UPSC Prelims 2021
Environment & Ecology Previous Year Questions (PYQs)

Detailed Explanation:

Answer: Option 4 — 1, 2 and 3

All three groups—cnidarians, fungi, and protozoa—have species capable of establishing symbiotic relationships with other organisms. This demonstrates the widespread occurrence of symbiosis across different taxonomic groups in nature.

✅ Statement 1 – Correct: Cnidarians like corals form symbiotic relationships with zooxanthellae (photosynthetic algae), which is essential for coral reef ecosystems.
✅ Statement 2 – Correct: Fungi form diverse symbiotic associations including mycorrhizae with plant roots and lichens with algae or cyanobacteria.
✅ Statement 3 – Correct: Protozoa establish symbiotic relationships, such as those living in termite guts that help digest cellulose.

📝 Short Notes: Symbiotic Relationships in Different Organisms

• Mutualism: Both organisms benefit from the relationship
• Symbiosis: Close and long-term biological interaction between different species
• Zooxanthellae: Single-celled dinoflagellates crucial for coral bleaching studies
• Ecological importance: Symbiotic relationships are fundamental to ecosystem functioning and biodiversity

Detailed Explanation:

Answer: Option 3 — 2 and 3

Lichens and mosses are uniquely adapted to survive on bare surfaces without soil. Lichens, being symbiotic associations of fungi and algae/cyanobacteria, can colonize rocks and tree bark by absorbing nutrients directly from air and rain. Mosses, though non-vascular plants, possess specialized structures that enable them to absorb water and nutrients from their immediate environment without requiring soil.

Analysis of Organisms:
1. Fern – Cannot survive without soil: Ferns are vascular plants requiring soil to anchor their roots and absorb water through their well-developed root systems.

2. Lichen – ✅ Can survive without soil: Lichens are symbiotic organisms (fungus + algae/cyanobacteria) that obtain nutrients directly from air and precipitation, thriving on rocks, bark, and other bare surfaces.

3. Moss – ✅ Can survive without soil: Mosses are bryophytes (non-vascular plants) with rhizoids that can attach to bare surfaces and absorb water and minerals directly from the environment.

4. Mushroom – Cannot survive without soil: Mushrooms are fungal fruiting bodies that require organic substrates (decaying matter, soil rich in nutrients) for the mycelium to grow and produce fruiting structures.

📝 Short Notes: Organisms Surviving Without Soil

• Lichens are pioneer species in ecological succession, colonizing bare rocks and weathering them into soil over time.
• Lichens as bioindicators: Highly sensitive to air pollutants like SO₂, NOₓ, and heavy metals; absence indicates poor air quality.
• Mosses reproduce via spores and require moisture for fertilization but can survive desiccation in dry conditions.
• Bryophytes (mosses, liverworts, hornworts) lack vascular tissues (xylem and phloem) but can thrive in moist, shaded environments on various substrates.
• Ferns are more advanced than mosses, having vascular tissues, but still require moist conditions and soil for optimal growth.

Detailed Explanation:

Answer: Option 2 — 3, 4 and 5

This question tests knowledge about Moringa (drumstick) and Tamarind trees, their characteristics, economic uses, and potential applications. Out of the five statements, only statements 3, 4, and 5 are correct regarding tamarind collection as minor forest produce, India's exports, and biofuel potential of both plants' seeds.

❌ Statement 1 – Incorrect: Moringa oleifera is a deciduous tree (not evergreen) and belongs to family Moringaceae (not a legume; it lacks nitrogen-fixing root nodules).

❌ Statement 2 – Incorrect: Tamarind (Tamarindus indica) is native to tropical Africa, not endemic to South Asia, though it is widely naturalized in India.

✅ Statement 3 – Correct: Tamarind is collected as Minor Forest Produce (MFP) in India and provides livelihood to tribal and forest-dwelling communities.

✅ Statement 4 – Correct: India is a major exporter of both tamarind and moringa seeds (valued for oil content and medicinal properties).

✅ Statement 5 – Correct: Moringa seeds contain 30-40% oil suitable for biodiesel production, and tamarind seed oil is also being researched for biofuel applications.

📝 Short Notes: Moringa and Tamarind

Detailed Explanation:

Answer: Option 4 — 1, 2 and 3

All three statements about furnace oil are correct. Furnace oil is a heavy residual fuel produced during petroleum refining, widely used in industries for power generation and heating, and its combustion releases significant sulfur emissions causing environmental pollution.

✅ Statement 1 – Correct: Furnace oil is a dark, viscous residual fuel product obtained from oil refineries by blending heavier fractions of crude oil distillation.
✅ Statement 2 – Correct: Industries such as cement, steel, textiles, and power plants use furnace oil in boilers and furnaces to generate heat and power.
✅ Statement 3 – Correct: Furnace oil contains high sulfur content (2-4%), and its combustion releases sulfur dioxide (SO₂) and other pollutants, contributing to air pollution and acid rain.

📝 Short Notes: Furnace Oil

• Definition: A dark, viscous residual fuel oil derived from petroleum refining, also known as heavy fuel oil or residual fuel oil.
• Production: Obtained by blending the heavier components left after distillation of crude oil; contains high carbon and sulfur content.
• Industrial Uses: Used in boilers, furnaces, and kilns in cement, steel, textile, chemical, and power generation industries.
• Sulfur Content: Contains 2-4% sulfur, significantly higher than diesel or petrol; releases SO₂ and particulate matter upon combustion.
• Environmental Impact: Major contributor to air pollution, acid rain, and respiratory health issues; many countries are phasing it out in favor of cleaner fuels.
• Alternatives: Natural gas, low-sulfur heavy oil, biomass, and renewable energy sources are being adopted to replace furnace oil in industries.

Detailed Explanation:

Answer: Option 1 — Carbon captured by oceans and coastal ecosystems.

Blue Carbon refers to the carbon captured and stored by oceanic and coastal ecosystems such as mangroves, seagrasses, and salt marshes. Although these vegetated coastal habitats cover less than 0.5% of the seabed, they are responsible for storing more than 50% (and potentially up to 70%) of all carbon stored in ocean sediments, making them highly efficient carbon sinks. This is distinct from 'green carbon' which is captured by terrestrial forests and agricultural soils.

📝 Short Notes: Blue Carbon Ecosystems

• Blue Carbon: Carbon captured and stored by coastal and marine ecosystems (mangroves, seagrasses, salt marshes, and tidal wetlands).
• High Efficiency: These ecosystems cover less than 0.5% of the seabed but store 50-70% of all carbon in ocean sediments.
• Sequestration Rate: Coastal ecosystems can sequester carbon at rates up to 40 times faster than terrestrial forests.
• Long-term Storage: Carbon can remain stored in coastal sediments for thousands of years if undisturbed.
• Co-benefits: Blue carbon ecosystems also provide coastal protection, biodiversity habitat, fisheries support, and water filtration.
• Threats: Coastal development, aquaculture expansion, pollution, and climate change threaten these ecosystems, releasing stored carbon back into the atmosphere.
• Conservation Importance: Protecting and restoring blue carbon ecosystems is crucial for climate mitigation and adaptation strategies.

Detailed Explanation:

Answer: Option 2 — 1, 2 and 4 Only

Magnetite nanoparticles are iron oxide particles released from high-temperature combustion and mechanical friction processes. They are generated from motor vehicle brakes (friction), engines (combustion), and power plants (fossil fuel burning), but not from microwave stoves or telephone lines which do not involve such processes.

✅ Statement 1 – Correct: Brakes of motor vehicles generate magnetite through friction and mechanical wear between brake pads and discs, producing iron-rich particulate matter that oxidizes to magnetite nanoparticles.

✅ Statement 2 – Correct: Engines of motor vehicles produce magnetite as a byproduct of high-temperature combustion of fuel and wear of engine components, releasing these particles as environmental pollutants.

❌ Statement 3 – Incorrect: Microwave stoves use electromagnetic radiation to heat food without combustion or mechanical abrasion, and therefore do not release magnetite particles.

✅ Statement 4 – Correct: Power plants, particularly coal-fired ones, emit magnetite through high-temperature combustion of fossil fuels, producing fly ash containing magnetic iron oxides.

❌ Statement 5 – Incorrect: Telephone lines carry electrical or optical signals and generate electromagnetic fields but do not undergo physical processes that emit magnetite particles.

📝 Short Notes: Magnetite Pollution Sources

• Magnetite (Fe₃O₄): Iron oxide nanoparticles suspected to cause neurodegenerative diseases like Alzheimer's when inhaled, as they can cross the blood-brain barrier.
• Vehicle Sources: Brake wear (friction-generated particles) and engine combustion are major urban sources of airborne magnetite.
• Industrial Sources: Coal-fired power plants and other fossil fuel combustion facilities release magnetite in fly ash and exhaust gases.
• Health Concern: Magnetite nanoparticles found in human brain tissue samples, particularly in urban populations exposed to traffic pollution.
• Non-Sources: Household appliances like microwaves and infrastructure like telephone lines do not generate magnetite as they lack combustion or mechanical friction processes.

Detailed Explanation:

Answer: Option 4 — Statement 1 is not correct but Statement 2 is correct

The question tests knowledge about the Tree City of the World recognition program and the specific case of Hyderabad. Statement 1 incorrectly attributes the recognition to UNCDF (United Nations Capital Development Fund) and Arbor Day Foundation, while in reality it is a joint program by the Food and Agriculture Organization (FAO) and the Arbor Day Foundation. Statement 2 correctly identifies Hyderabad's commitment to urban forestry as the basis for its recognition.

❌ Statement 1 – Incorrect: The Tree City of the World recognition is a joint initiative of the UN Food and Agriculture Organization (FAO) and the Arbor Day Foundation, not UNCDF and Arbor Day Foundation.
✅ Statement 2 – Correct: Hyderabad was indeed recognized as a 2020 Tree City of the World for its commitment to growing and maintaining urban forests.

📝 Short Notes: Tree City of the World Programme

• Organizing Bodies: Joint initiative by the Food and Agriculture Organization (FAO) and the Arbor Day Foundation
• Purpose: Recognition program to honor cities and towns committed to ensuring urban forests and trees are properly maintained, sustainably managed, and duly celebrated
• Recognition Criteria: Cities must demonstrate (1) Establish Responsibility, (2) Set the Rules, (3) Know What You Have, (4) Allocate Resources, and (5) Celebrate Achievements
• Indian Cities Recognized (2020): Hyderabad was among the first Indian cities to receive this recognition
• Significance: Promotes urban forestry as an essential component of healthy, sustainable cities and encourages tree planting and maintenance in urban areas

Detailed Explanation:

Answer: Option 2 — 1, 2 and 4

This question tests the understanding of differences between permaculture farming and conventional chemical farming. Permaculture emphasizes biodiversity, soil health, and sustainable practices like mulching, while conventional farming relies on monocultures and chemical inputs that can degrade soil quality.

✅ Statement 1 – Correct: Permaculture farming discourages monoculture and promotes polyculture systems with diverse crops, while conventional chemical farming predominantly uses monoculture for efficiency and ease of mechanization.

✅ Statement 2 – Correct: Conventional chemical farming causes soil salinity due to excessive use of synthetic fertilizers and poor irrigation practices, whereas permaculture maintains soil health through organic matter and prevents salinization.

❌ Statement 3 – Incorrect: Permaculture farming can be successfully practiced in semi-arid regions using water harvesting, mulching, and drought-resistant crops; it is not limited by aridity as the statement suggests.

✅ Statement 4 – Correct: Mulching is a core practice in permaculture to conserve moisture, suppress weeds, and improve soil fertility, but it is not essential in conventional chemical farming which relies on herbicides and irrigation.

📝 Short Notes: Permaculture vs Conventional Farming

Detailed Explanation:

Answer: Option 1 — assessing the carbon footprint of building operations around the world.

The Common Carbon Metric (CCM) is a protocol developed by UNEP's Sustainable Buildings & Climate Initiative (UNEP-SBCI) specifically for measuring energy use and reporting greenhouse gas (GHG) emissions from building operations. It provides a universal, standardized method to measure a building's carbon footprint, enabling consistent assessment and comparison of emissions from buildings worldwide, thereby supporting global efforts toward energy efficiency and emissions reductions in the built environment.

📝 Short Notes: Common Carbon Metric (CCM)

• Developed by: United Nations Environment Programme's Sustainable Buildings & Climate Initiative (UNEP-SBCI)
• Purpose: Standardized protocol for measuring energy consumption and GHG emissions from building operations globally
• Key Feature: Provides a universal methodology allowing consistent assessment and comparison of building carbon footprints across different countries and regions
• Focus Area: Building sector operations (not construction, but operational energy use)
• Objective: Support emissions reductions through accurate measurement of energy efficiency in buildings
• Scope: Enables benchmarking and performance tracking of buildings worldwide using a common measurement framework

Detailed Explanation:

Answer: Option 3 — Oyster

Oysters are filter feeders that strain suspended matter and food particles from water by passing it over specialized filtering structures called gills. Among the given options, only oysters possess this feeding mechanism, while catfish are bottom feeders, octopuses are predators, and pelicans are visual hunters that catch fish.

📝 Short Notes: Filter Feeders

• Definition: Filter feeders are organisms that feed by straining suspended matter and food particles from water through specialized filtering structures.
• Common Examples: Clams, oysters, mussels, sponges, krill, baleen whales, and certain fish including some sharks.
• Avian Filter Feeders: Flamingos and certain duck species use filter feeding mechanisms.
• Ecological Role: Filter feeders act as ecosystem engineers by clarifying water and removing suspended particles, improving water quality.
• Bioaccumulation: They serve as indicator organisms due to their role in bioaccumulation of pollutants and toxins from water.
• Pearl Production: Oysters and mussels are commercially important for pearl farming, practiced in states like Gujarat, Maharashtra, Kerala, Odisha, and others in India.

Detailed Explanation:

Answer: Option 1 — 1, 2, and 4

The New York Declaration on Forests (NYDF) is a voluntary political declaration endorsed at the 2014 UN Climate Summit. It sets a global timeline to cut natural forest loss in half by 2020 and end it by 2030, with endorsement from governments, major corporations, and indigenous communities. However, it is not legally binding and India has not endorsed it.

✅ Statement 1 – Correct: The NYDF was first endorsed at the United Nations Climate Summit in New York in September 2014.
✅ Statement 2 – Correct: It endorses a global timeline aiming to halve deforestation by 2020 and end natural forest loss by 2030.
❌ Statement 3 – Incorrect: The NYDF is a voluntary political declaration, not a legally binding international treaty.
✅ Statement 4 – Correct: It is endorsed by a multi-stakeholder coalition including governments, companies, civil society organizations, and indigenous communities (over 190 endorsers).
❌ Statement 5 – Incorrect: India is not among the signatories/endorsers of the New York Declaration on Forests.

Detailed Explanation:

Answer: Option 3 — 1, 3 and 5 Only

Detritivores are organisms that feed on dead and decaying organic matter (detritus), playing a crucial role in nutrient cycling and decomposition. Earthworms, millipedes, and woodlice are classic examples of detritivores that break down organic debris in soil and forest floors. Jellyfish are primarily carnivorous filter feeders that consume plankton and small fish, while seahorses are also carnivores feeding on small shrimp and invertebrates—neither are detritivores.

✅ Statement 1 (Earthworms) – Correct: Earthworms are classic detritivores that consume and break down organic matter in soil, enhancing soil fertility through decomposition.

❌ Statement 2 (Jellyfish) – Incorrect: Jellyfish are carnivorous organisms that primarily feed on plankton and small fish through filter feeding, not on detritus.

✅ Statement 3 (Millipedes) – Correct: Millipedes are detritivores that feed on decaying leaves and organic debris on forest floors, aiding in decomposition.

❌ Statement 4 (Seahorses) – Incorrect: Seahorses are carnivores that feed on small shrimp, plankton, and other small invertebrates, not on dead organic matter.

✅ Statement 5 (Woodlice) – Correct: Woodlice (also called pill bugs or roly-polies) are terrestrial crustaceans that feed on decaying plant and animal matter as detritivores.

📝 Short Notes: Detritivores and Decomposers

• Detritivores: Organisms that feed on dead organic matter (detritus), including dead plant material, animal carcasses, and feces. They physically break down organic matter into smaller pieces.
• Common Examples: Earthworms, millipedes, woodlice, dung beetles, termites, some snails, and certain types of flies and beetles.
• Role in Ecosystem: Detritivores accelerate decomposition by fragmenting organic matter, increasing surface area for microbial action, and mixing organic matter with soil.
• Difference from Decomposers: Decomposers (bacteria and fungi) chemically break down organic matter at molecular level, while detritivores physically consume and fragment it.
• Nutrient Cycling: Detritivores play a critical role in nutrient cycling by releasing nutrients locked in dead organic matter back into the soil for plant uptake.
• Habitat: Most detritivores are found in soil, leaf litter, forest floors, and aquatic sediments where organic debris accumulates.

Detailed Explanation:

Answer: Option 4 — 1 and 3

Among the given animals, hedgehogs and pangolins roll up into a tight ball as a defensive mechanism to protect their vulnerable parts from predators. Hedgehogs use their sharp spines, while pangolins use their tough keratin scales for protection. Marmots, on the other hand, do not roll up; they rely on burrows and alarm calls to avoid predators.

✅ Hedgehog – Correct: Rolls into a tight ball with spines covering vulnerable soft belly parts when threatened.
❌ Marmot – Incorrect: Does not roll up; uses burrows and vocal warning calls as primary defense against predators.
✅ Pangolin – Correct: Rolls into a protective ball, using overlapping keratin scales as armor to shield vulnerable underside.

📝 Short Notes: Defensive Mechanisms in Animals

• Hedgehog: Small spiny mammals found in Europe, Asia, and Africa; covered with about 5,000-7,000 spines made of keratin; rolls into a ball when threatened, making it difficult for predators to attack.
• Pangolin: Only mammal covered in scales; found in Asia and Africa; scales made of keratin (same material as human nails); listed as critically endangered due to illegal wildlife trade; all eight species protected under CITES.
• Marmot: Large ground squirrels found in mountainous areas; social animals living in burrows; use sentinel behavior where one individual watches for predators while others forage; emit high-pitched whistles as alarm calls.
• Other Rolling Animals: Armadillos (three-banded armadillo can roll completely), pill millipedes, pill bugs (woodlice), and some species of armored caterpillars also use rolling as defense.
• Types of Defense Mechanisms: Physical barriers (spines, scales, shells), camouflage, warning coloration, chemical defense (toxins), behavioral (fleeing, freezing, rolling), and group defense.

Detailed Explanation:

Answer: Option 2 — Polycarbonate

Bisphenol A (BPA) is a key structural component in the manufacture of polycarbonate plastics. Polycarbonates are widely used in consumer products such as water bottles, food storage containers, eyeglass lenses, and CDs/DVDs. BPA has raised significant health concerns as it can act as an endocrine disruptor, interfering with the body's hormonal system.

📝 Short Notes: Types of Plastics and BPA

• BPA Health Concerns: Acts as endocrine disruptor, mimics estrogen, linked to reproductive issues, developmental problems, and metabolic disorders
• BPA-Free Alternatives: Many manufacturers now produce BPA-free polycarbonates or use alternative plastics
• Regulation: Several countries have banned BPA in baby bottles and food contact materials

Detailed Explanation:

Answer: Option 4 — Toiletries

Triclosan (TCS) is an antibacterial and antifungal agent commonly found in various personal care and consumer products. It is most prevalently present in toiletries such as toothpaste, soaps, hand sanitizers, body washes, and deodorants. Long-term exposure to high levels of triclosan has raised health and environmental concerns, leading to restrictions and bans in several countries.

📝 Short Notes: Triclosan

• Chemical Nature: Triclosan is a synthetic antimicrobial compound used to reduce or prevent bacterial contamination.
• Common Products: Found in antibacterial soaps, toothpaste, hand sanitizers, body washes, detergents, cosmetics, and some medical equipment.
• Health Concerns: Prolonged exposure may disrupt hormone regulation (endocrine disruption), contribute to antibiotic resistance, and cause skin irritation.
• Environmental Impact: Triclosan can persist in the environment, contaminate water bodies, and harm aquatic ecosystems.
• Regulatory Actions: The US FDA banned triclosan from over-the-counter antiseptic wash products in 2016; the EU restricted its use in cosmetics in 2014.
• Alternatives: Manufacturers are increasingly using safer alternatives like alcohol-based sanitizers and natural antimicrobial agents.

Detailed Explanation:

Answer: Option 1 — environmentally responsible practices in electronics recycling industry.

The R2 (Responsible Recycling) Code of Practices is a comprehensive set of standards specifically designed for the electronics recycling industry to ensure environmentally sound and socially responsible handling of electronic waste. It provides guidelines for data security, material handling, downstream vendor management, and worker health and safety in e-waste recycling operations.

📝 Short Notes: R2 Code of Practices

• Full Form: R2 stands for Responsible Recycling, developed for electronics recycling industry.
• Purpose: Establishes standards for safe, ethical, and environmentally responsible recycling of electronic equipment and e-waste.
• Key Areas Covered: Data security and destruction, material handling, downstream vendor accountability, environmental health and safety, worker protection.
• Certification: R2 certification is recognized globally as a mark of responsible e-waste recycling practices.
• Focus on E-Waste: Addresses growing concerns about toxic materials in electronics (lead, mercury, cadmium) and promotes proper disposal and recycling methods.

Detailed Explanation:

Answer: Option 2 — 2 and 3 Only

Copper smelting plants are a major environmental concern due to their emission of sulfur dioxide and the potential for heavy metal leaching from copper slag. The primary gaseous pollutant is SO₂, not carbon monoxide.

❌ Statement 1 – Incorrect: Copper smelting plants do not typically release lethal quantities of carbon monoxide; the primary gaseous pollutant of concern is sulfur dioxide (SO₂).

✅ Statement 2 – Correct: Copper slag, a byproduct of smelting, contains heavy metals like arsenic, lead, and cadmium, which can leach into soil and groundwater, causing environmental contamination.

✅ Statement 3 – Correct: Copper smelting releases large amounts of sulfur dioxide because most copper ores are sulfide minerals (like chalcopyrite), and sulfur is oxidized to SO₂ during roasting and smelting processes.

📝 Short Notes: Environmental Concerns of Copper Smelting

• Primary Pollutant: Sulfur dioxide (SO₂) is the main gaseous emission from copper smelting, as copper ores are predominantly sulfide minerals.
• Copper Slag: Contains heavy metals (arsenic, lead, cadmium, mercury) that can leach into environment, contaminating soil and water resources.
• Acid Rain: SO₂ emissions contribute to acid rain formation, damaging ecosystems, crops, and infrastructure.
• Health Impacts: SO₂ causes respiratory problems; heavy metals from slag can bioaccumulate and cause neurological and developmental issues.
• Mitigation Measures: Use of scrubbers to capture SO₂, proper slag management, adoption of cleaner smelting technologies, and recovery of sulfur for sulfuric acid production.

Detailed Explanation:

Answer: Option 2 — 2 and 3

Primary producers in ocean food chains are organisms that produce organic compounds from inorganic substances through photosynthesis or chemosynthesis. Among the given organisms, cyanobacteria and diatoms are photosynthetic organisms that convert sunlight into chemical energy, forming the base of marine food webs. Copepods and foraminifera are consumers (zooplankton) that feed on primary producers.

❌ Statement 1 – Incorrect: Copepods are small crustacean zooplankton that function as primary consumers, feeding on phytoplankton and organic matter.
✅ Statement 2 – Correct: Cyanobacteria are photosynthetic bacteria capable of performing photosynthesis and are important primary producers in marine ecosystems.
✅ Statement 3 – Correct: Diatoms are unicellular algae (phytoplankton) that are among the most significant primary producers in oceans, contributing up to 40% of marine primary productivity.
❌ Statement 4 – Incorrect: Foraminifera are single-celled protists that are primarily consumers, feeding on organic particles and smaller organisms using pseudopodia.

📝 Short Notes: Marine Primary Producers and Plankton

• Primary Producers: Organisms that synthesize organic compounds from carbon dioxide through photosynthesis (using sunlight) or chemosynthesis, forming the base of food chains.
• Phytoplankton: Microscopic photosynthetic organisms drifting in water bodies; includes diatoms, dinoflagellates, cyanobacteria, and green algae.
• Diatoms: Most abundant phytoplankton with silica cell walls (frustules); responsible for 20-40% of Earth's oxygen production and major primary producers in oceans.
• Cyanobacteria: Photosynthetic bacteria (blue-green algae); can fix atmospheric nitrogen; some species form harmful algal blooms.
• Zooplankton: Heterotrophic organisms that feed on phytoplankton and organic matter; includes copepods, foraminifera, krill, and jellyfish larvae.
• Copepods: Small crustaceans that are the most abundant zooplankton; primary consumers linking phytoplankton to higher trophic levels.
• Foraminifera: Amoeboid protists with shells (tests); mostly marine; some are benthic while others are planktonic; feed on bacteria, algae, and detritus.