Coevolution in mutualistic communities

Plan

ts

Animals

Network structure for a plant-frugivore community in southeastern Spain. Bascompte and Jordano, 2007

Scott L. Nuismer

What is coevolution?

"Thus I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted to each other in the most perfect manner, by the continued preservation of all the individuals which presented slight deviations of structure mutually favourable to each other." — Charles Darwin, The Origin of Species

Coevolution: Reciprocal evolutionary change in interacting species (Janzen, 1980)

Species 1

Species 2

For interacting pairs of species, coevolution:

• Generates diversity both within and among species– Levels of genetic polymorphism– Phenotypic diversification among populations

• Drives major evolutionary transitions– Sexual vs. Asexual reproduction– Ploidy level

• Modulates epidemiological dynamics– Incidence and severity of disease

Coevolution matters for pairs of interacting species

Plan

tsAnim

als

Network structure for a plant-frugivore community in southeastern Spain.

But most species interact with many others

Bascompte and Jordano, 2007

What role does coevolution play in these complex communities?

Quantifying the impact of coevolution

We can answer these questions by developing a simple model…

Does coevolution alter trait distributions?

Does coevolution alter community function?

Does coevolution alter network structure?

Question 1:

Question 2:

Question 3:

A simple model of mutualistic communities

• • •

• • •

Animals

Plants

1x

Each species is defined by a phenotype distribution and abundance

1,AN2x

2,AN3x

3,ANnx

nAN ,

1y

1,PN2y

2,PN3y

3,PNny

nPN ,

Traits determine interaction probability

Phenotypic matching

e.g., Phenology

1 .0 0 .5 0 .0 0 .5 1 .0

0 .2

0 .4

0 .6

0 .8

1 .0

1 .0 0 .5 0 .0 0 .5 1 .0

0 .2

0 .4

0 .6

0 .8

1 .0

P[In

tera

ct]

x - y

P[In

tera

ct]

Phenotypic differences (threshold)

e.g., Corolla depth – proboscis length, Fruit size – beak size

x - y

• Consider two different functional forms of interaction

• • •

• • •

Animals

Plants

1y 2y 3y ny

1x 2x 3x nx

• Assumes mutualistic communities are organized by single “key” traits

Abundance + Traits + P[Interact] = Network structure

We want to predict how this network structure evolves

Predicting trait (co)evolutionLet’s start by defining individual fitness:

),()()( yxWxWxW BAT

Abiotic environment:

WA

Species interactions:

Phenotype, z

Optimal phenotype, • Assume all interactions are benefical

• Assume random encounters

• Focus on only inter-guild interactions

• Consider interactions mediated by either “matching” or “differences”

Predicting coevolution and community structure:

Two complementary approaches

Analytical approximation

Strong selectionEvolving G

Fitness

Individual based simulation

i

i

iii z

WW

Gz 1

Does coevolution alter network structure?

Does coevolution alter community function?

Answering the questions using the model

Does coevolution alter trait distributions?

Question 1:

Question 2:

Question 3:

• Coevolution causes trait distributions to converge among guilds

• Coevolution causes trait values to converge within guilds

Weak coevolution Moderate coevolution

Species mean trait value

Frequency within

community

Species mean trait value

Frequency within

community

PlantsAnimals

PlantsAnimals

Does coevolution alter trait distributions?Case I: Phenotype matching

• Coevolution causes animal trait values to increase and plant trait values to decrease (or vice versa)

Weak coevolution Moderate coevolution

Species mean trait value

Frequency within

community

Species mean trait value

Frequency within

community

PlantsAnimals

PlantsAnimals

Does coevolution alter trait distributions?Case II: Phenotype differences

Does coevolution alter trait distributions?

• Yes! But the details depend on the functional relationship between plant and animal traits

Phenotypic matching

e.g., Phenology

1 .0 0 .5 0 .0 0 .5 1 .0

0 .2

0 .4

0 .6

0 .8

1 .0

1 .0 0 .5 0 .0 0 .5 1 .0

0 .2

0 .4

0 .6

0 .8

1 .0

P[In

tera

ct]

x - y

P[In

tera

ct]

Phenotypic differences (threshold)

e.g., Corolla depth – proboscis length, Fruit size – beak size

x - y

Does coevolution alter network structure?

Does coevolution alter trait distributions?

Does coevolution alter community function?

Question 1:

Question 2:

Question 3:

Answering the questions using the model

• Study how coevolution influences interaction rate and connectance

Low interaction rateand connectance

High interaction rateand connectance

Does coevolution alter community function?

Phenotype matching

Phenotype differences

Coevolution increases the rate of interaction within communities. Why?

Average fitness consequences to plants

Average fitness consequences to plants

Aver

age

fitne

ss

cons

eque

nces

to a

nim

als

Aver

age

fitne

ss

cons

eque

nces

to a

nim

als

Does coevolution alter community function?

Interaction rate Interaction rate

Why does coevolution increase interaction rates?

Species mean trait value

Frequency within

community

Species mean trait value

Frequency within

community

Proportion Successful

interactions

Proportion Successful

interactions

Proportion Unsuccessful interactions

Proportion Unsuccessful interactions

PlantsAnimals

PlantsAnimals

Trait Evolution: Matching Interaction rate: Matching

Trait Evolution: Differences Interaction rate: Differences

Yes. In all cases, mutualistic coevolution increases the

rate of interaction and network connectance

Low interaction rateand connectance

High interaction rateand connectance

Coevolution

Does coevolution alter community function?

Does coevolution alter community function?

Does coevolution alter trait distributions?

Does coevolution alter network structure?

Question 1:

Question 2:

Question 3:

Answering the questions using the model

• Study how coevolution influences the nestedness of mutualistic communities

Low nestedness High nestedness

Does coevolution alter network structure?

Matching Differences

Strength of Coevolution

What explains these patterns?

Strength of Coevolution

Nestedness(Plant)

Nestedness(Animal)

Does coevolution alter network structure?

Does coevolution alter network structure?Case I: Phenotype matching

A.

Trait distributions Network structureN

umbe

r of s

peci

es

B.

PlantsAnimals

PlantsAnimals

Mean phenotype

Num

ber o

f spe

cies

IE = 0.134NA = -28.00NP = -25.41

IE = 0.061NA = -8.16NP = -6.59

Before coevolution

After coevolution

Does coevolution alter network structure?Case II: Phenotype differences

A.

B.

Trait distributions Network structure

PlantsAnimals

PlantsAnimals

Mean phenotype

Num

ber o

f spe

cies

Num

ber o

f spe

cies

IE = 0.90NA = -9.08NP = -16.44

IE = 0.52NA = 31.42NP = 32.20Before

coevolution

After coevolution

Does coevolution alter network structure?YES! Anti-nestedness &

Reciprocal specializationRandom community

Coevolution(phenotype matching)

Coevolution(phenotype differences)

Low nestedness &GeneralizationRandom community

Conclusions I

• Mutualistic coevolution drives rapid shifts in trait distributions

Species mean trait value

Frequency within

community

PlantsAnimals

Conclusions II

• Mutualistic coevolution increases rates of interaction and the connectance of networks

Low interaction rate High interaction rate

Coevolution

Conclusions III

• Coevolution drives rapid changes in network structure

Before coevoluti

on

After coevoluti

on

500 Generations

Acknowledgements

Collaborators

FundingNational Science Foundation

Jordi Bascompte Pedro Jordano