on the use (and misuse) of models in ecological research nicolas delpierre, ese (umr 8079)...

On the use (and misuse) of models in ecological research

Nicolas Delpierre, ESE (UMR 8079)[email protected] Ecology in English, October 2013

IPCC WG1 – published 2013, Sep. 13th

models +data

models

Models are central in current global change research

Pereira et al., 2010, Science

Models are central in current global change research

data models

A tentative chronology of ecological modelling

Mathematical models are the foundation of modern ecological theory



Population ecologyMalthus (1798), Verhulst (1838), Lotka (1925), Leslie (1945)

Biogeography / ecological communities Mc Arthur & Wilson (1967), Hubbel (2001)

Food webs Elton (1927)

Evolutionary ecology Wallace & Darwin (1858)

Ecosystem productivity Lieth (1972)



Population ecologyMalthus (1798), Verhulst (1838), Lotka (1925), Leslie (1945)

Biogeography / ecological communities Mc Arthur & Wilson (1967), Hubbel (2001)

Food webs Elton (1927)

Evolutionary ecology Wallace & Darwin (1858)

Ecosystem productivity Lieth (1972)

The complexity of ecological / biological systems prevents the discovery of simple yet powerful models

Different kinds of models

Empirical models

Mechanistic / deterministic models

Theoretical models

Different kinds of models

Empirical modelsstatistical

phenomenological

Mechanistic / deterministic modelsbased on the representation of

(known and described) biological / physical processes

Theoretical modelsgeneric, simple

How simple a model needs to be ?

«simple» means « general » means « good »…

William of Ockham 14th c.


«simple» means « general » means « good »… (?)


« some of the theoretical conclusions [from the model] can be pleasingly supported by hard data, while others remain more speculative» (May and Anderson, 1979)


«simple» means « general » means « good »… (?)


« some of the theoretical conclusions [from the model] can be pleasingly supported by hard data, while others remain more speculative» (May and Anderson, 1979)

« The generality of simple models is often superficial because they only demonstrate possible explanations rather than provide actual instances of explanation » (Evans et al., 2013, TREE)


Simple models may sometimes be misleading

Eisinger & Thulke, 2008

Eisinger & Thulke Anderson

Simple model (Eisinger & Thulke 2008):« 70% of the population needs immunization »

Spatially explicit model (Anderson 1981):« 60% …»

A difference of 15 M€ per annuum

How to build a model ?


Knowledge of processesand pre-existing models

Hypotheses

Model

Formulating equations Evaluation

parameterisation

parameterisation

data

Sim

ulati

ons

observations


Knowledge of processesand pre-existing models

New hypotheses

Model

Formulating equations Evaluation

parameterisation

parameterisation

data

Sim

ulati

ons

observations

How many processes should i consider ?Is there a limit to the reductionnist approach ?

« We have a tendency to incorporate more and more processes into models to improve fitness between simulated and observed data. »

How many processes should i consider ?Is there a limit to the reductionnist approach ?

« We have a tendency to incorporate more and more processes into models to improve fitness between simulated and observed data.Complicated models may integrate more process knowledge but make more parameters less identifiable given certain data sets. » (Luo, 2009)

IdentifiabilityWhen parameters can be constrained by a set of data with a given model structure, they are identifiable.

Equifinality different models / parameter values of the same model may fit the data equally well

Medlyn et al., 2005, TPBeven, 2006Luo, 2009, Ecol. Appl.

Model 1

Model 1 bis

Model 2

How to parameterize / validatea model ?

How to parameterize / validate a model ?

A question of (parameters and data) uncertainty…



Parameter uncertainty : different experimental sources report different values for the same parameter

Kattge et al., 2011Hollinger & Richardson, 2005



Parameter uncertainty : different experimental sources report different values for the same parameter

Data uncertainty:•Sampling error•Measurement precision / accuracy

Kattge et al., 2011Hollinger & Richardson, 2005

These uncertainties must be considered when parameterizing / validating the model


An example of data-assimilation techniquesBayesian optimisation approach

posterior parameter distribution

priorparameter distribution

Likelihood function= probability of the data given the model output generated through the

parameter vector q = « measurement of the

prediction error »

DppDp

Van Oijen et al., 2005Martin & Delpierre, 2011Keenan et al., 2012


Definition of the cost function

DppDp




posterior parameter distribution

priorparameter distribution

DppDp

Parameter value

Simulations + uncertainty



Kuppel, 2013, PhD Thesis


The more correlated… the less identifiable


How many data do i need ?

Keenan et al., 2013, Ecol. Appl.


Keenan et al., 2013, Ecol. Appl.

Beware of relying completely on the model !

Solar radiation

temperature

Radiation interceptionGlobal PAR

Photosynthesis

Carbon AllocationC leaves

C coarse roots

C fine roots

Growth Respiration

C litter

C surface

C deep

HeterotrophicRespiration

CO2

Stomatal Cond.

GPP

Reco

C aerial wood

C reserves Maintenance Respiration

Föobar model

Keenan et al., unpubl.

Need for considering uncertainty in projected trends

Assimilating more data reduces the uncertainty of projections

Keenan et al., 2012

Need for considering uncertainty in projected trends

Alternative model formulations…yield different trajectories in future projections

Vitasse et al., 2011, AFM

How to identify the « best » of 2 (n) models ?


Hirotugu Akaike1973

Use the Akaike information criterion !

The lowest the AIC, the best accuracy-parsimony trade-off

What a dataset will not tell…

Do experiments provide reliable data for informing my model ?

Wolkovich et al., 2012, Nature

Can model parameters be treated as constants ?

acclimation processes

Wythers et al., 2005, GCB

Using a model for prospective studies

Slide from Chris Yesson (Zoological Society of London)

My model can say many things… depending on what i ask !

Principles of niche modelling


Objective of the paper :

to assign species to extinction risk categories based on projected declines in population size.

Under a time scale of 80 years


Thuiller et al., 1005, PNASAkçakaya et al., 2006, GCB

What’s the problem with that ?


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Simulation of Oak productivity depends on the resolution of climate forcings

Martin et al., unpublished results

Take home ideas

• However detailed, models are idealized representations of the world

• Simple models are most of the time general… and not so good

• Complex models may not be parameterizable (… however complicated the data assimilation technique)

• Model forecasts are conditional on:model structure and parameters (and uncertainties)model forcings

• Models can only answer questions that one asks

Supplementary material

On the use and misuse of models in ecological / global change research

Keenan et al. rate my data, validation GCB (fails)Medlyn et al. 2005 perils and pitfallsEvans et al. 2013 « Simple means general means good»

What is a model?What is it used for?How valid are inferences from model simulations ?

Plan

Models are central in current global change researchExamples last ipcc reportExamples spp extinction from pereira et al. 2011Used for projections of what may happenRaises the question of reliability of the models… and of their uncertainties

What is a model ? (we’re not going to center on statistical models)How simple needs a model to be ? Does simple mean general mean true ? (Evans)I’m a researcher : how can i build my model ? (where do i start from ?)The question / problem of parameterisation. Data also are uncertain !Dealing with multiple uncertainties : MDF frameworksMy model is built. How can i check that its predictions are reliable?

Future trends : what do i need for running my model ?How accurate are the input data (Zhao, Nico + Evea)Simulations in a future / modified climate : what indexes of changes should i use (Akcakaya)

What a model can’t do : rate my data…

on the use (and misuse) of models in ecological research nicolas delpierre, ese (umr 8079)...

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