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Transcript
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