Life's First Foothold: Pioneer Species and Primary Succession from Glacier National Park to the Brooks Range

Across American mountain ranges right now, retreating glaciers and shrinking perennial snowpack are exposing rock that has not touched open air or sunlight in centuries, sometimes thousands of years. That rock is, ecologically, a blank slate: no soil, no organic matter, no established community of organisms. What happens next — and it happens in a remarkably predictable sequence — is called primary succession, and two real, actively monitored American ranges show it happening in very different climates right now: the Northern Rockies of Montana, and the Brooks Range on Alaska's North Slope.

Succession, defined precisely

Ecological succession is the predictable, gradual change in the species composition of a community over time. Ecologists distinguish two kinds by their starting point. Primary succession begins on substrate with no pre-existing soil and no living community — bare rock exposed by a retreating glacier, cooled volcanic rock, or a new sand dune are the classic examples. Secondary succession, by contrast, begins where a community already existed and soil is still present but the community itself has been disturbed or destroyed — after a wildfire, logging, or an abandoned farm field, for instance. The distinction matters because primary succession has to accomplish something secondary succession doesn't: building soil from nothing, which is why primary succession is dramatically slower, often unfolding over centuries.

The pioneer species: engineering soil out of bare rock

The first organisms to colonize bare rock are called pioneer species, and on newly exposed glacial rock, these are almost always lichens — a symbiotic partnership between a fungus and either an alga or a nitrogen-fixing cyanobacterium — sometimes joined by free-living cyanobacteria and mosses (bryophytes). Lichens survive with no soil at all by absorbing moisture and nutrients directly from rain, dust, and the rock surface itself, and they actively reshape that surface: lichens secrete weak organic acids that chemically break down (weather) the rock's mineral surface over time, a process called biological weathering. As lichens grow, die, and decompose, they deposit the first organic matter the rock has ever held. Nitrogen — essential for every living thing but absent from bare rock — enters the system through nitrogen fixation, the conversion of atmospheric nitrogen gas (N₂) into a biologically usable form, carried out by the cyanobacteria partnered within many pioneer lichens. Once even a thin layer of organic, nitrogen-enriched soil has accumulated, mosses can take root, followed by hardy herbaceous plants, and the system is on its way toward a more complex community.

The Northern Rockies: Grinnell Glacier, Montana

Grinnell Glacier, in Glacier National Park in Montana's Northern Rockies, is one of the most extensively photo-documented retreating glaciers in the country: repeat photography comparing images from 1938 to 2009 shows a dramatic, visually striking retreat, exposing large areas of bare bedrock and glacial till (the mixed rock and sediment debris a glacier leaves behind) that had been buried under ice. Studies of similarly retreating glacier forelands consistently document the same predictable sequence described above: an initial lichen and cyanobacteria stage, followed by moss colonization, followed by the slow arrival of grasses and, eventually, shrubs, as accumulating organic matter and nitrogen make the site hospitable to progressively larger and more demanding plants.

The Alaskan North Slope: the Brooks Range and McCall Glacier

On Alaska's North Slope, the Brooks Range presents an even harsher version of the same story. McCall Glacier, monitored for mass balance (the net gain or loss of ice and snow) since the late 1950s, has shown a persistent, worsening pattern of loss, averaging roughly −15 cm of water-equivalent ice loss per year from 1958–1972 and roughly −33 cm per year from 1972–1993 — a clear signature of long-term warming in one of the most remote mountain ranges in North America. The Brooks Range sits atop continuous permafrost, which adds a second layer of change beyond glacier retreat: as permafrost thaws, it exposes fresh mineral surfaces and can mobilize metals like iron and copper into meltwater, altering stream chemistry even before complex life has had a chance to establish itself. Warming in the region is also shifting the Arctic treeline (the boundary past which trees cannot survive) farther north, a slower, landscape-scale sign of the same succession process playing out at the boundary of an entire biome rather than on a single exposed rock face.

Where succession is heading: seral stages and the climax community

Ecologists describe each recognizable stopping point along a succession pathway as a seral stage (the whole sequence is called a sere): bare rock, then the lichen/cyanobacteria stage, then moss, then herbaceous plants, then shrubs, and eventually, if the process runs long enough and conditions allow, a relatively stable, self-perpetuating climax community — forest in much of the Northern Rockies, tundra vegetation in much of the Alaskan Arctic, where the treeline itself marks a fundamental climate limit on how far succession can proceed. In both ranges, what's notable is the timescale mismatch: glacier retreat driven by recent warming can expose new rock in a matter of years, while the primary succession needed to rebuild a mature community on that same rock is measured in centuries — a gap that is itself one of the clearest ecological signatures of how fast the current warming trend is moving relative to the pace at which ecosystems have historically rebuilt themselves.

Ecological succession

The predictable, gradual change in species composition of a community over time.

Primary succession

Succession that begins on substrate with no pre-existing soil or living community, such as bare rock exposed by a retreating glacier.

Secondary succession

Succession that begins where soil and some biological legacy already exist but the community has been disturbed, such as after a wildfire or logging.

Pioneer species

The first organisms — typically lichens, cyanobacteria, and mosses — to colonize a barren primary-succession site.

Biological weathering

The physical or chemical breakdown of rock caused by living organisms, such as acid secretion by lichens.

Nitrogen fixation

The conversion of atmospheric nitrogen gas (N₂) into a biologically usable form, carried out by certain bacteria and cyanobacteria, including those partnered within many pioneer lichens.

Seral stage / climax community

A seral stage is one recognizable step in a succession sequence; the climax community is the relatively stable, self-perpetuating community that persists once succession has run its course under a given climate.

Check your understanding

Sources: Wikipedia, "Grinnell Glacier"; repeat-photography documentation of Grinnell Glacier retreat (USGS); published studies of lichen and moss assembly patterns following glacier retreat in the European Alps and Scandinavia (used as representative primary-succession research, PMC); U.S. Forest Service ecosystem descriptions of the Brooks Range Tundra–Polar Desert province; published McCall Glacier mass balance records (1957–1993) and Brooks Range permafrost/treeline research; NPS documentation on Brooks Range perennial snowfields. DRAFT — verify current 2018 Virginia Science Standards of Learning biology codes (BIO.8, BIO.9) with the current Curriculum Framework before publishing.