Diversification and phylogeographic structure in widespread Azteca plant-ants from the northern Neotropics

The Neotropical myrmecophytic tree Cordia alliodora hosts symbiotic Azteca ants in most of its widespread range. The taxonomy of the genus Azteca is notoriously difficult, which has frequently obscured species identity in ecological studies. We used sequence data from one mitochondrial and four nuclear loci to infer phylogenetic relationships, patterns of geographic distribution, and timing of diversification for 182 colonies of five C. alliodora-dwelling Azteca species from Mexico to Colombia. All morphological species were recovered as monophyletic, but we identified at least five distinct genetic lineages within the most abundant and specialized species, Azteca pittieri. Mitochondrial and nuclear data were concordant at the species level, but not within species. Divergence time analyses estimated that C. alliodora-dwelling Azteca shared a common ancestor approximately 10-22million years ago, prior to the proposed arrival of the host tree in Middle America. Diversification in A. pittieri occurred in the Pleistocene and was not correlated with geographic distance, which suggests limited historical gene flow among geographically restricted populations. This contrasts with the previously reported lack of phylogeographic structure at this spatial scale in the host tree. Climatic niches, and particularly precipitation-related variables, did not overlap between the sites occupied by northern and southern lineages of A. pittieri. Together, these results suggest that restricted gene flow among ant populations may facilitate local adaptation to environmental heterogeneity. Differences in population structure between the ants and their host trees may profoundly affect the evolutionary dynamics of this widespread ant-plant mutualism.     

Biogeographical boundaries and Monmonier's algorithm: a case study in the northern Neotropics

Aim  To use Monmonier's algorithm, a spatially explicit technique, to elucidate positions of biogeographical boundaries in the northern Neotropics.
Location  The northern Neotropics (Isthmus of Tehuantepec, Mexico, south to trans-Andean Colombia).
Methods  We compiled avifaunal lists for 36 forested sites from the literature, museum records, field notes, and web sources. We constructed distance matrices as inverse Jaccard's similarity, used Monmonier's algorithm to place biogeographical boundaries, and created bootstrap matrices to determine the relative strength of boundaries.
Results  Biogeographical boundaries with the best support separated lowland (< 1000 m) and montane sites and areas with a distinct historical background, such as seaways, suture zones, volcanic peaks, and former islands.
Main conclusions  Monmonier's algorithm used with distance (dissimilarity) data effectively identified biogeographical boundaries consistent with historical processes and with past research. Montane sites tended to be circumscribed by sharp boundaries, emphasizing their isolation and higher endemism. Lowland sites, by contrast, tended to be homogeneous, suggesting that dispersal has played a much larger role at low elevations. Former seaways, as in the Nicaraguan Depression and extended Bay of Urabá, yielded boundaries, but typically for highland avifauna only. In addition to providing a rigorous (bootstrap support) and heuristic (direct mapping) means of locating biotic boundaries, Monmonier's algorithm can be a valuable tool for conservation planning.     

Tree diversity in the northern Neotropics: regional patterns in highly diverse Chiapas, Mexico

Physical factors that may account for regional patterns of plant species diversity remain controversial. We aim to determine the relationship of tree species diversity to environmental factors identifiable at regional scale in the northern Neotropics. We use a high-resolution data set based on herbarium collections of all native tree species known to occur in the highly diverse and physiographically heterogeneous Mexican state of Chiapas. We analyzed 114 grid cells (5 min latitude×5 min longitude each) with 40 or more vouchers. We obtained from maps (scale 1:250 000) data on temperature, rainfall, elevation, and soils, and calculated for each grid cell mean actual evapotranspiration (AET), its ratio during the rainy and dry seasons (RET), average fertility/quality of soils (SFQ), and elevation (coarse-scale topography) variance (SDE). These variables were largely independent of each other, and were entered in multiple regression models to predict species diversity assessed with Simpson's index of concentration. A model that accounted for 41.4% of the total variance in tree diversity showed positive effects of AET and seasonality (RET), whereas SFQ had a negative effect. A curvilinear model described well the relationship between tree diversity and AET (R²=0.45), and an intermediate maximum was detected. The data pattern suggested an asymptotic relationship as well, which was confirmed with a two-part regression. Regression quantiles provided better estimates of the effect of SFQ with the upper envelope of the data (0.85–0.90 quantiles). Minimum diversity at intermediate rainfall values hints at a bimodal model of tree diversity along a rainfall gradient, in opposition to the frequent contention of a positive linear relationship. We suggest that broad-scale climatic gradients interact with intraregional landscape-level influences, thus leading to the observed nonlinear responses of tree diversity to environmental predictors.     

Sedimentary diatom assemblages in the northern part of Lake Tanganyika

Surface sediment diatoms from the northern part of Lake Tanganyika were analysed with several ordination and classification techniques. Three sample groups characterized by four diatom assemblages were recognized. The first sample group occurs in a rather shallow area near the delta of the Rusizi River on a coarse-grained sand substrate. The second and most important sample group occurs on the silty bottom of an underwater depression situated off the delta of the Rusizi River in the north-eastern part of Lake Tanganyika; its diatom composition has a great affinity to the Rusizi River diatom population. The third sample group contains all the other samples and covers an area corresponding to the west-, east- and southward continuation of the area of the second group: its diatom composition is also related to the Rusizi River diatom population, but in a less pronounced manner.     

Macroecology: the organizing forces

The fundamental problem in ecology is the relationship between organisms and the physical world. This question is approached through the study of ecosystems as wholes. Pristine, autonomous Arctic lakes provide an invaluable starting point for such investigations. Studies on these lakes, initiated in 1958, indicate that the dominant fish populations assume a recognizable and repeatable structure, which if not disturbed by external forces, is maintained indefinitely. The observable characteristics are high biomass, large individual size, great age, uniformity of individuals despite great variation in age, and relatively few juveniles. This configuration expresses a state of least specific energy dissipation or least specific entropy production and may be regarded as a standing wave in the energy flow. Similar characteristics were found in the dominant species in a wide range of ecosystems worldwide. A state of least dissipation develops when two energy transport processes interfere with each other in the vicinity of thermodynamic equilibrium. Thus it is postulated that living organisms originated through the agonistic interaction between two energy transport processes within materials, initially occurring in an environment close to thermodynamic equilibrium. These two processes, fundamental to energy transfer, are recognized as the principle of least action and its diametrical opposite, the principle of most action. Each force dominates system behavior in a different time frame: most action in the short-term (ecological time) and least action over the long-term (evolutionary time). Interaction between the countervailing forces provides an understanding of a wide range of emergent ecological generalities such as succession, r- and K-selection, the stability and diversity of ecosystems, and the directionality of evolutionary change.     

Late Pleistocene environmental changes lead to unstable demography and population divergence of Anopheles albimanus in the northern Neotropics

We investigated the historical demography of Anopheles albimanus using mosquitoes from five countries and three different DNA regions, the mitochondrial cytochrome oxidase subunit I gene (COI), the single copy nuclear white gene and the ribosomal internal transcribed spacer two (ITS2). All the molecular markers supported the taxonomic status of a single species of An. albimanus. Furthermore, agreement between the COI and the white genes suggested a scenario of Pleistocene geographic fragmentation (i.e., population contraction) and subsequent range expansion across southern Central America.     

Macroecology of a host-parasite relationship

The larvae of freshwater mussels are obligate ectoparasites on fishes while adults are sedentary and benthic. Dispersal of mussels is dependent on the movement of fish hosts, a regional process, but growth and reproduction should be governed by local processes. Thus, mussel assemblage attributes should be predictable from the regional distribution and abundance of fishes. At a broad spatial scale in the Red River drainage, USA, mussel species richness and fish species richness were positively associated; maximum mussel richness was limited by fish richness, but was variable beneath that constraint. Measured environmental variables and the associated local fish assemblages each significantly accounted for the regional variation in mussel assemblages. Furthermore, mussel assemblages showed strong spatial autocorrelation. Variation partitioning revealed that pure fish effects accounted for 15.4% of the variation in mussel assemblages; pure spatial and environmental effects accounted for 16.1% and 7.8%, respectively. Shared variation among fish, space and environmental variables totaled 40%. Of this shared variation, 36.8% was associated with the fish matrix. Thus, the variation in mussel assemblages that was associated with the distribution and abundance of fishes was substantial (> 50%), indicating that fish community structure is an important determinant of mussel community structure. Although animals commonly disperse plants and, thus, influence the structure of plant communities, our results show a strong macroecological association between two disparate animal groups with one strongly affecting the assemblage structure of the other.     

Macroecology of local insect communities

The inter-relationships between animal body weight, range size, species richness and abundance are currently the basis of macroecology. Using 41 099 insects sampled from 31 Acacia tree canopies in north-east Tanzania, we first documented the basic macroecological patterns. The relationship between body weight and both species richness and abundance was polygonal with the highest insect species richness and abundance occurring at intermediate body weights. Across individual tree communities, the most statistically robust relationships were found between range size, abundance and species richness and they were all linear. In a second part, we focused on the positive abundance-range size relationship and we could test predictions of six of the eight proposed hypotheses to explain this widely documented pattern of community structure. The relationship is most likely explained by the metapopulation hypothesis stating that with more patches being occupied, local abundance in a given patch increases due to a higher rate of immigration from nearby patches. In addition, we found high slopes for the species-area relationship, typical of island systems and thus it seems reasonable to characterise Acacia trees in the savannah as habitat islands for insects.     

Changes in the microzooplankton assemblages in the northern Adriatic Sea during 1989 to 1992

Microzooplankton were sampled at five fixed stations in theopen waters of the northern Adriatic during four cruises in1989, nine in 1990, six in 1991 and 12 in 1992. Changes in thequalitative-quantitative composition, together with the spatial-temporaldistribution of the microzooplankton assemblages in the northernwaters of the open Adriatic, were the result of atypical oceanographicand productivity conditions in the entire region of this sea.Owing to low values of ciliated protozoa, which comprised only26% (1989) and 36% (1991) of the total number of microzooplankton,primary production cannot be controlled through grazing. Thismay possibly explain the occurrence of marine snow. The presenceof the larger mucous aggregates in the later phase causes asignificant decrease in the density of the naupliar copepodpopulation. Therefore, the usual dominance of copepods in summerdoes not occur and their development cycle is displaced to lateautumn. Consequently, only Olihona nana, a species of widerecological tolerance, comprised up to 73% of the total numberof postnaupliar copepods during the presence of mucous aggregatesin 1989. Thus, this small copepod plays an important role inthe processes of remineralization in the northern Adriatic.     

Macroecology and the hierarchical expansion of evolutionary theory

The constraint envelope describing the relationship between geographical range size and body size has usually been explained by a minimum viable population size model, furnishing a strong argument for species selection if geographical range size turns out to be ‘heritable’. Recent papers have questioned this assumption of nonzero geographical range heritability at a phylogenetic level, meaning that the logic that constraint envelopes provide support for higher‐level selection fails. However, I believe that analysis of constraint envelopes can still furnish insights for the hierarchical expansion of evolutionary theory because the fitness furnished by variation in body size, which is frequently measured as a highly ‘heritable’ trait at the species level, can be partitioned into anagenetic and cladogenetic components. The constraint envelope furnishes an explicit mechanism for large‐body biased extinction rates influencing the distribution of body size. More importantly, it is possible to envisage a scenario in which anagenetic trends driving an increase in body size in higher latitudes within species (Bergmann's rule) are counteracted by available habitat area or continental edges constraining overall species distribution in these higher latitudes, increasing the probability of extinction. Under this combined model, faunas at higher latitudes and under habitat constraints may reach equilibrium points between these opposing hierarchical adaptive forces at smaller body size than faunas with less intense higher‐level constraints and will tend to be more right‐skewed.