Density-dependent Mechanisms Research Articles

Everybody needs good neighbours: movement, habitat use, and conspecific association in a reintroduced mesopredator

As a reintroduction progresses through the establishment, growth, and regulation phases, density-dependent mechanisms increasingly drive population dynamics. This can complicate efforts to reinforce these populations if the translocated individuals (henceforth reinforcers) are excluded, or existing residents become displaced. This is especially pertinent for islands and fenced havens, where immigration is often only possible by translocation. Conspecific associations offer insights into how these density-dependent mechanisms manifest in space and time. We investigated how movement, habitat use, and conspecific associations differed between eastern quoll (Dasyurus viverrinus) residents and reinforcers at a conservation-fenced haven. We used GPS collars to quantify distances travelled per night, home and core ranges, nocturnal (activity) and diurnal (denning) habitat use and preference, and conspecific associations over three periods: baseline (residents only, days 3–21), release (both cohorts, days 22–32), and settlement (reinforcers only, days 33–52) for ≤ 42 days post-release (depending on GPS unit longevity). Eastern quolls travelled greater mean distances per night (< 2.15 km, x̄ 1.75 km) and had larger home ranges (< 251 ha, x̄ 178 ha) during the release period. Reinforcers had larger home ranges (249 ha) and greater overlap with other collared eastern quolls (115 ha) when compared to residents (range 90 ha, overlap 46 ha). We found a significant preference for grassland habitat across all animals and periods. During the settlement period, we found a preference for nocturnal activity in greater understory and south-west facing aspects, and lower movement correlation, sociality, and den sharing between collared eastern quolls. Finally, we found lower den sharing in reinforcers (29%) compared to residents (52%), and for fawn- animals compared to dark-morphs. Our results revealed short-term movements, habitat use, and conspecific associations at a greater spatiotemporal resolution than has ever been achieved for this species. Our findings offer important insights into the importance of appropriate habitat and conspecific cueing for reintroductions, and highlight the need to monitor density-dependent mechanisms to inform adaptive management to promote positive outcomes for both initial reintroductions and reinforcements.

Atomically dispersed cobalt activator with nitrogen and sulfur co-coordination for high-efficiency Fenton-like catalysis: Insights into density-dependent activity and mechanisms

Atomically dispersed metal activators (ADMAs) have demonstrated unique advantages in environmental remediation, but how to controllably regulate the active site density and electronic structure of ADMAs to further enhance activation efficiency remains challenging. Here, we introduce a sulfur-atom-doping approach that allows the fine-tuning of atomic Co site content and electronic structure, enabling exploration of density-dependent activation performance of ADMAs for peroxymonosulfate (PMS)-based Fenton-like catalysis. Our investigation reveals a direct correlation between activation capacity and single-Co-site density. The optimal SNC@CoSA-0.05 activator with densely populated Co–N3S1 sites (10.1 wt%) displays exceptional efficacy in eliminating Rhodamine B, with specific activity of 31.0 min−1 g−1 L, outperforming most previously published activators. Moreover, SNC@CoSA-0.05 showed a remarkedly reduced metal leaching (47.4 μg L−1) than its nanocluster counterpart (194 μg L−1) at pH 3.2. Experimental and theoretical analyses unveiled that coordinated sulfur actively modulates the electronic structure of the central Co atom, enhancing the adsorption and activation of PMS, thereby improving decontamination efficiency. Mechanistic studies further elucidate the predominant electron-transfer regime involved in oxidizing micropollutants by SNC@CoSA-0.05/PMS, with Co(IV)=O, •OH, and SO4•− being the auxiliary oxidizing species. This study not only offers a method for concurrent adjustment of active site density and electronic structure in ADMAs but also sheds light on the activation mechanisms of atomic metal sites.

A model for seagrass species competition: Dynamics of the symmetric case

We propose a general population dynamics model for two seagrass species growing and interacting in two spatial dimensions. The model includes spatial terms accounting for the clonal growth characteristics of seagrasses, and coupling between species through the net mortality rate. We consider both intraspecies and interspecies facilitative and competitive interactions, allowing density-dependent interaction mechanisms. Here we study the case of very similar species with reciprocal interactions, which allows reducing the number of the model parameters to just four, and whose bifurcation structure can be considered the backbone of the general case. We find that the parameter space can be divided into ten regions with qualitatively different bifurcation diagrams. These regions can be further grouped into just five regimes with different ecological interpretations. Our analysis allows the classification of all possible density distributions and dynamical behaviors of meadows with two coexisting species.

- Invited Review - Chemical signalling within the rumen microbiome.

Ruminants possess a specialized four-compartment forestomach, consisting of the reticulum, rumen, omasum, and abomasum. The rumen, the primary fermentative chamber, harbours a dynamic ecosystem comprising bacteria, protozoa, fungi, archaea, and bacteriophages. These microorganisms engage in diverse ecological interactions within the rumen microbiome, primarily benefiting the host animal by deriving energy from plant material breakdown. These interactions encompass symbiosis, such as mutualism and commensalism, as well as parasitism, predation, and competition. These ecological interactions are dependent on many factors, including the production of diverse molecules, such as those involved in quorum sensing (QS). QS is a density-dependent signalling mechanism involving the release of autoinducer (AIs) compounds, when cell density increases AIs bind to receptors causing the altered expression of certain genes. These AIs are classified as mainly being N-acyl-homoserine lactones (AHL; commonly used by Gram-negative bacteria) or autoinducer-2 based systems (AI-2; used by Gram-positive and Gram-negative bacteria); although other less common AI systems exist. Most of our understanding of QS at a gene-level comes from pure culture in vitro studies using bacterial pathogens, with much being unknown on a commensal bacterial and ecosystem level, especially in the context of the rumen microbiome. A small number of studies have explored QS in the rumen using 'omic' technologies, revealing a prevalence of AI-2 QS systems among rumen bacteria. Nevertheless, the implications of these signalling systems on gene regulation, rumen ecology, and ruminant characteristics are largely uncharted territory. Metatranscriptome data tracking the colonization of perennial ryegrass by rumen microbes suggest that these chemicals may influence transitions in bacterial diversity during colonization. The likelihood of undiscovered chemicals within the rumen microbial arsenal is high, with the identified chemicals representing only the tip of the iceberg. A comprehensive grasp of rumen microbial chemical signalling is crucial for addressing the challenges of food security and climate targets.

Unraveling population trends in Italy (1921–2021) with spatial econometrics

Testing density-dependence and path-dependence in long-term population dynamics under differentiated local contexts contributes to delineate the changing role of socioeconomic forces at the base of regional disparities. Despite a millenary settlement history, such issue has been rarely investigated in Europe, and especially in highly divided countries such as those in the Mediterranean region. Using econometric modeling to manage spatial heterogeneity, our study verifies the role of selected drivers of population growth at ten times between 1921 and 2021 in more than 8000 Italian municipalities verifying density-dependent and path-dependent dynamics. Results of global and quantile (spatial) regressions highlight a differential impact of density and (lagged) population growth on demographic dynamics along the urban cycle in Italy. Being weakly significant in the inter-war period (1921–1951), econometric models totalized a high goodness-of-fit in correspondence with compact urbanization (1951–1981). Model’s fit declined in the following decades (1981–2021) reflecting suburbanization and counter-urbanization. Density-dependence and path-dependence were found significant and, respectively, positive or negative, with compact urbanization, and much less intense with suburbanization and counter-urbanization. A spatial econometric investigation of density-dependent and path-dependent mechanisms of population dynamics provided an original explanation of metropolitan cycles, delineating the evolution of socioeconomic (local) systems along the urban-rural gradient.

Can regime shifts in reproduction be explained by changing climate and food availability?

Marine populations often show considerable variation in their productivity, including regime shifts. Of special interest are prolonged shifts to low recruitment and low abundance which occur in many fish populations despite reductions in fishing pressure. One of the possible causes for the lack of recovery has been suggested to be the Allee effect (depensation). Nonetheless, both regime shifts and the Allee effect are empirically emerging patterns but provide no explanation about the underlying mechanisms. Environmental forcing, on the other hand, is known to induce population fluctuations and has also been suggested as one of the primary challenges for recovery. In the present study, we build upon recently developed Bayesian change-point models to explore the contribution of food and climate as external drivers in recruitment regime shifts, while accounting for density-dependent mechanisms (compensation and depensation). Food availability is approximated by the copepod community. Temperature is included as a climatic driver. Three demersal fish populations in the Irish Sea are studied: Atlantic cod (Gadus morhua), whiting (Merlangius merlangus) and common sole (Solea solea). We demonstrate that, while spawning stock biomass undoubtedly impacts recruitment, abiotic and biotic drivers can have substantial additional impacts, which can explain regime shifts in recruitment dynamics or low recruitment at low population abundances. Our results stress the importance of environmental forcing to capture variability in fish recruitment.

Simulation of complex triaxial tests with HySand, a new multisurface constitutive model in the hyperplastic framework

Offshore wind has a key role to play in the energy transition. The majority of offshore wind turbines (OWTs) are bottom-founded with monopiles [1]. Monopiles account for up to 35% of the installation cost of the OWT [2, 3]. Optimising the design of monopiles can thus lead to significant savings and increased competitiveness of OWTs in the energy market.&#x0D; The PISA project [4] has led to the optimisation of the design under monotonic lateral loading of monopiles founded in sands, clays, and layered profiles [5 ,6, 7]. Because of this optimisation, along with the increasing size of OWTs, and the deeper waters where they are installed, design under cyclic loading is becoming crucial. There are, however, significant shortcomings in current design methods to predict the effects of cyclic loading on a monopile: accumulated rotation, changes in stiffness, energy dissipation and strength.&#x0D; The PICASO project, a joint research project between the University of Oxford and Ørsted, seeks to develop a new design method for monopiles under cyclic loading in sands and clays. Similarly to PISA [8,9], this design method will use the finite element method to simulate the behaviour of monopiles. For PICASO, such simulation requires constitutive models able to capture the behaviour of soil under cyclic loading. However, available advanced constitutive models for sand face severe limitations [10, 11] and are not satisfactory for the simulation of cyclic loading.&#x0D; HySand [12, 13], a family of models developed in the hyperplastic framework, fills this gap. HySand_base is a 14 parameter multisurface plasticity model with non-linear elasticity, shear plasticity and hardening, and two plastic volumetric mechanisms resulting in non-associative plasticity: one dilation mechanism which value depends on the evolving density and anisotropy of the sample, and one density dependent consolidation mechanism.&#x0D; This document presents comparisons between the database on Karlsruhe fine sand by Wichtmann [14] and results of simulations with HySand_base. The focus will be on complex tests that other models fail to simulate adequately, such as undrained strain-controlled cyclic tests. Data and simulation with HySand_base of such test on medium dense sand are presented in Figure 1.&#x0D; HySand_base performs well across densities and types of triaxial tests. This gives confidence in its existing three-dimensional implementation in finite element codes, and demonstrates its value to future design processes.&#x0D;

Testing Density-Dependent and Path-Dependent Population Dynamics in Greece with Spatial Quantile and Geographically Weighted Regressions

Regional variability in the spatial distribution of resident population and across-country density divides have consolidated heterogeneous demographic patterns at the base of modern urban systems in Europe. Although economic, historical, institutional, and cultural factors have demonstrated to affect the spatial distribution of resident population, density-dependence and path-dependence are mechanisms persistently shaping demographic dynamics at both local and regional scale. Analysis of density-dependent patterns of population growth (and decline) over sufficiently long time intervals allows a refined comprehension of socioeconomic processes underlying demographic divides. Despite a long settlement history, empirical investigation of the role of density-dependence in the long-term evolution of human populations along urban–rural gradients is relatively scarce especially in Mediterranean countries. The present study performs a comparative analysis of population distribution in 1033 Greek municipalities identifying (and testing the significance of) density-dependent and path-dependent mechanisms of population growth between 1961 and 2011, using spatially implicit and explicit econometric approaches. Results highlight a positive impact of density on population growth where settlements are concentrated. Assuming goodness of fit of the tested models as a proxy of density-dependence, the empirical findings clarify how density-dependent mechanisms were not significant all over the study period, being instead associated with specific phases of the city life cycle—basically urbanization with population concentrating in central locations. Density-dependence was less intense with suburbanization and counter-urbanization—when population sprawled over larger areas at medium–low density. An improved understanding of density-dependent and path-dependent mechanisms of population growth contributes to rethink spatial planning, regional development strategies, and socio-demographic policies adapting to heterogeneous (and rapidly changing) local contexts.

Increasing mussel transplantation success by initiating self-facilitating feedback mechanisms

Transplantation success of ecosystem-engineering species can be low in dynamic environments, as such ecosystem-engineers often require density-dependent positive feedback mechanisms to overcome environmental stressors. These self-facilitating feedback mechanisms play an important role in self-organization, whereby complex systems tend to organize and create patterns in order to ameliorating physical and/or biological stressors. In this study we used biodegradable structures to ameliorate self-facilitating feedback mechanisms to overcome environmental stressors in the initial post-transplantation phase. The biodegradable structures tested are an innovation of the traditional Seed Mussel Collectors (SMCs) used in mussel cultivation. The so-called “BioShell-SMC” does not contain any plastic, but is made up of a coconut fiber rope surrounded by empty cockle shells and held together by a biodegradable net based on a compound of aliphatic polyesters. We tested if the survival of two size classes of blue mussel (Mytilus edulis) transplants, on a tidal flat in the Oosterschelde estuary in the Netherlands, increased when mussel seed was transplanted attached to the BioShell-SMCs instead of single mussels in combination with empty cockle shells. The results of this study revealed that the survival of larger mussel seed significantly improved when attached to the BioShell-SMC compared to those transplanted loosely. Factors contributing to the difference in mussel loss between BioShell-SMC mussels and loosely transplanted mussels include predation, competition and dislodgement due to hydrodynamic forces. For small mussel seed, mussel biomass decreased strongly in the first three days of the experiment, irrespective of transplantation method. This is due the small size of the mussels in combination with low mussel densities. Overall, this study highlights the potential of using biodegradable structures to initiate self-facilitating feedback mechanisms in establishment of ecosystem engineers in dynamic environments.

Coupling dynamic energy budget and population dynamic models to inform stock enhancement in fisheries management

AbstractExtensive applications of fishery stock enhancement worldwide bring up broad concerns about its negative effects, creating a pivotal need for science‐based assessment and planning of enhancement strategies. However, the lack of mechanistic understanding of enhanced population dynamics, particularly the density‐dependent processes, leads to compromise in model development and limits the capacity in predicting enhancement effects. Here, we developed an individual‐based model based on dynamic energy budget theory and full life‐history processes, to understand the mechanism of density dependence in population dynamics that emerge from individual‐level processes. We demonstrated the utility of the model framework by applying it to an extensively enhanced species, Chinese prawn (Fenneropenaeus chinensis, Penaeidae). The model could yield projections reflecting the observed trajectory of population biomass and yields. The model also delineated the key effects of density dependence on the vital rates of growth, fecundity and starvation mortality. Regarding the manifold effects of stock enhancement, we demonstrated a dampened shape in population biomass and yields with increasing magnitude of enhancement, and trade‐offs between the ecological and economic objectives, that is, pursuing high benefit might compromise the wild population without proper management. Furthermore, we illustrated the possibility of combining stock enhancement and harvest regulation in promoting population recovery while maintaining fisheries yields. We highlight the potential of the proposed model for understanding density dependence in enhancement programme, and for designing integrated management strategies. The approach developed herein may serve as a general approach to assess the population dynamics in stock enhancement and inform enhancement management.

Habitat use of five sympatric predatory reef fishes at a remote island in the south-western Atlantic.

Density-dependent mechanisms and habitat use are important drivers of marine spatial distribution in complex ecosystems such as coral or rocky reefs. In the last decade, a few studies have assessed habitat use by reef fishes in nearshore and coastal environments along the Brazilian coast. Serranidae (groupers and sea basses) are regarded as excellent models for understanding habitat use patterns due to their diversity, long lifespan, wide distribution, morphological and functional diversity, and behavioural complexity. Their trophic position in the food web, from meso- to top-predators, grants them critical roles as top-down population controllers. Herein, we present the first assessment of habitat use by five sympatric Serranidae in a Brazilian oceanic island, Trindade. The model species selected for this assessment were the coney (Cephalopholis fulva), the rock hind (Epinephelus adscensionis), the greater soapfish (Rypticus saponaceus), the Creole-fish (Paranthias furcifer) and the hybrid between C. fulva and P. furcifer. Our findings revealed that the species showed specific associations with topographic characteristics related to shelter from predation, reproduction and feeding. Habitat use in Trindade was similar to that observed in nearshore coastal environments (where the hybrid is absent). The present work contributes to the knowledge of habitat use and niche partitioning among key species, which is a valuable tool to subsidize effective conservation initiatives such as designing marine protected areas focusing on the behaviour and habitat use of key ecological players.

Role of adaptive intraspecific competition on collective behavior in the rock–paper–scissors game

Density-dependent selection is a universal feature in the evolution of populations, and such an adaptive behavior can change the survival strategy of species during evolution. In this paper, we investigate the role of adaptive behavior in biodiversity in the system of cyclic competition. By incorporating a density-dependent mechanism into intraspecific competition, which is well-known as a key mechanism leading to biodiversity, we studied how such adaptive intraspecific competition can affect biodiversity and collective behavior during the evolution of cyclically competing species. Microscopically, we found that species can coexist strongly and are spirally entangled or collectively united by presenting two distinct pattern formations on spatially extended systems. While the adaptive mechanism can always promote species coexistence in a mean-field manner for particular sensitivity to the group scale, corresponding spatial dynamics exhibit nonmonotonic features for the robustness of extinction at moderately high mobility regime over the critical mobility when the associated mean-field system exhibits asymptotically stable heteroclinic cycles. The findings can shed light on a new aspect of the collective behavior of coexisting populations which may indicate the possibility of changing the survival strategies of each group to maintain the coexistence of cyclically competing populations.

An experimental analysis of density dependence in meadow voles: Within-season and delayed effects.

Wild mammal populations exhibit a variety of dynamics, ranging from fairly stable with little change in population size over time to high-amplitude cyclic or erratic fluctuations. A persistent question in population ecology is why populations fluctuate as they do. Answering this seemingly simple question has proven to be challenging. Broadly, density-dependent feedback mechanisms should allow populations to grow at low density and slow or halt growth at high density. However, experimental tests of what demographic processes result in density-dependent feedback and on what timescale have proven elusive. Here, we used replicated density perturbation experiments and capture-mark-recapture analyses to test density-dependent population growth in populations of meadow voles (Microtus pennsylvanicus) during the summer breeding season by manipulating founding population density and observing the pattern of survival, reproduction, and population growth. High population density had no consistent effect on survival rates but generally negatively influenced recruitment and population growth rates. However, these density-dependent effects varied within the breeding season and across years. Our study provides evidence that density-dependent feedback mechanisms operate at finer time scales than previously believed and that process, additively with delayed year effects, is key to understanding multiyear population demography.

Inferring density-dependent population dynamics mechanisms through rate disambiguation for logistic birth-death processes.

Density dependence is important in the ecology and evolution of microbial and cancer cells. Typically, we can only measure net growth rates, but the underlying density-dependent mechanisms that give rise to the observed dynamics can manifest in birth processes, death processes, or both. Therefore, we utilize the mean and variance of cell number fluctuations to separately identify birth and death rates from time series that follow stochastic birth-death processes with logistic growth. Our nonparametric method provides a novel perspective on stochastic parameter identifiability, which we validate by analyzing the accuracy in terms of the discretization bin size. We apply our method to the scenario where a homogeneous cell population goes through three stages: (1) grows naturally to its carrying capacity, (2) is treated with a drug that reduces its carrying capacity, and (3) overcomes the drug effect to restore its original carrying capacity. In each stage, we disambiguate whether the dynamics occur through the birth process, death process, or some combination of the two, which contributes to understanding drug resistance mechanisms. In the case of limited sample sizes, we provide an alternative method based on maximum likelihood and solve a constrained nonlinear optimization problem to identify the most likely density dependence parameter for a given cell number time series. Our methods can be applied to other biological systems at different scales to disambiguate density-dependent mechanisms underlying the same net growth rate.

Molecular Communication for Quorum Sensing Inspired Cooperative Drug Delivery

A cooperative drug delivery system is proposed, where quorum sensing (QS), a density-dependent bacterial behavior coordination mechanism, is employed by synthetic bacterium-based nanomachines (B-NMs) for controllable drug delivery. In our proposed system, drug delivery is only triggered when there are enough QS molecules, which in turn only happens when there are enough B-NMs. This makes the proposed system can be used to achieve a high release rate of drug molecules from a high number of B-NMs when the population density of B-NMs may not be known. Analytical expressions for i) the expected activation probability of the B-NM due to randomly-distributed B-NMs and ii) the expected aggregate absorption rate of drug molecules due to randomly-distributed QS activated B-NMs are derived. Analytical results are verified by particle-based simulations. The derived results can help to predict and control the impact of environmental factors (e.g., diffusion coefficient and degradation rate) on the absorption rate of drug molecules since rigorous diffusion-based molecular channels are considered. Our results show that the activation probability at the B-NM increases as this B-NM is located closer to the center of the B-NM population and the aggregate absorption rate of the drug molecules non-linearly increases as the population density increases.

Spatiotemporal outbreak dynamics of bark and wood-boring insects.

Bark and wood-boring insects (BWI) can cause important disturbances in forest ecosystems, and their impact depends on their spatiotemporal dynamics. Populations are usually at stable, low densities but can be disrupted by stochastic perturbations that trigger a transition to an epidemic phase. For less aggressive species, outbreaks die out quickly once the perturbation is removed, while aggressive species exhibit density-dependent feedback mechanisms facilitating persistent landscape-scale outbreaks. The interactions of attributes of tree, stand, landscape- and regional-scale stressors, and insect life history and behavior determine system-specific dynamics. However, most of our knowledge is based on a few species of mainly Holarctic bark beetles. With global change, it is becoming increasingly important to improve our understanding of the frequency and severity of BWI outbreaks.

Multimodal pathogen transmission as a limiting factor in host distribution.

Theoretical models suggest that infectious diseases could play a substantial role in determining the spatial extent of host species, but few studies have collected the empirical data required to test this hypothesis. Pathogens that sterilize their hosts or spread through frequency-dependent transmission could have especially strong effects on the limits of species' distributions because diseased hosts that are sterilized but not killed may continue to produce infectious stages and frequency-dependent transmission mechanisms are effective even at very low population densities. We collected spatial pathogen prevalence data and population abundance data for alpine carnations infected by the sterilizing pathogen Microbotryum dianthorum, a parasite that is spread through both frequency-dependent (vector-borne) and density-dependent (aerial spore transmission) mechanisms. Our 13-year study reveals rapid declines in population abundance without a compensatory decrease in pathogen prevalence. We apply a stochastic, spatial model of parasite spread that accommodates spatial habitat heterogeneity to investigate how the population dynamics depend on multimodal (frequency-dependent and density-dependent) transmission. We found that the observed rate of population decline could plausibly be explained by multimodal transmission, but is unlikely to be explained by either frequency-dependent or density-dependent mechanisms alone. Multimodal pathogen transmission rates high enough to explain the observed decline predicted that eventual local extinction of the host species is highly likely. Our results add to a growing body of literature showing how multimodal transmission can constrain species distributions in nature.

Relating plasticity to dislocation properties by data analysis: scaling vs. machine learning approaches

Plasticity modelling has long relied on phenomenological models based on ad-hoc assumption of constitutive relations, which are then fitted to limited data. Other work is based on the consideration of physical mechanisms which seek to establish a physical foundation of the observed plastic deformation behavior through identification of isolated defect processes (’mechanisms’) which are observed either experimentally or in simulations and then serve to formulate so-called physically based models. Neither of these approaches is adequate to capture the complexity of plastic deformation which belongs into the realm of emergent collective phenomena, and to understand the complex interplay of multiple deformation pathways which is at the core of modern high performance structural materials. Data based approaches offer alternative pathways towards plasticity modelling whose strengths and limitations we explore here for a simple example, namely the interplay between rate and dislocation density dependent strengthening mechanisms in fcc metals.

Population size regulation is density-dependent in Rhodnius prolixus (Hemiptera: Reduviidae) through an irritability mechanism.

Physical factors can determine the level of triatomine abundance, but do not regulate their population densities, and neither do natural enemies. To identify the processes associated with density-dependent triatomine population regulation. We set-up a laboratory experiment with four interconnected boxes; the central box harbored Rhodnius prolixus bugs and one hamster. Stage 5 and adult densities of 10, 20, 30, 40, and 60 bugs per hamster, were replicated four times (except the density of 60 bugs). Hamster's irritability and several triatomine responses were measured: feeding, development time and longevity, mortality, fecundity, dispersal, and the net reproductive value (R o ). Density had a statistically significant effect on irritability, but not on the percent of bugs feeding. Density was significant on blood meal size ingested in bugs that did not move between boxes, but not significant when the bugs moved. Density and irritability affected the proportion of stage 5 nymphs molting, and the proportion of adult bugs dying per day and over a three-week period. There was a highly significant effect of density and irritability on R o . We showed that a density-dependent mechanism, acting through the irritability of the host, seems the most plausible process regulating populations in triatomines.

Some of the Mechanisms for Coexistence of Tree Species Diversity in Tropical Forests: A Review of Effects of Tree Density Dependence

Tree communities contribute to maintenance of species diversity in tropical forests. Coexistence of many tree species is not without competition. Therefore, coexistence of tree species and size diversities occur sequentially or simultaneously in tropical natural forests. Understanding coexistence and competition mechanisms of tree species requires knowledge of interactions within and between species. However, many conservation efforts and strategies failed due to inability to identify and maintain functional coexistence mechanisms among tree species in the forest. Also, most trees died because of pressure on their habitats and not because of limiting growth resources. Hence, species identity, minimum distance and size of the neighbouring trees which are responsible for coexistence of competing trees in most tropical forests have not been explicitly reviewed. Therefore, this review evaluated some of the density dependent mechanisms for coexistence of tree species alpha diversity in tropical forests. Many interactive mechanisms are responsible for coexistence tree species in tropical forests. Inter- and intra-specific competitions are the most significant and both facilitate positive and negative density dependence. Therefore, switching from negative to positive density dependence may occur in some situations. Positive and negative density effects regulate species abundance and coexistence through conspecific and heterospecific structures. Aggregates of conspecific and heterospecific neighbours constitute forest spatial structure. Negative density interactions are mutually exclusive and basically ranged from effect of species identity of neighbours, distance to neighbours and tree size of the neighbours to reference trees in the community structures. Some mechanisms shorten distances for heterospecific than conspecific interactions. Conspecific structures improved survival and growth of rare tree species. Interactive mechanisms in tree community and population structures facilitate species diversity and size inequality, respectively.