A plan to keep carbon in check.
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Journal Article↗
Biomedical subjects
Stephen W Pacala
Publications and source records attributed to Stephen W Pacala.
The evolution of intergenerational discounting in offspring quality.
Intergenerational effects occur when an individual's actions affect not only its own survivorship and reproduction but also those of its offspring and possibly later descendants. In the presence of intergenerational effects, short-term and long-term measures of success (such as the expected numbers of surviving offspring and of farther descendants, respectively) may be in conflict. When such conflicts occur, life-history theory normally takes long-term measures to predict the outcome of selection. This ignores the fact that, because traits change in time--through mutation, sex, and recombination--long-term relations disintegrate. We study this issue with numerical simulations and analytical models combining intergenerational effects and evolutionary change. In the models, the parental investment per offspring, as well as the total reproductive effort, stand for investments in future generations. The models show that the rate of evolutionary change determines the level of those investments. Higher rates of mutation and of sexual as opposed to parthenogenetic reproduction favor lower parental investment per offspring and lower total reproductive effort. It follows that the level of investment of ancestors in descendants responds to the genetic relatedness between the generations of the lineage, in a manner unaccounted for by preexisting theory.
Animals↗
Demographic theory of coral reef fish populations with stochastic recruitment: comparing sources of population regulation.
The effects of three forms of density-dependent regulation were explored in model coral reef fish populations: top-down (predation), bottom-up (competition for food), and pelagic (non-reef-based mechanisms) control. We describe the demographic responses of both biomass and numbers of adult fish, predicting the mean and the variance of temporal fluctuations resulting from stochastic recruitment of juveniles. We find that top-down control acts by suppressing variability of numbers of fish, which in turn suppresses the variability of biomass. Bottom-up control has no effect on fluctuations of numbers of fish, though it strongly reduces fluctuations of biomass. Because fecundity of fish is directly linked to body mass, the regulation of biomass tightly regulates reproductive output independently of the number of individuals in the population. Finally, populations under pelagic control experience bounded fluctuations of biomass and numbers directly proportional to the bounded fluctuations of recruitment. The demographic signatures predicted from both bottom-up and pelagic control are consistent with current evidence supporting the recruitment limitation hypothesis in reef fish ecology. We propose tests to discriminate the dominant mode of density-dependent regulation using qualitative trends in time series demographic data across environmental clines.
Animals↗
Spatial dynamics in model plant communities: what do we really know?
A variety of models have shown that spatial dynamics and small-scale endogenous heterogeneity (e.g., forest gaps or local resource depletion zones) can change the rate and outcome of competition in communities of plants or other sessile organisms. However, the theory appears complicated and hard to connect to real systems. We synthesize results from three different kinds of models: interacting particle systems, moment equations for spatial point processes, and metapopulation or patch models. Studies using all three frameworks agree that spatial dynamics need not enhance coexistence nor slow down dynamics; their effects depend on the underlying competitive interactions in the community. When similar species would coexist in a nonspatial habitat, endogenous spatial structure inhibits coexistence and slows dynamics. When a dominant species disperses poorly and the weaker species has higher fecundity or better dispersal, competition-colonization trade-offs enhance coexistence. Even when species have equal dispersal and per-generation fecundity, spatial successional niches where the weaker and faster-growing species can rapidly exploit ephemeral local resources can enhance coexistence. When interspecific competition is strong, spatial dynamics reduce founder control at large scales and short dispersal becomes advantageous. We describe a series of empirical tests to detect and distinguish among the suggested scenarios.
Ecosystem↗
Coping with uncertainty: a call for a new science-policy forum.
The scientific and policy worlds have different goals, which can lead to different standards for what constitutes "proof" of a change or phenomena, and different approaches for characterizing and conveying uncertainty and risk. These differences can compromise effective communication among scientists, policymakers, and the public, and constrain the types of socially compelling questions scientists are willing to address. In this paper, we review a set of approaches for dealing with uncertainty, and illustrate some of the errors that arise when science and policy fail to coordinate correctly. We offer a set of recommendations, including restructuring of science curricula and establishment of science-policy forums populated by leaders in both arenas, and specifically constituted to address problems of uncertainty.
Curriculum↗
Mechanisms of long-distance dispersal of seeds by wind.
Long-distance dispersal (LDD) is central to species expansion following climate change, re-colonization of disturbed areas and control of pests. The current paradigm is that the frequency and spatial extent of LDD events are extremely difficult to predict. Here we show that mechanistic models coupling seed release and aerodynamics with turbulent transport processes provide accurate probabilistic descriptions of LDD of seeds by wind. The proposed model reliably predicts the vertical distribution of dispersed seeds of five tree species observed along a 45-m high tower in an eastern US deciduous forest. Simulations show that uplifting above the forest canopy is necessary and sufficient for LDD, hence, they provide the means to define LDD quantitatively rather than arbitrarily. Seed uplifting probability thus sets an upper bound on the probability of long-distance colonization. Uplifted yellow poplar seeds are on average lighter than seeds at the forest floor, but also include the heaviest seeds. Because uplifting probabilities are appreciable (as much as 1 5%), and tree seed crops are commonly massive, some LDD events will establish individuals that can critically affect plant dynamics on large scales.
Computer Simulation↗