Biomedical subjects

Claudia Neuhauser

Publications and source records attributed to Claudia Neuhauser.

6 recordsLinked to original sources

PubMed2007

Patch aging and the S-Allee effect: breeding system effects on the demographic response of plants to habitat fragmentation.

We used empirical and modeling approaches to examine effects of plant breeding systems on demographic responses to habitat fragmentation. Empirically, we investigated effects of local flowering plant density on pollination and of population size on mate availability in a common, self-incompatible purple coneflower, Echinacea angustifolia, growing in fragmented prairie habitat. Pollination and recruitment increased with weighted local density around individual flowering plants. This positive density dependence is an Allee effect. In addition, mean mate compatibility between pairs of plants increased with population size. Based on this empirical study, we developed an individual-based, spatially explicit demographic model that incorporates autosomal loci and an S locus. We simulated habitat fragmentation in populations identical except for their breeding system, self-incompatible (SI) or self-compatible (SC). Both populations suffered reduced reproduction in small patches because of scarcity of plants within pollination distance (potential mates) and inbreeding depression. But SI species experienced an additional, genetic contribution to the Allee effect (S-Allee effect) caused by allele loss at the S locus, which reduces mate availability, thereby decreasing reproduction. The strength of the S-Allee effect increases through time (i.e., patches age) because random genetic drift reduces S-allele richness. We investigate how patch aging influences extinction and discuss how the S-Allee effect influences communities in fragmented habitat.

Breeding↗

PubMed2006

Local migration promotes competitive restraint in a host-pathogen 'tragedy of the commons'.

Fragmented populations possess an intriguing duplicity: even if subpopulations are reliably extinction-prone, asynchrony in local extinctions and recolonizations makes global persistence possible. Migration is a double-edged sword in such cases: too little migration prevents recolonization of extinct patches, whereas too much synchronizes subpopulations, raising the likelihood of global extinction. Both edges of this proverbial sword have been explored by manipulating the rate of migration within experimental populations. However, few experiments have examined how the evolutionary ecology of fragmented populations depends on the pattern of migration. Here, we show that the migration pattern affects both coexistence and evolution within a community of bacterial hosts (Escherichia coli) and viral pathogens (T4 coliphage) distributed across a large network of subpopulations. In particular, different patterns of migration select for distinct pathogen strategies, which we term 'rapacious' and 'prudent'. These strategies define a 'tragedy of the commons': rapacious phage displace prudent variants for shared host resources, but prudent phage are more productive when alone. We find that prudent phage dominate when migration is spatially restricted, while rapacious phage evolve under unrestricted migration. Thus, migration pattern alone can determine whether a de novo tragedy of the commons is resolved in favour of restraint.

Animals↗

PubMed2003

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↗

PubMed2003

Founder control and coexistence in a simple model of asymmetric competition for light.

Size asymmetry in plant light acquisition complicates predictions of competitive outcomes in light-limited communities. We present a mathematically tractable model of asymmetric competition for light and discuss its implications for predicting outcomes of competition during establishment in two-, three-, and many-species communities. In contrast to the resource-reduction model of symmetric competition for a single resource, the model we present predicts that outcomes of asymmetric competition for light will sometimes depend on the timing of establishment and the consequent hierarchy among species in canopy position. Competitive outcomes in the model depend on the minimum light requirements (L(c)) and self-shading of species lower in the canopy compared to the light available (L(out)(*)) beneath species higher in the canopy. Succession progresses towards species with decreasing values for L(c), but arrested successions occur when initial dominants have relatively high values for L(c) but low values for L(out)(*), leading to founder control. A theoretically limitless number of species may coexist in competition for light when dominance is founder controlled. These model predictions have implications for an array of applied ecological questions, including methods to control invasive species in light-limited restored ecosystems.

Biomass↗

PubMed2002

Effects of local interactions and local migration on stability.

Mathematical models can help to resolve the longstanding question of whether more diverse communities are more stable. Here, I focus on how local dispersal and local interactions--hallmarks of spatial communities--affect stability in a spatially implicit model with demographic stochasticity. The results are based on a novel way to analyze moment equations. The main conclusion is that the type and strength of density-dependent factors, such as fecundity and competition, determine whether local dispersal and local interactions increase or decrease stability. Local dispersal has a stabilizing effect when fecundity is high, interspecific competition is either low or high, and the number of species is small. Effects of local migration on stability are amplified when space is explicit.

Animal Migration↗

PubMed2002

The pattern of amino acid replacements in alpha/beta-barrels.

The determinants of site-to-site variability in the rate of amino acid replacement in alpha/beta-barrel enzyme structures are investigated. Of 125 available alpha/beta-barrel structures, only 25 meet a variety of phylogenetic and statistical criteria necessary to ensure sufficient data for reliable analysis. These 25 enzyme structures (from a wide variety of taxa with diverse lifestyles in diverse habitats) differ greatly in size, number, and topology of domains in addition to the alpha/beta-barrel, quaternary structure, metabolic role, reaction catalyzed, presence of prosthetic groups, regulatory mechanisms, use of cofactors, and catalytic mechanisms. Yet, with the exception of ribulose-1,5-bisphosphate carboxylase, all structures have similar frequency distributions of amino acid replacement rates. Hence, site-specific variability in rates of evolution is largely independent of differences in biology, biochemistry, and molecular structure. A correlation between site-specific rate variation and (1) distance from the active site, (2) solvent accessibility, and (3) treating glycines in unusual main-chain conformations as a separate class, explains approximately half the causal variation. Secondary structure exerts little influence on the pattern and distribution of replacements. Additional domains and subunits, side-chain hydrogen bonds, unusual side-chain rotamers, nonplanar peptide bonds, strained main-chain conformations, and buried hydrophilic-charged residues contribute little to variability among sites because they are rare. Nonlinear models do not improve the fits. In several enzymes, deviations from the typical pattern of replacements suggest the possible action of natural selection. A statistical analysis shows that, in all cases, much of the remaining unexplained variation is not attributable to chance and that other, as yet unidentified, causal relations must exist.

Amino Acid Sequence↗