Ecological Succession
In 1943, in a cornfield in Michoacán, a crack opened in the ground and the Paricutín volcano was born. The lava covered cornfields and villages. Today, lichens, mosses, and even trees are already growing on those lava flows. How does life return to a place where it seemed impossible? Ecosystems are not still photographs: they change over time following a fairly predictable sequence, known as ecological succession.
Primary and Secondary Succession
Think of two devastated pieces of land. In the first, a freshly cooled lava flow covers everything. In the second, a milpa (a traditional cornfield) is abandoned and nobody plants it again. In both, "there is no vegetation," but the second fills up with plants in just a few months, while the first takes decades to show anything more than bare rock. What makes the difference? What is left underneath.
| Primary | Secondary | |
|---|---|---|
| Soil at the start | None | Present (sometimes damaged) |
| Seed bank | None | Usually present |
| Speed | Centuries | Years or decades |
| Mexican example | Xitle lava (Pedregal de San Ángel), Paricutín | Abandoned milpa, pine forest after a fire |
The reason is simple: in primary succession, soil has to be built from scratch, and that takes centuries. In secondary succession, the soil already exists and the seeds are waiting.
What Does the Pace of Primary Succession Depend On?
Not all lava flows recover at the same speed. The pace depends on several factors:
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Climate: with higher temperature and rainfall, rock wears down faster and plants grow better. A lava flow in a warm, humid area is covered much sooner than one in a cold or dry place.
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Moisture and nutrients: lava is poor in nitrogen and holds little water. The cracks where dust, water, and leaves collect are colonized first.
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Distance to sources of seeds and propagules: if there is forest nearby, wind, birds, and bats bring seeds. If the lava flow is very large, its center is far from any source and takes longer.
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Age and type of the lava flow: a rough, cracked surface holds more soil than a smooth one.
A Mexican example: at Paricutín, more than 70 years after the eruption, the lava flows have only lichens, mosses, and a few pines along the edges, near the forest that survived. In the Pedregal de San Ángel, the Xitle lava, about 1,700 years old, already has well-developed xerophytic scrub and soil in the cracks. With the same story behind them, time and conditions produce very different stages.
Differentiate between primary and secondary succession, and indicate which each of these cases corresponds to, justifying your answer: the colonisation of a recent lava…
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.
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…
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.
Pioneer Species and the Climax Community
Each stage prepares the next one. Lichens and mosses help break down the rock and, when they die, add organic matter. This forms a thin soil where herbs and ferns can live; these add more organic matter and shade, and so shrubs arrive, and then trees.
In the Pedregal de San Ángel (Mexico City), the lava from the Xitle volcano is now covered by xerophytic scrub with species such as the palo loco (Pittocaulon praecox). At Paricutín, by contrast, the 1943-1952 lava flows are still in very early stages.
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…
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…
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…
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.
Stability, Maturity, and Competition
An abandoned milpa fills up with grasses and scrub in a couple of years, but a mature oak forest can withstand a drought or a pest with almost no change. Why do some communities withstand disturbances better than others?
Why? In an ecosystem with many species, if one fails, another can take over its function. With few species, losing one affects the whole system. This is a form of feedback: a complex system corrects its own changes better.
And what drives the replacement of one stage by another? A large part of it is competition: organisms with similar needs for light, water, nutrients, or space compete and limit each other's growth and reproduction.
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Intraspecific: between individuals of the same species (two young oaks competing for the same light).
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Interspecific: between different species (grasses and shrubs competing for soil water).
As succession advances, the species that compete best for the scarce resource (for example, light under a closed canopy) displace the pioneers, which are poor competitors but very good colonizers.
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…
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,…
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 %.
- 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.
- 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.
Fire and Succession in Mexican Ecosystems
In many pine forests in Mexico, low-intensity fire is a natural part of the ecosystem. Some pines have thick bark that resists the heat, and certain grasses and shrubs resprout after a fire. In xerophytic scrub and grasslands there are also species adapted to frequent disturbances. That is why many ecologists speak of continuous disturbance rather than a fixed climax.
The intensity of the fire matters. A mild one spares the soil and the seeds: secondary succession is fast. A very severe one burns up the organic matter and the seed bank: recovery is much slower. On the other hand, putting out every fire for decades builds up fuel, and the next one can be devastating; that is why controlled burns and CONAFOR crews are used.
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…
In a protected natural area in southeastern Iberia, two adjacent sectors coexist on the same type of limestone substrate:
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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.
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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.
- 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).
- 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.
Human Pressure on Ecosystems
Succession shows that an ecosystem can recover from a disturbance if it is given time. But today human disturbances (deforestation, fires, pollution, climate change) are so large and frequent that there is often no time to recover. This is an ecological imbalance.
How can we measure that pressure? One indicator is the ecological footprint: the area of productive land and sea that a person, city, or country needs to obtain its resources and absorb its waste (including CO₂). It is expressed in global hectares (gha). If the footprint exceeds the biocapacity (what the territory can regenerate), there is an ecological deficit and the natural "capital" is being consumed.
For this reason, moving toward sustainable development requires combining three fronts: measuring the pressure (ecological footprint), slowing climate change, and managing urban solid waste well. You will see them in the following sections.
A shallow coastal lagoon receives continuous inputs of nitrates and phosphates from agricultural sources. The following table shows the ecosystem parameters before the…
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:
| Ecological parameter | Year 0 (Oligotrophic) | Year 20 (Eutrophic) |
|---|---|---|
| Gross production / Total biomass ratio ($P/B$, year$^{-1}$) | 0.32 | 4.60 |
| Phytoplankton diversity (Shannon Index, bits) | 3.9 | 0.9 |
| Secchi disk depth (water visibility) | 4.5 m (to the bottom) | 0.3 m |
| Dissolved oxygen at the bottom at night | 7.8 mg/L | 0.2 mg/L |
- 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.
- 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.