Comprehensive Notes – Tolerance, Biological Timing & Orientation

Tolerance range = comfort zone
- Zone of optimum physiological performance ➔ highest probability of survival, reproduction, & allele transmission.
- Plants/animals lacking human‐like behavioral or technological buffers must remain inside this range to retain their adaptive advantage.
Width of tolerance matters
- Wide range → “hardy” species capable of withstanding greater abiotic fluctuation.
- Wide range → broad geographic distribution (e.g., Tilapia in many African lakes).
- Narrow range → restricted distribution & higher extinction risk.
Extremes become limiting factors
- As conditions approach upper or lower boundary of tolerance, stress reduces energy for growth & reproduction.
- ALWAYS link abiotic stress back to reduced reproductive success.
Human example
- Optimal ambient temperature $20–30\,^{\circ}\text{C}$ .
- Heat/cold outside this span produces physiological stress; lethal at extremes.

Yearly (≈12-month) cycles driven by endogenous biological clocks but synchronised by external zeitgebers (especially photoperiod & temperature).
Major plant processes controlled
- Leaf fall / abscission (e.g., Oak, Liquid Amber) initiated by decreasing photoperiod.
- Winter dormancy (e.g., European Beech) initiated by decreasing day length + lower temperature.
- Flowering induced by species-specific photoperiod thresholds.
- Examples: Namaqualand daisies (short-day bloom), Pōhutukawa (long-day summer bloom).
- Emission of floral scents timed to coincide with pollinator activity (e.g., Sausage tree releasing strong odour at night for bat pollination).
- Seed maturation & dispersal coordinated with favourable seasonal conditions (e.g., Zing-Zing tree, Tipuana tree).
- Vernalisation-dependent seed germination: requirement for prolonged cold followed by warmth (e.g., Tree fuchsia, Cabbage tree) ensures seedlings emerge in spring.
Adaptive significance
- Maximises pollination success, seed set, and seedling survival by matching life-history events to predictable environmental windows.

Key wavelengths
- Daylight rich red light at $\lambda \approx 665\,\text{nm}$ .
- Twilight/night dominated by far-red light at $\lambda \approx 725\,\text{nm}$ .
Phytochrome pigment system (detected mainly in leaves)
- Exists in two interconvertible forms:
- $\text{Pr}$ – absorbs red light ( $660\,\text{nm}$ ) → converted to $\text{P}_{\text{fr}}$ .
- $\text{P}_{\text{fr}}$ – absorbs far-red light ( $730\,\text{nm}$ ) → converted back to $\text{Pr}$ .
- Light-dependent shape change operates as an internal hour-glass that measures length of darkness.
Diurnal dynamics
- Day: $\text{Pr} \xrightarrow{\text{red}} \text{P}_{\text{fr}}$ accumulates.
- Night: Slow reversion $\text{P}_{\text{fr}} \rightarrow \text{Pr}$ (via far-red absorption + thermal decay).
- Dawn ratio of $\text{Pr} : \text{P}_{\text{fr}}$ encodes length of preceding night.
Graphs (slides 13–15) illustrate % absorbance drop of $\text{P}_{\text{fr}}$ across night versus day.

Crucial variable is uninterrupted darkness, NOT daylight length.
Categories
1. Short-Day (Long-Night)
- Flower when night > critical length (≈ autumn/winter).
- Morning ratio: high $\text{Pr}$ / low $\text{P}_{\text{fr}}$ .
- Typical example: Chrysanthemum (~10 h photoperiod requirement).
1. Long-Day (Short-Night)
- Flower when night < critical length (≈ late spring/summer).
- Morning ratio: higher $\text{P}_{\text{fr}}$ retained.
- Examples: Petunia, Radish, Lettuce (~14 h daylight; failure to flower in tropics where 12 h nights exceed critical value).
1. Day-Neutral
- Flowering independent of photoperiod, regulated by other cues (e.g., temperature or developmental age).
Night-break experiments
- A few minutes of light during the night reconverts $\text{Pr} \rightarrow \text{P}_{\text{fr}}$ .
- Consequences
- Short-day plants: flowering inhibited (night effectively shortened).
- Long-day plants: flowering induced (night kept below critical length).

Definition: "For each abiotic factor an organism has a range within which it can survive."
Graphical zones (Slide 29)
- Unavailable niche (lethal extreme), Marginal niche (stress), Preferred/optimum niche (max fitness).
Abiotic variables influencing realised niche size
- Temperature, pH, salinity, water availability, light intensity, photoperiod, etc.
Always evaluate survival → reproductive success → gene flow.

Environment – all physical, chemical, & biotic factors acting on an organism.
Habitat – the specific place an organism lives.
Community – interacting group of various species (biotic only).
Niche – the role/function of a species ("its job").
Species – organisms able to interbreed & produce fertile offspring.
- Horse × Donkey → Mule (infertile) ⇒ two distinct species.
Population – members of the same species occupying the same habitat & time, capable of interbreeding.
Hierarchy recap: $\text{Organism} \subset \text{Population} \subset \text{Community} \subset \text{Ecosystem} \subset \text{Biosphere}$ .
Mnemonic summary (Slide 40)
- Environment = factors; Habitat = place; Community = living interactions; Niche = job; Species = babies work; Population = can reach each other.

Orientation responses (Topic section 1)
- Movements relative to abiotic cues (taxis, kinesis, etc.) – covered later.
Homing
- Regular return to a fixed site (nest, burrow, colony) after foraging or seasonal outing.
- Frequency: daily or episodic.
- Distance: often local to moderate.
- Usually involves all adults (e.g., pigeons, salmon returning to natal river).
Migration
- Persistent, straightened‐out movement between two habitats, typically seasonal, involving population or specific cohorts.
- Frequency: annual or lifetime.
- Distance: can be thousands of kilometres (monarch butterflies, Arctic terns).

Need for accurate navigation over complex or long routes (Slides 49–52).
Visual cues / landmarks
- Sequential orientation from one known feature to the next ("piloting").
- Effective over shorter distances ➔ common in homing.
- Multi-sensory landmarks: shapes, colours, odours, sounds.
Additional methods to be covered later (magnetic, celestial, olfactory, solar compass, path integration, etc.).

Hypothesis: Stripes deter biting flies.
Experiment: Paint cows with black & white stripes.
Result: >50 % reduction in fly landings.
Critical thinking prompt: “What extraneous variables might also reduce bites?” (e.g., paint chemicals, altered cow behaviour, observer bias).
Illustrates necessity of control treatments and replication.

Re-read SciPAD pages 54 & 99 for worked examples (leaf abscission, tolerance graphs).
Practise drawing tolerance curves and annotating where fitness decreases.
For photoperiodism, memorise the logic table:
- Long night = high $\text{Pr}$ ; Short night = high $\text{P}_{\text{fr}}$ .
Perform thought experiments: “Greenhouse night break – predict flowering outcome for each category.”
Relate every adaptation back to successful reproduction ➔ evolutionary fitness.