Functional Feeding Groups and the River Continuum Concept

Summary

In this engaging presentation Heather Geiss of the Farmington River Watershed Association introduces aquatic macroinvertebrates and their roles in stream ecosystems. She explains four functional feeding groups—shredders, collectors, grazers/scrapers, and predators—illustrating each with common insect examples and specialized adaptations. Geiss then links these roles to the River Continuum Concept, showing how community composition shifts from headwaters (more shredders and collectors) to larger downstream rivers (more grazers and collectors). The talk closes with an invitation to participate in FRWA’s volunteer macroinvertebrate sampling (September–October) and points viewers to additional resources and the organization’s website.

Takeaways

• 😀 Aquatic macroinvertebrates are small, visible creatures without backbones, and they live their entire lives in water, making them excellent indicators of water quality.
• 😀 Functional feeding groups categorize macroinvertebrates based on their feeding behaviors and structures: Shredders, Collectors, Grazers/Scrapers, and Predators.
• 😀 Shredders use specialized mouth parts and legs to break down organic matter, such as leaves, into smaller pieces, which is essential for downstream processing.
• 😀 Collectors gather smaller particles that have been processed by shredders, using gills or legs to capture or move the particles towards them.
• 😀 Grazers and Scrapers feed on algae and fine organic matter attached to rocks in streams, often having specialized structures to cling onto surfaces while feeding.
• 😀 Predators, such as Dragonflies and Dobsonflies, eat other insects or animal tissue, using specialized mouthparts for biting or piercing prey.
• 😀 The River Continuum Concept (RCC) explains how macroinvertebrate communities change as you move downstream, with different feeding groups dominating at different points in the river system.
• 😀 In headwater areas (upstream), shredders and collectors dominate due to abundant organic matter from trees and shrubs, while downstream areas see more grazers as sunlight promotes algae growth.
• 😀 In large rivers, collectors are more abundant as smaller organic particles from upstream or floating phytoplankton are the primary food sources, and shredders are less common.
• 😀 Predators are present throughout the river continuum, adapting to different niches in both small and large rivers.
• 😀 The Farmington River Watershed Association (FRWA) offers volunteer opportunities for macroinvertebrate sampling, with training available through webinars and field workshops.

Q & A

What are aquatic macroinvertebrates?

Aquatic macroinvertebrates are small organisms that live in the water, are large enough to be seen without a microscope, and lack a backbone. Many of them have a hard exoskeleton.

Why are aquatic macroinvertebrates important for water quality monitoring?

They are excellent indicators of water quality because they live their entire life in water, are highly sensitive to pollution, and are abundant and easy to catch. Their presence or absence can reveal much about the health of a river system.

What are the four functional feeding groups in aquatic macroinvertebrates?

The four functional feeding groups are Shredders, Collectors, Grazers/Scrapers, and Predators. These groups are based on the specialized feeding structures and behaviors of different macroinvertebrate species.

What do shredders do in aquatic ecosystems?

Shredders have specialized mouthparts and tarsal claws that allow them to tear apart organic matter such as leaves and other plant material in the water. This helps break down large pieces of organic matter into smaller ones.

Can you give examples of shredders in aquatic ecosystems?

Examples of shredders include Stoneflies, Caddisflies, Beetles, and True Flies. Species like the Roach-Like Stonefly and Plant Case Maker Caddisfly are known for their shredding behaviors.

What is the role of collectors in aquatic ecosystems?

Collectors gather smaller particles that have been broken down by shredders. They use specialized gills, legs, or even nets to capture fine particulate matter that flows downstream.

What are grazers and scrapers, and what do they feed on?

Grazers and scrapers feed on algae and fine organic matter found on the surfaces of rocks in streams. They have specialized features to cling to surfaces and mouthparts designed to scrape or chew up algae and debris.

What is the main food source for grazers and scrapers?

The primary food source for grazers and scrapers is algae and fine particulate matter growing on rocks and surfaces in the stream. As sunlight increases downstream, algae growth on rocks also increases.

What do predators do in aquatic ecosystems?

Predators in aquatic ecosystems hunt and consume other macroinvertebrates or smaller animals. They typically have specialized mouthparts for biting, chewing, or sucking prey.

How does the River Continuum Concept explain changes in macroinvertebrate communities downstream?

The River Continuum Concept suggests that as you move downstream in a river system, the composition of aquatic macroinvertebrate communities changes. In headwaters, shredders and collectors dominate, while in larger rivers, grazers and collectors become more prevalent. Predators are found throughout the river system.

Why are there fewer shredders in larger rivers according to the River Continuum Concept?

Larger rivers have wider and deeper channels, less tree cover, and reduced sunlight penetration to the riverbed. As a result, there is less organic material, such as leaves, for shredders to process. Most shredders are found in smaller streams with abundant organic matter.

How does sunlight affect the distribution of macroinvertebrates in a river?

Sunlight plays a role in algae growth on rocks. In larger, wider rivers, more sunlight reaches the water's surface, promoting algae growth, which in turn increases the number of grazers. In shaded, smaller streams, sunlight is limited, reducing algae growth and making shredders and collectors more common.

What volunteer opportunities does the Farmington River Watershed Association offer?

The FRWA offers volunteer opportunities for macroinvertebrate sampling, including a one-day field workshop held in September and October. An instructional webinar is also available, as part of the Connecticut DEEP's Riffle Bioassessment by volunteers program.