Research interest

Summary

In the context of global warming, the response of species range limits to changes in environmental conditions is a critical issue to understand biodiversity distribution and how it will react to environmental changes.

Two common ecological processes may be of a particular importance in determining the limits of species and their response to climate change : interspecific competition and local adaptation. The present PhD project aims at developing a process-based model to study the role of those two processes and their interplay in driving the response of population dynamics, population structure and species range limits to a changing environment. Parametrised for our model system, Folsomia candida, this model will be tested empirically and validated with independent laboratory data, then used to try to understand :

1. How population dynamics and species range limits respond to climate change in absence of adaptation ;
2. How local adaptation and gene flow affect a steady spatial distribution in a given environmental gradient ; and
3. How both interplay to determine the response of species range limitsto climate change.

Description

“Human alteration of the global environment has triggered the sixth major extinction event in the history of life...” (Chapin III et al., 2000)

This is how leading experts around the world concluded regarding the current status and future tenden- cies of biodiversity, including diversity of all taxonomic groups at every level : genetic diversity, species diversity, diversity of natural communities and ecosystems, in all natural habitats. In this context of global warming and biodiversity decline, it is particularly important to improve our understanding of how species respond to global change.

One important issue in this context is the response of species distributions and in of particular species range limits to changing environmental conditions such as climate change. Until now research has mainly focused on statistical, regression-based models (so-called "climate envelope models") (Parmesan, 2006). This approach does not take into account the role of ecological interactions (competition, predation, etc) in the determination of species distributions and range limits (Lavergne et al., ress). By contrast, in this PhD project we propose a more mechanistic, process-based approach to complement the climate-envelope approach. In a context of climate change, the project focuses on two aspects : competition-induced species range limits and the influence of adaptation on such range limits.

In particular, two common ecological processes may be of importance in determining the limits of species and their response to climate change : interspecific competition and local adaptation (Case and Taper, 2000 ; Lavergne et al., ress). Competition restricts the occurrence of a species to a smaller range than its hypothetical “climate envelope” (i.e. its physiological limit in terms of abiotic conditions). Yet the question how competitive interactions respond to climate change is not a priori clear, since the outcome of competition may depend on several –- possibly contradictory –- temperature-dependent processes (Vasseur and McCann, 2005). Local adaptation is likely to play a role, and generally ignored in climate- envelope models. Genotypes that are well-adapted to the centre of a species range may be maladapted at the species border (Kirkpatrick and Barton, 1997 ; Case and Taper, 2000). Invasion of new genotypes is hence likely to affect the competitive interaction at species border where it may be maladapted. The present proposal focuses on these two processes and their interplay. By studying these processes in the context of an ecological model of our model species (Folsomia candida) we aim at developing process- based and empirically testable hypothesis on the response of species dynamics and range limits to changing abiotic conditions.

To this end, the objectives of the PhD project are

1. to formulate and validate an ecological model that captures the essential ecological mechanisms (competition, predation, migration, etc) that regulate population size in response to climate factors (temperature in this case) for our model organism (Folsomia candida) ;
2. to derive single-species model predictions of population dynamics and test them with experimental time series ;
3. to extend the model to a spatial domain including dispersal ;
4. in the framework of adaptive dynamics, to extend the model to multiple genotypes to allow for the emergence of genetic clines and/or discrete species clusters ;
5. to study the dynamics of species range limits under constant or changing conditions.

The PhD proposal is based on a combination of modelling and experimental work, with an emphasis on the modelling part. In turn, the modelling will be based on a combination of two major ecological theories : physiological structured population (PSP) models and adaptive dynamics (AD). Physiologically structured population models (PSPMs) constitute an interesting approach to the development of such a forecasting tool. Indeed, considering the structure of a population modeled, in age or size classes for example, allows to model more complex ecological processes (Kooijman and Metz, 1984 ; Metz and Diekmann, 1986) leading to a rich models that can provide predictions of population dynamics in term of size and structure of the population (De Roos et al., 1992 ; Claessen et al., 2000), range of the species in case of spatialised models, and response of the population to environmental variability. Addition of adaptive dynamics to the theory of PSPMs gives access to an eco-evolutionary approach of population dynamics in the context of global warming and a better understanding of speciation process due to environmental change.

Previous work on adaptive dynamics has shown that in non-spatial models, density or frequency-dependent competition for resources may give rise to evolution towards a so-called “evolutionary branching point”, at which point selection becomes disruptive and which may result in sympatric speciation (Geritz et al., 1997 ; Dieckmann and Doebeli, 1999). Along a gradual environment, outcomes of adaptation and com- petition are more complex and can produce to two distinct situations : (1) a continuous distribution of genotypes, each one being the optimum for its “own” environment (a genetic cline) ; or (2) distinct clusters of genotypes, each one well adapted to a small part of the environmental gradient and spreading around it with only limited overlap with other clusters. The phenomenon causing the second situation is called “adaptive diversification” or “adaptive speciation” in sexual population with assortative mating (Doebeli and Dieckmann, 2003). Recent analytical studies of partial differential equation models of spatialy struc- tured population have shown that the outcome of adaptation, a cline or clusters, depends on the shapes of both competition and dispersal kernels (Leimar et al., 2008 ; Ispolatov and Doebeli, 2009).

This leads to several questions : how do species respond to environmental variability and global changes ? How species range limits depend on competition and how global warming would affect those limits ? What kind of spatial distribution is to be expected in a structured population and could alternative stable states be observed ?

To assess those questions, our research project intends to develop a spatially extended, physiologically structured population model for species of collembolans Folsomia candida, closely linked with experimen- tation. Our model would be studied in the framework of adaptive dynamics to investigate ecological and evolutionary responses of populations facing environmental changes. This project would take place in two steps.

The first aims at formulating and validating the model. The objective is to develop a single species PSP model for Folsomia candida, including temperature as a varying factor along a spatial gradient, and parameterised with laboratory data from experiments conducted under the supervision of Thomas Tully. Folsomia candida is a convenient model species because of a wealth of ecological, life history and population dynamics data already available, on a number of different clones. These clones, with different temperature preferences, can be used to mimick experimentally the adaptive dynamics processes of invasion and competition. The species is easy to manipulate and has a short generation times and is parthenogenetic. Missing parameters will be estimated with results from new experiments and fits of the model on experimental time series using the particle filter method. The model would then be tested against independent data to be validated.

The next step is the use of the model to assess the questions raised above. And more precisely, to answer the following interrogations :

1. In absence of adaptation, how do populations respond to climate change in terms of population dynamics and spatial distribution, and in particular its species range limits ?
2. For a given environmental gradient, how do local adaptation and gene flow affect the steady-state spatial distribution (cline or clustering) ?
3. Climate change moves the ecological system away from its attractor, in both the ecological and evolutionary sense. How do interspecific competition and adaptation interplay to determine the response of species range limits to climate change ?

This project is integrated in the ANR project EVORANGE [2010-2013, PI : Ophelie Ronce] in the “6th extinction” ANR program to which UMR 7625 participates (Claessen, Tully, Mallard, Ferrière, Legendre). The ANR project will provide a budget for missions and material (computational, experimental).

Références

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