The Theoretical Ecology lab teas are designed to be informal meetings for members of the research groups of Simon Levin , Steve Pacala , and Andy Dobson to give talks on their current research and receive feedback from their audience. The talks are usually 30 minutes, including the question and answer sessions, scheduled on Tuesdays at 2 PM. Additionally, other members of the Princeton University community and visitors are welcome to attend and to give presentations.
Talk schedules and email lists are maintained
by Nandi Leslie and Ben Strauss. Please contact nleslie@princeton.edu
or bstrauss@princeton.edu to have your name
added to the labtea email list so that you can receive reminders about upcoming
lab teas. Click here for Fall 2000
schedule and summaries or Spring
2001 schedule and summaries or Fall 2001
schedule and summaries or Spring
2002 schedule and summaries
Fall 2002
| Monday, August 26, at 4 PM | Nathaniel Newlands (Univ of British Columbia) |
| Friday, September 20, at 11 AM | Bernt Walther |
| Thursday, September 26, at 2 PM | Peter Walsh |
| Tuesday, October 1 at 2PM | Juan Keymer |
| Tuesday, October 8 at 2 PM | Junling Ma |
| Wednesday, October 16, at 3:30 PM | Tristram Seidler (NERC Centre for Population Biology) |
| Tuesday, October 22, at 2 PM | Kai Chan |
| Tuesday, October 29, at 2PM | Stefan Geritz (University of Turku) |
| Wednesday, October 30 at 2PM | Stefan Geritz |
| Thursday, November 7, at 2PM | Jean Carlson (UCSB) |
| Tuesday, November 12, at 2 PM | Juan Manuel Morales (University of Connecticut) |
| Tuesday, November 19, at 2 PM | Jonathan Dushoff |
| Tuesday, November 26, at 2 PM | Yoh Iwasa |
| Tuesday, December 3, at 2 PM | Marissa Baskett |
| Tuesday, December 10, at 2 PM | Adi Livnat |
Titles and abstracts
most recent first (posted approximately one week before the talk):
Nathaniel Newlands
Spatially-Explicit Individual-Based Modeling of
Bluefin Tuna,
Integrating New Data from Acoustic Tracking, Satellite
Tagging and Aerial
Survey Experiments
Biological interactions occur at specific locations involving the
spatial redistribution of organisms. From initially homogeneous
states, striking heterogeneous spatial patterns can emerge. Recognizing
the importance of space, biologists have struggled with the difficulties
of collecting data across spatial scales.
Recent advances in ocean monitoring technology are generating new insights on how fish move and interact. Mathematical modeling facilitates the exploration of the complexities in observed dynamics and aid in addressing hypotheses difficult to test in laboratory studies or open-ocean experiments. A spatially-explicit, individual-based model of bluefin tuna whereby interacting individuals coexist on a spatially heterogeneous ocean landscape will be presented.
With continued refinement and improvement, the model may lead to predictions of emergent patterns of bluefin spatial distribution, whereby spatial patterns are characterized on the basis of individual decision-making in schools, seeking to maintain survival and improve evolutionary fitness. Selected results from analyses of new experimental data and model simulations will be presented.
The talk will end by outlining several improvements required and two aspects
of the model where further research is now being focused.
Bernt Walther
GENOMIC vs. GENETIC RATIONALES FOR SEX: Accounting for the mysterious origin of Darwin's 2.principle of variation
Sexual biology consists of 2 distinct fields: Functions and adaptive uses of sexuality, and: the origin of sex in deep time. The former is an active field of study, while the latter has been ranked among the 4 major millenial scientific mysteries. Nonetheless, the advent of sex is generally taught according to the mechanism proposed by Weisman (1886) and Fisher (1930) as being due to the genetic advantages of sex.
Williams and Maynard Smith both showed around 1975 that such explanations were untenable: Sex at the cellular level is anti-reproductive and handicaps sexual relative to non-sexual species, &: Adaptation by sex is ephemeral compared to adaptive mutations. How then could sex evolve from non-sexual species ?
My analysis suggests that sexual syngamy evolved from microbial syntrophy present among primordial biota around hot vents. The advent of anoxygenetic photosynthesis allowed invasion of seasonal photic biotopes ("polar paradise") which accounts for emergence of proto-eukarya which are incompatible with extreme thermal environs. However, survival in seasonal biotopes mandates heightened biochemical mutualism that blend dormancy and seasonal autotrophy. Sex may reflect the seasonal union of microbes in such metabolic interdependency, which evolved to syngamy and proto-meiosis under selective pressure of toxic photosynthetic di-oxygen.
My model suggests that "Life’s dual roots" mirror the separate descent of the two sexes, where eukarya originated by the sexual union of these microbial genomes, becoming novel species when meiosis evolved into (non-nucleated) mitosis. Non-sexual species arose later by shedding sex due to its high costs. Sex as a non-mutational adaptation to toxic oxygen is now an exaptation, but the sexual life cycle may reflect the history of life’s transit from anaeroby to aeroby. I will discuss this and other falsifiable predictions from the model that sex is the "missing link’ in cell evolution from pro-karya to eukarya.
Ref: B.T. Walther (2000): Do Life’s Three Domains mirror the Origins of
Sex ?
www.ias.ac.in/jbiosci/sept2000/217.pdf
Thursday, September 26 at 2 PM
Peter Walsh
Ebola and the Catastrophic Decline of Central African Apes
About 80% of the world's gorillas and a slightly lower proportion of the
world's chimpanzees are thought to live in two countries, Gabon and the Republic
of Congo. My talk will discuss results from a new, largescale survey dataset
that suggest a catastrophic decline of apes in Gabon over the last 18 years.
The commercial trade in bushmeat appears to be the primary cause of decline
but several lines of evidence suggest that the ebola virus has also had a
major impact. I'll discuss this evidence and also some modeling I'm currently
in the midst of suggesting that differences between gorillas and chimps in
social group structure influence ebola transmission dynamics, with potentially
major impacts on group size distribution, sex ratio, and population resilience.
If additional data from several collaborators arrive in time, I will throw
out some seat of the pants estimates on the rate of spatial spread of ape
deaths. These estimates may help discriminate between primary transmission
from reservoir and secondary transmission between apes as major causes of
ebola outbreaks in apes. If time remains, the talk will end with some rank
speculation about the role that changes in the spatial grain of human settlement
(i.e. urbanization) may have played in the emergence of ebola as an important
disease in gorillas, chimpanzees,
and humans.
Juan Keymer
The benthic-pelagic linkage and coastal (meta)-populations: (i) invertebrates
I will be talking about the linkage between pelagic and benthic ecosystems.
These two different places function as home for two
different stages of an invertebrate life-cycle. Adults live on shore competing
for space while juveniles life in the water column. In up-welling ecosystems,
the exchange between benthic and costal populatios is episidic and stochastic---up-welling
intensity and frequeny.
Together with my collaborator, Bernardo Broitman, at UCSB, we have been
developing theoretical models about the demographic scenario described above
as well as analysing satellite image data from near shore sea variables (temperature,
chl-a) to gather some notion about the spatio-temporal nature of the linkage.
Junling Ma
Evolutionary branching and resource adaptation
I developed a framework for studying evolutionary branching of multiple
species with multiple traits using the adaptive dynamics method, and applied
it to study the evolution of resource adaptation. I introduced a
"double-dimension method" to study the branching conditions for both
multiple-species systems with a single trait per species and systems with
multiple traits per species. The branching conditions comprise a coexistence
condition for mutants and their parents, and a saddle condition of the
evolutionary equilibrium in the "doubled" system. The multiple-trait
systems
present more complexities than the single-trait-per-species systems because
in multiple-trait systems the branched species have random combinations
of
the branched traits. I then applied this framework to study the adaptation
to both continuously distributed and discrete resources. In both cases I
showed that evolutionary branching may occur if there are constraints on
the
resource utilization ability, so that multiple consumers with specialist
strategies for partitioning the resource distribution can evolve. Large
switching costs prevent consumers from branching and promote one specialist
strategy. But even with large switching costs a consumer may adapt to
extremely different resources either by branching through intermediate
resources or by draining the original resource so that switching to the
alternative is beneficial.
Tristram Seidler
Seed dispersal as a cause of aggregation in tropical
trees
It has often been speculated that limited seed dispersal leads to clumping
in tree populations, but this has never been directly demonstrated.
Interactions between dispersal and post-dispersal processes affecting
seedling and sapling survival, such as density dependence, facilitation,
and habitat associations, could in theory lead to a host of different
spatial distributions of juvenile and adult trees. In spite of this,
I
show that dispersal syndromes are broadly correlated with spatial
dispersion in a diverse tree community in Malaysia.
In order to determine the extent to which clumping is explained by seed
dispersal, I measured dispersal in five highly aggregated understory
species, and used these data to parameterize simulations of dispersal,
recruitment and death over multiple generations. Simulations predicted
dispersions very close to those observed in four of five species.
Further tests were required to eliminate alternative mechanisms leading
to
clumping: associations with patchy habitats, and facilitation. I
manipulated seedlings in the field, and measured effects of neighbors on
growth in saplings and adults of Mallotus penangensis, to show that for
at
least this species, only seed dispersal adequately explains the
distribution.
Overall, these tests represent minimum requirements for quantifying the
contribution of dispersal to spatial dispersion. They could with relative
ease be modified to apply to species with strong post-dispersal
constraints, such as negative density dependence or habitat specificity.
Kai Chan
What are porous genomes?
And what do they mean for species, speciation, and
phylogenetics?
Wu (2001) recently drew attention to the porosity of the genome: when
reproductive isolation is incomplete, lineages may remain distinct at
"speciation genes" while homogenizing at unlinked neutral genes. Using
simple analytical and more complex/realistic single-population models, I
previously showed that some components of the genome (e.g., mtDNA) are
especially vulnerable to introgression from immigrants. I speculated
that
the Lake Victoria cichlid flock could have resulted from low levels of
hybridization between several lineages during the re-founding of the lake,
rather than explosive radiation of a single lineage, as commonly believed.
In order to better consider the likelihood that introgression will mask
the
diversity of founding lineages, I now investigate a stochastic, explicitly
genetic individual-based model. Here organisms mate assortatively
in
ecologically divergent lineages. Surprisingly, such lineages are quite
likely to homogenize at some genes while maintaining their integrity at
others. The conditions characteristic of habitat colonization further
facilitate homogenization. I conclude by discussing the ramifications
for
the concepts of species and speciation, and the reconstruction of
phylogenies.
Stefan Geritz
Adaptive dynamics tutorial
Stefan Geritz
Invasion dynamics, attractor inheritance, and limiting
similarity in
evolution by small mutational steps
In adaptive dynamics it is commonly assumed that if a mutant can invade
a resident population while invasion under reversal of roles (i.e.,
with the resident as invader) is not possible, then the mutant will go
to fixation, i.e., the mutant will oust the resident and become the new
resident itself. A second common assumption is that if the mutant can
invade, and invasion under reversal of roles is also possible, then the
mutant and the resident will coexist as a protected dimorphism.
By means of examples I show that in general these assumptions need not
be true. I also show, however, that if the mutant and the resident have
only slightly different phenotypes or "strategies", then these
assumptions are readily justified for a very large class of
deterministic as well as stochastic ecological models. This is done in
two steps: First I show that if the mutant is introduced in
sufficiently low numbers in a resident population close to a population
attractor (equilibrium or limit cycle etc.), then the sum of the
resident and mutant population densities will stay arbitrarily close to
the resident attractor. In other words, with initial conditions typical
for an invasion event, the combined resident-mutant population dynamics
stays inside a narrow tube-shaped region of the resident-mutant
population state space. Next, I show that coexistence inside this tube
is either not possible (one-dimensional strategies) or can be made
arbitrarily unlikely (multi-dimensional strategies) provided the mutant
and resident strategies are sufficiently similar to one another but not
too close to a so-called singular strategy, i.e., a strategy where
directional selection vanishes. I then continue to show that this
implies that almost everywhere in the strategy space, invasion by the
mutant implies fixation of the mutant and moreover such that the mutant
stays on the same attractor as the former resident. The latter
phenomenon I call "attractor inheritance".
I conclude with two examples (evolutionary suicide and evolutionary
cycles) to demonstrate how these results can be applied to concrete
models.
Jean Carlson
Are forest fires HOT?
The catastrophic fires which have once again scorched US forests
underscore criticism that twentieth century wildfire (mis)management
disrupted nature's cycle of disturbance and renewal, leading to a
systematically increased risk and severity of large fires. This
real-world experience parallels debates involving abstract models
of forest fires which have become central metaphors for
illustrating universal features of complex systems. A popular link
between the worlds of real and model fires is the claim that both
exhibit roughly power law behavior in fire size versus frequency.
Two competing frameworks for power laws in complex systems apply
directly to forest fires. These are self-organized criticality
(SOC), which emphasizes fuel connectivity in random configurations
at a critical density, and highly optimized tolerance (HOT), which
emphasizes specialized high-density configurations which are tuned
for robustness in an uncertain environment. In this talk I use
observed statistics and fire scar shapes to (1) draw fundamental
distinctions between the SOC and HOT, and (2) evaluate the long
term effects of extreme weather and suppression in a detailed
high-fidelity forest fire simulation environment HFire.
Observations and the HFire simulations are both in striking
agreement with an abstract model based on HOT. This work suggests that
identifying robustness tradeoffs which underlie resilience in
different fire regimes may be key to understanding the long term
evolution of forest ecosystems, and evaluating sensitivities to
climate change and forest management strategies.
Juan Manuel Morales
Behavioral and landscape effects on redistribution
kernels: some theory,
some data
Movement behavior affects the way in which individuals redistribute
themselves over space and thus has the potential to affect many ecological
processes. Over limited time scales the movement of an individual can
often be characterized by relatively simple mathematical models. Examples
of such models include biased random walks and correlated random walks.
Over longer time-scales these simple models often fail to describe
patterns of movement because of the likelihood that the individual changes
its movement behavior (for example as a result of entering a different
habitat type). One way to accommodate these multiple behaviors is to
develop different movement models for a number of discrete movement
states. Analysis of simple models suggests that the redistribution kernel
of an individual moving according to different behaviors is related to the
total time spent in each movement state. This in turn depends on the
internal state dynamics of the individual and the structure of the
landscape. I will discuss ideas to approximate the results of this
interaction between behavior and landscapes. On the data side of my talk,
I will present Bayesian methods for partitioning elk movements into
different states based on ordered series of step lengths and turning
angles. This approach includes a method for estimating the switching rates
between movement states. Furthermore, landscape data can be integrated
into this approach to test whether certain landscape features are
associated with movement state transitions.
Tuesday, November
19 at 2PM
Jonathan Dushoff
Influenza seasonality and mortality
Influenza is a strongly seasonal disease which is directly and indirectly
responsible for a
large amount of mortality. Estimates of the total mortality burden
due to influenza in the
United States range from a few thousand people per year up to around a million
people per
year. I will discuss some ongoing efforts to understand the seasonality
and the mortality
burden of flu.
Tuesday, November
26 at 2PM
Yoh Iwasa
Spatial heterogeneity versus temporal fluctuation versus species coexistence
We study the effect of permanent spatial heterogeneity on species
coexistence in a lottery-competition model. The system consists of
multiple habitats, each composed of a number of sites occupied by adults
of
two species. Larvae (or seeds) produced from different habitats are
mixed
in a common pool. When an adult dies, the vacant site is filled by
an
individual randomly chosen from the larval (or seed) pool. We can
identify
all the equilibria and their stability. If there are n habitats, there
can
be up to n + 1 equilibria, and about half of them are locally stable.
Our
main result is that the between-habitat variation in the adult mortality
promotes the coexistence, but that in the larval production rate (or seed
production rate) discourages the coexistence. This conclusion is opposite
to the role of temporal variation in the standard lottery model, in which
the between-year variation in the reproductive rate promotes coexistence
of
competitors, but that in the adult mortality discourages the coexistence.
Next we study the fraction of randomly formed pairs of species that can
coexist. Species can coexist more easily for larger spatial heterogeneity
and smaller temporal fluctuation of adult mortality and for smaller spatial
heterogeneity and larger temporal fluctuation of larval (seed) production
rate. In general, in maintaining the diversity of species, the spatial
heterogeneity might be more likely to be important than temporal
fluctuation of the environment -- because [1] incomplete mixing of seeds
or
larval encourage the coexistence by spatial heterogeneity but not that by
temporal fluctuation, and [2] temporal fluctuation lottery model can work
only when the adult mortality is low, but the spatial heterogeneity model
can work irrespective of the absolute magnitude of mortality.
Marissa Baskett
Marine reserve design and the evolution of size
at maturation in
harvested fish
Size-selective harvesting in wild fish populations results in selection
pressure for earlier maturation, which reduces the biomass yield and
sustainability of fisheries. Marine reserves may counter this negative
effect, and my talk will focus on a model that addresses how marine
reserve design impacts the potential for marine reserves to reduce
selection for earlier maturation in harvested fish. In this model,
I
represent size at maturation as a quantitative genetic trait and
evaluate how marine reserve size, spacing, amount of take allowed
affects the evolution of size at maturation. To conclude, I will
present three possible extensions to the model, which is currently in
progress.
Adi Livnat
The Evolution of Subjective Future Discounting in Biological Systems
Some traits reflect a trade off between short term and long term
survival of the lineage carrying them. How is the balance determined?
The answer depends on the evolutionary rate of change of those traits.
Faster evolving ones will evolve to invest more in the short term and
less in the long term than slower evolving ones. I will try to explain
how this result comes by and discuss its meaning in the context of
evolutionary theory.