Succession
Primary Succession
Primary succession happens in places where no soil or living organisms existed before, such as newly formed volcanic islands, exposed rock after glacier retreat, or fresh lava flows. Because there's no soil to start with, this process is slow and can take hundreds or even thousands of years.
It begins with pioneer species like lichens and mosses. These organisms colonize bare rock and gradually break it down through physical and chemical weathering, creating thin layers of soil. As organic matter accumulates from dead pioneers, the soil deepens enough to support grasses and small shrubs. Those plants further enrich the soil, eventually allowing larger shrubs and trees to establish.
Over long periods, the community reaches what ecologists call a climax community, a relatively stable assemblage of species (like an old-growth forest) that persists until a major disturbance resets the process.
Secondary Succession
Secondary succession occurs where a disturbance (wildfire, flood, logging) has removed the existing community but left the soil intact. Because soil, seeds, and root systems often remain, this process moves much faster than primary succession.
- Pioneer species here are typically fast-growing grasses and herbaceous plants that thrive in full sunlight.
- Intermediate stages bring in shade-intolerant, fast-growing trees like aspens or pines.
- Eventually, slower-growing, shade-tolerant species (such as oaks and hickories) outcompete the early colonizers and form a mature climax community.
A useful comparison: after the 1988 Yellowstone fires, grasses and wildflowers returned within a year, shrubs within a few years, and lodgepole pines began reestablishing within a decade. Primary succession on new volcanic rock, by contrast, can take centuries just to build enough soil for grasses.
Community Interactions
Keystone Species
A keystone species has a disproportionately large effect on its ecosystem relative to its abundance. Remove it, and the community structure can collapse or shift dramatically.
Two classic examples:
- Sea otters in Pacific kelp forests prey on sea urchins. Without otters, urchin populations explode and overgraze kelp, turning diverse kelp forests into barren "urchin barrens."
- Beavers dam streams and create wetland habitats. These ponds support fish, amphibians, waterfowl, and riparian plants that wouldn't otherwise be present.
The key idea is that not all species contribute equally to community structure. Keystone species punch well above their weight.
Ecological Facilitation
Facilitation occurs when one species modifies the environment in a way that benefits another species. This is especially important during succession, where early colonizers often make conditions suitable for later arrivals.
- In deserts, nurse plants (shrubs like saguaro-sheltering palo verde trees) provide shade and trap moisture, allowing seedlings of other species to survive the harsh conditions beneath them.
- Legumes (clover, beans, alfalfa) host nitrogen-fixing bacteria in root nodules. These bacteria convert atmospheric into ammonia, enriching the soil with usable nitrogen and boosting growth of neighboring plants.
Facilitation differs from mutualism in that the benefit can be one-directional. The facilitating species doesn't necessarily gain anything from the interaction.
Ecological Communities
An ecological community consists of all the interacting populations of different species living in the same area at the same time. Ecologists describe communities using two measures:
- Species richness: the total number of different species present
- Species evenness: how evenly individuals are distributed among those species
A community with 10 species but 90% of individuals belonging to one species has high richness but low evenness. Both measures matter for understanding biodiversity.
The boundaries between communities can be sharp, like the edge of a lake meeting dry land, or gradual. A gradual transition zone between two communities is called an ecotone (for example, where grassland slowly shifts into forest). Ecotones often support high biodiversity because species from both adjacent communities overlap there.
Species Relationships
Niche and Competitive Exclusion
A species' niche is its total ecological role: the combination of habitat, food sources, environmental tolerances, and interactions with other organisms it needs to survive and reproduce. Think of it as the species' "job description" within the ecosystem.
The competitive exclusion principle (sometimes called Gause's principle, after the ecologist who demonstrated it) states that two species cannot occupy the exact same niche in the same habitat indefinitely. If two species compete for identical resources, one will inevitably outcompete the other. The weaker competitor either goes locally extinct, shifts to a different habitat, or evolves to use a slightly different set of resources.
This leads to an important distinction:
- Fundamental niche: the full range of conditions and resources a species could use in the absence of competition
- Realized niche: the narrower range it actually uses when competitors are present
Resource Partitioning
Resource partitioning is the evolutionary outcome of competitive exclusion. Instead of fighting over the same resources, coexisting species diverge to specialize on different portions of a shared resource. This reduces direct competition and allows similar species to live in the same habitat.
- MacArthur's warblers: Five warbler species in New England forests all eat insects in spruce trees, but each species feeds in a different zone of the tree (crown, middle branches, trunk base, etc.), minimizing overlap.
- Caribbean anole lizards: Different anole species on the same island partition their habitat by perch height and diameter. Some specialize on high, thin branches while others use low, thick trunks.
Resource partitioning can involve differences in food type, feeding location, activity time (day vs. night), or any other dimension of the niche. It's one of the main mechanisms that maintains species diversity within communities.