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.

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.

The abandonment of cropland in inland Spain 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.

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.

Following the eruption of the Tajogaite volcano (La Palma, 2021), a research team monitors two plots on sterile basaltic lava flows at an altitude of 800 m: Plot A, facing northeast (exposed to the trade wind sea of clouds), and Plot B, facing southwest (on the leeward slope, with higher radiation and aridity). Four years after the eruption, Plot A shows an 18 % cover of pioneer lichens (Stereocaulon vesuvianum) and bryophytes, with an accumulation of 1.2 mm of organic matter and dust in microfissures; in Plot B, living cover is below 1 %.

1. a) Justify, based on pedogenetic processes and ecological facilitation mechanisms, why the rate of primary succession differs so greatly between both plots given that the initial substrate is identical.
2. b) A restoration plan proposes planting Canary Island pine (Pinus canariensis) saplings directly onto both lava flows to "accelerate reaching the climax community". Predict the feasibility of this measure and refute the proposal using the relationship between net production and biomass ($P/B$), soil profile development, and r- and K-strategies.

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.

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 stabilization, 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, shrubs, forest) contributes to that accumulation and how the increase in soil relates to the increase in biodiversity.

To study primary succession following the retreat of a Pyrenean glacier, a group of students applies the chronosequence method by sampling four terminal moraines located along the valley: M1 (abandoned 15 years ago), M2 (40 years ago), M3 (75 years ago), and M4 (130 years ago, at the lowest elevation).

When processing the field data, they discover that M4 has a soil with a thinner organic horizon, a much more acidic pH (4.1 compared to 6.9 in M1-M3), and lower floristic richness than M3, contradicting the hypothesis of continuous successional progress over time.

1. a) Explain the biological and methodological principle on which a chronosequence is based ("substituting space for time") to investigate ecological succession.
2. b) A geological analysis reveals that moraine M4 formed over a band of sulfide-bearing slates, whereas M1, M2, and M3 originated from the weathering of limestone massifs. Demonstrate why this finding invalidates the conclusion that succession slowed down due to biological causes, and identify the key assumption of the method that has been violated.
3. c) Design an adjustment to the sampling scheme so that the chronosequence becomes methodologically valid again.

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) Analyze 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.

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.

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.