Download Equilibrium-based models of the maintenance of

Equilibrium-based models of the maintenance of
diversity
Hubbell: Emphasis on chance and history
is determinants of speies composition 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?
Terborgh looked at primary successional sequences on riverbends in the Manu river in SE Peru.
Meanders in the river generate
replicated ‘point bars’ – become
mature forest over ~300 yrs
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 <1
km from each other and two sites 30 and 40 km away
Point bars along the Manu river, Peru
Rank abundance for most common spp (out of 386) present in 1000 tree samples on five mature forest floodplains
Species
PLOT 1
PLOT 2
PLOT 3
Otoba
Astrocaryum
Iriartea
Quararibea
Scheelia
Theobroma
Guarea
Lonchocarpus
Pouteria
Oxandra
Malmea
Pseudolmedia
Calatola
Ruizodendron
Poulsenia
Unonopsis
Sorocea
Sapium
Trichilia
1
2
9
3
7
4
8
14
11
6
10
12
5
18
19
17
29
23
25
4
2
6
1
3
12
20
5
8
11
48
24
20
13
16
10
9
15
25
1
2
5
3
7
6
4
23
10
37
8
11
48
21
20
200
58
24
15
FAR
PLOT 1
3
2
1
4
5
6
13
14
15
12
43
17
11
24
9
40
FAR
PLOT 2
3
2
1
4
6
5
7
119
139
56
75
11
95
13
16
62
17
22
Longer-term evidence for constancy in community
composition comes from studies of coral reefs (Pandolfi
1996)
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.
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.
Uplift concurrent with 9 reef building episodes over 95K yr
Each reef building episode gave predictable species composition
across habitat types. Current composition is similar to pleistocene
Found that variation in space greater than in time...
Variation in coral reef assemblages among sites at any rebuilding
episode > 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? Diversity maintenance via density effects on recruitment
What do rubber and mahogany
have in common?
Density-dependent predation: increased rate of predation
when prey are abundant
Importance for diversity maintenance first recognized by
Janzen (1970) and Connell (1971) in two independent papers. Hence referred to as “Janzen-Connell Hypothesis” If pests are host-specific (…), then pests will reduce recruitment
where hosts are most abundant (for seeds and seedlings = near
adults) freeing up space for other plant species.
Interest is not population regulation but diversity maintenance
Janzen’s (1970) graphical representation of seed predation creating
recruitment space around conspecifics. How would you test this?
How common are ‘Janzen-Connell effects’?
Hammond and Brown (1998), Carson et al. (2008) reviewed
~50 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 generalists (polyphagous) and wider ranging?
• Until recently, little information about pathogens
What about community level effects of density-dependence?
J-C model is explicitly about maintenance of diversity not
density dependence therefore this hypothesis must be
tested at the community level.
Harms et al (2000) J-C envisioned that pests/predators have strongest effects on
seed and seedling survival.
If so, then should see impact of seed density on probability of
seedling recruitment.
Seeds (S) in traps
Seedlings (R) in soil What is the expectation for seeds/trap (S) vs seedlings/plot
(R) if no density dependence? Evaluated the relationship with a the power law:
logR = logc + blogS ask if the slope, b, differs from 1
What would a slope <1 indicate? Could you get a slope of >1?
Evaluated 53 species from seed captures at 200 census locations and
found that b <1 for every species (median value of b = 0.23)
Seedling recruit density
Relationship
between seed
density and
seedling density
for Trichilia
tuberculata
(Meliaceae)
Seed density
Number of trap sites
Community consequences of b<1 comes from comparing seed
rain and seedling recruit diversity Difference in diversity
indices comparing seed rain
to traps to seedling recruits
in plots
Difference in diversity index (plots-traps)
Higher diversity index values for plots than traps
(see Harms et al. (2000) for details on how they show that
difference in diversity is due to density-dependent effects) Comita et al. (2010) Science 329:330
Analysis of survival data of 30,000 seedlings of 180 spp in
20,000 sample quadrats.
Qu: How does the strength of density-dependence in survival
(NDD) vary among spp? Specifically what would predict for common versus rare species?
Bayesian statistical analysis to fit model predicting species-specific
effect sizes of NDD (ie effects on individual’s probability of
survival depending on frequency of conspecifics in its immediate
neighborhood – the quadrat). Strong negative
effects on
survival in
presence of
conspecific
Weak effects on
survival in
presence of
heterospecific
Key result: Strength of neighbor NDD effect correlated
with species abundance Rare species suffer
stronger negative effects
of having a neighbor of
the same species than do
common species
Mangan et al. (2010) Nature 466:752
Many different classes of ‘natural enemies’ could cause NDD effects
on growth or survivorship.
Most focus on seed predators and herbivores, not on microbes
Two experiments (field and greenhouse)
Grew seedlings in uniform soil with ‘inoculum’ collected from
either beneath that species or another species.
Measured ‘strength of negative feedback’ = how much more
growth is reduced with the presence of a conspecific vs
heterospecific inoculum
2 key results:
Pot experiment mirrors results of Comita et al. (2010)
Increasingly poor performance with soil
from beneath conspecific tree Field experiment: Examined contribution of measurable
(above-ground) natural enemies, versus ‘other’ on the
negative feedback So, soil pathogens can exert strong negative effects on seedling
survival – and can help explain differences in species relative
abundance
Does that mean other enemies are unimportant?
What happens to seeds in an empty forest - do they recruit or not?
Terborgh et al. (2008) Tree recruitment in an empty forest
Compared communities of plants at two sites in Peruvian Amazon:
Cocha Cashu (CC) - remote, intact fauna
Boca Manu (BM) - large mammals extirpated The two sites are structurally similar
More recruitment at undisturbed CC
What about groups of organisms 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 - a
hyperdiverse group of primary consumers… what would you
need to do to test equilibrium vs non-equilibrium models for the
maintenance of insect diversity??
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 (one insect, one host plant) was surprisingly rare
- Even phylogenetically distant hosts shared a third of herbivores
An average tree New Guinean tree hosted:
- 33 Coleoptera
- 26 Lepidoptera
- 20 orthopterids
FT= total number insect species associated with T host species
Mean number of insect spp/host * T hosts
Butterflies
Other lepidoptera
Coleoptera
orthopterids
Novotny et al. (2002) results challenged paradigm of strong host
specialization (niche partitioning) of insect communities
Suggest that non-equilibrium explanations needed to account
for insect diversity
Also need to re-evaluate estimates of insect diversity which
are based on host specialization.
Maybe ‘only’ 4-6 million insects, not 31 million insects? Summary:
Long term studies of communities have shown similarity in
species composition over long time scales that cannot be
accounted for by random sampling from a regional species pool.
Fine-scale niche partitioning however is probably not sufficient
to account for hyper-diverse communities. Non-equilibrial
processes must be important? Or…
Density-dependent processes may prevent superior competitors
from reaching high abundance
Rarity itself may indicate sensitivity to density dependent
processes.