Feeding Mechanics

For a general introduction to consumers, see Consumers.

Why Feeding Mechanics Drive Ecosystem Dynamics

Feeding is where energy enters the animal side of the food web, and the details of how it works matter enormously. A Daphnia that can filter-feed efficiently at low algal densities will suppress algae more effectively than a Copepods that needs high prey concentrations to feed well — and that difference determines whether algae bloom or stay controlled. The rate at which consumers find, capture, and digest food controls their survival, their reproduction rate, the grazing pressure they impose on producers, and the amount of nutrients they recycle back into the water through excretion and fecal production.

The mechanics described here — saturation curves that cap feeding at high food density, preference weights that split intake among multiple prey types, digestion costs that consume oxygen and produce CO₂ — are what create the predator-prey dynamics that can either stabilize an ecosystem through balanced grazing or crash it through overexploitation.

The Holling Type II Functional Response

The core feeding model is the Holling Type II functional response. In plain English: an animal's feeding rate goes up as food becomes more abundant, but it cannot increase forever. At some point, the animal is spending all its time handling, chewing, and digesting what it already caught, and adding more food to the environment does not help. The feeding rate levels off at a plateau.

Two parameters control this curve:

Maximum ingestion rate (Imax) -- The fastest the animal can possibly eat, measured as the amount of food carbon consumed per unit of body carbon per hour. Rotifers have the highest Imax at 0.07 per hour (about 1.7 times body carbon per day), reflecting their high weight-specific metabolism from small body size. Daphnia has an Imax of 0.06 per hour (about 1.4 times its body carbon per day). Copepods is lower at 0.035 per hour (about 0.84 per day), reflecting its more selective, raptorial feeding style.
Half-saturation constant (K) -- The food concentration (mol C per liter) at which the feeding rate is exactly half of maximum. A smaller K means the animal is efficient at finding food even when food is scarce. Copepods have the highest K at 5.0e-5 mol C/L (~0.6 mg C/L), reflecting that its raptorial feeding requires higher prey density for efficient capture. Ostracods have a K of 3.0e-5 mol C/L (~0.36 mg C/L). Daphnia's K is 2.5e-5 mol C/L (~0.3 mg C/L). Rotifers have a K of 2.0e-5 mol C/L (~0.24 mg C/L). The protists have the lowest K values: ciliates at 2.5e-5 mol C/L (~0.3 mg C/L) and nanoflagellates at 1.5e-5 mol C/L (~0.18 mg C/L), reflecting their ability to feed efficiently at low bacterial concentrations.

The actual feeding rate at any moment is: the maximum rate multiplied by the fraction (food concentration) / (K + food concentration). When food concentration is much greater than K, this fraction approaches 1 and feeding is near maximum. When food concentration equals K, feeding is at 50% of maximum. When food concentration is much less than K, feeding drops toward zero.

Multi-Food Selection

No consumer in this ecosystem survives on a single food type. A Daphnia filters whatever the water carries past its feeding appendages — algae, bacteria, detritus, the occasional ciliate — and its population dynamics depend on the combined availability of all of those sources, not any one in isolation. A copepod might switch from hunting ciliates to scraping periphyton when the plankton thins out. Modeling each food source independently and then picking the "best" one would miss these dietary overlaps entirely. Instead, the model calculates a single total "effective food" value by adding up all available food sources, each weighted by its preference value.

Food Sources and Preferences

Each consumer has a distinct set of food sources with specific preference weights, assimilation efficiencies, and access fractions. The complete per-consumer feeding tables for all species are in The Food Web.

Preference determines how much of the total intake comes from each food type. If a grazer has preference 1.0 for periphyton and 0.5 for detritus, and both are equally available (after accounting for access), it will eat roughly twice as much periphyton as detritus. The total intake is then split among food sources proportionally to each source's share of the total effective food pool.

Access Fractions

Not all of a food source is reachable. The access fraction determines what portion of a food pool the grazer can actually get to. Access depends on the surface type where the food is growing and may also change with prey density (see the refugia documentation for the full story). For prey tracked per-surface (like periphyton), access is calculated individually for each surface pool. For prey tracked as a single bulk pool (like bacteria and microzooplankton), the effective access is a surface-area-weighted average of the per-surface access values -- this means the scenario's surface composition directly influences how much of the prey is reachable. A scenario with mostly gravel will yield lower effective access to bacteria than one with mostly glass, because gravel surfaces provide more refugia. The values listed in the food web feeding tables are approximate typical values; actual effective access varies by scenario.

Assimilation

Only a fraction of ingested food is actually absorbed into the animal's body. This fraction is the assimilation efficiency, and it varies by food type:

Algae, ciliates, and HNF are relatively easy to digest (50-65% assimilated)
Bacteria are harder (40-45%)
Detritus is the worst (25-35% for Daphnia/rotifers, 35% for Copepods)

Importantly, nitrogen and phosphorus are assimilated with higher efficiency than carbon. This reflects the biology of digestion: cell contents (proteins, nucleic acids, phospholipids) are more easily digested than cell walls (cellulose, chitin). N assimilation is about 10% higher than C assimilation (multiplier 1.10), and P assimilation is about 15% higher (multiplier 1.15), both capped at 100%. This means that if a food type has 50% C assimilation, its effective N assimilation is 55% and P assimilation is 57.5%.

The unassimilated remainder -- the fecal pellets -- is ejected and becomes detritus. A portion stays suspended in the water column and the rest sinks to the bottom. Daphnia produces larger, denser pellets (75% settle, 25% stay suspended). Copepods produce smaller, more fragile pellets that break apart more easily (65% settle, 35% stay suspended). Rotifers produce very small pellets that mostly stay suspended (70% suspended, 30% settle).

Stoichiometric Homeostasis

Consumers maintain fixed body composition ratios: a carbon-to-nitrogen (C:N ratio) and a nitrogen-to-phosphorus (N:P ratio). Daphnia's body C:N ratio is 5.0; Copepods is 4.5 (slightly more protein-rich); rotifers are 5.0 (similar to Daphnia); ostracods are 5.5 (calcified carapace increases C content). Body N:P ratios are species-specific, reflecting the Growth Rate Hypothesis (Elser et al. 2000): fast-growing organisms with high ribosomal RNA content are P-rich (low N:P), while slower-growing organisms are N-rich (high N:P). Daphnia has N:P = 12 (P-rich, fast growth), rotifers, ciliates, and HNF use N:P = 16 (Redfield), ostracods have N:P = 20 (calcified), and Copepods have N:P = 22 (N-rich copepod).

After assimilation, the model checks whether the assimilated carbon, nitrogen, and phosphorus match the animal's body ratios:

If food is nitrogen-rich (C:N of food is lower than the body ratio): The animal absorbs all the carbon it can use but only keeps enough nitrogen to match. The excess nitrogen is excreted as ammonium (NH4) back into the water. This is a major pathway for nutrient recycling -- grazers eating nitrogen-rich algae and excreting ammonium that fertilizes more algae growth.
If food is phosphorus-rich (more P than needed relative to C): The animal keeps only the phosphorus it needs to maintain its body N:P ratio. The excess phosphorus is excreted as phosphate (PO4) back into the water.
If food is carbon-rich (C:N of food is higher than the body ratio): The animal absorbs all the nitrogen but has more carbon than it needs. The excess carbon is burned off through respiration.

Phosphorus is tracked through every step of the feeding pipeline: intake is calculated from each food source's C:P ratio, assimilation applies the same efficiency as for C and N, unassimilated P is egested as detritus, and excess assimilated P is excreted as PO4.

Specific Dynamic Action (SDA)

SDA is the metabolic cost of digesting a meal. Think of it as the energy your body spends breaking down and processing food -- it is a real overhead that reduces the net benefit of eating.

In the model, a fixed fraction of assimilated carbon is immediately burned off as the cost of digestion:

Daphnia SDA: 20% (0.20) -- Higher overhead for continuous filter pumping
Rotifer SDA: 20% (0.20) -- Similar overhead for ciliary filter feeding
Copepod SDA: 18% (0.18) -- Slightly lower, reflecting more efficient raptorial feeding
Ostracod SDA: 18% (0.18) -- Similar to Copepods (crustacean scraper)
Ciliate SDA: 15% (0.15) -- Lower overhead for protist phagocytosis
HNF SDA: 15% (0.15) -- Lowest overhead (smallest protist consumer)

This means that if Daphnia assimilates 100 units of carbon from food, 20 units are immediately burned for digestion, and only 80 units are available for growth.

Maintenance Respiration

Even when not eating, consumers need energy to stay alive — maintaining cell functions, swimming, osmoregulation. This is the maintenance respiration rate, measured as O2 consumed per unit of body carbon per hour.

Maintenance rates follow the classic allometric pattern: smaller organisms burn more energy per unit body mass. Nanoflagellates, the tiniest consumers, have the highest weight-specific rate (0.005/h), followed by rotifers and ciliates (both 0.004/h). The crustaceans are lower — Daphnia at 0.003/h, copepods at 0.0025/h — and ostracods, with their slow benthic lifestyle, are lowest at 0.0022/h. This roughly 2:1 spread from the smallest to largest consumer means that a population of nanoflagellates burns through its carbon reserves about twice as fast as an equivalent biomass of ostracods, which is why small consumers are the first to crash during food shortages.

Maintenance respiration is limited by dissolved oxygen availability, following the same saturation curve used for feeding. At low O2, the animal cannot respire fully, meaning it cannot meet its metabolic needs. The half-saturation for O2-limited respiration ranges from about 1.0 mg/L for rotifers (tolerant of mild hypoxia) to 1.6 mg/L for Daphnia (more sensitive).

If salinity deviates from the animal's optimum, osmoregulation costs increase, raising the effective maintenance rate.

The respiratory quotient (RQ) -- moles of CO2 produced per mole of O2 consumed -- is species-specific and depends on the substrate being catabolized. Pure carbohydrate catabolism gives RQ = 1.0, protein gives ~0.8, and lipid gives ~0.7. Zooplankton catabolize a mix of substrates (primarily protein and lipid), so RQ is typically below 1.0. The default RQ is 0.85 for crustaceans (Daphnia, Copepods, ostracods, rotifers) and 0.90 for protists (ciliates, HNF) which catabolize more mixed substrates. This means consumers produce slightly less CO2 than they consume O2.

Activity Modifiers

The actual feeding rate is not just a function of food availability — it is shaped by the animal's environment. Three factors scale the feeding rate, and they multiply together, so their effects compound.

All grazers slow down at night. Daphnia drops to about 70% of its daytime feeding rate in darkness, copepods to about 80%, and rotifers — nearly continuous feeders — to about 90%. This nocturnal slowdown reduces grazing pressure at exactly the time when algae are also not producing oxygen, which slightly softens the nighttime O₂ deficit.

Dissolved oxygen also matters. Activity follows a saturation curve: at healthy O₂ levels the animal feeds at full capacity, but as oxygen drops, feeding slows. Daphnia needs about 2 mg/L O₂ to reach half its maximum activity — it is the most oxygen-sensitive feeder. Copepods reach half activity at about 1.6 mg/L, and rotifers at about 1.3 mg/L. In a hypoxic jar, this means rotifers are the last grazers still actively feeding.

Finally, salinity modulates activity through a bell curve centered on the animal's optimum (around 0.5 PSU for all freshwater species). Feeding efficiency drops as salinity deviates from the optimum. Copepods tolerate a wider salinity range, while rotifers are the most sensitive (stressed above 3 PSU, unable to survive above 8 PSU).

These modifiers interact. Imagine a Daphnia at night, in warm water where dissolved oxygen has dropped to 2 mg/L. Its night activity factor is 0.70, and its oxygen factor — at the half-saturation point — is 0.50. Multiplied together, the animal is feeding at just 35% of its daytime, well-oxygenated capacity. That is not starvation yet, but a few more hours of declining O₂ could push it there, especially if it was already running a caloric deficit from days of heavy competition for a thinning algae population.

Mortality

Consumers face mortality from starvation, crowding, cannibalism (copepods), and environmental stressors (temperature, pH, oxygen, salinity, ammonia). All of these mechanisms — how they stack, what thresholds trigger them, and how they differ by species — are covered in Mortality Mechanisms.