Niche Structure Research Articles

When and why do non-neutral metacommunities appear neutral?

Hubbell's neutral theory assumes that all species in a community have the same per capita fitness. Despite the overwhelming evidence against this assumption in most communities the neutral theory has often been, though not always, successful at predicting patterns of diversity in nature. I analyze a non-neutral model in order to suggest conditions under which observed species-abundance distributions (SADs) could be expected to resemble neutral distributions. The non-neutral model consists of two guilds of species such that (1) individuals between guilds do not interact, (2) dynamics within guilds follow Hubbell's model and (3) neutral parameters between guilds differ. This two-guild model generates SADs that appear neutral in some cases and clearly non-neutral in other cases. This result suggests that SADs may be more informative about niche structure than previously thought. The two-guild model could be tested in communities composed of fairly well-defined guilds or functional groups.

In Vivo Depletion of Osteoblasts Reveals Roles in HSC Homing and B-Lymphopoiesis.

The identification of bone marrow (BM) niches controlling many aspects of hematopoiesis is of great interest. It is believed that these niches provide the necessary signals to maintain primitive hematopoietic cells including hematopoietic stem cells (HSC). Downstream of HSC are programs for both myelopoiesis and lymphopoiesis, although little is known about the niches that support these processes. Accumulating data implicating osteoblasts (OB) as a component of the HSC niche suggest that these cells are able to regulate HSC properties, and that the HSC are located in direct contact with OB at the bone endosteum. However, the interpretation of these data is controversial, and others have suggested that endothelial cells dictate the structure and function of the HSC niche. In order to determine whether OB are a critical component of hematopoietic niches, we employed Col2.3ΔTK mice, a transgenic mouse strain bearing a fusion gene composed of the rat type I collagen promoter and HSV-thymidine kinase. This construct allows for selective depletion of OB following in vivo gancyclovir (GCV) treatment. Mice received 100 mg/kg GCV for a period of 8, 21, or 28 days. Following 8 days of GCV, OB were depleted from the BM as determined by a lack of staining for osteocalcin. Concommitant with the loss of OB, there was a decrease in total BM cellularity of approximately 50% that included reductions in both myeloid and B lymphoid cells. Among the B cell populations that were measured, pre-pro-B and pro-B cells were reduced to the greatest extent, 64% and 81%, respectively. Despite this marked reduction in B lymphopoiesis, number of Lin− Sca-1+ cKit+ cells (LSK), a population enriched for HSCs, was unchanged immediately following OB depletion. This loss of B cell precursors, occurring before any observable effects on HSC numbers, suggests that OB may directly support the proliferation and maturation of early B cell precursors. Although minimal changes in BM LSK were observed immediately after GCV, BM LSK were reduced after 21 and 28 days of GCV treatment, when their numbers decreased 40% and 88%, respectively. During this same period, however, spleen LSK increased by 165% and 300%, respectively. Assuming that femurs and tibiae constitute 25% of total murine BM, the total number of LSK lost from the BM after 21 and 28 days of GCV is approximately 1.42 and 3.12×10^5 cells, respectively. These values are remarkably consistent with the increase in splenic LSK at the same time points, 1.2 and 3.7×10^5 cells, respectively. These data indicate that loss of OB plays a pivotal role in the partitioning of LSK into and out of the BM. Given the finding that the shift in LSK is most pronounced 2–3 weeks after the loss of OB, these data raise the intriguing possibility that the partitioning of LSK from the BM to the spleen following OB deletion is not the result of increased emigration from the BM but, rather, due to a decreased ability of HSC to home back to the BM following daily limited physiological mobilization. For example, it is plausible that OB supply chemotactic signals that control the ability of HSC to home to the BM. In conclusion, in vivo depletion of OBs causes an immediate reduction in the number of B cell precursors with a delayed shift in LSK from the BM to the periphery, thus demonstrating that OB play a major role in BM HSC trafficking and B lymphopoiesis.

THE EVOLUTION OF GENETIC ARCHITECTURE UNDER FREQUENCY-DEPENDENT DISRUPTIVE SELECTION

Abstract We propose a model to analyze a quantitative trait under frequency-dependent disruptive selection. Selection on the trait is a combination of stabilizing selection and intraspecific competition, where competition is maximal between individuals with equal phenotypes. In addition, there is a density-dependent component induced by population regulation. The trait is determined additively by a number of biallelic loci, which can have different effects on the trait value. In contrast to most previous models, we assume that the allelic effects at the loci can evolve due to epistatic interactions with the genetic background. Using a modifier approach, we derive analytical results under the assumption of weak selection and constant population size, and we investigate the full model by numerical simulations. We find that frequency-dependent disruptive selection favors the evolution of a highly asymmetric genetic architecture, where most of the genetic variation is concentrated on a small number of loci. We show that the evolution of genetic architecture can be understood in terms of the ecological niches created by competition. The phenotypic distribution of a population with an adapted genetic architecture closely matches this niche structure. Thus, evolution of the genetic architecture seems to be a plausible way for populations to adapt to regimes of frequency-dependent disruptive selection. As such, it should be seen as a potential evolutionary pathway to discrete polymorphisms and as a potential alternative to other evolutionary responses, such as the evolution of sexual dimorphism or assortative mating.

THE EVOLUTION OF GENETIC ARCHITECTURE UNDER FREQUENCY-DEPENDENT DISRUPTIVE SELECTION

We propose a model to analyze a quantitative trait under frequency-dependent disruptive selection. Selection on the trait is a combination of stabilizing selection and intraspecific competition, where competition is maximal between individuals with equal phenotypes. In addition, there is a density-dependent component induced by population regulation. The trait is determined additively by a number of biallelic loci, which can have different effects on the trait value. In contrast to most previous models, we assume that the allelic effects at the loci can evolve due to epistatic interactions with the genetic background. Using a modifier approach, we derive analytical results under the assumption of weak selection and constant population size, and we investigate the full model by numerical simulations. We find that frequency-dependent disruptive selection favors the evolution of a highly asymmetric genetic architecture, where most of the genetic variation is concentrated on a small number of loci. We show that the evolution of genetic architecture can be understood in terms of the ecological niches created by competition. The phenotypic distribution of a population with an adapted genetic architecture closely matches this niche structure. Thus, evolution of the genetic architecture seems to be a plausible way for populations to adapt to regimes of frequency-dependent disruptive selection. As such, it should be seen as a potential evolutionary pathway to discrete polymorphisms and as a potential alternative to other evolutionary responses, such as the evolution of sexual dimorphism or assortative mating.

STEM CELL NICHE: Structure and Function

Adult tissue-specific stem cells have the capacity to self-renew and generate functional differentiated cells that replenish lost cells throughout an organism's lifetime. Studies on stem cells from diverse systems have shown that stem cell function is controlled by extracellular cues from the niche and by intrinsic genetic programs within the stem cell. Here, we review the remarkable progress recently made in research regarding the stem cell niche. We compare the differences and commonalities of different stem cell niches in Drosophila ovary/testis and Caenorhabditis elegans distal tip, as well as in mammalian bone marrow, skin/hair follicle, intestine, brain, and testis. On the basis of this comparison, we summarize the common features, structure, and functions of the stem cell niche and highlight important niche signals that are conserved from Drosophila to mammals. We hope this comparative summary defines the basic elements of the stem cell niche, providing guiding principles for identification of the niche in other systems and pointing to areas for future studies.

Absence of phylogenetic signal in the niche structure of meadow plant communities.

A significant proportion of the global diversity of flowering plants has evolved in recent geological time, probably through adaptive radiation into new niches. However, rapid evolution is at odds with recent research which has suggested that plant ecological traits, including the beta- (or habitat) niche, evolve only slowly. We have quantified traits that determine within-habitat alpha diversity (alpha niches) in two communities in which species segregate on hydrological gradients. Molecular phylogenetic analysis of these data shows practically no evidence of a correlation between the ecological and evolutionary distances separating species, indicating that hydrological alpha niches are evolutionarily labile. We propose that contrasting patterns of evolutionary conservatism for alpha- and beta-niches is a general phenomenon necessitated by the hierarchical filtering of species during community assembly. This determines that species must have similar beta niches in order to occupy the same habitat, but different alpha niches in order to coexist.

Developing an Integrative International Human Resource Model: The contribution of Chinese Multinational Enterprises

This study develops an integrative international human resource IHRM framework in Chinese MNEs mainly by examining the relationship between the intervening factors and IHRM policies and practices. It confirms that IHRM is associated with a variety of contextual and firm-specific factors, including political, legal, economic, socio-cultural factors, international strategy, organizational structure, organizational culture, stage and mode of internationalization, type and niche of industry, international experience, size of international operation, the reliance on international markets and top management perceptions of home HRM systems. These factors have differentiated impact and are changing over time. It also explores the impact of IHRM policy and practice on each other, arguing that there is interplay between IHRM policies and practices.

LONG‐TERM EFFECTS OF UNGULATES ON PERFORMANCE, ABUNDANCE, AND SPATIAL DISTRIBUTION OF TWO MONTANE HERBS

Herbivores highly reduce the performance of many plant species. However, little empirical information exists on the real effect that these organisms have on plant populations. With a long‐term (seven‐year) ungulate exclusion experiment in two areas of southeastern Spain, I demonstrated that these organisms can affect not only individual performance, but also the population dynamics and spatial structure of two short‐lived monocarpic herbs, Erysimum mediohispanicum and E. baeticum. There was between‐year and among‐microhabitat variability in damage, with plants growing under shrubs being less damaged than those growing in open sites. Ungulates consumed flowers and fruits, severely decreasing plant reproductive output. The postdispersal seed predation rate increased after ungulates were excluded, presumably as a consequence of the relaxation of competition between seed predators and ungulates. The effect of ungulates on Erysimum early establishment was nonsignificant, although >50% of Erysimum seedlings died due to ungulate trampling. The exclusion experiment also revealed that, as a consequence of their impact on seed production, ungulates affected the population dynamics of their host plants; their removal produced a significant increase in the abundance of the two studied plant species. Furthermore, habitat distribution of plants was also influenced by the activity of ungulates. In control plots, most Erysimum individuals grew under the canopy of co‐occurring shrubs. In contrast, a dramatic spatial redistribution of plants occurred in ungulate‐excluded plots, where they started to colonize open sites. Consequently, the spatial structure of the plants excluded from ungulates significantly differed from that in control plots and became statistically similar to the structure expected according to the cover of every microhabitat. Finally, this experiment revealed that, for plants inhabiting heterogeneous landscapes, abundance and spatial structure are tightly related. Thus, I found that abundance increased in ungulate‐excluded plots due not to a general and homogeneous increase, but to a significant increase in those microhabitats inaccessible to ungulates in control plots. These findings suggest that herbivory effects on plants are intricate, affecting not only their performance or population dynamics, but also their habitat distribution and niche structure.

Phylogenies and Community Ecology

▪ Abstract As better phylogenetic hypotheses become available for many groups of organisms, studies in community ecology can be informed by knowledge of the evolutionary relationships among coexisting species. We note three primary approaches to integrating phylogenetic information into studies of community organization: 1. examining the phylogenetic structure of community assemblages, 2. exploring the phylogenetic basis of community niche structure, and 3. adding a community context to studies of trait evolution and biogeography. We recognize a common pattern of phylogenetic conservatism in ecological character and highlight the challenges of using phylogenies of partial lineages. We also review phylogenetic approaches to three emergent properties of communities: species diversity, relative abundance distributions, and range sizes. Methodological advances in phylogenetic supertree construction, character reconstruction, null models for community assembly and character evolution, and metrics of community phylogenetic structure underlie the recent progress in these areas. We highlight the potential for community ecologists to benefit from phylogenetic knowledge and suggest several avenues for future research.

Adaptive Ecology, Growth Strategies and the Global Bloom Expansion of Dinoflagellates

Dinoflagellates exhibit unique differences from diatoms in their adaptive ecologies that may be favoring their increasingly successful exploitation of coastal waters and global bloom expansion. Dinoflagellates behave as annual species, bloom soloists, are ecophysiologically diverse and habitat specialists, whereas diatoms behave as perennial species, guild members and are habitat cosmopolites. Diatoms have a relatively uniform bloom strategy based on species-rich pools and exhibit limited habitat specialization. Dinoflagellates have multiple life-form strategies consistent with their diverse habitat specializations, but rely on impoverished bloom species pools. Niche structure and dinoflagellate competition for niche space are considered. The “open niche period” formulated originally for Narragansett Bay is extrapolated as a general bloom paradigm. It is suggested that successful niche occupancy leading to blooms involves adaptive strategies at three heirarchic taxonomic elements: phylogenetic, generic and species-specific, and in that sequence. Transoceanic expatriation of emigrant species leading to indigenous status and blooms requires completion of a three-stage colonization process. Anthropogenic seedings are not, in themselves, bloom stimulation events; they are only the first phase of a multiple-step process. The organismal and niche features required for a hidden flora member to become a bloom species are considered, and the interplay between niche structure, habitat carrying capacity, colonization requirements and stochasticity as factors in the changing global bloom behavior of dinoflagellates discussed. The question is posed whether traditional perspectives of phytoplankton behavior apply completely to dinoflagellates.

Phylogeny and the niche structure of meadow plant communities

Summary Because related species tend to be ecologically similar, it has long been recognized that the phylogenetic relationships between species in a community are likely to influence its structure. Previous investigations of the influence of phylogeny on community structure have generally shown that congeneric species are more frequent than would be expected if communities were randomly assembled. Because, under most theories of coexistence, stable coexistence requires ecological differences between species, the apparent excess of coexisting congeners suggests that trait differences important to coexistence must be recently evolved, arising near the tips of phylogenetic trees. We test this hypothesis in two meadow plant communities for which we have good evidence of niche segregation along axes defined by hydrological conditions. Niche overlaps were calculated and compared against null models for (i) all species in the community, (ii) species within genera, (iii) genera within families, (iv) families within the eudicots and monocots, and (v) eudicots and monocots. Against expectation, we found that community structure was the cumulative result of niche separation arising at a range of phylogenetic levels and therefore that niche differences have been accumulated through the evolutionary history of species.

Leaf litter and the small‐scale distribution of carabid beetles (Coleoptera, Carabidae) in the boreal forest

Management practices favoring conifers at the expense of deciduous tree species, and the eradication of deciduous trees, especially aspen Populus tremula, from managed forests have resulted in population declines in several species in Fennoscandia. In addition to species depending on decaying wood of deciduous trees, earlier evidence suggests that leaf litter, especially that of aspen, is favored by many carabid species. We ran a four‐year experiment in order to compare carabid assemblages of unchanged forest floor with artificially created leaf‐litter plots in central Finland. A total of 18 plots (5 m in diameter) were established in three forest stands without aspen a few kilometers apart. Each stand had 3 litter plots (litter added) and 3 control plots, Pre‐treatment samples were compared with those collected alter litter addition.The litter addition affected the carabid‐assemblage structure by increasing the catches of some species and decreasing the catch of one species. The number of carabid species was similar in control and litter plots. The litter effect was smaller than variation among forest stands and year‐lo‐year fluctuations. There was a strong temporal constancy among the plots: ‘rich’ plots remained ‘rich’ from year to year and similarly, ‘poor’ plots remained ‘poor’.The significant influence of leaf litter on carabid abundance can be attributable to both abiotic factors (microenvironmental conditions, especially humidity and temperature), and biotic ones (changes in niche structure, improved food supply). Leal litter seems to have an effect on carabid distribution patterns, and deciduous trees scattered among conifers are likely to be of importance on carabid fauna in boreal forests.

Niche structure and evolutionin creodont (Mammalia) faunas of the European and North American Eocene

For the first time, comparative information is given for paleobiological patterns of creodonts. Three European (MP11–13 of the Geiseltal, MP13?–14 of Egerkingen, MP16–17b of Quercy) and three North American (Wa-1 to 3, Wa-4 to 6, Br-1) Eocene creodont faunas represented by totally 60 species are included in this study. The ecological niche of each species is defined for this study by the combination of body mass, diet, and locomotor pattern. Classes were built for each of these parameters by using described qualitative and semi-quantitative methods developed mainly for carnivorans which makes necessary some adjustments. Due to the newly found correlation between mean length of lower molars and estimated body mass, body weights could also be estimated for species from which postcranial remains are missing. Most creodonts are terrestrial to scansorial middle-sized animals with a large amount of meat in their diet. Not a single fully-arboreal, semiaquatic or hypocarnivorous species could be identified. With the exception of the Geiseltal, in each fauna at least one ossiphagous taxon was present. In North America, cursorial hypercarnivores were lacking, while such species were present in the European faunas. Remarkable different was the ecological role of proviverrines between the continents. While they showed a large diversity especially in size in Europe, they were restricted to weights lower than 10 kg in North America. Because higher taxa like limnocyonines or oxyaenids were not present in the investigated European faunas, proviverrines used niches here that are occupied by these groups in North America, e.g. the niche of a semifossorial large hypercarnivore. Moreover, the carnivoran families Miacidae and Viverravidae are much more diverse in North America, which, by comparison, opens European proviverrines additional autecological possibilities.

Examining the Role of Paneth Cells in the Small Intestine by Lineage Ablation in Transgenic Mice

The Paneth cell lineage is one of four epithelial lineages derived from the adult mouse small intestine's multipotent stem cell. Mature Paneth cells secrete antimicrobial peptides (cryptdins), growth factors, as well as two gene products, a secreted phospholipase A2 and matrilysin, that has been implicated as modifiers of adenoma formation in mice containing a mutation in the tumor suppressor Apc. Immature Paneth cells are located just above and below the cell layer, in intestinal crypts, that has been proposed to contain the multipotent stem cell. Paneth cells differentiate during a downward migration to the crypt base. The location and direction of Paneth cell migration, their high density and long residency time at the crypt base, and the nature of their secreted gene products, suggest that they may influence the structure and/or function of the stem cell niche. Paneth cell ablation can therefore be viewed as an experimental manipulation of the cellular microenvironment that purportedly contains the stem cell and its immediate descendants. Two types of ablation experiments were performed in transgenic mice. Nucleotides -6500 to +34 of the mouse cryptdin-2 gene (CR2) were used to express an attenuated diphtheria toxin A fragment. Light and electron microscopic immunohistochemical analyses of several pedigrees of postnatal day 28 to 180 animals established that ablation of Paneth cells is accompanied by an increase in the proportion of undifferentiated crypt base columnar cells. These cells normally co-exist with Paneth cells. The ablation does not produce a detectable effect on the proliferation or terminal differentiation programs of the other three lineages or on host-microbial interactions. The last conclusion is based on the ability of crypts to remain free of microbes detectable by Gram and Warthin-Starry stains and by retention of the normal crypt-villus distribution of components of the diffuse gut-associated lymphoid tissue. CR2-directed expression of simian virus 40 large T antigen also results in a loss of mature Paneth cells but produces a marked amplification of crypt cells having a morphology intermediate between Paneth and granule goblet cells. EM immunohistochemical analyses suggest that intermediate cells can differentiate to mature goblet cells but not to Paneth cells, as they migrate up the crypt-villus axis. Our findings suggest that (i) stemness in the crypt is not defined by instructive interactions involving the Paneth cell; (ii) expressing a Paneth cell fate may require that precursors migrate to the crypt base; (iii) antimicrobial factors produced by Paneth cells are not required to prevent colonization of small intestinal crypts; and (iv) this lineage does not function to maintain the asymmetric crypt-villus distribution of components of the diffuse gut-associated lymphoid tissue.

Effects of habitat fragmentation on foraging behaviour of tits and related species: does niche space vary in relation to size and degree of isolation of forest fragments?

We studied the winter foraging niches of tits and related species in deciduous forest fragments varying in size between 1 and 30 ha (plus one forest of 200 ha) in order to investigate the influence of forest fragmentation on foraging niches Very few correlations between niche structure (foraging niche, width and overlap) and forest size or isolation turned out to be significant This implies that either the species that disappear in small fragments are those that suffer most from competition (making the effect unmeasurable), or that competition is relatively unimportant for niche structure In any case we find no evidence that foraging niches are strongly affected by the changes (in habitat and/or community structure) associated with fragmentation

The Croonian lecture, 1995 Natural communities: structure and dynamics

A review is undertaken of various concepts in community ecology that relate to observed patterns in nature; these are species richness, diversity, guild structure, niche structure, constancy of structure, succession and the bionomic profile of communities. This understanding may allow us to better predict the effects of changes we are making in our environment.

Variation in species richness on small grassland quadrats: niche structure or small‐scale plant mobility?

Abstract. In this study we make clear that the significance of deviations from an expected variance in species richness as demonstrated in alvar grassland, is a function of spatial pattern at the scale richness was measured, i.e. 10 cm2. If corrected for spatial dependence, more than half of the significant cases become nonsignificant. As regards cases of significant variance deficit, which has been interpreted as an indication of niche limitation, we suggest that there may be a simple physical limitation to the number of species on the scale of observation in the form of a low number of plant units which can find a place because of the modal plant size of the species involved. In most cases of significant variance deficit the modal size of the plant units involved was probably bigger than average.Insofar as a significant variance deficit can be demonstrated, the resulting species richness distribution curve should be analyzed and we propose a skewness test, enabling us to differentiate between significantly left‐skewed curves (‘niche limitation’), significantly right‐skewed curves (‘niche facilitation’) and symmetrical curves. We present results obtained with the G‐test, a log‐likelihood ratio goodness of fit test. Only few cases of significantly left‐skewed curves and a majority of symmetrical curves were found.Attempts to demonstrate guild proportionality in grasslands suffer from the heterogeneity of usually distinguished guilds, such as annuals vs. perennials or graminoids vs. dicotyledons.We observe that niche limitation may occur in alvar grassland, but indications for niche facilitation are stronger. Finally, we conclude that deviations in species richness variance are interesting indications of community structure, but only of spatial structure. Niche structure resulting from assembly rules should be investigated through experiments.

Attraction Toward Feeding Conspecifics when Food Patches are Exhaustible

Animals that forage in groups commonly share patches of food discovered by one or a few individuals.

Developmental Niche Structure in the Gregarine Assemblage Parasitizing Tenebrio molitor

The exogenous developmental niche spaces of Gregarina cuneata and Gregarina polymorpha, parasites restricted to larval Tenebrio molitor, and Gregarina niphandrodes, a parasite restricted to adult T. molitor, were determined along temperature and humidity axes and associated with parasite niche space on the host resource axis. Gregarine gametocysts were incubated at controlled humidity (0, 10, and 20 mm Hg vapor pressure deficit) at 20, 25, 30, and 35 C to determine the environmental limits to survival and sporulation

An approach to modelling the dynamics of evolutionary self-organization

We have simulated the dynamics of species evolution in a systems context on a parallel supercomputer. Population dynamics are represented as generalized Lotka-Volterra systems defined as points in a generalized phenotype or character space T . Populations which are closest in T compete most strongly for resources. A variety of systems with varying assumptions, resource distributions, and number of trophic levels were simulated. Starting with a random initial seed proceeding through a complex temporal sequence, most cases converged to essentially the same configuration. The final equilibrium state consisted of a gridwork of localized population clusters in T , representing individual species. The intercluster spacing was roughly equal to the standard deviation of the resource utilization function. Thus the systems self-organize to an array of niches which maximally fills the available volume of resource space while minimizing the overlap of resource utilization functions. The simulations were performed on a Connection Machine (a massively parallel supercomputer) which allowed up to 32 000 distinct points in character space to be modelled in parallel. Simulation allows a more realistic treatment of evolutionary dynamics and greater flexibility in experimental manipulation than previous analytical approaches. We experimented with temporal variations in the resource base. In most cases the niche structure was not affected; species prospered or declined as a function of local resource availability but the niche pattern remained invariant. However, in the case in which each species depends on only one or two resources, increasing randomness in the resource base resulted in a decrease in the number of species.