Negative Density-dependent Effects Research Articles

Effects of stocking density and shelter on the behavior, growth, and survival of Stichopus cf. horrens juveniles

Stichopus cf. horrens is an emerging sea cucumber species in tropical aquaculture. However, stocking density must be optimized. This study investigated the effects of stocking density (278 ind. m−2, 556 ind. m−2, and 1111 ind. m−2) and the presence or absence of polyvinyl chloride (PVC) pipes as shelters on the growth performance and survival of juveniles (0.4 to 1.0 cm) in the field over 30 days. The activity pattern, sheltering behavior, and aggregation density of juveniles (1.5 to 2.0 cm) at different stocking densities (144 ind. m−2, 289 ind. m−2, and 578 ind. m−2) were further evaluated in the laboratory for 15 days. Results revealed negative density-dependent effects on the absolute growth rate (AGR) and the coefficient of variation in length among the juveniles. The presence of PVC pipes did not significantly improve AGR of juveniles in all density treatments by day 30. However, survival rates were higher in sheltered treatments compared to the non-sheltered. During inactive hours, aggregations of juveniles were observed on the undersides of the PVC pipes, with high-density aggregations prevalent only at medium and high stocking densities, but not at low stocking densities. The direct physical contact during these aggregations likely exacerbated the effects of crowding and increased the risk of exposure to the cytotoxic chemicals produced by conspecifics. Designing shelter models that reduce the probability of encounter during shelter-seeking, while enhancing epiphytic growth and grazing efficiency, is crucial for mitigating the negative effects of density on this high-value sea cucumber species.

Extreme population densities reduce reproductive effort of Atlantic sea scallops in high-density recruitment events

The Atlantic sea scallop Placopecten magellanicus supports one of the most valuable federal fisheries in the USA, with annual ex-vessel values of US$400-600 million since 2010. Among other strategies, the fishery utilizes rotational area management to protect juvenile sea scallops, increasing yield per recruit and spawning potential. While generally successful, area management was challenged by 2 extremely high-density recruitment events. Juveniles at both study sites, the Nantucket Lightship Closed Area and the Elephant Trunk portion of the Mid-Atlantic Access Area, persisted at high densities (up to 39 and 5 sea scallops m-2, respectively) and initially exhibited poor growth, yield, and gamete production. The effect of sea scallop population density on reproduction was investigated through quarterly sampling from May 2018 through January 2020 in low-, medium-, and high-density strata. Reproductive effort, i.e. the proportion of energy devoted to gamete production, was quantified to investigate differences in energy allocation across density, depth, shell height, reproductive stage, and sex. Reproductive activity was limited in the Nantucket Lightship high-density stratum, where the percentage of sea scallops staged as mature or spawning reached 50% during only 1 of 7 sampling trips, compared to 4-6 sampling trips in other strata. Population density was a significant predictor of reproductive effort, with a 28% reduction in reproductive effort from the highest densities to more typical densities. These results illustrate the complexities of managing fisheries for heterogeneous populations of sessile benthic invertebrates. Negative density-dependent effects should be monitored and accounted for in future extreme recruitment events.

Substrate scent-induced disproportionate seed dispersal by rodents.

Conspecific adults impose strong negative density-dependent effects on seed survival nearby parent trees, however, the underlying mechanisms are diversified and remain unclear. In this study, we presented consistent evidence that parent-scented forest floor masked seed odor, reduced cache recovery rate by scatter-hoarding animals, and then increased seed dispersal far away from mother trees. Our results showed that seed odors of Korean pine Pinus koraiensis match well with the volatile profile of their forest floor. Moreover, scatter-hoarding animals selectively transported P. koraiensis seeds toward the areas where seed odor was more contrasting against the background substrate, possibly due to the fact that accumulation of conspecific volatile compounds in caches hindered seed detection by scatter-hoarding animals. Our study provides insight into the role of leaf litter in directing seed dispersal process, representing a novel mechanism by which P. koraiensis increases selection for seed dispersal far away from the parent tree.

The delayed demographic responses of small mammals to habitat quality and density in the Brazilian Cerrado

Understanding the effects of exogenous and endogenous factors on species demographic rates is crucial to investigate their chances of persistence on natural ecosystems. We used a 9-year time series, based on monthly captures, to test the delayed effects of endogenous (density) and exogenous (rainfall, habitat cover/heterogeneity, fire) factors on the survival and recruitment rates of two small mammals inhabiting the Brazilian Cerrado: the semelparous, scansorial marsupial Gracilinanus agilis and the iteroparous, arboreal rodent Rhipidomys macrurus. As in most short-lived small mammals, we found negative density-dependent effects on the recruitment of both species (3 months delay), which could occur through reduced immigration or fecundity, in addition to the semelparous breeding strategy of G. agilis. Reduced habitat cover following fire events had a negative delayed effect on the survival of G. agilis (3 months after) and R. macrurus (1 year after), albeit it increased the recruitment rates (1-year lagged effect) of R. macrurus. We failed to find any effect of fire on demography of either species. Our results suggest that the irregular and non-cyclical fluctuations in R. macrurus abundance were driven by the interaction between delayed density dependence and changes in habitat quality, while the marked and repeated fluctuations in G. agilis abundance were mainly shaped by the occurrence of semelparity associated with direct density dependence. We emphasize that changes in habitat quality brought by human-made activities may have severe impacts on the dynamics and persistence of forest-dependent small mammals in the Cerrado.

Breeders’ age, nest-site characteristics and climatic conditions but not density-dependent effects determine Bonelli’s Eagle breeding performance: A long-term study (2002–2021)

Abstract The conservation of endangered species is a crucial topic in conservation biology due to the important ecological roles these species play in their respective ecosystems. Top predators are particularly vulnerable to the variation in environmental conditions. The Bonelli’s Eagle (Aquila fasciata) is a long-lived bird of prey that fulfills its role as a top predator in Mediterranean ecosystems, but its overall population has declined in the Iberian Peninsula in recent decades. As with other long-lived species, the Bonelli’s Eagle reaches its maximum fitness at maturity, highlighting the importance of assessing how intrinsic and extrinsic factors affect population dynamics over time. In this study, we conducted annual surveys of a Bonelli’s Eagle subpopulation in the eastern Iberian Peninsula from 2002 to 2021. Our results indicate that the large decline observed in the number of occupied territories, fledged chicks, and successful pairs is not due to negative density-dependent effects but rather influenced by various environmental and demographic factors. These factors include breeders’ age, nest-site characteristics, climatic conditions, and previous breeding costs. We found that productivity was higher in territories with high winter precipitation and temperatures, as well as in coastal areas, leading to earlier laying dates. Additionally, the presence of subadult individuals in the breeding pair decreased breeding performance. Interestingly, the variability in the number of fledglings in each territory was driven by previous breeding costs. Prioritizing efforts to mitigate adult mortality and conserve suitable habitats becomes imperative to halt the ongoing decline of this population and also to enhance productivity, ultimately facilitating the restoration of the species to previous population levels.

Male and female Brandt’s voles have higher reproductive success when they have more mating partners regardless of population density

Abstract Although population density and body mass are known to alter the mating strategies of individuals, their impacts on reproductive success have not been fully investigated. We examined genetic mating system variation (estimated number of mating partners), reproductive success (offspring number), and sperm competition intensity (sires per litter) in semi-natural populations of Brandt’s voles (Lasiopodomys brandtii) under low-, medium- and high-densities. We used 12 microsatellite genetic markers and parentage assignment of 3516 voles, with the aim of identifying the estimated number of mating partners and offspring produced by founder individuals. We provided strong evidence that individuals with more mating partners experienced increased reproductive success in all density groups, as measured by the production of more offspring, compared with individuals that had fewer mating partners. Further, we found that large males had more mating partners (especially in high-population density enclosures) and also produced more offspring relative to smaller males. In high-density (HD) enclosures, the average reproductive success per female vole was lower (a negative density-dependent effect), but voles had more mating partners, more sires per litter, but lower paternity skew compared to low-density enclosures. Our results suggests that Brandt’s voles increased reproductive success through increasing mating partners under HD stress which may have significant implications in offsetting the negative density-dependency effect on population growth.

Tree mycorrhizal type mediates conspecific negative density dependence effects on seedling herbivory, growth, and survival.

Tree mycorrhizal type plays an important role in promoting plant species diversity and coexistence, via its mediating role in conspecific negative density dependence (CNDD), i.e., the process by which an individual's performance is impaired by the density of conspecific plants. Previous findings suggest that ectomycorrhizal (EM) tree species are generally less susceptible to CNDD than arbuscular mycorrhizal (AM) tree species, due to the chemical and physical protection that EM fungi provide their host with. We examined how CNDD effects on leaf herbivory, seedling growth, and survival differ between AM and EM seedlings of ten tree species collected over 3years in an old-growth temperate forest in northeastern China. We found that AM and EM seedlings differed in how conspecific density affected their leaf herbivory, seedling growth, and survival. Specifically, AM seedlings leaf herbivory rates significantly increased with increasing conspecific seedling and adult density, and their growth and survival rates decreased with increasing conspecific adult density, these patterns were, however, absent in EM seedlings. Our work suggests that AM seedlings have a performance disadvantage relative to EM seedlings related to the negative effects from conspecific neighbors. We highlight the importance of integrating information on seedling leaf herbivory, seedling growth, to provide further understanding on potential mechanisms driving differences in CNDD between AM and EM tree seedlings.

Rare plant species are at a disadvantage when both herbivory and pollination interactions are considered in an alpine meadow.

Rare plant species often suffer less damage than common species because of positive density-dependent herbivory, and it has been suggested that this 'rare species advantage' fosters plant species coexistence. However, it is unknown whether rare species have an advantage when pollination interactions are also considered. We hypothesized that a 'positive density-dependent pollination success' across plant species would result in common plants experiencing higher seed set rates compared to rare species, and that positive density-dependent effects would negate or even override the positive density-dependent damage due to herbivory resulting in higher seed loss rates in common plant species. We tested this hypothesis by concurrently examining a plant-predispersal seed predator system and a plant-pollinator system for 24 Asteraceae species growing in an alpine meadow community (Sichuan Province, China). Having previously reported a positive density-dependent effect on seed loss rates due to seed predators, we here focus on the density-dependent effects on pollination success by investigating pollinator species richness, visitation frequencies and seed set rates for each plant species. We also estimated the seed output rate of each plant species as the product of seed set rate and the rate of surviving seeds (i.e. 1-the seed loss rate). Consistent with our hypothesis, a positive density-dependent effect was observed for pollinator species richness, visitation frequencies and seed set rates across plant species. Moreover, the positive effect overrode the negative density-dependent effect of herbivores on seed production, such that common species tended to have a higher seed output rate than rare species (i.e. we observed a 'rare species disadvantage'). These results indicate that the low seed output rate of rare species might result from a pollination limitation, and that both mutualistic and antagonistic interactions should be examined simultaneously to fully understand plant species coexistence in local communities.

Soil fungal networks moderate density-dependent survival and growth of seedlings.

Pathogenic and mutualistic fungi have contrasting effects on seedling establishment, but it remains unclear whether density-dependent survival and growth are regulated by access to different types of mycorrhizal fungal networks supported by neighbouring adult trees. Here, we conducted an extensive field survey to test how mycorrhizal and pathogenic fungal colonization of arbuscular mycorrhizal (AM) and ectomycorrhizal (ECM) seedlings in a subtropical forest respond to density of neighbouring adult trees. In addition, we undertook a hyphal exclusion experiment to explicitly test the role of soil fungal networks in driving density-dependent effects on seedling growth and survival. Conspecific adult density was a strong predictor for the relative abundance of putative pathogens, which was greater in roots of AM than of ECM seedlings, while mycorrhizal fungal abundance and colonization were not consistently affected by conspecific adult density. Both ECM and AM fungal networks counteracted conspecific density-dependent mortality, but ECM fungi were more effective at weakening the negative effects of high seedling density than AM fungi. Our findings reveal a critical role of common fungal networks in mitigating negative density-dependent effects of pathogenic fungi on seedling establishment, which provides mechanistic insights into how soil fungal diversity shapes plant community structure in subtropical forests.

Revisiting biotic and abiotic drivers of seedling establishment, natural enemies and survival in a tropical tree species in a West Africa semi-arid biosphere reserve

Biotic and abiotic drivers of seedling establishment and survival are fundamental not only for elucidating processes occurring at plant early life stages, but also for assisting species natural regeneration. Keystone, multipurpose and economically important tree species such as Afzelia africana Sm. are reportedly facing recruitment constraints, yet little is known about how abiotic and biotic factors shape the species seedling dynamics. Here, we monitored the species seedlings over one year across three seasons in West Africa savannahs to determine how conspecific and heterospecific biotic neighborhood and habitat heterogeneity correlate with initial seedling density, leaves’ fungal infection and herbivory and how all these factors combined, influence the species seedling survival.Seedling densities increased with increasing conspecific adult densities, and were highest in tree savannahs and on sandy-silt soils. Leaves’ fungal infection and herbivory were also positively associated with conspecific adult density, but were more abundantly observed in tree savannahs than in shrub savannahs. Seedling survival was constrained on higher slope, and negatively affected by conspecific adult density, especially in shrub savannahs.There was a strong evidence for negative density-dependence effects of conspecific adults on seedling survival, which operated through negative effects of herbivory and fungal infection. Habitat heterogeneity was also an important driver, which modulated biotic factors’ effects on seedling survival: tree savannahs promote positive conspecific density-dependence of seedling fungal infection and herbivory more than shrub savannahs. Nonetheless, seedlings were more sensitive to natural enemies in shrub savannahs, suggesting increased negative conspecific density-dependence effects on seedling survival in less dense vegetation, possibly as a result of enhanced specialization of predators and pathogens on a limited set of species. The study brings important insights into the mechanisms that drive the establishment and survival of the species seedling, which should be considered in the design of management activities aiming at the conservation of this endangered species.

Potential impact of mammal defaunation on the early regeneration of a large-seeded palm in the Brazilian Atlantic Forest

Defaunation, the decline in animal species and populations, is biased towards large-bodied animals that have unique roles as dispersers of large seeds. However, it is speculated that these roles may still be performed by smaller animals, such as small mammals like rodents and marsupials, that thrive in defaunated sites. We investigated if small mammals can disperse the large-seeded palm Attalea dubia. We performed the study in a well-conserved Atlantic Forest remnant in southeast Brazil that still harbours large mammals, such as tapirs. Focal observations showed that capuchin-monkeys consumed the mesocarp of the fruits and dropped the seeds beneath the plant crown thereafter. Mammals preyed on ca. 1% and removed ca. 15% of the fallen fruit/seed and deposited them up to 15 m away. Amongst them, small mammals (< 1 kg), such as the squirrel Guerlinguetus brasiliensis and non-identified nocturnal Sigmodontinae, as well as the marsupial Philander frenatus performed the bulk of interactions. Dispersal enhances recruitment, but the short distances of seed removal did not match the current spatial distribution of palm seedlings and juveniles. Recaching rates of hoarded seeds were small (2%) and unlikely to increase distances of seed dispersal achieved. Short distances of dispersal would increase plant clumpiness and negative density-dependent effects with time. Although small mammals can provide legitimate dispersal, they cannot fully replace larger frugivorous mammals and maintain long-distance seed dispersal that feeds plant metapopulation dynamics and seed gene flow.

Soil nitrogen availability intensifies negative density-dependent effects in a subtropical forest

Abstract Aims The importance of density-dependent mortality in maintaining tree species diversity is widely accepted. However, density-dependent effects may vary in magnitude and direction with different abiotic conditions in forests. Theoretical predictions surmise that density-dependent effects may vary with soil available nitrogen (AN), but this still needs to be tested. Methods We analyzed the density-dependent effects on survival of newly germinated seedlings for 18 common species based on a long-term seedling census across environmental gradients in a subtropical forest. We also conducted a root lesion detection experiment for five species to investigate the potential effects of pathogens on variation in density-dependent disease between rich and poor AN environments. Important Findings The seedling dynamics analysis revealed that the strength of density-dependent effects increased with AN, shifting from neutral or positive with low AN to negative with high AN. Three of the five tree species had stronger density-dependent effects on root lesions in rich AN environments than in poor AN environments, which is consistent with the results of a long-term seedling dynamics analysis. We also found higher species diversity in rich AN environments, which may be promoted by the stronger negative density-dependent effects. Both the seedling dynamic analysis and root lesion detection experiment revealed stronger negative density-dependent effects in higher AN environment, resulting from stronger disease pressure by soil pathogens. Our study emphasized the importance of considering context dependence when testing the density dependence hypotheses.

Effects of density dependence and climatic factors on population dynamics of Cricetulus barabensis: a 25-year field study

AbstractRodents often act as keystone species in communities and play important roles in shaping structures and functions of many ecosystems. Understanding the underlying mechanisms of population fluctuation in rodents is therefore of great interest. Using the data from a 25-year field survey carried out in Inner Mongolia, China, we explored the effects of density dependence, local climatic factors, and a large-scale climatic perturbation (El Niño–Southern Oscillation) on the population dynamics of the striped hamster (Cricetulus barabensis), a rodent widely distributed in northern China. We detected a strong negative density-dependent effect on the population dynamics of C. barabensis. Rainfall had a significant positive effect on population change with a 1-year lag. The pregnancy rate of C. barabensis was negatively affected by the annual mean temperature in the current year, but positively associated with the population density in the current year and the annual Southern Oscillation Index in the previous year. Moving-window analyses suggested that, with a window length of 12 years, there was a significant interaction between rainfall and density dependence, with increasing rainfall alleviating the negative effect of density dependence. As C. barabensis often causes agricultural damage and can transmit zoonotic diseases to human beings, our results also have implications for pest and disease control.

Janzen-Connell effects in a forest BEF experiment: Strong distance-dependent seedling establishment of multiple species.

The Janzen-Connell (JC) hypothesis is a major ecological explanation for high species richness, in particular in tropical forest ecosystems. Central components of the JC hypothesis are noncompetitive effects of distance and density dependence, two drivers that contribute independently to species coexistence, but are ultimately linked in the field. However, although numerous studies provide evidence for either distance- or density-dependent effects based on observational data, experimental testing of simultaneous and interactive effects of distance and density has rarely been conducted, especially in a comprehensive multispecies approach. Here, we make use of the forest Biodiversity-Ecosystem Functioning project (BEF) -China to estimate distance- and density-dependent effects in a reciprocal tree seedling transplant experiment of 11 tree species. We deployed 13,490 juveniles of all 11 species in their own (home) and in all foreign monocultures (away), as well as at three different levels of planting density, thereby testing for distance and density effects, respectively. In addition, to quantify the amount to which density effects were brought about by potential additional effects of intraspecific competition, we set up a common garden experiment with different levels of planting density, where an additional "shadow" treatment controlled for effects of canopy shading. Although the "away" and "high-density" treatments significantly impaired the performance and productivity of seedlings, leaf damage and survival was exclusively affected by either the home/away or the density treatment, respectively. Negative density-dependent effects on leaf damage were less pronounced in the "home" treatment, showing that the effects were not additive. In addition, results obtained in the Common Garden Experiment showed that negative effects of high density may be also brought about by intraspecific competition as an alternative density-responsive mechanism and less by true JC effects. Overall, our results provide strong support on a multispecies basis for the influence of host-specific effects already operating in early stages of a forest plantation. However, they also emphasize the need to account appropriately for potential additional density-responsive mechanisms such as intraspecific competition or microenvironmental conditions when addressing the role of JC effects for species coexistence.

Density-dependent growth in \u2018catch-and-wait\u2019 fisheries has implications for fisheries management and Marine Protected Areas

Stock enhancement activities provide an opportunity to examine density-dependent suppression of population biomass which is a fundamental issue for resource management and design of no-take-zones. We document ‘catch-and-wait’ fisheries enhancement where all but the largest lobsters are thrown back, recapturing them later after they have grown to a larger size. The residency, rate of return, and potential negative density-dependent effects of this activity are described using a combination of tagging and v-notching and by relating spatial growth patterns to population density defined with Catch Per Unit Effort. The results successfully demonstrated the concept of catch-and-wait practices. However, a density-dependent suppression of growth (in body size) was observed in male lobsters. This demonstrates a mechanism to explain differences in lobster sizes previously observed across EU fishing grounds with different stock densities. This negative effect of density could also affect individual biomass production in marine reserve or no-take zones.

Multiple density-dependent processes shape the dynamics of a spatially structured amphibian population.

Understanding the mechanisms that regulate the dynamics of spatially structured populations (SSP) is a critical challenge for ecologists and conservation managers. Internal population processes such as births and deaths occur at a local level, while external processes such as dispersal take place at an inter-population level. At both levels, density dependence is expected to play a critical role. At a patch scale, demographic traits (e.g., survival, breeding success) and the population growth rate can be influenced by density either negatively (e.g., competition effect) or positively (e.g., Allee effects). At the scale of an SSP, although positive density-dependent dispersal has been widely reported, an increasing number of studies have highlighted negative density-dependent dispersal. While many studies have investigated the effects of density on population growth or on dispersal, few have simultaneously examined density-dependent effects at the scale of both the local population and the entire SSP. In this study, we examine how density is related to demographic processes at both the pond level (survival and population growth) and the SSP level (between-pond dispersal) in a pond-breeding amphibian, the great crested newt (Triturus cristatus). The study was based on 20years of individual capture-recapture (CR) data (from 1996 to 2015) gathered from an SSP made up of 12 experimental ponds ("patches"). We first used a CR multievent model to estimate both survival and dispersal rates in specific ponds as a function of distance between ponds. Then, using a second CR multievent model, we examined whether survival and recapture rates were influenced by population density in a pond. Lastly, we used state-space time series models to investigate whether density affected population growth in each pond. Our results found a positive density-dependent effect on survival and a negative density-dependent effect on departure. In addition, the findings indicate that population growth was negatively related to density in all 12 ponds. These results support the hypothesis that in SSPs, density may have multiple and contrasting effects on demographic parameters and growth rates within local populations as well as on dispersal. This study underlines the need to better understand how density dependence may influence potential trade-offs between life-history strategies and life-history stages.

Resolving the paradox of clumped seed dispersal: positive density and distance dependence in a bat‐dispersed species

One of the hypothesized benefits of seed dispersal is to escape density- and distance-responsive, host-specific, natural enemies near maternal plants where conspecific seed and seedling densities are high. Such high conspecific neighbor densities typically result in lower offspring growth and survival (i.e., negative density-dependent effects), yet many dispersal modes result in clumped seed distributions. New World leaf-nosed bats transport fruits to their feeding roosts and deposit seeds, thereby creating high-density seed/seedling patches beneath feeding roosts in heterospecific trees away from maternal trees, which seemingly nullifies a key benefit of seed dispersal. Such dispersal may still be adaptive if negative density-dependent effects are reduced under feeding roosts or if the benefit of being dispersed away from maternal trees outweighs negative effects of conspecific seed/seedling density below roosts. We mapped the entire post-germination population of a bat-dispersed tree species Calophyllum longifolium (Calophyllaceae) in a 50-ha plot on Barro Colorado Island, Panama in each of three successive years. We tested two hypotheses: (1) distance-dependent effects are stronger than density-dependent effects on seedling performance because seedlings far from conspecific adults are more likely to escape natural enemies even when at high densities and (2) negative density-dependent effects will be reduced far from vs. near conspecific adults. Density and distance were naturally decoupled, as expected. However, in contrast to our expectation, we found positive density effects on seedling survival and density-dependent effects did not differ with distance from conspecific adults. Both density and distance had positive effects on seedling survival when considered together, while only year had a significant effect on seedling growth. Thus, both being dispersed under bat feeding roosts and escaping the vicinity of conspecific adults were beneficial for C.longifolium seedling survival, supporting the directed dispersal and escape hypotheses, respectively. Despite resulting in high densities of conspecific seedlings, favorable habitat under bat feeding roosts and lack of negative density-dependent effects appear to provide evolutionary advantages in C.longifolium.

Defaunation impacts on seed survival and its effect on the biomass of future tropical forests

Large animal species, which provide important ecological functions such as dispersal of seeds or top–down control of seed predators, are very vulnerable in fragmented forests, being unable to survive in small fragments, and facing increasing hunting pressure. The loss of large animals affects two main ecological processes crucial for the tree reproductive cycle: seed dispersal of large seeds (e.g. provided by tapirs) and control of seed predator population (e.g. provided by large cats). The changes in both processes are expected to increase seed mortality since seeds are not dispersed away from conspecifics (causing increased pre‐dispersal mortality due to negative density dependent effects) and/or face increased predation after a dispersal event (post‐dispersal mortality). Although an extensive body of empirical knowledge exists on seed predation, the link between seed loss and adult tree community composition and structure is not well established, as well as the temporal scale seed changes affect adults. Using an individual‐based forest model (FORMIND), we evaluate the long‐term consequences of increased pre and post‐dispersal seed mortality on the future forest biomass retention of a Brazilian northeastern Atlantic forest. Our results show that forest biomass is significantly affected after 80–93% pre‐dispersal loss of large seeds, or post‐dispersal predation densities of 20–25 predators per parent tree. Large‐seeded tree species are at increased risk of local extinction causing up to 26.2% loss of forest biomass when both pre and post‐dispersal processes are combined. However, these changes can last up to 100 years after the occurrence of defaunation. In summary we conclude that large animal loss has the potential to reduce future forest biomass and tree species‐richness by impacting seed survival, and should be considered in the planning of biodiversity friendly landscapes as well as in calculations of the global carbon budget.

Large trees are surrounded by more heterospecific neighboring trees in Korean pine broad-leaved natural forests

Negative conspecific density dependence is one of the principal mechanisms affecting plant performance and community spatial patterns. Although many studies identified the prevalence of density dependent effects in various vegetation types by analyzing conspecific spatial dispersal patterns (spatial patterning) of forest trees, interactions between individuals and heterospecific neighboring trees caused by density-dependent effects are often neglected. The effects of negative density dependence lead us to expect that neighbourhood species segregation would increase with increasing tree size and that larger trees would be surrounded by more heterospecific neighbours than would smaller trees. We studied four mapped 1-Ha plots on Changbaishan Mountain in North-eastern China and used marked point pattern analysis to explore whether trees of different sizes exhibited differences in neighbourhood species segregation; we also determined whether larger trees were more likely to have heterospecific neighbours than smaller trees were. Our results show that bigger trees generally have higher species mingling levels. Neighborhood species segregation ranged from lower than expected levels to random or nearly random patterns at small scales as tree size classes increased under heterogeneous Poisson null model tests. This study provides some evidence in support of negative density dependent effects in temperate forests.

Characterizing the strength of density dependence in at-risk species through Bayesian model averaging

Developing effective conservation plans for at-risk species requires an understanding of the relationship between numbers of breeding adults and their subsequent offspring. In particular, establishing the degree to which density-dependent effects limit population size can be difficult due to errors in the data themselves, uncertainty in model parameters, and possible misspecification of model structure. Here we develop a Bayesian model averaging framework to fit four simple models of adult-offspring production and estimate the probabilities that negative (i.e., decreasing survival with increasing density) and positive (i.e., Allee effects) density dependence exists. As an example, we analyzed 48 at-risk populations of anadromous Chinook salmon (Oncorhynchus tshawytscha) from the northwestern United States. We found strong evidence that more than two-thirds of the populations exhibit negative density-dependent effects of adults. This result was somewhat unexpected given the large reductions in adult numbers relative to historical benchmarks, indicating that carrying capacity of spawning habitat has been reduced considerably. Approximately two thirds of the populations also had non-zero probabilities of positive density-dependent effects of adults, which could suggest that cumulative losses of spawning adults over the past century has led to decreased nutrient and energy subsidies from semelparous carcasses, and diminished bio-physical disturbance from nest-digging activity. Importantly, our analysis highlights the utility of Bayesian model averaging in a conservation context wherein errors in choosing the best model may have more severe consequences than errors in estimating model parameters themselves.