by Andrew L. Rypel

Conservation science and restoration ecology are challenging and interdisciplinary fields. Managing for ecological function necessitates focus on multiple scales of ecological organization while simultaneously integrating feedback loops with critical environmental drivers like temperature, flow and habitat change. This means scientists working on these issues can emerge from diverse areas of inquiry including ecology, engineering, hydrological sciences, fisheries, geology, geography, and law. UC Davis’ Center for Watershed Sciences embraces this broad approach to water issues in California, but there are common challenges among fields. One particularly tricky proposition is the selection of appropriate “response metrics” when discussing restoration activities. 

A heuristic experience with Baldcypress trees

When I worked in rivers and reservoirs in the southeastern USA, I was always intrigued by the large Baldcypress trees (Taxodium distichum, a close relative to our California redwoods) growing in the middle of huge reservoirs. There were always only large trees – never any juveniles. It turns out these trees (which are long-lived, like redwoods) date back to the original floodplain, before the damming of rivers. Baldcypress are evolutionarily adapted to variable hydrological systems (Rypel et al. 2009). Thus while trees can persist as adults in reservoirs (flooded rivers), the seeds must fall into dry soil to germinate. In others words, reservoir cypress stands were now ecological sinks rather than sources – a form of extinction debt yet to be paid from dam construction. Yet a community ecologist might go out to reservoirs and, based on presence data, conclude that reservoirs are perfectly optimal habitats for Baldcypress trees. However these habitats clearly are not suitable to complete their life-cycle and these stands are in the process of decline because of hydrological impacts from dams – the decline is just very slow.

This example, while admittedly simple, offers a cautionary tale for how we can trick ourselves into focusing on pet ecological aspects and processes while ignoring critical ones. Holistic approaches are needed to isolate governing ecological dynamics and help guide management. In this blog, I’d like to briefly highlight one integrative metric I’ve gravitated to over time. The goal is not to convince that we should all measure or study secondary production. Rather, it is to illustrate the types of dynamics and thinking we might consider for evaluating effects of the vast water projects in California. While I focus on production, many other metrics could be equally relevant (e.g., nutrient cycling, decomposition, primary production, water circulation patterns, resilience) – essentially metrics classified as “ecosystem functions”.

What is “secondary production”?

Secondary production is the formation of biomass that crosses ecological trophic levels through time. It directly parallels the concept of primary production, but is intended for heterotrophic animals rather than plants. Both primary and secondary production are expressed in the exact same units (e.g., g m^-2 y^-1), meaning rates are comparable within or across food webs (Fig. 2, 3). It is also directly analogous to metrics of economic production such as GDP, only in this case, carbon is the currency of import. Although biomass is a representative static measure at a point in time, production is dynamic, and appropriately defined as a “flux”. And while correlated with biomass, production integrates a wider array of critical rate functions for animals including growth, reproduction, mortality, development time and lifespan. There is also the production-to-biomass ratio or “P/B”. P/B ratios provide an estimate of biomass turnover time and is a representative measure of productive capacity, expressed in units of inverse time (i.e., y^-1). For these reasons, secondary production captures underlying dynamics related to energy availability, trophic relationships and turnover. Interestingly, it also can be quantified at any level of organization – from an individual (essentially growth), to a population (probably the most common approach), to a community or assembly of species (much rarer but still done), or even whole ecosystems (quite rare).

In a recent paper, we suggest secondary production is an underutilized metric for tracking efficacy of restoration initiatives. Dating back to Lindeman (1942) and before, ecologists have been interested in understanding the dynamics of energy transfer in food webs (Fig. 2). Yet most of the literature over the last 30 years has focused on benthic macroinvertebrate communities. The relative paucity of studies for other taxa may have something to do with a perception among other ecologists that these approaches require extensive data that are too costly or laborious to obtain. For example, if one were “starting from scratch” with study design, it can be difficult and costly to obtain all the data necessary to estimate secondary production rates. However, often we are not starting from scratch. For high profile fisheries and ecosystems, all the data needed to estimate secondary production are usually already available (Rypel et al. 2018). Furthermore, when data are not available, simple shortcuts can still provide useful paths (Rypel and David 2017). And usually these analyses yield important insights and patterns. Jeffres et al. (2020) highlight how detrital food webs facilitate the dramatically higher rates of salmon productivity observed on floodplains in the Central Valley. A recent analysis found that valuable fish populations were experiencing hidden overfishing only visible after patterns in fish production were calculated and examined relative to harvest rates (Embke et al. 2019). Another study of California oil platforms found these to apparently be some of the more productive marine habitats known (Claisse et al. 2014). 

Scope for secondary production in California and the Delta

As we consistently discuss in this blog, California’s natural aquatic resources face unprecedented threats, most of which stem from the sizable existence of human populations. Given the challenges of designing and implementing complex restoration studies in this context, it is unsurprising that many do not initially focus on production as a tool. For example, regulatory mechanisms consistently guide us towards species-based management even though many increasingly recognize these approaches as ineffective and in need of a total re-rewrite (Fogerty 2014; Sass et al. 2017). Holistic, ecosystem-based approaches offer potential for broad understanding of conditions that can support many species (even non-fishes!). Furthermore, focusing on ecological aspects that support general functionality is a more promising and realistic path to improved management. Attempting complete recovery of Delta Smelt is likely impossible at this point. Understanding the conditions that once promoted smelt production, but now support production of natives and similar non-natives (e.g., striped bass) is a worthwhile approach. This context can generate ideas on what actions are needed to boost the type of ecosystem productivity we desire and value.

Several larger points come to mind when considering how a production approach could be useful in California:

Synthesis now! In some cases, we already have enough data for production-based approaches and vital rate analyses that leverage oodles of previously collected data. We simply lack synthesis capacity to fund ecologists to do the work. Transparent and open databases are needed to encourage capacity for diverse scientists to innovate new approaches. Training students and researchers in synthesis, data science, and ecosystem ecology will be key.

Design monitoring programs wisely. Careful choice of focal taxa is needed. It is noble to want to protect and study all species, but this isn’t practical. Ideal monitoring programs would include multiple species, but these might not always be the species at most risk of going away. They should however be representative of the functionality that is valued. Then for each focal species, data on vital rates would be needed so production or key elements of production can be studied over time. Analyses of relationships between production and environmental variables (e.g., flow) would be the next logical step. Indeed frameworks incorporating this type of thinking have emerged before (Fig. 4).

Inventory existing monitoring programs. How satisfied are we with data currently collected as part of our monitoring programs? Are these data yielding a critical depth of understanding to manage ecosystems? Without giving up on core datasets, can we make small tweaks to incorporate critical elements to help scale to the ecosystem level? Existing review efforts by the Delta Independent Science Board and Delta Science Program may be useful in revealing some of these gaps (Brandt and Canuel 2019). 

Water is central to ecological production, but ecological efficiency is key too. Because secondary production is expressed per-unit-area, it is a mathematical certitude that more habitat (water) yields more production. We don’t need additional studies to show this basic relationship. However, understanding how to increase production per unit area is less well-understood. This is similar to agriculture – we know more crops can be grown by having a larger acreage in rotation. The more important challenge is how we grow more crops on the same amount of land? How can we get the most bang per habitat buck in the Delta? This central question can be understood and answered firmly with an ecosystem- or production-based approach.

Water reliability likely won’t increase without improved ecosystem function. Increasing costs and decreased exports combine with uncertain climate change impacts to create water reliability issues for contractors. Given current constraints, reliability simply won’t increase without increased ecological function. How do we get that? A good start might be to focus on defining the ecological functionality that we value or are required to ensure – fish and other ecological production is likely high on this list. We are ultimately in this together and should focus on metrics that lead to the goal.

Study ecology – and management. As shown by the Lindeman 1942 example, these are ancient and fundamental quests in ecology. Data collected in California in conjunction with management-relevant research can contribute to advancing basic ecological understanding. Reciprocally, ecology can help advance management by providing clues about how human actions affect ecosystem functionality. A new focus on vital rate functions would provide a rare bridge between basic and applied sciences that few explore. Doing so will invite more talent into California water and provide greater insights and comparability for needed ecosystem management alternatives. We can’t get lost in our silos – there is too much work to be done.

Andrew Rypel is an Associate Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Co-Director of the Center for Watershed Sciences. 

Further Reading

Managing ecosystem restoration: what does success look like? https://www.pnnl.gov/news-media/managing-ecosystem-restoration-what-does-success-look

Are numbers of species a true measure of ecosystem health: https://e360.yale.edu/features/are-numbers-of-species-a-true-measure-of-ecosystem-health

https://californiawaterblog.com/2020/03/15/new-science-or-just-spin-science-charade-in-the-delta/

Futures for delta smelt. https://californiawaterblog.com/2019/12/15/futures-for-delta-smelt/

Functional flows can improve environmental water management in California. https://californiawaterblog.com/2020/11/29/functional-flows-can-improve-environmental-water-management-in-california/

Ten realities for managing the Delta. https://californiawaterblog.com/2015/07/23/ten-realities-for-managing-the-delta-2/

Knowing the Delta’s past offers new ideas forward. https://californiawaterblog.com/2012/09/26/knowing-the-deltas-past-offers-new-ideas-forward/

Striped bass control: cure worse than the disease? ps://californiawaterblog.com/2011/01/31/striped-bass-control-the-cure-worse-than-the-disease/

Benke, A. C. 2010. Secondary production. Nature Education Knowledge 3(10):23 https://www.nature.com/scitable/knowledge/library/secondary-production-13234142/.

Benke, A. C., and J. B. Wallace. 1997. Trophic basis of production among riverine caddisflies: implications for food web analysis. Ecology 78(4):1132-1145.

Brandt, S., and E. Canuel. 2019. Delta Independent Science Board: recent accomplishments and current activities. Delta ISB Update 8/22/2019. https://bit.ly/34nNRx5

Claisse, J. T., D. J. Pondella, M. Love, L. A. Zahn, C. M. Williams, J. P. Williams, and A. S. Bull. 2014. Oil platforms off California are among the most productive marine fish habitats globally. Proceedings of the National Academy of Sciences 111(43):15462-15467.

Embke, H. S., A. L. Rypel, S. R. Carpenter, G. G. Sass, D. Ogle, T. Cichosz, J. Hennessy, T. E. Essington, and M. J. Vander Zanden. 2019. Production dynamics reveal hidden overharvest of inland recreational fisheries. Proceedings of the National Academy of Sciences 116(49):24676-24681.

Fogarty, M. J. 2014. The art of ecosystem-based fishery management. Canadian Journal of Fisheries and Aquatic Sciences 71(3):479-490.

Jeffres, C. A., E. J. Holmes, T. R. Sommer, and J. V. E. Katz. 2020. Detrital food web contributes to aquatic ecosystem productivity and rapid salmon growth in a managed floodplain. PLoS ONE 15(9): e0216019.

Kwak, T. J., and T. F. Waters. 1997. Trout production dynamics and water quality in Minnesota streams. Transactions of the American Fisheries Society 126(1):35-48.

Layman, C. A., and A. L. Rypel. 2020. Secondary production is an underutilized metric to assess restoration initiatives. Food Webs 25:e00174.

Lindeman, R. L. 1942. The trophic-dynamic aspect of ecology. Ecology 23(4):399-417.

Lund, J., S. Brandt, T. Collier, B. Atwater, E. Canuel, H. J. Shemal Fernando, J. Meyer, R. Norgaard, V. Resh, J. Wiens, J. Zedler. 2015. Flows and fishes in the Sacramento-San Joaquin Delta: research needs in support of adaptive management. A Review by the Delta Independent Science Board.

Parks, T. P., and A. L. Rypel. 2018. Predator–prey dynamics mediate long-term production trends of cisco (Coregonus artedi) in a northern Wisconsin lake. Canadian Journal of Fisheries and Aquatic Sciences 75(11):1969-1976.

Rypel, A. L., W. R. Haag. and R. H. Findlay. 2009. Pervasive hydrologic effects on freshwater mussels and riparian trees in southeastern floodplain ecosystems. Wetlands 29: 497–504. 

Rypel, A. L., and S. R. David. 2017. Pattern and scale in latitude–production relationships for freshwater fishes. Ecosphere 8(1):e01660.

Rypel, A. L., D. Goto, G. G. Sass, and M. J. Vander Zanden. 2018. Eroding productivity of walleye populations in northern Wisconsin lakes. Canadian Journal of Fisheries and Aquatic Sciences 75(12):2291-2301.

Sass, G. G., A. L. Rypel, and J. D. Stafford. 2017. Inland fisheries habitat management: lessons learned from wildlife ecology and a proposal for change. Fisheries 42(4):197-209.