1,903 research outputs found
On the impact of dispersal asymmetry on metapopulation persistence
Metapopulation theory for a long time has assumed dispersal to be symmetric,
i.e. patches are connected through migrants dispersing bi-directionally without
a preferred direction. However, for natural populations symmetry is often
broken, e.g. for species in the marine environment dispersing through the
transport of pelagic larvae with ocean currents. The few recent studies of
asymmetric dispersal concluded, that asymmetry has a distinct negative impact
on the persistence of metapopulations. Detailed analysis however revealed, that
these previous studies might have been unable to properly disentangle the
effect of symmetry from other potentially confounding properties of dispersal
patterns. We resolve this issue by systematically investigating the symmetry of
dispersal patterns and its impact on metapopulation persistence. Our main
analysis based on a metapopulation model equivalent to previous studies but now
applied on regular dispersal patterns aims to isolate the effect of dispersal
symmetry on metapopulation persistence. Our results suggest, that asymmetry in
itself does not imply negative effects on metapopulation persistence. For this
reason we recommend to investigate it in connection with other properties of
dispersal instead of in isolation.Comment: 19 pages, 5 figure
Foreword
While several studies have explored how short-term ecological responses to disturbance vary among ecosystems, experimental studies of how contrasting ecosystems recover from disturbance in the longer term are few. We performed a simple long-term experiment on each of 30 contrasting forested islands in northern Sweden that vary in size; as size decreases, time since fire increases, soil fertility and ecosystem productivity declines, and plant species diversity increases. We predicted that resilience of understory plant community properties would be greatest on the larger, more productive islands, and that this would be paralleled by greater resilience of soil biotic and abiotic properties. For each island, we applied three disturbance treatments of increasing intensity to the forest understory once in 1998, i.e., light trimming, heavy trimming, and burning; a fourth treatment was an undisturbed control. We measured recovery of the understory vascular plant community annually over the following 14 years, and at that time also assessed recovery of mosses and several belowground variables. Consistent with our predictions, vascular plant whole-community variables (total cover, species richness, diversity [Shannon's HI, and community composition) recovered significantly more slowly on the smaller (least fertile) than the larger islands, but this difference was not substantial, and only noticeable in the most severely disturbed treatment. When an index of resilience was used, we were unable to detect effects of island size on the recovery of any property. We found that mosses and one shrub species (Empetrum hermaphroditum) recovered particularly slowly, and the higher abundance of this shrub on small islands was sufficient to explain any slower recovery of whole-ecosystem variables on those islands. Further, several belowground variables had not fully recovered from the most intense disturbance after 14 yr, and counter to our predictions, the degree of their recovery was never influenced by island size. While several studies have shown large variation among plant communities in their short-term response (notably resistance) to environmental perturbations, our results reveal that when perturbations are applied equally to highly contrasting ecosystems, differences in resilience among them in the longer term can be relatively minor, regardless of the severity of disturbance
Sparse Partially Collapsed MCMC for Parallel Inference in Topic Models
Topic models, and more specifically the class of Latent Dirichlet Allocation
(LDA), are widely used for probabilistic modeling of text. MCMC sampling from
the posterior distribution is typically performed using a collapsed Gibbs
sampler. We propose a parallel sparse partially collapsed Gibbs sampler and
compare its speed and efficiency to state-of-the-art samplers for topic models
on five well-known text corpora of differing sizes and properties. In
particular, we propose and compare two different strategies for sampling the
parameter block with latent topic indicators. The experiments show that the
increase in statistical inefficiency from only partial collapsing is smaller
than commonly assumed, and can be more than compensated by the speedup from
parallelization and sparsity on larger corpora. We also prove that the
partially collapsed samplers scale well with the size of the corpus. The
proposed algorithm is fast, efficient, exact, and can be used in more modeling
situations than the ordinary collapsed sampler.Comment: Accepted for publication in Journal of Computational and Graphical
Statistic
Mass distribution exponents for growing trees
We investigate the statistics of trees grown from some initial tree by
attaching links to preexisting vertices, with attachment probabilities
depending only on the valence of these vertices. We consider the asymptotic
mass distribution that measures the repartition of the mass of large trees
between their different subtrees. This distribution is shown to be a broad
distribution and we derive explicit expressions for scaling exponents that
characterize its behavior when one subtree is much smaller than the others. We
show in particular the existence of various regimes with different values of
these mass distribution exponents. Our results are corroborated by a number of
exact solutions for particular solvable cases, as well as by numerical
simulations
Recommended from our members
Conservative estimation of overvoltage-based PV hosting capacity
textThe primary objective of this work is to develop and demonstrate a steady-state
stochastic simulation method to estimate the PV hosting capacity of a given distribution,
based on the ANSI voltage regulation standard. The work discusses the key factors that
determine the voltage rise due to distributed PV. Load demand analysis is done to
determine statistically representative minimum daylight load demand for PV analysis. And
lastly, the steady-state, stochastic simulation method is discussed and implemented to
estimate the PV hosting capacity for small-scale and large-scale PV Deployments.Electrical and Computer Engineerin
Linking vegetation change, carbon sequestration and biodiversity
1. Despite recent interest in linkages between above- and belowground communities and their consequences for ecosystem processes, much remains unknown about their responses to long-term ecosystem change. We synthesize multiple lines of evidence from a long-term ‘natural experiment’ to illustrate how ecosystem retrogression (the decline in ecosystem processes due to long-term absence of major disturbance) drives vegetation change, and thus aboveground and belowground carbon (C) sequestration, and communities of consumer biota.
2. Our study system involves 30 islands in Swedish boreal forest that form a 5000 year fire-driven retrogressive chronosequence. Here, retrogression leads to lower plant productivity and slower decomposition, and a community shift from plants with traits associated with resource acquisition to those linked with resource conservation.
3. We present consistent evidence that aboveground ecosystem C sequestration declines, while belowground and total C storage increases linearly for at least 5000 years following fire absence. This increase is driven primarily by changes in vegetation characteristics, impairment of decomposer organisms and absence of humus combustion.
4. Data from contrasting trophic groups show that during retrogression, biomass or abundance of plants and decomposer biota decreases, while that of aboveground invertebrates and birds increases, due to different organisms accessing resources via distinct energy channels. Meanwhile, diversity measures of vascular plants and aboveground (but not belowground) consumers respond positively to retrogression.
5. We show that taxonomic richness of plants and aboveground consumers are positively correlated with total ecosystem C storage, suggesting that conserving old growth forests simultaneously maximizes biodiversity and C sequestration. However, we find little observational or experimental evidence that plant diversity is a major driver of ecosystem C storage on the islands relative to other biotic and abiotic factors.
6. Synthesis. Our study reveals that across contrasting islands differing in exposure to a key extrinsic driver (historical disturbance regime and resulting retrogression), there are coordinated responses of soil fertility, vegetation, consumer communities, and ecosystem C sequestration, which all feed back to one another. It also highlights the value of well replicated natural experiments for tackling questions about aboveground-belowground linkages over temporal and spatial scales that are otherwise unachievable
Branched polymers, complex spins and the freezing transition
We show that by coupling complex three-state systems to branched-polymer like
ensembles we can obtain models with gamma-string different from one half. It is
also possible to study the interpolation between dynamical and crystalline
graphs for these models; we find that only when geometry fluctuations are
completely forbidden is there a crystalline phase.Comment: 14 pages plain LateX2e, 4 eps figures included using eps
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