Trophic Systems & Ecology

Remove a predator and the whole ecosystem shifts. Add it back and rivers change course. Ecology is the study of those invisible threads — and how dangerously easy they are to cut.

Click any pyramid level to see what lives there, what it eats, and how energy flows.

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The 10% energy rule

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Only about 10% of the energy at one trophic level transfers to the next. The other 90% is lost as heat (through metabolism, movement, maintaining body temperature). This is why food chains rarely exceed 4–5 levels — there's not enough energy left to support another tier. It also explains why eating lower on the food chain (plants) is far more energy-efficient than eating higher (meat) — it takes roughly 10 kg of grain to produce 1 kg of beef.

💡 1,000 kg of grass → 100 kg of grasshoppers → 10 kg of frogs → 1 kg of snake → 0.1 kg of hawk.

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Detrital vs. grazing food chains

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Most ecosystems have two parallel pathways. The grazing chain goes: plants → herbivores → carnivores (the classic pyramid). The detrital chain goes: dead organic matter → decomposers (bacteria, fungi) → detritivores → larger consumers. Salt marshes are famous for being dominated by the detrital pathway — most Spartina grass dies and decomposes rather than being eaten directly by herbivores. The detrital chain is often overlooked but drives enormous energy flow in wetland and forest ecosystems.

Show ecosystem: State:

Salt marsh — predator present: Blue crabs control snail populations. Snails stay in balance. Spartina cordgrass thrives. Marsh ecosystem is healthy and productive.

Toggle between ecosystems and states to see the cascade effect

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Salt marsh — Brian Silliman's discovery

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Salt marshes were long assumed to be bottom-up regulated — controlled by nutrients and sunlight. Ecologist Brian Silliman challenged this using cage experiments. He found that periwinkle snails don't eat Spartina directly — they facilitate a fungal infection on the grass, killing it. Blue crabs control snail numbers, indirectly protecting the grass. Remove the crabs and snail populations explode, the fungal infection spreads, and the marsh dies. This is a top-down trophic cascade — a predator (blue crab) indirectly controls a producer (Spartina) through an intermediate consumer. The cage experiment approach makes this one of the cleanest demonstrations of trophic cascades ever documented.

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Yellowstone — how wolves changed rivers

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Wolves were hunted to extinction in Yellowstone by 1926. Without them, elk overgrazed riverbanks, stripping willows and aspens. Rivers eroded and straightened. By 1995 only one beaver colony remained in the park. When 31 wolves were reintroduced in 1995–96, elk changed both their numbers and their behavior — avoiding valleys where they'd be trapped. Vegetation recovered. Beavers returned (from 1 to 9 colonies). Their dams created wetland habitat. Songbirds came back. A 20-year study found a ~1,500% increase in willow crown volume. Stabilized riverbanks changed the physical course of rivers. One predator — wolves — reshaped an entire landscape. Note: some scientists argue drought and fire also contributed; the cascade is real but its magnitude is debated.

Control type:

Top-down control: Predators regulate the ecosystem. Consumer populations are kept in check by what eats them. Changes at the top cascade downward. Example: wolves control elk, elk control vegetation.

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Top-down control

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In top-down controlled systems, predators regulate consumer populations, which in turn regulate producers. The key feature: removing the top predator causes lower levels to explode. Examples include Yellowstone (wolves → elk → vegetation), the salt marsh (crabs → snails → grass), and sea otters → sea urchins → kelp forests. Top-down control is more common in simpler food webs with fewer species and in aquatic systems.

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Bottom-up control

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In bottom-up controlled systems, nutrients and primary productivity set the pace for the whole food web. If you add fertilizer (nutrients), phytoplankton blooms, zooplankton increases, fish increases. The energy flows up from the base. Changes at the bottom ripple upward. Most ecosystems have elements of both — bottom-up and top-down controls often act simultaneously, and their relative importance varies by system, season, and scale.

Salt marsh food web. Click any organism to highlight its connections — what it eats and what eats it.

Click any node to highlight its feeding relationships

Microbial ecology — the same trophic rules that govern wolves and elk operate at the microscopic level. Bacteriophages are the apex predators of the microbial world, and in your yogurt vat, they can collapse an entire culture in hours.

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What is a bacteriophage?

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A bacteriophage (phage) is a virus that specifically infects bacteria. It cannot infect human cells — it is a precision predator evolved to target one bacterial species, often one specific strain. Phages are the most abundant biological entity on Earth — estimated at 10³¹ (10 nonillion) particles — outnumbering bacteria 10-to-1 in most environments. They are the top-down regulators of bacterial populations worldwide.

💡 Phages have two life cycles. Lytic: infect → replicate → burst the cell → spread (the dangerous one for dairy). Lysogenic: integrate into bacterial DNA and replicate quietly with the host — can switch to lytic under stress.

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Bacteriophage in dairy — the real threat

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In cultured dairy production, bacteriophages that target Streptococcus thermophilus or Lactobacillus bulgaricus are one of the most significant causes of culture failure. Phage contamination causes slow acidification — the pH drop that signals healthy fermentation slows or stalls. The vat may look normal but acid production is collapsing. By the time the problem is visible it's often too late to save the batch.

Signs of phage attack:
Slow pH drop · Extended incubation time · Weak gel structure · Off flavor · High pH at set · Elevated post-pasteurization counts on next batch

Why it's so hard to control:
Phages survive pasteurization temps · Persist in biofilms on equipment · Spread through aerosols · Highly strain-specific but evolve rapidly

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Culture rotation — the ecological solution

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The primary defense against phage in dairy is culture rotation — cycling through phage-unrelated starter culture strains on a scheduled basis. Because phages are highly strain-specific (a phage that attacks Strain A usually cannot attack Strain B), rotating strains denies the phage its specific host. This is a direct application of ecological principles: you are managing predator-prey dynamics by removing the prey from the environment before the phage population can build up.

💡 This is exactly analogous to crop rotation in agriculture — you rotate species to break pest and pathogen cycles. The ecology is identical, just at a microscopic scale.

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Competitive exclusion — LAB vs the world

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In a yogurt vat, S. thermophilus and L. bulgaricus don't just ferment — they wage ecological warfare. By rapidly acidifying the environment to pH 4.0–4.5, they make it uninhabitable for most competing organisms. This is competitive exclusion — one species outcompeting others by changing the shared environment. Pathogens like Listeria and Salmonella are acid-sensitive. Yeasts and molds are more tolerant (hence why elevated YM counts on Petrifilm signal trouble — they can survive where most bacteria can't).

💡 The Gause competitive exclusion principle: two species competing for the same resource in the same niche cannot coexist indefinitely — one will outcompete the other. In your vat, LAB win by acidification. Unless phage tips the balance.

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The microbial loop & viral shunt

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In aquatic ecosystems, the microbial loop is a parallel energy pathway: dissolved organic matter → bacteria → protists (which eat bacteria) → zooplankton → the main food web. This loop recycles nutrients that would otherwise be lost. The viral shunt is what happens when phages intercept this loop: phages lyse bacteria before protists can eat them, releasing dissolved organic matter back into the water rather than up the food chain. This short-circuits the loop — phages redirect energy away from higher trophic levels and back to dissolved nutrients, fundamentally reshaping ocean ecosystems. Phages may process up to 20–40% of marine bacterial production daily.

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Quorum sensing — bacterial democracy

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Bacteria don't just compete — they communicate. Quorum sensing is the ability of bacteria to detect population density through chemical signaling molecules called autoinducers. When the autoinducer concentration reaches a threshold (a "quorum"), the whole population switches gene expression simultaneously — forming biofilms, producing virulence factors, or upregulating acid production. In dairy, LAB quorum sensing may help coordinate the acidification response. In pathogens, it controls when they express virulence — they "wait" until there are enough of them to overwhelm host defenses before revealing themselves.

💡 Biofilms on dairy equipment are quorum-sensing communities. Once established they're extremely resistant to cleaning and serve as persistent reservoirs for phage. This is why CIP protocols matter so much — you're disrupting microbial ecology, not just washing dishes.

Microbial ecology follows the same rules as macroscopic ecology — just at a scale invisible to the eye

Key vocabulary

Trophic level

Position in a food chain. Producers = level 1, primary consumers = level 2, and so on.

Trophic cascade

Indirect effects of predators rippling down through the food web, affecting organisms they don't directly eat.

Keystone species

A species with a disproportionately large effect on its ecosystem relative to its abundance. Wolves and sea otters are classic examples.

Top-down control

Predators regulate the whole system. Removing the top predator destabilizes everything below.

Bottom-up control

Nutrients and primary productivity drive the system. More nutrients → more producers → more consumers at each level.

Facilitation

One species indirectly helps another. Silliman's snails facilitate fungal infection — they don't eat the grass, but they enable its killer.

Detritivore

Organism that eats dead organic matter (detritus). Bacteria, fungi, earthworms. Critical for nutrient cycling.

Biomass

Total mass of living organisms in a given area or trophic level. Decreases at each level due to the 10% rule.

Bacteriophage

A virus that infects and kills bacteria. The apex predator of the microbial world. In dairy, phage attacking starter cultures causes slow acidification and culture failure.

Lytic cycle

Phage infects → hijacks replication → produces ~100–200 new phages → bursts the cell → spreads. The destructive cycle relevant to culture failure.

Competitive exclusion

Two species competing for the same niche can't coexist indefinitely. LAB win in yogurt by acidification — they change the environment to exclude all competitors.

Quorum sensing

Bacteria detect population density via chemical signals and coordinate behavior — forming biofilms, switching on virulence, or ramping up acid production.

Viral shunt

When phages lyse bacteria before predators can eat them, redirecting organic matter back to dissolved nutrients instead of up the food chain.

Exam reminders

Only ~10% of energy transfers between trophic levels. 90% is lost as heat. This limits food chain length to 4–5 levels.

A keystone species has impact far beyond its numbers. Wolves are 1% of Yellowstone's biomass but control the whole system.

Trophic cascades can be triggered by adding or removing a top predator. Wolves reintroduced = positive cascade. Sharks overfished = negative cascade.

Silliman's salt marsh work used cage experiments — controlled field manipulation. This is what makes it strong evidence rather than correlation.

Bottom-up and top-down controls are not mutually exclusive — most real ecosystems have both operating simultaneously.

The detrital food chain often moves more energy than the grazing chain in wetland and forest ecosystems. Don't forget decomposers!

The 3 famous cascades

Salt marsh: Crabs → snails → Spartina grass

Yellowstone: Wolves → elk → willows → rivers

Pacific kelp forest: Sea otters → sea urchins → kelp

Dairy vat (microbial): Phage → S. thermophilus → lactic acid → pH drop stalls