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Lecture 9

BIO120 lecture 9-12 note

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Doug Thomson

Lecture 9: Metapopulations, plant community composition Core Concepts 1. Application of spatial and metapopulation models to predict butterfly and pika dynamics t 2. Plant ecology statics: concept of the species association 3. The organismal hypothesis versus the individualistic hypothesis 4. Gradient studies as a resolution of the debate 5. Plant ecology dynamics: primary and secondary succession Case study Population persistence of a rare butterfly in habitat patches  A real case in conservation biology & spatial ecology  Fender’s Blue butterfly depends on a rare plant  Willamette Valley, Oregon; in 1850, all native prairie  Now, all but 0.5% converted to agriculture  Butterfly discovered in 1920, thought extinct by 1931, rediscovered 1989  Known from 13 prairie fragments  Annual pulses of reproduction followed by heavy larval mortality General conclusions on stability and coexistence 1. Model populations can be driven to extinction in several ways:  Strong density-dependence (chaos)  Unstable competition  Unstable predator-prey (disease-host)  Allee effects at low density **But these tendencies are countered by non-equilibrial conditions, habitat patchiness, rescue-by-migration, variation in life-history strategy Plant community ecology originally focused on discovering “community types”  What species regularly occur with each other?  Do find significant species associations, (e.g., beech-maple forest, oak-hickory forest, bur-oak savannah)  Mostly descriptive: experimentation is a recent addition Early controversy: Conflicting views of causes of associations  Organismal or holistic hypothesis: certain species found together because they are biologically integrated and depend on each other’s presence like tissues of an organism (typological community concept)  Individualistic hypothesis: species are distributed independently of each other; important limitations are: o 1. dispersal and; o 2. filtering by the physical environment Frederick Clements (1916): “The developmental study of vegetation necessarily rests upon the assumption that the [association] is an organic entity. As an organism the [association] arises, grows, matures, and dies.” Henry Gleason (1926): “…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. Its disseminules migrate everywhere, and grow wherever they find favorable conditions… Plant associations…depend solely on the coincidence of environmental selection and migration…” Other evidence for the individualistic hypothesis?  Curtis’s “indirect” gradient analysis reached same conclusion more objectively, using multivariate statistics (see EEB321!) to assign stands positions along a compositional continuum: did not find strong clustering  Margaret Davis used pollen data to reconstruct post-glacial migrations of tree species; found that different tree species migrated along different paths and at different times: therefore, tree communities didn’t migrate as units, membership varied continuously  Multiple lines of evidence often needed to topple an old paradigm Lecture 10: Spatial ecology, plant communities, and disturbance Core Conceptz 1. Plant succession: classification 2. Role of disturbance type, frequency, and size 3. Fire ecology 4. Intermediate disturbance hypothesis Community dynamics: predictable successional change in plant communities:  Pioneer species get in first (from dispersal or seed bank in soil)  Soil-building processes and shade thought to be critical  Happens at many levels, but most heavily studied in human-impacted landscapes in eastern North America, esp. “old-field” succession from abandoned land to forest  Vegetation changes spontaneously as the vegetation itself modifies the environment Classifications & Terminology (1)  Classic view of successional sequence  Starts with pioneer species (weedy, r-strategists)  Goes through temporary, non-equalibrium stages  Ends at climax stage, stable equilibrium, no more change Classification & Terminology (2)  Primary succession: new substrate created, no pre-existing vegetation  Secondary succession: pre-existing vegetation undergoes a disturbance  Disturbance = discrete event that cause abrupt change in ecosystem, community, population; sets back succession  E.g.: fire, windstorm, logging, agriculture Examples Primary succession:  New lava flows seeds and spores blow in Pioneer plants can establish themselves  Soil development: o solid lava erodes into finer particles; o dead plants contribute organic matter o more complex soil starts to develop o more plants can establish in the better soil  Plants attract birds; birds bring more seeds  Herbaceous plants cover ground, trees grow  Tree canopy closes in, soil is well developed shade becomes important Secondary succession:  Old field succession, year 1: Annual Weeds  Old field succession, stage 2: Perenniel weeds for several years  Old field succession, stage 3: Woody shrubs move in  Old field succession, stage 4: Tree saplings  Old field succession, stage FIVE: Tree canopy closes in shade becomes a main factor  Old field succession, stage 6: shrub layer thins, shade tolerant understory only  Old field succession, stage 7: Only shade-tolerant spp. Remain, including canopy tree spp. That are now replacing themselves – species turnover minimal Drivers of Terrestrial succession:  Soil development, especially accumulation of organic matter, N content, pH buffering, water retaining capacity (especially important in primary succession)  Shading (especially important in secondary succession, where soil is already developed); shade-tolerant species replace shade-tolerant ones  Succession may reach a stable climax configuration of dark shade, organic- rich soils (e.g., beech-maple in or region) – but often does not Succession where no climax-type equilibrium is attained:  Boreal forest: successional stages lead to spruce-fir forest, but it does not replace itself  Acid, sandy soils: pine-oak leaf litter can actually make soil more acid, not richer  Fire-prone ecosystems and biomes (many pinelands, grasslands, chaparral)  Systems driven by seasonality (plankton in temperate lakes)  Cycling of dominants (A replaces B, then B replaces A)  Transient substrates (decay of a log) More modern understanding of patterns:  Climax terminology in disuse: now “old-growth”  Regions and ecosystems have characteristics disturbance regimes; most “equilibria” are quasi-equilibria at most  Spatial scale matters: “gap-phase” succession  Habitats a mosaic of patches in different stages of regrowth after disturbance  Intermediate disturbance hypothesis for maximum species diversity Lecture 11: Trophic relationships in communities Core Conceptz: 1. Direct and Indirect interactions embedded in food webs; trophic cascades 2. Interaction strengths revealed by community experiments 3. Plant-herbivore interactions as biodiversity drivers 4. Insect-plant relationships often specialized, vertebrate-plant relationships less so Trophic Levels:  Primary producers = plants  Primary consumers = herbivores  Secondary consumers = carnivores who eat herbivores  Tertiary consumers = carnivores who eat secondary consumers  Detritivores = eat dead organic matter Special difficulties of herbivory:  Easy to be a carnivore: Animal tissues easy to convert into animal tissues  Plant tissues hard to convert into animal tissues o Cellulose and lignin tough, indigestible without microbial symbionts o Plant tissues heavily defended against herbivores, mechanically and chemically o Coevolutionary race between plants and insect herbivores is responsible for much of biodiversity: specialization is common Many different defensive chemicals, incl. about 10,000 alkaloids with diverse, potent biological activity: Some alkaloids important to humans: •  Caffeine •  Theobromine •  Strychnine •  Quinine •  Nicotine •  Morphine •  Cocaine •  Histamine •  Adrenaline •  Mescaline •  Ephedrine •  Dopamine •  Codeine •  Vincristine Challenges & solutions are different for vertebrate grazers & browsers:  Graminoids (grasses and similar plants) defended mechanically (silica) rather than chemically (meristems protected)  Chemically defended forbs (broad-leafed herbs) dealt with by dilution or food avoidance  Some detoxification by microbes in fermenting chambers Starting to put thingz together: Janzen-Connell hypothesis:  Why is plant species diversity in rainforests so phenomenal? o Partially due to unremitting attack from specialist insects and fungi in mild climate o Seedlings have a low chance of success in the vicinity of the mother plant o Strong density-dependence prevents any species from monopolizing habitat Lecture 12: Putting things together: Species interactions in subalpine meadows: 1. Zooming in: morphological and ecological characteristics of the glacier lily 2. Developing environmental variables for correlational study 3. Building an explanation consistent with the data on rocks, lilies, gophers 4. Zooming out evolutionarily via the comparative method: loss of elaiosomes? 5. Zooming out ecologically: the rock-refuge hypothesis expanded 6. Zooming out biogeographically: regional differences in systems? Basic ecology summary:  Long-lived, iteroparous; grows as a “vegetative” plant for years before flowering  Resource storage organ is underground corm  Seeds subject to desiccation unless in moist conditions  Seed dispersal distance minimal What do the experiments suggest about distribution?  Desiccation: Should find more plants away from thin soil around rock outcrops (We thought we saw the opposite)  Weak dispersal: Should find most seedlings near flowering plants (Can look for this)  Need to gather quantitative data on plant abundance and environmental factors What factors do we try to
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