Aspects of the ecology and natural history of Paraergasilus rylovi (Copepoda, Ergasilidae) parasitic in unionids of Finland

The distribution of Paraergasilus rylovi in 17 populations of unionids was investigated. In 1 unionid population, the parasite was studied regarding host age, size, sex, and the reproductive period (occurrence of egg sacs). Results from pooled material from the years 1987--1989 and 1996 (southern Finland, 11 populations) indicated that Anodonta piscinalis (n = 1,359) is the main host (total mean prevalence 71% and intensity +/-SE of infection 16.4+/-0.6). Pseudanodonta complanata (n = 106) was infected occasionally (3% and 1.3+/-0.3), whereas Unio pictorum (n = 108) and U. tumidus (n = 17) were not infected. Results from 17 A. piscinalis populations showed that P. rylovi occurs in southern Finland but not in northern Finland. In A. piscinalis, the mean intensity of infection was higher in lake populations than in river populations. Both host age and length had a negative relationship with the intensity of P. rylovi infection. Host sex did not affect the intensity of infection. Egg sacs of P. rylovi were found from June to August. There was a tendency for higher intensities of infection in autumn. Infection by the digenean Rhipidocotyle fennica had no effect on the intensity of P. rylovi infection.     

Reduction in the level of infection of the bivalve Anodonta piscinalis by the copepod Paraergasilus rylovi using high temperature and low oxygen

The aim of this study was to develop a method to kill or expel the gill-dwelling crustacean parasite Paraergasilus rylovi from a common freshwater clam, Anodonta piscinalis. Naturally infected clams were exposed to different water-quality treatments and monitoring in the laboratory. In a high-temperature treatment (26 C vs. control 18 C), the mean abundance of the parasite decreased to near zero in 7 days. Because only 2 clams of 72 died in this treatment during the 14-day experiment, the survival of the host was not seriously at risk at the high temperature. 'Low oxygen, no water change' (18 C) was the second most effective treatment, followed by a 'low-oxygen, water-flow' (18 C) treatment. At the end of the experiment, the mean parasite abundance was significantly lower in all the treatments than in the control clams (18 C). A few P. rylovi individuals abandoned the host at 26 C but died in a couple of days outside the host. However, the parasites lived on average (+/-SE) 12.7 +/- 0.9 days outside the clam, and were also shown to be capable of infecting another uninfected host individual, at 18 C. The results of the present study suggest that high temperature provides an effective, ecologically sustainable method to manipulate the intensity of P. rylovi infection.     

Decreased reproductive investment of female threespine stickleback Gasterosteus aculeatus infected with the cestode Schistocephalus solidus: parasite adaptation,host adaptation,or side effect?

Parasitic infections may cause alterations in host life history, including changes in reproductive investment (absolute amount of energy allocated to reproduction) and reproductive effort (proportion of available energy allocated to reproduction). Such changes in host life history may reflect: 1) a parasite tactic: the parasite adaptively manipulates energy flow within the host so that the host is induced to make a reduction in reproductive effort and reproductive investment, making more energy available to the parasite; 2) no tactic: there is no change in host reproductive effort and reproductive investment simply decreases as a side effect of the parasite depleting host energy stores; 3) a host tactic: the host adaptively increases reproductive effort in the face of infection and loss of body condition, reproductive investment possibly being reduced despite the increased reproductive effort. Females in Alaskan lake populations of threespine sticklebacks ( Gasterosteus aculeatus ) are capable of clutch production when parasitized by the cestode Schistocephalus solidus despite large relative parasite masses. We analyzed the somatic energy reserves, maturation stage and ovarian mass of female sticklebacks collected from an Alaska lake during a single reproductive season. We found that parasitized females were less likely to carry fully-matured gametes, had smaller ovarian masses, and had lower somatic energy stores than unparasitized females. The relationship between reproductive investment and energy storage did not differ between parasitized and unparasitized females. Thus, reproductive effort did not change in response to parasitic infection. We conclude there was no indication of either a parasite tactic or a host tactic. Simple nutrient theft is involved in the parasite's influence on host reproduction, consistent with an earlier hypothesis that reproductive curtailment in threespine sticklebacks is a side effect.     

Association between haematozoan infections and reproduction in the Pied Flycatcher

1. Parasites may affect breeding success of their host since they compete for the same resources as their hosts. Reproduction may also increase the susceptibility of a host to parasite infections owing to lowered resistance to parasites during breeding.
2. We studied the association between breeding performance and haematozoan parasite infection in the Pied Flycatcher ( Ficedula hypoleuca ) by using both natural data on reproduction and data from clutch size manipulations.
3. The most frequent blood parasites of the Pied Flycatcher in central Finland were Haemoproteus pallidus , Haemoproteus balmorali and Trypanosoma avium complex.
4. We did not find evidence that these haematozoan parasites have any debilitating effects on either reproduction or survival. The variation in reproductive effort did not seem to influence susceptibility to new blood parasite infections.
5. The intensity of Haemoproteus balmorali tended to increase in infected males as the brood size was artificially enlarged. Also, in females intensity of H. pallidus infection tended to increase with the level of clutch size manipulation. Thus, increased reproductive effort seems to debilitate the ability of Pied Flycatcher to control chronic infections.
6. Individuals with enlarged clutches/broods increased their reproductive effort at the expense of defence towards parasites. The cost of current reproduction may then be at least partly mediated by haematozoan infections.     

Parasitic mite and trematode abundance are associated with reduced reproductive output and physiological condition of freshwater mussels

Although historically understudied, parasites may play an important role in freshwater invertebrate population ecology and evolution. We quantified abundance of parasitic mites and trematodes in the freshwater mussel Pyganodon grandis, in a southeastern Alabama stream (USA), to assess parasite impact on adult mussel physiological condition and reproductive output. We used stepwise multiple regression analyses to assess the effects of mussel size and parasites on reproduction and condition. Multiple regression analysis found no multivariate models that predicted reproductive output or physiological condition. However, univariate models revealed that increased parasite densities predict reduced mussel reproductive output and physiological condition. These findings suggest that parasites may have important negative consequences for freshwater mussels. We hypothesize that elevated parasite loads may reduce mussel fitness in impounded or nutrient-enriched streams with high densities of intermediate hosts (chironomid midges).     

Resistance and tolerance in a host plant-holoparasitic plant interaction: genetic variation and costs

Host organisms are believed to evolve defense mechanisms (i.e., resistance and/or tolerance) under selective pressures exerted by natural enemies. A prerequisite for the evolution of resistance and tolerance is the existence of genetic variation in these traits for natural selection to act. However, selection for resistance and/or tolerance may be constrained by negative genetic correlations with other traits that affect host fitness. We studied genetic variation in resistance and tolerance against parasitic infection and the potential fitness costs associated with these traits using a novel study system, namely the interaction between a flowering plant and a parasitic plant. In this system, parasitic infection has significant negative effects on host growth and reproduction and may thus act as a selective agent. We conducted a greenhouse experiment in which we grew host plants, Urtica dioica, that originated from a single natural population and represented 20 maternal families either uninfected or infected with the holoparasitic dodder, Cuscuta europaea. that originated from the same site. We calculated correlations among resistance, tolerance, and host performance to test for costs of resistance and tolerance. We measured resistance as parasite performance (quantitative resistance) and tolerance as the slopes of regressions relating the vegetative and reproductive biomass of host plants to damage level (measured as parasite biomass). We observed significant differences among host families in parasite resistance and in parasite tolerance in terms of reproductive biomass, a result that suggests genetic variation in these traits. Furthermore, we found differences in resistance and tolerance between female and male host plants. In addition, the correlations indicate costs of resistance in terms of host growth and reproduction and costs of tolerance in terms of host reproduction. Our results thus indicate that host tolerance and resistance can evolve as a response to infection by a parasitic plant and that costs of resistance and tolerance may be one factor maintaining genetic variation in these traits.     

The role of host traits, season and group size on parasite burdens in a cooperative mammal

The distribution of parasites among hosts is often characterised by a high degree of heterogeneity with a small number of hosts harbouring the majority of parasites. Such patterns of aggregation have been linked to variation in host exposure and susceptibility as well as parasite traits and environmental factors. Host exposure and susceptibility may differ with sexes, reproductive effort and group size. Furthermore, environmental factors may affect both the host and parasite directly and contribute to temporal heterogeneities in parasite loads. We investigated the contributions of host and parasite traits as well as season on parasite loads in highveld mole-rats (Cryptomys hottentotus pretoriae). This cooperative breeder exhibits a reproductive division of labour and animals live in colonies of varying sizes that procreate seasonally. Mole-rats were parasitised by lice, mites, cestodes and nematodes with mites (Androlaelaps sp.) and cestodes (Mathevotaenia sp.) being the dominant ecto- and endoparasites, respectively. Sex and reproductive status contributed little to the observed parasite prevalence and abundances possibly as a result of the shared burrow system. Clear seasonal patterns of parasite prevalence and abundance emerged with peaks in summer for mites and in winter for cestodes. Group size correlated negatively with mite abundance while it had no effect on cestode burdens and group membership affected infestation with both parasites. We propose that the mode of transmission as well as social factors constrain parasite propagation generating parasite patterns deviating from those commonly predicted.     

From individual heterogeneity to population-level overdispersion: quantifying the relative roles of host exposure and parasite establishment in driving aggregated helminth distributions

In most host-parasite systems, variation in parasite burden among hosts drives transmission dynamics. Heavily infected individuals introduce disproportionate numbers of infective stages into host populations or surrounding environments, causing sharp increases in frequency of infection. Parasite aggregation within host populations may result from variation among hosts in exposure to infective propagules and probability of subsequent establishment of parasites in the host. This is because individual host heterogeneities contribute to a pattern of parasite overdispersion that emerges at the population level. We quantified relative roles of host exposure and parasite establishment in producing variation in parasite burdens, to predict which hosts are more likely to bear heavy burdens, using big brown bats (Eptesicus fuscus) and their helminths as a model system. We captured bats from seven colonies in Michigan and Indiana, USA, assessed their helminth burdens, and collected data on intrinsic and extrinsic variables related to exposure, establishment, or both. Digenetic trematodes had the highest prevalence and mean abundance while cestodes and nematodes had much lower prevalence and mean abundance. Structural equation modeling revealed that best-fitting models to explain variations in parasite burden included genetic heterozygosity and immunocompetence as well as distance to the nearest water source and the year of host capture. Thus, both differential host exposure and differential parasite establishment significantly influence heterogeneous helminth burdens, thus driving population-level patterns of parasite aggregation.     

Prevalence of the bopyrid isopod Probopyrus pandalicola in the grass shrimp, Palaemonetes pugio, in four tidal creeks on the South Carolina-Georgia coast

The grass shrimp, Palaemonetes pugio, is an important food source for many invertebrate and fish species, including several of commercial importance. The bopyrid Probopyrus pandalicola prevents reproduction in P. pugio by sexual sterilization. The purpose of our research was to determine bopyrid prevalence in grass shrimp over the course of a year. Shrimp were collected from 2 estuarine systems in South Carolina and 2 estuarine systems in Georgia and examined for parasite presence, sex, and gravidity. Site-specific monthly prevalence ranged from 0 to 6.3%. Country Club Creek had the maximum mean +/- SE prevalence of 3.1 +/- 0.3%, and Harbour Town had the minimum of 1.3 +/- 0.3%. Maximum prevalence was concurrent with peak gravidity for Moon River; thus, at this site the negative effect of this parasite on reproductive output may be greater. Reduced egg production may affect grass shrimp abundance and ultimately the recruitment success of its predators.     

Relationship between host abundance and parasite distribution: inferring regulating mechanisms from census data

1. We studied the effect of host abundance on parasite abundance and prevalence using data on 57 associations of fleas (Siphonaptera) and their mammalian hosts from Slovakia. 2. We assumed that flea-induced host mortality could be inferred from the relationship between flea aggregation and flea abundance, whereas host-induced flea mortality could be inferred from the relationship between flea abundance or aggregation and host abundance. 3. Relationships between flea abundance or prevalence and host abundance were either negative (in 23 flea-host associations) or absent (in 34 flea-host associations). Negative relationships between flea abundance and host abundance were always accompanied by negative relationships between flea prevalence and host abundance. 4. The link between flea abundance/prevalence and host abundance was evaluated as the coefficient of determination of the respective regressions. Across flea-host associations, this link decreased with an increase in the degree of flea aggregation (measured as a parameter b of Taylor's power law). 5. Mean crowding of fleas decreased with an increase of host abundance in eight flea-host associations, being asymptotic in four of them. On the other hand, mean crowding of fleas increased with an increase in flea abundance in 49 flea-host associations, being asymptotic in 15 of them. 6. Results of this study suggest that different flea-host associations are governed by different regulating mechanisms, but different regulation mechanisms may act simultaneously within the same flea-host associations.     

Temperature and multiple parasites combine to alter host community structure

Predicting the effects of climate change requires understanding complex interactions among multiple abiotic and biotic factors. By influencing key interactions among host species, parasites can affect community and ecosystem structuring. Yet, our understanding of how multiple parasites and abiotic factors interact to alter ecosystem structure remains limited. To empirically test the role of temperature variation and parasites in shaping communities, we used a multigenerational mesocosm experiment composed of four sympatric freshwater crustacean species (isopods and amphipods) that share up to four parasite species. Mesocosms were assigned to one of four different treatments with contrasting seasonal temperatures (normal and elevated) and parasite exposure levels (continuous and arrested (presence or absence of parasite larvae in mesocosm)). We found that parasite exposure and water temperature had interactive effects on the host community. Continuous exposure to parasites altered the community structure and differences in water temperature altered species abundance. The abundance of the amphipod Paracalliope fluviatilis decreased substantially when experiencing continuous parasite exposure and elevated water temperatures. Elevated temperatures also led to parasite-induced mortality in another amphipod host, Paracorophium excavatum. Contrastingly, isopod hosts were affected much less, suggesting increasing temperatures in conjunction with higher parasite exposure might increase their relative abundance in the community. Changes in invertebrate host populations have implications for other species such as fish and birds that consume crustaceans as well as having impacts on ecosystem processes, such as aquatic primary production and nutrient cycling. In light of climate change predictions, parasite exposure and rise in average temperatures may have substantial impacts on communities and ecosystems, altering ecosystem structure and dynamics.     

Coevolution between parasite virulence and host life-history traits

Epidemiological models generally explore the evolution of parasite life-history traits, namely, virulence and transmission, against a background of constant host life-history traits. However, life-history models have predicted the evolution of host traits in response to parasitism. The coevolution of host and parasite life-history traits remains largely unexplored. We present an epidemiological model, based on resource allocation theory, that provides an analysis of the coevolution between host reproductive effort and parasite virulence. This model allows for hosts with either a fixed (i.e., genetic) or conditional (i.e., a phenotypically plastic) response to parasitism. It also considers superinfections. We show that parasitism always favors increased allocation to host reproduction, but because of epidemiological feedbacks, the evolutionarily stable host reproductive effort does not always increase with parasite virulence. Superinfection drives the evolution of parasite virulence and acts on the evolution of the host through parasite evolution, generally leading to higher host reproductive effort. Coevolution, as opposed to cases where only one of the antagonists evolves, may generate correlations between host and parasite life-history traits across environmental gradients affecting the fecundity or the survival of the host. Our results provide a theoretical framework against which experimental coevolution outcomes or field observations can be contrasted.     

Geographic variation in life cycle strategies of a progenetic trematode

Numerous parasite species have evolved complex life cycles with multiple, subsequent hosts. In trematodes, each transmission event in multi-host life cycles selects for various adaptations, one of which is facultative life cycle abbreviation. This typically occurs through progenesis, i.e., precocious maturity and reproduction via self-fertilization within the second intermediate host. Progenesis eliminates the need for the definitive host and facilitates life cycle completion. Adopting a progenetic cycle may be a conditional strategy in response to environmental cues related to low probability of transmission to the definitive host. Here, the effects of environmental factors on the reproductive strategy of the progenetic trematode Stegodexamene anguillae were investigated using comparisons among populations. In the 3-host life cycle, S. anguillae sexually reproduces within eel definitive hosts, whereas in the progenetic life cycle, S. anguillae reproduces by selfing within the metacercaria cyst in tissues of small fish intermediate hosts. Geographic variation was found in the frequency of progenesis, independent of eel abundance. Progenesis was affected by abundance and length of the second intermediate fish host as well as encystment site within the host. The present study is the first to compare life cycle strategies among parasite populations, providing insight into the often unrecognized plasticity in parasite developmental strategies and transmission.     

Differential precocious sexual development of Proctoeces lintoni (Digenea: Fellodistomidae) in three sympatric species of keyhole limpets Fissurella spp. may affect transmission to the final host.

The prevalence, abundance, and developmental status of the digenetic trematode Proctoeces lintoni Siddiqui et Cable 1960 were compared in 3 species of keyhole limpets Fissurella. A total of 197 limpets was collected at Caleta Chome, south-central Chile. Fissurella picta and F. costata had the highest prevalence of infection, whereas F. picta showed the greatest abundance of parasites, which increased with host shell length. However, the frequency of P. lintoni specimens with eggs in the uterus was greatest in F. costata. These results suggest that an increased rate of development of a parasite in the intermediate host may shorten the residence time necessary for maturation in the final host. Thus, faster development of the parasite in F. costata suggests the possibility that the parasites transmitted through this host species have shorter maturation times in clingfishes than individuals transmitted via other limpet species.     

Sexual size dimorphism in parasitic oxyurid nematodes

As in many invertebrates, female oxyurids are larger than male. Sexual size dimorphism (SSD) of oxyurid nematodes (the hosts of which are both invertebrate and vertebrate), is investigated regarding body size of both host and parasite. SSD of parasites appeared to be weakly, but not significandy, correlated with invertebrate and vertebrate host body size. However, this study reveals a different pattern for SSD with respect to either type of host. SSD does not increase in tandem with body size in vertebrate parasites either at the level of species or genus. SSD is much more pronounced in Syphaciidae than in Heteroxynematidae, two families of vertebrate parasites exhibiting different modes of transmission (members of the Syphaciidae are transmitted through perianal contamination). SSD is investigated in one monophyletic group of parasites of primates, for which a phylogeny is known. Independent comparisons method is used and we find that the body size of female parasite is strongly correlated with that of the male. The hypoallometry (slope<1) of the relationship suggests that the SSD is not linked to an increase of parasite body size. Moreover, there is no influence of host body size on parasite SSD. The pattern in parasites of invertebrates is different. First, SSD has been found to increase with parasite body size in two groups of invertebrate parasites: the oxyurids of Dictyoptera and Coleoptera. Second, female body size of invertebrate parasites is not correlated with male body size either at genus or species level. Finally, the evolution of SSD is discussed in relation to the demographic patterns of invertebrate parasites and the haplodiploid mode of reproduction of these parasitic nematodes.     

Fruit abundance may fine-tune timing of reproduction of the Chilean Elaenia (Elaenia chilensis), a long-distance migratory bird in South America

Timing of reproduction has a great impact on the breeding success of birds because a mismatch with the moment when environmental conditions are warm or when food is most plentiful can reduce nestling survival and increase the energetic cost of parental care. Consequently, birds synchronize gonadal maturation with the most favourable environmental conditions, using photoperiod changes throughout the year as an initial proximal cue. Additionally, non-photic cues, such as temperature and food abundance, may be necessary to fine-tune reproductive timing. However, the influence of non-photic cues on finely tuning reproductive timing is not yet fully understood for migratory birds. Here, we evaluate how much non-photic cues influence the reproductive timing of the Chilean Elaenia Elaenia chilensis, a long-distance migrant that reproduces in the Andean–Patagonian Forest. We assessed associations of mean temperature, ripe fruit and arthropod abundances with the number of nests in the laying period, and also with the number of nests with hatchlings. In both analyses we used cross-correlations and partial least squares path modelling. Mean temperature was not consistently associated with the breeding phenology of Chilean Elaenias. The increase in number of nests in the laying period was preceded by the increase in caterpillar abundance and coincided with the increase in ripe fruit abundance. The timing of nests with hatchlings matched with the timing of highest ripe fruit abundance. Both types of food could contribute to the beginning of reproduction of birds and be used as proximal cues by Chilean Elaenias. Ripe fruits would also be beneficial for nestling growth because parents feed them with fruits, and might potentially play an ultimate role in reproduction. Because ripe fruit abundance was related to the egg-laying and hatching stages, it may be the main cue used by Chilean Elaenias to fine-tune reproductive timing. These findings allow advances in our understanding of the importance of non-photic cues in the reproductive phenology of migratory birds and also generalize our knowledge among regions and taxa, as most studies on this topic focus on the Northern Hemisphere.     

Effects of parasitic castration on plant resource allocation

It has been proposed that host castration is a parasite strategy to reallocate host resources from reproductive to vegetative functions to increase parasite fitness. Since resource partitioning between reproduction and vegetative growth can affect host life-history traits, parasite effects on resource allocation can affect both plant fitness and host-parasite coevolution. Field and greenhouse experiments were used to investigate the effects of host castration by the fungus Atkinsonella hypoxylon on the resource allocation and architecture of the grass Danthonia spicata. The results indicate that non-infected D. spicata can reallocate resources from reproduction to vegetative growth when resource allocation to reproduction is prevented. However, I found no evidence that fungal castration causes reallocation of resources from host reproduction to vegetative growth. Instead, infection reduces host biomass and the fungus directly utilizes resources that would have been used for host reproduction for its own reproduction. Received: 25 March 1999 / Accepted: 24 October 1999     

Distribution and abundance of parasite nematodes: ecological specialisation,phylogenetic constraint or simply epidemiology?

We investigate the patterns of abundance‐spatial occupancy relationships of adult parasite nematodes in mammal host populations (828 populations of nematodes from 66 different species of terrestrial mammals). A positive relationship between mean parasite abundance and host occupancy, i.e. prevalence, is found which suggests that local abundance is linked to spatial distribution across species. Moreover, the frequency distribution of the parasite prevalence is bimodal, which is consistent with a core‐satellite species distribution. In addition, a strong positive relationship between the abundance (log‐transformed) and its variance (log‐transformed) is observed, the distribution of worm abundance being lognormally distributed when abundance values have been corrected for host body size.
Hanski et al. proposed three distinct hypotheses, which might account for the positive relationship between abundance and prevalence in free and associated organisms: 1) ecological specialisation, 2) sampling artefact, and 3) metapopulation dynamics. In addition, Gaston and co‐workers listed five additional hypotheses. Four solutions were not applicable to our parasitological data due to the lack of relevant information in most host‐parasite studies. The fifth hypothesis, i.e. the confounded effects exerted by common history on observed patterns of parasite distributions, was considered using a phylogeny‐based comparison method. Testing the four possible hypotheses, we obtained the following results: 1) the variation of parasite distribution across host species is not due to phylogenetic confounding effects; 2) the positive relationship between mean abundance and prevalence of nematodes may not result from an ecological specialisation, i.e. host specificity, of these parasites; 3) both a positive abundance‐prevalence relationship and a negative coefficient of variation of abundance‐prevalence relationship are likely to occur which corroborates the sampling model developed by Hanski et al. We argue that demographic explanations may be of particular importance to explain the patterns of bimodality of prevalence when testing Monte‐Carlo simulations using epidemiological modelling frameworks, and when considering empirical findings. We conclude that both the bimodal distribution of parasite prevalence and the mean‐variance power function simply result from demographic and stochastic patterns (highlighted by the sampling model), which present compelling evidence that nematode parasite species might adjust their spatial distribution and burden in mammal hosts for simple epidemiological reasons.     

Fecundity compensation in the three‐spined stickleback Gasterosteus aculeatus infected by the diphyllobothriidean cestode Schistocephalus solidus

Causal explanations for host reproductive phenotypes influenced by parasitism fit into three broad evolutionary models: (1) non‐adaptive side effect; (2) adaptive parasitic manipulation; and (3) adaptive host defence. This study demonstrates fecundity compensation, an adaptive non‐immunological host defence, in the three‐spined stickleback fish (Gasterosteus aculeatus) infected by the diphyllobothriidean cestode Schistocephalus solidus. Both infected and uninfected female sticklebacks produced egg clutches at the same age and size. The reproductive capacity of infected females decreased rapidly with increased parasite : host body mass ratio. Body condition was lower in infected females than uninfected females and decreased with increasing parasite : host mass ratio. Females with clutches had greater body condition than those without clutches. A point biserial correlation showed that there was a body condition threshold necessary for clutch production to occur. Host females apparently had the capacity to produce egg clutches until the prolonged effects of nutrient theft by the parasite and the drain on resources from reproduction precluded clutch formation. Clutch mass, adjusted for female body mass, did not differ significantly between infected and uninfected females. Infected females apparently maintained the same level of reproductive allotment (egg mass as proportion of body mass) as uninfected females. Infected females produced larger clutches of smaller eggs than uninfected females, revealing a trade‐off between egg mass and egg number, consistent with the fecundity compensation hypothesis. The rapid loss of reproductive capacity with severity of infection probably reflects the influence of the parasite combined with a trade‐off between current and future reproduction in the host. Inter‐annual differences in reproductive performance may have reflected ecological influences on host pathology and/or intra‐annual seasonal changes. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

Effects of food abundance, density and climate change on reproduction in the sparrowhawk Accipiter nisus

The reproductive success of predators depends on abiotic environmental conditions, food abundance and population density, and food abundance, density and their interactions may respond to changes in climatic conditions. Timing of reproduction by five of the eight numerically most common prey of the sparrowhawk Accipiter nisus advanced significantly since 1971, during a period of temperature increase. There was no evidence that mean laying date or any other reproductive parameter of sparrowhawks changed consistently during the study period 1977–1997. Laying date advanced and percentage of unsuccessful female sparrowhawks decreased with beech mast in the current year, an index of food abundance for avian prey. Mean laying date of sparrowhawks was advanced in warmer springs, and although mean clutch size was not larger in warm than in cold springs, mean brood size of successful pairs and breeding success increased in such springs, showing that sparrowhawks enjoyed a fitness gain when reproducing early. The timing of sparrowhawk reproduction with respect to the peak in abundance of fledgling prey increased, from a good match between mean timing of fledging by prey and maximum demand for food by the predator in 1977, to reproduction occurring later than the peak in fledging prey availability in 1997. The size of the breeding population of sparrowhawks was not predicted by mean spring temperature, the size of the breeding population the previous year or beech mast crop. The size of the post-breeding population was predicted by size of the breeding and post-breeding population the previous year and by the proportion of unsuccessful females the current year. These findings imply that sparrowhawks did not respond to change in climate, although climate changed the timing of reproduction by the main prey species.