Download Lecture 22. Succession Reconsidered

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