Density dependence and the control of helminth parasites

1. The transient dynamics and stability of a population are determined by the interplay between species density, its spatial distribution and the positive and negative density-dependent processes regulating population growth. 2. Using the human-helminth parasite system as an example, we propose that the life-stage upon which negative density dependence operates will influence the rate of host reinfection following anthelmintic chemotherapy, and the likely success of control programmes. 3. Simple deterministic models are developed which highlight how a parasite species whose population size is down-regulated by density-dependent establishment will reinfect a host population at a faster rate than a species with density-dependent parasite fecundity. 4. Different forms of density dependence can produce the same equilibrium behaviour but different transient dynamics. Under-representing the nature and magnitude of density-dependent mechanisms, and in particular those operating upon establishing life-stages, may cause the resilience of the parasite population to a control perturbation to be underestimated.     

Environmental Stochasticity and Long-Term Livestock Viability—Herd-Accumulation as a Risk Reducing Strategy

This study tests the hypothesis that herd accumulation can be a risk reducing strategy aimed at increasing security in an unpredictable environment. Saami reindeer husbandry in Norway is characterized by environmental unpredictability and occasionally harsh winters can have dramatic negative effects on reindeer population densities. While herd accumulation has been found to be an adaptive risk reducing strategy in stochastic environments (i.e., individually rational), the accumulation of large herds may also result in collectively negative density dependent effects, which may negatively affect individual herders (i.e., collectively irrational). We found that individual husbandry units’ strategies, such as accumulating reindeer, have a larger effect on individual husbandry units’ herd size than a negative density-dependent effect.     

Distribution of life cycle stages in a lithophytic and epiphytic orchid

Density-dependent processes may have multiple effects on populations, which among other things include the regulation of population abundance and of the relative distribution of life-cycle stages within populations. The epiphytic habitat is often characterized as highly ephemeral and therefore epiphytic orchid populations may never achieve density-dependent regulation. In this study, we investigated the potential for density-dependent regulation in epiphytic and lithophytic orchids by examining the association between seedlings, juvenile and adult life-history stages in the Caribbean endemic orchid,Lepanthes rupestris in a cross-sectional study of 179 populations surveyed in the Luquillo National Forest along a riparian area where it is locally abundant. Under density-dependent regulation we expected a negative association between the ratio of seedling/adults and juveniles/adults and total population density. Population density was in the range of 140 individuals per m², however patch sizes were small and mostly limited to less than 0.5 m² with a maximum of 3 m². We found no evidence of reduction of the ratio of seedlings or juveniles to adults as population size increased in either tree or boulder populations suggesting negative density dependence for population regulation inL. rupestris is either rare or occurs at even higher densities than those measured here. Moreover, we found positive (although weak) relationship between the ratio of seedlings and juveniles to adults and population size, suggesting that facilitation may be occurring.     

Density-dependent migration and stability in a system of linked populations

The effect of adding density-dependent migration between nearest neighbour populations of a single discrete-generation species in a chain of habitat fragments is investigated. The larger the population on a particular habitat fragment, the greater the fraction of inhabitants who migrate before reproducing. It has previously been shown for similar models with density-independent migration that coupling populations in this way has no effect on the stability of these populations. Here, it is demonstrated that this effect is also generally true if migration is density-dependent. However, if the migration rate is large enough and has density dependence of the correct form, then the steady state (with all the populations remaining at the same constant value through time) can be destabilised. The conditions for this to occur are obtained analytically. When this “destabilisation” occurs, the system settles down to an alternative steady state where half of the populations take one constant value which is below that of an equivalent isolated system, and the other populations all share a population value which is greater than the steady state of the isolated populations. Once this configuration is reached, the population size on each patch remains constant over time. hence the change might more properly be described as a decrease in homogeneity rather than in stability.     

Rapid growth and large body size in annual fish populations are compromised by density-dependent regulation

We tested the effect of population density on maximum body size in three sympatric species of annual killifishes Nothobranchius spp. from African ephemeral pools. We found a clear negative effect of population density on body size, limiting their capacity for extremely fast development and rapid growth. This suggests that density-dependent population regulation and the ephemeral character of their habitat impose contrasting selective pressures on the life history of annual killifishes.     

Spatial variation and density-dependent dispersal in competitive coexistence

It is well known that dispersal from localities favourable to a species' growth and reproduction (sources) can prevent competitive exclusion in unfavourable localities (sinks). What is perhaps less well known is that too much emigration can undermine the viability of sources and cause regional competitive exclusion. Here, I investigate two biological mechanisms that reduce the cost of dispersal to source communities. The first involves increasing the spatial variation in the strength of competition such that sources can withstand high rates of emigration; the second involves reducing emigration from sources via density-dependent dispersal. I compare how different forms of spatial variation and modes of dispersal influence source viability, and hence source-sink coexistence, under dominance and pre-emptive competition. A key finding is that, while spatial variation substantially reduces dispersal costs under both types of competition, density-dependent dispersal does so only under dominance competition. For instance, when spatial variation in the strength of competition is high, coexistence is possible (regardless of the type of competition) even when sources experience high emigration rates; when spatial variation is low, coexistence is restricted even under low emigration rates. Under dominance competition, density-dependent dispersal has a strong effect on coexistence. For instance, when the emigration rate increases with density at an accelerating rate (Type III density-dependent dispersal), coexistence is possible even when spatial variation is quite low; when the emigration rate increases with density at a decelerating rate (Type II density-dependent dispersal), coexistence is restricted even when spatial variation is quite high. Under pre-emptive competition, density-dependent dispersal has only a marginal effect on coexistence. Thus, the diversity-reducing effects of high dispersal rates persist under pre-emptive competition even when dispersal is density dependent, but can be significantly mitigated under dominance competition if density-dependent dispersal is Type III rather than Type II. These results lead to testable predictions about source-sink coexistence under different regimes of competition, spatial variation and dispersal. They identify situations in which density-independent dispersal provides a reasonable approximation to species' dispersal patterns, and those under which consideration of density-dependent dispersal is crucial to predicting long-term coexistence.     

Analysis of polymorphic variation in different types of habitat

A correlation between the distribution of an organism and features of its environment can be taken as indirect evidence of natural selection. Biologists may therefore collect samples from polymorphic populations at a number of locations, classify the locations into habitat types, and consider whether the distribution of morphs varies with the habitat. Statistical aspects of this type of study are discussed in this paper. A randomization test for habitat effects is proposed and a negative binomial model is suggested for the distribution of morphs from random locations within one type of habitat. Data on the distribution of Cepaea hortensis and C. nemoralis snails in southern England provide an example. For both species there is clear evidence of differences between habitats, although the morph distributions are rather variable within habitats. The negative binomial model suggests that, for the snail data, variation in morph proportions is mainly due to location differences. The binomial sampling error is relatively unimportant unless the sample size at a location is very small. Therefore it is reasonable to analyse morph proportions by standard methods without giving different weights to data from different locations. The snail data are analysed in this way. Discriminant function analyses are used to test for habitat effects. The relationships between C. hortensis and C. nemoralis morph frequencies within one habitat are examined by a canonical correlation analysis.     

Field evidence for density-dependent effects in the trematode Microphallus papillorobustus in its manipulated host, Gammarus insensibilis

Numerous studies have demonstrated that parasites with complex life cycles frequently manipulate the phenotype of their hosts to increase their transmission rate. Little is known, however, concerning density-dependent processes within infrapopulations of manipulative parasites--whether parasites cooperate to manipulate the host, whether competition counteracts with these potential cooperative benefits, or both. Here we explored these ideas, focusing on the association between the manipulative trematode Microphallus papillorobustus and its second intermediate host, the gammarid Gammarus insensibilis. From the data collected in the field, we found no evidence that co-occurring M. papillorobustus individuals benefit from the presence of conspecifics; instead, individuals in larger infrapopulations suffered reduced size and fecundity. Thus, the net effect of increasing density suggests that competition rather than cooperation is the dominant force in infrapopulations of M. papillorobustus.     

Effects of experimental stress on the glycogen content of Cepaea hortensis (Mollusca)

A method for measuring the glycogen content of the hepatopancreas of Cepaea hortensis is described. Glycogen content was determined for snails from field population cages and from a nearby natural population. Results showed that caged snails possessed lower levels of glycogen than did the natural population, while the lowest levels of glycogen were associated with higher snail densities in the cages. There is evidence for an increase in glycogen content of C. hortensis during the late summer in nature and in experimental populations. The results are discussed with regard to the selective effect that glycogen may have on Cepaea and to the interpretation of events occurring in populations of Cepaea that glycogen assay could provide.     

Density-dependent effects in a parasitic nematode,Elaphostrongylus rangiferi,in the snnail intermediate host

Summary Density-dependent effects in Elaphostrongylus rangiferi, a parasitic nematode in the CNS and muscular system of reindeer, were studied in a laboratory population of the snail intermediate host, Arianta arbustorum. The rates in parasite growth, development and mortality were all affected by parasite density. The effects on growth and development were, however, much more marked, than the effect on mortality.All density-dependent rates were intensified by decreasing snail size, and by snail starvation. The snail host showed marked tissue reactions against infection, and the intensity of these reactions increased with increasing parasite density. The mechanism behind the observed density-dependent rates is discussed, and is tentatively concluded to be competition for nutritive substances in the host tissue.The importance of a density-dependent developmental rate in natural populations of this parasite is discussed, and it is hypothesized that this effect may counteract the strong temperature-dependent developmental rate of E. rangiferi In a more general context it is pointed out that density-dependent developmental rates, although common amongst animal populations, has been neglected in models of population dynamics. Developmental rates are usually represented by a constant time lag in such models, but should be treated as a density-dependent variable.     

Density-dependent growth responses in two clonal herbs: regulation of shoot density

Summary It has been shown in clonal perennial herbs that shoot natality decreases, and shoot mortality increases, in stands of increasing density. In a two-year garden experiment, we have tested Hutchings' (1979) hypothesis that these responses are the result of physiological integration, i.e. the exchange of resources and growth substances between shoots of a single clone. Dense monocultures of two rhizomatous graminoids, Brachypodium pinnatum and Carex flacca, were created that differed more than 10-fold in the density of clones (genets), but that had similar densities of shoots. A more effective shoot density control was expected in stands with the smaller clone densities (larger clones) due to more extensive clonal connections. Shoot turnover was evaluated by counting living and dead shoots at different times. In the summer of the second year, when shoot densities and stand structure were similar between treatments, shoot natality (the number of shoots born per plot) and shoot mortality (the number of shoots that died per plot) were usually unrelated to clone density in either species. If there was a significant treatment effect, it could be attributed to (small) differences in shoot density. Over the whole range of shoot densities, natality was negatively density-dependent. The number of shoots that died in a given growth period was proportional to the number of shoots present, suggesting that mortality rates were density independent. In Carex, however, there were some indications that mortality rate increased with increasing density. Our study confirms that clonal herbaceous species can effectively prevent an overproduction of shoots, but in contrast to Hutchings' (1979) propositions, we found no evidence that physiological integration may be the responsible mechanism. An alternative explanation for the observed patterns is proposed.     

Evidence for density-dependent growth in North Sea gadoids

The classical view on density-dependent growth is that an increase in population density heightens intraspecific competition for food, leading to reduced food intake and depressed growth. This paper summarizes the analysis of English Groundfish Survey, International Young Fish Survey and ICES Roundfish Working Group data for evidence of density-dependent growth of I- and II-group cod, haddock and whiting in the North Sea.
Within Roundfish areas there was a negative relationship between growth and density in some instances, but it was small. However, there were persistent differences in both growth rate and density between Roundfish areas. In all cases, except for I-group cod, there appeared to be a tendency for fish in low-density areas to be larger than fish in high-density areas; this might be due to feeding competition but other explanations are possible. The results urge caution in interpreting commercial catch data for evidence of density-dependent growth, since changes in fishing effort in particular areas could significantly bias the conclusions.
There was evidence of enhanced survival of small fish in good year-classes; this would depress the mean size at age, thereby mimicking the effects of density-dependent growth.     

Body weight as an indication of density-dependent population regulation in badgers (Meles meles) at Woodchester Park, Gloucestershire

The body weights of badgers were examined to look for density-dependent effects of the increase in group size at Woodchester Park, Gloucestershire. The weight of badgers in 21 groups were studied from 1978–1993. A significant negative relationship was found between weight and group size in breeding adult females in autumn and winter, and adult males in summer, such that increase in group size caused a decrease in weight. However, these relationships were affected by year-to-year changes and random variation, and overall there was no downward trend in body weight. The existence of density-dependent effects on badger populations at high density was consistent with previous findings     

Density-dependent sexual selection in the monogamous fish Archocentrus nigrofasciatus

To date, density-dependent effects on sexual selection have been studied only scantly. In this study we experimentally assessed the effect of breeding site density on the size distribution of mating convict cichlids Archocentrus nigrofasciatus in the field. We found that females were larger under low than high nest density. These results are better explained by density-dependent mate choice and mating competition than competing hypotheses of resource competition and predation pressure. We did not find differences in brood survival between the two nest density regimes. Nevertheless, convict cichlids avoided breeding in each other's close proximity, indicating that a high density of breeding pairs entails some other costs, such as energy loss through increased territorial aggression. Our results stress the importance of considering evolutionary effects of mate choice and mate competition in the context of availability of resources that determine the density of individuals that succeed to mate.     

Spatial synchrony through density-independent versus density-dependent dispersal

Many theoretical studies support the notion that strong dispersal fosters spatial synchrony. Nonetheless, the effect of conditional vs. unconditional dispersal has remained a matter of controversy. We scrutinize recent findings on a desynchronizing effect of negative density-dependent dispersal based on spatially explicit simulation models. Keeping net emigration rates equivalent, we compared density-independent and density-dependent dispersal for different types of intraspecific density regulation, ranging from under-compensation to over-compensation. In general, density-independent dispersal possessed a slightly higher synchronizing potential but this effect was very small and sensitive compared to the influence of the type of local density regulation. Notably, consistent outcomes for the comparison of conditional dispersal strategies strongly relied on the control of equivalent emigration rates. We conclude that the strength of dispersal is more important for spatial synchrony than its density dependence. Most important is the mode of intraspecific density regulation.     

Plants as reef fish: fitting the functional form of seedling recruitment

The life histories of many species depend first on dispersal to local sites and then on establishment. After dispersal, density-independent and density-dependent mortalities modify propagule supply, determining the number of individuals that establish. Because multiple factors influence recruitment, the dichotomy of propagule versus establishment limitation is best viewed as a continuum along which the strength of propagule or establishment limitation changes with propagule input. To evaluate the relative importance of seed and establishment limitation for plants, we (1) describe the shape of the recruitment function and (2) use limitation and elasticity analyses to quantify the sensitivity of recruitment to perturbations in seed limitation and density-independent and density-dependent mortality. Using 36 seed augmentation studies for 18 species, we tested four recruitment functions against one another. Although the linear model (accounting for seed limitation and density-independent mortality) fitted the largest number of studies, the nonlinear Beverton-Holt model (accounting for density-dependent mortality) performed better at high densities of seed augmentation. For the 18 species, seed limitation constrained population size more than other sources of limitation at ambient conditions. Seedling density reached saturation with increasing seed density in many studies, but at such high densities that seedling density was primarily limited by seed availability rather than microsite availability or density dependence.     

Spatial synchrony through density-independent versus density-dependent dispersal

Many theoretical studies support the notion that strong dispersal fosters spatial synchrony. Nonetheless, the effect of conditional vs. unconditional dispersal has remained a matter of controversy. We scrutinize recent findings on a desynchronizing effect of negative density-dependent dispersal based on spatially explicit simulation models. Keeping net emigration rates equivalent, we compared density-independent and density-dependent dispersal for different types of intraspecific density regulation, ranging from under-compensation to over-compensation. In general, density-independent dispersal possessed a slightly higher synchronizing potential but this effect was very small and sensitive compared to the influence of the type of local density regulation. Notably, consistent outcomes for the comparison of conditional dispersal strategies strongly relied on the control of equivalent emigration rates. We conclude that the strength of dispersal is more important for spatial synchrony than its density dependence. Most important is the mode of intraspecific density regulation.     

Density-dependent demography in a Japanese temperate broad-leaved forest

A method of detecting the density-dependent dynamics of a size-structured population is developed. The method is applied to a Japanese broad-leaved forest and the density-dependent and-independent projection matrix models are constructed based on the data of the forest. Then, the difference between the density-independent and density-dependent dynamics is compared in terms of several statistical quantities obtained from the matrices. Three kinds of sensitivity matrices are proposed for the density-dependent matrix model. At earlier stages, the sensitivity when the population density is low are higher than at the equilibrium. On the other hand, the inverse result is obtained at later stages. Moreover, the responses of the forest is analyzed to a decrease in survival rate and to an increase in the probability of gap formation. The decrease in survival rate leads to an extreme decrease in the density of the forest. However, the decrease in recruitment rate gives little effect on it because the effect of density-dependence in recruitment of new individuals is strong. The forest has the optimal rate of gap formation such that leads to the maximum population density.     

Multiple sources of evidence for density dependence in the endangered Hawaiian stilt (Himantopus mexicanus knudseni)

Hawaiian stilts (Himantopus mexicanus knudseni) are an endangered subspecies of the Black-necked stilt endemic to the Hawaiian Islands. Despite long-term study, the main drivers of Hawaiian stilt population dynamics are poorly understood. We tested for density dependence using two sources of evidence: a 30-year time series of annual estimated range-wide abundance, and two 15+ year time series of reproductive success. Using separate methods with independent data, sources allowed us to make up for the potentially positive bias of one approach with the more conservative nature of the second. We compared nonlinear density-dependent and density-independent population model fits to our time-series data, using both frequentist and Bayesian state-space approaches. Across both approaches, density-dependent models best fit observed population dynamics, with lower AICc and cross-validation statistics compared to density-independent models. Among density-dependent models, a conditional model in which density-independent dynamics occur below a population size threshold (~850–1,000 birds), and then density-dependent dynamics occur above that threshold, performed best across Bayesian and frequentist model comparisons, with the Ricker model ranked next or equivalently. Our analysis of reproduction data revealed a strong negative effect of local adult density on nest success (proportion of nests hatching at least one chick) at Kealia National Wildlife Refuge on Maui, where few alternative breeding habitats are available, but no such effect at another site where many nearby alternative wetlands are available. These congruent results across independent datasets and analytical approaches support the hypothesis that Hawaiian stilts exhibit density dependence across their range.     

Adaptive plasticity in ontogenetic niche shifts stabilizes consumer-resource dynamics

Ontogenetic niche shifts, changes in the diet or habitats of organisms during their ontogeny, are widespread among various animal taxa. Ontogenetic niche shifts introduce stage structure in a population with different stages interacting with different communities and can substantially affect their dynamics. In this article, I use mathematical models to test the hypothesis that adaptive plasticity in the timing of ontogenetic niche shifts has a stabilizing effect on consumer-resource dynamics. Adaptive plasticity allows consumers in one ontogenetic niche to perform an early shift to the next ontogenetic niche if the resource density of the first niche is low. The early shift will reduce predation by the consumer on the scarce resource. On the other hand, adaptive plasticity allows consumers to delay their shift to the next niche if the resource density of the first niche is high. The delayed shift will increase the predation on the abundant resource. As a result, the scarce resource will tend to increase, and the abundant resource will tend to decrease. This causes density-dependent negative feedback in the resource dynamics, which stabilizes the consumer-resource dynamics. To test this hypothesis, I compare three consumer-resource models differing in terms of mechanisms controlling the timing of the ontogenetic niche shift: the fixed-age model assumes that the age at which the ontogenetic niche shift occurs is fixed; the fixed-size model assumes that the size at the shift is fixed; and the adaptive plasticity model assumes that the timing of the shift is such that the individual fitness of the consumer is maximized. I show that only the adaptive plasticity model has a locally stable equilibrium and that the stabilizing effect is due to the density-dependent negative feedback in the resource dynamics. I discuss the ontogenetic niche shifts of lake fish in light of the obtained result.