Reduction In Population Growth Rate Research Articles

Effect of growth temperature on photosynthetic capacity and respiration in three ecotypes of Eriophorum vaginatum.

Ecotypic differentiation in the tussock‐forming sedge Eriophorum vaginatum has led to the development of populations that are locally adapted to climate in Alaska's moist tussock tundra. As a foundation species, E. vaginatum plays a central role in providing topographic and microclimatic variation essential to these ecosystems, but a changing climate could diminish the importance of this species. As Arctic temperatures have increased, there is evidence of adaptational lag in E. vaginatum, as locally adapted ecotypes now exhibit reduced population growth rates. Whether there is a physiological underpinning to adaptational lag is unknown. Accordingly, this possibility was investigated in reciprocal transplant gardens. Tussocks of E. vaginatum from sites separated by ~1° latitude (Coldfoot: 67°15′N, Toolik Lake: 68°37′, Sagwon: 69°25′) were transplanted into the Toolik Lake and Sagwon sites and exposed to either an ambient or an experimental warming treatment. Five tussocks pertreatment combination were measured at each garden to determine photosynthetic capacity (i.e., V cmax and J max) and dark respiration rate (R d) at measurement temperatures of 15, 20, and 25°C. Photosynthetic enhancements or homeostasis were observed for all ecotypes at both gardens under increased growth temperature, indicating no negative effect of elevated temperature on photosynthetic capacity. Further, no evidence of thermal acclimation in R d was observed for any ecotype, and there was little evidence of ecotypic variation in R d. As such, no physiological contribution to adaptational lag was observed given the increase in growth temperature (up to ~2°C) provided by this study. Despite neutral to positive effects of increased growth temperature on photosynthesis in E. vaginatum, it appears to confer no lasting advantage to the species.

Accumulation of Mutational Load at the Edges of a Species Range.

Why species have geographically restricted distributions is an unresolved question in ecology and evolutionary biology. Here, we test a new explanation that mutation accumulation due to small population size or a history of range expansion can contribute to restricting distributions by reducing population growth rate at the edge. We examined genomic diversity and mutational load across the entire geographic range of the North American plant Arabidopsis lyrata, including old, isolated populations predominantly at the southern edge and regions of postglacial range expansion at the northern and southern edges. Genomic diversity in intergenic regions declined toward distribution edges and signatures of mutational load in exon regions increased. Genomic signatures of mutational load were highly linked to phenotypically expressed load, measured as reduced performance of individual plants and lower estimated rate of population growth. The geographic pattern of load and the connection between load and population growth demonstrate that mutation accumulation reduces fitness at the edge and helps restrict species' distributions.

Identifying critical life stage transitions for biological control of long‐lived perennial Vincetoxicum species

Abstract Demographic matrix modelling of invasive plant populations can be a powerful tool to identify life stage transitions for targeted disruption, in order to cause population decline. This approach can provide quantitative estimates of reductions in select vital rates needed to reduce population growth rate (λ) below 1 and can inform the prerelease selection of effective biological control agents, as in the case of two invasive Vincetoxicum species in North America. We parameterized a five life stage matrix model using vital rate data from six populations (field and forest) of Vincetoxicum nigrum and Vincetoxicum rossicum. Elasticity analyses were used to identify life stage transitions and associated vital rates for perturbation through the incorporation of per capita impact data of candidate agents. Several survival, growth and fecundity‐related transitions were identified that were mostly similar across species, habitats and locations, although two populations showed unique population dynamics. Reductions in associated vital rates needed to prevent population growth varied greatly among populations. Defoliation damage and predispersal seed predation can be effective against slower growing forest and field populations of Vincetoxicum species. A fly and bivoltine moth are recommended as priority agents. However, biological control with these same agents will be ineffective against other field populations as well as some forest populations if the intensity of damage is not severe. Synthesis and applications. Control of long‐lived perennial plants, such as species of Vincetoxicum, is projected to occur through disruption of some individual vital rates of survival, growth or fecundity if population increase is low to moderate (λ < 1.6). However, without significant and simultaneous reductions in multiple lower level vital rates, faster growing populations will not be contained with biological control or other management tools.

High juvenile mortality in amphibians during overwintering related to fungal pathogen exposure.

The result of pathogen exposures may depend upon trade-offs in energetic demands for immune responses against host growth and survival. Environmental conditions may influence these trade-offs by affecting host size, or trade-offs may change across seasons, altering impacts of pathogens. We exposed northern leopard frog Lithobates pipiens tadpoles to different larval environments (low leaf litter, high density of conspecifics, atrazine, caged fish, or controls) that influenced size at metamorphosis. Subsequently, we exposed metamorphs to Batrachochytrium dendrobatidis (Bd), a fungal pathogen, just after metamorphosis and/or prior to overwintering 12 wk later. Bd exposure dramatically reduced survival during overwintering, with the strongest effects when hosts were exposed at both time points. Larval environments resulted in differences in host size. Those exposed to caged fish were 2.5 times larger than the smallest (those exposed to high density of conspecifics), but larval environment did not influence Bd effects on growth and survival. The largest frogs exposed to caged fish had greater survival through overwintering, but in the absence of Bd. We built stage-structured models to evaluate if overwinter mortality from Bd is capable of having effects on host populations. Our models suggest that Bd exposure after metamorphosis or before overwintering can reduce population growth rates. Our study demonstrates that hosts suffer little effects of Bd exposures following metamorphosis and that small body size did not hamper growth and survival. Instead, we provide evidence that winter mortality from Bd exposure is capable of reducing population sizes, providing a plausible mechanism for amphibian declines in temperate regions.

Elephant population growth in Kruger National Park, South Africa, under a landscape management approach

South African National Parks (SANParks) manage landscapes rather than numbers of elephants (Loxodonta africana) to mitigate the effects that elephants may have on biodiversity, tourism and stakeholder conservation values associated with protected areas. This management philosophy imposes spatial variability of critical resources on elephants. Restoration of such ecological processes through less intensive management predicts a reduction in population growth rates from the eras of intensive management. We collated aerial survey data since 1995 and conducted an aerial total count using a helicopter observation platform during 2015. A minimum of 17 086 elephants were resident in the Kruger National Park (KNP) in 2015, growing at 4.2% per annum over the last generation of elephants (i.e. 12 years), compared to 6.5% annual population growth noted during the intensive management era ending in 1994. This may come from responses of elephants to density and environmental factors manifested through reduced birth rates and increased mortality rates. Authorities should continue to evaluate the demographic responses of elephants to landscape scale interventions directed at restoring the limitation of spatial variance in resource distribution on elephant spatiotemporal dynamics and the consequences that may have for other conservation values.Conservation implications: Conservation managers should continue with surveying elephants in a way that allows the extraction of key variables. Such variables should focus on measures that reflect on how theory predicts elephants should respond to management interventions.

Tree species vary widely in their tolerance for liana infestation: A case study of differential host response to generalist parasites

AbstractLianas are structural parasites of trees and reduce individual host tree growth, survival and fecundity. Thus, liana infestation is expected to affect tree population growth rates, with potentially different effects in different species depending on the frequency of liana infestation and the impact of liana infestation on population growth rates. Previous studies have documented the myriad negative effects of lianas on trees and variation in liana infestation among tree species; however, no study has quantified the impact of liana infestation on individual tree species population growth rates. Lianas are increasing in abundance in multiple Neotropical sites, which may have profound consequences for tree species composition if lianas differentially affect host tree species population growth.Here, we use long‐term data to evaluate the effects of liana infestation on the reproduction, growth, survival and ultimately population growth rates of dozens of tree species from Barro Colorado Island, Panama. We then test whether liana infestation affects tree species differentially with respect to two axes of life‐history variation: adult stature and position along the fast–slow axis, a measure of shade tolerance.Liana infestation decreased tree growth, survival and reproduction, with the strongest effects on survival in fast‐growing, light‐demanding species and on reproduction in large‐statured species. In combination, these effects reduced tree population growth rates such that liana‐infested populations declined by an average of 1.4% annually relative to conspecific liana‐free populations. The reduction in population growth rates was greatest among fast‐growing species and smaller in slow‐growing species.Synthesis. Our results demonstrate that liana infestation has strong negative effects on tree population growth rates, which vary systematically among tree species with tree life history. The finding that liana infestation is more harmful to fast‐growing tree species appears to be at odds with the general expectations in the literature. We propose that this is likely due to survivorship bias, as infestation greatly decreases survival in fast‐growing species such that the observable sample is biased towards those that survived and liana‐free. In combination with data on how tree species vary in liana infestation rates, these results provide a basis for predicting the impacts of changes in liana abundance on tree species composition.

Chronic toxicity of five metals to the polar marine microalga Cryothecomonas armigera – Application of a new bioassay

The paucity of ecotoxicological data for Antarctic organisms is impeding the development of region-specific water quality guidelines. To address this limitation, toxicity testing protocols need to be developed to account for the unique physiology of polar organisms, in particular their slow growth rates. In this study, a toxicity test protocol was developed to investigate the toxicities of five metals to the polar marine microalga Cryothecomonas armigera. The concentrations which reduced population growth rate by 10% (EC10) after 24-d for Cu, Pb, Zn, Cd and Ni were 21.6, 152, 366, 454, and 1220 μg.L−1, respectively. At the concentrations used in tests, only Cu and Ni were sufficiently toxic to enable the derivation of EC50 values of 63.1 and 1570 μg.L−1 respectively. All metals affected C. armigera's cellular physiology including cellular chlorophyll a fluorescence, cell complexity and size, and lipid concentrations. However, no changes to cellular membrane permeability were observed. The reduction in cellular lipid concentrations was a more sensitive indicator of toxicity for Cd, Ni, and Pb than growth rate inhibition, with EC10 values of 89, 894, and 11 μg.L−1, respectively, highlighting its potential as a sensitive measure of metal toxicity.

Life-shortening Wolbachia infection reduces population growth of Aedes aegypti

Wolbachia bacteria are being introduced into natural populations of vector mosquitoes, with the goal of reducing the transmission of human diseases such as Zika and dengue fever. The successful establishment of Wolbachia infection is largely dependent on the effects of Wolbachia infection to host fitness, but the effects of Wolbachia infection on the individual life-history traits of immature mosquitoes can vary. Here, the effects of life-shortening Wolbachia (wMelPop) on population growth of infected individuals were evaluated by measuring larval survival, developmental time and adult size of Aedes aegypti in intra- (infected or uninfected only) and inter-group (mixed with infected and uninfected) larval competition assays. At low larval density conditions, the population growth of wMelPop infected and uninfected individuals was similar. At high larval densities, wMelPop infected individuals had a significantly reduced population growth rate relative to uninfected individuals, regardless of competition type. We discuss the results in relation to the invasion of the wMelPop Wolbachia infection into naturally uninfected populations.

Pesticide increases transgenerational cost of inducible defenses in a freshwater rotifer

In addition to natural stressors such as predation risk, aquatic organisms receive the simultaneous impact of anthropogenic stressors like pollution. In order to advance our understanding of multiple stressor effects, we evaluated the potential costs in the population growth rate derived from the sub-lethal effect of exposure to the pesticide methamidophos and from the expression of morphological defenses front to predation risk, in the rotifer Brachionus calyciflorus. Costs were evaluated both in the organisms that were exposed to the stressors and in their offspring. Our hypotheses were (1) plastic morphological defenses under exposure to pesticides have fitness costs, which may be transmitted from the parental to the filial generation, and (2) interactive effects between pesticides and predation are dependent of the mother’s age. Our results indicate that pesticide exposure increased the costs, expressed as reduction in population growth rate, generated by the induction of defenses. Interestingly, these costs were detected only in the offspring of the treated organisms and were depended on the age of the mother. This indicates that the sub-lethal effects of predation were increased by pesticide pollution. This study provides experimental evidence of the transgenerational costs of phenotypic plasticity linked to interaction of natural and anthropic stressors.

White-tailed deer florivory influences the population demography of Polemonium vanbruntiae

White-tailed deer (Odocoileus virginianus Zimm.) may influence reproduction and dispersal of plant species through herbivory of flowering stems. We examined the effects of white-tailed deer herbivory on the seed production of Bog Jacob’s-ladder (Polemonium vanbruntiae Britt.), a facultative wetland plant considered rare throughout its range. We monitored life-stage transitions in 10 local populations in Canaan Valley, West Virginia, from 2005–2007, modeled the population growth rates, and estimated extinction rates for each population accounting for the loss of seeds due to white-tailed deer florivory. Seed loss due to consumption of flowering stems ranged between 0 and 96% within individual populations ( = 52 ± 4.5%). A significant difference in seed production occurred between browsed ( = 0.6 ± 0.18) and unbrowsed ( = 24 ± 1.43) plants. Predicted seed loss was significantly higher ( = 57 ± 19%), where no hunting was allowed, than where deer hunting occurred ( = 40 ± 18%). The observed levels of white-tailed deer florivory have the potential to significantly reduce population growth rates (p < 0.0001). Although white-tailed deer florivory may not increase local population extinction rates, loss of seed production may result in a loss of the potential for colonization of new patches.

Ecological and evolutionary effects of selective harvest of non‐lactating female ungulates

Summary Female ungulates are often selectively harvested according to their reproductive status. Because ungulate population growth depends heavily on adult female survival, it is crucial to understand the effects of this selective harvest. Recent studies revealed persistent individual differences in female reproductive potential, with a positive correlation of reproductive success over consecutive years. If current reproduction is correlated with lifetime reproductive success, then selective harvest of non‐lactating females should remove individuals of low reproductive potential, with lower impact on population growth than random harvest. If lifetime reproductive success has a genetic basis, selective harvest may also increase the proportion of successful females. We used an individual‐based model to understand the short‐term effects of harvest intensity and hunter selectivity on population dynamics, accounting for both heterogeneity in reproductive potential and orphan survival. We also explored the long‐term effect of harvest as a selective pressure on female heterogeneity. Selective harvest of non‐lactating females reduced survival to primiparity compared to random harvest, because of high harvest rates of pre‐reproductive females. After primiparity, however, females of higher reproductive potential had higher survival under selective than random harvest. Therefore, the overall effect on population dynamics depends on a trade‐off between a high harvest of pre‐reproductive females and a reduced harvest of reproductive females with high reproductive potential. Female heterogeneity and the length of the pre‐reproductive period affected this trade‐off. Over the short term, high heterogeneity in reproductive potential of pre‐reproductive females made selective harvest the most effective strategy to maintain a high population growth rate. With low heterogeneity and little effects of orphaning on juvenile mortality, however, random harvest had a lower impact on population growth than selective harvest. Over the long term, selective female harvest may increase the proportion of successful reproducers in the population. Synthesis and applications. Selective harvests of non‐lactating females appear justified only if female heterogeneity in reproductive potential and/or orphan mortality are very high. Because pre‐reproductive females will be subject to intense harvest, selective harvest may reduce population growth rate compared to random harvest in species with late primiparity, especially if most pre‐reproductive female normally survive to primiparity. When heterogeneity in reproductive potential and orphan mortality are low, random female harvest appears preferable to selective harvest.

Impacts of changing rainfall regime on the demography of tropical birds

Increasing dry season length in central Panama reduced population growth rates and viability in nearly one-third of the 20 tropical bird species investigated. Such changes are projected to alter tropical bird community structure in protected areas. Biodiversity in tropical regions is particularly high and may be highly sensitive to climate change1,2. Unfortunately, a lack of long-term data hampers understanding of how tropical species, especially animals, may react to projected environmental changes. The amount and timing of rainfall is key to the function of tropical ecosystems and, although specific model predictions differ3,4, there is general agreement that rainfall regimes will change over large areas of the tropics5,6. Here, we estimate associations between dry season length (DSL) and the population biology of 20 bird species sampled in central Panama over a 33-year period. Longer dry seasons decreased the population growth rates and viability of nearly one-third of the species sampled. Simulations with modest increases in DSL suggest that consistently longer dry seasons will change the structure of tropical bird communities. Such change may occur even without direct loss of habitat—a finding with fundamental implications for conservation planning. Systematic changes in rainfall regime may threaten some populations and communities of tropical animals even in large tracts of protected habitat. These findings suggest the need for collaboration between climate scientists and conservation biologists to identify areas where rainfall regimes will be able to plausibly maintain wildlife populations.

Prioritizing management actions for invasive populations using cost, efficacy, demography and expert opinion for 14 plant species world-wide.

Summary Management of invasive populations is typically investigated case‐by‐case. Comparative approaches have been applied to single aspects of management, such as demography, with cost or efficacy rarely incorporated.We present an analysis of the ranks of management actions for 14 species in five countries that extends beyond the use of demography alone to include multiple metrics for ranking management actions, which integrate cost, efficacy and demography (cost‐effectiveness) and managers’ expert opinion of ranks. We use content analysis of manager surveys to assess the multiple criteria managers use to rank management strategies.Analysis of the matrix models for managed populations showed that all management actions led to reductions in population growth rate (λ), with a median 48% reduction in λ across all management units; however, only 66% of the actions led to declining populations (λ < 1).Each management action ranked by cost‐effectiveness and cost had a unique rank; however, elasticity ranks were often tied, providing less discrimination among management actions. Ranking management actions by cost alone aligned well with cost‐effectiveness ranks and demographic elasticity ranks were also well aligned with cost‐effectiveness. In contrast, efficacy ranks were aligned with managers’ ranks and managers identified efficacy and demography as important. 80% of managers identified off‐target effects of management as important, which was not captured using any of the other metrics. Synthesis and applications. A multidimensional view of the benefits and costs of management options provides a range of single and integrated metrics. These rankings, and the relationships between them, can be used to assess management actions for invasive plants. The integrated cost‐effectiveness approach goes well ‘beyond demography’ and provides additional information for managers; however, cost‐effectiveness needs to be augmented with information on off‐target effects and social impacts of management in order to provide greater benefits for on‐the‐ground management.

Contrasting effects of spatial heterogeneity and environmental stochasticity on population dynamics of a perennial wildflower

Summary Understanding how variation in growth, survival and reproduction affect population dynamics is a fundamental question in ecology. Although the effects of among‐year variation (environmental stochasticity) are well understood, the effects of among‐site variation (spatial heterogeneity) are less clearly defined. I evaluated the effects of spatial and temporal variation on the population dynamics of Pulsatilla patens, pasqueflower, a perennial prairie forb. I conducted a 10‐year demographic monitoring study, and quantified vital rate variation among sites and years using generalized linear models. I incorporated vital rate functions using this variation into integral projection models for stochastic and spatially heterogeneous environments. I also explored the effects of temporal and spatial autocorrelation, by exploring model predictions over the range of possible values for temporal autocorrelation and local seed dispersal. Vital rates varied more among years than among sites. However, environmental stochasticity and spatial heterogeneity had similar magnitude effects on population dynamics. These effects were also qualitatively different: environmental stochasticity reduced population growth rates relative to the average, whereas spatial heterogeneity increased population growth rates. Spatial autocorrelation and negative temporal autocorrelation led to higher population growth rates, although environmental stochasticity still reduced growth rates for all autocorrelation values, and spatial heterogeneity increased growth rates for all autocorrelation values. Some form of autocorrelation would be necessary for model projections to match observed population trends. Synthesis. Spatial heterogeneity is as important as environmental stochasticity for population dynamics, but it is much less often incorporated into population projection models. This study points to a number of interesting avenues for future research into the roles of spatial heterogeneity and spatiotemporal variation for long‐term population dynamics.

Field-evolved resistance to Cry1Ab maize by Spodoptera frugiperda in Brazil.

The first Bt maize in Brazil was launched in 2008 and contained the MON 810 event, which expresses Cry1Ab protein. Although the Cry1Ab dose in MON 810 is not high against fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), MON 810 provided commercial levels of control. To support insect resistance management in Brazil, the baseline and ongoing susceptibility of FAW was examined using protein bioassays, and the level of control and life history parameters of FAW were evaluated on MON 810 maize. Baseline diet overlay assays with Cry1Ab (16 µg cm(-2) ) caused 76.3% mortality to field FAW populations sampled in 2009. Moderate mortality (48.8%) and significant growth inhibition (88.4%) were verified in leaf-disc bioassays. In greenhouse trials, MON 810 had significantly less damage than non-Bt maize. The surviving FAW larvae on MON 810 (22.4%) had a 5.5 day increase in life cycle time and a 24% reduction in population growth rate. Resistance monitoring (2010-2015) showed a significant reduction in Cry1Ab susceptibility of FAW over time. Additionally, a significant reduction in the field efficacy of MON 810 maize against FAW was observed in different regions from crop season 2009 to 2013. The decrease in susceptibility to Cry1Ab was expected, but the specific contributions to this resistance by MON 810 maize cannot be distinguished from cross-resistance to Cry1Ab caused by exposure to Cry1F maize. Technologies combining multiple novel insecticidal traits with no cross-resistance to the current Cry1 proteins and high activity against the same target pests should be pursued in Brazil and similar environments. © 2015 Society of Chemical Industry.

Maybe Next Month? Temperature Shocks, Climate Change, and Dynamic Adjustments in Birth Rates

Dynamic adjustments could be a useful strategy for mitigating the costs of acute environmental shocks when timing is not a strictly binding constraint. To investigate whether such adjustments could apply to fertility, we estimate the effects of temperature shocks on birth rates in the United States between 1931 and 2010. Our innovative approach allows for presumably random variation in the distribution of daily temperatures to affect birth rates up to 24 months into the future. We find that additional days above 80 °F cause a large decline in birth rates approximately 8 to 10 months later. The initial decline is followed by a partial rebound in births over the next few months implying that populations can mitigate the fertility cost of temperature shocks by shifting conception month. This dynamic adjustment helps explain the observed decline in birth rates during the spring and subsequent increase during the summer. The lack of a full rebound suggests that increased temperatures due to climate change may reduce population growth rates in the coming century. As an added cost, climate change will shift even more births to the summer months when third trimester exposure to dangerously high temperatures increases. Based on our analysis of historical changes in the temperature-fertility relationship, we conclude air conditioning could be used to substantially offset the fertility costs of climate change.

Plant fitness in a rapidly changing world

Modern reliance on fossil fuels has ushered in extreme temperatures globally and abnormal precipitation patterns in many regions. Although the climate is changing rapidly, other agents of natural selection such as photoperiod remain constant. This decoupling of previously reliable environmental cues shifts adaptive landscapes, favors novel suites of traits and likely increases the extinction risk of local populations. Here, I examine the fitness consequences of changing climates. Meta-analyses demonstrate that simulated future climates depress viability and fecundity components of fitness for native plant species in the short term, which could reduce population growth rates. Contracting populations that cannot adapt or adjust plastically to new climates might not be capable of producing sufficient migrants to track changing conditions.

Effects of precipitation change and neighboring plants on population dynamics of Bromus tectorum.

Shifting precipitation patterns resulting from global climate change will influence the success of invasive plant species. In the Front Range of Colorado, Bromus tectorum (cheatgrass) and other non-native winter annuals have invaded grassland communities and are becoming more abundant. As the global climate warms, more precipitation may fall as rain rather than snow in winter, and an increase in winter rain could benefit early-growing winter annuals, such as B. tectorum, to the detriment of native species. In this study we measured the effects of simulated changes in seasonal precipitation and presence of other plant species on population growth of B. tectorum in a grassland ecosystem near Boulder, Colorado, USA. We also performed elasticity analyses to identify life transitions that were most sensitive to precipitation differences. In both study years, population growth rates were highest for B. tectorum growing in treatments receiving supplemental winter precipitation and lowest for those receiving the summer drought treatment. Survival of seedlings to flowering and seed production contributed most to population growth in all treatments. Biomass of neighboring native plants was positively correlated with reduced population growth rates of B. tectorum. However, exotic plant biomass had no effect on population growth rates. This study demonstrates how interacting effects of climate change and presence of native plants can influence the population growth of an invasive species. Overall, our results suggest that B. tectorum will become more invasive in grasslands if the seasonality of precipitation shifts towards wetter winters and allows B. tectorum to grow when competition from native species is low.

Integral projection models show exotic thistle is more limited than native thistle by ambient competition and herbivory

Both competitors and natural enemies can limit plant population growth. However, demographic comparisons of the effects of these interactions on introduced versus co‐occurring, related native species are uncommon. We asked: (1) does plant competition, insect herbivory, or their combination reduce population growth rate, log λ, of the Eurasian thistle Cirsium vulgare sufficiently to explain its limited invasiveness in western tallgrass prairie; and (2) how do the effects of these interactions compare to those for C. altissimum, its co‐occurring, synchronously‐flowering native congener? We developed integral projection models (IPMs) to estimate log λ for both species, using parameter estimates from field experiments. Our models predicted that the growth potential (growth rate at minimal competition and herbivory) for the introduced thistle (log λ = 3.5 (2.5, 4.6)) was twice as large as for the native thistle (log λ = 1.6 (0.4, 3.1)); however, a high level of competition and ambient insect herbivory together reduced log λ to similar values for both thistle species (C. vulgare: log λ = −1.3 (−2.4, −0.3) vs C. altissimum: log λ = −0.9 (−1.4, −0.3)). This suggests that the introduced thistle was more affected by competition and insect herbivory. For the introduced thistle, neither competition nor insect herbivory alone led to negative log λ. In contrast, for the native thistle, high competition alone also led to negative population growth (log λ = −0.8, percentile limits do not overlap with zero). Ambient herbivory alone prevented the spread for both thistle species (percentile limits include zero). Overall, the results show that interspecific competition followed by ambient levels of insect herbivory strongly constrained log λ for both thistles, limiting C. vulgare invasiveness and C. altissimum abundance. The outcome highlights the importance of synergy between the two biological interactions in limiting plant population growth. Improved understanding of mechanisms limiting log λ for weedy plants enhances our ability to predict when biotic resistance will contribute to invasive plant species management.

Context-dependent survival, fecundity and predicted population-level consequences of brucellosis in African buffalo.

Chronic infections may have negative impacts on wildlife populations, yet their effects are difficult to detect in the absence of long-term population monitoring. Brucella abortus, the bacteria responsible for bovine brucellosis, causes chronic infections and abortions in wild and domestic ungulates, but its impact on population dynamics is not well understood. We report infection patterns and fitness correlates of bovine brucellosis in African buffalo based on (1) 7years of cross-sectional disease surveys and (2) a 4-year longitudinal study in Kruger National Park (KNP), South Africa. We then used a matrix population model to translate these observed patterns into predicted population-level effects. Annual brucellosis seroprevalence ranged from 8·7% (95% CI=1·8-15·6) to 47·6% (95% CI=35·1-60·1) increased with age until adulthood (>6) and varied by location within KNP. Animals were on average in worse condition after testing positive for brucellosis (F=-5·074, P<0·0001), and infection was associated with a 2·0 (95% CI=1·1-3·7) fold increase in mortality (χ(2) =2·039, P=0·036). Buffalo in low body condition were associated with lower reproductive success (F=2·683, P=0·034), but there was no association between brucellosis and pregnancy or being observed with a calf. For the range of body condition scores observed in the population, the model-predicted growth rate was λ=1·11 (95% CI=1·02-1·21) in herds without brucellosis and λ=1·00 (95% CI=0·85-1·16) when brucellosis seroprevalence was 30%. Our results suggest that brucellosis infection can potentially result in reduced population growth rates, but because these effects varied with demographic and environmental conditions, they may remain unseen without intensive, longitudinal monitoring.