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Lecture 6

Lecture 6 on Successional Dynamics in relation to Landscape ecosystems

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Environmental Studies
ENVS 3710
Sam Benvie

Successional dynamics in relation to landscape ecosystems - Succession bad rep in the 50s and 60s to deterministic of its use o Locked into the use of the deterministic process o Succession and dynamics – used interchangablely Succession (ecological community dynamics) in its broadest most comprehensive sense is the gradual replacement of one community of organism in an ecosystem by another - Gradual process – not aware that it is occurring - Within a particular ecosystem - Not fully understood why it happens; but it occurs over and over again in ecosystems There are two broadly recognized types of succession: primary and secondary. Primary succession involves gradual community change on larger extents of land that has been newly created (lava flows)or newly exposed (glacial tills or landslides) - Things that happen in primary also happen in the secondary - A sort of “hybrid succession - Gap – phase succession  a micro succession (occurs on a very small piece of land); variant of second type of succession - Primary succession  devolved of life; or newly created; single cell organisms that could stay underground for a long period of time Secondary secession occurs where the standing vegetation has been removed over an extensive area by major or catastrophic disturbances (fire, agriculture, mining, flood etc.) - All natural: fire and flooding - catastrophic - Human intervention: fire (aboriginal- traditional firing of the forests) and mining - Disturbances of ecosystems – fire and agriculture The dynamic process by which community change occurs is highly site and context specific, and will not occur exactly the same even on very similar sites in the same region - Not 100% understood – why the species change - Many factors has effect on how succession occurs – how plants replace the next one - The context is also important – what is on it? What causes it to change? And time (directional flow)? – unfolding into the future; the memory (the site holds all that it has been) Secondary succession in particular will depend on the history of what has been on the site in terms of ecosystems prior to the major or catastrophic disturbances, and the history of the types, intensities, frequencies and extents of prior disturbances. - Past disturbances are important; type of disturbances - How much of the land was affected – would be in the history of the land - Would play out after the last disturbances it had on the land – previous landscape disasters might be important to the landscape of the “NOW” Secondary succession will also depend on what ecological systems exist as context to the site undergoing secondary succession, their sizes, configurations and conditions. - If abandon; it would be a different succession vs. a desert and a lake on either side of the succession - Context is not as important in the first 5 – 10 years, the most important is the memory of what has been grown on it Secondary succession has been intensely studied in ecosystems with little human intervention. In human altered ecosystems, successional processes unfold as much in elation of human use of the land as on natural processes of community change. IMG: parking lot – abandoned for 20 years (grass and trees started to grow) – primary succession o Soil underneath would harbor some seeds before the contraction took place o Start to pop up in the crakes below o Forces it to follow a slower succession process for secondary Because successional process are highly site and context specific and each site has unique history, the pattern of community change will be variable. However, there are trends, or trajectories of change which can be very broadly understood. - One general plant community (would be replaced by another) and another - Trajectory – size, species type The classic example of a secondary successional trajectory is the abandoned pasture field changing to forest - Trajectory might be the same but the species might not be the same IMG: abandon pastures In broad outline old – field succession proceeds slowly through arguable six or seven discernibly communities of organism: old field to perennial forbs and grasses to short lived shrubs and trees to longer lived shrubs and trees to very long lived trees and shrubs. Forbs of different plant species and attendant herbivores, carnivores and soil biota all change as the larger vegetated community changes - Annually or bi annually species would dominate - After 3 years – plants (perennials) – longer lived (3 – 7 or 8 years) - The plants that you would see are : Seeds within the soil (since the last time the ground was disturbed) or - Herbaceous plants – stay there for about 15 years (might come after community and community) - Endogenous – the seed has been in the soil that kick start the succession; what seed is found in the soil (the seed bank) – the memory of what has grown in that disturbed landscape; - Memory wipe – - Ecosystems are changing – when we get the first woody plants that shows up = change; sun loving shrubs (shed their seeds) – short lived; last 10 – 15 years; 50 years or 70 years o Fewer of them are going to start disappearing after many years o Depend on sunlight o Germinate away from the parent tree – the babies need full sun o Pure stands of shrubs that are even aged – age are the same; underneath – longer lived plants (trees and shrubs); outside the ecosystem (create shade); first seeds are wind dependent that need sun o Small trees – brought in by animals: have by eaten in a developing ecosystem; others need to be caught to feathers and wings to get spread around; by flooding (natural and seasonal phenomena)  Viable and need to germinate quickly  Don’t start to appear till 30 years and in 60 years they become dominant – settled changed and o Climax community – everyone that perseveres it is called a secres it; change of the above ground successions (there are also a species that changes with the species)  What happens above ground is what happens below ground Each successive community is called a sere while… (Finish) The length of time it takes for each successive community to appear is quite variable from site to site and from disturbance to disturbance on the same site. The species involved in each community will also vary under the same circumstances. Predicting the species composition of any given sere and the length of time before any given species will appear is not really possible for any given site, though again there are tendencies for certain species to be found associated with one another. - First closed community succession - With no animal or bird species - it will be a slow succession Example: monarch butterfly and milkweed; sugar maple, basswood and spring ephemeral - Tend to find; where you have specific species there are specific animals/bird/butterfly species - Not able to predict specifics The processes by which succession takes place are in part dependent on the site in question and in part on its context - Context also includes boundaries within ecosystems IMG: boundary between the woods and the mowed grass is called the ecotone (contributes to succession – to the pasture and the ecosystem); the ecotone is usually pretty sharp (mowing – sharp and straight edge); vertical, lateral and horizontal (the pasture on the other side would spread quickly outwards) - Some species might only be found in this specific boundary and nothing else - Found their way there – by insect, birds and small animals that parallel with the boundaries (responsible for moving species like plants and trees) - Light, organism that uses ecosystem that uses more than one lifecycle, soil and organic development  reason for moving outwards - Change plant community within the disturbed ecosystem and the spreading of the ecotones Site specific processes are significan
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