BIOL1030 Chap.14 Ecology: Distributions, Niches and Populations
Ecology: Distributions, Niches and Populations
Ecology: Distributions, Niches and Populations covers the Unit 8 lectures that open in Week 10. It starts with why climate alone cannot predict which species live somewhere: fire can lock a site into grassland or forest, and shared climate gives Mediterranean shrublands the same structure but different species because their lineages have different histories.
It then works through the textbook sequence for explaining a missing species, from dispersal and habitat choice to other species and physical tolerance, with grazing urchins on Sydney shores and Connell's barnacles as evidence. The final part turns to process models of population growth: exponential, logistic and time-lagged growth, the carrying capacity K, and the difference between deterministic and stochastic models.
What this chapter covers
- 01
Climate, fire feedback and alternative vegetation states
- 02
Ecosystem structure versus species composition
- 03
Dispersal, behaviour, biotic and abiotic limits to distribution
- 04
Fundamental and realised niches in Connell's barnacles
- 05
Natural experiments and their confounds
- 06
Exponential, logistic and time-lagged growth models
- 07
Lambda for annual breeders and stochastic models
Worked example · free
Explain why a native fish is missing from one creek
- 1Dispersal: ask whether the fish can reach the sixth creek, for example whether a waterfall or culvert blocks movement upstream.
- 1Behaviour: ask whether adults avoid the creek when choosing habitat, for example by offering it in a choice trial.
- 1Biotic factors: check for an introduced predator or competitor, and compare survival in caged and uncaged groups placed in the creek.
- 1Abiotic factors: measure temperature, salinity and water chemistry against the fish's tolerance range.
Key terms
- Fundamental niche
- Every condition and resource a species could exploit, limited only by what its body can tolerate.
- Realised niche
- The part of the fundamental niche a species actually occupies once competitors and other species act on it.
- Carrying capacity
- The largest population an environment can support, written K in the logistic model.
- Natural experiment
- A comparison created by an event that was not designed as an experiment, such as a fence or a fire boundary.
- Stochastic model
- A model that includes chance, so each run differs and results are reported as ranges and probabilities.
Ecology: Distributions, Niches and Populations FAQ
Why can two places with the same climate carry different vegetation?
Because climate is not the only control. Fire can hold a site in a grassy state: grass fuels fire, fire kills tree seedlings, and the grass returns. History matters too, since lineages that never reached a region cannot appear there however suitable the conditions are.
What is the difference between a fundamental and a realised niche?
The fundamental niche is everything a species could use given its physiology, while the realised niche is the narrower range it occupies after competition, predation and other interactions. Connell's barnacles show both: one species is pushed out of the lower shore by a rival, the other is stopped higher up by drying.
Where is logistic growth fastest?
At half the carrying capacity. A small population has few individuals to breed, and a crowded one has little room left, so the product of the two terms peaks at K over two. With r of 0.5 and K of 1,000 that peak is 125 new individuals a year.
Why run a stochastic model many times?
Each run draws different chance events, so one run shows a single possible future. Repeating it hundreds or thousands of times lets you report the average, the spread and how often an outcome such as extinction occurs.
Why do time lags make populations cycle?
With a lag, a population responds to how crowded it was in the past rather than now. It keeps growing after passing K because it is still reacting to an earlier, roomier time, then keeps shrinking below K for the same reason. Fast growth combined with a long delay gives the largest boom-and-bust swings.
Exam move
Learn this chapter as two skills. For distributions, rehearse the four questions in order on organisms you invent, and attach one concrete test to each, so that an exam scenario becomes a checklist rather than a guess. For the models, write each rule in words and in symbols, compute growth at three population sizes by hand, and sketch the J-curve, S-curve and overshoot so you can recognise them on a graph.
Keep the ecology practice questions and the models handout open while you revise, and check every answer against the structure-versus-composition and fundamental-versus-realised distinctions before moving on.
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