Reconciling niche and neutrality: the continuum hypothesis

In this study, we ask if instead of being fundamentally opposed, niche and neutral theories could simply be located at the extremes of a continuum. First, we present a model of recruitment probabilities that combines both niche and neutral processes. From this model, we predict and test whether the relative importance of niche vs. neutral processes in controlling community dynamics will vary depending on community species richness, niche overlap and dispersal capabilities of species (both local and long distance). Results demonstrate that niche and neutrality form ends of a continuum from competitive to stochastic exclusion. In the absence of immigration, competitive exclusion tends to create a regular spacing of niches. However, immigration prevents the establishment of a limiting similarity. The equilibrium community consists of a set of complementary and redundant species, with their abundance determined, respectively, by the distribution of environmental conditions and the amount of immigration.     

Architecture,seasonal growth and interference in three grass species with different flowering phenologies in a tropical savanna

In a field experiment we studied the relationships between architecture, seasonal aerial biomass growth and interference from neighbours in three savanna grass species differing in time of flowering: a precocious species (Elyonurus adustus), an early species (Leptocoryphium lanatum) and a late species (Andropogon semiberbis). To detect the neighbour interference upon architecture and seasonal regrowth, we measured the leaf interception of a plane at different heights, when the species grew alone, in pairs and in groups of three. Although the three species differed widely in the spatial and temporal patterns of occupation of above-ground space, important levels of interference among neighbouring plants were detected. The species differ in: a) the pattern of space occupation when growing alone; b) the magnitude of the interference effect by the companion species; c) the change in pattern as a consequence of interference; d) their reaction to fire.The interference is not symmetric and it is not related to phenological similarities based on flowering season.L. lanatum was both the most impaired and the least impairing of the three species and the opposite is valid forA. semiberbis. The basal species (E. adustus andL. lanatum) showed a higher potential to produce leaf surface during the growth season than the tall species (A. semiberbis), but the stronger interference from the latter tended to equate their growth when the species grew in mixtures.In all three species interference led to a reduction of the growing period. Based on the analysis of growth at the various heights and the architectural peculiarities of the species we concluded that neighbour interference is probably reducing both tillering and leafing in the basal species but only culm elongation and leafing in the erect species.The species also differed in their reaction to fire.A. semiberbis andE. adustus showed a pulse of regrowth after the passing of fire which is missing inL. lanatum. In all cases the neighbour interference affected both the intensity of growth and the length of the growing period, but it did not affect either the height of the plants nor the reproductive phenology of these three grass species.     

The fitness cost of generalization: present limitations and future possible solutions

One of the longest and liveliest debates in the evolutionary and ecological literature has centred on the existence and magnitude of constraints that can also be described by a proverb 'jack-of-all-trades is a master of none'. Often assumed, rarely tested, this proverb/assumption states that evolution of generalization necessarily entails a cost. The cost is expressed in terms of fitness loss elsewhere along an environmental gradient that leads to a genetic fitness trade-off between a generalist and a specialist. Although there is a well-developed body of knowledge that documents the cost of adaptation in general, the genetic fitness cost of generalization remains unclear. An empirical test of such cost is not a trivial task because it requires knowledge of a genotype's fundamental ecological niche breadth to document the process of generalization. The estimation of genetic fitness correlation between environments, a commonly used method in the literature, has a limited explanatory power regarding the cost of generalization, and new approaches are needed to further clarify the existence as well as the nature/pattern of constraints in evolution of generalization and specialization. A new approach is proposed to examine experimentally the genetic fitness cost of generalization, which is based on statistical analysis of tolerance curve properties. The approach can be used to study natural populations of both unicellular and multicellular organisms.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 583–590.     

Group Composition, Home Range Size, and Diet of Three Sympatric Bamboo Lemur Species (Genus Hapalemur) in Ranomafana National Park, Madagascar

This is the first long-term, simultaneous, comparative study of three bamboo lemur species (Hapalemur griseus, H. aureus, and H. simus) at a site in southeastern-central Madagascar where they occur in sympatry. At Talatakely, Ranomafana National Park, the three Hapalemur spp. share overlapping home ranges. Hapalemur griseus has flexible group sizes, varying from three to nine individuals (n = 6). The home range of Hapalemur griseus averages 15 ha (n = 2). Hapalemur aureus forms family groups of 4 individuals (n = 3); they have a home range on average of 26 ha (n = 2). The single group of Hapalemur simus is composed of one or three adult males, two adult females, and their offspring; they occupy a home range of 62 ha. The three species of Hapalemur are year-round bamboo specialists: >88% of their diets consist of bamboo and grass in the Family Poaceae. Contrary to earlier findings, all three Hapalemur spp. consume the cynogenic parts—young leaf bases, young pseudopetioles, and young shoots—of the giant bamboo, Cathariostachys madagascariensis. They rely heavily on this plant, which comprises 72–95% of their diets. Hapalemur griseus and H. aureus consume similar proportions of bamboo vs. nonbamboo plants, though they differ in the species of bamboo they prefer. Hapalemur simus has the most distinct diet of the three bamboo lemurs. They exploit the young shoots of Cathariostachys madagascariensis during the austral summer rainy season, between November and April. From June to November, Hapalemur simus shifts its diet to eating the mature culm pith of Cathariostachys madagascariensis; the proportion of pith represented in their diet reaches a maximum of 89% in October. Seasonal availability of food resources, feeding competition, and factors related to body size may provide clues to the understanding of diet selection among the three sympatric Hapalemur spp.     

Linking climate change and species invasion: an illustration using insect herbivores

Climate change and invasive species are two of the most important ecological issues facing the world today. Yet, to date these two factors have largely been viewed independently. In order to prevent large-scale economic and environmental damage and as a first step towards predicting and preventing invasions, it is important to understand the factors affecting invasions. Here, we focus on insect herbivores and link the climate change and invasive research fields. We illustrate using existing published research that life history traits can be useful indicators of future invasive potential. However, climate change will also affect propagule pressure and the communities into which invaders will arrive. With the aid of a meta-analysis we show that climate-induced community changes are likely to increase niche-availability in the future, further exacerbating the problem of invasive species. It is timely and important that further research linking these two important ecological threats is undertaken.     

Functional diversity of crustacean zooplankton communities: towards a trait-based classification

1. While studies of phytoplankton and terrestrial plant communities have increasingly emphasised the use of functional traits in ecological research, few have yet to apply this approach to zooplankton communities.
2. This study reviews laboratory and observational studies on zooplankton feeding and life history and provides a series of functional trait tables for the North American freshwater zooplankton. Qualitative and quantitative trait tables highlight areas where data were more scarce and point to which types of studies could fill in gaps in our knowledge of zooplankton niches.
3. Data were most complete for the Cladocera across most traits, while feeding information for cyclopoids was most sparse. Qualitative data that distinguished congeneric species were lacking for most groups.
4. A regional community dendrogram for common north-eastern North American zooplankton species was generated and shows that taxonomic differences between species do not capture fully functional differences based on the traits of body length, habitat, trophic group and feeding type.
5. The data collected here, combined with readily measurable species attributes, can be used to generate a multivariate measure of the functional niche of each species found in a community. Armed with this information, functional relationships that are useful for ecological studies of lake ecosystems can be more easily conducted.     

Assembly rules of ground-foraging ant assemblages are contingent on disturbance, habitat and spatial scale

Aim A major endeavour of community ecology is documenting non‐random patterns in the composition and body size of coexisting species, and inferring the processes, or assembly rules, that may have given rise to the observed patterns. Such assembly rules include species sorting resulting from interspecific competition, aggregation at patchily distributed resources, and co‐evolutionary dynamics. However, for any given taxon, relatively little is known about how these patterns and processes change through time and vary with habitat type, disturbance history, and spatial scale. Here, we tested for non‐random patterns of species co‐occurrence and body size in assemblages of ground‐foraging ants and asked whether those patterns varied with habitat type, disturbance history, and spatial scale. Location Burned and unburned forests and fens in the Siskiyou Mountains of southern Oregon and northern California, USA. Methods We describe ground‐foraging ant assemblages sampled over two years in two discrete habitat types, namely Darlingtonia fens and upland forests. Half of these sites had been subject to a large‐scale, discrete disturbance – a major fire – in the year prior to our first sample. We used null model analyses to compare observed species co‐occurrence patterns and body‐size distributions in these assemblages with randomly generated assemblages unstructured by competition both within (i.e. at a local spatial scale) and among (i.e. at a regional scale) sites. Results At local spatial scales, species co‐occurrence patterns and body‐size ratios did not differ from randomness. At regional scales, co‐occurrence patterns were random or aggregated, and there was evidence for constant body‐size ratios of forest ants. Although these patterns varied between habitats and years, they did not differ between burned and unburned sites. Main conclusions Our results suggest that the operation of assembly rules depends on spatial scale and habitat type, but that it was not affected by disturbance history from fire.     

Correlates of range size and dispersal ability: a comparative analysis of sphingid moths from the Indo-Australian tropics

Aim  We tested various species-level traits for their potential to explain species' range sizes and dispersal abilities.
Location  Southeast Asia and Malay Archipelago.
Methods  We used published maps of geographical distribution estimates for sphingid moths to calculate range areas and classify species according to their dispersion across (present or historical) water straits in the Malay Archipelago. We tested forewing length (FWL), wing load (thorax width/FWL), presence or absence of a functional proboscis (i.e. adult feeding), larval diet breadth and larval diet composition for univariate correlations with range size and inter-island dispersion. We used multivariate, phylogenetically controlled models to test for independent effects of parameters.
Results  Range size correlated strongly with larval diet breadth, a result that was also confirmed in the multivariate model. Adult feeding had a significant impact on range sizes only within the multivariate model, but not in the univariate correlation. Dispersal class also correlated with larval diet breadth, but was additionally influenced by forewing length, wing load and larval diet composition. A univariate effect of adult feeding became non-significant in the multivariate, phylogenetically controlled model.
Main conclusions  Larval diet breadth is the best predictor of range size as well as inter-island dispersion, confirming the importance of niche breadth on the geographic ranges of species. A number of other factors are shown to have additional impact on predictions of range size or inter-island dispersal ability. Our analyses cannot determine the causal mechanisms of these correlations, but may stimulate further research on the adaptive significance of traits affecting range size and dispersal in this system.     

Ultrastructure of the putative stem cell niche in rat mammary epithelium

There is now strong evidence that the stem cells of many tissues reside in specialized structures termed niches. The stem cell niche functions to house and regulate symmetric and asymmetric mitosis of stem cells in mammalian skin, mouse and human bone marrow, mouse brain, gut, and hair follicle, and Drosophila ovary and testis. This regulation is effected through the action of various signaling pathways such as Notch, Hedgehog, Wnt and others. The hormones of the estrous cycle, pregnancy and lactation that initiate growth in mouse mammary epithelium appear to act at a paracrine level to regulate mitosis through Notch receptors. Previous work has established that the putative stem cells of the mammary epithelium in several animal species reside near the basement membrane and never make contact with the ductal lumen. We show that these putative stem cells are found in anatomically specialized places created by the cytoplasmic extensions and modifications of neighboring differentiated cells. Such specializations may help to regulate stem cell activity by modulating molecular traffic to putative stem cells and contact with signaling molecules in the basement membrane. The histological characteristics of these putative niches vary as to the kinds of relationships the cells can have with the basement membrane and neighboring cells and as to how many stem or progenitor cells they may contain. This suggests a plasticity that may be relevant to the response of niches to tissue demands, such as wound healing, the periodic growth and regression of mammary epithelium, the process of mammary tumorigenesis therapeutic strategies for breast cancer.     

Molecular phylogeny of congeneric monogenean parasites (Dactylogyrus): a case of intrahost speciation

Dactylogyrus species (Dactylogyridae: Monogenea) are a group of monogenean gill parasites that are highly specific to freshwater fish of the family Cyprinidae. Dactylogyrus species were sampled from 19 cyprinids and one percid collected in Europe. Using partial 18S rDNA and ITS1 sequences, a phylogeny of 51 Dactylogyrus species was reconstructed to investigate the patterns of parasite speciation and diversification. Three main Dactylogyrus lineages were recognized from all phylogenetic trees, that is, analysis of 18S rDNA alone and combined 18SrDNA and ITS1. The first lineage associates the Dactylogyrus species of Cyprinus carpio and Carassius auratus of the Cyprininae; the second associates Dactylogyrus species of the Gobioninae, Pseudorasbora parva of the Rasborinae, and Ctenopharyngodon idella of the Cyprininae; and the third associates Dactylogyrus species of the Leuciscinae and Alburninae and Barbus barbus of the Cyprininae. Our results suggest that the genus Dactylogyrus is of quite recent origin and that these three lineages separated from each other in a very short period of time. Host subfamily mapping onto the parasite tree inferred from analysis of the combined dataset showed that the Cyprininae could be plesiomorphic hosts for Dactylogyrus. Dactylogyrus parasites would have secondarily colonized the Percidae and representatives of the Leuciscinae, Alburninae, Gobioninae, and Rasborinae. Comparison of host and parasite phylogenetic relationships indicated that a very high number of parasite duplications occurred within two of the three Dactylogyrus lineages. Dactylogyrus diversification can be mainly explained by sympatric intrahost speciation events that seem to be correlated to strict host specificity. Moreover, the present study shows that the congeneric parasites speciating within one host tend to occupy niches within hosts differing at least in one niche parameter.