Complex discrete dynamics from simple continuous population models

Nonoverlapping generations have been classically modelled as difference equations in order to account for the discrete nature of reproductive events. However, other events such as resource consumption or mortality are continuous and take place in the within-generation time. We have realistically assumed a hybrid ODE bidimensional model of resources and consumers with discrete events for reproduction. Numerical and analytical approaches showed that the resulting dynamics resembles a Ricker map, including the doubling route to chaos. Stochastic simulations with a handling-time parameter for indirect competition of juveniles may affect the qualitative behaviour of the model.     

Biomass-diversity responses and spatial dependencies in disturbed tallgrass prairies

Monotonic, hump-shaped and zero-correlation productivity-diversity relationships have been found to date in many ecosystems. This diversity of responses has puzzled ecologists in their search for general principles on ecosystem functioning. Some state that the scale of observation is crucial in defining this relationship. We have developed a spatial model of tallgrass prairies where biomass and litter dynamics are defined by uncoupled difference equations. In this system, we periodically apply prescribed fire as a disturbance that propagates through neighboring cells. The model shows percolation thresholds at points where small-scale spatial heterogeneity and large-scale, global correlation coexist, resulting in power-law distributions in available areas for non-dominant species. These points maximize the biomass-diversity relationship. Our results suggest that spatial dependencies and the disturbance heterogeneity hypothesis are the cornerstone processes accounting for unimodality in productivity-diversity relationships.     

Age-Related Tooth Wear Differs between Forest and Savanna Primates

Tooth wear in primates is caused by aging and ecological factors. However, comparative data that would allow us to delineate the contribution of each of these factors are lacking. Here, we contrast age-dependent molar tooth wear by scoring percent of dentine exposure (PDE) in two wild African primate populations from Gabonese forest and Kenyan savanna habitats. We found that forest-dwelling mandrills exhibited significantly higher PDE with age than savanna yellow baboons. Mandrills mainly feed on large tough food items, such as hard-shell fruits, and inhabit an ecosystem with a high presence of mineral quartz. By contrast, baboons consume large amounts of exogenous grit that adheres to underground storage organs but the proportion of quartz in the soils where baboons live is low. Our results support the hypothesis that not only age but also physical food properties and soil composition, particularly quartz richness, are factors that significantly impact tooth wear. We further propose that the accelerated dental wear in mandrills resulting in flatter molars with old age may represent an adaptation to process hard food items present in their environment.     

Density and population structure of the jaguar (<Emphasis Type="Italic">Panthera onca</Emphasis>) in a protected area of Los Llanos,Venezuela, from 1 year of camera trap monitoring

Density is crucial for understanding large carnivore ecology and conservation, but estimating it has proven methodologically difficult. We conducted 1 year of camera trapping to estimate jaguar (Panthera onca) density and population structure in the Los Llanos region of Venezuela on the Hato Piñero ranch, where hunting is prohibited and livestock are excluded from half of ranch lands. We identified 42 different jaguars and determined their sex, age class, and reproductive status. We estimated adult jaguar densities with spatial capture-recapture models, using sex/reproductive state and session as covariates. Models without temporal variation received more support than models that allowed variation between sessions. Males, reproductive females, and nonreproductive females differed in their density, baseline detectability, and movement. The best estimate of total adult jaguar population density was 4.44 individuals/100 km². Based on reproductive female density and mean number of offspring per female, we estimated cub density at 3.23 individuals/100 km² and an overall density of 7.67 jaguars/100 km². Estimated jaguar population structure was 21% males, 11% nonreproductive females, 26% reproductive females, and 42% cubs. We conclude that extending the sampling period to 1 year increases the detectability of females and cubs and makes density estimates more robust as compared to the more common short studies. Our results demonstrate that the Venezuelan Llanos represent important jaguar habitat, and further, they emphasize the importance of protected areas and hunting restrictions for carnivore conservation.     

The early adaptive evolution of calmodulin

Interaction between gene duplication and natural selection in molecular evolution was investigated utilizing a phylogenetic tree constructed by the parsimony procedure from amino acid sequences of 50 calmodulin- family protein members. The 50 sequences, belonging to seven protein lineages related by gene duplication (calmodulin itself, troponin-C, alkali and regulatory light chains of myosin, parvalbumin, intestinal calcium-binding protein, and glial S-100 phenylalanine-rich protein), came from a wide range of eukaryotic taxa and yielded a denser tree (more branch points within each lineage) than in earlier studies. Evidence obtained from the reconstructed pattern of base substitutions and deletions in these ancestral loci suggests that, during the early history of the family, selection acted as a transforming force on expressed genes among the duplicates to encode molecular sites with new or modified functions. In later stages of descent, however, selection was a conserving force that preserved the structures of many coadapted functional sites. Each branch of the family was found to have a unique average tempo of evolutionary change, apparently regulated through functional constraints. Proteins whose functions dictate multiple interaction with several other macromolecules evolved more slowly than those which display fewer protein-protein and protein-ion interactions, e.g., calmodulin and next troponin-C evolved at the slowest average rates, whereas parvalbumin evolved at the fastest. The history of all lineages, however, appears to be characterized by rapid rates of evolutionary change in earlier periods, followed by slower rates in more recent periods. A particularly sharp contrast between such fast and slow rates is found in the evolution of calmodulin, whose rate of change in earlier eukaryotes was manyfold faster than the average rate over the past 1 billion years. In fact, the amino acid replacements in the nascent calmodulin lineage occurred at residue positions that in extant metazoans are largely invariable, lending further support to the Darwinian hypothesis that natural selection is both a creative and a conserving force in molecular evolution.