Theoretical Ecology Lab Tea

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 2:00 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 Marissa Baskett and Jeremy Lichstein. Please contact mbaskett@princeton.edu or jwl@princeton.edu to have your name added to the labtea email list so that you can receive reminders about upcoming lab teas.

Spring 2004


Tuesday, February 3, at 2:00 PM	Steve Pacala
Tuesday, February 10, at 2:00 PM	Joe Wright
Tuesday, February 17, at 2:00 PM	Pierre-Yves Henry
Tuesday, February 24, at 2:00 PM	Drew Purves
Tuesday, March 2 at 2:00 PM	Marco Janssen
Tuesday, March 9, at 2:00 PM	Tom Doak
Tuesday, March 16, at 2:00 PM	(Spring break)
Tuesday, March 23 at 2:00 PM	Helene Muller-Landau
Tuesday, March 30 at 2:00 PM	Jeremy Lichstein
Tuesday, Arpil 6, at 2:00 PM	Julie Pulliam
Tuesday, April 13, at 2:00 PM	Ben Strauss
Tuesday, April 20 at 2:00 PM	Katie Hampson
Tuesday, April 27 at 2:00 PM	Simon Donner
Tuesday, May 4 at 2:00 PM	Jerome Chave
Tuesday, May 11 at 2:00 PM	Tanguy Daufrasne
Tuesday, May 18 at 2:00 PM	Simon Levin
Tuesday, May 25 at 2:00 PM	Anping Chen

Titles and abstracts most recent last (posted approximately one week before the talk):

We analyze tree growth data from Wisconsin forest inventories completed in 1968, 1983, 1996 and 2002. These show that the rate of forest growth decreased steadily over the period, in contrast to the increases predicted by CO2 fertilization models. Measured growth rate changed an average of -0.27% y-1 (95% confidence range: -0.05% to -0.49% y-1), whereas the prediction for CO2 fertilization is 0.16% y-1 (corresponding to a β of 0.36). The high statistical precision is due both to large sample sizes and positive correlations among the growth rates from different time periods within the same plot. Decreased growth occurred in stands of all ages, and so our results are not caused by age-related declines in growth (although highly significant age-related declines were also detected).
Data allowing a direct examination of growth rates over several decades are available only for Wisconsin, but Caspersen et al. (2000) introduced an indirect method for detecting past changes in growth rate using only two sequential inventories. This method was criticized by Joos et al. (2002), who claimed that it lacked the statistical power to falsify state-of–the-art ecosystem models of CO2 fertilization. We explain both the sound points and the critical errors in Joos et al.’s argument, introduce a transparent and analytically tractable version of Caspersen et al.’s method, and check its ability to detect the decreasing growth rates in the Wisconsin data. The results show that the indirect method accurately characterizes the past changes that actually occurred, and has sufficient statistical power to falsify CO2 fertilization models, including the model in Joos et al. (2002).
We discuss the implications of decreasing Wisconsin growth rates, together with other reasons for skepticism about the future magnitude of CO2 fertilization. In particular, the steep reductions in fossil fuel emissions required to stabilize atmospheric CO2 at 500+50 ppm must begin more than a decade sooner if the predictions of the CO2 fertilization models in the IPCC Third Assessment (Prentice et al. 2001) are incorrect. The difference between a terrestrial carbon sink that grows because of CO2 fertilization, and one that shrinks because it is caused by recovery from past land use, is the difference between the luxury of a substantial delay and the need to act now.

Variable reproduction by tropical forest plants: causes and consequences of among-year variation

Seed production and seedling recruitment are being monitored for forests in tropical Australia, Ecuador, Malaysia, Panama and Puerto Rico. More than 1,000,000 fruit and seeds and 40,000 seedlings have been identified to species in Panama alone over the past 17 years. Today, I will discuss among year variation in seed production, its likely abiotic cause, and its consequences for seedling recruitment and for pollinating bees in Panama. These are ongoing studies, three members of the Horn-Levin-Pacala labs have worked with the data, and new collaborations are always welcome.

Role of gregariousness and inter-specific aggressivity in the process of biological invasion

Biological invasions are the second cause of biodiversity loss after habitat destruction. It makes invasion biology one of the priority fields of integrative research for population biologists. However, the evolutionary consequences of invasions have been poorly investigated. Our project aims at investigating some behavioral-based selective mechanisms that are likely to be involved in the invasion process. How do they shape the evolutionary potential, life history and behavioral traits of invasive species? We identified two behavioral aspects that have been disregarded : (1) Is gregariousness selected for during the establishment phase of an alien species ? (2) Is inter-specific aggressiveness selected for in response to competition during invasion?
These two lines of research will be addressed through (i) comparative analyses of invasion success at the inter-specific level, and (ii) by implementing experimental introductions at invasion front with different group size as experimental treatments. The model species is the House Sparrow Passer domesticus. Because of the lower “dilution” of propagules during the introduction event (Allee effect), we expect (i) gregarious species, and (ii) high group size (10 indivdiuals), to be more successful in getting established than non-gregarious species and low group size (2 individuals). Introduced individuals will be radio-tracked continuously during the first three days after release (and then once per weak for three weaks) to document how group size affects their dispersal behavior and their interactions with the native “ecological sparrow” (Zonotrichia capensis).

Sapling growth and light availability in tropical forests: analyzing general patterns and sources of variation

Plant species vary in the dependence of juvenile growth and survival on light availability, and this variation is an important axis of niche
differentiation in virtually all plant communities. The overall objective of this study is to characterize the distribution of tropical forest trees
in a light-dependence trait space -- both to quantify the contribution of such niche differentiation to coexistence and to use the information on the
distribution of traits in ecosystem models. I use three years of detailed growth and light availability measurements on 1500 saplings of 23 species
on Barro Colorado Island, Panama, to investigate the dependence of diameter growth on light. Light availability alone explains over 51% of the
variation in growth among all saplings, and 16-78% of the variation in individual species, with individual species response parameters strongly
related to wood density. As is usually the case in such studies, there is extensive scatter around the fitted relationship for each individual
species, and overlapping scatter among species. Traditional analyses implicitly assume this scatter is measurement error, while a recent paper
by Jim Clark suggests it should be attributed almost entirely to intraspecific differences in light response parameters among individual
trees. Here, I use explicit measurements of measurement error in both light and growth to quantitatively assess the contribution of these errors
to scatter in growth-light relationships, and to better constrain the potential variation among individuals within species. I close with
discussion of different sources of variability in observed relationships, and their implications for species niche differentiation.

Relating ecological parameters to biogeographic patterns in species diversity

After well over 100 years of careful study, there is still little consensus regarding broad scale patterns in species diversity. Why are there are so many species in the tropics? Or, why are there so few species at high latitudes. I will briefly review the three major competing hypotheses: (1) area, (2) current climate/productivity, and (3) history (time/stability). The neutral theory is, in my opinion, neutral on this issue; i.e., without invoking one of the other 3 hypotheses, neutral theory tells us nothing about geographic patterns in diversity. I will argue that Area, is a poor primary candidate. Current climate/productivity is a good candidate, but its popular proposed mechanism (that more productivity allows higher total abundance and therefore diversity) makes no sense for plants. As for history, Pleistocene extinctions have undoubtedly affected contemporary diversity patterns (e.g., European trees), but many groups and regions (e.g., marine invertebrates; N. American and E. Asian trees) appear to have suffered few extinctions in the last several million years. Climate history on longer time scales (10-100 Myrs) is probably more important in explaining contemporary diversity patterns; i.e., cold climates at high latitudes arose only in the last 20 Myrs.

So, we have 2 tenable hypotheses: climate history over 10-100 Myrs, and contemporary climate/productivity. Developing a mechanistic basis for the latter hypothesis requires that we study the ecological attributes of communities across climatic and environmental gradients. I will present some analyses that are part of a nascent effort to do that for N. American trees.

[back to schedule]

Tuesday, April 13 @ 2:00 PM

Ben Strauss

Horizontal gene transfer in American crows via shared tofu resources

If intrinsic constraints on evolution are absent, what can cause the formation of species range boundaries within an environmental gradient? I will describe a demographic and quantitative genetic model in which gene flow from a range center can swamp local adaptation at more peripheral locations, leading to the formation of a stable range together with limits on the extremes to which a species evolves. This result holds assuming random, diffusive dispersal. However, some organisms’ ability to disperse may change with the environment, such as may be the case for many stream or river fauna, because the force of water is generally stronger upstream. In the second part of my talk, I will use the same modeling framework to explore the possible consequences for distribution of organisms within streams, including cases where stream organisms have the potential to evolve their dispersal abilities in response to the changing dispersal environment. Finally, I will synthesize all of these results with a view toward suggesting the important consequences for the American crow and its preferred tofu resource.

[back to schedule]

Tuesday, May 25 @ 2:00 PM

Anping Chen

What limits tree distribution ranges?

Mechanisms responsible for contemporary patterns of species diversity remain unresolved. MacArthur (1972) believed that the ranges of single species would seem to be the basic unit of biogeography. Studies on species distribution ranges, particularly their northern/upper and southern edges, may therefore provide insight into the mechanisms of latitudinal and/or altitudinal biodiversity gradients. Here I will discuss several inherently related stories (but neither of them are complete) on tree species ranges and their range limits. The altitudinal ranges of tree species in subtropical China makes the expanded Rapoport?s Rule doubtful as an explanation for altitudinal biodiversity gradients. Analysis on North Hemisphere forest lines confirms the climatic constraints on forest limits. Finally I will discuss the tradeoff between cold tolerance and diameter growth in determining northern and southern edges for trees using FIA data.