Gretchen Hansen's research on aquatic food webs
Understanding changes in populations of interest often requires an understanding of the food webs in which they are embedded. Benthic invertebrates, zooplankton, phytoplankton, aquatic plants, and even terrestrial biota are all linked to fish populations in lakes. Additionally, aquatic systems may exhibit unexpected responses to environmental change due to indirect effects, and these responses can sometimes only be understood in a food web context.
Interactions between sport fish populations
In many lakes of northern Wisconsin, walleye recruitment and overall abundance is declining while black bass (smallmouth bass and largemouth bass) populations are increasing. This species transition has enormous implications for angling and tourism centered on walleye, traditionally the most popular sport fish of this region. However, the causes of these trends are unknown, and this uncertainty makes designing the most appropriate management actions difficult. In particular, it is unclear if the growing black bass populations have directly caused the walleye declines and thus whether management to reduce black bass numbers might benefit walleye.
Several environmental and human factors could be driving the bass-walleye shift, either singly or in combination. Changes in angler harvest of black bass may be significant. Angler exploitation of black bass has dropped substantially over the last 20 years through a combination of more stringent harvest regulations and widespread adoption of catch-and-release angling practices, presumably contributing to increased bass populations. Conversely, walleye harvest via both angling and tribal spear fisheries has remained fairly steady over the same time period and cannot explain the walleye declines. Conceivably, increased black bass populations might be depressing walleye recruitment and abundance.
Short-term climate cycles and long-term climate trends may also be important. In the short term, many of the affected lakes have been subject to a prolonged (since 2006) drought that has brought inflows and water levels down below values seen during the Dust Bowl era. In the longer term, the climate of northern Wisconsin has been gradually warming over the last 40 years, a trend predicted to continue and perhaps intensify in the future. Warming of surface waters and compression of cooler midwater habitats are likely to positively influence Centrarchid abundance whereas their effects on walleye are less clear and could be negative. Even if the climate-driven changes have been beneficial for walleye, their probable greater positive effects on black bass may have increased black bass numbers to a point at which the bass began to suppress walleye abundance through predation or competition.
Overall, it is unlikely that a single environmental or human factor can explain the bass-walleye transition in all northern Wisconsin lakes. Rather, it is probable that each factor varies in importance among lakes and that the various factors interact in complex ways. Multiplicity of causes is common in ecological changes that occur over broad landscapes. Estimating the importance and understanding the interactions of the various factors that may be driving this species shift is a major goal of my research.
Together with multiple collaborators, we have developed a multi-faceted approach to examine the multiple complex and inter-related hypotheses that have been proposed to explain the bass-walleye transition. Field studies have evaluated the level of direct predation between bass and walleye in Wisconsin Lakes. We have used historical data collected across Wisconsin to evaluate trends in walleye and black bass and predictors of these trends. We have also developed a model of lake temperatures in Wisconsin that we have used to link water temperature to walleye recruitment, and that will be used to understand changes in the thermal habitat of bass as well.
To complement the above statistical approach, we will also use results of the historical data analysis to inform a food web simulation model. Simulation modeling allows alternative hypotheses about the causes of species shifts to be compared, and potential management scenarios to be explored. A variety of management scenarios have already been proposed to address growing black bass and shrinking walleye populations, including changes in regulations to encourage greater angler harvest of black bass, direct removal of black bass from lakes by Wisconsin DNR biologists, more restrictive angling regulations for walleye, and stocking of walleye big enough to avoid black bass predation. Statistical analysis of historical data and ecological simulation modeling can help determine which of these scenarios are most likely to be successful. Finally, we will work with fish management staff to design and evaluate an adaptive management experimental change in regulations to better understand the response of walleye and black bass to regulation changes. Due to the complex nature of this problem, a combination of observational, experimental, and modeling approaches to scientific understanding maximizes our chances of disentangling the multiple hypothesized drivers responsible for the bass-walleye transition and thus identifying appropriate management responses.
Hansen, G. J. A., J. W. Gaeta, J. F. Hansen, and S.R. Carpenter. 2015. Learning to manage and managing to learn: sustaining freshwater recreational fisheries in a changing environment. Fisheries 40(2): 56-64.
Hansen, J. F., G.G. Sass, J. W. Gaeta, G. J. A. Hansen, D. A. Isermann, J. Lyons , and M. J. Vander Zanden. 2015. Largemouth bass management in Wisconsin: Intra-and inter-specific implications of abundance increases. pp 193-206 in W. Pine, ed. Black Bass Diversity: Multidisciplinary Science for Conservation. American Fisheries Society Symposium 82, Bethesda, MD.
Food web consequences of invasive species control
Controlling invasive species can restore ecosystems to uninvaded conditions, but can also result in unexpected effects due to complex food web interactions. We experimentally removed invasive rusty crayfish from a Wisconsin lake. Rusty crayfish abundance declined by 99% in eight years, did not significantly increase four years post-harvest, and no compensatory recruitment response was observed. Native crayfish and sunfish (Lepomis spp.) abundances increased by two orders of magnitude as rusty crayfish abundance declined, and macrophyte cover increased significantly in 2-4 m waters. Rusty crayfish cause well documented declines of benthic invertebrates, and as such we expected benthic invertebrate densities to increase as rusty crayfish were removed. However, fish consumption of invertebrates increased as rusty crayfish density declined and fish were forced to switch to alternative food sources, and invertebrate responses to the crayfish removal were quite variable. Total snail density increased 300-fold in rocky habitats. Mayfly larvae (Ephemeroptera), dragonfly larvae (Odonata), and scuds (Amphipoda) densities also declined in certain habitats as rusty crayfish were removed, suggesting that they are indirectly facilitated by rusty crayfish. Our study highlights the importance of considering indirect effects when assessing the impacts of invasive species, and demonstrates that these impacts may be reversed over relatively short timescales.
Hansen, G. J. A., C. L. Hein, B. M. Roth, M. J. Vander Zanden, J. W. Gaeta, A. W. Latzka, S. R. Carpenter. 2013. Food web consequences of long-term invasive crayfish control. Canadian Journal of Fisheries and Aquatic Sciences. 70: 1109-1122
Alternative stable states
Rapid transitions in ecosystem structure, or regime shifts, can cause complex systems such as food webs to enter an undesirable state and stay there. Such rapid transitions occur in systems that have alternative stable states - that is, two or more stable equilibria. However, regime shifts can occur even when feedbacks are not strong enough to cause alternative stable states. We investigated the potential role of alternative stable states in explaining transitions between dominance of an invasive species, rusty crayfish (Orconectes rusticus), and native sunfishes (Lepomis spp.) in northern Wisconsin lakes. A rapid transition from Lepomis to rusty crayfish dominance occurred as rusty crayfish invaded Trout Lake, WI, and the reverse transition resulted from an eight-year experimental removal of rusty crayfish from Sparkling Lake, WI. We fit a stage-structured population model of species interactions to 31 years of time-series data from each lake. The model identified water level as an important driver, with drought conditions reducing rusty crayfish recruitment and allowing Lepomis dominance. The maximum likelihood parameter estimates of the negative interaction between rusty crayfish and Lepomis led to alternative stable states in the model, where each species was capable of excluding the other within a narrow range of environmental conditions. However, uncertainty in parameter estimates made it impossible to exclude the potential that rapid transitions were caused by a simpler threshold response lacking alternative equilibria. Simulated forward and backward transitions between species dominance occurred at different environmental conditions (i.e., hysteresis) even when the parameters used for simulation did not predict alternative states as a result of slow species responses to environmental drivers. Thus, alternative stable states are possible, but by no means certain, explanations for rapid transitions in this system, and our results highlight the difficulties associated with distinguishing alternative stable states from other types of threshold responses. However, whether regime shifts are caused by alternative stable states may be relatively unimportant in this system, as the range of conditions over which transitions occur is narrow, and under most conditions the system is predicted to exist in only a single state.
Hansen, G. J. A., A. R. Ives, M. J. Vander Zanden, S. R. Carpenter.2013. Are rapid transitions between invasive and native species caused by alternative stable states, and does it matter? Ecology 94:2207–2219.