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Lecture 14

BIOL 3170 Lecture 14: Lecture 14 - Succession

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York University
BIOL 3170
Mark Vicari

Succession Succession = predictable sequence of changes in species composition and/or dominance over time • is temporal change in community structure • Sere = the complete sequence of changes for a particular community o e.g. grassland --> shrubland --> forest • Seral stage = one stage in the sere o e.g. grassland Early succession species (aka pioneer species) • Species which tend to be the first species to establish in an area • Characteristics: o Fast growing o Small, short-lived o Strong dispersal o Rapid population growth potential • Often r-selected species Late succession species (aka climax species) • Species which tend to establish in an area that’s already established with other species Characteristics: o Slow-growing o Large, long-lived o Poor dispersal o Slow population growth potential o Strong competitors • Often k-selected species Primary succession = establishment of species on land that has not been previously inhabited • on a new surface/substrate with no seed bank or propagules • Must receive seeds or other propagules from dispersion onto the site Secondary succession = establishment of species on land that has previously been inhabited • E.g. on an old soil with an existing seed bank and/or other propagules • Both dispersal and regeneration from seed bank important Successional sequence • Can use known successional sequences to estimate age of areas • E.g. Glacier Bay moraines in Alaska o Glaciers retreating last ~200 years o Retreating glaciers leave moraines behind o Can date moraines by age of vegetation growing on them • 5-10 years after exposure to new surface/substrate o = blue-green algae, horsetail gametophytes, lichens, liverworts, fireweed • 35-45 yrs after o = Dryas drummondii, scattered willows, cottonwood, alder, sitka spruce • 60-70 yrs o = ONLY alder thickets • 150 yrs o = Sitka Spruce • 200-225 yrs o One possible “climax”: mixed spruce-hemlock forest o Another possible “climax” (for poorly drained areas): muskeg bog of Sphagnum spp. What causes successional change? • Autogenic environmental change o = change caused by organisms within the community o E.g. Change in conditions / availability of resources due to exploitation • Allogenic change o = change due to factors not affected by organisms in the community (e.g. climate change or disturbances) Autogenic change • E.g. change in light availability o Early stages: high light availability ▪ Many species establish ▪ Good dispersers most abundant o Fast growers (shade intolerant) overtop and suppress slow growers (shade tolerant) o Fast growers form closed canopy ▪ Own seedlings can’t establish o When a canopy plant dies, it is replaced by a formerly suppressed shade tolerant plant ▪ Over time, shade tolerant species replace shade intolerant ones • E.g. change in nutrient availability o Colonization of a new, nutrient-poor site by N-fixing organisms (organisms that make their own N) ▪ e.g. bacteria, algae, or even plants with these as N-fixing bacteria as symbionts ▪ these enrich the soil with N over time o Soil eventually has enough nutrients for plants to establish • E.g. change in pH condition o Bare soil usually ~8.0 pH o Certain species, tolerant of higher pH establish ▪ Dead leaves and decaying matter lower pH o New species, tolerant of lower pH establish Mechanisms/models of succession 1. Facilitation = early arrivals benefit later arrivals o Early species modify the environment making it more suitable for later species, and/or less suitable for themselves ▪ E.g. all examples above o Especially common in primary succession 2. Inhibition = early arrivals suppress later arrival o Early species establish canopy, suppress later arrivals o Replacement occurs only after an established plant dies (from disturbance, herbivory, etc.) o Succession is from short-lived to long-lived o May be the most common mechanism 3. Tolerance = early arrivals are gradually outcompeted by later arrivals o Intermediate between (1) and (2) o Early colonists (fast-growing, resource-demanding) establish, deplete local resources  gradually outcompeted by species more tolerant of low resources o E.g. Tolerant species grow to adult size in presence of closed canopy E.g. succession in a North Carolina abandoned farmland • Secondary succession: all seeds already present in soil and in nearby areas (can disperse there) • 1 year o Immediately, crabgrass establishes o Same yr, horseweeds become dominant ▪ Shade from horseweeds inhibit asters and broomsedge nd • 2 year o Horseweed self-inhibits (pathogen buildup) o Asters establish closed canopy rd • 3 year o Asters deplete soil water --> broomsedge (more drought tolerant) outcompetes aster • 5-10 years o Shortleaf pine (even more drought tolerant) replaces broomsedge --> closed can
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