Population-level Interactions Research Articles

Population level interactions between an invasive woodwasp, an invasive nematode and a community of native parasitoids

Parasitic nematodes and hymenopteran parasitoids have been introduced and used extensively to control invasive Eurasian Sirex noctilio woodwasps in pine plantations in the Southern Hemisphere where no members of this community are native. Sirex noctilio has more recently invaded North America where Sirex-associated communities are native. Sirex noctilio and its parasitic nematode, Deladenus siricidicola, plus six native hymenopteran woodwasp parasitoids in New York and Pennsylvania, were sampled from 204 pines in 2011–2019. Sirex noctilio had become the most common woodwasp in this region and the native parasitoids associated with the native woodwasps had expanded their host ranges to use this invader. We investigated the distributions of these species among occupied trees and the interactions between S. noctilio and natural enemies as well as among the natural enemies. Sirex noctilio were strongly aggregated, with a few of the occupied trees hosting hundreds of woodwasps. Nematode parasitism was positively associated with S. noctilio density, and negatively associated with the density of rhyssine parasitoids. Parasitism by the parasitoid Ibalia leucospoides was positively associated with host (S. noctilio) density, while parasitism by the rhyssine parasitoids was negatively associated with density of S. noctilio. Thus, most S. noctilio come from a few attacked trees in a forest, and S. noctilio from those high-density trees experienced high parasitism by both the invasive nematode and the most abundant native parasitoid, I. l. ensiger. There is little evidence for direct competition between the nematodes and parasitoids. The negative association occurring between rhyssine parasitoids and I. l. ensiger suggests rhyssines may suffer from competition with I. l. ensiger which parasitize the host at an earlier life stage. In addition to direct competition with the native woodwasp Sirex nigricornis for suitable larval habitat within weakened trees, the large S. noctilio population increases the parasitoid and nematode populations, which may increase parasitism of S. nigricornis.

The community-function landscape of microbial consortia.

Quantitatively linking the composition and function of microbial communities is a major aspiration of microbial ecology. Microbial community functions emerge from a complex web of molecular interactions between cells, which give rise to population-level interactions among strains and species. Incorporating this complexity into predictive models is highly challenging. Inspired by a similar problem in genetics of predicting quantitative phenotypes from genotypes, an ecological community-function (or structure-function) landscape could be defined that maps community composition and function. In this piece, we present an overview of our current understanding of these community landscapes, their uses, limitations, and open questions. We argue that exploiting the parallels between both landscapes could bring powerful predictive methodologies from evolution and genetics into ecology, providing a boost to our ability to engineer and optimize microbial consortia.

Shifting agriculture and a depleting aquifer: implications of row-crop farming on mule deer population performance

Context Conversion of native vegetation to cropland is one of the most widespread anthropogenic landscape alterations, particularly in the Great Plains region of the United States. Mule deer occur throughout the Great Plains; however, it is the south-eastern edge of their geographical distribution, and few populations coincide with dense cropland. The rapidly depleting Ogallala Aquifer supplies irrigation to row-crops throughout the region, which will likely shift towards dryland agricultural practices in the near future. Aims We sought to understand how cropland use influences morphology, body condition indices, reproductive output, and survival of free-ranging mule deer. Methods We accumulated a multi-year, longitudinal dataset of movement and morphology for 146 mule deer in the Texas Panhandle. We linked seasonal cropland use with observed morphology, body condition metrics, and reproductive output via linear mixed-effect modelling and assessed the influence of cropland on annual survival by using Cox proportional hazard models. Key results Mule deer that did not use cropland at any time during the year exhibited morphological and nutritional indices similar to those that did; except body-fat percentage being greater for mature (≥4-year-old) males that used cropland. Further, cropland use did not predict survival probability. Analyses of cropland use during seasons defined by life-stage showed context-dependent nutritional benefits. Use of cropland during winter following reproduction demonstrated an increase in young (≤3-year-old) male antler size and body mass and summer crop use increased body condition for all males. Female mule deer that utilised cropland before pregnancy had increased probability of successful reproduction, demonstrating a potential capital investment strategy in reproduction. Conclusions Cropland does not limit morphology or survival of mule deer; however, additive use of row-crops can provide a nutritional buffer and enhanced reproductive output for individuals that choose to utilise it. Implications Our study demonstrates important population-level interactions with the environment for a species near the extent of their geographical distribution. Conversion of row-crop farming from aquifer depletion or climate shifts may not diminish mule deer populations, but these changes may alter specific habitat-nutritional health relationships that can influence population performance and future conservation efforts.

Syndemics of intimate partner violence among women in HIV endemic South Africa: geospatial analysis of nationally representative data

Despite some improvement in lowering HIV incidence, HIV-related challenges, such as intimate partner violence (IPV), remain unacceptably high among women in South Africa. For decades, researchers and activists have pointed to the complex and intertwined reality of the substance abuse, violence and AIDS (SAVA) syndemic that endangers women. However, more recent systematic review/meta-analysis evidence points to inconclusive association between IPV and alcohol use. Furthermore, much of the evidence is often non-population-based that focuses on the co-occurrence rather than synergistic SAVA interaction. In this study, using the latest data from the South Africa Demographic and Health Survey (SA-DHS), we identified geographic synergistic clustering of IPV associated with HIV and substance abuse in South Africa as a measure of population-level interactions among these factors. The SA-DHS is a nationally representative sample that includes wide-ranging data on health, social challenges and household geo-locations of 5,874 women who participated in the domestic violence module. First, geographical IPV, harmful alcohol use (as the substance abuse measure available in SA-DHS) and HIV clusters were identified using the Kulldorff spatial scan statistic in SaTScan. Second, synergistic interactions related to recent IPV (i.e. recent physical, sexual, emotional violence during the last 12 months) with harmful alcohol use and HIV challenge were measured using RERI [Relative excess risk due to interaction], AP [attributable proportion] and S [Synergy index]. In our results, we spatially identified geographical physical IPV syndemic interactions in parts of the Eastern Cape/Free State Provinces (RERI = 4.42 [95% CI: 2.34–6.51], AP = 0.56 [95% CI: 0.44–0.68], S = 2.77 [95% CI: 2.01–3.84], but not in other forms of IPV. Although IPV, based on decade old concept of SAVA syndemic, was less common/widespread than expected from the national scale population-based data, we identified population-level physical violence syndemic occurring in South Africa. Our study highlights the need to prioritize public health response targeting vulnerable populations residing in these high-risk areas of syndemic mechanisms linking these synergistic epidemics that women face in South Africa.

Dynamic population stage structure due to juvenile–adult asymmetry stabilizes complex ecological communities

Natural ecological communities are diverse, complex, and often surprisingly stable, but the mechanisms underlying their stability remain a theoretical enigma. Interactions such as competition and predation presumably structure communities, yet theory predicts that complex communities are stable only when species growth rates are mostly limited by intraspecific self-regulation rather than by interactions with resources, competitors, and predators. Current theory, however, considers only the network topology of population-level interactions between species and ignores within-population differences, such as between juvenile and adult individuals. Here, using model simulations and analysis, I show that including commonly observed differences in vulnerability to predation and foraging efficiency between juvenile and adult individuals results in up to 10 times larger, more complex communities than observed in simulations without population stage structure. These diverse communities are stable or fluctuate with limited amplitude, although in the model only a single basal species is self-regulated, and the population-level interaction network is highly connected. Analysis of the species interaction matrix predicts the simulated communities to be unstable but for the interaction with the population-structure subsystem, which completely cancels out these instabilities through dynamic changes in population stage structure. Common differences between juveniles and adults and fluctuations in their relative abundance may hence have a decisive influence on the stability of complex natural communities and their vulnerability when environmental conditions change. To explain community persistence, it may not be sufficient to consider only the network of interactions between the constituting species.

Generative modelling of the thalamo-cortical circuit mechanisms underlying the neurophysiological effects of ketamine

Cortical recordings of task-induced oscillations following subanaesthetic ketamine administration demonstrate alterations in amplitude, including increases at high-frequencies (gamma) and reductions at low frequencies (theta, alpha). To investigate the population-level interactions underlying these changes, we implemented a thalamo-cortical model (TCM) capable of recapitulating broadband spectral responses. Compared with an existing cortex-only 4-population model, Bayesian Model Selection preferred the TCM. The model was able to accurately and significantly recapitulate ketamine-induced reductions in alpha amplitude and increases in gamma amplitude. Parameter analysis revealed no change in receptor time-constants but significant increases in select synaptic connectivity with ketamine. Significantly increased connections included both AMPA and NMDA mediated connections from layer 2/3 superficial pyramidal cells to inhibitory interneurons and both GABAA and NMDA mediated within-population gain control of layer 5 pyramidal cells. These results support the use of extended generative models for explaining oscillatory data and provide in silico support for ketamine's ability to alter local coupling mediated by NMDA, AMPA and GABA-A.

Intraglomerular gap junctions enhance interglomerular synchrony in a sparsely connected olfactory bulb network.

Despite sparse connectivity, population-level interactions between mitral cells (MCs) and granule cells (GCs) can generate synchronized oscillations in the rodent olfactory bulb. Intraglomerular gap junctions between MCs at the same glomerulus can greatly enhance synchronized activity of MCs at different glomeruli. The facilitating effect of intraglomerular gap junctions on interglomerular synchrony is through triggering of mutually synchronizing interactions between MCs and GCs. Divergent connections between MCs and GCs make minimal direct contribution to synchronous activity. A dominant feature of the olfactory bulb response to odour is fast synchronized oscillations at beta (15-40Hz) or gamma (40-90Hz) frequencies, thought to be involved in integration of olfactory signals. Mechanistically, the bulb presents an interesting case study for understanding how beta/gamma oscillations arise. Fast oscillatory synchrony in the activity of output mitral cells (MCs) appears to result from interactions with GABAergic granule cells (GCs), yet the incidence of MC-GC connections is very low, around 4%. Here, we combined computational and experimental approaches to examine how oscillatory synchrony can nevertheless arise, focusing mainly on activity between 'non-sister' MCs affiliated with different glomeruli (interglomerular synchrony). In a sparsely connected model of MCs and GCs, we found first that interglomerular synchrony was generally quite low, but could be increased by a factor of 4 by physiological levels of gap junctional coupling between sister MCs at the same glomerulus. This effect was due to enhanced mutually synchronizing interactions between MC and GC populations. The potent role of gap junctions was confirmed in patch-clamp recordings in bulb slices from wild-type and connexin 36-knockout (KO) mice. KO reduced both beta and gamma local field potential oscillations as well as synchrony of inhibitory signals in pairs of non-sister MCs. These effects were independent of potential KO actions on network excitation. Divergent synaptic connections did not contribute directly to the vast majority of synchronized signals. Thus, in a sparsely connected network, gap junctions between a small subset of cells can, through population effects, greatly amplify oscillatory synchrony amongst unconnected cells.

Differential Intrinsic Coupling Modes in Psychological and Physical Trauma.

Despite each disorder having a distinct etiology, post-traumatic stress disorder (PTSD), and mild traumatic brain injury (mTBI) often exhibit overlapping symptomatology that makes clinical diagnosis difficult. Furthermore, identification using structural imaging is impractical because anatomical alterations, if they exist at all, can be subtle, or lie beyond the resolution of current technology. Functional MRI, which relies on an indirect measure of brain function (that of blood hemodynamics), has revealed that aberrant functional connectivity (FC) is prevalent in these disorders (1, 2), and machine learning/pattern classification shows promise that these injuries can be objectively classified on certain feature parameters within the spontaneous fluctuations of these signals (3). Although these analyses could potentially aid diagnosis, theories of how these disorders impact underlying neurophysiological interactions and neural network function remain scant. Generating the questions driving answers to this knowledge gap, basic neuroscience research is increasingly revealing the critical role that neurophysiological networks and their dynamics play in cognition and behavior (4). From a philosophical viewpoint, there has been a Kuhnian epistemological paradigm shift from a reductionist, and historically segregative approach toward, or at least combined with, an integrative neural doctrine (5). In other words, ontological standpoints are being driven by neuroscience moving toward the view that mental states are the population-level interactions of neurons, rather than simply the activity of “independent” neurophysiological units; the emergent properties of these networks ultimately give rise to our inner mental life. Understanding how perturbations to these networks results in psychiatric and neurological disorders will be crucial in future explanations, and ultimately the efficacy of diagnostics, intervention, and prognostication. In this short opinion piece, I discuss some of the Taylor and Pang laboratory’s recent exploratory studies using resting-state paradigms with magnetoencephalography (MEG) that have investigated FC and spontaneous networks in two groups of these patients by examining “intrinsic coupling modes” [ICMs (6)]. These putative types of network interactions comprise distinct mechanisms that facilitate the spatiotemporal organization of ongoing and spontaneous brain activity that defines our psychological state (these modes also subserve goal-directed action, but this is outside the scope of the current piece). I will describe the phenomenology of discrete neurophysiological connectivity profiles evident in the specific cohorts we tested with these disorders, and how they differ in subtle but important ways as well as theorize on the underlying alterations to connectivity that drive these macroscopic markers of disease. Language: en

Multiple cell and population-level interactions with mouse embryonic stem cell heterogeneity.

Much of development and disease concerns the generation of gene expression differences between related cells sharing similar niches. However, most analyses of gene expression only assess population and time-averaged levels of steady-state transcription. The mechanisms driving differentiation are buried within snapshots of the average cell, lacking dynamic information and the diverse regulatory history experienced by individual cells. Here, we use a quantitative imaging platform with large time series data sets to determine the regulation of developmental gene expression by cell cycle, lineage, motility and environment. We apply this technology to the regulation of the pluripotency gene Nanog in mouse embryonic stem cells. Our data reveal the diversity of cell and population-level interactions with Nanog dynamics and heterogeneity, and how this regulation responds to triggers of pluripotency. Cell cycles are highly heterogeneous and cycle time increases with Nanog reporter expression, with longer, more variable cycle times as cells approach ground-state pluripotency. Nanog reporter expression is highly stable over multiple cell generations, with fluctuations within cycles confined by an attractor state. Modelling reveals an environmental component to expression stability, in addition to any cell-autonomous behaviour, and we identify interactions of cell density with both cycle behaviour and Nanog. Rex1 expression dynamics showed shared and distinct regulatory effects. Overall, our observations of multiple partially overlapping dynamic heterogeneities imply complex cell and environmental regulation of pluripotent cell behaviour, and suggest simple deterministic views of stem cell states are inappropriate.

Naturalizing Institutions: Evolutionary Principles and Application on the Case of Money

Summary In recent extensions of Darwinism into economics, the replicator-interactor duality looms large. I propose a naturalistic approach to this duality in the context of the theory of institutions, which means that its use is necessarily dependent on identifying a physical realization. I introduce a general framework, which synthesizes Searle’s and Aoki’s theories, especially with regard to the role of public representations (signs) in the coordination of actions, and the function of cognitive processes that underlie rule-following as a behavioural disposition. Institutions are causal circuits that connect the population-level dynamics of interactions with cognitive phenomena on the individual level which ultimately root in neuronal structures. I propose a new conceptualization of the replicator in the context of institutions: the replicator is a causal conjunction between (physical) signs and neuronal structures which undergirds the dispositions that generate rule-following actions. Signs, in turn, are outcomes of population-level interactions. I apply this framework on the case of money, analysing the emotions that go along with the use of money, and presenting a stylized account of the emergence of money in terms of the naturalized Searle-Aoki model. In this view, money is a neuronally anchored sign for emotions relating with social exchange and reciprocity. The money replicator is physically realized in a causal conjunction of money artefacts and money emotions.

The systems biology of mitochondrial fission and fusion and implications for disease and aging

Mitochondria organize themselves as dynamic populations within a cell, by undergoing continuous cycles of fission and fusion. The spatio-temporal distribution and abundance of mitochondria determines the cell’s energy budget and is thus intimately linked to the cell’s response to environmental stimuli during aging. The dynamic balance of mitochondrial fission and fusion can be studied in terms of antagonistic subpopulations that regulate the mitochondrial responses in space and time. The dynamic nature of these processes motivates mathematical modelling and the simulation of such complex process. In several neurodegenerative and metabolic diseases the dynamic balance of fission and fusion is disturbed. However, how this dynamics plays a role in the progression of diseases is largely unclear. Fission and fusion help mitochondria to regulate cellular energy (ATP) levels, and minimize accumulation of harmful oxidized material called reactive oxygen species which accelerate mutations in mitochondrial DNA (mtDNA) during aging. We discuss how systems biology approaches can be used to investigate the mechanisms controlling the fission–fusion dynamics under two categories: dissecting the design of its molecular regulatory motifs, and understanding complex mitochondrial responses through their population level interactions. This will help us to understand how different regulatory mechanisms regulate the ATP and mutation (mtDNA) landscape of mitochondria to a variety of environmental stimuli in order to maintain their function during aging.

The mutacins of Streptococcus mutans: regulation and ecology

Streptococcus mutans is generally recognized as a causative agent of human dental caries. The production of mutacins (bacteriocins) by S.mutans is considered to be an important factor in the colonization and establishment of S.mutans in the dental biofilm. Two types of mutacins have been characterized: the lantibiotics and the non-lantibiotics. The lantibiotics generally have a wider spectrum of activity than the non-lantibiotics, which make them attractive targets for development into new antimicrobial modalities. The non-lantibiotics are much more prevalent among strains of S.mutans and play a significant role in both community-level and population-level interactions in the dental biofilm. These interactions are directly mediated through the ComCDE two-component system and the newly characterized LytTR Regulation Systems HdrRM and BrsRM. These systems coordinate natural competence development and mutacin production as a means to acquire transforming DNA either by killing closely related streptococcal species in the vicinity of S.mutans, or through an altruistic suicide mechanism among a subpopulation of competent cells within the S.mutans community. As more S.mutans strains are sequenced, it is anticipated that additional mutacins with novel functions will be discovered, which may yield further insights into the ecological role of mutacins within the oral biofilm.

Naturalizing Institutions: Evolutionary Principles and Application on the Case of Money

In recent extensions of the Darwinian paradigm into economics, the replicator-interactor duality looms large. I propose a strictly naturalistic approach to this duality in the context of the theory of institutions, which means that its use is seen as being always and necessarily dependent on identifying a physical realization. I introduce a general framework for the analysis of institutions, which synthesizes Searle’s and Aoki’s theories, especially with regard to the role of public representations (signs) in the coordination of actions, and the function of cognitive processes that underly rule-following as a behavioral disposition. This allows to conceive institutions as causal circuits that connect the population-level dynamics of interactions with cognitive phenomena on the individual level. Those cognitive phenomena ultimately root in neuronal structures. So, I draw on a critical restatement of the concept of the ‘meme’ by Aunger to propose a new conceptualization of the replicator in the context of institutions, namely, the replicator is a causal conjunction between signs and neuronal structures which undergirds the dispositions that generate rule-following actions. Signs, in turn, are outcomes of population-level interactions. I apply this framework on the case of money, analyzing the emotions that go along with the use of money, and presenting a stylized account of the emergence of money in terms of the naturalized Searle-Aoki model. In this view, money is a neuronally anchored metaphor for emotions relating with social exchange and reciprocity. Money as a meme is physically realized in a replicator which is a causal conjunction of money artifacts and money emotions.

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m−2) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).

Single-cell quantification of IL-2 response by effector and regulatory T cells reveals critical plasticity in immune response.

Understanding how the immune system decides between tolerance and activation by antigens requires addressing cytokine regulation as a highly dynamic process. We quantified the dynamics of interleukin-2 (IL-2) signaling in a population of T cells during an immune response by combining in silico modeling and single-cell measurements in vitro. We demonstrate that IL-2 receptor expression levels vary widely among T cells creating a large variability in the ability of the individual cells to consume, produce and participate in IL-2 signaling within the population. Our model reveals that at the population level, these heterogeneous cells are engaged in a tug-of-war for IL-2 between regulatory (T(reg)) and effector (T(eff)) T cells, whereby access to IL-2 can either increase the survival of T(eff) cells or the suppressive capacity of T(reg) cells. This tug-of-war is the mechanism enforcing, at the systems level, a core function of T(reg) cells, namely the specific suppression of survival signals for weakly activated T(eff) cells but not for strongly activated cells. Our integrated model yields quantitative, experimentally validated predictions for the manipulation of T(reg) suppression.

Seasonal Interactions, Habitat Quality, and Population Dynamics in Migratory Birds

AbstractAbstract. Historically, studies of habitat selection have focused on quantifying how current patterns of habitat occupancy influence condition and survival within a season. This approach, however, is overly simplistic, especially for migratory birds that spend different periods of the year in geographically distinct places. Habitat occupancy and the resulting condition of individual birds is likely to be affected by events in the previous season, and the consequences of habitat occupancy will influence individuals and populations in subsequent seasons. Thus, for migratory birds, variation in habitat quality (and quantity) needs to be understood in the context of how events interact throughout periods of the annual cycle. Seasonal interactions can occur at the individual level or population level. Individual-level interactions occur when events in one season produce nonlethal, residual effects that carry over to influence individuals the following season. Population-level interactions occur when a change in population size in one season influences per capita rates the following season. We review various methods for estimating seasonal interactions and highlight a number of examples in the literature. Using a variety of techniques, including intrinsic and extrinsic markers, the vast majority of studies to date have measured seasonal interactions at the individual level. Obtaining estimates of density and changes in per capita rates across multiple seasons to determine population-level interactions has been more challenging. Both types of seasonal interactions can influence population dynamics, but predicting their effects requires detailed knowledge of how populations are geographically connected (i.e., migratory connectivity). We recommend that researchers studying habitat occupancy and habitat selection consider how events in previous seasons influence events within a season.

SEASONAL INTERACTIONS, HABITAT QUALITY, AND POPULATION DYNAMICS IN MIGRATORY BIRDS

Abstract Abstract. Historically, studies of habitat selection have focused on quantifying how current patterns of habitat occupancy influence condition and survival within a season. This approach, however, is overly simplistic, especially for migratory birds that spend different periods of the year in geographically distinct places. Habitat occupancy and the resulting condition of individual birds is likely to be affected by events in the previous season, and the consequences of habitat occupancy will influence individuals and populations in subsequent seasons. Thus, for migratory birds, variation in habitat quality (and quantity) needs to be understood in the context of how events interact throughout periods of the annual cycle. Seasonal interactions can occur at the individual level or population level. Individual-level interactions occur when events in one season produce nonlethal, residual effects that carry over to influence individuals the following season. Population-level interactions occur when a...

Prehistoric Native Americans and ecological change: human ecosystems in eastern North America since the Pleistocene

Acknowledgements Part I. Panarchy as an Integrative Paradigm: Overview 1. The need for a new synthesis 2. Panarchy theory and Quaternary ecosystems 3. Holocene human ecosystems Part II. Ecological Feedbacks and Processes: Overview 4. Gene-level interactions 5. Population-level interactions 6. Community-level interactions 7. Landscape-level interactions 8. Regional-level interactions Part III. Application and Synthesis: Overview 9. The ecological legacy of prehistoric Native Americans References Index.

A phylogenetic approach to assessing the targets of microbial warfare.

Bacteriocins are the most abundant and diverse defense systems in bacteria. As a result of the specific mechanisms of bacteriocin recognition and translocation into the target cell it is assumed that these toxins mediate intra-specific or population-level interactions. However, no published studies specifically address this question. We present here a survey of bacteriocin production in a collection of enteric bacteria isolated from wild mammals in Australia. A subset of the bacteriocin-producing strains was assayed for the ability to kill a broad range of enteric bacteria from the same bacterial collection. A novel method of estimating killing breadth was developed and used to compare the surveyed bacteriocins in terms of the phylogenetic range over which they kill. The most striking result is that although bacteriocin-producers kill members of their own species most frequently, some kill phylogenetically distant taxa more frequently than they kill closer relatives. This study calls into question the role these toxins play in natural populations. A significant number of bacteriocins are highly effective in killing inter-specific strains and thus bacteriocins may serve to mediate bacterial community interactions.

Evaluating effects of an expanding mountain goat population on native bighorn sheep: a simulation model of competition and disease

Biological invasions create serious conservation problems at local to global scales, and decisions about their management require evaluation of the likely environmental consequences of an expanding population. An expanding population of exotic mountain goats ( Oreamnos americanus) occupy an area near Rocky Mountain National Park, Colorado, USA (RMNP), an area managed for preservation of natural communities and processes. If mountain goats colonized RMNP, they could have detrimental impacts on native bighorn sheep ( Ovis canadensis) in the Park. A simulation model was used to evaluate population-level interactions of these species. Simulated mountain goats and bighorn sheep each attained total population sizes of about 950 in the absence of disease or competition. Disease and competition resulted in bighorn sheep populations that were an average of 31% (moderate competition) and 47% (strong competition) smaller than populations without these influences, while competition reduced mountain goat populations by an average of 9%. Competition was responsible for most (>75%) of the reduction in average size of bighorn sheep populations, but disease increased variation in population size by a factor of 5. The combined population-level effects of competition (reduced average size) and disease (increased variance) represent a substantial risk to native bighorn sheep populations in RMNP.