Lecture 19 –

Succession is the response of a community to a disturbance that opens a relatively large area for invasion, or reoccupation by organisms. The disturbance should sufficiently large such that intermediate replacement by neighbors is not possible.

Succession – ‘nonseasonal’ directional change and continuous pattern of colonization and extinction at a site by populations. The local replacement of populations in a regular sequence following the opening of a space by some disturbance.

Basic question of interest – Are patterns of replacement predictable and what is the underlying mechanism(s) that accounts for these changes?

Not in the sense of succession over geological time where changes in biota reflect changes in the physical enviroment.

For examples changes in sea level (e.g. lowering) can result in marshes becoming prairie. These patterns of ‘succession’ are not determined by inherent properties of the system because they results from major changes in the environment.

Within the context of changes over ecological time succession can be identifies or classified as:

Primary succession – sequence of species replacements that follow often create a totally new habitat (i.e. no organic matter present). E.g. flood that strips top soil, volcanic eruptions that establish new substrate, etc)

Secondary succession – occurs when there is some form of life already present, either as living or as decaying substance as in well developed soil. Most succession studies address this type.

Climax – stage in community development in the successional process where the composition of the community is relatively permanent, as long as further disturbances do not occur. Usually composed of a ‘cline’ – gradient in

distribution of organisms.

Historical background:

Studies on temporal changes in communities began in the early 20^th century by plant ecologists.

In 1916 Clementss the foremost proponent of the view that succession represented an orderly and predictable process analogous to embryological development. Succession in his view was highly deterministic and predictable and represented an emergent property of the system (i.e. the whole is more than the sum of the parts).

Each stage was required for the next culminating in establishment on a stable ecosystem - homeostatic view – where the climax stage achieves maximum resistance to disturbance.

The mechanism envisioned for this succession was facilitation. One stage paving the way to the next. Each stage was necessary and sufficient.

Clements was a plant ecologist who studies plant communities in North America at a time when there were many disturbed forest areas. Establishment of farms formed patch work of habitats in varying stages of recovery from human disturbances.

His view of succession was developed from this patchwork pattern. He saw a homology between spatial sequences of zones of vegetation visible at one time in a landscape, and the long-term sequences of vegetative types on a single site.

For example: Lake Michigan

Creeping annuals ® grass ® cotton trees ® pines ® oaks (young dunes) (old dunes)

Wherever indicators were out of sequence succession was occurring.

Do these patterns represent succession over ecological time scales or do the merely reflect the biota specific to a particular physical climate independent of the previous occupant?

Difficult to address in vegetated communities because the intervals of change are longer than the life-span of the individual investigator.

Early stages ® extrapolated to later stages.

Clementsian view was challenged by Gleason who argued that changes could be explained by random spreading and establishment by individual plant species – doesn’t require a "successional" process.

"No super-organism"

Clements argued for a single climax species. View modified by Gleason who noted that locations quite close to on another could have different dominant species. E.g. slopes on hill dominated by hickory and oak, in valleys by beech and maple.

Gleason view was term "poly-climax" – some physical factor that determined the climax species.

Clements’ view prevailed for many decades, but the prevalence of facilitation was not as high as Clement envisioned. Clements’ view however was "group selectionist"

In general, early and late successional species have the following traits:

Adult size small large

Growth rate fast slow

Number many few

Size small large

Frequency of reproduction many few

Dispersal ability high low

Competitive ability low(?) high (?)

Resistance to disturbance low (?) high (?)

Classic life history augment is that late successional species have high competitive ability and resist disturbance at the cost of higher investment in growth, low investment in reproduction (not necessarily true in terms of number of offspring.

Earliest colonists however are likely to be best at colonizing, many propagules with frequent periods of reproduction.

So, succession may simply reflect life-history patterns not facilitation.

Mechanisms of succession suggested by Connell and Slatyer.

• only certain species can establish (pioneers) – must start with these species.

• modify environment making it more suitable for later species.

• early species eliminated through competition for resources by later arrivals.

• Sequence continues until current residents no longer facilitate invasion and growth of next species or until no species exists to invade and grow in presence of current adult population.

• Individuals on any species can arrive and exist as adult under prevailing conditions (early recruiting species fill up spaces due to rapid recruitment).

• modification of environment has little to do with subsequent recruitment of later successional spp.

• early species eliminated through competition for resources by later arrivals. (as in facilitation model)

• sequence continues until current residents no longer facilitate invasion and growth of next species or until no species exists to invade and grow in presence of current adult population (as in facilitation model)

• One less step in contrast to the Facilitation model

• modification of the environment by early occupants makes it less suitable for recruitment of later successional species (early w/ good colonizing abilty).

• early species persist as long as undamaged.

• replacement only occurs when damaged (external force) replaced by more resistant species

• no climax per se

Sousa (1979)

In comparison with terrestrial environment, with low turnover rates in species composition and a prolonged succession, Sousa examined succession in a marine community. Algal communities in the rocky intertidal zone. He was able to address these models because:

1. rapid turnover – disturbance frequent
2. stages of succession all represented in the same area
3. could use artificial substrates to simulate bare space openings

He could describe patterns of successional change and experimentally determine the mechanisms for these changes.

In the system:

Algae recruit by spores, physical factors are wave action and desiccation, biological factors is grazing.

Found earliest colonists were green algae – Ulva

Rapid colonizer with continuous breeding throughout the year.

Fig. 1

Was able to demonstrate that Ulva could prevent other algae from invading.

Fig. 2

Results suggest inhibition

To be true: later successional species could only become established if early successional species were more susceptible to physical damage or stress or predation (still possible to be tolerance if sufficient time were allowed)

To test:

Determine if later colonist succeed by differential resistance to physical environment vs. later species outcompete early arriving species.

Observation of mortality during long tidal exposure.

Fig 3:

Ulva recruits the year-around, others particularly Gigartina (fleshy red algae) recruit only in the Fall, coincident with high Ulva mortality resulting from desiccation from extreme low tide exposures.

Predation experiment – Ulva differentially susceptible to grazing by crabs and mollusks.

Middle successional trend explained by some interspecific competition when large clearing become a mosaic of small opening, middle successional species can extent vegetatively better than Ulva.

So, in Sousa’s study life-history traits are important in determining sequences of succession. Ulva rapid colonizer, rapid growth, non-resistant to grazers vs. Gigartina with opposite attributes.

For tolerance model competition is key element in succession during early stages Þ early species give way to late spp by competitive displacement.

For inhibition model differential resistance to environmental stress and /or predation Þ Ulva while not a poor competitor, still has lots of other characteristics of early colonists.

Old field succession in abandoned farms in eastern US.

Pre-colonial hardwood-conifer forest mix. Sites abandoned for known periods of time.

Typical sequence:

Annual weeds ® rapid colonizers (disturbance ecology) dormant in soil, disturbance key to germination.

Herbaceous perennials ® win out over annuals by having winter growth – usurp space.

Shrubs ® slow growth but eventually shade out perennials. Also produce chemical inhibitors to deter predator/grazers.

Early and Late successional trees ® differences in shade tolerance, multi-layered (early) vs monolayered species (late).

Example of facilitation model

Primary succession – Glacial retreat ® leaves boulder clay till, nutrient deficient.

Soil pH

0 – 50 years 8.0 Lichens, mosses ®

Willow

¯

50 –100 years Alder (nitrogen fixer)

¯

¯

Facilitation in marine system Harris et al. 1984

Fig 4

Sporophytes

Bitten (%)

Filamentous algal height

Kelp beds disturbed by storms ® rapid invasion by filamentous algae ® provides protection of kelp sporophytes (recruits) from grazers ® cover, or refuge.