Effects of tick population dynamics and host densities on the persistence of tick-borne infections

The transmission and the persistence of tick-borne infections are strongly influenced by the densities and the structure of host populations. By extending previous models and analysis, in this paper we analyse how the persistence of ticks and pathogens, is affected by the dynamics of tick populations, and by their host densities. The effect of host densities on infection persistence is explored through the analysis and simulation of a series of models that include different assumptions on tick-host dynamics and consider different routes of infection transmission. Ticks are assumed to feed on two types of host species which vary in their reservoir competence. Too low densities of competent hosts (i.e., hosts where transmission can occur) do not sustain the infection cycle, while too high densities of incompetent hosts may dilute the competent hosts so much to make infection persistence impossible. A dilution effect may occur also for competent hosts as a consequence of reduced tick to host ratio; this is possible only if the regulation of tick populations is such that tick density does not increase linearly with host densities.     

Impact of biodiversity and seasonality on Lyme-pathogen transmission

Lyme disease imposes increasing global public health challenges. To better understand the joint effects of seasonal temperature variation and host community composition on the pathogen transmission, a stage-structured periodic model is proposed by integrating seasonal tick development and activity, multiple host species and complex pathogen transmission routes between ticks and reservoirs. Two thresholds, one for tick population dynamics and the other for Lyme-pathogen transmission dynamics, are identified and shown to fully classify the long-term outcomes of the tick invasion and disease persistence. Seeding with the realistic parameters, the tick reproduction threshold and Lyme disease spread threshold are estimated to illustrate the joint effects of the climate change and host community diversity on the pattern of Lyme disease risk. It is shown that climate warming can amplify the disease risk and slightly change the seasonality of disease risk. Both the “dilution effect” and “amplification effect” are observed by feeding the model with different possible alternative hosts. Therefore, the relationship between the host community biodiversity and disease risk varies, calling for more accurate measurements on the local environment, both biotic and abiotic such as the temperature and the host community composition.     

Host defence versus intraspecific competition in the regulation of infrapopulations of the flea Xenopsylla conformis on its rodent host Meriones crassus

Mechanisms that regulate parasite populations may influence the evolution of hosts and parasites, as well as the stability of host-parasite dynamics but are still poorly understood. A manipulation experiment on the grooming ability of rodent hosts (Meriones crassus) and flea (Xenopsylla conformis) densities on these hosts successfully disentangled two possible regulating mechanisms: (i) behavioural defence of the host and (ii) intraspecific competition among parasites, and revealed their importance in suppressing the feeding of fleas. Moreover, the results suggest that flea competition is direct and is not mediated by host grooming, immune response, or parasite-induced damage to the host. These mechanisms, together with interspecific competition and density-dependent parasite-induced host damage, may limit the parasite burden on an individual host and may prevent parasites from overexploiting their host population.     

Indirect evidence of density‐dependent population regulation in Aponomma hydrosauri (Acari: Ixodidae), an ectoparasite of reptiles

Abstract The extent to which density‐dependent processes regulate natural populations is the subject of an ongoing debate. We contribute evidence to this debate showing that density‐dependent processes influence the population dynamics of the ectoparasite Aponomma hydrosauri (Acari: Ixodidae), a tick species that infests reptiles in Australia. The first piece of evidence comes from an unusually long‐term dataset on the distribution of ticks among individual hosts. If density‐dependent processes are influencing either host mortality or vital rates of the parasite population, and those distributions can be approximated with negative binomial distributions, then general host–parasite models predict that the aggregation coefficient of the parasite distribution will increase with the average intensity of infections. We fit negative binomial distributions to the frequency distributions of ticks on hosts, and find that the estimated aggregation coefficient k increases with increasing average tick density. This pattern indirectly implies that one or more vital rates of the tick population must be changing with increasing tick density, because mortality rates of the tick's main host, the sleepy lizard, Tiliqua rugosa, are unaffected by changes in tick burdens. Our second piece of evidence is a re‐analysis of experimental data on the attachment success of individual ticks to lizard hosts using generalized linear modelling. The probability of successful engorgement decreases with increasing numbers of ticks attached to a host. This is direct evidence of a density‐dependent process that could lead to an increase in the aggregation coefficient of tick distributions described earlier. The population‐scale increase in the aggregation coefficient is indirect evidence of a density‐dependent process or processes sufficiently strong to produce a population‐wide pattern, and thus also likely to influence population regulation. The direct observation of a density‐dependent process is evidence of at least part of the responsible mechanism.     

An invasive plant-fungal mutualism reduces arthropod diversity

Ecological theory holds that competition and predation are the most important biotic forces affecting the composition of communities. Here, we expand this framework by demonstrating that mutualism can fundamentally alter community and food web structure. In large, replicated field plots, we manipulated the mutualism between a dominant plant ( Lolium arundinaceum ) and symbiotic fungal endophyte ( Neotyphodium coenophialum ). The presence of the mutualism reduced arthropod abundance up to 70%, reduced arthropod diversity nearly 20%, shifted arthropod species composition relative to endophyte-free plots and suppressed the biomass and richness of other plant species in the community. Herbivorous arthropods were more strongly affected than carnivores, and for both herbivores and carnivores, effects of the mutualism appeared to propagate indirectly via organisms occurring more basally in the food web. The influence of the mutualism was as great or greater than previously documented effects of competition and predation on arthropod communities. Our work demonstrates that a keystone mutualism can significantly reduce arthropod biodiversity at a broad community scale.     

Host community structure and infestation by ixodid ticks: repeatability,dilution effect and ecological specialization

Abundance of a species in a location results from the interplay between the intrinsic properties of that species and the extrinsic properties, both biotic and abiotic, of the local habitat. Intrinsic factors promote among-population stability in abundance, whereas extrinsic factors generate variation among populations of a species. We studied (a) repeatability and (b) the effect of abundance and species richness of small mammals on the level of their infestation by larvae and nymphs of Ixodes ricinus (ecological generalist) and Ixodes trianguliceps (ecological specialist). We asked if tick infestation parameters are characteristic (=repeatable) for a particular host species or a particular stage of a particular tick species. We also asked how abundance and diversity of hosts affect the level of tick infestation on a particular host species. We predicted that the dilution effect (decrease in tick infestation levels with an increase of host abundance and/or species richness) will be better expressed in an ecological generalist, I. ricinus, than in an ecological specialist, I. trianguliceps. We found that (a) tick abundance, prevalence and aggregation were generally repeatable within tick species/stage; (b) tick abundance and prevalence, but not the aggregation level, were repeatable within host species; (c) the proportion of variance among samples explained by the differences between tick species and stages (as opposed to within-tick species and stage) was higher than that explained by the differences between host species (as opposed to within host species); and (d) the relationship between tick infestation parameters and host abundance and diversity revealed the dilution effect for I. ricinus but not for I. trianguliceps.     

Simulation of climate‐tick‐host‐landscape interactions: Effects of shifts in the seasonality of host population fluctuations on tick densities

Tick vector systems are comprised of complex climate‐tick‐host‐landscape interactions that are difficult to identify and estimate from empirical observations alone. We developed a spatially‐explicit, individual‐based model, parameterized to represent ecological conditions typical of the south‐central United States, to examine effects of shifts in the seasonal occurrence of fluctuations of host densities on tick densities. Simulated shifts in the seasonal occurrence of periods of high and low host densities affected both the magnitude of unfed tick densities and the seasonality of tick development. When shifting the seasonal densities of all size classes of hosts (small, medium, and large) synchronously, densities of nymphs were affected more by smaller shifts away from the baseline host seasonality than were densities of larval and adult life stages. When shifting the seasonal densities of only a single size‐class of hosts while holding other size classes at their baseline levels, densities of larval, nymph, and adult life stages responded differently. Shifting seasonal densities of any single host‐class earlier resulted in a greater increase in adult tick density than when seasonal densities of all host classes were shifted earlier simultaneously. The mean densities of tick life stages associated with shifts in host densities resulted from system‐level interactions of host availability with tick phenology. For example, shifting the seasonality of all hosts ten weeks earlier resulted in an approximately 30% increase in the relative degree of temporal co‐occurrence of actively host‐seeking ticks and hosts compared to baseline, whereas shifting the seasonality of all hosts ten weeks later resulted in an approximately 70% decrease compared to baseline. Differences among scenarios in the overall presence of active host‐seeking ticks in the system were due primarily to the degree of co‐occurrence of periods of high densities of unfed ticks and periods of high densities of hosts.     

Epidemic size determines population-level effects of fungal parasites on Daphnia hosts

Parasites frequently reduce the fecundity, growth, and survival of individual hosts. How often do these virulent effects reduce the density of host populations? Spectacular examples show that recently invaded parasites can severely impact host populations—but what about parasites persisting long-term in host populations? We have addressed this issue using a zooplankton host (Daphnia dentifera) that becomes infected with a fungal microparasite (Metschnikowia bicuspidata). We combined observations of epidemics in nine lakes over 6 years, fine-scale sampling of three epidemics, and a mesocosm experiment. Most epidemics remained small (<10% maximum prevalence) and exerted little influence on host densities. However, larger epidemics more severely depressed the populations of their hosts. These large/severe epidemics started and peaked earlier than smaller/benign ones. The larger epidemics also exerted particularly negative effects on host densities at certain lags, reflecting the delayed consequences of infection on fecundity reduction and host mortality. Notably, negative effects on the juvenile stage class manifested later than those on the adult stage class. The results of the experiment further emphasized depression of host density by the fungus, especially on the density of the juvenile stage class. Consequently, this common parasite reduces the density of host populations when conditions foster larger outbreaks characterized by an earlier start and earlier peak. Given these considerable effects on host density seen in a number of large epidemics, parasitism may sometimes rank highly among other factors (predation, resource availability) driving the population dynamics of these hosts.     

Opposing deer and caterpillar foraging preferences may prevent reductions in songbird prey biomass in historically overbrowsed forests

Overbrowsing by ungulates decimates plant populations and reduces diversity in a variety of ecosystems, but the mechanisms by which changes to plant community composition influence other trophic levels are poorly understood. In addition to removal of avian nesting habitat, browsing is hypothesized to reduce bird density and diversity through reduction of insect prey on browse‐tolerant hosts left behind by deer. In this study, we excluded birds from branches of six tree species to quantify differences in songbird prey removal across trees that vary in deer browse preference. Early in the breeding season, birds preyed on caterpillars at levels proportional to their abundance on each host. Combining these data with tree species composition data from stands exposed to experimentally controlled deer densities over 30 years ago, we tested whether overbrowsing by white‐tailed deer reduces prey biomass long after deer densities are reduced. Our analysis predicts total prey availability in the canopy of regenerating forests is fairly robust to historic exposure to high deer densities, though distribution of prey available from host species changes dramatically. This predicted compensatory effect was unexpected and is driven by high prey abundance on a single host tree species avoided by browsing deer, Prunus serotina. Thus, while we confirm that prey abundance on host trees can act as a reliable predictor for relative prey availability, this study shows that quantifying prey abundance across host trees is essential to understanding how changes in tree species composition interact with ungulate browse preference to determine prey availability for songbirds.     

Persistence of host and parasite populations subject to experimental size-selective removal

Predators have the potential to limit the spread of pathogens not only by selecting infected prey but also by shaping prey demographics. We tested this idea with an epidemiological experiment in which we simulated variable levels of size-selective predation on zooplankton hosts and monitored the persistence of host and parasite populations. In the absence of simulated predation, the virulent protozoan Caullerya mesnili frequently drove its host Daphnia galeata to extinction. Uninfected control populations showed lower extinction rates and higher average densities than infected populations in the absence of simulated predation (all of the latter went extinct or remained infected). With a weak removal rate of the largest hosts, the proportion of populations in which the parasite drove the host to extinction decreased, while the number of populations in which the host persisted and the parasite went extinct increased. Host-parasite coexistence was also observed in some cases. With intermediate levels of removal, most of the parasite populations went extinct, while the host populations persisted. With an even higher removal rate, Daphnia were driven to extinction as well. Thus, variation in one factor, size-selective mortality, resulted in four different patterns of population dynamics. Our results highlight the potential role of predation in shaping the epidemiology and community structure of host-parasite systems.     

Effects of climate‐driven primary production change on marine food webs: implications for fisheries and conservation

Climate change is altering the rate and distribution of primary production in the world's oceans. Primary production is critical to maintaining biodiversity and supporting fishery catches, but predicting the response of populations to primary production change is complicated by predation and competition interactions. We simulated the effects of change in primary production on diverse marine ecosystems across a wide latitudinal range in Australia using the marine food web model Ecosim. We link models of primary production of lower trophic levels (phytoplankton and benthic producers) under climate change with Ecosim to predict changes in fishery catch, fishery value, biomass of animals of conservation interest, and indicators of community composition. Under a plausible climate change scenario, primary production will increase around Australia and generally this benefits fisheries catch and value and leads to increased biomass of threatened marine animals such as turtles and sharks. However, community composition is not strongly affected. Sensitivity analyses indicate overall positive linear responses of functional groups to primary production change. Responses are robust to the ecosystem type and the complexity of the model used. However, model formulations with more complex predation and competition interactions can reverse the expected responses for some species, resulting in catch declines for some fished species and localized declines of turtle and marine mammal populations under primary productivity increases. We conclude that climate‐driven primary production change needs to be considered by marine ecosystem managers and more specifically, that production increases can simultaneously benefit fisheries and conservation. Greater focus on incorporating predation and competition interactions into models will significantly improve the ability to identify species and industries most at risk from climate change.     

Competition,facilitation or mediation via host? Patterns of infestation of small European mammals by two taxa of haematophagous arthropods

1. We studied the effect of flea infestation on the pattern of tick (Ixodes ricinus and Ixodes trianguliceps) infestation on small mammals. 2. We asked (1) whether the probability of an individual host being infested by ticks was affected by its infestation of fleas (number of individuals and species) and (2) whether the abundance and prevalence of ticks in a host population was affected by the abundance, prevalence, level of aggregation, and species richness of fleas. 3. The probability of a host individual being infested by ticks was affected negatively by flea infestation. At the level of host populations, flea abundance and prevalence had a predominantly positive effect on tick infestation, whereas flea species richness had a negative effect on tick infestation. 4. The effect of flea infestation on tick infestation was generally greater in I. ricinus than in I. trianguliceps, but varied among host species. 5. It can be concluded that the effect of fleas on tick infestation of small mammals may be either negative or positive depending on the level of consideration and parameters involved. The results did not provide support for direct interactions between the two ectoparasite taxa, but suggested population and community dynamics and the defence system of the hosts as possible factors.     

Resource limitation,competition and the influence of life history in a freshwater snail community

Summary Previous work on a snail community occurring throughout lakes in southwestern Michigan showed that predation by molluscivorous sunfish had large impacts on only the rarest snail species. Thus, competition might play a major role in population limitation because dominant members of the snail community are relatively immune to predation. The present experiments were conducted to determine the extent to which the snail community depleted the abundance of food resources (epiphytes) and the extent to which epiphyte abundances limited snail production. An experimental gradient in snail densities showed that removal of snails increased epiphyte biomass by approximately 3-fold relative to that observed at natural snail densities. Enrichment of the environment with phosphorus fertilizer increased epiphyte biomass by approximately 20-fold and provided a test of food limitation in the snail community. All snail taxa exhibited positive numerical or growth responses to enrichment. The observations that snails depleted resources and that resources limited snail production demonstrated that snails competed exploitatively for epiphytes. The response of each snail species to increased food abundance differed depending on the timing of fertilization relative to the snails' life histories. Snails hatched before the experiment began were larger in fertilized treatments, due to increased growth, but their densities were similar among treatments. On the other hand, densities of snails born during the experiment were up to 15-fold greater in fertilized treatments, due in part to increased survival of newborn snails. Comparison of the responses of snails to food addition and to predator removals (based on prior experiments) suggested that food availability limits snail production more than predators do. Additionally, the large responses by algae and snails to fertilization demonstrated that both the producers and herbivores in this simplified food chain were strongly resource limited.     

Two different strategies of host manipulation allow parasites to persist in intermediate–definitive host systems

Trophically transmitted parasites start their development in an intermediate host, before they finish the development in their definitive host when the definitive host preys on the intermediate host. In intermediate–definitive host systems, two strategies of host manipulation have been evolved: increasing the rate of transmission to the definitive host by increasing the chance that the definitive host will prey on the intermediate host, or increasing the lifespan of the parasite in the intermediate host by decreasing the predation chance when the intermediate host is not yet infectious. As the second strategy is less well studied than the first, it is unknown under what conditions each of these strategies is prevailed and evolved. We analysed the effect of both strategies on the presence of parasites in intermediate–definitive host systems with a structured population model. We show that the parasite can increase the parameter space where it can persist in the intermediate–definitive host system using one of these two strategies of host manipulation. We found that when the intermediate host or the definitive host has life‐history traits that allow the definitive host to reach large population densities, that is high reproduction rate of the intermediate host or high conversion efficiency of the definitive host (efficiency at which the uninfected definitive host converts caught intermediate hosts into offspring), respectively, evolving manipulation to decrease the predation chance of the intermediate host will be more beneficial than manipulation to increase the predation chance to enhance transmission. Furthermore, manipulation to decrease the predation chance of the intermediate host results in higher population densities of infected intermediate hosts than manipulation that increases the predation chance to enhance transmission. Our study shows that host manipulation in early stages of the parasite development to decrease predation might be a more frequently evolved way of host manipulation than is currently assumed.     

Species traits and environmental characteristics together regulate ant‐associated biodiversity

Host‐associated organisms (e.g., parasites, commensals, and mutualists) may rely on their hosts for only a portion of their life cycle. The life‐history traits and physiology of hosts are well‐known determinants of the biodiversity of their associated organisms. The environmental context may strongly influence this interaction, but the relative roles of host traits and the environment are poorly known for host‐associated communities. We studied the roles of host traits and environmental characteristics affecting ant‐associated mites in semi‐natural constructed grasslands in agricultural landscapes of the Midwest USA. Mites are frequently found in ant nests and also riding on ants in a commensal dispersal relationship known as phoresy. During nonphoretic stages of their development, ant‐associated mites rely on soil or nest resources, which may vary depending on host traits and the environmental context of the colony. We hypothesized that mite diversity is determined by availability of suitable host ant species, soil detrital resources and texture, and habitat disturbance. Results showed that that large‐bodied and widely distributed ant species within grasslands support the most diverse mite assemblages. Mite richness and abundance were predicted by overall ant richness and grassland area, but host traits and environmental predictors varied among ant hosts: mites associated with Aphaenogaster rudis depended on litter depth, while Myrmica americana associates were predicted by host frequency and grassland age. Multivariate ordinations of mite community composition constructed with host ant species as predictors demonstrated host specialization at both the ant species and genus levels, while ordinations with environmental variables showed that ant richness, soil texture, and grassland age also contributed to mite community structure. Our results demonstrate that large‐bodied, locally abundant, and cosmopolitan ant species are especially important regulators of phoretic mite diversity and that their role as hosts is also dependent on the context of the interaction, especially soil resources, texture, site age, and area.     

Competing for blood: the ecology of parasite resource competition in human malaria–helminth co‐infections

Ecological theory suggests that co‐infecting parasite species can interact within hosts directly, via host immunity and/or via resource competition. In mice, competition for red blood cells (RBCs) between malaria and bloodsucking helminths can regulate malaria population dynamics, but the importance of RBC competition in human hosts was unknown. We analysed infection density (i.e. the concentration of parasites in infected hosts), from a 2‐year deworming study of over 4000 human subjects. After accounting for resource‐use differences among parasites, we find evidence of resource competition, priority effects and a competitive hierarchy within co‐infected individuals. For example reducing competition via deworming increased Plasmodium vivax densities 2.8‐fold, and this effect is limited to bloodsucking hookworms. Our ecological, resource‐based perspective sheds new light into decades of conflicting outcomes of malaria–helminth co‐infection studies with significant health and transmission consequences. Beyond blood, investigating within‐human resource competition may bring new insights for improving human health.     

Food Webs and Disease: Is Pathogen Diversity Limited by Vector Diversity?

Classical predator–prey and host–parasite systems have been extensively studied in a food web context. Less attention has been paid to communities that include pathogens and their vectors. We present a coarse-grained, pan-African analysis of the relationships between the abiotic environment (location, precipitation, temperature), the species richness and community composition of ixodid ticks, and the species richness and community composition of pathogens that ticks transmit to humans. We found strong correlations between the abiotic environment and tick species richness, and a weak but significant correlation between the abiotic environment and pathogen species richness. A substantial amount of variation in community composition of parasites and pathogens was not explained by the variables that we considered. A structural equation model that compensated for the indirect effects of climate on the pathogen community via tick community composition suggested that while the environment strongly regulates tick community composition and tick community composition strongly regulates pathogen community composition, abiotic influences on pathogen species richness and community composition are weak. Our results support the view that changes in the broader environment will influence tick-borne pathogens primarily via the influence of the environment on ticks. The interactions that regulate host–vector–pathogen dynamics are of particular relevance in understanding the relationships between environmental change and health concerns, such as the impact of climate change on the occurrence of vector-borne diseases.     

Effects of interspecific competition on asexual proliferation and clonal genetic diversity in larval trematode infections of snails

Interactions among different parasite species within hosts can be important factors shaping the evolution of parasite and host populations. Within snail hosts, antagonistic interactions among trematode species, such as competition and predation, can influence parasite abundance and diversity. In the present study we examined the strength of antagonistic interactions between 2 marine trematodes (Maritrema novaezealandensis and Philophthalmus sp.) in naturally infected Zeacumantus subcarinatus snails. We found approximately the same number of snails harbouring both species as would be expected by chance given the prevalence of each. However, snails infected with only M. novaezealandensis and snails with M. novaezealandensis and Philophthalmus sp. co-occurring were smaller than snails harbouring only Philophthalmus sp. In addition, the number of Philophthalmus sp. rediae was not affected by the presence of M. novaezealandensis sporocysts and the within-host clonal diversity of M. novaezealandensis was not influenced by the presence of Philophthalmus sp. Our results suggest that antagonistic interactions may not be a major force influencing the evolution of these trematodes and that characteristics such as host size and parasite infection longevity are shaping their abundance and population dynamics.     

Come with me if you want to live: sympatric parasites follow different transmission routes through aquatic host communities

Community composition, including the relative density of each host species, plays a vital role in the transmission of parasites or disease in freshwater ecosystems. Whereas some host species can effectively transmit parasites, others can act as dead ends (non-viable transmission routes), accumulating large numbers of parasites throughout their life, thus becoming important sinks for parasite populations. Although population sinks have been identified in certain host-parasite systems, robust field estimates of the proportions of parasites that are lost to these hosts are lacking. Here, we quantified the distribution of encysted larval hairworms (phylum Nematomorpha), common parasites in lotic ecosystems, in two subalpine stream communities of New Zealand. With parasite and host population densities calculated per m², we identified which host species most likely contributed to the transmission of three sympatric hairworm morphotypes identified in both streams, and which species acted as population sinks. We also tested for seasonal patterns and peaks in the abundance of each morphotype in the two communities over the sampling season. Finally, we tested whether hosts emerging from the streams had comparable abundances of hairworm morphotypes throughout the sampling period. For each morphotype, different key sets of host species harboured more hairworms on average (abundance) than others, depending on the stream. For one morphotype in particular, two species of hosts were found to be important population sinks that inhibited over a third of these parasites from completing their life cycle. We also observed a clear peak in abundance for another hairworm morphotype during summer. Our data suggest that hosts emerging from the streams matched their aquatic counterparts with respect to hairworm abundance, indicating no infection-dependent reduction in emergence success. Our findings suggest that, depending on relative community composition, sympatric parasites follow different host transmission pathways, some of which lead to dead ends that potentially impact overall infection dynamics. In turn, this information can help us understand the spread or emergence of disease in both freshwater and terrestrial environments, since hairworms infect terrestrial arthropods to complete their life cycle.     

The impact of raptors on the abundance of upland passerines and waders

The issue of predator limitation of vertebrate prey populations is contentious, particularly when it involves species of economic or conservation value. In this paper, we examine the case of raptor predation on upland passerines and waders in Scotland. We analysed the abundance of five wader and passerine species on an upland sporting estate in southern Scotland during an eight-year period when hen harrier, peregrine and merlin numbers increased due to strict law enforcement. The abundance of meadow pipit and skylark declined significantly during this time. Golden plover also showed a declining trend, whereas curlew increased significantly and there was a near significant increase in lapwings. Contrasting the local population trends of these species with trends on nearby areas revealed higher rates of decline for meadow pipit and skylark at the site where raptors increased, but no differences in trends for any of the three wader species. There was a negative relationship between the number of breeding harriers and meadow pipit abundance the same year and between total annual raptor numbers and meadow pipit abundance. Predation rates of meadow pipit and skylark determined from observations at harrier nests suggested that predation in June was sufficient to remove up to 40% of the June meadow pipit population and up to 34% of the June skylark population. This 'quasi-natural' experiment suggests that harrier predation limited the abundance of their main prey, meadow pipit, and possibly the abundance of skylark. Thus, high densities of harriers may in theory reduce the abundance of the prey species which determine their breeding densities, potentially leading to lower harrier breeding densities in subsequent years. We found no evidence to suggest that raptor predation limited the populations of any of the three wader species. We infer that concerns over the impact of natural densities of hen harriers on vulnerable upland waders are unjustified.