Food Webs

Food Webs

Using Food Web Analysis to Estimate Carrying Capacity, Diagnose Limitations, Prioritize Treatments and Evaluate Success


Dramatic declines of native salmon and steelhead populations throughout the Pacific Northwest (PNW) during the past century have led to the extinction of certain stocks and the imperilment and federal listing of many others [1, 2]. Significant efforts have been undertaken to restore natal salmon streams throughout the PNW during recent decades. Although these efforts are massive and well-funded, little evidence currently suggests that such efforts are actually increasing the abundance of naturally produced juvenile salmonids [3]. Past and current restoration prescriptions often focus solely on correcting physical habitat conditions and geomorphic processes (abiotic) and focus less frequently on the complete set of biological drivers needed to increase and sustain natural production of juvenile salmonids [4].

My doctoral research focuses on identifying density dependent mechanisms and how they limit the production of imperiled fish species in anadromous streams. Changes in trophic energy routing as a result of 1) geomorphic based restoration prescriptions 2) reduced nutrient inputs and 3) competition from non-native fish species are done through the construction and application of a transferable, empirically based, food web analysis tool. Results from this study aim to quantify biological limiting factors in natal streams to better inform recovery.

Density Dependence: An Altered Food Web

Current research suggests that many natal streams inhabited by rearing anadromous salmonids are experiencing density dependent conditions. These conditions may be the result of reduced energetic carrying capacity, meaning that fewer fish can now rear and survive in areas that were historically more productive [5]. Although interior PNW streams are often naturally oligotrophic, this feature, played a key role in evolutionary processes supporting anadromy and the semelparous life history strategy (dying after spawning) exhibited by Pacific Salmon [6]. These carcasses naturally provided a key marine-derived nutrient source that completed the nutrient cycle, energetically justified anadromy, and dramatically increased biological production in oligotrophic streams throughout the interior PNW[7] However, decades of habitat loss and degradation, migration corridor challenges and migration-blocking dams in the Columbia Basin, have greatly reduced nutrient loading to a fraction (< 7%) of its historical contribution [2].

In addition to these reduced nutrient inputs, is the wide spread dispersion of non-native fish species, which directly compete with native fish species (anadromous and resident)  for limited prey resources. In some cases large diet overlap between native and non-native species exist and may be a large contribution to density dependence  [9, 10].

Empirically Based, Quantitative Food Web Analysis to Estimate Carrying Capacity and Detangle Density Dependence

Accurately detangling density dependence in lotic ecosystems is imperative to identifying limiting factors and prescribing treatments that prioritize the corrections of those limitations. Geomorphic and habitat-based monitoring metrics, models and assessments (primarily abiotic) have been developed and employed as tools to assess current and ongoing physical limitations in riverine environments. Though needed and useful, their application does little to understand biological limitations. Furthermore, biological assessment tools based on static metrics (abundance, biomass and diversity) have some value, but lack the ability to quantify energetic limitations (food availability and trophic routing) and ultimately density dependence. In order to identify and then quantify density dependent factors, monitoring protocols and subsequent analysis procedures must capture dynamic, complex processes that directly affect energetic routing.

An extensive, multi-year, food web sampling and analysis plan allows me to develop an empirical, mass balance, bio-energetic approach. This approach   uses a series of estimators and analyses to produce dynamic metrics over time (annually), including taxa level production for fish and aquatic insect communities as well as the energetic relationships between fish and prey communities. This mass balance modelling approach is expected to address a series of questions, currently considered to be large data gaps in restoration science.

Used as a pre- and post-restoration monitoring tool, a food web approach allows researchers and managers to evaluate the effectiveness of specific restoration actions. Empirically derived estimates allow for accurate assessments in a wide range of lotic environments, across watersheds. While empirically derived food web models have been applied in small academic studies, there is a lack of available, applicable analysis tools, to directly answer questions regarding density dependent concerns in a large, holistic, restorative context.

Study Site “Natural Lab”

In order to understand the complex ecological effects of restoration treatments as they relate to food specific, density dependent limitations, I use a small, stable stream to conduct food web experiments. This site allows me to compare a highly degraded reach to an adjacent reach that was recently restored. Follow link to view detailed site information, photos and sampling methods.

(Link to Natural Lab)

Broad Dissertation Questions:

1) How does energetic carrying capacity limit the success of current restoration approaches?
2) How does an altered food web decrease the carrying capacity in restored streams?

Constructing a Usable Analysis Tool as an R Package

My dissertation will provide a rigorous alternative analytical tool to evaluate restoration actions for salmon and steelhead throughout the Columbia River Basin. Empirically based quantitative food web studies require intensive field sampling and lab work. Though most prohibitive as a management tool is the ability to gleam accurate, usable results through the analysis of large, stochastic data sets to answer difficult multivariate questions. Historically, small scale research studies used spreadsheet analyses that required time consuming, repeated calculations. In order for researchers and managers to utilize a quantitative food web approach at a resolution applicable to regional restoration programs, all required analyses are being constructed as a series of module packages using R coding language, which should greatly assist researchers and managers in answering important restoration questions pertaining to specific density dependent limitations. For more detailed information on analysis methods and specific deliverables, follow link to analysis tools.

(Link to Analysis Tools)

  1. NPCC, Columbia River Basin Fish and Wildlife Program: 2009 Amendments. 2009, Northwest Power and Planning Council: Portland, OR.
  2. Gresh, T., J. Lichatowich, and P. Schoonmaker, An estimation of historic and current levels of salmon production in the Northeast Pacific ecosystem: evidence of a nutrient deficit in the freshwater systems of the Pacific Northwest. Fisheries, 2000. 25(1): p. 15-21.
  3. Stewart, G.B., et al., Effectiveness of Engineered In-Stream Structure Mitigation Measures to Increase Salmonid Abundance: A Systematic Review. Ecological Applications, 2009. 19(4): p. 931-941.
  4. Naiman, R.J., et al., Developing a broader scientific foundation for river restoration: Columbia River food webs. Proceedings of the National Academy of Sciences, 2012. 109(52): p. 21201-21207.
  5. ISAB, Density Dependence and its Implications for Fish Management and Restoration Programs in the Columbia River Basin. Independent Scientific Advisory Board, 2015(ISAB-2015-1).
  6. Gross, M.R., Evolution of diadromy in fishes. American fisheries society symposium 1987. 1: p. 14-25.
  7. Stockner, J.G. and Q.I.C.o.R.N.t.S.E. International Conference on Restoring Nutrients to Salmonid Ecosystems, Nutrients in salmonid ecosystems : sustaining production and biodiversity, ed. J.G. Stockner. 2003, Bethesda, Md.: Bethesda, Md. : American Fisheries Society.
  8. Stockner, J.G., Nutrients in salmonid ecosystems: sustaining production and biodiversity. Transactions of the American Fisheries Society 2003. 34.
  9. ISAB, Non-native Species Impacts on Native Salmonids in the Columbia River Basin Independent Scienttific Advisory Board 2008(ISAB 2008-4).
  10. Sanderson, B.L., K.A. Barnas, and A. Michelle Wargo Rub, Nonindigenous species of the Pacific Northwest: An overlooked risk to endangered salmon? BioScience, 2009. 59(3): p. 245-256.