Consumers (Grazers and Predators)

Consumers are the animals in the ecosystem. Unlike algae and plants, which make their own food from light and dissolved nutrients (photosynthesis), consumers are heterotrophs -- they survive by eating other organisms. In this simulator, consumers fill the critical role of keeping algae and bacteria populations in check, recycling nutrients back into the water, and creating detritus (dead organic matter) that feeds the decomposer community.

The model includes seven freshwater grazers, ranging from microscopic rotifers (~100 micrometers) to cherry shrimp at 1.5-3 cm. For detailed profiles of each species -- size, diet, tolerances, reproduction, and parameter values -- see the Species Catalog: Consumers.

Three Feeding Strategies

Every consumer in the model uses one of three basic strategies to find food. The strategy determines what kinds of food the animal can reach and how effectively.

Filter feeders

Filter feeders sweep water through a fine mesh and capture whatever suspended particles pass through. They are excellent at harvesting planktonic algae, bacteria, and fine detritus from the water column, but poor at scraping surfaces. Daphnia and rotifers are the freshwater filter feeders.

Filter feeders are non-selective -- they eat whatever their mesh captures, including ciliates and nanoflagellates. This means dense filter-feeder populations can suppress the microbial loop directly by removing its participants from the water column. Daphnia are the most important controller of planktonic algae blooms in the model, with 95% access to free-floating cells.

Raptorial hunters

Raptorial hunters actively detect and grab individual prey items rather than passively filtering. Cyclopoid copepods are the model's raptorial hunter. They target periphyton on surfaces and are the most versatile feeders in the model -- eating algae, bacteria, ciliates, rotifers, nanoflagellates, and suspended detritus. Their predation on ciliates is what completes the microbial loop, channeling energy from the decomposer pathway back into the main food web.

Benthic scrapers

Benthic scrapers crawl on surfaces and scrape off biofilm, settled detritus, and anything else growing on the substrate. Bladder snails, cherry shrimp, and ostracods all use this strategy. Among them, bladder snails are the most effective biofilm scrapers on smooth surfaces (using a muscular radula), cherry shrimp are the largest-bodied consumer and the dominant processor of settled detritus, and ostracods fill a unique niche as the most effective settled detritus consumer among the small-bodied grazers. Benthic scrapers have little access to food suspended in the water column -- their world is the substrate.

How Feeding Works

All consumers follow the same general pipeline:

Find food -- The consumer encounters available food in the water or on surfaces. Not all food is equally reachable; some is hidden in crevices or protected by surface texture (this is the refugia concept -- see the detailed refugia doc).
Eat -- The feeding rate increases with the amount of food available, but it hits a ceiling. Even in a food-rich environment, there is a maximum rate at which the animal can process meals. Each food type has a preference weight that determines how much attention the consumer gives it.
Digest -- Only a fraction of what is eaten actually gets absorbed into the body (the assimilation efficiency). The rest is ejected as fecal pellets, which become detritus -- some stays suspended in the water, some sinks to the bottom.
Grow or excrete -- Consumers maintain fixed ratios of carbon to nitrogen and nitrogen to phosphorus in their bodies (stoichiometric homeostasis). If the food they eat is richer in nitrogen or phosphorus than they need, the surplus is excreted as ammonium (NH4) and phosphate (PO4), which algae can then use. If the food is carbon-rich, extra carbon is burned off through respiration.

Three numbers define each feeding relationship: the preference (how much the consumer wants that food type relative to others), the assimilation efficiency (what fraction of ingested food is actually absorbed), and the access (what fraction of the food is physically reachable). See food_web.md for the complete map of who eats whom.

Respiration

Like all animals, consumers burn carbon for energy and consume dissolved oxygen (O2) in the process, releasing carbon dioxide (CO2). This happens in two parts:

Maintenance respiration -- the baseline cost of staying alive, even when not eating. This is like the idle fuel burn of an engine.
Specific dynamic action (SDA) -- the extra energy cost of digesting food. Every meal has an overhead.

If oxygen in the water drops too low, respiration becomes limited and the animal cannot meet its energy needs, leading to stress and potentially death.

Activity Patterns

All consumers are less active at night than during the day, though they do not stop entirely. Nighttime feeding rates range from about 70% of daytime rates (Daphnia) to about 95% (ciliates and nanoflagellates, which are nearly continuous feeders). Activity is also reduced by low oxygen and unfavorable salinity.

Density-Dependent Controls

Without some form of population control beyond starvation, consumer populations tend to overshoot wildly -- booming until food is gone, then crashing. The model includes several density-dependent mechanisms that prevent this. Each consumer species has at least one.

Crowding mortality. At high population densities, competition for space, waste buildup, and disease transmission cause extra mortality. For water-column species (Daphnia, rotifers), crowding scales with volumetric density (biomass per liter). For benthic species (ostracods, bladder snails, cherry shrimp), crowding scales with areal density (biomass per cm² of crawlable substrate), reflecting the fact that bottom-dwellers compete for floor space rather than water volume. Larger-bodied species saturate their available space at lower absolute biomass.

Cannibalism. Copepods eat their own young. At high population densities, encounter rates between adults and nauplii increase, making cannibalism a significant source of mortality. This creates a natural negative feedback loop.

Viral lysis. Ciliates and nanoflagellates (HNF) are killed by giant viruses (Nucleocytoviricota). Contact rates scale with population density, creating stabilizing density-dependent mortality. This prevents unchecked blooms of these microbial consumers from suppressing the bacterial populations they feed on.

Mechanical interference. Large Daphnia populations cause physical interference mortality on rotifers -- filtering currents damage or dislodge the smaller organisms. This is not predation but interference competition that scales with Daphnia density (Gilbert 1988).

Reproduction suppression. Cherry shrimp (Neocaridina davidi) have a unique mechanism: under poor water quality (elevated nitrate, unfavorable pH or temperature, elevated ammonia), adults feed and survive normally but stop breeding entirely. The population stagnates rather than crashing -- a biologically distinct response from mortality.

Habitat Quality

Food alone does not determine how fast a benthic consumer can grow. Some species also need structurally complex biofilm to shelter and brood in — a requirement that is independent of food availability. The model captures this through a habitat factor derived from the biofilm maturity index (M), which tracks the structural complexity of the EPS scaffold on each surface.

The habitat factor multiplies the consumer's maximum ingestion rate. Species that depend heavily on mature biofilm (ostracods) can barely grow in a young tank: at M = 0.3 (roughly 2 months), their habitat factor is only 0.09. Species with mild dependence (bladder snails) are less affected — the same M = 0.3 gives them a habitat factor of 0.55. Planktonic and water-column consumers are unaffected entirely. This produces the characteristic pattern in new aquariums where benthic meiofauna lag months behind the initial microbial colonization. For the full mechanics — how M develops, what drives it, and how grazing damages it — see Biofilm Maturity.

Refugia (Brief Overview)

Not all food is equally accessible. Algae hiding in the crevices of rough gravel, bacteria embedded deep in a biofilm, or ciliates tucked inside a tangle of filamentous algae are all partially or fully protected from grazing. This "refugia" effect is critical for ecosystem stability: it prevents grazers from completely wiping out their food supply. The amount of protection depends on the surface type, the grazer species, and how much prey is present. For the full details, see the refugia documentation.