Revisiting tamarisk biocontrol in the western United States: ecological and societal implications
Pamela L. Nagler1*, Emily Palmquist2, Keirith Snyder3, Matthew J. Johnson4, Mary Anne McLeod5, Eduardo Jimenez-Hernandez6, Christian Edwards7, and Kevin R. Hultine8
1U.S. Geological Survey, Southwest Biological Science Center, 520 N. Park Ave., Tucson, AZ 85719 USA
2U.S. Geological Survey, Southwest Biological Science Center, Grand Canyon Monitoring and Research Center, 2255 N. Gemini Drive, Flagstaff, AZ 86001 USA
3U.S. Department of Agriculture − Agricultural Research Service, Great Basin Rangelands Research Unit, 920 Valley Road, Reno, NV 89512 USA
4EcoPlateau Research, 2251 NE Lakeridge Drive, Bend, OR 97701 USA
5SWCA Environmental Consultants, 1750 South Woodlands Village Boulevard, Suite 200, Flagstaff, AZ 86001 USA
6Department of Biosystems Engineering, The University of Arizona, 1177 E 4th, Suite 403, Tucson, AZ 85721 USA
7Utah Department of Natural Resources, Division of Wildlife Resources, Washington County Field Office, 451 N SR-318, Hurricane, UT 84737 USA
8Department of Research, Conservation and Collections, Desert Botanical Garden, 1201 N. Galvin Parkway, Phoenix, AZ 85008 USA
In 2001, the tamarisk leaf beetle (“beetles,” Diorhabda spp) was released to control invasive tamarisk (Tamarix spp), which dominates western United States (US) floodplains and alters riparian ecosystems. The beetles now inhabit thousands of river miles across the western US, radiating far from original release sites and repeatedly defoliating tamarisk stands. While biological control offers an alternative to mechanical or chemical removal, the benefits from tamarisk biological control remain an open question. We review the current knowledge of one of the most consequential biological control programs ever implemented in North America. We evaluate: 1) impacts on riparian ecosystem evapotranspiration and riverine hydrology, 2) changes to ecosystem-scale carbon and nutrient cycling, 3) vegetation community dynamics and secondary invasions, and 4) avian habitat quality and impacts on at-risk bird species. We assess current challenges and opportunities from tamarisk biocontrol, and anticipate how western US riparian forests may evolve with reduced tamarisk cover. We discuss four lessons learned related to beetle distribution, impacts on tamarisk, hosting switching, beetle hybridization and impacts on fire risk. Some river reaches now have more native shrubs where tamarisk cover has declined due to beetles, but native tree recovery remains rare. From 2014–2023, 52% of studied sites in the Colorado River Basin showed modest reductions in evapotranspiration from tamarisk biocontrol, while evapotranspiration (ET) of remaining sites increased slightly, yielding inconsistent water savings. Carbon and nutrient cycling accelerated during initial outbreak conditions, but successive years of defoliation did not produce directional cumulative impacts. Beetle-impacted tamarisk stands support a depauperate avian community and negatively affect breeding in rare birds. Restored native vegetation provides important replacement habitat. In our effort to re-examine the ramifications of tamarisk control after the Hultine et al. (2010) assessment, we acknowledge that the biological complexity of riparian ecosystems and the vast geographical scope of tamarisk biological control will make any predictions regarding how riparian forests across the western US will look and function in the future require myriad caveats.
Given these caveats, a few likely scenarios are as follows: 1) periodic tamarisk defoliation over the next 15 years results in limited mortality and so a lesser change in riparian plant communities in general. The riparian plant communities are a mix of healthy tamarisk, defoliated tamarisk, and other species able to live alongside tamarisk. In this case, the plant communities continue to look a lot like what we currently have. 2) similar to scenario 1, except hotter/drier climate conditions give the remaining tamarisk a competitive advantage. This could result in steady, dominant stands of tamarisk and (potentially) associated xeroriparian plant communities. 3) regular tamarisk defoliation results in high mortality of many tamarisk stands across the western US. In this case, riparian areas have low tamarisk cover over the next 15 years, but may be characterized by large, dead stands of tamarisk. These stands remain on the landscape unless removed through fire, flood, or restoration. The associated riparian plant communities adjust to hydrologic conditions that can support them - mesoriparian in wetter conditions, xeroriparian in hotter/drier conditions.