8.4 Functional feeding groups

Functional feeding groups categorize aquatic macroinvertebrates based on their feeding strategies. These groups include shredders, collectors, scrapers, and predators, each playing unique roles in processing organic matter and transferring nutrients within aquatic ecosystems.

Understanding functional feeding groups provides insights into stream ecosystem health and trophic structure. Their composition and interactions reflect resource availability, habitat characteristics, and environmental conditions, making them valuable indicators for biomonitoring and assessing anthropogenic impacts on aquatic systems.

Types of functional feeding groups

• Functional feeding groups categorize aquatic macroinvertebrates based on their feeding strategies and the food resources they utilize
• Understanding the different types of functional feeding groups provides insights into the trophic structure and ecological processes within aquatic ecosystems
• The main functional feeding groups include shredders, collectors, scrapers, and predators, each playing distinct roles in the processing and transfer of organic matter and nutrients

Shredders

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• Shredders feed on coarse particulate organic matter (CPOM) such as leaves, wood, and other plant debris
• They possess specialized mouthparts adapted for cutting and chewing tough plant material
• Shredders play a crucial role in the initial breakdown of allochthonous inputs (leaf litter) in stream ecosystems
• Examples of shredders include caddisflies (Trichoptera) and crane flies (Tipulidae)

Collectors

• Collectors feed on fine particulate organic matter (FPOM) suspended in the water column or deposited on the substrate
• They can be further divided into filtering collectors and gathering collectors based on their feeding mechanisms
• Filtering collectors use specialized structures (e.g., nets, fans) to capture FPOM from the water column
• Gathering collectors actively forage and collect FPOM from the stream bed or other surfaces

Scrapers

• Scrapers, also known as grazers, feed on periphyton (attached algae and associated microorganisms) growing on submerged surfaces
• They possess specialized mouthparts adapted for scraping and grazing periphyton from rocks, wood, and other substrates
• Scrapers play a significant role in controlling periphyton growth and nutrient cycling in stream ecosystems
• Examples of scrapers include certain mayflies (Ephemeroptera) and snails (Gastropoda)

Predators

• Predators feed on other aquatic organisms, including macroinvertebrates and small fish
• They employ various prey capture strategies, such as active pursuit, ambush, or sit-and-wait tactics
• Predators possess specialized mouthparts and adaptations for capturing and consuming prey
• Examples of predators include dragonflies (Odonata), stoneflies (Plecoptera), and some caddisflies (Trichoptera)

Shredder characteristics

• Shredders possess specialized mouthparts, such as robust mandibles and maxillae, adapted for cutting and chewing tough plant material
• Their feeding activities contribute significantly to the breakdown of leaf litter and other coarse particulate organic matter in stream ecosystems
• Shredders convert CPOM into smaller particles (FPOM) through their feeding activities, making it more accessible to other functional feeding groups

Mouthpart adaptations

• Shredders have strong, heavily sclerotized mandibles with sharp cutting edges suitable for shredding and chewing plant material
• They often possess enlarged maxillae with brush-like structures (e.g., galeal brushes) that aid in handling and processing leaf litter
• Some shredders, such as certain caddisflies, have additional adaptations like silk-spinning glands used to construct protective cases or nets

Role in leaf litter breakdown

• Shredders initiate the breakdown of leaf litter by fragmenting it into smaller particles
• Their feeding activities enhance microbial colonization and decomposition of leaf litter by increasing surface area and exposing more tissue to microbial attack
• Shredders accelerate the release of nutrients from leaf litter, making them available to other organisms in the stream ecosystem

Importance in stream ecosystems

• Shredders play a vital role in the processing of allochthonous inputs and the transfer of energy from terrestrial to aquatic ecosystems
• They serve as a link between coarse particulate organic matter and other functional feeding groups by converting CPOM into FPOM
• Shredders contribute to nutrient cycling and the overall productivity of stream ecosystems by facilitating the breakdown and decomposition of leaf litter

Collector characteristics

• Collectors feed on fine particulate organic matter (FPOM) suspended in the water column or deposited on the substrate
• They utilize different feeding mechanisms to capture and consume FPOM, depending on their specific adaptations and the size of the particles

Filtering collectors vs gathering collectors

• Filtering collectors use specialized structures, such as nets or fans, to capture FPOM from the water column as it passes by
• They often have adaptations like setae or bristles on their appendages that aid in trapping and retaining particles
• Examples of filtering collectors include black flies (Simuliidae) and some caddisflies (Hydropsychidae)
• Gathering collectors actively forage and collect FPOM from the stream bed or other surfaces
• They use their mouthparts to scrape or gather deposited particles and may have adaptations like specialized setae or brushes for efficient particle capture
• Examples of gathering collectors include certain mayflies (Ephemeroptera) and midges (Chironomidae)

Mouthpart adaptations

• Collectors possess mouthparts adapted for capturing and processing fine particulate organic matter
• Filtering collectors often have fan-like or net-like structures formed by modified appendages or mouthparts to strain particles from the water
• Gathering collectors have mouthparts equipped with brushes, combs, or rakes to efficiently gather and collect deposited particles
• Some collectors have specialized setae or bristles on their mouthparts that aid in trapping and retaining small particles

Role in fine particulate organic matter processing

• Collectors play a crucial role in the processing and utilization of FPOM in stream ecosystems
• They consume FPOM that has been generated by the breakdown of CPOM by shredders or from other sources such as algal production or terrestrial inputs
• Collectors convert FPOM into biomass and make it available to higher trophic levels through their feeding activities
• Their feeding helps in the transfer of energy and nutrients from FPOM to other organisms in the stream food web

Scraper characteristics

• Scrapers, also known as grazers, feed on periphyton (attached algae and associated microorganisms) growing on submerged surfaces
• They possess specialized mouthparts and adaptations for scraping and grazing periphyton efficiently

Mouthpart adaptations

• Scrapers have mouthparts adapted for removing and consuming periphyton from surfaces
• They often have radula (toothed tongues) or specialized mandibles with cutting edges suitable for scraping algae
• Some scrapers, such as certain mayflies, have flattened bodies and modified labrum (upper lip) that aid in close contact with the substrate during grazing
• Snails, a common scraper group, have a rasping radula that allows them to scrape periphyton effectively

Role in periphyton grazing

• Scrapers play a significant role in controlling the growth and distribution of periphyton in stream ecosystems
• Their grazing activities help in maintaining the balance between periphyton production and consumption
• Scrapers can influence the community structure and composition of periphyton by selectively grazing on certain algal species
• Their feeding can stimulate periphyton growth and productivity by removing senescent cells and promoting nutrient cycling

Importance in nutrient cycling

• Scrapers contribute to nutrient cycling in stream ecosystems through their grazing activities
• They consume periphyton, which is rich in nutrients such as nitrogen and phosphorus
• Through their feeding, scrapers convert the nutrients locked in periphyton into their own biomass and make them available to higher trophic levels
• Scrapers' excretion and fecal matter also release nutrients back into the water, supporting primary production and other ecosystem processes

Predator characteristics

• Predators feed on other aquatic organisms, including macroinvertebrates and small fish
• They employ various prey capture strategies and possess specialized adaptations for capturing and consuming prey

Prey capture strategies

• Predators use different strategies to capture their prey depending on their hunting abilities and the characteristics of their prey
• Some predators, such as dragonfly nymphs (Odonata), are active pursuers that actively chase and capture their prey
• Others, like ambush predators (e.g., some stoneflies), wait for prey to come within striking distance before attacking
• Sit-and-wait predators, such as certain caddisflies (Rhyacophilidae), construct nets or retreats to trap passing prey

Mouthpart adaptations

• Predators possess mouthparts adapted for capturing, killing, and consuming prey
• Many predators have strong, sharp mandibles and maxillae suitable for grasping and piercing prey
• Some predators, like dragonfly nymphs, have extendable labium (lower lip) with hooks or spines that aid in prey capture
• Predatory caddisflies often have long, curved mandibles that allow them to grasp and hold onto their prey effectively

Role in top-down control

• Predators exert top-down control on the populations of their prey species in stream ecosystems
• They help in regulating the abundance and distribution of other macroinvertebrates and small fish
• Predators can influence the structure and composition of macroinvertebrate communities through selective predation on certain species
• The presence of predators can also induce behavioral changes in prey species, such as altered foraging patterns or increased use of refugia

Factors influencing functional feeding group composition

• The composition and relative abundance of functional feeding groups in stream ecosystems are influenced by various environmental factors
• Understanding these factors is crucial for predicting and interpreting changes in the structure and function of aquatic communities

Resource availability

• The availability and quality of food resources play a significant role in shaping functional feeding group composition
• Shredders are more abundant in streams with high inputs of coarse particulate organic matter (CPOM) from riparian vegetation
• Collectors tend to dominate in streams with high levels of fine particulate organic matter (FPOM) from upstream processing or autochthonous production
• Scrapers are more prevalent in streams with abundant periphyton growth, often associated with high light availability and nutrient concentrations

Physical habitat characteristics

• Physical habitat characteristics, such as substrate type, flow velocity, and stream morphology, influence the distribution and abundance of functional feeding groups
• Streams with diverse substrate types (e.g., cobbles, boulders, leaf packs) provide a range of microhabitats for different functional feeding groups
• Flow velocity affects the availability and transport of food resources, with higher velocities favoring filtering collectors and lower velocities favoring gathering collectors and scrapers
• Stream morphology, including pool-riffle sequences and channel complexity, creates heterogeneous habitats that support diverse functional feeding groups

Seasonal variations

• Seasonal changes in environmental conditions and resource availability can lead to shifts in functional feeding group composition
• Leaf litter inputs from riparian vegetation often peak in autumn, supporting higher abundances of shredders during this period
• Algal growth and periphyton biomass may increase during spring and summer due to higher light availability and warmer temperatures, favoring scrapers
• Seasonal variations in flow regimes, such as high flows during snowmelt or rainfall events, can influence the distribution and abundance of functional feeding groups

Functional feeding groups as indicators

• Functional feeding groups can serve as valuable indicators of ecosystem health and anthropogenic disturbance in stream ecosystems
• The relative abundance and composition of functional feeding groups reflect the availability and quality of food resources, as well as the prevailing environmental conditions

Indicators of ecosystem health

• A diverse and balanced representation of functional feeding groups is often indicative of a healthy and functional stream ecosystem
• The presence of all major functional feeding groups suggests a well-developed trophic structure and efficient nutrient cycling
• Shifts in the relative abundance of functional feeding groups can indicate changes in resource availability or environmental conditions

Indicators of anthropogenic disturbance

• Anthropogenic disturbances, such as pollution, habitat modification, or changes in land use, can alter the composition of functional feeding groups
• Organic pollution, for example, can lead to an increase in the abundance of collectors and a decrease in the abundance of shredders and scrapers
• Sedimentation from erosion or land disturbance can negatively impact scrapers by smothering periphyton and altering substrate characteristics
• Changes in riparian vegetation due to deforestation or land use practices can affect the availability of CPOM and the abundance of shredders

Applications in biomonitoring

• Functional feeding groups are commonly used in biomonitoring programs to assess the ecological integrity of stream ecosystems
• The relative proportions of functional feeding groups can be compared to reference conditions or historical data to detect changes over time
• Biomonitoring indices, such as the ratio of shredders to collectors or the percentage of sensitive functional feeding groups, provide insights into ecosystem health
• Incorporating functional feeding group analysis into biomonitoring protocols enhances the understanding of ecosystem functioning and the impacts of anthropogenic stressors

Functional feeding group interactions

• Functional feeding groups do not exist in isolation but interact with each other through various ecological processes
• These interactions, including competition, facilitation, and trophic cascades, shape the structure and dynamics of stream communities

Competition within and between groups

• Competition can occur within functional feeding groups when resources are limited, leading to niche partitioning and resource specialization
• For example, different species of scrapers may specialize in grazing on different types of periphyton or occupy distinct microhabitats to minimize competition
• Competition can also occur between functional feeding groups when they utilize similar resources or occupy overlapping niches
• Collectors and scrapers may compete for space on substrate surfaces, while predators may compete for prey resources

Facilitation and mutualism

• Facilitative interactions occur when the activities of one functional feeding group benefit another group
• Shredders, by breaking down leaf litter, facilitate the availability of FPOM for collectors downstream
• Scrapers, through their grazing activities, can stimulate periphyton growth and productivity, benefiting collectors and other groups that feed on algae
• Mutualistic relationships, such as the association between certain collectors (e.g., chironomids) and periphyton, can enhance resource availability and ecosystem functioning

Trophic cascades

• Trophic cascades occur when changes in the abundance or activity of predators indirectly affect the lower trophic levels
• Predators, by controlling the populations of their prey (e.g., scrapers or collectors), can indirectly influence the abundance and composition of periphyton or FPOM
• The removal of predators can lead to an increase in the abundance of their prey, which in turn can alter the dynamics of primary producers and detrital resources
• Trophic cascades highlight the complex interactions and feedbacks between functional feeding groups in stream ecosystems

Functional feeding groups in food webs

• Functional feeding groups play crucial roles in the structure and dynamics of stream food webs
• They contribute to energy flow, nutrient cycling, and the linkages between aquatic and terrestrial ecosystems

Contributions to energy flow

• Functional feeding groups facilitate the transfer of energy from primary producers and detrital resources to higher trophic levels
• Shredders and collectors process and convert organic matter into biomass, making it available to predators and other consumers
• Scrapers transfer energy from periphyton to higher trophic levels through their grazing activities
• Predators, as top consumers, regulate the populations of other functional feeding groups and influence energy flow through the food web

Linkages to terrestrial ecosystems

• Functional feeding groups play a vital role in connecting aquatic and terrestrial ecosystems
• Shredders rely on allochthonous inputs, such as leaf litter from riparian vegetation, establishing a direct link between terrestrial and aquatic food webs
• Emerging aquatic insects, including representatives from various functional feeding groups, provide a significant food source for terrestrial predators like birds and bats
• The exchange of nutrients and organic matter between aquatic and terrestrial ecosystems is mediated by the activities of functional feeding groups

Role in nutrient spiraling

• Functional feeding groups contribute to nutrient spiraling, the downstream transport and recycling of nutrients in stream ecosystems
• Shredders and collectors process organic matter, releasing nutrients that can be taken up by primary producers or transported downstream
• Scrapers, through their grazing activities, promote nutrient cycling by consuming periphyton and excreting nutrients back into the water
• Predators indirectly influence nutrient dynamics by regulating the populations of other functional feeding groups and altering their feeding activities
• The interactions and feedbacks among functional feeding groups drive nutrient spiraling and support the productivity and functioning of stream ecosystems