Direct Competition For Space Research Articles

Edge effects shape the spatial distribution of lianas and epiphytic ferns in Australian tropical rain forest fragments

AbstractQuestionIn fragmented forests, edge effects lead to changes in the distribution of plant species. In particular, tropical forest edges are increasingly dominated by lianas. Will this increase in lianas lead to changes in their interactions with other plant morphological groups? If so, will this alter the local distributions and abundance of other species?LocationPlots located at increasing distances from the nearest forest edge and in remnant fragments of rain forest in the Atherton Tablelands, far northeast Queensland, Australia.MethodsWe mapped the distribution of trees, lianas and epiphytic ferns to better understand the role of forest disturbance in shaping their competitive and facilitative interactions. We then used specific spatial point‐process analyses to examine the effects of the spatial distribution of trees on the presence and abundance of lianas and epiphytic ferns.ResultsTree aggregation near forest edges was lower than that in the interior. The higher abundance of lianas near edges was associated with increased spatial segregation between lianas and epiphytic ferns. This segregation suggests there is competition between these two functional groups, and that lianas, being much more abundant, probably outcompete epiphytic ferns.ConclusionsThe ability of lianas to thrive in disturbed tropical rain forests appears to reduce the abundance of epiphytic ferns, probably via direct competition for space. Epiphytic ferns provide unique microclimates and harbour much biodiversity, and their decline could negatively affect many animals and plants that rely upon them.

The effects of chemical interactions and culture history on the colonization of structured habitats by competing bacterial populations

BackgroundBacterial habitats, such as soil and the gut, are structured at the micrometer scale. Important aspects of microbial life in such spatial ecosystems are migration and colonization. Here we explore the colonization of a structured ecosystem by two neutrally labeled strains of Escherichia coli. Using time-lapse microscopy we studied the colonization of one-dimensional arrays of habitat patches linked by connectors, which were invaded by the two E. coli strains from opposite sides.ResultsThe two strains colonize a habitat from opposite sides by a series of traveling waves followed by an expansion front. When population waves collide, they branch into a continuing traveling wave, a reflected wave and a stationary population. When the two strains invade the landscape from opposite sides, they remain segregated in space and often one population will displace the other from most of the habitat. However, when the strains are co-cultured before entering the habitats, they colonize the habitat together and do not separate spatially. Using physically separated, but diffusionally coupled, habitats we show that colonization waves and expansion fronts interact trough diffusible molecules, and not by direct competition for space. Furthermore, we found that colonization outcome is influenced by a culture’s history, as the culture with the longest doubling time in bulk conditions tends to take over the largest fraction of the habitat. Finally, we observed that population distributions in parallel habitats located on the same device and inoculated with cells from the same overnight culture are significantly more similar to each other than to patterns in identical habitats located on different devices inoculated with cells from different overnight cultures, even tough all cultures were started from the same −80°C frozen stock.ConclusionsWe found that the colonization of spatially structure habitats by two interacting populations can lead to the formation of complex, but reproducible, spatiotemporal patterns. Furthermore, we showed that chemical interactions between two populations cause them to remain spatially segregated while they compete for habitat space. Finally, we observed that growth properties in bulk conditions correlate with the outcome of habitat colonization. Together, our data show the crucial roles of chemical interactions between populations and a culture’s history in determining the outcome of habitat colonization.

Infection of HIV-specific CD4 T helper cells and the clonal composition of the response

A hallmark of human immunodeficiency virus is its ability to infect CD4+ T helper cells, thus impairing helper cell responses and consequently effector responses whose maintenance depends on help (such as killer T cells and B cells). In particular, the virus has been shown to infect HIV-specific helper cells preferentially. Using mathematical models, we investigate the consequence of this assumption for the basic dynamics between HIV and its target cells, assuming the existence of two independently regulated helper cell clones, directed against different epitopes of the virus. In contrast to previous studies, we examine a relatively simple scenario, only concentrating on the interactions between the virus and its target cells, not taking into account any helper-dependent effector responses. Further, there is no direct competition for space or antigenic stimulation in the model. Yet, a set of interesting outcomes is observed that provide further insights into factors that shape helper cell responses. Despite the absence of competition, a stronger helper cell clone can still exclude a weaker one because the two clones are infected by the same pathogen, an ecological concept called “apparent competition”. Moreover, we also observe “facilitation”: if one of the helper cell clones is too weak to become established in isolation, the presence of a stronger clone can provide enhanced antigenic stimulation, thus allowing the weaker clone to persist. The dependencies of these outcomes on parameters is explored. Factors that reduce viral infectivity and increase the death rate of infected cells promote coexistence, which is in agreement with the observation that stronger immunity correlates with broader helper cell responses. The basic model is extended to explicitly take into account helper-dependent CTL responses and direct competition. This study sheds further light onto the factors that can influence the clonal composition of HIV-specific helper cell responses, which has implications for the overall pattern of disease progression.

Intra- and inter-specific competition for food in infaunal coral reef gastropods

Deposit-feeding gastropods have mostly been studied in the temperate northern hemisphere where these animals are typically found in fine muddy sediments, presumably as a response to the amount of food available. At One Tree Reef, in the southern Great Barrier Reef, deposit-feeding cerithiid gastropods, Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguière, 1792) and R. vertagus (Linnaeus, 1758), are most abundant in an atypical environment, the coarse sandy sediments of a back-reef sandflat. Densities in the finer sediments of the lagoon are several orders of magnitude smaller than on the sandflat. In this study, these three species were enclosed at a range of naturally occurring densities in fine and coarse sediments to investigate their competitive interactions and the influence of sediment on their growth. The type of substratum in experimental enclosures influenced the growth of each species. Growth was greatest in the fine sediments from the lagoon, despite the fact that recruitment of each species had been shown to be greatest into the coarse sediments of the sandflat. Adult R. aspera significantly reduced the growth of juveniles of the three species, but adults of R. fasciata and R. vertagus had no significant effect on the growth of juveniles of R. aspera. These results are probably a function of the large disparity in the densities of the three species: R. aspera occur in densities an order of magnitude greater than the other two species and naturally occurring densities were used in the experiments. Competition for food or other expendable resources was distinguished from direct interference competition for space by including in our experiments treatments with dead shells occupying the same volume as live adults, but not utilizing expendable resources. Direct competition for space between adults and juveniles of R. aspera is unlikely to be important as a factor affecting the growth of juvenile R. aspera. Dead shells had no significant effect on the growth of juveniles in coarse or fine sediment. Direct competition for space may be more important for R. fasciata and R. vertagus because, in some cases, the presence of dead shells reduced the growth of juveniles. These experiments demonstrate how complex competitive interactions among Rhinoclavis spp. are modified by the type of sediment and vary with the particular combination of competitors. Research in rocky intertidal systems has similarly shown how the physical environment may modify the nature of biological interactions such as competition and predation. Our results are consistent with findings from temperate systems in that growth of Rhinoclavis spp. was limited by the availability of food or a related resource such as the availability of suitably sized particles of sediment for ingestion. This system differs, however, because deposit-feeding snails occur in their greatest densities in a habitat where they have the slowest rates of growth. Further work is needed to explain these patterns of distribution and abundance and why they differ from those typical of studies in temperate systems in the northern hemisphere.

Adsorption of tetrahexylammonium – bromothymol blue ion-pairs at the chloroform–water interface

Porous membrane phase separators are used to study the adsorption of the cation tetrahexylammonium (Q+), of the anion bromothymol blue (HB−) and of the ion-pair formed between them (QHB) at the liquid–liquid interface in a rapidly stirred mixture of chloroform and aqueous buffer. Adsorption isotherms in all three cases follow the Langmuir equation. The anion HB− is much more strongly adsorbed than the ion-pair QHB. The porous membrane technique readily permits measurement of simultaneous adsorption of the two species HB− and QHB, and thereby allows a study of their competitive adsorption. When QHB is adsorbed in the presence of an excess of HB− both the saturated (monolayer) interfacial concentration of QHB and the logarithm of the adsorption equilibrium constant for QHB decrease linearly with an increase in interfacial concentration of HB−. This shows quantitatively that coadsorption of QHB and HB− involves a direct competition for space at the interface and also that the presence of adsorbed HB− changes the adsorbent properties of the interface. Analytical implications for solvent extraction are discussed. Key words: interfacial adsorption, ion-pairs, liquid–liquid interface.