Ecology, Ecosystems, and Conservation

This is Unit Fourteen of Biology, Part Two: community ecology and behaviour. Part One dealt with single populations — how they are counted and how they grow. This part is what happens when populations of different species occupy the same place, and then how individual animals behave within that. The through-line is that an interaction is defined by its outcome rather than by how it looks: two species in close contact are not necessarily harming each other, a declining population is not necessarily being eaten, and a behaviour that appears self-sacrificing is not necessarily against the individual's interest. By the end you will be able to say what competition requires and what it does not; say what classifies a symbiosis; say what triggers competitive exclusion; tell a proximate cause from an ultimate one; and say why sterile workers persist.

Three entries. Two species wanting the same thing, one species eating another, and two species living in close and lasting contact.

Wanting the same thing.

Bio 3591 Community Interactions: Competition

Competition is a community interaction where two species or individuals use the same limited resource, reducing the fitness of both, whether through direct interference or indirect resource depletion. Two plant species drawing on the same limited soil nitrogen compete without any direct contact. Competition need not involve physical confrontation — depletion alone produces the same fitness cost as interference.

Eating another.

Bio 3593 Predation and Herbivory

Predation is an interaction where a predator kills and consumes another organism, the prey, while herbivory is the analogous interaction where an animal consumes plant tissue. A wolf killing a deer is predation; a caterpillar eating leaf tissue without killing the plant is herbivory. Herbivory usually does not kill the plant, unlike predation's typical outcome for the prey.

Living together.

Bio 3595 Symbiosis: Mutualism, Commensalism, and Parasitism

Symbiosis is a close, long-term relationship between two species: mutualism when both benefit, commensalism when one benefits and the other is unaffected, parasitism when one benefits at the other's expense. A tapeworm consuming a host's nutrients is parasitism; a bird nesting harmlessly in a tree is commensalism. Symbiosis names any close relationship — it does not specify which outcome applies.

Two entries, a misconception and its correction, and the distinction the last entry set up. Closeness is not the same as harm, and the test that tells them apart is not about how intimate the relationship looks.

The assumption.

Bio 3599 "Any Close Species Relationship Is Parasitic"

Assuming any close, sustained relationship between two species must be parasitic overstates a real pattern — symbiosis includes mutualism and commensalism as equally valid, non-parasitic outcomes. A pollinator and the flowering plant it visits both benefit, fitting mutualism rather than parasitism. The test this assumption overlooks comes next.

Check the fitness.

Bio 3601 Fitness Outcome, Not Closeness Alone, Classifies a Symbiosis

Classifying a symbiotic relationship requires checking each species' fitness outcome — closeness alone does not indicate mutualism, commensalism or parasitism. A cleaner fish and the fish it grooms both gain, so it classifies as mutualism despite one depending on visiting the other. The parasitic assumption ignores that many intimate pairings are mutualistic.

Two entries. Why two species that need exactly the same things cannot both persist, and the arrangement that lets very similar species live side by side anyway.

Identical needs.

Bio 3605 Competitive Exclusion and the Ecological Niche

The competitive exclusion principle holds that two species occupying the identical ecological niche — the full set of resources and conditions a species uses — cannot stably coexist. Two species needing the same limited food source and nesting site cannot both persist long-term. Species sharing a habitat routinely coexist by dividing resources along another axis: full niche overlap, not habitat overlap, triggers exclusion.

Dividing it up.

Bio 3607 Resource Partitioning

Resource partitioning is the differentiation of niches — in space, time or resource type — allowing similar competing species to coexist by reducing the overlap competitive exclusion would otherwise penalise. Several warbler species feeding in different vertical zones of the same tree partition its insects by location. Partitioning reduces competitive overlap; it does not eliminate the underlying competition.

Four entries. How a community rebuilds after disturbance, who arrives first and what they make possible, the two different questions any behaviour can be asked about, and the chapter's worked interpretation of a population that is falling for no obvious reason.

With soil, or without.

Bio 3609 Primary and Secondary Ecological Succession

Primary succession begins on newly exposed substrate with no prior soil or life, such as bare rock; secondary succession begins where soil remains after life is disturbed or removed. Bare rock left by a retreating glacier undergoes primary succession, a burned forest with intact soil secondary. Secondary succession proceeds faster mainly because soil, the slowest resource to rebuild, is already present.

First arrivals.

Bio 3613 Pioneer Species and Facilitation

Pioneer species are the first to colonise bare substrate during primary succession, and facilitation is the process by which their presence, often by building soil, makes colonisation by later species possible. Lichens on bare rock slowly break it down, facilitating the mosses and small plants that follow. Facilitation describes pioneers enabling later colonisers, not assisting any specific one.

Two questions.

Bio 3615 Proximate versus Ultimate Causation in Behavior

A proximate cause explains the immediate physiological or environmental trigger of a behaviour, while an ultimate cause explains the evolutionary reason selection favoured it. Shortening day length is the proximate trigger for bird migration; avoiding winter scarcity is its ultimate explanation. The two answer different questions about the same behaviour — neither substitutes for nor contradicts the other.

Why is it falling.

Bio 3617 Interpreting a Population Change Through Community Interactions, Worked

A plant population grew rapidly after colonising a disturbed field and is now declining, with no new predator or disease. The early growth fits the exponential phase of an early coloniser. The decline, with no predator, points first to density-dependent competition as the population neared carrying capacity — light, water and nutrients tightening. The timing also fits secondary succession, with later arrivals outcompeting the first colonisers. Telling self-limitation from successional replacement means checking resource overlap with a newly abundant competitor. Assuming a decline means predation is the error.

Five entries. Behaviour that arrives complete and needs no experience, and then four ways experience changes what an animal does — one that weakens a response, one locked to a narrow window of life, and two that build associations.

Complete on arrival.

Bio 3619 Innate Behavior and Fixed Action Patterns

Innate behaviour is inherited and performed correctly without experience, and a fixed action pattern is a stereotyped sequence that, once triggered, runs to completion. A graylag goose retrieves an egg-shaped object with the same rolling motion even after the object is removed mid-retrieval. The pattern runs to completion once triggered — removing the stimulus does not stop it.

Learning to ignore.

Bio 3621 Habituation

Habituation is a simple form of learning in which an animal's response to a repeated, harmless stimulus weakens over time. A prairie dog stops giving alarm calls at a scarecrow once repeated exposure shows it poses no threat. Habituation is a decreased response to one specific stimulus, not general fatigue affecting all responses.

A narrow window.

Bio 3623 Imprinting

Imprinting is a rapid, irreversible learning restricted to a critical early-life period, in which an animal attaches to the first suitable stimulus it encounters. Lorenz's goslings followed him after hatching because he was the first moving object they saw during their critical period. Imprinting occurs only within that narrow period — the same stimulus presented later fails to produce it.

A bell, and a reflex.

Bio 3625 Classical Conditioning

Classical conditioning is learning in which a neutral stimulus, repeatedly paired with one that already triggers a response, comes to trigger that response alone. Pavlov's dogs salivated at a bell alone after the bell was repeatedly paired with food. It links a new stimulus to an existing involuntary response — it does not teach a new behaviour.

Consequences.

Bio 3627 Operant Conditioning

Operant conditioning is learning in which a voluntary behaviour's frequency changes because of the consequence — reinforcement or punishment — that follows it. A rat presses a lever more often after one press delivered a food pellet. Reinforcement increases a behaviour's frequency and punishment decreases it, and both can be achieved by adding or by removing a stimulus.

Four entries. How an animal decides what to eat, how it signals to others, how mating arrangements follow from investment, and then the case that looks like selection working against the individual — and is not.

Worth the chase.

Bio 3629 Foraging and Optimality

Optimal foraging theory predicts that selection favours foraging which maximises net energy gain per time spent searching, handling and consuming food. A predator preferentially attacks the prey size yielding the greatest energy return for pursuit and handling time, rather than the largest prey available. Optimal foraging is favoured by selection, not consciously calculated by the animal.

Chemical messages.

Bio 3631 Communication and Pheromones

Animal communication transmits information between a signaller and a receiver, often through a pheromone — a chemical released to trigger a response in another individual of the same species. A foraging ant lays a pheromone trail other ants follow to a food source. A pheromone signals within a species; a chemical cue read by a different species is not one.

How many mates.

Bio 3633 Mating Systems

A mating system describes how many mates an individual takes: monogamy pairs one male with one female, polygyny one male with several females, polyandry the reverse. Where male investment is far lower than female investment, a species tends toward polygyny, since one male can father many broods. A mating system reflects parental investment rather than a fixed taxonomic trait.

Sterile, and favoured.

Bio 3635 Eusociality and Kin Selection

Eusociality is cooperative brood care, overlapping generations and division of labour, and kin selection explains sterile workers through inclusive fitness. A sterile worker bee raises her queen's brood, sharing many of her own genes and raising inclusive fitness despite zero direct reproduction. Selection does not act for the species' good — sterility persists because it raises the worker's own gene copies, not because it benefits the colony.

Five things to carry out of this part. Competition requires only that two species or individuals draw on the same limited resource, reducing the fitness of both — it does not require any physical confrontation, since depletion alone imposes the same cost. What classifies a symbiosis is the fitness outcome for each species, not how close or how dependent the relationship looks: mutualism where both gain, commensalism where one gains and the other is unaffected, parasitism where one gains at the other's expense, and a great many intimate pairings turn out mutualistic. Competitive exclusion is triggered by full niche overlap rather than shared habitat, which is why similar species routinely coexist by partitioning resources in space, time or type — reducing the overlap without eliminating the competition underneath. A proximate cause is the immediate trigger of a behaviour and an ultimate cause is the evolutionary reason selection favoured it, and they answer different questions rather than competing to answer the same one. And sterile workers persist because raising close relatives propagates the worker's own genes through inclusive fitness — selection does not act for the good of the species, and reading eusociality that way inverts the mechanism. Part Three takes up ecosystems: energy flow, the nutrient cycles, and biodiversity.