Zonation, succession and change in ecosystems practice questions.

A grazed upland pasture is abandoned. Within a year herbs and grasses cover the site, shrubs follow, and after several decades woodland develops. This sequence is

• primary succession, because there were no trees on the site at the start
• zonation, because the vegetation changes along the slope
• secondary succession, because soil and a seed bank were already present
• a plagioclimax, because the grazing animals were removed

Answer: secondary succession, because soil and a seed bank were already present

Whether a succession is primary or secondary depends on the starting surface, not on the vegetation present: soil, nutrients and a surviving seed bank make secondary succession far faster than colonisation of bare rock.

As a sere approaches its climax community, the relationship between gross primary productivity (GPP) and community respiration (R) changes so that

• GPP falls far below R, so total biomass declines steadily
• GPP and R become approximately equal, so biomass stops increasing
• GPP rises far above R, so net productivity reaches its maximum
• R falls to almost zero, so all the energy fixed is stored as biomass

Answer: GPP and R become approximately equal, so biomass stops increasing

Through the earlier stages of a sere GPP exceeds R, so the biomass storage fills; at the climax the community respires almost everything it fixes, so net productivity approaches zero while standing biomass is at its largest.

A transect across a rocky shore recorded on a single day shows distinct bands of seaweed and limpet species from the low-water mark up to the splash zone. This banding is best described as

• primary succession driven by the age of the rock surface
• secondary succession following the disturbance caused by winter storm waves
• a plagioclimax maintained by grazing limpets
• zonation caused by a gradient in exposure to air and desiccation

Answer: zonation caused by a gradient in exposure to air and desiccation

Zonation is a spatial pattern along an abiotic gradient recorded at one moment, whereas succession is change in one place over time; a survey taken on a single day cannot demonstrate succession.

Heather moorland in upland Britain is maintained by regular burning and sheep grazing; without that management birch and pine woodland would develop. The moorland is therefore

• a plagioclimax, because human activity arrests the sere before the climax
• a climatic climax, because the upland climate is too cold and wet for trees
• a pioneer community, because heather is the first species to colonise
• an example of zonation, because heather occupies a particular altitude

Answer: a plagioclimax, because human activity arrests the sere before the climax

The label depends on what arrests the sere: where the arresting factor is human, such as burning, grazing or mowing, the stable community is a plagioclimax and it reverts to woodland once the management stops.

Woodland is felled and the ground is left bare; nearby, a lava flow cools to bare rock. Vegetation cover returns to the felled woodland far faster than to the lava. The main reason is that the felled site retains

• a warmer local climate than the lava flow
• a higher rainfall than the surrounding area
• soil together with a seed and root bank
• a larger number of grazing animals nearby

Answer: soil together with a seed and root bank

Secondary succession starts from an existing soil storage with buried seeds, roots and soil organisms, so the slow soil-building stages are skipped entirely. Primary succession on bare rock has to generate the soil first, which is why it is measured in centuries rather than decades.

Marram grass is among the first plants to establish on mobile sand at the back of a beach. Which of its traits most directly allows it to do so?

• It needs a deep humus-rich soil to germinate
• It survives burial and binds the sand with rhizomes
• It requires permanently moist, sheltered ground away from wind
• It grows only where salt spray never reaches it

Answer: It survives burial and binds the sand with rhizomes

A pioneer must tolerate the conditions that exclude everything else, and on a dune those are burial, drought and salt. By trapping and binding sand the marram also changes the site, which is what allows the next species in and makes it a pioneer rather than merely a survivor.