Lecture 22. Succession Reconsidered

-concept of succession was introduced in Intro Ecology

-we look at succession specifically in the context of forests and soils in my senior Soils course

-here, will look at succession as an ecological phenomenon in different ecosystem types

-to see if we can find common mechanisms and principles guiding this process

**my thesis:

succession is a special case of continuous ecosystem response to external driving variables

-succession is best understood by looking at how ecosystems change in response to changes

in the physical environment at multiple time scales

Review:

-this outline present a succinct review of what we know about succession

Succession: A sequential, directional, biotically driven series of changes in community structure

following disturbance

L Succession follows disturbance that changes population densities or availability of resources

L Succession may be primary (from nothing) or secondary (after disturbance of a community)

L Autotrophic succession may culminate in a stable, self-perpetuating community, the climax

L A successional sequence from disturbance to climax is a sere; any community along the way

is a seral stage

-all these definitions refer to terrestrial vegetation

** one change inserted to the definition of succession: it is biotically driven

-succession happens because species already present in the community change

environmental conditions so that other species may colonize or thrive

-it is the biotic community re-arranging and restructuring itself

-that is why it is called autotrophic (“self-feeding”) succession

-technically, succession driven by outside forces is allogenic succession

-field is full of such useless terminology that doesn’t really explain anything

recall: primary succession for seres that occur on new habitat being created for the first time

-e.g., receding glaciers, volcanic debris, exposed rock faces

-classic Maritime example of primary succession is sand dunes along the ocean

-secondary succession follows disturbance of an established ecosystem;

-differs from primary succession in that some species survive the disturbance,

-and habitat features such as soil usually persist

-recovery of a forest after a fire is a good example of secondary succession

-division between primary and secondary succession is arbitrary,

-merely represent extremes of a continuum of disturbance intensity

-Example: a very hot forest fire may eliminate virtually everything but a few seeds and microbes

-while other fires may only burn some of the aboveground vegetation.

-primary succession is just most extreme example of biological disturbance

** most research on succession, and most conspicuous examples, arise from terrestrial, forested

ecosystems of eastern North America

-and most of this work concentrates on vegetation with little regard for other species

-we have fewer examples from other terrestrial biomes,

-some from freshwater and marine ecosystems, such as rocky intertidal zones

-this bias may be part of the reason our theories of succession are so incomplete

The Role of Disturbance

-when does succession occur?

-conventional theory says that succession always follows a disturbance

-disturbance is at the heart of successional theory; so let’s examine this idea in detail

-recall the definition of disturbance from Intro Ecology:

(1) Any relatively discrete event in time, that

(2) disrupts ecosystem, community or population structure, and

(3) changes resource availability or the physical environment

-lots of debate about what constitutes a disturbance

-from catastrophic events like forest fires or hurricanes or tsunami

-to smaller events like a local storm, a flood, a severe winter

-even a local windstorm that throws over a few trees could be considered a disturbance

-Recall from Intro Ecology:

Magnitude of a disturbance depends on:

(1) Frequency: how often it returns

(2) Severity: how great are its effects

(3) Extent: how large an area it affects

-using these characteristics we can categorize disturbances by their magnitude

-recall that frequency is negatively correlated with severity and extent

-ecosystems suffer a few, large, widespread disturbances and frequent small, local ones.

** textbook argues two other characteristics: type of disturbance and its timing

-same disturbance striking a mature ecosystem may have different effects

than if the ecosystem were recovering from another disturbance

** disturbances must be defined in terms of the normal range of environmental variation

in any given ecosystem

-events that constitute a major disturbance in one ecosystem would be entirely normal in another

-Example: freezing temperatures are an annual event here but a disturbance in Florida

** dividing line between normal variation and a disturbance is an arbitrary one

Text says: herbivory is considered a normal part of the functioning of most ecosystems

-but stand-destroying insect outbreaks like spruce budworm or pine bark beetle are disturbances

-therefore difficult to define “disturbance” unambiguously

** this is an important issue to which I will return later

-for now, emphasize that disturbances large and small are normal events in any ecosystem

-most terrestrial disturbances do two things:

(1) reduce live plant biomass and (2) change the pool of actively cycling soil organic matter

-succession can then be viewed as “a directional change in ecosystem structure and function

resulting from biotically driven changes in resource supply.” (Text, p. 285)

-resources such as light, water, nutrients drive succession on land

-Text Figure 12.7 (p. 348) shows a range of disturbance types,

-with magnitude indicated by the amount of soil organic material that each removes

-disturbance removes some or all species from the ecosystem

-and relieves competition for resources, allowing new species to colonize

-in a sense, the disturbance “resets” the ecosystem to an earlier successional state

-succession then begins to move it back toward a later successional state again

-according to original theory of Clements, succession proceeds to a climax

-at which time the structure of the ecosystem is not changing

-and resource demand matches resource supply

-in practice, most ecosystems are disturbed again before they reach the climax

-in fact, most local variation in terrestrial ecosystems is accounted for by differing

frequencies of disturbance and different stages of succession

-recall from Intro Ecology: “Every ecosystem is recovering from the last disturbance”

Community Changes During Succession

**succession is driven, mechanistically, by different resource requirements of different species

-for example, certain plant species are better at colonizing open ground than others

-these species are usually tolerant of wildly varying physical conditions

-tend to be ruderals: wide-dispersing, fast growing, small, annual plants

-but these pioneer species, by their presence, change the habitat of the site

-so that it no longer has all the characteristics to which they are best adapted

-hence, these species render the site more suitable for other species than for their own offspring.

[Aside: Remember, species change the habitat because they can’t help it, not because they benefit

from succession. A spruce tree, adapted to growing in full sunlight, cannot help but cast shade.]

-original species are replaced in succeeding generations by new species, driving succession.

-as the ecosystem develops, environmental conditions become more stable,

-competitive ability becomes more important than dispersal or rapid growth

-that is, community shifts from r-selected species to K-selected species

** one qualification: in secondary succession, resources are usually abundant,

-left over from the previous ecosystem; hence rapid growth of colonizers is an asset

-in primary succession, resources may be very scarce,

-so ability to grow in a poor environment at whatever speed, is more important:

-compare lichens (1 succession) and weeds (2 succession)o o

-this difference is also shown in Text Figure 12.10 (p. 353)

-average seed mass increases from primary to secondary to late succession (left graphs)

-but highest growth rates are in secondary succession, when nutrients are most available

-growth limited in primary succession by supply (nutrient capital)

-and in late succession by competition

** because of these differences in colonization rates and growth rates,

-succession would proceed even in the absence of any interactions among species

Succession is more complicated than one set of species replacing another, however

-recall there are three different kinds of species interactions that govern succession:

Facilitation: Established species favour colonizing species

Inhibition: Established species impede colonizing species

Tolerance: No interaction between established species and colonizing species

-most obvious example of facilitation is N-fixing species

-these enrich the soil with N, increase resource availability and allow other species to survive

-particularly important in primary succession

-shade-intolerant trees that permit other species to grow in their shade are also facilitators

** inhibition is also surprisingly important

-established species frequently prevent colonizing species from establishing

-or compete with them when they do, slowing population growth

-inhibition can be thought of as effect of competition at the community level

-we seem many instances of inhibition

-because competition is so widespread, and established species have an advantage

-finally, we have tolerance, which isn’t really an interaction at all

-tolerance occurs when one species becomes established independently of another

-Example: in succession of old fields to forest, young spruce trees arrive relatively early,

-when site is covered with grass and perennial forbs

-they suffer no severe interactions with other species:

-removing competitors makes no difference

-but we don’t notice them for a long time because they are small

Herbivores

-a false picture to suggest that succession is driven entirely by plant interactions

-herbivores and pathogens may also have a strong influence on the rate and pattern

-selective browsing by mammals (deer, mice, rabbits) accounts for much mortality of early

successional species in northern forests, favouring later species

or, browsing-intolerant species are replaced by browsing-tolerant species

-we have seen many examples of indirect effects,

-in which selective grazing or predation on one species shifts the competitive balance

among prey (plant) species

-at community level, these indirect effects change community composition

-here again we see herbivores and predators as important regulators of community processes

-same effect may arise from pathogens, which may weaken or even remove an early species

-trembling aspen, an early successional species, is prone to Armillaria root rot,

-from a pathogenic mushroom:

-it kills the trees leading to replacement of the infected stand

-black knot of cherries (Apiosporina morbosa) may do the same thing around here

Above: Black knot fungus on cherry branches. Below left, Armillaria mushrooms growing out

of a root-rot infected aspen tree. Below right: Fallen aspen trees killed by Ganoderma sp.,

another common cause of fatal root rot.

I thought it was about time we had some pictures.

Varying Trajectories

-classically think of succession following a single, predictable sequence from pioneers to climax

-in fact, successional sequence depends a great deal on chance events

-most important of these is initial colonizers

** plants that colonize after a disturbance can vary greatly from one disturbance to another

-these initial species have a strong influence on the direction in which succession proceeds,

-through their interactions with later colonizing species

-especially true in primary succession,

-because opportunities for colonization decline as succession proceeds

** in many forests, the dominant species interaction is tolerance:

-all tree species colonize at essentially the same time;

-successional changes in dominance reflect differences in size and growth rate

-also examples from marine environments:

-where first alga to colonize a bare spot effectively prevent other species from colonizing

-called pre-emptive competition

-environment may also modify the successional pathway after a disturbance

-Example: in Nova Scotia, spruce and grey birch colonize upland sites after fire or logging

-wet sites come back in alder, red maple and larch after the same disturbance

-these differences in initial colonizers then combine with different possible pathways

-at each stage of succession, there may be a possibility of several different pathways

-figures on next page show examples of multiple pathways for Nova Scotia forests

-any particular site may go through many different paths

-depending on initial colonizers, nature of the site and disturbance, and random chance

** As a consequence of unpredictable pathways, a given site may have more than one climax

-half a dozen climax forests in Nova Scotia, for example

-in addition, succession may be modified along the way by new disturbances, large and small

-many sites never reach equilibrium because they are always disturbed before they get there

Ecosystem Changes

-succession is more than a change in community composition

-also profound changes in ecosystem structure and function

-Text goes into this in great detail; read p. 356-364 to learn more about it

-look at Figure 12.16, (p. 359), which summarizes idealized patterns of carbon flux

during succession on land

-in primary succession, soil C and plant C both begin from zero and climb steadily,

-eventually reaching an asymptote as the climax is reached

-see Text Figures 12.12 and 12.15

NPP, NEP and respiration (litter decomposition) increase, level off, and then decline

-at climax, by definition, NEP should be zero,

-because decomposition exactly matches production

-in secondary succession, soil C and plant C begin at a high level and are sharply reduced

-because that is what disturbances do

-both plant and soil C then recover to the original level through successional time

-disturbance also depresses NPP and NEP,

-knocking NEP below zero

-these then recover more or less as for primary succession

Lecture 23. Succession: Continued

Aquatic Succession

-everything to this point has applied to terrestrial ecosystems

-succession also occurs in freshwater and marine ecosystems

-Figure 15.2, (p. 300 in Levinton, Marine Biology) shows colonization

of Thalassia sea-grass beds in Florida

-bare sand deposited by wave action is first colonized by seaweeds

-these stabilize the sediments, deposit organic matter and nutrients

-only when sediments are stable and N-rich do vascular plants colonize

-also see successions of algal species in the open water every year

-very-well studied sequences of succession on rocky intertidal zones

-succession known even in mud-bottomed sediments (Figure next page)

-as shown on this [Figure 16.7, p. 331 in Levinton)

-recent work has demonstrated a succession on artificial substrates presented

to benthic communities around deep-sea submarine vents (Figure next page)

-in general, marine succession follows same general rules and patterns as on land:

1. Begins from disturbance

2. Governed by many factors, largely nutrient availability and grazing

3. Inhibition, facilitation and tolerance between species may occur

4. Changes from fast-growing colonizers to slow-growing competitors

5. Many complications and exceptions, including multiple pathways and repeated disturbance

-succession in rocky intertidal zones has been intensively studied

-these ecosystems undergo frequent, intense disturbance, so a natural place to study succession

-also easy to get at

** general sequence is shown below:

SUCCESSION ON A ROCKY SHORE

1. Bare rock

2. Bacterial slime layer

L intense wave action may maintain this stage perpetually

3. Ulva or Enteromorpha (green seaweeds)

L establish where wave action is less intense than in stage 2

L good colonizers, but prone to intense grazing

L limpets and periwinkles may graze back to stages 1 or 2

L cycle may repeat numerous times

4. Fucus (brown seaweed)

L good competitor

L appears later because of slow colonization

L resists grazing by toughness and toxins

L once established, only removed by disturbance

** change in community composition depends only on life history of the species involved

-Fucus establishes only on bare rock,

-but gets there later because it is slow to disperse

-there is no facilitation, merely a colonization sequence

-if Ulva is prevented from colonizing (by a scientist with a razor blade),

Fucus will colonize anyway

Differences from Land

-succession in marine ecosystems differs from that on land

-in the same way that these types of system differ in general structure

-some tentative generalizations follow: we have too little data to be sure

(1) first, classic examples of facilitation, such as nitrogen fixers and shade-intolerant trees,

do not occur in the water

-successional sequences driven more by colonization, species traits than by species interactions

(2) second, often no clear concept of a climax, self-perpetuating community

-many communities appear to be equally stable at any step along the way,

-or are so frequently reset by disturbance that the concept of a climax never enters the picture

(3) greater influence from grazers:

-grazers on algae and seaweeds an important force structuring those communities

-on land, competitive interactions among plant species appear to be more important

(4) a corollary: because of the large number of “plant-like” sessile animals,

-we have to look at “vegetation” in a very broad sense:

-corals, barnacles, mussels, are animals, but they act like vegetation

-hence terms like structural species or foundation species

(5) absence of a solid phase (soil) in pelagic ecosystems limits their successional scope

-marine and freshwater algae do undergo a predictable sequence of community compositions

-these sequences are reset every winter and repeat each growing season

-are these true autogenic successions?

-or merely a seasonal response to climate (phenology)?

-seasonal succession mostly driven by seasonal changes in light, temperature, nutrient availability

-but grazers and disease (micro-fungi) may also be important

**many concepts underlying succession simply do not apply in pelagic systems

-no soil development, no loss of organic matter, no change in NEP

-terrestrial theory (again) doesn’t seem to have been developed for aquatic ecosystems

\Aside: go back for a moment to discussion of trophic cascades

-Pace et al. (1999, Trends Ecol. Evol. 14:483-488), suggest that:

-mature ecosystems (diverse, longer time from disturbance) should have fewer trophic cascades

than simpler, early-successional, low diversity systems

-community-level trophic cascades are far more common in aquatic ecosystems than on land

-suggesting that aquatic ecosystems are not capable of becoming mature like terrestrial systems

-congruent with idea aquatic ecosystems cannot become mature because they lack a solid phase

\end Aside

Transitional Ecosystems

-appears that, again, pelagic ecosystems differ from terrestrial systems through lack of soil

-therefore, moving from deep oceans to nearshore environments, we anticipate that succession

will proceed more like on land

-this is indeed the case, as was shown earlier for salt marshes

-although species are different, system follows a recognizable sere, pioneer to climax

-there are recognizable changes in nutrient cycling, accumulation of organic matter,

-that parallel those on land

** in general, transitional ecosystems (wetlands, tidal marshes, coral reefs)

more or less follow terrestrial rules of succession

-many small wetlands can be thought of as merely a spot of wet habitat within larger woods

-these systems succeed as larger, emergent plants establish, organic matter accumulates,

water depth decreases

An Alternative Explanation for Succession

-reading the literature on succession, one is struck by the difficulty of making generalizations

-and how frequent and pervasive are exceptions to any general model

-we can predict succession in forest with reasonable accuracy

-but models that do so are immensely complicated and require gallons of input data

-in addition, generalizations from land appear to be only weakly applicable to water

-suggests that we do not have a good general theory of succession

-therefore, I have been forced to make one up

Here it is: Succession does not exist

-or more exactly, the model of terrestrial succession that we currently have is useless

-to see how I come to this conclusion, go back to the idea of disturbance and legacy effects

-ecosystems are always changing

-processes of growth, decay, nutrient cycling, are sensitive to condition of physical environment

** ecosystems are constantly responding to varying physical conditions of environment

-responding on every temporal scale, and from smallest to the largest physical scales,

-Text Figure 12.19 (p. 365) shows enormous temporal range of variables influencing ecosystems

-in addition to this temporal range, similar range in size and severity of disturbance

-this range is reflected in difficulty we have defining “disturbance” accurately

-text points out a continuum of disturbance severity

-between (say) large-scale defoliation and the loss of a single leaf

-or between a landslide and burial of a leaf by an earthworm

** therefore a continuum in severity of disturbances

-between day-to-day functioning of ecosystems at one extreme,

-and events that initiate primary succession at the other

-Text Figure 12.7 (p. 286) is an example of what I mean

** in addition to current situation, ecosystems are always recovering from previous changes

-may be time lags great and small between when an external variable changes

and when ecosystem fully responds to that change

-these include relatively predictable, fine-scale changes like daily and seasonal patterns,

-rather less predictable changes such as weather systems (frontal systems),

-long-term changes like atmospheric warming and cooling, and glacier advances and retreats,

-and finally, unpredictable, random events such as fires, volcanoes, floods, hurricanes,

-which also may occur frequently or rarely

** behaviour of any ecosystem is a reflection of both current environment

and past changes to the environment to which ecosystem is still responding

- persistent effect of a disturbance in the past is called a legacy

-legacies are very common, especially in terrestrial ecosystems

-Example: many trees live for hundreds of years; aspen clones can be up to 10 000 years old

-current distribution of these trees is a product of current environmental forces,

**-but also a legacy of past influences

-Example: much marginal farmland in the eastern N. America and Europe has been abandoned

-over the past 200 years, as agriculture intensified and people moved to the cities

-these lands have reverted to forest,

-but legacy of past agriculture remains in the soil

-these forests are still accruing organic matter (NEP >1) and will do so for the foreseeable future

-soil physical structure, drainage, nutrient retention, biota, still reflect disturbance by ploughing

-Example: many plant species, especially long-lived trees are still migrating northward

following the retreat of the glaciers.

-present distribution is limited not by environmental tolerances but by their migration speed

-nut-bearing trees may move northward only as far as a squirrel travels, each generation

-Example: Freschet et al. 2014. Aboveground and belowground legacies of native Sami land use

on boreal forest in northern Sweden, 100 yr after abandonment. Ecology 95(4): 963-977.

-Example: researchers have recently discovered 2000-year-old Roman farms in central France

-these farms were unearthed in mature forest that everyone thought had never been disturbed

-soil structure, compaction, microbiota, even tree diversity is different in the farmed ground

-a legacy persisting two millennia after disturbance

-so, we know that ecosystems respond to external forces (state variables);

-and we know that these changes may involve enormous time lags

-especially if long-lived species or soil development are involved

** soil is why these legacies are so much more common on land than in water

-look at a classic successional sequence, from abandoned farmland to beech-maple forest

-after perhaps 200 hundred years, we finally arrive at a forest of mature trees

-are we at equilibrium?

-first test is to see if the understorey (seedlings) is the same as the overstorey

-if not, then the vegetation is not yet at equilibrium

this example shows closed-canopy forest may still

be a long way from climax

-west coast rainforest takes >1000 years before a self-perpetuating climax appears

-if we go forward to stable vegetation, are we at equilibrium?

-probably not: in most ecosystems, soil would still be accumulating biomass

that is, NEP > 0, still

-will the soil ever reach true equilibrium? Who knows?

-conditions never stay stable for that long:

new species invade (glacial rebound) or the climate changes again

-or the system is disturbed by a fire or a flood or an earthquake or etc.

Here is a summary of my alternative view of succession:

1. Ecosystems respond to external forces

2. Different ecosystem components respond at different rates

3. Slowest ecosystem components respond to external forces at rates congruent

with long-term climatic and landform changes

4. Because climate and landform are never in equilibrium, ecosystems are never in equilibrium

5. “Succession” is a convenience term for ecosystem responses at medium rates which cause

conspicuous changes in community structure

-I argue that succession in the Clementsian sense is a misconception

-instead, acknowledge that ecosystems change in response to state variables

-some of these responses are slow because they involve long-lived species or soil

-slowest of them are not much faster than climate change

-it follows that, since these driving factors alway change, that ecosystems are always changing

** we see one small part of range of possible responses,

-those that are conspicuous and occur within a human lifetime, and we call that succession

-but we are really talking about an arbitrary section of a wider span,

-rather like visible light within the whole electromagnetic spectrum

-if we examine succession, senso latto, in terms of how ecosystems respond to external variables,

then we have a better chance of building a predictive model that will apply to all ecosystems, dry,

wet or salty

Primary succession at it’s finest: ferns and flowering plants colonizing lava flow in Hawaii