Exercises: Ecological succession

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.

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.

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.

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.

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 colonisation of the coastal platform is a primary one, highlighting the role of soil and the seed bank.

The abandonment of cropland in central Mexico is followed by the appearance of scrubland and brush. 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.

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 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.

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.