Widespread dam construction, flow diversions and water abstraction for growing human populations in the Colorado River Basin have significantly altered environmental conditions in the region, creating situations that threaten native species persistence and promote the proliferation of invasive fishes (Minckley and Deacon 1968, Olden and Poff 2005, Strecker et al. 2011). Consequently, the number of invasive species meets or exceeds native species in watersheds throughout the southwest (Pool and Olden 2015, Walsworth and Budy 2015). Given climate forecasts predict warmer temperatures and drier conditions during the summer, projecting community changes for novel communities is a primary research challenge (Hobbs et al. 2009).

In our ongoing research, myself, Julian Olden, and colleagues from OSU and USFS are developing a coupled multispecies demographic model to project changes for the novel community of native and invasive fishes in the Verde River, Arizona. Demographic approaches to modeling communities have recently demonstrated a striking ability to recover realistic patterns of community dynamics in response to flow variability for riparian vegetation (Lytle et al. 2017). By linking populations of commonly occurring fish species together through a community-wide carrying capacity and using recorded flow data from the Verde River, we are simulating community dynamics over the past half century. Hydrologic floods and droughts are critical bottlenecks that affect recruitment and survival of fishes, so these flow events are used to vary annual rates of recruitment and survival for each species based on published flow-response relationships.

Preliminary results suggest increasing drought frequency favors the dominance of invasive species in the fish communities. Native species made up about 80% of total abundance at the beginning of the time series, but in recent decades, invasive species have become much more abundant. The switch to invasive species dominance coincides with the increasing frequency of drought events. Fluctuations in the abundance of native and invasive fish predicted by the model reflects differences in flow dependence and the survival of young-of-year fishes. Invasive fishes reproduce during summer low-flow conditions when smaller volumes of water concentrate prey, providing a food subsidy and increasing survival probability (Schlosser 1985, Humphries et al. 1999, Craven et al. 2010). Additionally, drought results in crowding intensifying predation and competition for resources among fishes (Magoulick and Kobza 2003, Matthews and Marsh-Matthews 2003). In the southwest USA, competition and predation with invasive species is known to negatively impact native fish populations (Dudley and Matter 2000, Stefferud et al. 2011). For all these reasons, unfortunately, invasive species do well in drought years.

If droughts continue to be as severe and frequent as the past decade, native fishes are likely to face serious challenges in the future. In general, opportunities to enhance the probability for native fish persistence include the control of invasive species populations through removal efforts, and maintaining flow connectivity or refuge pools during dry summer months by limiting water extractions.

— Jane S. Rogosch



Craven, S. W., J. T. Peterson, M. C. Freeman, T. J. Kwak, and E. Irwin. 2010. Modeling the Relations Between Flow Regime Components, Species Traits, and Spawning Success of Fishes in Warmwater Streams. Environmental Management; New York 46:181–94.

Dudley, R. K., and W. J. Matter. 2000. Effects of Small Green Sunfish (Lepomis cyanellus) on Recruitment of Gila Chub (Gila intermedia) in Sabino Creek, Arizona. The Southwestern Naturalist 45:24–29.

Hobbs, R. J., E. Higgs, and J. A. Harris. 2009. Novel ecosystems: implications for conservation and restoration. Trends in Ecology & Evolution 24:599–605.

Humphries, P., A. J. King, and J. D. Koehn. 1999. Fish, Flows and Flood Plains: Links between Freshwater Fishes and their Environment in the Murray-Darling River System, Australia. Environmental Biology of Fishes 56:129–151.

Lytle, D. A., D. M. Merritt, J. D. Tonkin, J. D. Olden, and L. V. Reynolds. 2017. Linking river flow regimes to riparian plant guilds: a community-wide modeling approach. Ecological Applications 27:1338–1350.

Magoulick, D. D., and R. M. Kobza. 2003. The role of refugia for fishes during drought: a review and synthesis. Freshwater Biology 48:1186–1198.

Matthews, W. J., and E. Marsh-Matthews. 2003. Effects of drought on fish across axes of space, time and ecological complexity. Freshwater Biology 48:1232–1253.

Minckley, W. L., and J. E. Deacon. 1968. Southwestern Fishes and the Enigma of “Endangered Species.” Science 159:1424–1432.

Olden, J. D., and N. L. Poff. 2005. Long-term trends of native and non-native fish faunas in the American Southwest. Animal Biodiversity and Conservation 28.1:75–89.

Pool, T. K., and J. D. Olden. 2015. Assessing long-term fish responses and short-term solutions to flow regulation in a dryland river basin. Ecology of Freshwater Fish 24:56–66.

Schlosser, I. J. 1985. Flow Regime, Juvenile Abundance, and the Assemblage Structure of Stream Fishes. Ecology 66:1484–1490.

Stefferud, J. A., K. B. Gido, and D. L. Propst. 2011. Spatially variable response of native fish assemblages to discharge, predators and habitat characteristics in an arid-land river. Freshwater Biology 56:1403–1416.

Strecker, A. L., J. D. Olden, J. B. Whittier, and C. P. Paukert. 2011. Defining conservation priorities for freshwater fishes according to taxonomic, functional, and phylogenetic diversity. Ecological Applications 21:3002–3013.

Walsworth, T. E., and P. Budy. 2015. Integrating Nonnative Species in Niche Models to Prioritize Native Fish Restoration Activity Locations along a Desert River Corridor. Transactions of the American Fisheries Society 144:667–681.