Ecology: Chapter 5 — Interactions between species

5.1 Signs, and the signs that lie #

The usual ledger is competition (−/−), predation, herbivory, and parasitism (+/−), mutualism (+/+), commensalism (+/0), and amensalism (−/0 or 0/−). The sign can change with conditions. A mycorrhizal fungus can raise plant growth in poor soil and cost the plant in rich soil. A classification is a start.

Apparent competition (Holt) has the sign of competition and the mechanism of a shared enemy. Prey A increases, the predator increases, and prey B declines. Removing A helps B, which looks like release from competition. The test is whether the predator, not a shared resource, is the link. Two prey of one pathogen give the same pattern.

Intraspecific effects are Chapter 4. Interspecific effects are this chapter. Exclusion in the models below works only because each species also inhibits itself.

5.2 Competition #

5.2.1 Exploitation, interference, and a caution about allelopathy #

Exploitation is depletion of a shared resource. Interference is direct harm: territorial defence, overgrowth, or a chemical. Allelopathy, harm by a released chemical, is real in some plants and is often claimed without a demonstration that the compound, at the concentration found in soil, is what reduces the neighbour. A smell is not a mechanism. These notes require the field concentration and the response. They do not catalogue toxins.

Intraspecific competition is what makes K exist. Among plants, crowded stands lose individuals as the survivors grow. A proposed slope of −3/2 on a log plot of mean mass against density, the self-thinning rule, is a historical generalisation. Measured slopes scatter. It is not an exact law.

5.2.2 Lotka–Volterra competition #

Each species grows logistically, and the other species’ individuals are converted into equivalents by a competition coefficient:

dN₁/dt = r₁ N₁ (1 − (N₁ + α₁₂ N₂) / K₁),

and the symmetric equation for species 2. α₁₂ is the effect of one individual of species 2 on species 1, in units of individuals of species 1. Species 1 can increase when rare if the resident species 2, sitting at K₂, has left it enough resource:

K₁ > α₁₂ K₂.

Species 2 can invade species 1 at K₁ if K₂ > α₂₁ K₁. The four outcomes follow.

Coexistence requires each species to increase when the other is already at its own carrying capacity. When the two carrying capacities are equal, that is α₁₂ < 1 and α₂₁ < 1: each species then inhibits its own growth more than it inhibits the other's. A product α₁₂ α₂₁ < 1 is necessary for that pair of inequalities and is not sufficient on its own, because one coefficient can still exceed 1. The assumptions are logistic growth of each species alone, constant coefficients, a closed and well-mixed system, no other species, no age structure, and a constant environment. Gause's Paramecium cultures are the classic illustration, not a proof that nature is a test tube. P. aurelia excluded P. caudatum on one food. P. bursaria coexisted with P. aurelia by using the culture differently. Change the assumptions and the inequalities do not govern the outcome.

The competitive exclusion principle is the verbal form: two species cannot coexist indefinitely on one limiting resource in a constant, well-mixed environment. The caveats are the content. One resource. Constant. Well mixed. No other interactions. Chapter 6 is what happens when those fail, which in the field is most of the time. The principle is a limiting case that tells you which assumption you broke when you see coexistence.

5.2.3 How coexistence is possible #

Resource partitioning puts species on different resources, or on different parts of one gradient: rooting depth, seed size, feeding height, night and day. Temporal and spatial variation mean the environment is not the constant the model assumed. A competition–colonisation trade-off lets a poor competitor persist if it reaches new gaps before the better competitor does, provided gaps open often enough.

R (Tilman) restates exclusion for a single resource. The species that can hold r* = 0 at the lowest resource level draws the resource down and excludes the other. For two resources, coexistence is possible if each species consumes more of the resource that limits it. That is the modern form of “intraspecific exceeds interspecific.”

Predation that falls more heavily on the competitive dominant can hold it below the density at which it would exclude others. Paine’s seastar is the experiment (Chapter 6). The interaction type belongs here.

The storage effect (Chesson) needs three ingredients: species respond differently to the environment, competition is stronger in favourable periods, and a long-lived stage (a seed bank, a long adult life) stores the gains from a good year through a bad one. Bet-hedging in Chapter 4 is the life-history piece. The storage effect is the community consequence: fluctuating conditions plus storage stabilise coexistence.

Hutchinson’s paradox of the plankton is the exclusion principle applied to a lake and found embarrassing. Many phytoplankton species persist on few resources in water that looks well mixed. Fluctuations, incomplete mixing, predation, and limitation by different factors are all candidate resolutions. The paradox is a problem for the simplest model. It is not a finding that exclusion never happens.

5.2.4 Frequency dependence #

If a species is at an advantage when rare — because its specialist enemy is scarce, or because the resource it uses is then abundant — that advantage is stabilising. It is the invasion criterion in words. An advantage when common is destabilising and produces priority effects, the bottom row of the table above. Evolutionary frequency dependence, including selection for rare mates or rare genotypes, is Evolutionary Biology. The ecological pattern can exist with no genetic change at all, only a change in numbers.

5.3 Predation, herbivory, and parasitism #

5.3.1 The predator–prey equations, and the lynx #

The simplest Lotka–Volterra predator–prey model is

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dN/dt = r N − a N P,

dP/dt = b a N P − m P.

Prey grow exponentially except for predation. Predators decline exponentially except when they convert prey into predator births. The equilibrium is a cycle whose amplitude is set by the starting numbers. The cycles are neutrally stable: a nudge does not return the system to the old orbit or send it to a new kind of behaviour. It puts it on a different orbit. That is a property of these equations, not of nature. Add a prey carrying capacity, or a type II functional response, and the qualitative result changes.

The Rosenzweig–MacArthur model gives the prey a carrying capacity and the predator a saturating functional response. Enriching the prey’s K can then destabilise the equilibrium into a large cycle. That is the paradox of enrichment: a more productive system can be less stable, and the cycle can drive one or both species to very low numbers. It is a model warning. It has been seen in some laboratory systems. It is not a universal field law, and it is not an argument against nutrients in every ecosystem.

The Canada lynx and the snowshoe hare, with a cycle of about a decade in the boreal record, are not a demonstration of the neutral Lotka–Volterra orbit. Food shortage and predation both act on the hare, and the lynx follows the hare. A two-species cartoon drops the plants. Huffaker’s mite experiments make a related point from the other direction. In a simple arena the predators found all the prey and then starved. When the arena gave prey a dispersal advantage and temporary refuges, the interaction persisted. Persistence needed space. The equations in this section have none.

5.3.2 Defences, mimicry, and herbivory #

Prey reduce encounters (crypsis, a refuge in habitat or in size, nocturnality), survive encounters (spines, shells, chemical defence, flight), or dilute them (groups). Group living also makes prey easier to find. The net effect is empirical. Aposematism is a warning signal paired with a defence. Batesian mimicry is a palatable species resembling an unpalatable model. The mimic’s advantage falls as it becomes common, because predators encounter the honest defence less often. Müllerian mimicry is two unpalatable species converging on one signal, so they share the cost of teaching predators. The direction of frequency dependence is the difference. Batesian is stabilising for the mimic’s rarity. Müllerian pushes the two signals together.

Herbivory is consumption of a victim that often survives. Plants defend constitutively or only after damage. Induced defence saves the cost in years when the herbivore is absent, and it pays a lag. The hormones of induction are Plant Physiology. The classes of secondary metabolites are Plant Biology. Compensation, regrowth that replaces what was eaten, occurs in some plants under some conditions. It is not a general absolution of herbivory. Overcompensation, more production when grazed than when ungrazed, is rarer still and is a measured result in particular systems, not a slogan for grazing.

Hairston, Smith, and Slobodkin asked why the world is green if herbivores eat plants. Their answer was that predators keep herbivores rare, so plants compete with each other and herbivores are enemy-limited. That is a hypothesis about a trophic cascade, tested in Chapter 6. Green tissue is also poor food and is defended. The world can be green for more than one reason at once.

5.3.3 Parasites #

A parasite lives in or on a host and harms it. A parasitoid, typically an insect, kills its host as a necessary part of development. Microparasites (viruses, bacteria, many protozoa) are often counted as infected hosts. Macroparasites (worms, parasitic arthropods) are counted as a burden on a host. The immune response is Immunology. The life cycle of Plasmodium is Microbiology. The epidemiological R₀ is Microbiology, and it is still not given a number.

The ecological consequences are these. A parasite can regulate a host. It can mediate apparent competition. It can reverse the outcome of competition if it hits the dominant harder. It can change behaviour and so change who gets eaten. Virulence, the harm done, is not required by any principle to evolve towards mildness. The trade-off hypothesis says transmission and virulence are linked, so a parasite that never harms may also never spread. Support is strong in some systems and absent in others. Vector-borne and water-borne parasites can remain severe because the host does not have to stay mobile to deliver them.

Myxoma virus, released against European rabbits in Australia in the mid-twentieth century, is the historical case. The introduced virus was highly lethal. Later samples of the virus were less lethal, and the rabbits became more resistant. It is a natural experiment in the evolution of virulence and of host defence, told in Evolutionary Biology from the selection side. It is not a protocol, and these notes give none: no strain, no dose, no release method.

5.4 Mutualism and the leftover signs #

5.4.1 Reciprocal benefit, cheating, and the word “symbiosis” #

A mutualism raises the fitness of both partners relative to each partner alone, under the conditions measured. It is often reciprocal exploitation that stays positive. Cheating, taking the reward without paying the cost, is expected whenever a mutant can do it. Mutualisms persist because of sanctions against cheats, partner choice, or vertical transmission that ties the partner’s offspring to the host’s offspring. That evolutionary stability is Evolutionary Biology. The natural history is here.

Obligate mutualists cannot persist apart. Facultative ones can. Symbiosis, as de Bary defined it, means living together. It includes parasites. These notes say mutualist, parasite, or partner, and they use “symbiosis” only with the sign stated. “Symbiotic” used as a synonym of “mutually beneficial” hides the ledger.

5.4.2 Cases worth knowing as ecology #

Mycorrhizae and the rhizobium–legume nodule move limiting nutrients. The structures and the biochemistry are Plant Physiology and Microbiology. The ecological fact is that they change which plants win when nutrients are scarce, and they can move resources among plants that share a fungal network. Whether a given network is a cooperation or a drain is a measurement, not a moral.

Pollination and seed dispersal are mutualisms when the animal moves the pollen or the seed and receives a reward. Syndromes — the matching of flower or fruit traits to a kind of animal — are tendencies, and Plant Biology owns the structures. A flower pollinated by several kinds of visitor has not read the syndrome.

Ant–acacia mutualisms, in which the plant houses and feeds ants that remove herbivores and sometimes neighbouring plants, have a cost: the ants may also drive off pollinators. Fig–fig wasp and yucca–yucca moth systems are pollination by a seed parasite. They are the clearest obligate cases, and the wasp or the moth is also an herbivore of some of the seeds. One interaction, two signs, depending on which currency is counted. Cleaning mutualisms, in which one animal removes parasites from another, are facultative in many reef fishes and are kept honest, imperfectly, by the client’s ability to leave.

5.4.3 Commensalism and amensalism #

Commensalism is often a mutualism or a parasitism that has not been measured. An epiphyte that only occupies a branch is the textbook commensal, and a heavy load of epiphytes is sometimes costly. Amensalism is harm with no benefit to the harmer: trampling, or a chemical effect that does not help the source. If the sign has not been measured, these notes do not assign one. A zero in the ledger is a result, not a default.

Key terms. Apparent competition; exploitation; interference; competition coefficient; competitive exclusion; R*; storage effect; paradox of the plankton; neutrally stable cycle; paradox of enrichment; Batesian mimicry; Müllerian mimicry; parasitoid; mutualism; symbiosis.

Summary. Signs classify interactions and mechanisms test them. Apparent competition looks like resource competition and is a shared enemy. Lotka–Volterra competition gives coexistence only when each species can invade the other, which is intraspecific limitation exceeding interspecific limitation. Exclusion on one resource in a constant mixed system is a limiting case; partitioning, fluctuation, storage, and predators on the dominant are how real coexistence happens. The simplest predator–prey cycles are neutral and structurally fragile. Type II responses and enrichment can destabilise; refuges and type III responses can stabilise. Virulence need not evolve to mildness. Mutualism is reciprocal fitness benefit with cheating held in check, and symbiosis is not its synonym.

Self-check.

1. Two plants both decline when either is common, and removing a shared herbivore stops the pattern. Is this resource competition or apparent competition, and which result decides it?

2. State the condition for species 1 to invade species 2 at K₂. Why does coexistence in this model require each species to limit itself more than it limits the other?

3. Why are Lotka–Volterra predator–prey cycles called neutrally stable, and why do lynx and hare not confirm that equation?

4. How does the frequency dependence of Batesian mimicry differ from that of Müllerian mimicry?

5. Why is a fig wasp both a mutualist and a seed predator? Why is “symbiosis” alone an inadequate description?