Chaire Modélisation Mathématique et Biodiversité

École Polytechnique, Muséum national d'Histoire naturelle
Fondation de l'École Polytechnique
VEOLIA Environnement

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16 mars 2011, École Polytechnique, Amphithéâtre CURIE

13h30 Larry Venable (University of Arizona)
Ecological population and community dynamics: long-term data from desert annual plants (Partie II)
15h00 Pause

15h30-17h00 Judith Becerra (University of Arizona)
Timing the origin and expansion of the Mexican tropical dry forest
(Un second exposé de Judith aura lieu lors de la rencontre du 29 mars 2011)

Summary of Lawrence's course: Ecological population and community dynamics: long-term data from desert annual plants

Desert annual plants are frequently used to illustrate the principles of adaptation to variable environments, the population dynamic functions of dispersal and dormancy, and how temporal variation may promote species coexistence. All of these topics involve ecological and evolutionary responses to environmental variability. High levels of environmental variation driven by rainfall is a signature characteristic of hot deserts. Desert annuals have provided useful conceptual models because they have very simple life cycles and respond on a rapid time scale to such environmental variation. "Good wildflower years," when showy flowered annuals blanket the desert, often occur in association with abrupt desert annual population increases. Such years are correlated with greater than average germination-season rainfall and global climatic cycles, such as El Niño and Pacific Decadal Oscillations, in the case of US Southwestern deserts. Desert annuals spend most of their lives as seeds and may even go unnoticed during their normal flowering season in years of little germination or high mortality. Persistent seed banks play an important role in population and community dynamics and it is not uncommon for species to reappear following years of absence. While desert annuals are small and short-lived, they occur as members of mature, persistent communities. This means that it is relatively easy to monitor multiple generations during the course of a single long-term project. Thus, in addition to being good conceptual models, desert annuals make good empirical models for exploring ecological and evolutionary dynamics in variable environments. Here, I will present the results of our work combining the collection of long-term population dynamic data with several short-term focused approaches to understanding the ecology of Sonoran Desert winter annuals. I demonstrating how our data provide evidence for bet hedging and coexistence via the storage effect. Next, I describe a fundamental functional tradeoff that structures the dominant members of our community and determines the degree of inter-annual variation in fecundity. Finally, I explain long-term trends in response to climate change.

Summary of Judith's course: Timing the origin and expansion of the Mexican tropical dry forest

Macroevolution examines the temporal patterns of biological diversity in deep time. When combined with biogeography, it can provide unique information about the historical changes in the distribution of communities and biomes. For the first part of my course I will present a documentation of the temporal and spatial changes of diversity in the plant genus Bursera and relate them to the origin and expansion of the tropical dry forests of Mexico. Bursera is very old, highly adapted to warm dry conditions, and a dominant member of the Mexican tropical dry forest. These characteristics make it a useful indicator of the history of this vegetation. I used a time-calibrated phylogeny to estimate Bursera's diversification rate at different times over the last 60 million years. I also reconstructed the geographic center and time of origin of all species and nodes from information on current distributions. Results show that between 30 and 20 million years ago, Bursera began a relatively rapid diversification. This suggests that conditions were favorable for its radiation and thus, very probably for the establishment of the dry forest as well. The oldest lineages diverged mostly in Western Mexico while the more recently lineages diverged in the south-central part of the country. This suggests that the tropical dry forest probably first established in the west and then expanded south and east. The timing of the radiations in these areas correspond to that suggested for formations of the mountainous systems in Western and Central Mexico, which have been previously recognized as critical for the persistence of the Mexican dry forest.

Programme des rencontres de la Chaire