This study will address a fundamental issue about how species interactions influence communities. Classical approaches, including the vast majority of experimental and modeling studies, focus on how species affect the density of other species through predator-prey interactions. This approach ignores a growing body of work that documents how prey modify their phenotype (traits) in response to predators to reduce predation risk, and how this, in turn, can profoundly influence the consequences of species interactions. There is abundant evidence that such nonconsumptive effects (NCEs, also trait-mediated or nonlethal) of predators could be important in pelagic Great Lakes and marine ecosystems. Whereas, the underlying mechanism is well known, there is little understanding of the influence of NCEs, and almost no studies of NCEs in large pelagic ecosystems. The existence of NCEs could strongly affect the qualitative and quantitative nature of species interactions and the properties of communities.
This project will examine the role and operation of NCEs in marine and Great Lake systems. The investigators will examine NCEs caused by an invertebrate predator, the invasive spiny water flea Bythotrephes. The project will attempt to answer the broad questions: How do Bythotrephes induced effects on prey influence their net effect on Great Lakes communities? Are NCEs influential in complex field settings? What is the influence of NCEs of an invertebrate predator on the zooplankton prey assemblages or on the young-of-year fish competitors? How do factors such as resource level, temperature, and light affect the influence of a predator?s NCE on a prey assemblage (via effects on competition)?
A comprehensive approach based on three objectives will be used that integrates laboratory studies, field surveys and modeling. The investigators will: (1) Determine the phenotypic responses of key zooplankton species to Bythothrephes. This will involve laboratory studies that examine behavioral, morphological and life history responses. Laboratory studies will also quantify Bythotrephes predation rates. Optimization modeling will guide and allow interpretation of these laboratory studies. (2) Develop predictive models of the NCEs of Bythothrephes on zooplankton prey growth rate and competitive outcomes and test predictions with microcosm and mesocosm experiments. This is a key methodological component: If models accurately predict induced trait changes, and their consequences to species interactions in mesocosm experiments, this increases the ability to form predictive models in natural systems. (3) Determine NCEs of Bythotrephese in the field using ecological models based on parameters derived from the aforementioned exercises and field surveys. Field surveys will examine the effect of Bythotrephes on zooplankton prey in the field, and classify abundances and parameters such as young of the year fish density and position and abiotic factors.
Significant intellectual contributions will result from this study by addressing a key, but poorly understood, component of species interactions in Great Lakes and marine systems. Societal concern and interest over the influence of invasive species including Bythotrephes in Great Lakes food webs will facilitate educational outreach. In addition, close associations with institutions (e.g., the NOAA Great Lakes Environmental Research Laboratory, GLERL) will develop from the project and there will be strong outreach components that foster educational opportunities. For example, many individuals and groups from high schools and management agencies come to GLERL for education and training. Further, a Great Lakes Sea Grant Network extension educator has agreed to work to bridge the research with educators by helping the PIs engage with the Center for Ocean Sciences and Education Excellence Great Lakes (COSEE) Great Lakes and Sea Grant. The investigators on this project will work with professional educational outreach partners to enhance the broader impacts from the research. COSEE Great Lakes already has a framework in place to reach teachers throughout the region and includes an emphasis on underrepresented groups including tribal educators and COSEE and Sea Grant also are working on a variety of curriculum pieces. This project will contribute to these efforts. Finally, graduate and undergraduate training in research and outreach is part of this activity.
Predators impact prey by killing them for food. This is the popular conception. It is also the prevalent view in ecological and environmental sciences, in regards to theoretical models built to investigate general phenomenon such as biodiversity and in management models designed to e.g., investigate the effect of fishing or invasive species on e.g., marine and freshwater ecosystems. But we know prey don't passively sit around and get eaten; they display many means to detect and avoid predation. What we are learning is that these changes are far more extensive than avoiding nearby predators: prey change their behavioral characteristics, such as spending more time in refuges, moving slower for the entire day, or even growing larger protective spines. We are further learning that whereas such responses may save the prey, they can come at a very large cost to the prey and have large effects on ecosystems. Such effects caused by a predator by "scaring prey" (rather than eating them) are called non-consumptive effects (NCEs). We investigated this problem in the Lake Michigan food web. Zooplankton -- small crustaceans 1 to 6mm long -- are animals lowest in marine and freshwater food webs that serve as the major resource for the entire upper food web, including fish. The spiny water flea (SWF) is an invasive invertebrate that preys on zooplankton, and thus has been hypothesized to disrupt Great Lake's food webs and fisheries. We investigated whether non-consumptive effects may be at play. We had previous evidence that suggested Daphnia (a zooplankton prey) can perceive SWF scent in the water, and respond by swimming deeper in laboratory water columns. In the present work, we have extended our previous finding: at the higher end of SWF densities in the field, Daphnia are found on average 11 meters deeper. Pinning this finding down was not simply a matter of measuring Daphnia at low and high SWF densities, because there are many different factors that correlate with SWF densities. For example, SWF densities are higher later in the season, so perhaps Daphnia are just lower late in the season but not due to the SWF. Our statistical analysis allowed us to show that indeed Daphnia swim much lower in the day, but come up at night when the threat of SWF is lower. We further found that this effect extends to many other zooplankton, including various types of copepods (the major crustacean in ocean waters). We further found in laboratory experiments that copepod prey may respond so strongly to the scent of SWF that they reduce egg production, and may even die prematurely! Meanwhile, since SWF is an invasive species, and potential prey for fish, we investigated if SWF itself is evolving to its new environment. We found indeed that there is strong selection on its long conspicuous spine (a protective feature against fish), but that selection in opposite directions at different periods of the year essentially even out. Our results have implications for how managers view the effects of SWF in Lake Michigan, but also predators on prey in general. For example, at the heart of most ecological models is the need to represent how many prey predators kill as a function of prey density. Our work is showing that the type of response zooplankton display in response to predators dramatically alters this relationship, which can have a surprisingly large influence on model predictions. This work had a number of broader impacts. As described above, the research is uncovering a potentially general feature of food webs. Training included 1 post-doctoral associate in writing, data-analysis, and mentoring that will be applied at a faculty position secured during tenure on this project. It also supported 1 graduate student, and eight undergraduate students. Mentorship of undergraduate students included independent projects that included oral presentations and co-authorship on journal articles. Research was presented to both scientific and broader audiences. For example the PI organized an international meeting where we presented our research, and the PI was an organizer and moderator of a public outreach symposium on Great Lakes issues. At this symposium, the PI presented the role of basic research in informing managers of the Great Lakes fisheries to a diverse audience of ~200 attendees includes academics, managers, K-12 teachers, and other stakeholders. Further, research presentations and guided tours of laboratory/field operation were given to a number of groups ranging from a local sailing club interested in the health and welfare of Lake Michigan, a local community center for the aged; and students from Western Michigan University and Grand Valley State University. Lastly, The PI presented aquatic ecosystems in aquarium to elementary school classes (including a special education classroom). Food web dynamics, ecosystem processes, and Great Lakes ecology were reviewed over the course of several presentations each year.
