Growth response dynamics

When active, individuals of a species are actively searching for prey, or adequate substrate for growth. The exceptions are immotile species, mostly prokaryotes, that are active only when resources are available in the immediate vicinity. Even fungal hyphae search the soil for adequate substrates and respond directionally by chemotaxis. Feeding on primary saprotrophs is best described as grazing, and resembles herbivory by terrestrial consumers. Grazing is different from predation by consumers because it does not usually cause death or depletion of the grazed indi vidual. Fungivores only graze a portion of the mycelium or thallus, or ingest cytoplasm from a small number of hyphae. As for herbivory, the fungus mycelium is immobile relative to the mobile grazer. It is possible under intense grazing pressure by consumers for the eaten individual to die. These are extreme events that occur when consumer populations are exceptionally abundant, or the grazed species exceptionally rare. In predation, consumption of an individual usually leads to death because the whole individual is killed and consumed. An unsuccessful predation attempt (prey capture) may cause wounding or escape of the prey. Predation occurs as cytotrophy on motile protists, or as consumption of nematodes, microarthropods or other invertebrates. In bacterivory and fungivory on yeast cells, it is almost impossible for the consumers to ingest all the clonal descendants. The soil matrix provides too many refuges from consumers (see below). Whether consumption of a prokaryote or eukaryote cell is considered predation or grazing depends on the scale of observation. If ingestion of a single cell is considered, predation is correct. If consumption of bacteria or protozoa in general is considered, grazing is probably the correct term.

The relationship between a predator and its prey is described theoretically by the functional response (Holling, 1959). Three types are hypothetically possible (Fig. 5.3). In the simplest case, the rate of prey ingestion increases linearly with the abundance of prey individuals, without an apparent upper or lower limit (type I). Realistically, most consumers have minimum prey abundance requirements (to sustain growth and reproduction), as well as maximum ingestion rates. This relationship is described in part in the type II response, where prey capture rate increases rapidly at first but levels off. As more prey are available for capture, search time decreases and capture frequency increases. However, handling time and ingestion rate become rate limiting, and the predator may become satiated. These cause the levelling off of the curve to a maximum rate. The type II response does not account for a minimum prey abundance requirement. Its shape is similar to that of enzyme kinetics as described by the Michaelis-Menton equation, and several derivations have been proposed through the years (Williams, 1980). At very low prey abundances, search time is so long between captures that predator growth may not be sustainable. In the natural habitat, prey abundance below a minimum level for an extended period causes a behavioural shift in the predator. The predator can switch to an alternative prey, it can become quiescent (or encyst) or it can enter a dispersal phase. The type III response is often the best descriptor of predator-prey interactions. Its S-shape is the same as that of the logistic growth equation. The type III functional response is more common than assumed, even with microscopic species. It is often an indication of optimal foraging behaviour by the predator or grazer (Krebs, 2001).

Fig. 5.3. Functional response curves. (A) Theoretical functional response curves (types I, II and III) observed in biological systems. (B) Type III functional response, with similarities to enzyme kinetics and the logistic growth equation. (C) Growth of Sterkiella histriomuscorum on the soil alga Chlamydomonas reinhardtii at two different temperatures.

Fig. 5.3. Functional response curves. (A) Theoretical functional response curves (types I, II and III) observed in biological systems. (B) Type III functional response, with similarities to enzyme kinetics and the logistic growth equation. (C) Growth of Sterkiella histriomuscorum on the soil alga Chlamydomonas reinhardtii at two different temperatures.

The response of the common soil ciliate Sterkiella histriomuscorum to variations in conditions can illustrate predator-prey dynamics. This species can ingest bacteria and protists of a variety of size ranges up to about 25 ^m. When grown on single species, cell cycle duration and cell size are affected. Most efficient growth results in large cells with short cell cycles (Fig. 5.3). Less efficient growth results in smaller cells with longer cell cycles. When grown at different temperatures, the preferred prey varies. Furthermore, the minimum required prey abundance, maximum ingestion rates and growth efficiency vary with changes in temperature. This is because the handling time and encounter rate also vary with temperature for different prey species. When grown in the presence of several prey species, Sterkiella show a clear preference for one prey over others. This preference depends on the temperature and on the cell size at the beginning of the experiments, as well as the abundance of each prey. These functional response curves are similar to enzyme kinetic curves in their response to temperature changes, prey abundance changes and initial predator:prey ratio. One simply substitutes the prey for the substrate and the predator for the enzyme. Prey preference at different temperatures is then analogous to activation energy barriers in kinetic reactions: reactions with high activation energy are less probable (slower rate) as the temperature decreases.

In the soil habitat, the dynamics of population interactions are complicated further by the heterogeneity of the habitat. Dispersal distance through the soil reticulum is slow, and patchiness (heterogeneity) is observed at all scales from submillimetre distances and larger. At the microscopic scale, each piece of microdetritus is a microhabitat. There are micro-climatic effects which vary with soil depth, shade from a tree, presence of a rock or pebble, aspect of a hill slope, and so on. The accumulation of a particular type of litter becomes a resource patch. A twig, pine needle, dead rootlet, shed insect cuticle or empty test of a testate amoeba become the habitat for other species. One can quickly imagine a multitude of microhabitats and microclimates in the soil, over short distances and depth. These have repercussions on the natural dynamics of interstitial populations. It is unclear what dispersal distances and dispersal rates are inside the soil matrix. Clearly, dispersal occurs as decomposition of litter proceeds every year, and succession of local species activity through seasons is observable. However, patch dynamics and local immigrations of sapro-trophs in decomposition food webs have received very little attention.

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