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 a secondary succession is and name two frequent disturbances in Spanish ecosystems 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.

Marram grass (Ammophila arenaria) stabilises 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.

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.

In a protected natural area in southeastern Iberia, two adjacent sectors coexist on the same type of limestone substrate:

• Sector X: has experienced recurrent fires every 15-20 years over the last half century; it is occupied by rockrose-rosemary scrubland with an abundant soil seed bank and resprouter species.

• Sector Y: has not recorded fires in more than 120 years; it hosts a dense holm oak forest (Quercus ilex subsp. ballota) with sparse understory and very low diversity of annual herbaceous plants.

1. a) A technical report claims that Sector X is "ecologically degraded" and that the management goal should be to force its transition to the holm oak forest of Sector Y. Evaluate this statement by contrasting the classical equilibrium model (single climax) against the continuous disturbance dynamics model (mosaic of alternative stable states).
2. b) If during an extreme drought a large, high-intensity wildfire sweeps through both sectors equally, predict with justification which of the two sectors will show greater resilience (rate of plant cover recovery and soil retention) during the first three post-fire years.

In two neighbouring Mediterranean Aleppo pine (Pinus halepensis) woodlands with an understorey of mastic tree (Pistacia lentiscus, resprouter) and gum rockrose (Cistus ladanifer, obligate seeder), two management models have been applied for 45 years:

• Woodland A: Total and strict suppression of any fire outbreak ("zero fire") without grazing or pruning.

• Woodland B: Controlled goat grazing and periodic, low-intensity prescribed burns every 10 years.

During an extraordinarily dry summer, a natural fire burns both woodlands.

1. a) Predict how fire intensity will affect the viability of the underground Cistus seed bank and the resprouting ability of Pistacia in Woodland A compared to Woodland B.
2. b) Explain the mechanism behind the "fire suppression paradox": why does the total fire exclusion policy in Woodland A alter the successional trajectory and potentially turn a regulatory disturbance into irreversible soil degradation?
3. c) Justify why grazing with goats (browsing) is biologically more efficient than uniform mechanical clearing for maintaining a heterogeneous successional mosaic.