteed recovery for damaged areas. In the Mojave I have shown climate and landscape restrictions on reproduction and movement of Acacia greggii. These results indicate that changing environmental conditions will further restrict species in these areas and emphasize the need for swift action to halt possibly irreversible damage.
Detecting landscape change in a desert environment:
The last 100 years have seen drastic environmental changes in the Sahara and Sahel where a prolonged drought beginning in the 1970s led to pronounced vegetation loss. Previous work with coarse scale remote sensing imagery has shown dramatic expansion of the Sahara during the drought (1). However, more recent work shows a greening trend since the mid-1990s (1). I sought to determine if this indicates actual recovery of lost vegetative area, or greening of refugial patches that were never lost during the drought. To do this, I examined a 32,000 km2 area in The Islamic Republic of Mauritania reported to have a recent increase in vegetation cover. I used high-resolution satellite images collected every 15 days from 1990 to 2010. Like previous work, I found vegetation recovery in this region, but significant patterns of continued loss are ongoing in areas away from refugia.
Reproductive limitations of Acacia greggii:
Movement of a Mojave Desert shrub at multiple spatial scales:
Identifying past and present movement patterns is essential to understanding the influence of a changing landscape and climate on the viability of a species. The rate of movement determines the ability of a species to reestablish a damaged population after disturbance, or maintain genetic variation when populations are separated. Acacia greggii occurs across a broad elevation gradient, and has populations separated or defined by distinct geographic features. It is possible, that this species has succeeded due to its ability to disperse widely in spite of these factors. If, however, its movement is affected by them, it will indicate susceptibility to future change. Using molecular markers I developed in the lab, I addressed these alternative hypotheses for A. greggii within the Mojave National Preserve. To identify broad and fine-scale influences Isampled individuals at two spatial scales. I genotyped 460 trees from 23 regional sites (6,200km2), and 200 trees along three parallel dry washes (8km2).
1) Anyamba, A. and C. J. Tucker (2005) Journal of Arid Environments 63(3): 596-614.
2) Agular, R. et al. (2006) Ecology Letters 9: 968-980.
3) McCarthy, J. (2002) Conservation Biology 15 (2): 320-331.