Foodwebs


 * Consideration of Food Web Dynamics in River Restoration **  By Betsy Shafer   River and Watershed Restoration   3/30/2012

Studies on aquatic food webs are typically interested in identifying sources of energy, or carbon, nutrients, and rates of energy transfer that act to shape biotic community assemblages. Food webs are driven by the availability of resources, trophic levels and roles (meaning functional feeding groups), and environmental conditions that range in availability of nutrients and light, geomorphology, hydrologic flow, and other physicochemical parameters. These variables co-vary within and among watersheds and shape biotic community structure (abundance and biodiversity) and function (nutrient cycles and uptake rates). The interplay between biotic (living) and abiotic (nonliving) variables shape food web structure and function. Due to the complexity of trophic interactions, studies have analyzed food web structure on spatial (Finlay et al. 2002) and temporal scales (Woodward and Hildrew 2002) in attempt to describe controls of energy dynamics and ecosystem processes. For example, spatial scales of biological communities range from microscopic biofilms on large woody debri in a stream reach to watershed scale of aquatic species migrating upstream to spawn in tributary stream reaches and served as a food source to carnivores. Food web dynamics is an important component when characterizing ecosystem processes and provides a description of potential pathways of energy transfer through the interaction of trophic levels. Conducting quantitative and qualitative measurements of food webs provide an estimate of ecosystem structure and function at a specific point in time. Food webs are dynamic in nature because they respond to environmental conditions and provide useful information when addressing ecosystem level processes, structure and function.
 * Introduction of food webs in lotic systems **
 * Definition and description of food webs **

Ecosystem structure and function can be used as a measure of river health (Young et al. 2008) and serves to indicate the integrity of a system. Studies show human activities alter food web dynamics directly or indirectly resulting in a shift in basal food resources that gets transferred along trophic levels and changes biological community structure when compared to natural conditions of the system. Before restoration activities are set into motion, it is of equal importance to identify historical food web structure and function prior to anthropogenic impact of the system and to use historical conditions as a restoration guideline. Additionally, reference streams or sites are a useful when comparing river health, particularly biotic community structure, because they represent 'stream health'. Yet, the lack of food web dynamics incorporated into restoration goals is a shortcoming that can impact the success of restoration. Public interest can be narrowly focused on a particular life-stage for species. For example, adult Chinook Salmon in the Northwest have to migrate upstream through fish passages at dams, but juveniles traveling downstream have difficulty using fish passages to travel downstream. An overall understanding of food web in river restoration is slowly evolving, however, the concept is complex. The objectives of this paper are to 1) describe the importance of food web function and structure in riverine systems and 2) evaluate how food web concepts have been implemented in river restoration projects.

Stream ecologists study ecosystem processes in river systems with a perspective of terrestrial and aquatic ecosystem connections (Hynes 1975). In addition, longitudinal, lateral, and vertical spatial gradients along streams create complex food web structures due to the variability in environmental conditions (see floodplain connectivity and hyporeic zone pages for additional information). Two well known drivers of biological communities are stream flow and geomorphic patterns. In disturbed systems, the extent of altered flow regimes in streams from upstream impoundments is known to largely affect biotic and abiotic components that are adapted to historical stream conditions (Poff et al. 1997). For greater depth of natural flow regimes and evironmental flows visit Ryan's page.
 * Implications of disturbed ecosystems on aquatic community composition **

Each stream is controlled by specific environmental factors. As a result restoration goals and approaches will vary due to climate forcing and degraded conditions specific to stream locations. The response community composition to environmental disruption can be seen in declines of populations of aquatic species. Efforts have been made to restore stream systems by targeting aquatic species with collapsed populations, especially species listed as ‘threatened’ or endangered’. A common method to restore species is to simulate historical patterns of variable flow through the release of impoundment water to simulate environmental cues for spawning. Most importantly, tactics of restoring a single species require knowledge of life-histories, habitat preference and roles in the food web. If a species population crashes, then a holistic approach is needed to identify ‘cause and effect’ relationships that are influencing a particular species survival. Food webs account for direct and indirect trophic and intratrophic (within the same trophic level) interactions providing valuable information to assist restoration of a species of ecosystem. Ways to measure the impairment of food web structure and function include a quantitative assessment of biotic populations, nutrient uptake, and rates of energy transfer. This approach allows one to address such questions like will restoration efforts benefit the abundance of one species at the trade-off of another; or how will nutrient cycles be altered in floodplains from the manipulation of flow by dams upstream, how does physical habitat construction influence community assemblages, and can basal food resources be supplemented in a stream? This paper will address some of these questions in context of restoration. Overall, restoration projects need to assess ecosystem structure and function to understand how food web dynamics drive biological diversity and abundance.

Restoration efforts are initiated for a variety of reasons, but are commonly implemented to improve endangered species of fish that experience decline in populations, water quality conditions caused by invasive aquatic species and/or anthropogenic disturbance, and/or ecosystem health within a stream system. These are due to a myriad of human activities along the stream and within the watershed, acting to disrupt food web structure and function, in both space and time. Impairment can manifest along the entire stream or at segmented reaches depending on the degree of human activities. In addition, a particular species life-stage may be more vulnerable than other life-stages to environmental disturbance or invasive species. Finally, important biogeochemical cycles that occur between connective landscape-river ecosystems are known to become altered from anthropogenic disturbance.
 * What is considered a disturbed system in need of restoration? **

Two dominate sources of carbon for lotic systems are allochthonous and autochthonous material derived from terrestrial vegetation and in-stream primary production, respectively. Allochthonous and autochthonous sources represent the base of lotic food pyramids that support higher trophic levels (herbivores and carnivores). Sources of energy available in a stream can vary spatially and depend on canopy coverage (leaf litter) and light availability for in-stream photosynthetic organisms like algae and macrophytes, vascular aquatic plants. The uptake of energy by consumer organisms is transferred through the food web at each trophic level, meaning the role of organisms as producer, consumer, and decomposer. The quality of food resources available, rates of energy transfer, and how consumers utilize energy is important when describing food webs. Trophic levels can be delineated further into functional feed groups (FFG) that represent the functional role of organisms in transporting energy within the food web. Identification of basal food sources should be considered when assessing what drives structure and function of biological communities. Studies on deforestation have conveyed aquatic community assemblages shift in response to change in carbon food source, from leaf litter to algae, and loss of FFG specialized to forage on terrestrial inputs (Benstead and Pringle 2004).
 * Energy Budgets (carbon) **

Food resources vary in size and structure along the stream continuum. The river continuum concept theorizes the shift in community structure of organisms is to capitalize on upstream community inefficiencies in processing organic matter (Vannote et al. 1980). Elemental isotopic studies have been conducted to identify nutrient origins, such as allochthonous and autochthonous sources, and its relative importance in structuring food webs. A comparison of upstream and local sources of nutrients in streams was studied by Finlay et al. (2002) using stable isotopic ratios. They found aquatic invertebrate to consume food resources from either localized habitat or through the partitioning of drifting sources depending on the FFG of the organism. In conjunction, invertebrate were found to drift between riffle and pool habitats serving to link trophic interaction in adjacent habitat and play an important role in the transfer of energy on multiple spatial scales in food webs. The use of isotopic research is to explain the partitioning of nutrients and/or feeding behavior of organisms. Scientific research linking nutrient uptake and food web structure will progress restoration attempts to restore species recovery and ecosystem health.

Ecosystem linkages occur on multiple spatial scales from macro to micro scales. Direct and indirect interactions between terrestrial and aquatic ecosystems have been assessed by Woodward and Hildrew (2002) to characterize the effects of spatial scales on community structure and food webs. For example, the dispersal of adult insects (ie dragonfly) upon emergence of aquatic larval life-stage connects aquatic-terrestrial ecosystems. Dispersal of organisms within and among watersheds contributes to the complexity of food webs and tracking how energy is utilized. Another example is migratory species, like anadramonous fish, play an important role linking oceanic and river food webs. Anadramous species spend the adult life-stage in ocean environments and migrate up freshwater rivers to spawn, the young are reared in freshwater ecosystems for usually a year then migrate to the ocean.
 * Biotic interactions **



Energy is transferred through trophic interaction at a rate of 2 to 40 percent between each step in trophic level (Horne and Goldman chapt. 15 pg 333). The inefficiency of energy transfer is represented in the decrease in biomass with an increase in trophic level. A typical diagram that displays energy transfer rates is the pyramid of numbers (Figure 1). The basic idea is that primary producers compose more biomass than top carnivores. Shifts in the food base (primary producers) cascades up the trophic levels. Predators are known to show declines in population because of shifts in the food base structure. For example, Vander Zanden et al. (2003) found food base shifted in Lake Tahoe from historical conditions of benthic (bottom) production to pelagic producers (phytoplankton) due to fish introduction and extirpation of native fish species.

Food web structure is patchy at multiple scales due to the availability of basal food resources (carbon) and biotic interactions (Figure 2). This figure also implies restoration strategies for food webs would differ due to different trophic linkages and possible environmental/biotic controls on the food web. When predator trophic levels display control over trophic interaction then a top-down response occurs in the food web; in contrast, when basal food sources control food web dynamics then a bottom-up response occurs. When biotic components are altered then food web structure shifts due to responses of organisms that make up the community. Temporal variability in available food resources also influence food web dynamics that can be observed in an increase of population or growth rates of biological organisms. Trends typically show high growth rates when limiting nutrients are more readily available and during warmer seasons due to light and temperature affects on primary productivity. Biomass and growth rates are constrained by environmental stoichiometry, elemental C:N:P ratios, because nutrient ratios affect the transfer of energy through interspecific interaction, or between different species (Elser et al. 2000). A study done by Elser et al. (2000) found that as C:nutrient ratios increased, meaning more carbon atoms versus nitrogen or phosphorous, there was a decline in “gross growth efficiency” (GGE). If there is a change in basal food sources, such as a shift from allochthonous to autochthonous, then GGE in herbivore trophic levels may change and cause trophic cascade through the food web, meaning multiple trophic levels respond to a disturbance.
 * Trophic interactions and cascade affects **



How can trophic relationships be measured? Wallace (1997) manipulated inputs of terrestrial leaf litter in a headwater stream and measured community response through consumer abundance, biomass, and diversity in habitats of bedrock covered by moss and mixed substrates. Results showed FFG of invertebrate communities to shift in specific habitats, thus, the change in energy sources impact invertebrate FFG differently and is important when identifying specific sites for restoration. This research shows the terrestrial input of allochthonous material influence ecosystem productivity and also shows that community response by FFG vary among habitat within the same stream system.

Riparian vegetation directly serves as a carbon energy source to streams as well as controls the availability of sunlight, or photosynthetically active radiation (PAR), for instream primary producers. PAR ranges from 400 to 700 nm and is the best wavelength for photosynthesis to occur. Forested streams usually are heterotrophic when allochthonous material is a significant part of basal food source. Conversely, streams with reduced riparian zones usually support autochthonous production of carbon and are typically autotrophic. Both basal food sources drive community structure differently. Furthermore, community structures vary along a stream gradient (Vannote et al. 1980). A method to determine the origin of carbon material for a stream using ecosystem processes is through P/R ratio. Heterotrophic/allochthonous conditions are P/R <1 and autotrophic/autochthonous conditions are >1 and indicate whether a reach is a sink or source of energy, respectively. To learn more on restoration of riparian zones specficially, visit Nassam and McCoy.
 * Riparian zones **

“Environmental homogeneity” is induced by dams that reduce natural flow regime characteristics on regional scales and was investigated by Poff et al. (2007) by comparing stream flow for pre-dam and post-dam rivers. They found significant changes in stream flow towards homogenization following dam establishment on third to seventh order streams. Implications are pointed towards the management of regulated rivers in a way to sustain ecological biodiversity dependent on natural flow regimes, and simultaneously, preventing non-native, cosmopolitan aquatic species from becoming established and altering food web dynamics.
 * Effects of disturbance **

Decline in fish population is occurring worldwide in freshwater and oceanic ecosystems and is a concern to resource managers. Identifying stressors on these populations becomes increasingly complex when species are anadromous, like the pacific salmon species. Due to these complexities it is important to assess resource availability for all life-stage niches. Further, alteration of physical habitat and basal resources from deforestation are found to impact biodiversity and abundance and should be addressed holistically (Fausch et al 2010). This example exemplifies the need for a holistic perspective of a watershed in a way that acknowledges linkages between oceanic, freshwater, and terrestrial ecosystems. Restoration projects are able to use endangered species as a foothold to gain momentum in addressing food web shifts and ecosystem degradation. Projects are usually designed to manipulate and enchance 'critical habitat' associated with an endangered species. For example, riparian corridors along segments of the MRG have been manipulted by lowering the channel and engineering backwater habitats to create prime spawning and refuge habitat for the silvery minnow during spring-summer flow pulses. In Northwest U.S. salmonids are of particular interest in regards to species restoration. For example, Wipfli et al. (2004) experimented with food resource supplementation for coho salmon at young-of-the-year lifestage by enriching stream reaches in Bridget Cove Creek, Alaska with adult salmon carcasses and analogs ("dried, processed hatchery salmon"). They found substantial increases in young coho production, body condition, and lipid content due to stream enrichment. In addition, they found a decline in production rates following a two week period, suggesting the timing of enrichment is important in attempt to optimize salmon survival and fitness. Salmon carcasses provide nutrients for juvenile salmon through direct forage and once carcasses are decomposed these elemental nutrients transfer through the food web. Carcass placement upstream in tributary spawning and rearing reaches is a growing restoration practice to improve salmon survival and fitness in the Northwest.
 * Restoration of food web dynamics **

It is suggested that natural resource management and restoration be implemented when ecosystem impairment occurs, and in the case of a listed endangered species, restoration is mandatory to attempt to restore populations. However, detailed strategies needed are limited and conflict with other stakeholders. To read further on restoration goals visit Patrick Swazo-Hinds.
 * Projects and goals to restore food webs or ecosystem structure and function **

Young et al. (2008) suggest additional in-stream parameters beyond standard physical and chemical assessment be measured to better assess the state of the ecosystem. Visit Terri Austin for greater depth on 'assessment techinques' but I will briefly example the relevance of stream health assessment here. Extensive research has been done on structural ecosystem components such as 'Index of Biotic Integrity' and 'Rapid Bioassessment' to provide a snapshot of biotic community composition. The benefit of including stream indicators that are representative of ecosystem processes like decomposition of organic matter or whole stream metabolism (primary production and community respiration) will expound our ability to assess the functionality of ecosystems (Young et al. 2008). In-stream decomposition of allochthonous material provides rates of organic matter processed by herbivores. This energy is ultimately transferred to higher trophic levels, whether by organisms in-stream or terrestrially. Similarly, stream metabolism provides rates of primary production and consumer consumption in relation to carbon. The ratio of production to respiration (P/R) is a proxy for determining if a stream is a source or sink for energy in the food base. Changes in the rates of organic matter processing allow us to predict how events or development impact the food base in stream systems. These rates are naturally dynamic and influenced by environmental variables. Yet, scientific understanding of ecosystem alterations induced by anthropogenic stressors is important to determine what factors or events lead to past, present and desired stream conditions. This is imperative in order to begin designing a recovered system.
 * Ecosystem health assessments by use of indicators **

Ecosystem connectivity between terrestrial and aquatic systems should be incorporated into restoration projects, such as floodplain connectivity addressed by Bramlett. Predictions should be made concerning ecosystem response to restoration projects during early planning stages to understand the potential impacts of manipulating a feature, whether it be physical, chemical, or biological, in one ecosystem (ie river) or another ecosystem (ie floodplain).
 * Efforts to restore food web dynamics **

Restoration goals, when defined, should heavily consider including food web dynamics into the project objectives and to assess the potential impact of the project on the existing food web. For example, a study on gravel bed reconstruction to enhance Chinook spawning sites showed there were adverse impacts on food-base sources for juvenile life stages in the Merced River, California (Monaghan and Milner 2008). This study demonstrates restoration projects of reconstructing stream channel substrate can have negative impacts on food web dynamics and can actually work against the project goal indirectly. Thus, the decline of food resources in abundance and biomass, ie shift in community assemblage, can result in slower growth of juvenile Chinook salmon and contrasts with restoration efforts to increase population. Manipulation of river habitat is a common approach to restore lotic systems and, in particular, threatened or endangered species. Indirect impacts of habitat modification on community composition should be acknowledged to ensure restoration projects are not acting working against the goal.

Anthropogenic disturbance have altered ecosystem structure and function and is apparent through the loss of biological diversity. Though documentation of food web response to anthropogenic disturbance is well established, it is limited concerning the response of food webs to restoration projects in lotic systems. Restoration projects typically address declines in a single species population and attempts to restore species of concern have focused primarily on enhancing physical habitat, water quantity, and/or water quality to increase biological populations. Little is known how restoration projects impact food webs, or if this concept is incorporated into such efforts.
 * Conclusions **


 * Literature Cited **

Benstead, J.P. and C.M. Pringle. 2004. Deforestation alters the resource base and biomass of endemic stream insects in eastern Madagascar. Freshwater Biology 49:490-501.

Elser, J.J., W.F. Fagan, R.F. Denno, D.R. Dobberfuhl, A. Folarin, A. Huberty, S. Interlandi,S.S. Kilham, E. McCauley, K.L. Schulz, E.H. Siemann, R.W. Sterner. 2000. Nutritional constraints in terrestrial and freshwater food webs. Nature 408:578-580.

Fausch, K.D., C.V. Baxter, M. Murakami. 2010. Multiple stressors in north temperate streams: lessons from linked forest-stream ecosystems in northern Japan. Freshwater Biology 55:120-134.

Finlay J.C., S. Khandwala, M.E. Power. 2002. Spatial scales of carbon flow in a river food web. Ecology 83:1845-1859.

Horne, A.J. and C.R. Goldman. 1994. Chapter 15: Food-chain dynamics in //Limnology// 2nd edition. McGraw-Hill, Inc. United States of America.

Hynes, H.B.N. 1975. The stream and its valley. Verhandleungen der Internationalen Vereinigung fur Theoretische und Angewandte Limnologie 19:1-15.

Monaghan, K.A. and A.M. Milner. 2008. Salmon carcase retention in recently formed stream habitat. Fundamental and Applied Limnology 170:281-289.

Poff, N.L., J.D. Allan, M.B. Bain, J.R. Karr, K.L. Prestegaard, B.D. Richter, R.E. Sparks, J.C. Stromberg. 1997. The natural flow regime. Bioscience 47:769-787.

Poff, N.L., J.D. Olden, D.M. Merritt, D.M. Pepin. 2007. Homogenization of regional river dynamics by dams and global biodiversity implications. Proceedings of the National Academy of Sciences 104:5732-5737.

Power, M.E. and W.E. Dietrich. 2002. Food webs in river networks. Ecological Research 17:451-471

Vander Zanden, M.J., S. Chandra, B.C. Allen, J.E. Reuter, C.R. Goldman. 2003. Historical food web structure and restoration of native aquatic communities in Lake Tahoe (California-Nevada) Basin. Ecosystems 6:274-288

Vannote, R.L., G.W. Minshall, K.W. Cummins, J.R. Sedell, C.E. Cushing. 1980. The river continuum concept. Candian Journal of Fisheries and Aquatic Sciences 37:130-137.

Wallace, J.B., S.L. Eggert, J.L. Meyer, J.R Webster. 1997. Multiple trophic levels of a forest stream linked to terrestrial litter inputs. American Association for the Advancement of Science 277:102-104.

Wipflie, M.S., J.P. Hudson, J.P. Caquette. 2004. Restoring productivity of salmon-based food webs: contrasting effects of salmon carcass and salmon carcass analog additions on stream-resident salmonids. Transactions of the American Fisheries Society 133:1440-1454.

Woodward, G. and A.G. Hildrew. 2002. Food web structure in riverine landscapes. Freshwater Biology 47:777-798.

<span style="font-family: Arial,sans-serif; font-size: 11pt;">Young, R.G., C.D. Matthaei, C.R. Townsend. 2008. Organic matter breakdown and ecosystem metabolism: functional indicators for assessing river ecosystem health. Journal of North American Benthological Society 27:605-625.