Lecture 16 - Aquatic-Terrestrial Linkages and Food Web Subsidies
Introduction to Aquatic-Terrestrial Linkages
Context and Connectivity: Streams are considered open ecosystems within a meta-ecosystem framework, meaning they are intrinsically connected to other environments rather than being isolated.
Ecotones and Boundaries: Significant focus is placed on the aquatic-terrestrial ecotone, exploring the exchanges and subsidies between freshwater and land-based food webs.
Marine Connections: Connectivity extends to the ocean through processes like diadromy. New Zealand freshwater fish, for example, move between the ocean and streams, connecting marine and freshwater food webs.
The Orangutukiya Stream Case Study: Following a flood event, eels in the Orangutukiya Stream showed a massive recovery in biomass despite incredibly low densities of aquatic macroinvertebrates. This discrepancy suggests that the eels are sustained by external subsidies, such as terrestrial prey or even anthropogenic sources (e.g., cat food), rather than local aquatic primary production.
Sources of Energy in Stream Food Webs
Autochthonous Production (The Green Channel):
Energy produced within the stream itself by primary producers.
Examples: Algae, diatoms, and macrophytes.
Diatoms: These are highlighted as a highly nutritious and rapidly turning over energy source that sustains a wide range of stream consumers.
Allochthonous Inputs (The Brown Channel):
Energy entering the stream from the terrestrial environment.
Example: Leaf litter falling into the water.
Microbial Enrichment: Leaf litter is colonized by bacteria, archaea, and aquatic hyphomycete fungi. These microbes enrich the detritus, making it more nutritious for consumers.
Metaphor: The speaker describes the microbes as "peanut butter on a cracker," where the leaf is the cracker and the nutritious microbes are the peanut butter consumed by shredding macroinvertebrates and detritivores.
The River Continuum Concept (RCC) and Land Use
Longitudinal Framework: The RCC describes how physical habitats and community structures change from headwaters to lower reaches in a predictable manner.
Headwaters: Dominated by allochthonous detrital inputs due to riparian shading. Functional feeding groups are dominated by shredders.
Mid-reaches: Streams are wider and shallower, allowing for more sunlight. Autochthonous production increases, leading to a dominance of grazers and scrapers (algal feeders).
Lower Reaches: Streams become deep and turbid (floodplain). Fine Particulate Organic Matter (FPOM) is transported from upstream. Functional feeding groups are dominated by collector-gatherers and filter feeders.
Metabolic Ratios: The ratio of primary production () to respiration () shifts along the network.
Land Use Variations:
Forested Streams: High detrital (allochthonous) inputs.
Open Streams (Pasture/Tussock): Higher sunlight and temperatures lead to increased autochthonous production (the green channel).
Otago Study (Ross Thompson and Colin Townsend): Quantified basal resources across four land uses. Algal production spiked in pasture and tussock sites, while detrital inputs (Coarse Particulate Organic Matter) were significantly higher in forested sites.
Meta-Ecosystem Theory and Landscape Ecology
Definition of Meta-Ecosystems: A system where different ecosystems are connected in space via the movement of subsidies (organic matter, nutrients, or organisms). The prefix "meta" implies a comprehensive view that transcends local dynamics.
Subsidy Definition: The flow of energy or nutrients from one ecosystem to another.
Desert Island Study (Gulf of California): Research by Gary Polis on islands off the Baja Peninsula demonstrated the power of marine subsidies in desert ecosystems.
Inputs: Kelp ("wrack") washing ashore and nutrient-rich guano from nesting seabirds.
Trophic Cascade: Marine detritus fuels kelp flies, which in turn sustain high densities of spiders. The effect is more pronounced on smaller islands due to a higher edge-to-area ratio.
The Salmon Run as a Meta-Ecosystem Subsidy
Anadromous Behavior: Salmon spawn in freshwater, grow to maturity in the marine environment, and return to streams to reproduce.
Nutrient Pulse: Salmon are semelparous (they die after spawning), resulting in a massive influx of marine-derived nutrients into the freshwater system.
Bottom-Up Effects: Nitrogen and phosphorus from carcasses enrich the stream. This leads to an increase in the Ash-Free Dry Mass () of algae, which then fuels higher densities of macroinvertebrates.
Feedback Loop: Juvenile salmon hatching in the stream benefit from the increased macroinvertebrate production fueled by the previous generation's carcasses.
Terrestrial Impact: Stable isotope analysis () shows that marine nitrogen spreads into terrestrial food webs.
Consumers: Shrews, voles, squirrels, mice, and bears (grizzlies and brown bears) scavenge carcasses or consume emerging aquatic insects.
Avian Insectivores: Studies show that in areas with high salmon biomass, bird territory sizes are smaller and bird density is higher because the environment is more productive.
New Zealand Context: Diadromous galaxiids (whitebait) historically provided a similar subsidy. In the past, runs were so large that whitebait were used as garden fertilizer.
Experimental Studies on Detrital and Prey Subsidies
The Bear Brook Study (Fisher and Likens, 1973): Established that in small forested headwaters, approximately of the energy budget comes from allochthonous inputs.
Litter Exclusion Experiment (Bruce Wallace):
Conducted in a forested headwater stream by installing nets to block leaf litter.
Results: Secondary production (macroinvertebrate growth) crashed across most trophic levels.
Top Predator Impact: The biomass of top predators, such as newts, declined in direct correlation with the reduction in prey production.
Exception: Moss patches acted as independent "compartments," sustaining miniature food webs regardless of leaf litter availability.
Wood Manipulation: Removing large woody debris and replacing it with PVC plastic surrogates reduced energy availability further. Removing the nets and restoring the wood eventually allowed the system to recover.
The Allen Paradox (Fred Allen, Horokiwi Stream):
Allen found a mismatch in the between the biomass of trout and the production of benthic (aquatic-dwelling) macroinvertebrates in the stream.
Resolution: The "paradox" was solved by recognizing that terrestrial prey falling into the stream provides the missing energy required to sustain fish populations.
The Japan Exclusion Experiment (Shigeru Nakano)
The Greenhouse Experiment: In the Horonui Stream (Hokkaido), Shigeru Nakano built greenhouse-like structures over stream reaches to exclude terrestrial prey (insects falling from the canopy).
Experimental Design: A factorial design manipulating both fish presence and terrestrial prey inputs.
Findings:
When terrestrial prey was excluded, fish (Dolly Varden) shifted their diet to consume more benthic aquatic invertebrates, such as Gammaridae (amphipods).
Trophic Cascade: This increased predation on aquatic invertebrates released primary producers (algae) from grazing pressure, resulting in an increase in algal biomass in the exclusion reaches.
Energy Efficiency: Feeding on drifting terrestrial prey is more energetically advantageous for fish than scouring the substrate for small aquatic prey.
Parasite-Mediated Subsidies and Reciprocal Linkages
Nematomorphic (Gordian) Worms: These parasites infect terrestrial insects (like crickets) and manipulate their behavior, causing them to jump into the water.
Ecological Role: This host manipulation facilitates the movement of terrestrial energy into the aquatic food web, making the insects available as prey for fish while the adult worm returns to the water to complete its life cycle.
Reciprocal Subsidy (Aquatic to Terrestrial): Insects like mayflies, stoneflies, and caddisflies emerge as adults and fly into the terrestrial environment.
Nakano's Findings: In Japanese forest systems, aquatic subsidies can account for up to of the energy budget for forest birds and roughly of the annual energy budget for fish (via terrestrial inputs).
Seasonality:
Spring: A massive pulse of aquatic insects emerges, aiding nesting birds.
Summer: The forest canopy is full, and terrestrial caterpillars/insects fall into the stream, aiding fish.
Lizard Study (California):
Closing off aquatic emergence caused lizards to shift their diet to terrestrial invertebrates.
Consequence: This dietary shift led to lower growth rates and potential fitness declines in lizards due to the loss of the high-energy aquatic subsidy.
Connectivity Footprints and Biomarkers
Dispersal Ability: The distance a stream's biological "signature" reaches into the forest is determined by the dispersal ability of emerging insects (e.g., large-bodied flyers penetrate further).
Abundance: High numbers of insects (e.g., midges/chironomids) can saturate the terrestrial environment even if the insects are poor fliers, often aided by wind.
Fatty Acid Biomarkers: Polyunsaturated Fatty Acids (PUFAs), especially those derived from diatoms, can be used as tracers. Finding these PUFAs in terrestrial spiders or birds indicates a strong nutritional link to the aquatic environment.
Pollution Impacts: In acid mine drainage sites or highly polluted streams, the loss of aquatic emergence leads to a corresponding decline in terrestrial predators (spiders/birds) nearby.
Questions & Discussion
The Legacy of Shigeru Nakano:
Tragedy in the Gulf of California: Shigeru Nakano and Gary Polis both passed away in a tragic boat accident involving a freak storm while visiting islands in the Gulf of California. Only a few survivors, including a postdoc who swam to shore, remained.
Contributions: Nakano is remembered as a "rockstar" of Japanese ecology who bridged the gap between Japanese-language research and international journals, mastering English to share his groundbreaking work on linkages.
Tomakomai Experimental Forest: The student mentions growing up near the site where Nakano performed much of his research in Hokkaido.
Administrative Notes: Discussion regarding the Hamilton and Tauranga streams data workshops, R script availability on Moodle, and flexibility for the literature review deadline for students not taking the final exam. Suggestions included moving the due date to approximately the or .