alpha diversity

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Transcript alpha diversity

Global biodiversity patterns and changes
Richness, S = number of species
Diversity, H’ = pi log2 pi
a, b, g diversity:
alpha diversity: the diversity of a site
beta diversity: the change in species
composition from place to place, or along
environmental gradients (turnover)
gamma diversity: the diversity of a region or
landscape due to either or both a and b.
(Begon et al. 1996)
In sea, diversity increases to
6000 m, then decreases
(Sanders et al. attribute to
stability, then resource
Macan’s Filter: If a species isn’t present at a site, is it
because of…
Dispersal (age, distance/isolation) ?
yes, propagules haven’t arrived and established.
no, propagules arrived, but populations don’t persist…
Behavior (specialization/heterogeneity) ?
yes, colonists avoid this habitat.
no, colonists settle, but don’t persist…
Abiotic factors (stress, productivity, heterogeneity) ?
yes, physico-chemical conditions outside tolerable
no, abiotic conditions sufficiently benign…
Biotic factors (productivity, disturbance, heterogeneity) ?
yes: predators, pathogens, parasites, competitors or
lack of mutualists exclude species.
Stanley 1976 hypothesis: breakthrough that ended the
Pre-Cambrian was the invention of ‘jaws’—the first
herbivorous protists grazed holes in algal monocultures,
producing environmental heterogeneity…
Tropical vs temperate niches
Are niches narrower
(species more
specialized) in the
tropics (a)?
Can more niche overlap be
tolerated without
competitive exclusion (b)?
Are resource axes longer
in the tropics?
Is niche space more
completely filled?
LawtonEmpty niches?
Papua New
New Mexico
Bracken fern
Milankovitch theory: orbital oscillations of Earth cause cyclical
changes in temperature and precipitation:
Eccentricity of orbit: 100 ky cycle
Annual timing of aphelion (point in planet’s orbit when it is
furthest from the sun) relative to summer solstice:
23 ky cycle
Tilt of wobbling pole: 41 ky cycle
Ruddiman 2000
Dynesius and Jansson 2000
Orbitally forced species range dynamics (Dynesius and Jansson 2000)
• Mean duration of species 1-30 my, so species have endured many
Milankovitch oscillations
• Oscillations more severe at higher latitudes than around
equator, selecting in temperate, boreal and arctic species for
– More vagility
– Less specialization
Range restricted = < 50,000 sq. km
Dynesius and Jansson 2000
Larger effect
change) at
latitudes than
near equator
Brown and Maurer 1989: Hypoth:
N. Am. species with small
geographic ranges are limited by
local environmental conditions so
should run NW, while species with
large ranges limited by large scale
patterns of vegetational or climatic
zones that correspond to latitudinal
bands (run EW)
North American reptiles
should run
Diversity vs
Productivity: a humpshaped relationship,
like intermediate
Longest running ecological experiment: 1843 - present
Science 293 p. 625 (2001)
Light triangles: extinct
Dark triangles: extant
Parmesan, C. 1996
Censused Edith’s
checkerspot butterfly
throughout its range for
151 previously located
populations. Striking
latitudinal trends in
extinction: populations in
Mexico 4x more likely to
have gone extinct than
those in Canada. Habitat
loss was similar at
southern and northern
ends of range, suggesting
range contraction due to
climate change. This
hypothesis also supported
by less frequent
extinctions at higher
Ice age 25-25 K y bp
Invasive species…
SF Chronicle Nov 28 2003
David Schindler
whole lake
manipulations in
Lakes Region of
southern Ontario
Over a wide range of pH in
acidification experiments, and
of nutrient concentrations in
eutrophication experiments,
ecosystem respiration and
photosynthesis stayed fairly
constant, while species
composition varied markedly.
Schindler et al. 1985
Plots with higher
plant diversity
maintained more
constant biomass
across severe 1987
Drought resistance
(biomass 1988/biomass1986)
Cedar Creek LTER
Tilman and Downing 1994
Duffy et al. 2003: more diverse grazer assemblage more
effective in removing algal epiphytes from sea grass beds
Ecosystem consequences of diversity loss…
(Chapin et al. 2000).
• “Rivet hypothesis” (Ehrlich): which species are crucial to ‘holding
the wings on the plane’, esp. in changing environments?
• Grasslands: less productivity, resilience after drought
• Plant production enhanced linearly by mycorrhizal species
• Consortia of diverse microbes each contribute different
enzymes to speed organic matter decomposition and nutrient
• Societal costs of diversity loss (some speculative examples…)
– Passenger pigeons may have suppressed deer mice by competing for
acorns, reducing Lyme disease reservoirs (Ostfeld)
– Cheatgrass fires reduce rangeland value and air quality (D’Antonio)
– Teosinte, wild relative of corn, needed periodically to develop virusresistant strains—nearly lost to land conversion around Jalisco, MX,
and now possibly subject to genetic engineering introgression
Higher species diversity in tropical
rainforest vertebrates because of
higher rates of speciation, or lower
rates of extinction?
Most sister taxa arose before
Pleistocene, but recent species
occur in heterogeneous,
geologically or climatically unstable
Evidence for both gradient and
allopatric diversification
Conservation strategies need to
preserve both the important
genetic lineages (variation) and the
ecological processes that generate
species (e.g. refuges and barriers:
edges and forest core)
Moritz et al. 2000: