Evidence for equilibrium forces structuring communities

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Equilibrium models of species coexistence
Evidence for equilibrium forces structuring
communities If community composition is
dictated by niche assembly then... 1. Pattern
of species occupancy in a community should be
predictable and stable. 2. Perturbations to a
community should result in a return to a stable
equilibrium composition after disturbance.
BUT... How do you determine through
observation if natural communities are at or near
equilibrium when rates of change in composition
are slow?
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What evidence is there for community stability?
- Several examples with insect communities in
which species have annual life cycles Lawton
and Gaston (1989) looked at composition of 20
spp of insects living on plots of bracken fern in
the UK over 7 years and at 2 sites (woodland and
open). Found that species relative abundances
remained remarkably similar year after year, and
that rank timing of appearance of insect species
was similar each year Unclear how much
disturbance to the system over the
study. Composition apparently not driven by
interspecific competition. Community is composed
of herbivores with different feeding niches.
Density-dependent predation important?
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Several other examples of long term stability in
community composition eg Ocean plankton,
(McGowan and Walker 1979) Odonates in lakes
(Crowley and Johnson 1992), Rothamstead Park
grassland experiment (Silvertown 1987) Fewer
examples of predictable responses to community
disturbance Terborgh (1996) Substitute space
for time by examining chronosequences. What
evidence that successional sequences culminate in
mature phase forests with similar species
composition and similar species abundance
patterns?
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Terborgh looked at primary successional sequences
on river-bends in the Manu river in SE
Peru. Over long distances the Manu river
produces many point bars - often separated by
several km of upland forest differing in species
composition. Individual bars are largely isolated
from one-another by a matrix of upland
forest Looked at composition in plots in
floodplain forest at 3 sites lt1 km from each
other and two sites 30 and 40 km away
Meanders in the river generate replicated point
bars of deposition of silt on the inside of the
bend and erosion on the outer bend. Over
approximately 300 yr vegetation on the bar
attains a mature stage floodplain forest.
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Point bars along the Manu river, Peru
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Rank abundance for most common spp (out of 386)
present in 1000 tree samples on five mature
forest floodplains
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Terborgh paper concludes The non-equilibrium
model, although useful as a null hypothesis is
exasperatingly devoid of biology. There is a vast
difference between saying that we do not exist,
and saying that we do not fully understand what
they are. Although null models based on random
processes can serve as useful guides, they are no
substitute for hypotheses based on explicit
biological mechanisms. Only through the latter
will significant progress be made toward
understanding diversity
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Perhaps even better evidence for constancy in
community composition comes from studies of coral
reefs (Pandolfi 1996) - For each site
found no significant difference in taxonomic
composition of the reefs over time even though
during any one reef-building episode only about
25 of spp present in the available species pool
actually occupied a particular reef environment.
Looked at coral composition of reef communities
at 3 sites in Papua New Guinea over a 95,000 yr
period during which sea level fluctuated up to
120 m and sea temp 6oC. This resulted in 9
cycles of perturbation and reef re-assembly.
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Found that variation in space greater than in
time... - Variation in coral reef assemblages
among sites at any rebuilding episode gt that at
any one site among the 9 rebuilding
episodes Evidence from these coral reefs
suggests that some marine communities exhibit
consistent patterns of assembly - more so than
comparable terrestrial systems from the
quaternary period Why?
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Processes allowing persistence of numerous
species in equilibrium communities Resource
variation?? What conditions are necessary? (Think
R) Variation in physical or biotic factors
influencing a species resource requirements
Fine scale partitioning of food resources
(animals) Fine scale partitioning of
regeneration requirements (plants) Density
dependent population regulation
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Density-dependent predation increased rate of
predation when prey are abundant First
recognized by Janzen (1970) and Connell (1971) in
two independent papers. Hence referred to as
Janzen-Connell Hypothesis They predicted
that host-specific pests reduce recruitment near
susceptible adults (where seed density is
greatest) thereby freeing up space for other
plant species. Janzen-Connell mechanism could
provide a limit on abundance of common species
(and conversely an advantage to rare species) -
why?
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Janzens (1970) graphical representation of seed
predation creating recruitment space around
conspecifics. (Hubbell has a rescaled version of
this allowing recruitment below the crown)
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How common are Janzen-Connell effects?
Hammond and Brown (1998) reviewed 46 studies that
compared seed or seedling performance near or far
from conspecifics. Performance lower near the
conspecific for 15/19 populations whose
principal herbivore was an insect 2/27
populations with principal herbivore a vertebrate
(maybe because polyphagous and wider ranging?)
Very little information about pathogens Could
J-C effects be responsible for high diversity in
the tropics?
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Packer and Clay (2000) Spatial patterns of
seedling mortality
Looked at limitation to recruitment of black
cherry (Prunus serotina) in Indiana by soil
pathogens Some pathogens might generate strong
Janzen-Connell effects if they host specific
because they can increase rapidly in abundance
and can persist in the soil
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Packer and Clay observed that for cherry a)
seedling and b) juvenile survival increased with
distance from an adult cherry tree and with
decreasing cherry neighbourhood density
(seedlings only)
Hypothesized that lower mortality was due to soil
borne pathogens How would you test this
hypothesis?
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Isolate soil effects from other potential factors
that might covary with distance from parent
Conduct pot experiment with soil from different
sources (Cherry density and distance from the
parent)
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Strongest evidence that pathogens are involved
requires fulfilling Kochs postulates
C1 Potting mix only C2 Potting mix sterile
fungal growth medium P1-3 5ml of inoculum
potting mix
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What about community level effects of
density-dependence? Harms et al (2000) evaluated
density dependence at the critical seed-seedling
transition phase by comparing numbers of seedling
recruits R appearing in plots, to the number of
seeds S, captured in adjacent traps. Evaluated
the relationship with a the power law R cSb
where c and b are constants so, logR logc
blogS ask if the slope, b, differs from 1
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Seeds (S) in traps
Seedlings (R) in soil
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Evaluated 53 species from seed captures at 200
census locations and found that b lt1 for every
species indicating negative density dependence
(median value of b 0.23)
Relationship between seed density and seedling
density for Trichilia tuberculata (Meliaceae)
Seedling recruit density
Seed density
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Okay all spp show density-dependent recruitment
- what is the community consequence?
Ask how does local community diversity of trapped
seeds compare to that of seedling recruits. -
First took into account mean transition
probabilities of different species (on average,
how many seeds are needed to generate a seedling
recruit). - Use mean transition probabilities
to calculate expected seedling numbers of each
species in each seedling plot. Calculated the
Shannon-Weiner diversity index for each plot
based on these predicted seedling numbers. -
Second, used the slope of regressions of
seed-seedling plots to calculate predicted
seedling number for given seed number
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Difference in diversity indices for 53 spp
comparing seed rain to traps to seedling
recruits in plots Adding mean values for
seed-seedling transition probabilities helps a
bit Using the predicted seedling abundance
values from regressions centers the 2
distributions...
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Hubbell et al. (2001) looked at density effects
at next stage (sapling stage) on BCI - Critical
to this study is that they have data on the fate
of huge numbers of saplings (250,000) monitored
over 13 years (previous analyses with shorter
census periods did not yield the same results) -
Used logistic regression analysis to calculate
the odds of survival of a large number of species
as a function of the structure and composition of
their close biotic neighbourhood - Neighbourhood
variables stem density, relative plant size,
number of conspecific neighbours, and species
richness. - Strongest neighbourhood effects for
the 2.5 m annulus around each focal plant
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Survival probability of pioneers more strongly
affected by presence of conspecific neighbours...
X axis proportion of trees within 2.5 m that
are the same species (conspecific)
Rare species much more strongly affected than
are common species might explain why rare
species are rare??
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What about groups of organism other than plants??
Classical theory (Hutchinson 1957, 1959) argues
that fine partitioning of feeding niches can
support the diversity of herbivores Increasing
body size of organisms at higher trophic levels
limits population density and may limit how
finely feeding niches are divided - constraining
diversity? Very few tests of theories of
diversity maintenance for insects - what would
you need to do to test equilibrium vs
non-equilibrium models for the maintenance of
insect diversity??
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Novotny et al. (2002) Host specificity of
herbivorous insects in New Guinea

• Analyzed host preferences of 900 insects on 51
  plant species
• Most species feed on several (congeneric) host
  species. Tight specificity is rare
• Even phylogenetically distant hosts shared a
  third of herbivores
• An average tree hosted
• 33 Coleoptera
• 26 Lepidoptera
• 20 orthopterids

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FT total number herbivores found on all hosts
total host plant records involving those hosts
Butterflies
Other lepidoptera
Coleoptera
orthopterids