resilience and stability of ecological networksresilience and stability of ecological networks elisa...

Resilience and stability of ecological networks

Elisa Thébault

elisa.thebault@upmc.fr

Why study ecological interaction webs?

Why study ecological interaction webs?

Factors which determine interactions

between species

Network structural patterns

Naisbit et al. 2012

Rezende et al. 2007

Why study ecological interaction webs?

Consequences on ecosystem functioning

and stability

Network structural patterns

Dunne et al. (2004)

Why study ecological interaction webs?

Effects of global change on networks and consequences

Cavalheiro et al. (2008)

Before 1970s, the view is that complexity begets stability

Several arguments:

- Theoretical and experimental evidence that

simple model ecosystems are inherently

unstable

- species-poor islands and artificial

agricultural ecosystems are more prone to

invasions by new species and pests than are

their continental and natural counterparts.

Elton 1958

Before 1970s, the view is that complexity begets stability

Elton 1958 MacArthur 1955 Several arguments:

- Theoretical and experimental evidence that

simple model ecosystems are inherently

unstable

- species-poor islands and artificial

agricultural ecosystems are more prone to

invasions by new species and pests than are

their continental and natural counterparts.

Before 1970s, the view is that complexity begets stability

– Consider a community of k species with population density Ni

– Population dynamics can be described by:

kii NNF

dt

dN,...,1

This traditional view is challenged in the early 1970s

May 1972

A mathematical formulation

n …

… 2 1

1 2 ….. n

– Near equilibrium, the behaviour of the community is related to the eigenvalues of the Jacobian of the system at this point

A = community matrix

aij = interaction strength eqj

iij

N

Fa

This traditional view is challenged in the early 1970s

n …

… 2 1

1 2 ….. n

– Near equilibrium, the behaviour of the community is related to the eigenvalues of the Jacobian of the system at this point

– For a random network with a proportion C of nonzero interaction strengths (connectance), and s the mean interaction strength, the condition for local stability is:

A = community matrix

aij = interaction strength eqj

iij

N

Fa

1kCs

This traditional view is challenged in the early 1970s

These contrasting views are at the origin of the complexity – stability

debate

Tackling this debate is fundamental to understand how the structure of ecological networks affect ecosystem resilience

These contrasting views are at the origin of the complexity – stability

debate

New perspectives on this debate with different definitions of stability

Going beyond May’s work: ecological networks are not random and new model assumptions

Today we will focus on two questions from recent works in this field:

These contrasting views are at the origin of the complexity – stability

debate

New perspectives on this debate with different definitions of stability

Going beyond May’s work: ecological networks are not random and new model assumptions

Today we will focus on two questions from recent works in this field:

These contrasting views are at the origin of the complexity – stability

debate

Stability: a multifaceted concept …

Dynamic stability

Local stability: definition often used in theoretical studies See May

Cycles and Chaos: Associated measures of stability: Variability, amplitude of fluctuations, etc.

Stability: a multifaceted concept …

Dynamic stability Response to disturbances

Resilience and resistance of the community, species persistance, ….

… in relation with many diversity-stability relationships

Ives and Carpenter (2007)

Model of randomly structured competitive communities:

Investigate 13 relationships between diversity and stability

… in relation with many diversity-stability relationships

Ives and Carpenter (2007)

… in relation with many diversity-stability relationships

… in relation with many diversity-stability relationships

Temporal variability of ecosystem functioning: a new perspective on the

diversity-stability debate

Tilman et al. (2006)

Insurance hypothesis

Diversity decreases variability of ecosystem properties through asynchronous responses of species to environmental perturbation Higher diversity

levels

Low diversity

Yachi & Loreau (1999)

Tem

pe

ratu

re T

(°C

)

Time

Niche differentiation

Pla

nts

an

d h

erb

ivo

res

mo

rtal

ity

rate

s m

T(t)

5 15 25

5 15 25

5 15 25

0

5

10

15

20

25

30

0 500 1000 1500 2000

Degree of niche differentiation

Insurance hypothesis and interactions between species

1 1 P

1 … P

n - 1 P

n

H 1

… H n - 1

H n

P 1

… P n - 1

P n

H 1

… H n - 1

H n

P 1

… P n - 1

P n

H 1

… H n - 1

H n

Environmental fluctuations

Thébault & Loreau (2005)

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

Plant diversity

CV

of

tota

l he

rbiv

ore

b

iom

ass

P 1 P n - 1 P n

H 1 H n - 1 H n

Specialist herbivores

CV

of

pla

nt

and

he

rbiv

ore

bio

mas

s

Higher degree of niche

differentiation

Plant diversity

synchronous responses

asynchronous responses

highly asynchronous responses

CV

of

tota

l pla

nt

bio

mas

s

0

0,1

0,2

0,3

1 2 4 8 16

Insurance hypothesis and interactions between species

Thébault & Loreau (2005)

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

Plant diversity

CV

of

tota

l pla

nt

bio

mas

s C

V o

f to

tal h

erb

ivo

re

bio

mas

s

CV

of

pla

nt

and

he

rbiv

ore

bio

mas

s

Plant diversity

synchronous responses

asynchronous responses

highly asynchronous responses

Insurance hypothesis and interactions between species

Thébault & Loreau (2005)

P 1 P n - 1 P n

H 1 H n - 1 H n

P 1

H

P 1

H

P 1

H

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

Plant diversity

CV

of

tota

l pla

nt

bio

mas

s C

V o

f to

tal h

erb

ivo

re

bio

mas

s

CV

of

pla

nt

and

he

rbiv

ore

bio

mas

s

Plant diversity

synchronous responses

asynchronous responses

highly asynchronous responses

Insurance hypothesis and interactions between species

Thébault & Loreau (2005)

P 1 P n - 1 P n

H 1 H n - 1 H n

P 1 … P n - 1

H 1 … H n - 1

P 1 … P n - 1

H 1 … H n - 1

P 1 … P n - 1

H 1 … H n - 1

P 1 P n - 1 P n

H 1 H n - 1 H n

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

Plant diversity

CV

of

tota

l pla

nt

bio

mas

s C

V o

f to

tal h

erb

ivo

re

bio

mas

s

CV

of

pla

nt

and

he

rbiv

ore

bio

mas

s

Plant diversity

1 1

synchronous responses

asynchronous responses

highly asynchronous responses

P 1 … P n - 1 P n

H 1 … H n - 1 H n

P 1 … P n - 1 P n

H 1 … H n - 1 H n

P 1 … P n - 1 P n

H 1 … H n - 1 H n

Insurance hypothesis and interactions between species

Thébault & Loreau (2005)

P 1 P n - 1 P n

H 1 H n - 1 H n

0

0,1

0,2

0,3

1 2 4 8 16

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

P 1 … P n - 1 P n

H 1 … H n - 1 H n

P 1 … P n - 1 P n

H 1 … H n - 1 H n

P 1 … P n - 1 P n

H 1 … H n - 1 H n

0

0,1

0,2

0,3

1 2 4 8 16

0

0,2

0,4

0,6

0,8

1

1 2 4 8 16

Plant diversity

CV

of

tota

l pla

nt

bio

mas

s C

V o

f to

tal h

erb

ivo

re

bio

mas

s

Plant diversity

synchronous responses

asynchronous responses

highly asynchronous responses

0

0,1

0,2

0,3

1 2 4 8 16

CV

of

pla

nt

and

he

rbiv

ore

bio

mas

s

Insurance hypothesis and interactions between species

Thébault & Loreau (2005)

Jiang et al. 2009

Haddad et al. 2010

Diversity promotes ecosystem stability

These contrasting views are at the origin of the complexity – stability

debate

Going beyond May’s work: ecological networks are not random and new model assumptions

- effects of interaction type and network structure - importance of foraging adaptation

Today we will focus on two questions from recent works in this field:

A diversity of interaction networks

Food web

Plant-pollinator network

Host-parasitoid network

Food web

94% food

webs

1% mutualistic

webs

4% parasitic

webs

Proportions of papers on ecological

networks published in the last 50 years

that were related to food webs,

mutualistic webs and parasitic webs

A focus on food webs

A focus on food webs

Schmid-Araya et al. 2002

A focus on food webs

Gauzens et al. 2015

Trophic groups Modules

Gross et al. (2009)

Food web structure and stability

Stouffer et al. (2011)

A growing number of studies on mutualistic webs

Seed dispersal pollination

Nested structure

• Continuum between specialist and generalist species

• Presence of a core of highly connected species

• Asymmetrical specialization Bascompte et al. 2003

A growing number of studies on mutualistic webs

Okuyama & Holland (2008)

Nestedness of mutualistic webs and stability

Rohr et al. (2014)

Gross et al. (2009) Okuyama & Holland (2008)

The links between network structure and stability

The model: dynamics of mutualistic and trophic webs

Na

j

AjpreyPk

kij

ijij

iPiiPii

Np

j

AipreyPk

kji

jiji

iAiiAii

P

PAcPIPr

dt

dP

P

PAcAIAr

dt

dA

1

)(

1

2

1

)(

1

2

Na

j

PimutAk

kij

ijij

iPiiPii

Np

j

AimutPk

kji

jiji

iAiiAii

A

PAcPIPr

dt

dP

P

PAcAIAr

dt

dA

1

)(

1

2

1

)(

1

2

-intrinsic growth rates rP and rA < 0 obligate mutualism

-intrinsic growth rates rP > 0 and rA < 0

-density dependence term -density dependence term

-interaction term saturates with mutualistic partner densities

-interaction term saturates with prey densities

Mutualistic Antagonistic

diversity

con

ne

ctan

ce

n = 24

n = 80

C =

0.0

5

C =

0.2

nestedness

modularity

time

Spec

ies

den

siti

es

The model: network structure and stability

diversity

con

ne

ctan

ce

n = 24

n = 80

C =

0.0

5

C =

0.2

nestedness

modularity

time

Spec

ies

den

siti

es Persistence:

proportion of species persisting at the equilibrium

Resilience:

measure of the speed at which a system returns to its original state after a perturbation

The model: network structure and stability

Results: impact of network structure on species persistence

Antagonistic networks Mutualistic networks

Diversity

Co

nn

ecta

nce

Diversity

Nestedness

Mo

du

lari

ty

Nestedness Thébault & Fontaine 2010

Results: impact of network structure on species persistence

Connectance Diversity

Modularity Nestedness

Persistence

0.07 0.31

- 0.53 - 0.03

- 0.81

0.87 -0.36

0.32

Connectance Diversity

Modularity Nestedness

Persistence

0.12 -0.29

- 0.01 - 0.89

- 0.81

0.86 -0.36

0.35

Mutualistic

opposite effect of network structure on the persistence of mutualistic and trophic networks

Antagonistic

Thébault & Fontaine 2010

Results: impact of network structure on species persistence

indirect effect: 0.40 indirect effect: 0.18 indirect effect: -0.76 indirect effect: -0.31

Importance of nestedness and modularity for network stability

Connectance Diversity

Modularity Nestedness

Persistence

0.07 0.31

- 0.53 - 0.03

- 0.81

0.87 -0.36

0.32

Connectance Diversity

Modularity Nestedness

Persistence

0.12 -0.29

- 0.01 - 0.89

- 0.81

0.86 -0.36

0.35

Mutualistic Antagonistic

Thébault & Fontaine 2010

Results: impact of network structure on resilience

Mutualistic Antagonistic

Connectance Diversity

Modularity Nestedness

Resilience

-0.17 0.49

- 0.17 0.25

- 0.73

0.83 -0.42

0.27

Connectance Diversity

Modularity Nestedness

Resilience

-0.62 -0.28

0.23 0.10

- 0.86

0.90 -0.30

0.26

indirect effect: 0. 32 indirect effect: 0.13 indirect effect: -0.10 indirect effect: -0.04

opposite effect of network structure on the resilience of mutualistic and trophic networks

Thébault & Fontaine 2010

Results: network structure at equilibrium

Thébault & Fontaine 2010

Summary: comparing networks with mutualistic and antagonistic interactions

The relation between structure and stability strongly differ between mutualistic and antagonistic webs Observed differences between herbivory and pollination networks suggest that network structure might differ between mutualistic and antagonistic interactions Importance of the particular architecture of interaction networks in determining their stability

Importance of foraging adaptation

Consumers can’t consume different resource species simultaneously because of the prey’s patchy distribution, the capturing strategy for different preys, consumer’s sensory and cognitive constraints for discriminating between preys.

Bernays and Funk 1999

Kondoh 2003

Importance of foraging adaptation

Kondoh 2003

Importance of foraging adaptation

Take home message Resilience, stability and the structure

of ecological networks

The relationship between network structure and stability depends on the definition of stability and on the mechanisms that determine species interactions

Need to be very careful about stability measures and model assumptions when you consider a study

Is resilience the most appropriate measure to assess the stability of ecological networks and their response to perturbations?

Take home message Resilience, stability and the structure

of ecological networks

Thank you for your attention