Density-dependent Mortality Factors Research Articles

The Greenhouse Gas Crisis and the Logistic Growth Curve

The greatest challenge of the coming century will be the consequences of an imbalanced atmosphere. Currently, projections of global heating due to an increasingly imbalanced atmosphere are dire, but they underestimate the near-term heating impacts of the growing concentrations of methane. Industrially mediated carbon capture and storage sometimes gets raised as a promising solution on the CO2 front, but it is presently commercially inviable. Despite these facts, we nonetheless need to act globally to reduce the atmospheric concentrations of greenhouse gases, although our increasingly separate information ecosystems make finding a way to express the reality of the atmospheric imbalance crisis to a wide audience daunting. One approach to presenting the atmospheric imbalances leading to global heating is to strip the discussion down initially to its bare bones with a sharp focus on the variables of the logistic growth equation. Although virtually anything can be politicized, the logistic growth equation’s variables are at least apolitical in their origin. After examining those variables, we can proceed to focus on density-dependent mortality factors (DDMFs) and their relationship to visible climatic changes driven by atmospheric imbalances. Both the Global North and the Global South need to do all that we do to reduce atmospheric greenhouse gas accumulation, reducing DDMFs, while paying careful attention to Indigenous rights and to the need for global gender equity, so that our efforts to control DDMFs do not produce a new expression of colonialism.

From structure to function in mutualistic interaction networks: Topologically important frugivores have greater potential as seed dispersers.

Networks of mutualistic interactions between animals and plants are considered a pivotal part of ecological communities. However, mutualistic networks are rarely studied from the perspective of species-specific roles, and it remains to be established whether those animal species more relevant for network structure also contribute more to the ecological functions derived from interactions. Here, we relate the contribution to seed dispersal of vertebrate species with their topological role in frugivore-plant interaction networks. For one year in two localities with remnant patches of Colombian tropical dry forest, we sampled abundance, morphology, behaviour and fruit consumption from fleshy-fruited plants of various frugivore species. We assessed the network topological role of each frugivore species by integrating their degree of generalization in interactions with plants with their contributions to network nestedness and modularity. We estimated the potential contribution of each frugivore species to community-wide seed dispersal, on the basis of a set of frugivore ecological, morphological and behavioural characteristics important for seed dispersal, together with frugivore abundance and frugivory degree. The various frugivore species showed strong differences in their network structural roles, with generalist species contributing the most to network modularity and nestedness. Frugivores also showed strong variability in terms of potential contribution to seed dispersal, depending on the specific combinations of frugivore abundance, frugivory degree and the different traits and behaviours. For both localities, the seed dispersal potential of a frugivore species responded positively to its contribution to network structure, evidencing that the most important frugivore species in the network topology were also those making the strongest contribution as seed dispersers. Contribution to network structure was correlated with frugivore abundance, diet and behavioural characteristics. This suggests that the species-level link between structure and function is due to the fact that the occurrence of frugivore-plant interactions depends largely on the characteristics of the frugivore involved, which also condition its ultimate role in seed dispersal.

Functional Response of Generalist Predators to Halyomorpha halys (Hemiptera: Pentatomidae) Eggs

The brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), is an invasive agricultural pest in the United States with the potential to become a serious economic threat to Michigan agriculture. As a novel pest in Michigan's agroecosystems, the potential for a biological control program that utilizes existing natural enemies for H. halys is currently unknown. The present study identified potential H. halys egg predators and determined their effectiveness as biological control agents through functional response testing. Four generalist predators were selected based on video surveillance of sentinel egg masses in the field and through preliminary experiments. The predators were Acheta domesticus (L.) (Orthoptera: Gryllidae), Melanoplus femurrubrum (DeGeer) (Orthoptera: Acrididae), Orius insidiosus (Say) (Hemiptera: Anthocoridae), and Conocephalus fasciatus (DeGeer) (Orthoptera: Tettigoniidae), and each was tested against four densities of H. halys eggs: 26, 52, 78, and 104 eggs. A. domesticus was the only predator tested that exhibited a Type II functional response with non-negative estimates of handling time and attack rate, suggesting the potential to be a density-dependent mortality factor when H. halys egg densities are low. The theoretical maximum predation rates for female A. domesticus were 189 eggs (±95), or roughly seven egg masses. For males, the theoretical maximum was 116 eggs (±35), or 4.5 egg masses. The remaining predators tested exhibited a Type I functional response and are unlikely to be a stabilizing force in H. halys population dynamics.

Delayed density-dependent parasitism of eggs and pupae as a contributor to the cyclic population dynamics of the autumnal moth.

Many populations of forest Lepidoptera exhibit 10-year cycles in densities, with impressive outbreaks across large regions. Delayed density-dependent interactions with natural enemies are recognized as key factors driving these cyclic population dynamics, but emphasis has typically been on the larval stages. Eggs, pupae and adults also suffer mortality from predators, parasitoids and pathogens, but little is known about possible density relationships between mortality factors and these non-feeding life stages. In a long-term field study, we experimentally deployed autumnal moth (Epirrita autumnata) eggs and pupae to their natural enemies yearly throughout the 10-year population cycle in northern Norway. The abundance of another geometrid, the winter moth (Operophtera brumata), increased in the study area, permitting comparisons between the two moth species in predation and parasitism. Survival of autumnal moth eggs and pupae was related to the moth abundance in an inverse and delayed manner. Egg and pupal parasitoids dominated as density-dependent mortality factors and predicted the subsequent growth rate of the host population size. In contrast, effects of egg and pupal predators were weakly density dependent, and generally predation remained low. Parasitism rates did not differ between the autumnal and winter moth pupae, whereas predators preferred winter moth pupae over those of the autumnal moth. We conclude that parasitism of the autumnal moth by egg and pupal parasitoids can be related to the changes of the moth density in a delayed density-dependent manner. Furthermore, egg and pupal parasitoids cannot be overlooked as causal factors for the population cycles of forest Lepidoptera in general.

Rodent seed predation: effects on seed survival, recruitment, abundance, and dispersion of bird-dispersed tropical trees

Tropical tree species vary widely in their pattern of spatial dispersion. We focus on how seed predation may modify seed deposition patterns and affect the abundance and dispersion of adult trees in a tropical forest in India. Using plots across a range of seed densities, we examined whether seed predation levels by terrestrial rodents varied across six large-seeded, bird-dispersed tree species. Since inter-specific variation in density-dependent seed mortality may have downstream effects on recruitment and adult tree stages, we determined recruitment patterns close to and away from parent trees, along with adult tree abundance and dispersion patterns. Four species (Canarium resiniferum, Dysoxylum binectariferum, Horsfieldia kingii, and Prunus ceylanica) showed high predation levels (78.5-98.7%) and increased mortality with increasing seed density, while two species, Chisocheton cumingianus and Polyalthia simiarum, showed significantly lower seed predation levels and weak density-dependent mortality. The latter two species also had the highest recruitment near parent trees, with most abundant and aggregated adults. The four species that had high seed mortality had low recruitment under parent trees, were rare, and had more spaced adult tree dispersion. Biotic dispersal may be vital for species that suffer density-dependent mortality factors under parent trees. In tropical forests where large vertebrate seed dispersers but not seed predators are hunted, differences in seed vulnerability to rodent seed predation and density-dependent mortality can affect forest structure and composition.

Dispersal Modes and Spatial Patterns of Tree Species in a Tropical Forest in Arunachal Pradesh, Northeast India

We characterized the dispersal modes and spatial patterns of 128 tree species in a tropical semi-evergreen forest of Arunachal Pradesh, northeastern India. A high percentage of species are animal-dispersed (78%), while about 22% (28) are dispersed abiotically (wind or gravity-dispersed). Of the animal-dispersed species, 54 species were primarily bird-dispersed, 25 were mammal-dispersed, and 21 were dispersed by both groups. We hypothesized that adult tree distribution patterns were related to dispersal mode and fruit size. We predicted that tree species with mechanisms for long-distance seed dispersal are likely to show more uniform or random spatial patterns than those with limited seed dispersal. Tree species with large fruits were also predicted to have greater levels of clumping than those that have small fruits. However, all tree species had a clumped distribution pattern. At the community level, we found no differences in spatial patterns based on dispersal mode for a subset of 50 tree species. Fruit size was, however, positively correlated with higher levels of spatial aggregation suggesting that tree species distributions are to an extent limited by dispersal. The importance of dispersal mode in determining adult tree distribution patterns at the community level may be obscured by interacting effects of other factors such as patchy habitat conditions and density-dependent mortality factors at different life-history stages that ultimately determine adult tree distributions.

Density-dependent resistance of the gypsy moth Lymantria dispar to its nucleopolyhedrovirus, and the consequences for population dynamics

The processes controlling disease resistance can strongly influence the population dynamics of insect outbreaks. Evidence that disease resistance is density-dependent is accumulating, but the exact form of this relationship is highly variable from species to species. It has been hypothesized that insects experiencing high population densities might allocate more energy to disease resistance than those at lower densities, because they are more likely to encounter density-dependent pathogens. In contrast, the increased stress of high-density conditions might leave insects more vulnerable to disease. Both scenarios have been reported for various outbreak Lepidoptera in the literature. We tested the relationship between larval density and disease resistance with the gypsy moth (Lymantria dispar) and one of its most important density-dependent mortality factors, the nucleopolyhedrovirus (NPV) LdMNPV, in a series of bioassays. Larvae were reared in groups at different densities, fed the virus individually, and then reared individually to evaluate response to infection. In this system, resistance to the virus decreased with increasing larval density. Similarly, time to death was faster at high densities than at lower densities. Implications of density-resistance relationships for insect-pathogen population dynamics were explored in a mathematical model. In general, an inverse relationship between rearing density and disease resistance has a stabilizing effect on population dynamics.

Factors Influencing Population Dynamics of the Southwestern Corn Borer (Lepidoptera: Crambidae): A Reassessment

Mortality data from the literature on the southwestern corn borer, Diatraea grandiosella Dyar, were analyzed to determine which factors have the most impact on populations of this insect. Adult mortality, presumed to occur primarily as a result of dispersal, appears to have been the largest source of mortality, as well as the key factor in the annual variance of mortality. Nonpredation mortality during diapause was also an important factor. Of the density-dependent mortality factors, predation by the flicker, Colaptes aruaus (L.) appears to be the factor that limits the size of overwintering populations in the southeastern United States, while cannibalism among prediapausing larvae limits the size of overwintering populations in the western Great Plains. Based on the analysis of mortality rates and historical field data, the annual finite capacity for increase (multiplication rate) was estimated to be ≈10 in the central United States. This analysis suggests that to comprehend more clearly the factors that influence population variation in D. grandiosella, future research efforts should be focused on understanding adult dispersal, and the interaction between flicker predation and nonpredation mortality in the diapausing larvae stage.

The Current Status of the Biocontrol of Ceroplastes destructor Newstead (Hemiptera: Coccidae) on Citrus and Syzygium in South Africa

The incidence of parasitism and predation of the third instar and adult Ceroplastes destructor Newstead (Hemiptera: Coccidae) on Citrus reticulata (Blanco) and Syzygium (= Eugenia ) malaccensis (L.) in the Western Cape, South Africa, was assessed from June 1997 to December 1999. Seven primary and three secondary parasitoids, as well as four predator species were identified. Aprostocetus (= Tetrastichus ) ceroplastae (Girault) was the predominant parasitoid species, accounting for 78.9% of the total primary parasitoids. Peak numbers of parasitoids and predators were synchronized with peak emergence of the scale stages susceptible to parasitism and predation, indicating that the scale-parasitoid/predator association contained a density-dependent regulatory mechanism. Parasitoids demonstrated density-dependence at the third instar and pre-ovipositing female stage for scales on C. reticulata and S. malaccensis , respectively, whereas predators acted as a density-dependent mortality factor only during the pre-ovipositing female stage of scales from C. reticulata . However, the density-dependent process of parasitism and predation fluctuated between generations, indicating that the regulatory effects of these mortality agents may not be strong enough to provide long-term suppression of scales under the current citricultural practices. Mortality due to disease was either inversely related or not related to scale density. Body-length of A. ceroplastae varied significantly between sampling periods, parasitoid sex and scale stage from which it was reared (P< 0.001). The sex ratio of A. ceroplastae reared from early third instar C. destructor was male biased while that of parasitoids reared from mature adult female was slightly female biased.

Temporal and spatial variation of larval parasitism in non-outbreaking populations of a folivorous moth.

In order to assess the role of parasitoids in the regulation of non-outbreaking populations of Epirrita autumnata, a geometrid lepidopteran with outbreaking populations in northern Europe, we examined the temporal and spatial variation of larval parasitism in southwestern Finland during 6 successive years. The study was carried out on two spatial scales, among trees within sites of about 1 ha and among sites separated by distances of 2-10 km, using experimental and observational approaches respectively. The overall percent parasitism was independent of host density on both spatial scales, while temporally it fluctuated only little. Of the two main parasitoids, the commoner one, Protapanteles immunis, showed a variable response to host density on the larger spatial scale and negative density dependence on the smaller scale. Temporally, parasitism caused by this species was independent of host density. Another parasitoid, Phobocampe bicingulata, showed positive density dependence on the smaller spatial scale and had a variable response on the larger scale, but exhibited negative density dependence over time. The results of this study caution against drawing conclusions concerning population regulation on the grounds of spatial density dependence alone. Larval parasitoids apparently do not maintain low densities in the E. autumnata populations studied. However, they may suppress E. autumnata densities to a level low enough for density-dependent mortality factor(s) to become regulating. Among other mortality factors of E. autumnata, pupal predation has been found to be temporally positively density-dependent.

On the stabilizing effect of density-dependent mortality factors.

For nearly twenty years den Boer (1968, 1971, 1973, 1982, 1985 and 1986 a) has been trying to stimulate ecologists to investigate the possibilities of "spreading of risk". He has shown that spatial heterogeneity has a stabilizing effect on animal populations (den Boer 1982). Together with density-dependent processes spatial heterogeneity is thus responsible for the persistence of populations. The relative importance of each of these two stabilizing influences may differ between populations, but so far population ecologists have focused their interest on density-dependent processes. It was thus of particular interest when den Boer (1986b) showed that density-dependent mortality does not necessarily have any stabilizing effect at all. Comparing the original data (Varley and Grad well 1968) with some null model simulations he showed that density-dependent pupal pr?dation in the winter moth did not regulate density. More generally he concluded that "the range of variation of any supposed regulating factor should be as great, or at least nearly as great, as the range of variation of all other factors together influencing density, i.e. it should be the key factor".

Density dependence and the stabilization of animal numbers : 1. The winter moth.

The claim of Varley and Gradwell that the highly density-dependent pupal predation (k 5) in the population of the winter moth in Wytham Wood, England would keep density within limits (regulate) is compared in this paper with the density limits in the null model: pupal predation causes the same mean generation mortality (35%) as in the field, but is not density-dependent, i.e. either constant or randomly fluctuating between years according to the actual frequency distribution, ceteris paribus. According to this null model the winter moth would have fluctuated between narrower limits than in the field; the claim of Varley & Gradwell must thus be rejected. It is more generally concluded that a regulating factor should be the key factor, but this is not a sufficient condition. It should also prevent in some way the low throughs in the time series that usually accompany the operation of a dominating density-dependent mortality factor.