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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