Examples of ecological succession so far concern how communities develop following disturbance of a pre-existing community. Ecologists often distinguish between (re)development of communities following disturbance and development of ecological communities on new substrates where conditions are not shaped by a preexisting community. The former process is called secondary succession (e.g., dynamics following a forest fire or blowdown); the latter is primary succession (the initial development of ecological systems where none existed previously). Unstabilized dunes of pure sand can be a substrate for primary succession. 1
Historical interlude: Primary succession on dunes was first studied by Henry C. Cowles (middle, upper photo) at the University Chicago in the years around 1900. Many photos of his botany classes are archived at the Library of Congress American Memory website. They re interesting for the field costumes. Also note the high proportion of women in the botany classes; at this time, botany was considered an appropriate field of study for women; conversely, it was sometimes regarded as inappropriate for men 2
In large dune fields along the south and west shores of L. Michigan, Cowles recognized that dunes closest to the lake were youngest and dunes progressively further from the lake were older. He reasoned that the vegetation along such a series of dunes might represent different stages along a sequence of vegetation development (assuming that the different dunes have developed under relatively similar circumstances). This approach to studying long-term dynamics comparing current samples from different places that are presumed to be at different stages of a similar process is a common approach now, and is referred to as a space-for-time substitution (or, sometimes, a chronosequence study). The youngest dunes are a stressful place for plant growth. They re not very fertile, but, more importantly, the substrate is extremely unstable; the sand is continuously shifting in the wind, and roots of plants might be exposed, or the plants smothered. Most types of plants can t survive this. Some have special adaptations that allow them to cope. Ammophila breviligulata or beach grass (found also along the east coast), for example, is able to send out new roots along the stem as it gets buried, and to spread by sending out horizontal underground stems (or rhizomes). As beach grass spreads across the dune, its roots tend to bind the loose sand and gradually stabilize it. 3
Other plants can do similar things; wild strawberry (top) can also spread with aboveground stolons. As the sand is stabilized, woody plants can become established. Willows are one of the earliest types of woody plants that can colonize the dunes; they, too, are able to root along buried stems and survive some burying. Note that accumulation of a mound of wind-blown sand around the willow shrub at lower right. 4
This process is not irreversible; severe storms (or foot or vehicular traffic) can disrupt the root systems and cause blow-outs where the sand becomes loose and mobile again. 5
But, generally, older, stabilized dunes are colonized by additional species of woody plants, including trees; as these grow and spread, they shade out the early successional grasses and willows. 6
The forest in the background occupies one of the older dunes, and is dominated by beech and maple shade-tolerant species. Cowles was one of the first people to recognize the potential for plants in early succession to alter habitat conditions in such a way as to make the establishment of later-successional species possible when those species might have been entirely unable to survive on the site initially. This is very different from most secondary successional dynamics; there is no reason why spruce and fir can t grow on a newly burned site, for example; they just, typically, don t get there and grow as quickly as jack pine and aspen. But beech and maple trees would be entirely unable to survive on a new sand dune. The process of habitat alteration that permits (and is essential for) establishment of later-successional species (superior competitors) is called facilitation. It can be important in primary successions. 7
Frederic (no k ; the photo caption is wrong) Clements was particularly impressed by what seemed to be the orderliness and predictability of successional sequences like those described by Cowles. The notion of facilitation (he called it reaction of organisms on the environment) struck him as particularly important. It made the process of succession look like a programmed developmental sequence, like that of a developing organism. Each stage prepared the way for the next, and was necessary before the next could develop. Succession would continue to a mature stage, which Clements called the climax community; the climax community should be stable. Ultimately, Clements believed successional dynamics would alter site conditions so that all communities in a given region would succeed towards a single climax type, determined only by regional climate (the climatic climax ). However, many trajectories were possible, depending on starting point and other things; he gave all of these names He extended the analogy of communities (= formations) with organisms by suggesting that communities came in discreet and deterministic types which could be named and organized in a taxonomic structure, just as individual organisms could be grouped like with like into species. For this reason, Clements model is often referred to as the pseudoorganism model of communities. This also implies that there should be distinct boundaries between communities; he called them ecotones. 8
Thus, under the same climate, successional processes on a lava flow or in a pond should eventually modify their respective habitats (by building soils on weathering rock in one case, by filling in basins and developing new soils in sediments/peats in the other) until communities converged on the same climax. Clements, of course, gave these successional sequences names xerarch for dry starting points, hydrarch for wet. Each sequence of successional change was called a sere, and successional communities along the way might be referred to as seral. 9
Clements models prevailed for decades, from about 1900 until the 1960s, perhaps because they were practically and esthetically appealing, perhaps because he had a dominating personality, and trained dozens of students who were influential in various agencies and organizations. The map, here, is from a book by E. Lucy Braun on the deciduous forests of eastern North America; the mapping units are climatic climax formations in the Clementsian sense. Schematically (lower left), Clementsian communities should have relatively sharp boundaries/transitions (ecotones) along an environmental gradient (note that horizonal axis is NOT geographical, but represents continuous change in some environmental factor). The curves represent abundances of different species, and three distinct associations or communities occur along this section of gradient. 10
Henry Allan Gleason challenged Clements model in his 1926 paper, The individualistic concept of the plant association, in which he suggested that ecological and evolutionary understanding indicate that species should be distributed according to their individual tolerances and adaptations, and that there is no reason that their distributions along a gradient should be coherent with one another that there s no mechanism that would tend to make communities act as integrated wholes (he was also one of the first to recognize that long-term climate change and range shifts would make the Clements model more unlikely). Species distributions might be limited, he thought, by competition with neighboring species of similar life-style (this foreshadows the notion of ecological niche and guild), but this would simply have individual species replacing one another not whole communities. Gleason also believed that chance factors (like seed dispersal and weather) might have large influence on successional sequences, so that they would be less predictable than Clements thought. (It s not clear whether he thought there would ultimately be a deterministic climax ). The graph here is similar to the one on previous page, but reflecting the species individualistic hypothesis. 11
Contrasting predictions of species distribution patterns across environmental gradients provide a means of testing between Gleasonian and Clementsian models but it was a long time before anybody gave serious effort to implementing such tests; the prevailing Clementsian model was simply taken as dogma 12
But in the late 1950s, Robert Whittaker, as a graduate student at the University of Illinois, conducted a massive study of plant communities in the Great Smoky Mountains in Tennessee. He looked at distribution of woody species with elevation (vertical axis in these graphs) and across what he called a topographic moisture gradient from moist protected coves, to progressively more exposed slopes, to dry ridgetops (left to right on the horizontal axis). The dashed lines indicate general habitat categories; the solid lines are contour maps of species abundance (stem density per sample plot). Acer rubrum, for example (bottom left), is most abundant (the highest topographic contour) at lower elevations in mid-moisture conditions. Whittaker plotted abundances for over 100 species, and found that they were individualistically distributed; there were no coherent groupings with multiple species showing congruent topographies of abundance. He concluded that the Gleasonian model was more accurate, at least for the spatial patterns he observed. At the same time, John T. Curtis, at the University of Wisconsin, was examining patterns of plant communities across the state with respect to moisture and climatic gradients. He ultimately concluded the same thing; plant species were individualistically distributed across moisture gradients. 13
Whittaker continued to analyze distributions of tree species in other regions of the country and found similar results repeatedly 14
And one more similar study. 15
Margaret Davis s work (see paleoecology notes), showing that tree species that currently co-occur broadly had strongly differing distributions during the glacial maximum >14,000 ybp, first published in the mid-70s, was considered by many to be the final straw in rejecting Clementsian models of coherent community-types configurations of species are reorganized when climate changes, and no communitytypes remain consistent over time. 16
The predictions of Clements model for successional patterns would look something like the upper graph; those of Gleason might look something more like the lower (species arrive when they happen to get there, and they increase and decrease in abundance or dominance depending on their individual competitive abilities relative to other species. In the upper model, facilitation is necessary: later groups (or seral communities ) can t establish until earlier communities have altered environment to permit it. In the lower model, facilitation might be involved, but it s not inherently required. Dispersal and chance can plan a large role as well. Frank Egler used a different terminology: he called the upper (Clementsian) model relay floristics, implying groups of plants handing over dominance from one community to the next, and he called the lower an initial floristics model, implying that most of the species involved in succession were there from the beginning they jus 17
Empirical studies seem to be more consistent with an individualistic model of community change during succession as well. This is a study of bird populations during old-field succession in Georgia; species do not enter and leave the community in concert, and their abundance peaks at different times in succession. The cluster of sharp ups and downs at about 15 years corresponds to when a more or less continuous canopy of trees was established in other words, from a bird s perspective, an abrupt change in environment ( individualistic models say community composition should change gradually with environment; if environment changes abruptly, so will community composition) 18
In some instances successional trajectories are not what s expected. For example, there are situations where forest regeneration has failed following major disturbances here logging followed by slash-fire about 100 years ago in northern Michigan (the stumps are remains of large white pine; large hardwood stumps have completely rotted away). Hypotheses proposed for absence of normal successional dynamics include: earlyestablished species prevent tree seedling survival by toxic (allelochemical) or mechanical (barriers to rooting) effects; changes to soil properties (loss of organic matter and resulting loss of fertility) due largely to intense post-logging fires; loss of water-holding capacity for same reasons so that soil is too dry for seedling establishment. The first mechanism would be an example of inhibition slowing of successional dynamics by early colonists; the latter two would suggest that re-establishment of forests will require facilitation (rebuilding of soil properties by earlier successional species). 19
In the northeast, some old fields show similarly slow establishment of tree canopies; here, shrubby species of dogwood and viburnums (Cornus, Viburnum) may either establish very dense shade or perhaps exert allechemical effects, again inhibiting succession. 20
Noting phenomena like all of those described above, Connell and Slatyer suggested THREE categories of successional processes facilitation (as suggested in Clements original model) is alteration of habitat by stress-tolerant early successional species allowing establishment of other species; tolerance is more related to Egler s notion of initial floristics species establish when they can get there, and those more tolerant of competition simply out-persist the others; and inhibition where the effects of some earlyarriving species actually modify environment so as to inhibit establishment of species that would otherwise become established and dominant. These increasingly complex conceptual models reflect recognition that successional dynamics are driven by diverse interactions among species of diverse life-histories and changing site environments. Other models (including the SORTIE simulation, for example) are much more complex. 21