Ecological Succession

Differentiate between primary and secondary succession, and indicate which each of these cases corresponds to, justifying your answer: the colonisation of a recent lava flow and the recovery of a pine forest after a fire.

Explain what secondary succession is and name two frequent disturbances in ecosystems in the Dominican Republic that trigger it, indicating what remains of the previous ecosystem that speeds up recovery.

Explain what the soil seed bank is and why its presence makes recovery after a fire much faster than the colonisation of a completely new surface.

Define ecological succession and explain what pioneer species and climax communities are, giving a real example of a pioneer species and the trait that allows it to establish itself first.

Order these communities in a successional series from earliest to latest and explain what each contributes to the one replacing it: mature forest, lichens and mosses on rock, grasslands, and scrubland.

Define climax community and explain why it is considered the most stable and complex state of a successional series, indicating what its biomass and biodiversity are like compared with the initial stages.

Name two examples of pioneer species able to settle on bare rock and explain two traits (resistance to desiccation, effective dispersal, low nutrient requirements) that allow them to arrive first.

On a newly formed coastal dune, grasses such as marram grass settle first, followed by shrubs and finally pines. Order this successional series and explain what each stage contributes to the next (sand stabilisation, input of organic matter, shade).

The diagram shows, out of order, the four stages (A to D) of a primary succession on bare rock. Arrange them chronologically and explain how the soil, biomass, and biodiversity change throughout the series.

In a primary succession on rock, the soil's organic matter increases century after century. Explain what each community (lichens, mosses, herbaceous plants, scrub, forest) contributes to that accumulation and how the increase in soil relates to the increase in biodiversity.

The eruption of Krakatoa (1883) left the island almost sterile, yet within a few decades it was covered in vegetation again. Explain what type of succession occurred, which agents (wind, sea currents, birds) brought the first colonisers, and why the distance to the neighbouring islands determined the order in which species arrived.

Marram grass (Ammophila arenaria) stabilizes the mobile dunes of the coastline. Explain the phenomenon of ecological facilitation using this example: how the pioneer plant modifies the wind, the sand and the moisture, making the arrival of more demanding species possible.

Describe the three mechanisms ecologists propose to explain species replacement during succession — facilitation, tolerance and inhibition — and give a possible example of each in the recovery of an abandoned field.

The abandonment of cropland in a Mediterranean region is followed by the appearance of thyme fields and scrubland. Interpret this process as a succession, predict which community could establish itself in the long term, and comment on what disturbances could reset the series.

Two pine forests burn: one in a low-intensity fire that spares the soil, and another in a severe fire that also burns the seed bank and the humus. Compare the successional pathways of both after the fire and discuss which will take longer to recover and why.

The abandonment of farmland in rural areas of the Dominican Republic is covering former croplands with forest. a) Describe the expected successional series from stubble to forest. b) Analyze which factors can slow down or divert the series (herbivory, fires, prior soil erosion). c) Discuss the two management strategies: letting nature take its course (rewilding) versus replanting with chosen species.

A shallow coastal lagoon receives continuous inputs of nitrates and phosphates from agricultural sources. The following table shows the ecosystem parameters before the discharges and 20 years later:

1. a) In normal maturational ecological succession, biomass accumulates, the $P/B$ ratio decreases, and food webs become more complex. Based on the data, justify why the eutrophication process is functionally interpreted as an "ecological regression" or regressive succession.
2. b) In Year 21, agricultural runoff is completely diverted, eliminating the external nutrient input. However, 10 years later the lagoon still exhibits turbid waters and bottom anoxia. Formulate a biogeochemical hypothesis based on sediment-water feedback (ecological hysteresis) to explain why the ecosystem does not return to its initial state.

In a quarry abandoned 60 years ago, two vegetation-free environments emerged: an artificial pond 3 m deep created when the water table was breached (Aquatic Environment) and a vertical cliff face of compact limestone (Rocky Environment).

Today, the Aquatic Environment has lost 80% of its open water surface, transforming into a reed bed with 2.2 m of organic sediments and a high rate of biomass accumulation. In contrast, in the Rocky Environment, more than 90% of the cliff face remains dominated by crustose lichens and a few chasmophytic clumps in isolated crevices.

1. a) Justify, taking into account water availability, nutrient retention, and biotic feedback loops, why the hydrosere (infilling) progresses on a decadal timescale while the xerosere on rock requires centuries or millennia.
2. b) Analyse the role of helophytes (such as Phragmites or Typha) in the sediment balance of the pond: in what way does their own photosynthetic and structural activity turn the ecosystem into a sink that hastens its own transition toward a terrestrial community?
3. c) If an invasive herbivorous species such as the red swamp crayfish (Procambarus clarkii) is introduced into the pond, massively destroying macrophyte beds and disturbing the bottom sediment, predict with justification how the rate of infilling and water transparency will change.

In the Sierra de Guadarrama National Park, the ecotone located at an elevation of 1,600 m between Pyrenean oak (Quercus pyrenaica) and Scots pine (Pinus sylvestris) forests has been monitored since 1980. Over the past 45 years, the regional average temperature has risen by 1.5 °C and summer droughts have become longer. Forest surveys reveal the following changes:

• Scots pine recruitment (established seedlings) has fallen by 70% at that elevation, and adult trees are suffering decline and pests.

• Acorns dispersed uphill by corvids germinate successfully: the density of juvenile Pyrenean oaks has increased by 85% beneath the pine canopy.

1. a) Interpret these results in light of the ecological concepts of climax community: why does the conception of climax as a "final and permanent state of equilibrium" become unsustainable in the face of global change? Contrast Clements' organismic view of climax with Gleason's individualistic model.
2. b) Predict what will happen to the psychro-xerophytic high-summit grasslands (above 2,200 m) as shrub and tree formations shift their optimum toward higher elevations. Justify why this phenomenon is described as an "escalator to extinction".
3. c) Two management criteria for the park are in conflict: 1) Artificially treating and watering the pine forests at 1,600 m to conserve the botanical composition that motivated the park's declaration; 2) Facilitating the natural transition toward Pyrenean oak forest. Argue which of the two stances is ecologically consistent with successional dynamics.