Theories so far….

Recruitment into communities is a lottery – which ever species has recruits at the time that a territory becomes available will ‘win’

Storage effect (Daphnia and desert plants) – requires two conditions: ___________ and _________________

Intermediate Disturbance Hypothesis – where diversity is highest when disturbance-favored species (ruderals, pioneers etc) coexist with ________. Can’t explain forest diversity because….

Recruitment limitation: species don’t compete because of two processes – __________ and ___________

Degree of dispersal limitation can vary in communities because of a ______ - _______ trade-off

Hurtt and Pacala (1995) Simulation: rank species according to competitive ability but give all species equivalent and restricted dispersal ability.

Limited dispersal slows population and community dynamics in species rich communities.

As richness increases, individual species become rarer and more dispersal limited. If a species becomes super-abundant then it can escape dispersal limitation

Pure dispersal assembly of communities

If propagules don’t arrive, organisms can’t compete. “Winner” of a site or territory is the best competitor among those species that were able to provide propagules.

Long-term seed trapping studies can be used to see how frequently seeds arrive at potential recruitment sites

Barro Colorado Island 50 ha plot – best dataset for evaluating dispersal limitation in tree communities - 20+ years of data.

BCI 50 ha plot Two hundred 1 m2 seed traps arrayed along trails over 50 ha. Traps emptied every week…for last 20 years.

Hubbell et al. (1999)

Analysis of seed trap captures over 10 years

260 plant species identified based on seed characters

1.3 million seeds collected over 10 years!

On average, 31 species were recorded in a single trap over 10 YEARS

A small number of wind-dispersed species reach nearly all traps.

HALF of the species had seeds dispersed to 6 traps or fewer

40 species no captures at all

Conclude: most individual trees will disperse seeds to only a handful of potential recruitment sites during their reproductive lifetimes

No captures

Seed limitation: Proportion of sites not receiving seeds of a given species

Can be broken down into: Source limitation: Proportion of sites not receiving seeds when seeds are randomly distributed among sites

Dispersal limitation (strict definition): is defined as 1 – (proportion of sites receiving seeds)/(proportion of sites that would receive seeds if seeds were randomly distributed). Degree to which aggregated dispersal further contributes to limitation beyond source limitation.

Decomposing seed limitation

Seed limitation arises because of: insufficient seed production, or inadequate seed distribution.

How dispersal limited are pioneer trees in the forest?

Example of 2 pioneer species from Panama growing in the Barro Colorado Island Forest.

Jacaranda copaia (Bignoniaceae)

Canopy tree

Wind dispersed

Seed mass 4.7 mg

Reprod. DBH = 20 cm

n=193 adult trees in 50 hectares

Croton billbergianus (Euphorbiaceae)

Sub-canopy tree

Seeds explosively dispersed

Seed mass 24 mg

Repro. DBH = 3 cm

n=367 reproductive trees in 50 ha

Can we model seed dispersal at the landscape scale?

Sort of…

Traditional approach - measure seed patterns around isolated trees and scale up to the forest.

- Need lots of isolated trees to describe the relationship between tree size and reproductive output, as well as inter-annual variation in seed output

- Additional problem that isolated trees may differ from the overall population in the activity of pollinators, seed predators, or dispersers.

Start with a known pattern of seed rain (e.g. seed captures in traps), a map of the location of the traps, and a map of trees (and their sizes) that could have contributed seeds to those traps…

Can we model seed dispersal at the landscape scale?

Use maximum-likelihood analysis to locate parameter values for dispersal functions that best fit observed see rain data

Start by assuming some relationship between tree size and seed production, and some relationship between distance from tree and probability of dispersal and then calculate expected captures of seeds to each of the traps... This approach is called ‘inverse-modeling’.

Jacaranda Pearson r = 0.73 Median dispersal 7m

Croton

Pearson r = 0.87 Median dispersal 1 m

Problems with inverse-modelling approach

Assumes a distribution of dispersal probabilities as function of distance from source

Cebus mean seed dispersal distance >200 m

25 % of dispersal events were outside the 50 ha plot

Wehncke et al. (2003)

Jones et al. (2005) Genetic evaluation of dispersal of Jacaranda

Microsatellite genotypes of maternal tissue to identify seed sources

Grey bars are minimum dispersal distances for trees with no parent assigned to them

Turnbull et al (2000) reviewed results of seed sowing experiments.

Seed limitation: Failure to regenerate because inadequate supply of seeds reached favorable recruitment sites

- Found that about half of them showed evidence for seed limitation.

- More evidence for seed limitation in early successional habitats, and for early successional species.

Experimental tests of the importance of seed limitation

Two hypothetical results of a seed addition experiment for 3 spp at low (L), medium (M) and high (H) sowing densities

���What would you conclude about the way spp interact if result (a) arose?

If result (b) arose?

Interacting effects of dispersal and competition

Outcome of competition among species may depend on whether a competitive hierarchy is transitive or not.

a b c

a b c

transitive

non-transitive

Recognize this game??

Non-transitive competition in

bacteria 3 bacterial types:

Colicinogenic bacteria (C) produce colicin (toxin) and producing this toxin is costly

Resistant bacteria (R) avoid the competitive cost of carrying ‘col’ plasmid but suffer because colicin receptor is involved in nutrient metabolism

Colicin-sensitive (S) bacteria are killed by colicin

Dispersal and non-transitive Competition���

(Kerr et al. 2002) Strains grown in three environments: ���

(1) flask (well-mixed environment where dispersal and interaction not local) (2) static plate (environment in which dispersal and interaction are primarily local; and (3) mixed plate (intermediate environment).

What would you predict the outcome will be?

S R

S wins =

R C

R wins =

C S

C wins =

Competition

Kerr et al. 2002

Coexistence occurs when local structure is maintained (Fig. 2a)

With no spatial structure (in flask or mixed plate), R strain wins (Fig. 2b-2c)

Results show local interaction and dispersal in this non-hierarchical competitive interactions promoted species coexistence (rem: tree neighborhoods?)

Evidence for equilibrium forces structuring communities

If community composition is dictated by niche assembly then not a lottery…

And predict??

Equilibrium models of species coexistence

- Several examples with insect communities in which species have annual life cycles:

e.g., Lawton and Gaston (1989) constant composition of ~20 spp of insects living on plots of bracken fern over 7 years and at 2 sites (woodland and open).

What evidence is there for community stability?

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 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?

Terborgh looked at primary successional sequences on river-bends in the Manu river in SE Peru.

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

Meanders in the river generate replicated ‘point bars’ – become mature forest over ~300 yrs

Point bars along the Manu river, Peru

Species PLOT 1 PLOT 2 PLOT 3 FARPLOT 1

FARPLOT 2

Otoba 1 4 1 3 3Astrocaryum 2 2 2 2 2Iriartea 9 6 5 1 1Quararibea 3 1 3 4 4Scheelia 7 3 7 5 6Theobroma 4 12 6 6 5Guarea 8 20 4 13 7Lonchocarpus 14 5 23 14 119Pouteria 11 8 10 - 139Oxandra 6 11 37 15 56Malmea 10 48 8 - 75Pseudolmedia 12 24 11 12 11Calatola 5 20 48 43 95Ruizodendron 18 13 21 17 13Poulsenia 19 16 20 11 16Unonopsis 17 10 200 - 62Sorocea 29 9 58 24 17Sapium 23 15 24 9 -Trichilia 25 25 15 40 22

Rank abundance for most common spp (out of 386) present in 1000 tree samples on five mature forest floodplains

Longer-term 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.

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?

Summary: Many theories for the maintenance of diversity explore mechanisms that can prevent species exclusion through limiting similarity.

Classical niche differentiation may not be necessary.

Species coexistence can be mediated by repeated disturbance (IDH), temporal fluctuations in recruitment success (Lottery model and storage effect), competition-colonization trade-offs or simply by steady-state dispersal limitation.

Models need not stop competitive exclusion, just slow it down sufficiently such that species loss is balanced by species replacement