3.1 Conditions and resources #
3.1.1 The distinction Begon uses #
A condition is an abiotic factor an organism experiences and does not consume. Temperature and pH are conditions. A resource is consumed, and use by one individual leaves less for another. Light intercepted, prey eaten, nitrate taken up, and space occupied are resources. Water is a resource for a terrestrial plant because transpiration spends it. Oxygen is a resource for an aerobe because respiration spends it. The temperature of the same water is still a condition. The distinction matters: individuals compete for resources. They do not compete for a temperature, though a temperature can decide who wins a competition for something else.
3.1.2 Tolerance curves, acclimation, and ecotypes #
Performance against a condition is often unimodal. There is a lower lethal limit, a range in which survival, growth, or reproduction is possible, an optimum, and an upper lethal limit. The three optima need not coincide. A temperature that maximises growth can shorten life. Shelford’s law is this curve read as a distribution: a species is absent where a condition is outside its tolerance, but absence inside the tolerance still needs a biotic explanation.
Gause'S Experiment And Fitted Logistic Curves
Acclimation is a reversible shift of the curve within one lifetime, for example a change in enzymes or in membrane lipids after a cool period. Adaptation in the evolutionary sense is a genetic change across generations, and the test for it is Evolutionary Biology. A phenotypic plasticity is the set of phenotypes one genotype can produce. It includes acclimation and also developmental responses that are not later reversed.
Turesson called a genetically distinct population from a particular habitat an ecotype. Clausen, Keck, and Hiesey’s transplant experiments, moving alpine and lowland plants among gardens, showed that some of the differences survived the new garden and were therefore genetic, and that some did not and were plastic. A plant that is short on a mountain is not, on that fact alone, an alpine ecotype. The transplant is the test. Common-garden logic is the same idea: raise individuals from different places in one environment, and the differences that remain are genetic. Maternal effects and the particular garden’s conditions are the reasons a single garden is not the whole answer.
3.1.3 Endotherms and ectotherms, only as ecological consequences #
The machinery of temperature regulation is Animal and Human Physiology. The ecological consequences are these. An endotherm pays a high respiratory cost to hold a body temperature and can remain active in the cold. An ectotherm’s activity tracks the environment, and its production efficiency is often higher because it does not pay that heat bill. “Often” is required: a basking ectotherm in a warm, food-rich site is not a general law of superiority. Torpor, hibernation, migration, and seasonal reproduction are ecological solutions to a temporal mismatch between conditions and resources. The hormonal control of any one of them is physiology.
3.2 How resources are taken #
3.2.1 What autotrophs and heterotrophs hand to later chapters #
Photoautotrophs fix carbon using light. The light reactions, the Calvin cycle, and the C4 and CAM variants are Plant Physiology. Chemoautotrophs, including those at hydrothermal vents, are Microbiology. These notes use the accounting. Gross primary production is the carbon fixed. Net primary production is what remains after the producer’s own respiration. Heterotrophs live on organic matter that already exists. A food web is that dependence drawn out. The full definitions, including net ecosystem production, are Chapter 7, because they are properties of fluxes, not of one individual’s diet.
3.2.2 Functional and numerical responses #
Holling distinguished three functional responses, the amount a predator (or herbivore, or parasitoid) consumes as prey density rises.
| Type | Shape | Ecological consequence in simple models |
|---|---|---|
| I | Linear up to a plateau | A filter-feeder idealisation; handling does not slow the rise until the plateau |
| II | Decelerating, then a plateau | Handling time. Per-capita risk to the prey is highest when prey are rare, which destabilises predator–prey models |
| III | Sigmoid | Low attack rate at low prey density, from learning, switching, or a refuge. That refuge can stabilise the interaction |
The per-capita risk is the point students miss. A type II predator that is hardly ever handling, because prey are scarce, spends its time searching. Each remaining prey is at greater risk, not less. A type III response reverses that at the low end.
The numerical response is the change in predator numbers as prey numbers change. It is demographic, so it lags. Aggregation of predators where prey are locally dense is a spatial numerical response and can protect prey in the patches the predators have not found.
3.2.3 Foraging models and what they assume #
Optimal foraging treats a choice as a currency maximised under constraints. The usual currency is energy per unit time. The marginal value theorem (Charnov) says leave a patch when the intake rate inside it falls to the average rate for the habitat, including travel. If travel is long, the forager should stay longer and accept a lower leaving rate. The animal in the theorem knows the habitat average, travel has a cost, and predation risk is absent unless it is added to the currency. Real foragers fail the simplest prediction in informative ways: they leave earlier when danger is high, or they select nutrients rather than calories. A giving-up density, the food left when a forager quits a patch, is the field trace of the leaving rule. The theorem is a null for behaviour, in the same spirit as Chapter 1’s null models.
3.3 The niche #
3.3.1 Three uses of one word #
Grinnell’s niche is the set of habitats and requirements: the species’ place in the environment, close to what would now be called its abiotic distribution. Elton’s niche is the role, especially the trophic role: what it eats and what eats it. Two species can share a Grinnellian habitat and split an Eltonian role, or the reverse. Hutchinson’s niche is an n-dimensional hypervolume of conditions and resources. The fundamental niche is the volume in which the population can persist without negative interactions, in the original wording. The realised niche is the volume it occupies once interactions are included.
A modern restatement is worth using because it can be measured. The niche is the set of environments in which the intrinsic rate of increase is at least zero. Persistence, not a job description, is the criterion. A species does not “fill its niche” as a person fills a post. If the population cannot replace itself, that combination of conditions is outside the niche, however suitable it looks.
3.3.2 Realised is not always a subset #
Competition and predation usually shrink the occupied volume inside the fundamental niche. Connell’s barnacles (Chapter 6) are the standard case: the upper-shore species can live lower down when the lower-shore competitor is removed. Facilitation does the opposite. A nurse plant, a mycorrhizal fungus, or a canopy that softens frost can let a population persist where it cannot persist alone. If the fundamental niche was defined on abiotic axes only, the occupied range can extend beyond it. Bruno and colleagues made this correction. “Realised inside fundamental” is true for negative interactions and unfinished as a general rule.
Mutualists create a further case. An obligate pollinator’s absence is an environment in which r is negative, so the partner is an axis of the niche, not an optional decoration.
3.3.3 Breadth, overlap, and the mistake overlap invites #
Niche breadth is the range of an axis the population uses. A specialist is narrow on the axis that matters. A generalist is broad. The labels are axis-specific. A bee can be broad in habitat and narrow in the pollen it can feed its larvae.
Overlap on a resource axis is necessary for exploitation competition if that resource limits both species. It is not sufficient to prove that competition is happening. High overlap may mean the resource is abundant and is not limiting. Low overlap may be the ghost of past competition, or it may be two species that never shared a limit. Character displacement, a greater difference where ranges overlap than where they do not, is evidence of past competition only after heritability, a shared limiting resource, and simpler explanations (sorting of species that were already different) have been checked. The finch measurements usually cited are Evolutionary Biology. The standard of evidence is the point here.
Key terms. Condition; resource; tolerance curve; acclimation; ecotype; functional response; numerical response; marginal value theorem; fundamental niche; realised niche; niche overlap.
Summary. Organisms compete for resources, which are consumed, and they tolerate conditions, which are not. Performance curves are typically unimodal, and a transplant or a common garden is what separates a genetic ecotype from a plastic response. Type II functional responses destabilise simple predator–prey models because rare prey suffer higher per-capita risk; type III responses can do the reverse. Grinnell, Elton, and Hutchinson meant different things by niche. The usable modern version is the set of environments where r is at least zero. Overlap does not prove competition, and facilitation can put a species outside its purely abiotic fundamental niche.
Self-check.
1. Why is temperature a condition and nitrate a resource? Which of the two can be the object of exploitation competition?
2. A species from a cold site stays small when grown in a warm garden, and a warm-site population stays large. What does that show, and what would the opposite result have shown?
3. In a type II functional response, why is the per-capita risk to prey higher, not lower, when prey are rare?
4. State the marginal value theorem’s leaving rule, and one assumption that fails if the forager is also avoiding predators.
5. Give a case in which the occupied range is smaller than the fundamental niche, and a case in which facilitation makes “smaller” the wrong expectation.
