Nicolas LoeuilleLaboratoire Ecologie & Evolution,

Université Pierre et Marie Curienicolas.loeuille@upmc.fr

Effects of local negative feedbacks on the evolution of species within

metacommunities

Starting with an old debate

Gause 1934

Hutchinson 1961

How do we explain the maintenance of diversity within a group of species ?

Spatial components in the maintenance of diversity

● Total « regional » diversity :– Colinization-Competition Trade-off (Tilman 1994)

– Spatial heterogeneity and niche partitioning (Leibold et al. 2004)

– Storage effects (Chesson 1994)

– Random speciation, ecological drift (Hubbell 2001)

● Local diversity– Mass effects (source-sink relationships) Mouquet &

Loreau (2004)

– Janzen-Connell (Janzen 1970)

The puzzling diversity of tropical forests

● Neutral dynamics

Speciation, migration, ecological drift (Hubbell 2001)

● Niche deterioration

Competitive hierarchy, mediated by enemy attraction (Janzen 1970)

The Janzen-Connell hypothesis

Janzen 1970

Local negative feedbacks

Enemy presence

Presence of one individual

Decrease in survival, decrease in competitive ability

An illustrative example

Diez et al. 2010

Importance for community structure and diversity

Klironomos et al. 2002

Consumption constraints also yield local negative feedback (1/2)

Plant N:P=12:1

SoilN:P=15:1

Nu

trie

nt u

pta

keN

itrog e

n re

lease

N:P in soil increasesFavors incoming competitor species with a higher N:P

Consumption constraints also yield local negative feedback (2/2)

P

BA

P

BA

Habitat becomes less suitable for predator AAn alternative predator with high affinity for B would be favored

Critical conditions under which feedbacks act on diversity

1)Local environment is largely driven by the species presence (vs, eg, external abiotic constraints): niche construction (Laland et al. 1999, Kylafis & Loreau 2008)

2)Spatial constraints: low diffusion of enemies or nutrient compared to the species dispersal

3)Evolutionary constraints: Evolutionary dynamics lag behind ecological dynamics

What is the amount of diversity emerging from the interplay of these three constraints?

Ecological dynamics of the model● 30*30 patches on a torus

● Each patch i has an environmental state zi

● Each morph has a trait xi

● (a) Extinction, with probability e● (b) Colonization from neighbor with probability c,

success if the invader j is better adapted (xj closer to

zi)

Initial Situation(a)

(b)

Ecological Dynamics

Evolutionary dynamics

● Each local population has a probabilityμ to undergo a beneficial mutation.

● (c) If so, the new trait of the population xi becomes

closer to environmental state zi by less than dx

Initial Situation

(c)

Evolutionary Dynamics

Environmental dynamicsLocal environment z

i changes :

d) Away from xi by a fixed step

dz if the patch is full (negative feedback)

d) Toward 0 by a step dz if the patch is empty (global averaging)

e) Toward the local average by a step αdz in all instances (contamination)

Initial Situation

(d)

(e)

Initial conditions and possible dynamics

● Only one population, of trait xi=0.5

● For each patch, an environmental value z is picked at random between 0 and 1

● One million time steps considered

Q1 Conditions for the emergence of diversity ?

Q2 Effects on the environmental structure ?

Q3 Effects on the community patterns at various scales ?

Permanent generalism

One species only, generalist(here, high e)

Permanent specialization

DiversificationMaintenance of specialist morphs(here, e=0)

Taxon cycles

DiversificationPeriodic extinction of specialist morphs(here, intermediate e)

On taxon cycles

Considering the occupation of islands by lizards :-one lizard species on one island-invasion by larger lizard species is possible-evolution toward smaller sizes of the ancestral species (specialization)-Same for the invader-Eventual extinction of the ancestral species

Roughgarden & Pacala 1979Ricklefs & Bermingham 2002

Effects on the grain of the environment

Permanent Generalism

Taxon cycles

Permanent specialization

The succession of the three patterns is deterministic

Conditions for emerging diversity

● From the succession of the three patterns, it is possible to distinguish parameters favoring diversity (PG->TC->PS)

● Such parameters include : colonization rate c, negative feedback dz

● Parameters that prevent the emergence of structure and diversity : extinction rate e, mutation rate μ (or mutation amplitude dx), contamination from surrounding patches α

What is the amount of diversity obtained ?

What is the amount of diversity obtained ?

The succession of the three patterns is deterministic

How is diversity organized in space ?

● Species-Area relationship (Preston 1962) : D=D0As

Kodric-Brown & Brown (1993)

How is diversity organized in space ?

Species abundance distribution

From McGill et al. 2007

On an arithmetic scale, the model reproduces the traditional empty curve

PGTCPS PGTCPS

On a logarithmic scale, multimode distributions

PGTCPS PGTCPS

Conclusions

● Negative feedbacks may allow the emergence of diversity, but not under all conditions

● Taxon cycle one possible outcome, but in fact part of a continuum

● The model also provides classical shapes for macroecological patterns (species abundance distribution, species area curve)

● Associated with the temporal dynamics of diversity, such patterns can allow more robust testing of the model (McGill et al. 2003)

Acknowledgements

Mathew Leibold