Ecological succession: lecture topics

1. 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

2. Kinds & qualitative characteristics of disturbances that impact communities Asteroid impact volcanism Disturbance magnitude Glaciation Hurricane Landslide Animal carcass ice storm fire treefall Disturbance frequency

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

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

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

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

7. 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

8. 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

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

10. Succession examples: 1º succession, Indiana sand dunes Marram grass settlement Accumulation organic nutrients Establishment shrubs Shrubs replaced by trees

11. 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

12. 2º Succession examples: bog succession, Ontario, Canada Beaver dam forms pond Bog fills in from edges Quaking bog covers almost entire wetland

13. 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

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

15. Coralline algae facilitate seedling establishment of surfgrass (Phylospadix)

16. 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)

17. 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

18. 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)

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

20. 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?

21. 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

22. 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

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

24. 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)

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

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

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

28. 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

29. 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

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