Ecological succession

Differentiate between primary and secondary succession, and indicate which each of these cases corresponds to, justifying your answer: the colonization 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 Spanish ecosystems that trigger it, indicating what remains of the previous ecosystem that speeds up recovery.

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 colonizers, and why the distance to the neighboring islands determined the order in which species arrived.

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

A fire devastates a pine forest and, in the same year, a lava flow covers a coastal platform (fajana). Explain why the recovery of the pine forest is a secondary succession and the colonization of the coastal platform is a primary one, highlighting the role of soil and the seed bank.

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.

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.

In 1990, two adjacent 4-hectare agricultural plots on the Iberian plateau were abandoned:

• Plot 1: Deep soil on a river terrace, 80 m away from a mature holm oak dehesa with the presence of seed-dispersing birds (jays and corvids).

• Plot 2: Medium-slope hillside (15%) whose A horizon was lost due to erosion after decades of plowing parallel to the slope; the area also sustains an overpopulation of roe deer and rabbits.

In 2025, Plot 1 has transformed into dense scrubland dominated by shrubby legumes with numerous holm oak (Quercus ilex) saplings and young seedlings. In contrast, Plot 2 remains a sparse grassland of annual herbs and scattered thyme shrubs over open gullies.

1. a) Analyze the causes of this contrast: what ecological factors have allowed secondary succession in Plot 1, and what mechanisms have caused the "successional arrest" (ecological trap) in Plot 2?
2. b) An environmental group proposes planting 500 holm oak saplings directly in Plot 2 under the idea that "reintroducing the climax species is enough to restore the forest". Predict the foreseeable outcome of this planting and propose two prerequisite interventions on the physical environment or biotic interactions that are essential for succession to resume.

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 neighboring Mediterranean Aleppo pine (Pinus halepensis) woodlands with an understory 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.

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 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.