Evidence For Negative Density Dependence Research Articles

Can livestock grazing dampen density‐dependent fluctuations in wild herbivore populations?

Abstract Conservation policy for the high mountains of Asia increasingly recognises the need to encompass large multi‐use landscapes beyond the protected area network. Due to limited long‐term research in this region, our understanding of even fundamental processes, such as factors regulating large mammal populations is poor. Understanding the factors that regulate animal populations, especially those generating cyclicity, is a long‐standing problem in ecology. Long‐term research across multiple taxa (mainly from Europe and North America) has focussed on the relative roles of food and predation in generating cyclicity in population dynamics. It remains unclear how trophic interactions that are influenced by anthropogenic stressors can affect population dynamics in human‐modified landscapes. We present a 10‐year study to compare the effects of livestock grazing on density‐dependent dynamics in two populations of bharal, Pseudois nayaur, in the Himalayas. We combine this with a mechanistic understanding of whether density dependence in these two sites acts predominantly by affecting adult survival or recruitment. We compared and quantified density dependence in the bharal population by fitting Bayesian Gompertz state‐space models. We found evidence for negative density dependence which indicates possible cyclic dynamics in the bharal population of the site (Tabo) with low livestock density. The population dynamics of this site were driven by recruited offspring—with a 2‐year density‐dependent lag effect—rather than adult survival. In the site with high livestock density (Kibber), this density dependence was not detected. We postulate the potential role of excessive grazing by livestock in affecting offspring recruitment, thereby affecting the bharal population in Kibber. Synthesis and applications: Our results suggest that conservation action to facilitate wild herbivore population recovery, such as the development of protected areas and village reserves, needs to account for density‐dependent regulation. Sites with trophy hunting require continuous monitoring to understand the effects of density dependence so that appropriate hunting quotas can be formulated.

A test of the Janzen‐Connell hypothesis in a species‐rich Mediterranean woodland

AbstractThe Janzen‐Connell (JC) hypothesis predicts that conspecific negative density dependence contributes to the maintenance of plant diversity by lowering the recruitment of locally abundant plant species. The JC hypothesis is a widely evoked explanation for the high species diversity in tropical forests, but remains poorly tested in other species‐rich systems such as Mediterranean woodlands. As such, we tested if the JC operates and the role of soil‐borne oomycetes in a species‐rich Mediterranean woodland of Western Australia, where post‐fire recruitment can lead to high seedling densities, using the common Banksia attenuata as a case study. We attempted to decipher the effects of oomycete pathogens and distance from conspecific trees on intraspecific seedling survival and growth. Contrary to the JC hypothesis, we found little evidence of negative density dependence, but our results suggest positive density dependence survival under conspecific trees. Oomycete‐driven mortality in seedlings was also found regardless of the type of tree. Our results suggest that short‐term seedling recruitment patterns in this species‐rich, fire‐prone ecosystem do not follow the JC hypothesis. Future studies should explore whether negative conspecific density and distance dependence could play a greater role in later stages of the post‐fire recovery process.

Different ecological mechanisms lead to similar grazer controls on the functioning of periphyton Antarctic and sub-Antarctic communities

The processes underpinning the differences between Antarctic and sub-Antarctic ecological communities are still unclear. Dispersal, drift, speciation, and abiotic environmental filtering have been considered to explain these differences; biotic interactions, however, have received less attention. Antarctic intertidal assemblages of macrobenthic grazers are characterised by numerically abundant populations of a single species, Nacella concinna. In contrast, sub-Antarctic habitats display a diverse assemblage of herbivores like chitons, keyhole limpets, and several species of Nacella. Thus, it was hypothesised that herbivores would have significant density-dependent effects of a single species on benthic primary productivity in Antarctica, but stronger effects of the whole assemblage in the sub-Antarctic. Field grazer inclusion-exclusion experiments showed that chlorophyll-a (chl-a) concentration was one order of magnitude lower in Fildes Bay (King George Island, Antarctica) than in the Strait of Magellan (Chilean South Patagonia). Still, grazers had significant and negative effects on chl-a accrual, a proxy for productivity, in both sites. In Fildes Bay, these effects were similar between experimental levels or grazer density. Accordingly, evidence for negative density dependence of per capita interaction strength was detected in this region. In the Strait of Magellan, only the open access treatments, exposed to the diverse assemblage of grazers, significantly decreased chl-a accrual. Grazers negatively affected the relative abundance of browns (i.e. diatoms, dinoflagellates, and early stages of brown algae) and cyanobacteria at both sites, but favoured green algae and bare substratum in Fildes Bay and Strait of Magellan, respectively. These results suggest that different mechanisms, such as negative density dependence and resource partitioning, can lead to similar grazing controls of the productivity of Antarctic and sub-Antarctic periphyton communities. Herbivory should be incorporated as a local biotic filter into a comprehensive model of community structure and functioning for these ecosystems.

Negative canopy–understorey interaction shapes the sapling bank of Fagus crenata in a cool-temperate, conifer–hardwood mixed forest

Negative density-dependence (NDD) is a major mechanism for shaping forest communities. However, its influence on species coexistence in conifer–hardwood mixed forests remains unclear. We aimed to determine whether NDD influences establishment of the seedling and/or sapling bank of Fagus crenata in an evergreen, coniferous Cryptomeria japonica-dominated stand, focusing on canopy structure (openness, distance from mature plants and species composition). For F. crenata trees, we analysed the spatial pattern using the pair-correlation function g(r) to test for NDD and spatial correspondence to canopy structure in two study plots established in a cool-temperate, old-growth conifer–hardwood mixed forest in western Japan. We also analysed the height growth of offspring and understorey light condition with respect to canopy structure. We detected evidence of NDD among neighbours during the transition of seedlings (age ≤ 4 years) to saplings (age > 4 years, stem length 4 years) was elevated under the canopy of C. japonica, which was lighter than under that of F. crenata. Our results suggest that F. crenata recruitment was affected by NDD at the seedling-to-sapling transition under the conspecifics and allowed for the formation of the sapling bank under C. japonica canopies beyond the drip lines of conspecifics.

Positive interactions between herbivores and plant diversity shape forest regeneration

The effects of herbivores and diversity on plant communities have been studied separately but rarely in combination. We conducted two concurrent experiments over 3 years to examine how tree seedling diversity, density and herbivory affected forest regeneration. One experiment factorially manipulated plant diversity (one versus 15 species) and the presence/absence of deer (Odocoileus virginianus). We found that mixtures outperformed monocultures only in the presence of deer. Selective browsing on competitive dominants and associational protection from less palatable species appear responsible for this herbivore-driven diversity effect. The other experiment manipulated monospecific plant density and found little evidence for negative density dependence. Combined, these experiments suggest that the higher performance in mixture was owing to the acquisition of positive interspecific interactions rather than the loss of negative intraspecific interactions. Overall, we emphasize that realistic predictions about the consequences of changing biodiversity will require a deeper understanding of the interaction between plant diversity and higher trophic levels. If we had manipulated only plant diversity, we would have missed an important positive interaction across trophic levels: diverse seedling communities better resist herbivores, and herbivores help to maintain seedling diversity.

Effects of forest fragmentation on the recruitment success of the tropical tree Poulsenia armata at Los Tuxtlas, Veracruz, Mexico

Abstract:Recruitment success of individual plants is limited by an array of biotic and abiotic factors. Seedling survival may experience high mortality due to negative density dependence or altered microclimatic conditions. This study reports regeneration of Poulsenia armata (Moraceae), in the fragmented landscape of the Los Tuxtlas region in south-eastern Mexico. Density, survival and growth of seedlings (<1 y) and juveniles (<150 cm height) of P. armata were predicted to be significantly lower in forest fragments compared with extensive continuous forest. Contrary to expectation, density did not vary between habitats; however, we found twice the number of seedlings (n = 82) in forest fragments than in the continuous forest (n = 35). Forest fragments were associated with higher seedling densities close to adult trees. Unexpectedly, we found no evidence for negative density dependence in plant survival or growth. Survival and growth of P. armata were negatively impacted in forest fragments, with desiccation by warmer daily temperatures likely the cause of mortality, independent of density. Of the 111 individuals recorded in 2010 in forest fragments, 38% died over 2 y of census (n = 42), while 9% (n = 12) of the 127 individuals from the continuous forest died. Higher rates of mortality suggest that conditions in forest fragments are detrimental to seedling cohorts over time. Low juvenile recruitment jeopardizes persistence of P. armata, in fragmented populations of this rain-forest tree.

Gemma output in the liverwort Lophozia ventricosa (Dicks.) Dumort: spatial variation, density dependence, and relationships among production components

Although gemma production is common among liverworts, and thought to play a key role in their population dynamics, quantitative data on numbers produced at shoot and colony levels, or on spatial variation in these numbers, are very sparse. Here we investigate gemma output of a boreal and arctic liverwort species, Lophozia ventricosa, at populations in Trøndelag, a boreal region of central Norway, and in arctic tundra on Svalbard. Production parameters investigated include colony size, shoot density, number of gemmae per shoot, proportion of shoots gemmiferous, and gemmiferous shoot density. Colony-level output was calculated as the product of the number of gemmae per shoot, colony size, and gemmiferous shoot density. Nested analysis of variance, used to spatially partition variance in these gemma-production components, reveals that the vast majority of the variation lies at the measurement level, i.e. among shoots within colonies or among colonies within plots. The only significant difference between the two geographic areas is that in boreal sites shoots tend to produce slightly more gemmae, and form larger, denser, colonies. This translates to somewhat higher colony-level output in boreal, compared to arctic, sites. Evaluation of a structural equations model reveals intricate patterns of relationship among the production components and provides strong evidence of negative density dependence of shoot-level production. We argue that explicit evaluation of size–number tradeoffs, allometric scaling of production with shoot size, and detailed studies of genetic versus phenotypic effects on production levels, are necessary to understand how asexual propagule production is shaped by selection on life histories.

Does supplemental feeding affect the viability of translocated populations? The example of the hihi

AbstractTranslocations are commonly used conservation actions that aim at establishing new, self‐sustaining populations of threatened species. However, many translocated populations are not self‐sustaining but managed through supplemental feeding from the onset. Often, the decision to start managing is ad hoc, but managers will eventually have to make decisions for the future, for example, stop intervening, continue as it is or change the quantity of food provided. Such a decision requires managers to quantify the importance of supplemental feeding in determining the performance and population dynamics of translocated populations, information that is rarely available in the published literature. Using the hihi as a case study, we examined the importance of supplemental feeding for the viability of a translocated population in New Zealand. We found that supplemental feeding positively affected the survival and abundance of translocated adult hihi but also found evidence of negative density dependence on recruitment. We present two stochastic population models that project the hihi population under different management scenarios, quantitatively assessing the impact supplemental feeding has had on the population. Our results illustrate how important long‐term targeted monitoring is for robust decision making about adaptive management.

Combining ground count, telemetry, and mark–resight data to infer population dynamics in an endangered species

Summary 1. To successfully manipulate populations for management and conservation purposes, managers must be able to track changes in demographic rates and determine the factors driving spatial and temporal variation in those rates. For populations of management concern, however, data deficiencies frequently limit the use of traditional statistical methods for such analyses. Long‐term demographic data are often piecemeal, having small sample sizes, inconsistent methodologies, intermittent data, and information on only a subset of important parameters and covariates. 2. We evaluated the effectiveness of Bayesian state‐space models for meeting these data limitations in elucidating dynamics of federally endangered Sierra Nevada bighorn sheep Ovis canadensis sierrae. We combined ground count, telemetry, and mark–resight data to: (1) estimate demographic parameters in three populations (including stage‐specific abundances and vital rates); and (2) determine whether density, summer precipitation, or winter severity were driving variation in key demographic rates. 3. Models combining all existing data types increased the precision and accuracy in parameter estimates and fit covariates to vital rates driving population performance. They also provided estimates for all years of interest (including years in which field data were not collected) and standardized the error structure across data types. 4. Demographic rates indicated that recovery efforts should focus on increasing adult and yearling survival in the smallest bighorn sheep population. In evaluating covariates we found evidence of negative density dependence in the larger herds, but a trend of positive density dependence in the smallest herd suggesting that an augmentation may be needed to boost performance. We also found that vital rates in all populations were positively associated with summer precipitation, but that winter severity only had a negative effect on the smallest herd, the herd most strongly impacted by environmental stochasticity. 5. Synthesis and applications. For populations with piecemeal data, a problem common to both endangered and harvested species, obtaining precise demographic parameter estimates is one of the greatest challenges in detecting population trends, diagnosing the causes of decline, and directing management. Data on Sierra Nevada bighorn sheep provide an example of the application of Bayesian state‐space models for combining all existing data to meet these objectives and better inform important management and conservation decisions.

Large-scale noninvasive genetic monitoring of wolverines using scats reveals density dependent adult survival

Noninvasive genetic monitoring has the potential to estimate vital rates essential for conservation and management of many species. In a long-term genetic capture-mark-recapture study using scats we evaluated temporal variation in adult survival in a wolverine ( Gulo gulo) population in southern Norway. In contrast to most previous studies of large mammals we found evidence for negative density dependence in adult survival in this large carnivore. Both sexes showed the same pattern of density dependence, with higher annual survival rates in adult females than males. In addition, we also found an additive mortality effect of harvesting in the population, resulting in the lowest adult survival rates at a combination of high population density and high harvest rate. The additive effects of density and harvest on adult survival of wolverines have relevance to the conservation and management of solitary carnivores with strong intrasexual territoriality, especially for species where combats among conspecifics can cause serious injury or even mortality.

An evaluation of density‐dependent and density‐independent influences on population growth rates in Weddell seals

Much of the existing literature that evaluates the roles of density-dependent and density-independent factors on population dynamics has been called into question in recent years because measurement errors were not properly dealt with in analyses. Using state-space models to account for measurement errors, we evaluated a set of competing models for a 22-year time series of mark-resight estimates of abundance for a breeding population of female Weddell seals (Leptonychotes weddellii) studied in Erebus Bay, Antarctica. We tested for evidence of direct density dependence in growth rates and evaluated whether equilibrium population size was related to seasonal sea-ice extent and the Southern Oscillation Index (SOI). We found strong evidence of negative density dependence in annual growth rates for a population whose estimated size ranged from 438 to 623 females during the study. Based on Bayes factors, a density-dependence-only model was favored over models that also included environmental covariates. According to the favored model, the population had a stationary distribution with a mean of 497 females (SD = 60.5), an expected growth rate of 1.10 (95% credible interval = 1.08-1.15) when population size was 441 females, and a rate of 0.90 (95% credible interval = 0.87-.93) for a population of 553 females. A model including effects of SOI did receive some support and indicated a positive relationship between SOI and population size. However, effects of SOI were not large, and including the effect did not greatly reduce our estimate of process variation. We speculate that direct density dependence occurred because rates of adult survival, breeding, and temporary emigration were affected by limitations on per capita food resources and space for parturition and pup-rearing. To improve understanding of the relative roles of various demographic components and their associated vital rates to population growth rate, mark-recapture methods can be applied that incorporate both environmental covariates and the seal abundance estimates that were developed here. An improved understanding of why vital rates change with changing population abundance will only come as we develop a better understanding of the processes affecting marine food resources in the Southern Ocean.

A unified capture-recapture framework

This article develops a hierarchical framework for capture-recapture data that separates the capture process from the demographic processes of interest, such as birth and survival. This allows users to parameterize in terms of meaningful demographic parameters. The framework is very flexible with many of the current capture-recapture models included as special cases. The hierarchical nature of the model allows natural expression of relationships, both between parameters and between parameters and the realization of random variables, such as population size. Previously, many of these relationships, such as density dependence have been unable to be explored using capture-recapture data. Density dependence, where survival and birth rates depend on the population size, is an interesting special case. We fit a density-dependent model to male Gonodontis bidentata data and report evidence of negative density dependence in percapita birth rates and weak evidence of negative density dependence in survival.