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Submitted to: Diversity and Distributions
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/2/2004 Publication Date: 1/1/2005 Citation: Bestelmeyer, B.T. 2005. Does desertification diminish biodiversity? Enhancement to ant diversity by shrub invasion in southwestern USA. Diversity and Distributions. 11(1):45-55. Interpretive Summary: The conversion of desert grasslands to shrublands is a long-standing concern in the southwestern United States, but the effects of this change on native animals defy generalization. For most types of animals, desertification is assumed to lead to reduced diversity. Here, I consider evidence that shrub invasion and encroachment, particularly that of honey mesquite (Prosopis glandulosa), has led to increasing ecological dominance and diversity of ants in general, as well as increases in specific native taxa. The mesquite-dominated Jornada site harbored four times the number of ant foragers found at the relatively 'pristine' Sevilleta site. Species richness and ecological dominance was also greatest at the Jornada. Within the Jornada landscape, turnover in species composition was related to mesquite density, but local richness and abundance was unrelated to mesquite density. Coupled with the results of previous studies, there is support for the notion that ant diversity is not negatively affected by shrub invasion but that several taxa prosper from it. Technical Abstract: The conversion of desert grasslands to shrublands is a long-standing concern in the southwestern United States, but the effects of this change on native animals defy generalization. Here, I consider evidence that shrub invasion and encroachment, particularly that of honey mesquite (Prosopis glandulosa), has led to increasing ecological dominance and diversity of ants in general, as well as increases in specific native taxa. The effects of shrub invasion on ants were measured at two scales: (1) between Chihuahuan Desert landscapes that vary slightly in temperature and strongly in the dominance of mesquite, and (2) across variation in mesquite density occurring within a generally mesquite-dominated landscape. Ant richness and numerical dominance was measured at pitfall traps over 2 years and baits were used to assess ecological dominance across different temperatures. The mesquite-dominated Jornada site harbored four times the number of ant foragers found at the relatively 'pristine' Sevilleta site with several ecologically-dominant taxa driving this pattern, especially Dorymyrmex bicolor. Species richness and ecological dominance was also greatest at the Jornada. Within the Jornada landscape, turnover in species composition was related to mesquite density, but local richness and abundance was unrelated to mesquite density. Coupled with the results of previous manipulative experiments and comparative studies, there is support for the notion that ant diversity is not negatively affected by shrub invasion but that several taxa prosper from it. The Jornada is uniquely saturated by dominant ant taxa, perhaps as a consequence of an overall high level of shrub availability that provides a reliable source of carbohydrate-rich plant exudates. This raises important questions about the relationship between environmental degradation, ecosystem productivity, and animal diversity. |
