## Big idea: Biomes and food webs

Biomes help scientists describe life on Earth at a large scale. If you know the climate and geography of a place, you can predict what kinds of producers (plants, algae), consumers (animals), and decomposers (fungi, bacteria) can survive there.

A food web is a model that shows many feeding relationships in an ecosystem. Food webs are more realistic than a single food chain because most organisms eat more than one type of food and are eaten by more than one type of predator.

### Key vocabulary (used throughout) - Producer (autotroph): makes its own food using sunlight (photosynthesis) or chemicals (chemosynthesis). - Consumer (heterotroph): gets energy by eating other organisms. - Decomposer: breaks down dead organisms and waste, returning nutrients to the environment. - Trophic level: a feeding level in a food web (producer, primary consumer, secondary consumer, etc.). - Detritus: dead organic material (leaf litter, dead bodies, waste) that fuels decomposer-based pathways.

### Two rules you should remember 1. Energy flows one-way: from producers to consumers to decomposers. Energy is used for life processes and is released as heat, so less usable energy is available at higher trophic levels. 2. Matter cycles: atoms like carbon, nitrogen, and phosphorus move through organisms and return to air, water, and soil through waste and decomposition.

## How to read a food web (and avoid common mistakes)

### 1) Arrows show the direction of energy transfer In food webs, arrows point from the organism being eaten (the energy source) to the organism that eats it.

Example: grass -> rabbit means the rabbit gets energy by eating grass.

### 2) Trophic levels are helpful labels, but real webs overlap - Many consumers are omnivores, so they can occupy more than one trophic level depending on what they eat. - Life stages matter. A frog tadpole may eat algae (primary consumer) but an adult frog may eat insects (secondary consumer).

### 3) Decomposers are not optional Every biome has decomposers. Without them, nutrients would get locked up in dead material, soils would lose fertility, and producers would decline.

### 4) More links usually means more stability Food webs with multiple food sources can be more resilient because organisms can switch foods when one resource becomes scarce. However, this only works if habitats remain intact and biodiversity stays high.

## Earth's major biomes and their food webs

Below are major terrestrial and aquatic biomes. Each section includes (1) where the biome is found, (2) what conditions shape it, and (3) a food web sketch from producers up through trophic levels, including decomposers.

Important: These are examples, not complete species lists. Real food webs vary by location and season.

### 1) Tropical rainforest

Where found: Near the equator where warm temperatures and frequent rain occur year-round. Examples include parts of the Amazon Basin (South America), Congo Basin (Africa), and Southeast Asia.

Conditions that shape the web: High sunlight, high rainfall, rapid decomposition, intense competition for light. Many species specialize in narrow niches.

Food web by trophic level (examples): - Producers: tall canopy trees, understory shrubs, vines, epiphytes, ferns. - Primary consumers (herbivores): leafcutter ants (eat leaves and farm fungi), caterpillars, fruit bats, monkeys that eat fruit and leaves. - Secondary consumers: insect-eating frogs and lizards, birds that eat insects, small snakes. - Tertiary and apex predators: large cats (for example jaguar), large snakes, raptors. - Decomposers and detritivores: fungi, bacteria, termites, beetles, earthworms.

Multiple pathways (examples): - Leaves -> caterpillars -> birds -> raptors - Fruit -> monkeys -> big cat - Seeds -> rodents -> snake -> raptor - Dead wood and leaf litter -> fungi and termites -> nutrients -> trees

Key idea: Many rainforest nutrients are stored in living biomass, not deep soil. Fast decomposition and rapid plant uptake keep the cycle moving.

### 2) Temperate deciduous forest

Where found: Mid-latitudes with four seasons. Examples include eastern North America, parts of Europe, and eastern Asia.

Conditions that shape the web: Moderate precipitation, warm summers, cold winters, strong seasonal changes in plant growth and animal behavior (migration, hibernation).

Food web by trophic level (examples): - Producers: oak, maple, beech trees; shrubs; spring wildflowers; mosses. - Primary consumers: deer (browse leaves), rabbits, caterpillars, squirrels (eat seeds and nuts). - Secondary consumers: foxes, snakes, insect-eating birds, raccoons (often omnivores). - Tertiary and apex predators: coyotes in many regions; large raptors. - Decomposers and detritivores: fungi, bacteria, millipedes, earthworms.

Multiple pathways (examples): - Acorns -> mice and squirrels -> snakes and owls - Leaves -> caterpillars -> songbirds -> hawks - Berries -> rabbits -> foxes - Leaf litter -> fungi and worms -> nutrients -> spring wildflowers

Key idea: Seasonal leaf drop creates a strong detritus pathway that fuels decomposers and helps build fertile soil.

### 3) Boreal forest (taiga)

Where found: High northern latitudes below the tundra. Large areas of Canada, Alaska, Scandinavia, and Russia.

Conditions that shape the web: Long cold winters, short growing season, conifer trees, slower decomposition, and often acidic soils.

Food web by trophic level (examples): - Producers: spruce, fir, pine; mosses; lichens. - Primary consumers: snowshoe hare, moose (browsers), voles, insects in summer. - Secondary consumers: foxes, martens, owls, small predators. - Tertiary and apex predators: wolves; large raptors. - Decomposers: fungi and bacteria; many decomposers work slowly due to cold.

Multiple pathways (examples): - Conifer needles and shrubs -> hare -> lynx or fox - Aquatic plants in forest ponds -> insects -> fish -> birds - Moose calves -> wolves - Dead conifer wood -> fungi -> nutrients (slow) -> new seedlings

Key idea: Because decomposition is slow, organic material can build up. Nutrient cycling is slower than in tropical forests.

### 4) Tundra

Where found: Near the poles and at high mountain elevations (alpine tundra). Arctic tundra occurs across northern Alaska, Canada, Greenland, and Siberia.

Conditions that shape the web: Very cold, low precipitation, strong winds, permafrost (frozen ground) in many areas, and a very short growing season.

Food web by trophic level (examples): - Producers: lichens, mosses, sedges, dwarf shrubs. - Primary consumers: lemmings, hares, caribou (reindeer), insects in summer. - Secondary consumers: arctic fox, weasels, insect-eating birds. - Tertiary and apex predators: snowy owl; in some areas polar bear (often tied to marine food webs). - Decomposers: bacteria and fungi, but decomposition is limited by cold.

Multiple pathways (examples): - Lichens -> caribou -> wolves (where present) - Sedges -> lemmings -> arctic fox -> snowy owl - Dead plant material -> microbes -> nutrients (slow) -> sedges

Key idea: Small changes in temperature can strongly change plant growth and the timing of insect and bird life cycles.

### 5) Desert (hot and cold deserts)

Where found: Often around 30 degrees latitude where dry air descends, plus interior rain-shadow regions. Hot deserts include parts of the Sahara and Arabian deserts. Cold deserts include parts of the Great Basin and Gobi.

Conditions that shape the web: Very low precipitation, high evaporation, large day-night temperature swings in many deserts, and strong adaptations for water conservation.

Food web by trophic level (examples): - Producers: cacti and succulents, drought-tolerant shrubs, short-lived wildflowers after rain, algae in soil crusts. - Primary consumers: insects, rodents, rabbits, tortoises, seed-eating birds. - Secondary consumers: lizards, snakes, scorpions, small carnivores. - Tertiary and apex predators: hawks, owls, coyotes in some regions. - Decomposers: bacteria and fungi; decomposition can be slow when extremely dry.

Multiple pathways (examples): - Seeds -> rodents -> snake -> hawk - Nectar -> insects -> lizards -> owl - Plant leaves -> rabbit -> coyote - Dead organisms -> decomposers -> nutrients -> desert shrubs

Key idea: Food webs can be bursty. After rain, producers and insects increase quickly, which temporarily supports higher consumers.

### 6) Temperate grassland (prairie, steppe)

Where found: Interiors of continents with moderate precipitation that is often too low for dense forests. Examples include the Great Plains (North America) and Eurasian steppe.

Conditions that shape the web: Seasonal drought, frequent fires, and grazing maintain grasses as dominant producers.

Food web by trophic level (examples): - Producers: grasses, wildflowers, shrubs in some areas. - Primary consumers: grazing mammals (bison historically in North America), rabbits, grasshoppers, seed-eating rodents. - Secondary consumers: coyotes or foxes, snakes, insect-eating birds. - Tertiary and apex predators: large raptors; top predators vary by region. - Decomposers: soil bacteria and fungi; earthworms and insects help break down grass litter.

Multiple pathways (examples): - Grass -> grasshopper -> bird -> hawk - Grass -> rabbit -> fox - Seeds -> mouse -> snake -> hawk - Dead grasses -> decomposers -> nutrients -> new grasses

Key idea: Deep grass roots store carbon and help soils stay fertile. This biome often depends on fire and grazing to keep trees from taking over.

### 7) Savanna (tropical grassland)

Where found: Tropical regions with distinct wet and dry seasons. Large savannas occur in parts of Africa, South America, and Australia.

Conditions that shape the web: Seasonal rain, frequent fires, and grazing pressure support grasses with scattered trees.

Food web by trophic level (examples): - Producers: grasses, acacia-like trees, shrubs. - Primary consumers: large grazers and browsers (for example zebra-like grazers in some regions), insects, seed-eating birds. - Secondary consumers: medium predators, snakes, insect-eating birds. - Tertiary and apex predators: large cats in some regions; large pack predators in others. - Decomposers: fungi, bacteria, dung beetles and other detritivores.

Multiple pathways (examples): - Grass -> large grazer -> apex predator - Leaves -> insects -> birds -> raptors - Seeds -> rodents -> snake -> raptor - Dung and dead material -> detritivores -> nutrients -> grasses

Key idea: The detritus pathway is strong because large herbivores produce lots of waste that feeds decomposers and returns nutrients to soil.

### 8) Chaparral (Mediterranean shrubland)

Where found: West coasts in mid-latitudes with mild wet winters and hot dry summers. Examples include coastal California, parts of the Mediterranean region, central Chile, parts of South Africa, and southwestern Australia.

Conditions that shape the web: Drought in summer and periodic fires. Many plants are adapted to fire (resprout or release seeds after fire).

Food web by trophic level (examples): - Producers: drought-tolerant shrubs, grasses, seasonal wildflowers. - Primary consumers: rabbits, deer in some areas, insects, seed-eating birds. - Secondary consumers: snakes, foxes, bobcat-like predators in some regions. - Tertiary and apex predators: large raptors and top carnivores depending on region. - Decomposers: soil microbes; fungi; detritivores.

Multiple pathways (examples): - Shrub leaves -> insects -> lizards -> hawks - Seeds -> rodents -> snakes -> owls - Shrubs -> rabbits -> foxes - Burned plant material -> decomposers -> nutrients -> regrowth

Key idea: Fire can reset the system. After fire, plant regrowth can increase herbivores and then predators, but repeated too-frequent fires can reduce biodiversity.

### 9) Freshwater (lakes, rivers, wetlands)

Where found: On all continents where water collects or flows. Wetlands occur where soils stay saturated.

Conditions that shape the web: Light penetration, water temperature, oxygen levels, and nutrient inputs. Wetlands often have high productivity.

Food web by trophic level (examples): - Producers: algae (phytoplankton), aquatic plants, shoreline plants. - Primary consumers: zooplankton, insect larvae, snails, small fish that filter-feed. - Secondary consumers: larger fish, frogs, turtles, insect-eating birds. - Tertiary and apex predators: fish-eating birds, large fish, alligator-like predators in some regions. - Decomposers: bacteria and fungi that break down dead plants and animals in water and sediments.

Multiple pathways (examples): - Phytoplankton -> zooplankton -> small fish -> larger fish -> bird - Aquatic plants -> snails -> fish -> bird - Insect larvae -> frogs -> snakes -> raptors - Dead material -> decomposers -> dissolved nutrients -> algae and plants

Key idea: Too many nutrients (from runoff) can cause algae blooms that reduce oxygen and harm fish, changing the whole web.

### 10) Estuary (where rivers meet the ocean)

Where found: Coastal areas worldwide where freshwater mixes with saltwater. Estuaries often include salt marshes or mangroves.

Conditions that shape the web: Mixing water creates changing salinity. Estuaries trap nutrients and sediments, making them among the most productive ecosystems.

Food web by trophic level (examples): - Producers: marsh grasses, mangrove trees (in warm regions), algae and phytoplankton. - Primary consumers: zooplankton, filter-feeders (oysters and clams in many regions), crabs, small fish. - Secondary consumers: larger fish, shorebirds that eat crabs and small fish. - Tertiary and apex predators: larger fish, marine mammals in some regions, birds of prey. - Decomposers: microbes in mud and water; detritivores that feed on marsh plant litter.

Multiple pathways (examples): - Phytoplankton -> zooplankton -> small fish -> larger fish - Marsh grass (detritus) -> microbes -> worms/crabs -> fish -> birds - Oysters (filter-feeders) -> crabs/fish -> larger fish

Key idea: Many organisms use estuaries as nurseries. If wetlands are destroyed, young fish survival drops and ocean food webs can be affected.

### 11) Coral reef (a marine biome with high biodiversity)

Where found: Warm, clear, shallow tropical ocean water with enough light for photosynthesis.

Conditions that shape the web: High sunlight, stable warm temperatures, and complex habitat structure. Many reef builders depend on symbiotic algae.

Food web by trophic level (examples): - Producers: algae, seagrass nearby, phytoplankton; symbiotic algae living in coral. - Primary consumers: herbivorous fish, sea urchins, zooplankton feeders. - Secondary consumers: small predatory fish, reef invertebrates, larger fish that eat smaller fish. - Tertiary and apex predators: large predatory fish. - Decomposers: microbes and detritivores recycling organic material within reef crevices and sand.

Multiple pathways (examples): - Algae -> herbivorous fish -> larger fish -> apex predator - Phytoplankton -> zooplankton -> small fish -> larger fish - Detritus trapped in reef -> detritivores -> small fish -> predators

Key idea: If herbivores decline, algae can overgrow corals, reducing habitat complexity and biodiversity.

### 12) Open ocean (pelagic) and deep sea

Where found: Most of Earth's surface is ocean. Open ocean food webs vary from sunlit surface waters to dark deep waters.

Conditions that shape the web: Light is limited to the surface zone. Nutrient availability depends on upwelling and currents. In the deep ocean, food often arrives as sinking detritus.

Food web by trophic level (examples): - Producers (mainly surface): phytoplankton. - Primary consumers: zooplankton that eat phytoplankton. - Secondary consumers: small fish and squid that eat zooplankton. - Tertiary and apex predators: larger fish and marine mammals in some regions. - Decomposers and detritivores: microbes and scavengers that break down sinking organic material (sometimes called marine snow).

Multiple pathways (examples): - Phytoplankton -> zooplankton -> small fish -> large fish - Phytoplankton -> zooplankton -> squid -> large fish - Dead organisms and waste -> microbes -> nutrients (surface mixing can return some nutrients) - Sinking detritus -> deep scavengers -> microbes

Key idea: Even though the deep sea is dark, it is connected to surface food webs through sinking organic matter.

## Cause and effect in food webs (what happens when things change)

When one species changes (declines, increases, moves to a new region), effects can spread through the web.

### Examples of ecosystem effects you should be able to explain - Producer decline (drought, disease, deforestation): fewer herbivores, then fewer predators. - Loss of an apex predator: mid-level consumers may increase, which can reduce herbivores and even reduce producers (a trophic cascade). - Nutrient increase in water: algae can increase, oxygen can drop, fish populations can crash. - Invasive species: can outcompete native producers or consumers and rewire the web.

### What makes a good scientific prediction A strong prediction names (1) the species changing, (2) what it eats and what eats it, and (3) at least two expected population changes linked by energy flow or resource availability.