Succession and Biodiversity

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Information about Succession and Biodiversity

Published on March 3, 2008

Author: Petronilla


Succession, disturbance, & Biodiversity:  Succession, disturbance, & Biodiversity Slide2: Slide3: Ecological Succession:  Ecological Succession Progressive change in species composition, ecosystem function and structure following a disturbance minor changes in structure and function accumulate over time Ecological function of ecosystem changes Initiated by a disturbance Directional change in structure Follows a predictable pattern First ideas of succession:  First ideas of succession Frederick Clements (first in 1916) Often called Clementsian succession Viewed succession as a deterministic phenomenon Ended each step is a sere, and succession ends with a climax community What does this mean?? Clementsian succession:  Clementsian succession Species established in an area ALTER THE ENVIRONMENT Allows invasion by other species which otherwise would not have been able to colonize the area Directional change in species composition (community structure) maintained by continual alteration of the environment until the climax community is reached Clementsian Succession:  Clementsian Succession Given a specific community: Succession will ALWAYS have the same seres/communities leading to a specific climax community (e.g., a boreal forest) If a certain sere is interrupted, the community will follow the same path again Hence, the idea of predictability and being deterministic Clements’ Climax Community:  Clements’ Climax Community The concept of a climax community assumes: the species colonizing & establishing themselves in a given region can achieve stable equilibrium Stable equilibrium  when the forces to change the system are equal to the forces to keep the system the same; therefore, there is not change in the system over time Therefore, the climax community = stable equalibrium Clementsian Example:  Clementsian Example Hardwood trees, a climax community, has a disturbance in the form of deforestation The community will ALWAYS (deterministic) proceed in this way and will be predictable Barring new disturbances, the hardwood community will be reached and will be stable Problems with Clementsian:  Problems with Clementsian “Always” is a very strong word...will it “always” happen this way? Idea of a deterministic pathway fell out of favor Idea of a stable community (climax community) fell out of favor Organismic and Individualistic Views of Succession:  Organismic and Individualistic Views of Succession Clements  community as a superorganism Member species tightly bound together both now and in their common evolutionary history Organismic view: Individuals  cells Populations of species  tissues Communities  organs Ecosystem  organism Organismic and Individualistic Views of Succession -- Clements, cont.:  Organismic and Individualistic Views of Succession -- Clements, cont. Clements organismic view: successional concept  whole is greater than the sum of the parts directional change in the species composition of the community to a climax community primarily by climate This is why Clements’ view is deterministic Organismic and Individualistic Views of Succession:  Organismic and Individualistic Views of Succession Gleason (1926) – Individualistic view He saw the relationship between coexisting species (communities) as the result of similarities in their requirements and tolerance NOT as “organs” of a “superorganism” partly the result of chance (random) Clementsian view challenged:  Clementsian view challenged Gleason (1926) and others Viewed communities as RANDOM aggregations of individuals THEREFORE, succession is NOT deterministic Clementsian view challenged:  Clementsian view challenged Gleason argued: Clements organismic view did not explain the mechanism of succession Example: Gleason view could not explain phenomena such as retrogressive successions Gleason stated that ".... every species of plant is a law unto itself, the distribution of which in space depends upon its individual peculiarities of migration and environmental requirements“ Thus, associations of plants, or communities, were not highly organized, but aggregations of independent plant species, each specialized to survive on habitats they were adapted for. Retrogressive succession:  Retrogressive succession Succession where the plant community becomes simplistic and contains fewer species and less biomass over time. So, Gleason would say to Clements: Did the “superorganism” no longer need that “organ” (relationship between species lost)? How will the “superorganism” live without it? Clements vs Gleason:  Clements vs Gleason Clements: assumed long-term climatic stability so short-term changes like retrogressive succession was not possible Gleason: assumed that environmental variables can deteriorate over time Deterioration would change the pattern of establishment, growth and reproduction of plants in a habitat Clements vs Gleason - summary:  Clements vs Gleason - summary Clements: Superorganism, deterministic, interdependence among species/communities, predictable Gleason: Species together due to chance & adaptations to environment, no relationships between species, just together because have similar environmental requirement Stable climax state refuted:  Stable climax state refuted Began in 1920s, by 1950: succession viewed as a phenomenon that rarely attains equilibrium Why?  related to the nature of disturbance Disturbance  acts on communities at a variety of spatial and temporal scales (scale dependent) Magnitude of disturbance varies Many disturbances remove only a part of the previous plant community. Two classifications of seres:  Two classifications of seres Primary succession  the establishment of plant communities on NEWLY FORMED HABITATS previously lacking plants Lava flows, sand dunes, landslides, etc. Secondary succession  return to vegetation to its former state following a disturbance Some blurring…(related to view of disturbance on previous slide) Blurring of primary and secondary:  Blurring of primary and secondary Example: Tornado levels a strip of forest Even though trees/all plants gone, seed bank and nutrients are still there  secondary succession follows Example: Severe fire may burn through the organic layer of the soil, destroying the seed bank, nutrients, etc. May be like a primary succession even though plants were there before Unusual way to think of succession:  Unusual way to think of succession Animal carrion (dead animals) Succession of animals (invertebrates) that break down body Can use ‘sere’ to age when a body was deposited (given temperatures, seasons, location, etc) Moral of the story…:  Moral of the story… Disturbances can vary in the degree that they impact a community What is disturbance, anyway? Disturbance:  Disturbance Disturbance: little agreement in definition "any relatively discrete event in space and time that disrupts ecosystem, community, or population structure and changes resources, substrate, or the physical environment" (Pickett and White 1985) discrete in time (vs. chronic stress or background environmental variability) cause a notable change (a perturbation) in the state of the system Examples of disturbance:  Examples of disturbance Fires Mudslides Tree gaps in forests Insect blights Flooding Human-caused as well...more later Consequences of disturbance:  Consequences of disturbance total habitat destruction/extinction creation of new habitat (i.e., habitat transformation [replacement]) habitat fragmentation (which implies habitat loss, isolation of habitat remnants, and habitat transformation) increase patch number, isolation, edge decrease patch size, connectivity, interior alter local climate/microclimate, hydrology, biota (in terms of diversity, behavior, health, fitness, and persistence) Are all changes disturbances?:  Are all changes disturbances? NO A disturbance falls outside the system’s natural variance BUT we often do not know what the natural variance of a system is Example Short time/localized spatial scale: forest fire is a disturbance But long time scale: some forests REQUIRE fires for seedling regeneration (e.g., in Florida) Moral of the story: your frame of reference is important  must state spatial and temporal time scales Intermediate disturbance hypothesis:  Intermediate disturbance hypothesis Highest diversity at intermediate level of disturbance (not too rare or too frequent, or not too intense or too light) # of species Disturbance summary:  Disturbance summary Time & spatial scale important Intensity of disturbance important Type of disturbance important How often a disturbance happens (frequency) important (Intermediate Disturbance Hypothesis) Much more on this topic not covered here... Excellent place to watch succession – abandoned fields:  Excellent place to watch succession – abandoned fields Abandoned fields such as the Piedmont of North Carolina Annual plants  Herbaceous perennials and shrubs  Pines  Hardwoods Change rapidly at first Annuals grow quickly Change in species composition slows down as annuals are outcompeted by slower-growing plants (perennials, shrubs) Then pines, which require decades to grow, to eventually be replaced by hardwoods Slide31:  Piedmont of North Carolina Slide32: In terms of succession::  In terms of succession: Disturbance happened often enough that the ‘climax community’ (e.g., hardwood forest) was not reached Transitional community would have a higher species diversity Mount St. Helens Eruption, May 1980:  Mount St. Helens Eruption, May 1980 Natural laboratory for research on succession Can make observations to see importance of competing theories Different kinds of disturbance associated with eruption Mount St. Helens Eruption:  Mount St. Helens Eruption Survival of organisms was strongly influenced by characteristics of disturbance processes, local site conditions, and biological factors Pyroclastic flow and avalanche debris: almost no organisms survived Elsewhere, diverse refuges facilitated survival: In the blast area, survivors included plants with underground buds, burrowing animals, and organisms protected by snow, topography, or other features thin tephra fall, organisms that were able to stand above or penetrate the deposits had a greater potential for survival Certain life-history attributes important:  Certain life-history attributes important not present during the eruption (May) anadromous fish (which return from the sea to rivers to reproduce) migratory bird populations were Organism size also proved important Large species and individuals suffered greater mortality than did small ones Food webs changed, but still functioned:  Food webs changed, but still functioned Surviving organisms included all of the primary trophic groups--herbivores, predators, scavengers, and decomposers Complex foodwebs quickly developed in the emerging ecosystems Surviving species provided source populations, ameliorated site conditions, processed dead biological legacies (such as toppled trees) of the pre-eruption system, and established ecological interactions Timing of eruption important:  Timing of eruption important Timing of eruption strongly influenced patterns of survival and succession Eruption in early morning allowed nocturnal animals to be protected in their subterranean retreats Eruption in early spring: snow and ice created refuges and that plants had not yet broken winter dormancy at high elevations Early successional stage of many recently harvested forest sites Profusion of wind-dispersed seeds of pioneer plant species Timing and chance:  Timing and chance The importance of timing underscores the significance of chance in survival and successional pathways. More supports Gleason’s view of succession than Clements’ Key successional processes after major disturbance:  Key successional processes after major disturbance Dispersal Site amelioration Establishment of organisms, species accrual, biotic interactions Changing community structure Key successional processes after major disturbance:  Key successional processes after major disturbance Dispersal pattern and rate influenced by distance from source populations wind patterns landscape permeability mobility of seeds, spores, and organisms Key successional processes after major disturbance:  Key successional processes after major disturbance Site amelioration was critical The new volcanic substrates: low nutrient status little moisture holding capacity limited shade BUT differed from lava flow substrates common in other volcanic terrains in that they were readily penetrated by plants and animals Site amelioration occurred due to: weathering, decomposition, addition of nutrients to soils, microbial activity, mixing of soils by animals, and clearing of suspended particulates from lakes Key successional processes after major disturbance:  Key successional processes after major disturbance Over time, community structure increased across all disturbance zones as species accrued, individuals and populations grew and spread, and interactions developed Rate of ecological response varied greatly among different environments:  Rate of ecological response varied greatly among different environments Lakes and most streams: largely returned to ecological conditions Blast area landscape: barren gray  mostly green with vegetation dominated by herbs, shrubs, and small patches of surviving trees Debris-avalanche and pyroclastic-flow deposits: vegetation is lush around ponds and wetlands, but sparse herb and shrub cover characterizes uplands and actively eroding sites Life history of species important in recolonization:  Life history of species important in recolonization The number of vertebrate species and their population sizes have increased dramatically since 1980 Birds have colonized with increased habitat structure, but species are limited by lack of forest structure Small mammals associated with undisturbed forests have returned to even the most disturbed areas Secondary disturbances:  Secondary disturbances Modified pathways of succession Shifting river channels small landslides and mudflows  repeatedly reset succession increases heterogeneity of vegetation patterns and thus consumers associated with them Human activities & Mount St. Helens:  Human activities & Mount St. Helens Human activities have greatly altered ecological processes in many areas. However, Congress established a National Volcanic Monument so natural processes can proceed unimpeded Environmental scientists: provide advice regarding protection of natural features; management of erosion, floods, and natural resources interpretive programs and educational programs However, often potential risks to human life or property outweigh ecological concerns Lessons learned from Mount St. Helens:  Lessons learned from Mount St. Helens First, living and dead biological legacies are integral to the ecological response, even after severe disturbances E.g., dead trees and rotten logs Second, ecological succession is very complex: happens at varying paces along diverse paths, & with periodic rollbacks via secondary disturbances Conclusion: no single, overarching succession theory provides a good explanation to successional processes Lessons learned, cont.:  Lessons learned, cont. Third, chance factors, can strongly influence survival and the course of succession E.g., timing of disturbance at various scales Finally, environmental scientists have provided a long-term & broad-scale view of how human actions can affect ecological systems Guidance is important but not always an integral component of decision-making (politics involved) What if disturbance regime altered?:  What if disturbance regime altered? Example: Reduce the frequency of fires and grazing in prairies Red cedar/mesquite trees grow more dense Use lots of water Lower water table Reduce stream flow Compounded effects What if landscape severely altered by disturbance?:  What if landscape severely altered by disturbance? Human or natural Hurricanes wiping out islands/marshes Haiti: deforestation has caused several feet of soil lost to erosion  big trees can no longer grow there Both cases: cannot just ‘rewind’ and start over New ecosytems will need to be created Playa wetlands:  Playa wetlands Drying essential to maintaining ecosystem function Some sources:  Some sources Ricklefs’ The economy of nature, 4th ed. 1990 Putman’s Community ecology 1994

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