Inverse density dependent parasitism in a patchy environment: a laboratory system

Abstract. 1. Two species of parasitoids (Anisopteromalus calandrae (Howard) and Heterospilus prosopidis Vier) attacking the bruchid beetle, Callosobruchus chinensis (L.), show marked inverse density dependent relationships between per cent parasitism and host density per patch.
2. These patterns are well described quantitatively using data on the spatial distribution of searching time by the parasitoids and their attack rates on patches of different host density.
3. A model of optimal foraging predicts just the opposite (i.e. density dependent) patterns of parasitism.
4. Both density dependent and inversely density dependent spatial patterns of parasitism can be explained mechanistically in terms of (a) the allocation of searching time in patches of different host density and (b) the maximum attack rate per parasitoid that constrains the extent of host exploitation within a patch.     

Patterns of parasitism by Trybliographa rapae, a cynipid parasitoid of the cabbage root fly, under laboratory and field conditions

ABSTRACT.

• 1 The spatial patterns of parasitism of the cabbage root fly caused by the cynipid parasitoid Trybliographa rapae (Westw.) have been studied in a laboratory system, within field cages and in a natural situation.
• 2 Continuous observations during the laboratory experiments showed the parasitoids to spend proportionately more time on the patches of high host density. This resulted in the per cent parasitism per patch being directly density dependent.
• 3 Similar patterns of parasitism were found from the field cage system, and also from experiments using the natural parasitoid population and either manipulated or natural host densities.
• 4 While mutual interference was marked in the laboratory experiments, there was little or no sign of it within the larger field cages.

     

Patterns of parasitism by insect parasitoids in patchy environments

Abstract. 1. This paper shows how the different spatial patterns of per cent parasitism in patches of different host density can be explained within a single model framework that takes into account the parasitoid's aggregative response, and the factors limiting the degree of host exploitation within patches.
2. Two contrasting laboratory examples are presented in which the distribution of searching parasitoids and the resulting levels of parasitism in different patches are both known for a range of parasitoid densities.
3. A model is described predicting the number of hosts parasitized per patch, in which the number of parasitoids searching is determined from a simple expression allowing different degrees of aggregation.
4. The model generates patterns of parasitism encompassing the two laboratory examples and a wide range of examples from the field.
5. The importance of density dependent spatial distributions of parasitism to population stability is briefly discussed.     

Spatial distribution of limited resources and local density regulation in juvenile Atlantic salmon

1. Spatial heterogeneity of resources may influence competition among individuals and thus have a fundamental role in shaping population dynamics and carrying capacity. In the present study, we identify shelter opportunities as a limiting resource for juvenile Atlantic salmon (Salmo salar L.). Experimental and field studies are combined in order to demonstrate how the spatial distribution of shelters may influence population dynamics on both within and among population scales. 2. In closed experimental streams, fish performance scaled negatively with decreasing shelter availability and increasing densities. In contrast, the fish in open stream channels dispersed according to shelter availability and performance of fish remaining in the streams did not depend on initial density or shelters. 3. The field study confirmed that spatial variation in densities of 1-year-old juveniles was governed both by initial recruit density and shelter availability. Strength of density-dependent population regulation, measured as carrying capacity, increased with decreasing number of shelters. 4. Nine rivers were surveyed for spatial variation in shelter availability and increased shelter heterogeneity tended to decrease maximum observed population size (measured using catch statistics of adult salmon as a proxy). 5. Our studies highlight the importance of small-scale within-population spatial structure in population dynamics and demonstrate that not only the absolute amount of limiting resources but also their spatial arrangement can be an important factor influencing population carrying capacity.     

Experimental evidence for density dependence of reproduction in great tits

1. Density dependence of avian reproduction has often been analysed using correlations between annual mean reproductive output and population density. Experiments are necessary to prove that density is the cause of the observed patterns, but so far, three out of four experimental studies do not support a direct causal effect of density on reproduction.
2. This paper presents experimental evidence that reproductive decisions in great tits, Parus major L., are causally affected by breeding density. The breeding density of great tits was manipulated by providing nest-boxes at different densities in an ecologically homogeneous area.
3. Within years the densities in the high and low density plots differed approximately 8-fold. During the 11 years of the experiment, clutch size, nestling mass and the proportion of birds starting a second brood were all lower in the high density plot. In 5 years with equal breeding densities in both parts, clutch size did not differ between the plots. The patterns found were consistent with the density effects as predicted from the non-experimental data.     

Influence of density dependence on the detection of trends in unobserved life-history stages

In many species, certain life-history stages are difficult or impossible to observe directly, hampering management. Often more easily observed stages are monitored instead, but the extent to which various forms of uncertainty cloud our ability to discern trends in one critical life-history stage by observing another is poorly studied. We develop a stochastic simulation model for threatened California coho salmon Oncorhynchus kisutch to examine how well trends in one stage can be detected from observations of another. In particular, we use the model to examine the effect density dependence has on our ability to detect trends. We present a structural form for the transition between life-history stages that encompasses the common functional forms: density independence, Beverton–Holt compensatory density dependence and Ricker-type over-compensation. In small populations, such density dependence is often ignored. However, it may in fact be extremely important, for example if population decline was caused by a decrease in carrying capacity. Our results show that density dependence in any life-history transition significantly reduces the ability to detect trends in abundance; critical but inaccessible stages cannot generally be studied by monitoring more easily observed stages, especially if density dependence is present for any life-cycle transition.     

Cycles, stochasticity and density dependence in pink salmon population dynamics

Complex dynamics of animal populations often involve deterministic and stochastic components. A fascinating example is the variation in magnitude of 2-year cycles in abundances of pink salmon (Oncorhynchus gorbuscha) stocks along the North Pacific rim. Pink salmon have a 2-year anadromous and semelparous life cycle, resulting in odd- and even-year lineages that occupy the same habitats but are reproductively isolated in time. One lineage is often much more abundant than the other in a given river, and there are phase switches in dominance between odd- and even-year lines. In some regions, the weak line is absent and in others both lines are abundant. Our analysis of 33 stocks indicates that these patterns probably result from stochastic perturbations of damped oscillations owing to density-dependent mortality caused by interactions between lineages. Possible mechanisms are cannibalism, disease transmission, food depletion and habitat degradation by which one lineage affects the other, although no mechanism has been well-studied. Our results provide comprehensive empirical estimates of lagged density-dependent mortality in salmon populations and suggest that a combination of stochasticity and density dependence drives cyclical dynamics of pink salmon stocks.     

Restricted dispersal reduces the strength of spatial density dependence in a tropical bird population

Spatial processes could play an important role in density-dependent population regulation because the disproportionate use of poor quality habitats as population size increases is widespread in animal populations-the so-called buffer effect. While the buffer effect patterns and their demographic consequences have been described in a number of wild populations, much less is known about how dispersal affects distribution patterns and ultimately density dependence. Here, we investigated the role of dispersal in spatial density dependence using an extraordinarily detailed dataset from a reintroduced Mauritius kestrel (Falco punctatus) population with a territorial (despotic) breeding system. We show that recruitment rates varied significantly between territories, and that territory occupancy was related to its recruitment rate, both of which are consistent with the buffer effect theory. However, we also show that restricted dispersal affects the patterns of territory occupancy with the territories close to release sites being occupied sooner and for longer as the population has grown than the territories further away. As a result of these dispersal patterns, the strength of spatial density dependence is significantly reduced. We conclude that restricted dispersal can modify spatial density dependence in the wild, which has implications for the way population dynamics are likely to be impacted by environmental change.     

Modeling density dependence in heterogeneous landscapes: Dispersal as a case study

Population models often pose density-dependent rates as relations between current population size on a habitat patch, n, and some threshold size defined by limiting resources, r. In fourteen recent modeling studies incorporating density-dependent dispersal, formulations of the density-dependent rate (or probability) fall into two distinct groups, expressing the rate as a function of n-r or n/r. These two depictions of the same process differ fundamentally: they can cause strikingly different dynamics in otherwise identical systems and they have different scaling properties in heterogeneous landscapes. Here I consider the implications of the two formulations under two broad ecological scenarios: scramble competition for an equally divided resource (e.g. food) and contest competition for an unequally divided resource (e.g. nest sites). In both cases, simple arguments show that the n/r form is preferable when density dependence is driven by individual access to resources. Other circumstances may require different formulations, but modelers must ensure that these have appropriate scaling and non-equilibrium behavior.     

景观生态学的核心:生态学系统的时空异质性

1　景观生态学与景观异质性景观生态学是研究在一个相当大的区域内 ,由许多不同生态系统所组成的整体 (即景观 )的空间结构、相互作用、协调功能以及动态变化的生态学新分支[1 ] 。它的出现促进了空间关系模型和理论、空间格局与动态的数据类型的获取以及经典生态学很少涉及的空间尺度检测等方面的发展[2 ] 。Ｒｉｓｓｅｒ等认为景观生态学研究就是异质性的研究[3] 。其实 ,“景观”本身就具有“变化的异质性整体”的含义[4] 。景观生态学集中关注于对生态系统空间关系的研究 ,它把景观视为空间上镶嵌出现和紧密联系的生态系统组合 ,景观可…     

Analysing the performance of a micro soil solution sampling device in a laboratory examination and a field experiment

The performance of a micro soil solution sampling device was tested in a laboratory examination and in a field experiment. The instrument allows detection of temporal and spatial changes in soil solution chemistry at a spatially high resolution. The flexible tube of the suction cell is made of a porous polymer with a diameter of 2.3 mm. To achieve more stability and to minimize disturbance of the instrument during field installation, the original device was modified by embedding the suction cell in a stainless steel and pressure absorbing corpus. During a laboratory test the new sampling system was compared to ceramic P-80 suction cells. Solution samples taken with the new device adapted more quickly to the given concentrations compared to the ceramic suction cells. In a field test, micro samplers were implanted in an existing soil solution monitoring plot, equipped with standard ceramic samplers. Bi-weekly sampling using the micro cells indicated high temporal and spatial variation, and in June 1995 it was possible, to identify a distinct nitrification. However, in a statistical comparison of the entire sampling period and respective sub-sampling areas the two sampler types indicated identical concentration ranges for nitrate. It is concluded that the new micro samplers can help to identify processes in soils which may cause short-term changes in the soil solution chemistry, whereas the standard sampling technique with ceramic cells seems to be still a suitable tool if long-term mean soil solution concentrations are to be measured.     

Comment arising from a paper by Wolda and Dennis: using and interpreting the results of tests for density dependence

We argue that tests for density dependence are useful in analyses of population dynamics and suggest guide lines for their use and interpretation of results which avoid many of the problems discussed by Wolda and Dennis (1993). Processes other than density dependence per se can cause statistical tests to indicate the presence of density dependence (Wolda and Dennis 1993 and unpublished simulations). Tests for density dependence cannot reveal the mechanism of regulation, but they do indicate the nature of long-term population dynamics. Tests for density dependence give misleading results if sampling is not at generation intervals; however, this problem is avoided if we only use tests on data collected in each generation (Holyoak 1993a). Similarly, species should be semelparous. Non-delayed density dependence should not be considered without looking for delayed density dependence, since the presence of delayed density dependence can lead to over-detection of non-delayed density dependence (Woiwod and Hanski 1992; Holyoak 1993b). The partial autocorrelation function and knowledge of life-history are more useful than tests for density dependence for indicating whether any density dependence is delayed or not (Royama 1992; Holyoak 1993b). Estimation error with a constant upper size limit causes tests for density dependence to overestimate the frequency of delayed density dependence; however we do not know whether estimation error is bounded in real populations. Work in progress suggests that 20–40 generations (depending on the nature of population dynamics) gives a moderate level of accuracy with tests for density dependence, and >40 generations are necessary for tests to be accurate in their assessment of the strength of density dependence. We conclude that tests are useful indicators of whether density dependence, or other feedback mechanisms are likely to be acting.     

The frequency of detection of density dependence in insect orders

Abstract.

• 1 A priori, there are no obvious reasons why patterns should exist in the frequency of density dependence across insect orders. However, orders may reflect related factors which influence population regulation (e.g. life-history patterns and ecology) and are difficult to quantify. The frequency of occurrence of density dependence is compared in 171 time series (of ten or more generations) from Lepidoptera, Hemiptera, Diptera, Odonata, Hymenoptera and Coleoptera. A posteriori attempts are made to identify the cause of observed patterns.
• 2 Buhner's (1975) test found non-delayed density dependence more frequently in Odonata than Lepidoptera and Hymenoptera, which in turn showed non-delayed density dependence more frequently than Diptera, Hemiptera and Coleoptera. Similarly, detection was greater for Odonata than other orders using Dennis & Taper's (1993) test for density dependence and Crowley's (1992) test for attraction. Varley & Gradwell's (1960) test found density dependence less frequently in Hemiptera than other orders. These differences were independent of time series length, temporal trends and numbers of generations per year.
• 3 The reasons for observed patterns in detection of density dependence (and attraction) in insect orders are not clear; however, plausible explanations are differences in: (i) intrinsic growth rate, which is correlated with body size (although evidence to support this hypothesis is weak); (ii) the sampling method used; or (iii) whether individuals come from a single population or many populations.
• 4 Using Turchin's (1990) test, delayed (lag 2) density dependence was detected most frequently in Hymenoptera, which often show delayed diapause or are parasitoids.

     

Individual differences, density dependence and offspring birth traits in a population of red deer

Variation between individuals is an essential component of natural selection and evolutionary change, but it is only recently that the consequences of persistent differences between individuals on population dynamics have been considered. In particular, few authors have addressed whether interactions exist between individual quality and environmental variation. In part, this is due to the difficulties of collecting sufficient data, but also the challenge of defining individual quality. Using a long-established study population of red deer, Cervus elaphus, inhabiting the North Block of the Isle of Rum, and three quality measures, this paper investigates how differences in maternal quality affect variation in birth body mass and date, as population density varies, and how this differs depending on the sex of the offspring and the maternal quality measure used. Significant interactions between maternal quality, measured as a hind's total contribution to population growth, and population density are reported for birth mass, but only for male calves. Analyses using dominance or age at primiparity to define maternal quality showed no significant interactions with population density, highlighting the difficulties of defining a consistent measure of individual quality.     

Negative density dependence can offset the effect of species competitive asymmetry: a niche-based mechanism for neutral-like patterns

The debate on the role of species differences in shaping biodiversity patterns, with its two extremes of pure niche theory and neutral theory, is still ongoing. It has been demonstrated that a slight difference in competitive ability of species severely affects the predictions of the neutral model. At the same time, neutral patterns seem to be ubiquitous. Here, we model both negative density dependence (NDD) and competitive asymmetry (CA) simultaneously. Our simulation results show that an appropriate intensity of NDD can offset the negative effect of CA (modeled as fecundity difference) on species coexistence and produce a neutral-like species abundance distribution. Therefore, our model provides a plausible mechanistic explanation of neutral-like patterns, but contrary to the neutral model, a species' relative abundance is positively related to its competitive ability in our model.     

Post-dispersal seed predation: consequences for plant demography and evolution

Post-dispersal seed predation is only one of many factors underlying plant demography and evolution. Nevertheless, the generalist feeding habits of many post-dispersal seed predators and the limited ability of plants either to compensate for or to respond to post-dispersal seed losses directly suggest that post-dispersal seed predation may have a considerable impact on plant populations. Seed predators probably have little direct influence on the demography of plants that regenerate exclusively by vegetative means or are buffered by a large active seed bank, but such species are only a minority in most plant communities.In general, ants are significant post-dispersal seed predators in arid and semi-arid ecosystems while they act mainly as seed dispersers rather than as predators in temperate ecosystems. Although studies have probably underestimated the importance of invertebrates and birds as seed predators, rodents appear to have greater potential to influence seed dynamics, and are particularly important in temperate ecosystems. For example, production of mast seed crops is more effective at satiating specialist invertebrate seed predators than generalist vertebrates, and recruitment may be limited by post-dispersal seed predation even during mast years.Both spatial variation in post-dispersal seed predation and differences in predation between species are important elements which facilitate the coexistence of different plant species. Where microsites are limiting, selective post-dispersal seed predators can influence pre-emptive competition for these microsites. Seed size determines the extent of density-dependent predation and the exploitation of buried seed. This suggests that post-dispersal seed predators may also play a role in the evolution of seed characteristics. However, conclusions regarding the ecological and evolutionary impact of post-dispersal seed predators will remain speculative without a more substantial empirical base.     

Predators reverse the direction of density dependence for juvenile salmon mortality

The effect of predators on prey populations depends on how predator-caused mortality changes with prey population density. Predators can enforce density-dependent prey mortality and contribute to population stability, but only if they have a positive numerical or behavioral response to increased prey density. Otherwise, predator saturation can result in inversely density-dependent mortality, destabilizing prey populations and increasing extinction risk. Juvenile salmon and trout provide some of the clearest empirical examples of density-dependent mortality in animal populations. However, although juvenile salmon are very vulnerable to predators, the demographic effects of predators on juvenile salmon are unknown. We tested the interactive effects of predators and population density on the mortality of juvenile Atlantic salmon (Salmo salar) using controlled releases of salmon in natural streams. We introduced newly hatched juvenile salmon at three population density treatments in six study streams, half of which contained slimy sculpin (Cottus cognatus), a common generalist predator (18 release sites in total, repeated over two summers). Sculpin reversed the direction of density dependence for juvenile salmon mortality. Salmon mortality was density dependent in streams with no sculpin, but inversely density dependent in streams where sculpin were abundant. Such predator-mediated inverse density dependence is especially problematic for prey populations suppressed by other factors, thereby presenting a fundamental challenge to persistence of rare populations and restoration of extirpated populations.     

Micro‐scale structure in the chemistry and biology of a shallow lake

1. Repeat sampling in daytime within a lily (Nuphar lutea) bed and in open water showed distinct heterogeneities in the three‐dimensional distributions of water chemistry and planktonic organisms on centimetre to decimetre scales. 2. Vertical gradients of physico‐chemical variables that did not exist at dawn developed during the day in both sites, as available nutrients were released from the sediments and were consumed towards the surface. 3. Distributions of algal standing crop suggest limitation by both nutrients and grazing. 4. Marked variability in distributions may question the assumptions often made about the homogeneity of plankton and available nutrient distributions in open water and in macrophyte stands of shallow lakes. Although simple sampling regimes for monitoring of water quality may be adequate for many purposes, they miss a fine structure in the water that is inherently interesting in understanding the underlying processes of plankton function.