Succession Defined
Ecological succession is the process by which an ecosystem or community experiences change. A climax community is found when this ecosystem stops experiencing sporadic change and instead reaches stability. This means that, under the current climate conditions, little to no changes are occurring. Most diagrams depict this as a forest, though the Sahara Desert has remained unchanged for almost five thousand years.
If conditions change, the climax community will change and succession will begin again. Think what would happen if the desert suddenly had more consistent rainfall or the rainforests experienced long term drought. Those conditions then become "unideal" or "unstable" for its residents and it must change (experience succession) again until it reaches another climax community.
Primary Succession
Primary succession occurs after a disaster when only bedrock is in the area. Think about if a parking lot was left alone to reclaim the environment and grow vegetation; the process for plants to overtake cement or rock is a slow and tedious one. Since soil and nutrients in the soil are required to foster plant growth, pioneer species like lichens are important. These are the first complex organisms involved in succession, and with the help of erosion and weathering, these species will break down rock and create soil from the remains. Since this process is so long and arduous, it will take hundreds (or thousands!) of years for other plants (like grasses) to take root and start growing. As individual organisms die and decompose, this promotes nutrient increase and soil development, which helps succession move faster. More and more complex plants, like shrubs and trees, will start to compete for resources, and forest will eventually form when hardwood trees win the interspecies "competition." This creates, in many years, a pipeline from rock to pioneer species to much larger, more complex, and ecologically more tolerant plant species.
Secondary Succession
There is little difference between primary and secondary succession after they start. Secondary succession is only different in that the soil is not wiped out; it remains, and bare bedrock is not exposed. Often, a natural disaster is more tame (such as a wildfire or a flood) and wipes away the living organisms, leaving only soil. Now the pioneer species will be grasses and ‘weeds’ like dandelions. The rest of the process remains the same, and will eventually foster competition after the ecosystem's entire rebuild.
Fauna: A Part of Succession
Most diagrams of succession, primary or secondary, only show the vegetation (flora) that is changing (replaced) over time. As mentioned before, the plants present become larger or more complex the further along succession processes are. You must also realize that the animals (fauna) will also be changed (replaced). Buffalo are at home in the open grasslands, for example, but would have a hard time navigating the redwood forest.
Keystone Species
A keystone species is depended upon by other species in the ecosystem such that if it were removed, the ecosystem would undergo dramatic changes. These keystone species are disproportionate in size, meaning that they have relatively low population density for how extremely important their effect on their ecosystem is. T__he wolves of Yellowstone National Park were reintroduced in 1995__ after nearly a century of absence. The video link explains some of the changes that the park experienced because of the wolves (including the shape of the rivers). The changes to the park showed that the wolves are an extremely valuable species. Keystone species are often undervalued and, as a result of their importance combined with their numbers, are much more prone to going extinct.
Indicator Species
An indicator species is one that reflects the health of its ecosystem. If an indicator species experiences death or much lower population density, this can indicate that an ecosystem's health is poor or failing. For example, amphibians like frogs experience life both on land and in water, so their health could indicate whether or not both parts of an ecosystem are healthy and functioning properly.
🎥 Watch: AP Environmental Science - Adaptations and Ecological Succession
Frequently Asked Questions
What is ecological succession and how does it work?
Ecological succession is the predictable, directional change in species composition and ecosystem structure over time after a new or disturbed area is available. Two main types: primary succession starts on lifeless substrate (bare rock after a glacier or lava) where pioneer species like lichen and moss build soil; secondary succession follows disturbances that leave soil intact (fire, farming). Succession proceeds through seral stages as pioneers facilitate (or sometimes inhibit/tolerate) later species until a relatively stable climax community forms. Keystone species and indicator species can shape or signal those community changes. On the APES exam, you should link succession to changes in biomass, species richness, and net primary productivity over time (CED ERT-2.I and ERT-2.J). Know models (facilitation, inhibition, tolerance), examples (lichen → moss → grasses → shrubs → trees), and how disturbance resets stages. For a quick topic review, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr). More unit review and practice problems are at (https://library.fiveable.me/ap-environmental-science/unit-2) and (https://library.fiveable.me/practice/ap-environmental-science).
What's the difference between primary and secondary succession?
Primary succession starts on lifeless ground with no soil—like after a lava flow or bare rock. Pioneer species (lichens, mosses) colonize first, break down rock, and help form soil; over time you get more plants, increasing biomass, species richness, and net primary productivity until a more stable climax community develops. Secondary succession happens where a disturbance (fire, farming, storm) removes vegetation but leaves soil intact. Because soil (and often seeds/root systems) remain, recovery is faster: grasses and herbs colonize quickly, followed by shrubs and trees through seral stages. Key differences: presence of soil (none vs. existing), speed (slow vs. faster), and typical pioneer species (lichens/moss on rock vs. annuals/grasses in soil). For APES, know effects on biomass, species richness, resilience, and examples—these show up on multiple-choice and FRQs. Review Topic 2.7 on Fiveable (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why does succession happen in ecosystems?
Succession happens because ecosystems are rarely static—when new habitat opens (bare rock after a glacier = primary succession) or when disturbance removes most life but leaves soil (fire, farming = secondary succession), species colonize and change conditions over time. Pioneer species (lichens, mosses, annual plants) modify the environment (build soil, add organic matter) which lets later, more competitive species establish. Models like facilitation, inhibition, and tolerance describe how early species can speed up, slow down, or be neutral to later arrivals. As succession proceeds, soil formation, total biomass, species richness, and net primary productivity typically increase until a more stable climax community (or shifting steady state) forms. Keystone and indicator species can shape or reveal successional stages. This topic links directly to CED EKs (primary/secondary succession, pioneer species, effects on biomass/richness) and is commonly tested on the AP exam—review the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) for examples and AP-style problems.
Can someone explain pioneer species in simple terms?
Pioneer species are the first organisms that move into a bare or disturbed area and start the process of succession. In primary succession (no soil, like after a lava flow) pioneers are usually lichens and some mosses that can live on rock; they break down rock and help form soil. In secondary succession (soil already present, like after a fire) pioneers are fast-growing grasses, weeds, and shrubs that quickly colonize open ground. Pioneer species often follow the facilitation model: they change conditions (add organic matter, hold moisture, fix nitrogen) so other, less-tolerant species can move in. Over time this increases biomass, species richness, and net productivity until a later seral stage or climax community forms (CED EKs ERT-2.I.1 and ERT-2.J.1). This is a common APES topic—review the ecological succession study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to see examples on the exam.
I'm confused about keystone species vs indicator species - what's the difference?
Keystone species and indicator species sound similar but do different jobs in an ecosystem. A keystone species is one whose activities have a particularly significant role in determining community structure—remove it and the whole community can change (classic example: sea otters controlling sea urchins and preserving kelp forests). An indicator species, by contrast, signals something about ecosystem quality by its presence, abundance, scarcity, or chemistry (lichens or certain amphibians indicate air or water quality). In succession, keystone species can shape which species persist as communities move toward later seral stages, while indicator species help you detect ecosystem conditions (like pollution or soil development) during primary or secondary succession. For APES, remember the CED definitions: keystone = big effect on community structure (EK ERT-2.I.2); indicator = reveals ecosystem condition (EK ERT-2.I.3). Need more examples or practice questions? Check the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and extra practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How long does ecological succession take to complete?
Short answer: it depends. Primary succession (starting on bare rock after lava or glaciation) can take hundreds to thousands of years to reach a late seral or “climax” community because soil must form from weathering and pioneer species (lichens, mosses) slowly build organic matter. Secondary succession (after a disturbance that leaves soil, like fire, farm abandonment, or storm) is much faster—often decades to a few hundred years—to regain high species richness and biomass. Old-field succession examples often show major community changes in ~50–150 years. On the AP exam, be ready to use terms from the CED—pioneer species, seral stage, climax community, biomass accumulation—and to compare primary vs. secondary succession rates. For a quick review, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What happens to biodiversity during succession?
During succession species composition and biodiversity change predictably. Right after a disturbance (or in primary succession on bare rock) pioneer species—lichens, mosses, fast-growing plants—colonize. Species richness, total biomass, and net primary productivity (NPP) generally increase through seral stages as soils form and more niches appear. Over time, mid- to late-successional species (shrubs, trees) replace pioneers; diversity often peaks during intermediate stages and may stabilize in a climax community. Biomass tends to keep increasing even after richness levels off, while NPP can rise then plateau or decline slightly at very late stages. Secondary succession is faster than primary because soil and seed banks remain. Succession also affects community structure: keystone or indicator species can appear and reshape diversity, and, per EK ERT-2.J.1–2, pioneer colonization can even drive speciation over long timescales. For AP prep, expect questions about changes in species richness, biomass, and NPP during succession (see the Topic 2.7 study guide: https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr). For more unit review and practice, check Unit 2 (https://library.fiveable.me/ap-environmental-science/unit-2) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
Why are pioneer species important in primary succession?
Pioneer species are crucial in primary succession because they’re the first organisms that colonize lifeless surfaces (bare rock after a glacier or lava). Species like lichens and some mosses break down rock, trap dust, and start forming the first thin soils—that soil formation is what lets later plants (herbaceous then woody) move in. Pioneer species also add organic matter when they die, increasing biomass and nutrients and raising species richness over successive seral stages (EK ERT-2.J.1). Their presence follows the facilitation model: they modify harsh conditions so other species can establish, sometimes leading to long-term community changes or even new species. On the AP exam you might be asked to describe this process or link succession to changes in biomass and species richness (Topic 2.7). For a quick review, check the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and try practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How does a forest fire affect secondary succession?
A forest fire is a classic disturbance that triggers secondary succession because soil and often seeds/root stock remain. Right after a fire, pioneer (early-successional) species—fast-growing grasses, herbaceous plants, and fire-adapted shrubs or trees—colonize the burned area. Over time seral stages progress: species richness and biomass typically increase as shrubs and young trees establish, then shade-tolerant species replace early colonizers toward a climax community (if no new disturbance occurs). Net primary productivity (NPP) is usually low immediately after fire, rises during mid-successional stages as biomass accumulates, and may level off later. Fire can also favor keystone or indicator species adapted to fire (e.g., certain pines with serotinous cones), altering community structure and resilience. For AP review, focus on differences from primary succession (soil present → faster recovery), changes in species richness/biomass, and examples of pioneer species (see the Topic 2.7 study guide: https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr). For more practice, check unit resources (https://library.fiveable.me/ap-environmental-science/unit-2) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What's an example of a keystone species and why is it so important?
A classic keystone species example is the sea otter. Sea otters eat sea urchins; without otters urchin populations explode and overgraze kelp, collapsing kelp forest communities. Because otters have a disproportionately large effect on community structure, their presence maintains high species richness and biomass—exactly what EK ERT-2.I.2 means. In succession terms, a keystone species can shape which species persist during seral stages and influence whether an ecosystem reaches a particular climax community. For the AP exam, be ready to connect a keystone species to changes in species richness, biomass, and community structure (ERT-2.J.1–2). Want a quick review? Check the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) to see keystone examples and how they’re tested.
I don't understand how succession increases biomass over time - can someone explain?
Succession increases biomass because communities become more productive and build more living tissue over time. In primary succession (bare rock) pioneer species like lichens and mosses break down rock and start forming soil. As soil depth and nutrients increase, grasses, shrubs, and eventually trees can establish—each stage (seral stage) adds more plant mass and supports more consumers. Net primary productivity (NPP) generally rises because bigger, longer-lived plants photosynthesize more and store more carbon as biomass. Secondary succession (after a disturbance) is faster because soil and seeds remain, so biomass rebounds quicker. Models like facilitation explain how early species modify conditions to allow later, higher-biomass species to move in. For AP, remember EK ERT-2.J.1–.2: succession affects total biomass, species richness, and productivity—a common free-response concept. For more review and practice, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and practice problems (https://library.fiveable.me/practice/ap-environmental-science).
What are indicator species used for in environmental science?
An indicator species is a plant or animal whose presence, absence, abundance, or chemical makeup tells you something specific about an ecosystem’s condition (CED EK ERT-2.I.3). You use them as ecological “early warning” signals—for example, lichens signal good air quality (sensitive to SO2), many amphibians indicate clean freshwater because their permeable skin reacts fast to toxins, and certain algae or diatoms show nutrient loading or pH shifts. On the APES exam, expect to connect indicator species to succession, disturbance, and ecosystem health (Topic 2.7 and Topic 2.5 links). They’re practical for monitoring succession stages, guiding restoration or conservation actions, and detecting pollution before whole communities collapse. For a quick review, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr). More unit resources and practice questions are at (https://library.fiveable.me/ap-environmental-science/unit-2) and (https://library.fiveable.me/practice/ap-environmental-science).
How does net productivity change during different stages of succession?
Net productivity (net primary productivity, NPP) changes predictably during succession. In primary succession (no soil), early pioneer species (lichens, mosses) give very low NPP because little biomass and harsh conditions exist. As soil forms and more plants establish (early → mid seral stages), NPP rises—more producers, faster growth, and increasing species richness and biomass. NPP often peaks in mid-successional communities where fast-growing herbs, shrubs, and young trees dominate. As succession moves toward a climax community, total biomass keeps increasing but NPP usually levels off or declines because mature forests have high respiration rates and slower growth per unit biomass (so NPP decreases even though standing biomass is large). Secondary succession follows the same pattern but starts higher (soil present), so NPP rises faster. This pattern (biomass up, NPP up then down) is a CED concept you should know for Topic 2.7. For a concise review, see the Fiveable study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr). More unit review (https://library.fiveable.me/ap-environmental-science/unit-2) and practice questions (https://library.fiveable.me/practice/ap-environmental-science) help solidify this for the exam.
What happens when you remove a keystone species from an ecosystem?
A keystone species has an outsized role in shaping community structure (CED EK ERT-2.I.2). If you remove one, the ecosystem often changes a lot: you can get a trophic cascade where populations of prey or competing species explode or crash, which alters species richness, total biomass, and net productivity over time (ERT-2.J.2). Habitats or food-web links can shift, allowing different successional paths or favoring invasive or opportunistic species; over time that can push the community to a different seral stage or new climax assemblage. Because keystone losses change which species facilitate or inhibit others, succession after the disturbance may follow different facilitation/inhibition/tolerance models. This is a common AP question type—know the terms (keystone, succession, trophic cascade, species richness, biomass) and expect applied examples on the exam. For a quick review, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and more practice at (https://library.fiveable.me/practice/ap-environmental-science).
Why does species richness increase during succession?
Species richness increases during succession because the environment becomes more habitable and complex over time. In primary succession (bare rock) pioneers like lichens and mosses start soil formation; in secondary succession (soil remains) pioneers recolonize faster. As soil depth, nutrients, and biomass build up, more niches form (more microhabitats, food sources, and shelter), so more species can establish. Interactions change too: facilitation lets later species move in after pioneers improve conditions; tolerance and inhibition alter which species persist, increasing turnover and overall richness through seral stages until a more stable climax community. More structural complexity and keystone species also support higher diversity and net productivity (CED EK ERT-2.I.1, ERT-2.J.2). For AP review, see the Topic 2.7 study guide (https://library.fiveable.me/ap-environmental-science/unit-2/ecological-succession/study-guide/9nK9VTaGMUhfWvoRySDr) and Unit 2 overview (https://library.fiveable.me/ap-environmental-science/unit-2). For practice Qs, try the Fiveable practice bank (https://library.fiveable.me/practice/ap-environmental-science).