Ecological succession: lecture topics

Plant (and animal) communities develop in often predictable ways following disturbances, a process known as succession

Several kinds of species interactions alter the process of succession Facilitation Inhibition Tolerance

Species richness changes predictably with successional changes

Kinds & qualitative characteristics of disturbances that impact communities

Disturbance frequency

Dis

turb

ance

mag

nitu

de

treefallice storm fire

Glaciation

volcanism

Asteroid impact

LandslideHurricane

Animal carcass

Glaciers are a kind of disturbance that influences the development of biological communities (Grinnell Glacier, Glacier N.P.--photo by T. W. Sherry)

Thin soils on glacial till indicate short time since release from glaciation (photo by T.W. Sherry)

Aftermath of extensive fire in Yellowstone N.P. (photo T. W. Sherry)

Fir waves in alpine zone are self-perpetuating dead zones (disturbances) that create cyclic successional sequences (from Ricklefs 2001)

Community succession The flipside of disturbance is community development Define succession = community development in

response to disturbance Primary succession involves colonization of new sites,

devoid of plants (e.g., bare soil from glacial recession; bare rocks)

Secondary succession involves the re-colonization, regrowth, and/or germination of recently cleared ground

Sere or seral stage is a “unit” or subset of succession Succession is omnipresent set of complex processes

Organism adaptations to disturbed environments attests to importance, persistence of succession

Succession may end in climax community

Kinds & examples of succession Most ecological communities are probably always in

some phase of recovery from disturbance, i.e., undergoing systematic change = a kind of resilience

Terrestrial succession best studied, but aquatic, & marine succession certainly important (e.g., animal fauna changes in deep sea whale carcass; progression of invertebrate species on rocks)

Most studies deal with plant succession, but animal succession is also well known (as in above marine examples; also succession of detritivores in rotting tree trunks)

Examples of succession in next few slides

Lichens on rocks, initiating primary succession; Glacier National Park, Montana (photo by T. W. Sherry)

Succession examples: 1º succession, Indiana sand dunes

Marram grass settlement

Establishment shrubs

Accumulation organic nutrients

Shrubs replaced by trees

Examples of succession: old-field (2º) succession, oak-hornbeam, Poland

After forest cleared After 7-years After 15 years

After 30 years After 95 years After 150 years

2º Succession examples: bog succession, Ontario, Canada

Beaver dam forms pond

Quaking bog covers almost entire wetland

Bog fills in from edges

Several models of succession...

Facilitation = a process in which one or more species alter environment, making it more suitable for one or more other species to invade Earliest ecologists to study succession thought this

process was predominant; this was only original model

Facilitation thought to lead progressively and inexorably to climax community (stable end point, comprised of K-selected species that replace themselves, maintaining identical community)

Climax then determined by local climate, soil characteristics

Example: Alder trees (shrubs) facilitate succession by fixing nitrogen in soils, making them more suitable for invasion by birch, aspen spruce trees

Coralline algae facilitate

seedling establishment

of surfgrass (Phylospadix)

Climax community is

rarely any deterministic,

fixed endpoint of succession,

but rather a continuum of

endpoints, depending on

soil conditions, among others

(from Ricklefs 2001)

Ecological models of succession, continued

Second of these models is inhibition Defined as the negative effect of one species on

another, preventing it from establishing as quickly (or at all) during succession

Basically involves interspecific competition as mechanism Often involves preemption of space Alternatively, may involve allelochemical

interactions Third mechanism involves soil-borne pathogens

associated with particular plant that helps out-compete plant earlier in succession

Seral position of plants is related to life-history adaptations: In this

2º succession of N. Carolina old fields, Crabgrass (summer) and

Horseweed (winter) disperse well (r-selected), and tolerate sunny,

bare soil conditions, but are easily outshaded by perennials

such as broomsedge (example of inhibition by broomsedge)

Broomsedge inhibits establishment of aster by scavenging water, nutrients from soil near parent stems

Ecological models of succession, continued

Third such model = tolerance Defined by a lack of an effect on other species According to this model of succession, sequence can

start out with any of several disturbance-tolerant species, and it eventually ends up with a climax species that can persist indefinitely

Example? Floral succession (2º), in which all species present as seed bank in soil

Fourth such model = random colonization Accordingly, any species can colonize, and any species

can be endpoint--it is determined stochastically (i.e., colonization lottery)

Example--colonization of corals by herbivorous fish?

Any concensus today about models of succession?

Succession is complex, and no one process is pre-eminent

Multiple processes involved in many successions, and these processes manifest themselves at different successional (seral) stages

E.g., 2º succession of abandoned tobacco fields in North Carolina Piedmont region Inhibition of white aster, ragweed by crabgrass Facilitation of broomsedge by white aster, ragweed Random colonization by loblolly pine, hardwoods,

white oak, dogwood, hickory in later seral stages

Early successional species tend to be r-selected

Note tradeoff: shade tolerance of late-successional plants at expense of slow growth rate, few (large) seeds

Survival of seedlings in shade is directly related to seed weight--later seral stages tend to have larger seeds

Some “climax” communities are maintained by extreme conditions, such as frequent fires -> “fire climax”

Longleaf pine savannas of southeast are maintained indefinitely as the endpoint of succession, as long as fires are allowed to burn Depend on summer fires to burn off (kill) fire-

intolerant hardwood tree species Adapted exquisitely to tolerate (even promote?) fire In absence of fire, hardwoods replace pine savanna

E.g., river bottoms such as Camp Independence along Tangipahoa River

Covington & Mandeville, where fires are suppressed (see slides)

Fire used to control shrubbery, restore pine savanna in Kisatchie Nat’l Forest, LA (photo T. W. Sherry)

Longleaf pine savanna, showing aftermath of ground fire; & seedlings that survived light burning (from Ricklefs 2001)

Hardwoods replacing longleaf pine, Kisatchie Nat’l Forest, LA (photo T. W. Sherry)

Patterns of species richness with succession?

Species richness often increases with seral stages This results from the increased time available for

different species to colonize sequence Species richness often reaches asymptote at later

successional stages, and can even decrease dramatically (to one or a few species that are competitive dominants, depending on level of disturbance)

Dominance of fire-disturbed longleaf pine savanna is example of how diversity decreases However, shrubs and grasses in understory remain

very diverse--due to disturbance preventing any particular species from dominating completely

Conclusions: Disturbance and community succession are flip sides

of same coin Succession involves complex combination of multiple

processes: facilitation, inhibition, tolerance, random colonization

Succession can be primary changes associated with newly created environment, or much more frequent secondary changes associated with disturbances to well developed community

Climax communities can result from facilitation in late seral stages, or from frequent, severe disturbances such as fire; however few communities have any one type of climax

Life history traits closely associated with seral stages

Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs. 2001. The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.