Presentation on theme: "Recap Food web structure influences the stability of communities Constancy and resilience; diversity and stability Constancy and resilience; diversity."— Presentation transcript:
Recap Food web structure influences the stability of communities Constancy and resilience; diversity and stability Constancy and resilience; diversity and stability Communities can switch between alternative stable states Alternative stable states Alternative stable states Trophic levels are influenced from above by predation (top-down control) and from below by production (bottom-up control)
Topographic distribution of forest communities in the Great Smoky Mountains National Park (west-facing) OH: red- oak-pignut hickory OCH: chestnut oak-chestnut heath OCF: chestnut oak-chestnut forest ROC: red oak-chestnut H: Helmock forest; P: pine; F: Frazir fir; SF: spruce-fir; S: Red spruce; GB: grassy balds HB:Heath balds An example of forest zonation
Patterns of co-occurrence for 4 plant species on a landscape along a gradient of altitude
August 27, 1883 Island of Krakatau Volcano erupted followed by tsunamis Catastrophe natural lab Ecologists studied the colonization of species Source islands: Sumatra and Java (40km from Krakatau) Sea-dispersal arrived first (10 of 24 by 1886) Others, wind-dispersed grasses and ferns and moved away from beach Then wind-d trees, birds and bats arrived …
Plants dispersed by physical forces are the first to arrive in primary succession
Outline (Chapter 19) 16.1 The concept of the sere includes all the stages of successional change 16.2 Succession ensures as colonists alter environmental conditions 16.3 Succession becomes self-limiting as it approaches the climax
16.1 Community structure changes through time Successional changes over 30 years in a western Pennsylvania field. Cropland or Grazed grassland grasses, goldenrod, weedy herbaceous plants shrubs (blackberry, hawthorn) fire cherry, pine, aspen forest of maple, oak, cherry or pine. Grazed grassland in 1942 aspen and maple in 1972
Cleaning of oak- hornbeam forest 7 years after15 years after 30 years after 95 years after150 years after
Succession Definition: The process of gradual and (seemingly) directional change in the structural of the community through time (from open field to forest) Temporal change in community structure Sere (from the word series): The sequence of communities from grass to shrub to forest Seral stage: each of the changes is a seral stage, is a point of continuum of vegetation through time, can be short or long (1 or 2 yrs to several decades) Climax community: ultimate association of species achieved
Species dominance change along time William Sousa Process of succession in a rocky intertidal algal community in southern California (by W. Sousa) Use concrete blocks for algae to colonize Panel b shows succession Early successional species (pioneer species): High growth rate, small size, high degree of dispersal, high rates of population growth Late successional species: Low rate of dispersal, slower growth rate, larger and live longer (competition?).
Species dominance change along time Hubbard Brook Experimental Forest, New Hampshire Process of succession after forest clearing Prior to forest clearing, beech and sugar maple seedling dominate understory Following clearing, pin cherry, yellow birch etc. will replace. After many years, sugar maple and beech will dominate. Primary succession (example before, a site unoccupied by a community) and secondary succession (this example, occurs on previous occupied site after disturbance, remove part or all vegetation)
Primary succession occurs on newly exposed substrates Primary succession begins on sites that never have supported a community, such as rock outcrops and cliffs, sand dunes, and newly exposed glacial till. Primary succession on a coastal sand dune colonized by beach grass Later on, shrub, then trees (pines and oak if they can survive)
Primary succession occurs on newly exposed substrates
Glacier Bay fjord complex in southeastern Alaska. Ice retreats, primary succession occurs
Secondary succession occurs after disturbances Terrestrial environment: Oldfield succession in the Piedmont region of North Carolina by Dwight Billings in the late 1930s Abandoned farmland 1st yr: crabgrass, horseweed 2nd yr: horseweed, white aster and ragweed 3rd yr: broomsedge, pine seedling 5-10th yr pine Later oak and ash Decline in pine and increase in hardwood (oak and hickory)
Plant life history influence old-field succession Tolerance and inhibition interact with life history to shape details of the species sequence during succession. Oosting and Keever, Duke University, 1950s, old field study, Piedmont, NC
Old field on the Piedmont of North Carolina. Abandoned agricultural fields undergo a series of successional changes. (shrubs start to replace annual plants)
The climax community Friderick Clements (1916, 1936): Monoclimax hypothesis view community as a highly integrated superorganism, the process of succession represents gradual and progressive development of community to ultimate or climax stage (similar as development of an individual organism) Clements’ climax community (Closed system): Community is like an organism, it arises, grows, matures, and dies. Seres are different stage of development, ultimately lead to similar climax community Fourteen climaxes: 2 grassland: prairie and tundra 3 scrub: sagebrush, desert scrub and chaparral 9 forest: pine-juniper woodland to beech-oak forest
The climax community New views consider climax communities represent a continuum of vegetation types (open system) Communities were ordered along a continuum index Continnum index: scale of an environmental gradient based on the changes in physical characteristics or community composition along the gradient Stages in the sere will lead to sugar maple climax community, but so-called climax vegetation actually represents a continuum of forest types Curtis and McIntosh (1951)
16.2 Succession ensures as colonists alter environmental conditions Two factors determines a species’s present in a sere: 1.How readily it invades a newly formed or disturbed habitat 2. Its response to environment over the course of succession Early pioneering species hypothesis F. Egler (1954):Initial floristic composition Succession at any site depends on which species gets there first. No species is competitively superior to another. Once the original dies, the site becomes available to others. Joesph Connell and Ralph Slatyer (1977): three models (facilitation model, inhibition model, and tolerance model)
Alder facilitates succession by adding nitrogen to soils Alder trees harbor nitrogen-fixing bacteria in their roots, providing nutrient to soils, facilitate the establishes of nitrogen-limited plants such as spruce, which will replace alder later.
Facilitation, inhibition, and invasive species Mycorrhizae fungi and plants: mutualistic sybmioses Fungi can facilitate or inhibit plant growth (positive and negative) Effects are larger in home-home than home-foreign (Black cherry, Maple with soil microorganisms)
The differing adaptations of early and late successional species Early-stage species and later inhabitants tend to have different strategies of growth and reproduction. Early-arriving species: High dispersal ability. (dandelion, milkweed) Climax species: large size, slow growth, but shade-tolerance as seedling (oak, maple)
The survival of tree seedlings in shade is directly related to seed weight
16.3 Succession becomes self-limiting as it approaches the climax Succession continues until the addition of new species to the sere and the exclusion of established species no longer change the environment of the developing community. The progression from small to large growth form modifies the conditions of light, temperature, moisture and soil nutrients. Conditions change slowly after the vegetations achieves the largest growth form that the environment can support. Final dimensions of a climax community are limited by climate independently of events during succession.
Succession becomes self-limiting as it approaches the climax Time required for succession from a new or disturbed habitat to a climax community depends on nature of climax and initial quality of habitat Mature oak-hickory climax forest from old field in North Carolina: 150 yrs Climax stage of grasslands in western North America: 20-40 years Humid tropics, reach climax within 100 years from clear cut, but may take a few more centuries to achieves a fully mature structure and species composition. Sand dune beech-maple climax, up to 1,000 years Climax is an elusive concept: Communities also change in response to climate change, hunting, fire, and logging, disappearance of keystone consumers (wolf, passenger pigeon) and trees (chestnuts, eastern hemlock)
Climax communities under extreme environmental conditions Fire is an important feature of many climax communities, favoring fire-resistant species and excluding species that would otherwise dominate. Longleaf pine after a fire Seedling may be badly burned, but the growing shoot is protected by the long, dense needles.
Grazing pressure also modify a climax community Grassland can be turned into shrubland by intense grazing Herbivivores may kill or severely damage perennial grasses and allow shrubs and cacti that are unsuitable for forage to invade. Selective grazing Some species prefer to feed on areas previously grazed by others. Both zebras and Thompson’s gazelles feed on Serengeti ecosystem of east Africa, but eating different plants. In North America, cattle grazing may lead to invasion by alien cheatgrass, which promote fire.
Transient and cyclic climaxes Succession is a series of changes leading to a stable climax, whose character is determined by local environment. Once established, a beech-maple forest perpetuates itself, and its general appearance changes little despite constant replacement of individuals within the community. Transient climaxes: such as communities in seasonal ponds – small bodies of water that either dry up in summer, or freeze solid in winter. The extreme seasonal changes regularly destroy the communities that become established in the ponds each year. On African savannas, carcasses of large mammals are devoured by a succession of vultures including: large, aggressive species smaller species that glen smaller bits of meat from bone species that cracks open bone to feed on marrow.
Cyclic climax: Suppose, for example, species A can only germinate under species B, B only under C, and C only under A. The relationships create a regular cycle of species dominance in the order of A, C, B, A, C, B, A, …, in which the length of each stage is determined by the life span of the dominant species. Cyclic succession is usually driven by stressful environmental conditions.
When high winds damage heaths and other types of vegetation in northern Scotland, shredded foliage and broken twigs create openings for further damage, and soon a wide swath is opened in the vegetation. Regeneration occurs on the protected side of damaged area while wind damage further encroaches on exposed vegetation. Temporal: wind damage and regenerate, cycling Spatial: mosaic patches
18.4 Succession is associated with autogenic changes in environmental conditions Environmental changes can be grouped into two classes: Autogenic (direct result of organisms within community) Created by organisms in community Light in a forest Allogenic (a feature of physical environment) Created by physical environment Elevation on a mountain (decline temperature with elevation) Stress in a salt marsh, seasons Succession: changes in community structure through time; specifically, changes in species dominance. During plant succession, it creates autogenic environmental change in a place Both primary and secondary succession, colonization alters environmental conditions How do autogenic environmental changes influence succession?
During plant succession, it creates autogenic environmental change in a place. For example, light environment (vertical distribution) Initial colonization, the light at ground level is high, seedlings are able to establish themselves. As plants grow, their leaves intercept sunlight, light availability declines from canopy to ground levels, reducing light to short plants The reduction in light enables fast-growing plants to out-compete the other species that dominate the site Sun-adapted, shade-intolerant plants exhibit high rates of photosynthesis and growth under high-light conditions (Under low light, they can not survive). Only shade-tolerant species exhibit much low photosynthesis rate and growth under high-light conditions, are able to continue photosynthesis and growth, and survival under low-light (trade-off)
In the early stage, shade-intolerant species dominate because of their high growth rate. They grow and shade the slower growing, shade-tolerant species. As time progresses and light level decline below the canopy, seedlings of the shade-intolerant species can’t grow and survive in the shaded conditions. At this time, although shade- intolerant species dominate the canopy, no new individual are being recruited. In contrast, shade-tolerant species will germinate and grow, and replace the old, dead shade-intolerant spp.
Example of succession 1 st Early successional species, pine, eventually dominate only in canopy Pine seedling regeneration declines as light decreases in the understory 2 nd Shade tolerant species (oak and hickory) seedlings established Shade intolerant pines die out due to no seedlings Shade tolerant take over Density of species change
18.6 Species diversity changes during succession In addition to shifts in species dominance, patterns of plant species diversity change over the course of succession. Comparison of species diversity at different sites within an area that are at different stages of succession; such groups of sites are: Chronosequences (or chronoseres) For example, farmland abandoned at different times (herbaceous: crabgrass, horseweed, broomsedge; wood: pine, oak) Colonization and replacement processes
Changes in plant diversity during secondary succession of an oak-pine forest in Brookhaven, New York. Species richness increases into the late herbaceous stage, declines into the shrub stage, then increases in the early forest stages, then decreases thereafter. Colonization increases species richness, replacement decreases spp richness.
Hypothetical succession involving five plant species (Huston, ORNL) Species diversity increases initially as new species colonize the site. As autogenically changing environments and competition result in the displacement of early successional species, diversity declines Normal growth rateDouble growth rate
Pattern of succession under three different disturbance frequencies Intermediate disturbance hypothesis: community with intermediate disturbance has large species diversity High to low?
18.7 Succession involves heterotrophic species Not only autotrophic component of community (plant succession) show succession, changes in heterotrophic component of the community also occur. Decomposition (tree soil) Tree fall bark beetle, wood boring beetle fungi broke down lignin bacteria predator insects (centipedes, mites, pseudoscorpions, beetles) fungi moss and lichens seedlings
As plant succession advances, changes in structure and composition of the vegetation result in changes in the animals life that depends on vegetation as habitats.
18.8 Systematic changes in community structure occur as a result of allogeneic environmental change at a variety of timescales Shifting patterns of community structure in response to autogenic environmental changes often occur at time scales relating to the establishment and growth of the vegetation. Purely abiotic environmental change can produce patterns of succession over time scales ranging from days to millennia.
Short-term change: temporal changes in abundance of dominate phytoplankton species during the period of May to Oct every year. Optimal temperature controls the abundance Lawrence Lake, MI
Paleoecology: study of distribution and abundance of ancient organisms and their relationship to the environment Pleistocene was an epoch of great climate fluctuations (1.8m to 10k yr BP). 7 ice-sheet (4 in N. America 3 in Europe) advanced and retreated. Last great ice sheet reached maximum advance at 18,000 BP Cold state, tundralike vegetation dominated; glaciers retreated, light- demanding forests(birch, pine) advanced. As soil improved, climate warmed, shade- tolerant trees (oak, ash) dominated.
Paleoecology: study of distribution and abundance of ancient organisms and their relationship to the environment Study fossil: bone, insect exoskelecton, plant part pollen grains Pleistocene was an epoch of great climate fluctuations. 7 ice-sheet advanced and retreated. Last great ice sheet reached maximum advance at 18,000 BP
Post-glacial migration of four tree species Numbers are thousands of year before present
Changes in distribution of plant communities during and after the retreat of the Wisconsin ice sheet (pollen analysis). mixed- hardwood and oak-hickory- southern pine shrieked Southern pine expanded