Chapter Eight: Trophic Relationships
Chapter Eight: Trophic Relationships
Overview of Fluvial Food Webs
Fluvial food webs comprise a network of consumers and resources supported by a diverse mix of energy supplies originating within the stream and from surrounding terrestrial ecosystems. Key resources include:
Living resources: algae and higher plants
Nonliving resources: particulate organic matter (POM) and dissolved organic matter (DOM)
Microorganisms: play a crucial role in mediating organic matter availability and serve as a resource for both small and large consumers.
Energy subsidies: These include terrestrial arthropods and the eggs and carcasses of migrating fish, which contribute energy to many stream-dwellers.
Additionally, energy provided within a stream may not be entirely consumed there; downstream export, insect emergence, and fish movements can supply energy to distant ecosystems.
Complexity of Trophic Organization
Trophic organization in river ecosystems can be complex and indistinct:
Many consumers are polyphagous (non-specific) rather than monophagous (specialized), exhibiting extensive dietary overlap.
The gut contents of invertebrates are often difficult to distinguish, leading to the use of the broad term herbivore-detritivore.
In temperate waters, most fishes typically consume invertebrates, making classification based solely on diet less useful.
Distinction among feeding roles based on the method of food acquisition is emphasized through the guild concept.
Fish that capture invertebrate prey from the bottom form a different guild compared to those that do so from the water column.
This offers better resolution of feeding roles in both invertebrate and vertebrate consumers in streams.
Functional Feeding Groups (FFGs)
Functional Feeding Groups were defined by Cummins in 1973 to classify consumers based on what they consume and how they obtain it.
Categories include:
Shredders: feed on coarse particulate organic matter (CPOM)
Collectors: target fine particulate organic matter (FPOM) either from the water column or streambed
Grazers: consume periphyton
Predators: feed on animal prey
Limitations exist, as many macroinvertebrates function as collectors in early stages, and prospective feeding roles may differ by species. Consequently, FFG designations adhere primarily to late instars.
Microbial Food Webs
Importance of Microorganisms
The study of trophic relationships has traditionally focused on larger consumers, but a growing appreciation of the role of microorganisms has emerged:
Biofilms are crucial energy complexes enabling the interplay between autotrophic (algae) and heterotrophic (microorganisms) pathways.
Microbial food webs involve bacteria and fungi that utilize nonliving organic matter for carbon (C) sources, fueling microbial production and remineralizing it as CO2.
Microbial Loop: The interactions between microorganisms and larger consumers can often lead to energy being dissipated primarily within microbial food webs, raising questions about the efficiency and significance of nutrient recycling. Current evidence suggests that microbial production serves both as nutrient recycling and energy pathways to higher trophic levels.
Meiofauna
The meiofauna (<0.5 mm) includes protozoans and metazoans that significantly contribute to the diversity and abundance of stream ecosystems. These organisms are often overlooked yet perform vital roles in energy transfer within the food web.
Predation and the role of meiofauna in consuming bacteria have seen limited study, despite their potentially significant effects on organic matter cycling and energy transfer.
Invertebrate Feeding Roles
Functional Feeding Group Classification
Invertebrates in fluvial ecosystems are categorized according to their food sources and feeding mechanisms, as reflected in Table 8.1:
Shredder:
Food Resource: Nonwoody CPOM (e.g., leaves) and associated microbiota
Feeding Mechanism: Chewing and mining
Example Taxa: Various Trichoptera, Plecoptera, and certain Crustaceans and Diptera.
Filterer-Collector/Suspension Feeder:
Food Resource: FPOM from water column
Feeding Mechanism: Collecting particles via filtering apparatus
Example Taxa: Trichoptera, Simuliidae, and Ephemeroptera.
Collector-Gatherer/Deposit Feeder:
Food Resource: FPOM and biofilm
Feeding Mechanism: Collecting surface deposits and browsing
Example Taxa: Ephemeroptera, Chironomidae, and Ceratopogonidae.
Grazer:
Food Resource: Periphyton, especially diatoms
Feeding Mechanism: Scraping and rasping adaptations
Example Taxa: Several families of Ephemeroptera and Trichoptera.
Predator:
Food Resource: Animal prey
Feeding Mechanism: Piercing and biting
Example Taxa: Odonata, Megaloptera, and some Plecoptera, Trichoptera, and Diptera.
Interactions and Resource Availability
The relative distribution and abundance of these resources in streams can change predictably from headwaters to the river mouth, impacting which food webs dominate.
Trophic interactions among these functional feeding groups provide insights into ecosystem health and help predict responses to environmental changes.
CPOM and Shredders:
Shredding of leaves enriched by microorganisms enhances the nutritional quality for detritivores.
Secondary production relies heavily on microbial degradation of leaf litter to facilitate energy transfer within the ecosystem. So, not only the consumers themselves are essential to the flow of energy but the microbial communities on which they depend are equally relevant in assessing ecosystem productivity.
Vertebrates in Lotic Food Webs
Overview of Fishes
Fish represent the primary vertebrate component of most riverine food webs:
Attempts to categorize stream fishes have led to identifying multiple trophic guilds as indicated in Table 8.3, a summary of feeding categories based on Horwitz (1978) and Welcomme (1979).
There is infrequent herbivory among North American fishes, with most relying on invertebrates or small fish. Examples include:
Piscivores: Consuming primarily fish and large invertebrates
Benthic Invertebrate Feeders: Targeting immature insects from benthic zones
Surface and Water Column Feeders: Consuming prey from surface sources
Omnivores and Detritivores: Ingesting a variety of material including plant matter and detritus.
Other Vertebrates
In addition to fishes, amphibians, reptiles, birds, and mammals play roles in trophic dynamics:
Salamanders are often top predators in headwater streams, acting as significant consumers of invertebrates and small fish.
Reptiles and Birds: Some are specialized predators of fish or invertebrates, such as turtles and diving ducks.
Mammals: Various species, including otters and raccoons, commonly forage on aquatic resources.
Secondary Production
Definition and Measurement
Secondary production refers to the biomass produced by microbial and animal growth and reproduction in specific time periods within aquatic ecosystems. High turnover rates (P/B ratios) often correlate with rapid growth and life cycles. Final estimates can be based on standard mass measurements (e.g. g m⁻² year⁻¹).
Influencing Factors
Heightened secondary production is influenced by temperature, food availability, and various habitat conditions. For example:
Secondary production estimates show substantial variation across ecosystems and various taxa, often yielding low values reflected in inadequate resource availability.
Studies indicate that primary production levels influence consumer biomass, and thus consecutive trophic turnovers, meaning energy must be effectively monitored and modeled to understand ecosystem dynamics better.
Summary of Trophic Relationships
Trophic interactions reflect a product of basal resources in rivers, alongside adaptations that maximize feeding efficiency across groups. Functional classifications, such as FFGs and guilds, enrich our understanding of ecological interactions and energy flow through aquatic ecosystems, from primary production through to the apex predators in the food web. Gains in microbial ecology research continue to reveal insights into energy transfers, emphasizing the complexity and interconnectedness within fluvial food webs.