Impact of three co-occurring physical ecosystem engineers on soil Collembola communities.

The interplay between organisms with their abiotic environment may have profound effects within ecological networks, but are still poorly understood. Soil physical ecosystem engineers (EEs) modify the abiotic environment, thereby potentially affecting the distribution of other species, such as microarthropods. We focus on three co-occurring physical EEs (i.e. cattle, vegetation, macrodetritivore) known for their profound effect on soil properties (e.g. pore volume, microclimate, litter thickness). We determined their effects on Collembola community composition and life-form strategy (a proxy for vertical distribution in soil) in a European salt marsh. Soil cores were collected in grazed (compacted soil, under short and tall vegetation) and non-grazed areas (decompacted soil, under short and tall vegetation), their pore structure analysed using X-ray computed tomography, after which Collembola were extracted. Collembola species richness was lower in grazed sites, but abundances were not affected by soil compaction or vegetation height. Community composition differed between ungrazed sites with short vegetation and the other treatments, due to a greater dominance of epigeic Collembola and lower abundance of euedaphic species in this treatment. We found that the three co-occurring EEs and their interactions modify the physical environment of soil fauna, particularly through changes in soil porosity and availability of litter. This alters the relative abundance of Collembola life-forms, and thus the community composition within the soil. As Collembola are known to play a crucial role in decomposition processes, these compositional changes in litter and soil layers are expected to affect ecosystem processes and functioning.

Population genetic structure of the tree-hole tick Ixodes arboricola (Acari: Ixodidae) at different spatial scales.

The endophilic tick Ixodes arboricola infests cavity-nesting birds, and its dispersal strongly depends on the movements of its host. Population genetic structure of I. arboricola was studied with seven polymorphic microsatellite markers. We collected 268 ticks from 76 nest boxes in four woodlots near Antwerp, Belgium. These nest boxes are mainly used by the principal hosts of I. arboricola, the great tit Parus major and the blue tit Cyanistes caeruleus. As these birds typically return to the same cavity for roosting or breeding, ticks within nest boxes were expected to be highly related, and tick populations were expected to be spatially structured among woodlots and among nest boxes within woodlots. In line with the expectations, genetic population structure was found among woodlots and among nest boxes within woodlots. Surprisingly, there was considerable genetic variation among ticks within nest boxes. This could be explained by continuous gene flow from ticks from nearby tree holes, yet this remains to be tested. A pairwise relatedness analysis conducted for all pairs of ticks within nest boxes showed that relatedness among larvae was much higher than among later instars, which suggests that larvae are the most important instar for tick dispersal. Overall, tick populations at the studied spatial scale are not as differentiated as predicted, which may influence the scale at which host-parasite evolution occurs.

Seasonal feeding activity of the tree-hole tick, Ixodes arboricola.

Bird-specific ticks do not infest humans and livestock, but these ticks often share their avian hosts with generalist ticks that do. Therefore, their feeding activity may have an impact on the transmission of pathogens outside bird-tick transmission cycles. Here we examined the seasonal feeding activity of the tree-hole tick (Ixodes arboricola) in relation to the activity of its hole-breeding hosts (Parus major and Cyanistes caeruleus). We analysed data on ticks derived from birds, on the abundance of engorged ticks inside nest boxes, and on bird nests that were experimentally exposed to ticks. We observed a non-random pattern of feeding associated with the tick instar and host age. The majority of adult ticks fed on nestlings, while nymphs and larvae fed on both free-flying birds and nestlings. Due to their fast development, some ticks were able to feed twice within the same breeding season. The highest infestation rates in free-flying birds were found during the pre-breeding period and during autumn and winter when birds roost inside cavities. Except during winter, feeding of I. arboricola overlapped in time with the generalist Ixodes ricinus, implying that tick-borne microorganisms that are maintained by I. arboricola and birds could be bridged by I. ricinus to other hosts.

Host specificity of a bird-specialised endophilic ectoparasite, the tree-hole tick Ixodes arboricola.

Host specialisation in parasites can be due to either limited exposure or limited adaptation to different host types. When the first barrier is lifted experimentally, the degree of adaptive specialisation can be studied. The tree-hole tick Ixodes arboricola is an endophilic parasite with a narrow host range, found in nest boxes used mainly by great and blue tits (Parus major, Cyanistes caeruleus) and to a lesser extent by pied flycatchers (Ficedula hypoleuca) and nuthatches (Sitta europaea). In the current study, we exposed two nestlings per nestbox of pied flycatchers (N = 14), blue tits (N = 18), great tits (N = 14), and nuthatches (N = 16) to ten I. arboricola nymphs each. We found no differences in attachment success 2 days after infestation (56 ± 4% across species) nor were there any differences in tick engorgement weight (1.95 ± 0.03 mg across species), and moulting success was >90% for ticks from all bird species. Hence, our data suggest that all bird species investigated here are suitable host species. This may enhance the ticks' chances for persistence in cavities and dispersal among cavities inhabited by multiple host species, and supports the hypothesis that host use by ticks is limited by host ecology rather than by host specialisation.

Isolation and characterization of ten polymorphic microsatellite loci in Ixodes arboricola, and cross-amplification in three other Ixodes species.

We characterized ten polymorphic microsatellite loci from the tree-hole tick, Ixodes arboricola. Loci were screened in 11-18 individuals from three Belgian populations and five to ten alleles were found at each locus. Seven loci did not show deviations from Hardy-Weinberg equilibrium conditions and there were no indications for null alleles at these loci. The three other loci showed significant heterozygote deficiencies in at least one population, and a high potential for the occurrence of null alleles. We observed no effect of potential host DNA on the scoring of the microsatellites. Cross-amplification of the microsatellites was tested in eight specimens of three congeneric species: I. ricinus, I. hexagonus and I. frontalis. Depending on the species, six or seven of the loci were amplified in ≥ 4 of the 8 specimens and were polymorphic in each of these species (except for Ixaf 11 in I. frontalis and I. ricinus). These loci thus provide a tool for population genetic analysis of I. arboricola. The suitability of these markers needs to be further investigated in its congeners.

Plasticity in the timing of detachment of an Eurasian-African songbird tick, Ixodes frontalis.

In non-permanent parasites, host detachment should take place in an environment that ensures the continuation of their life cycle. Timing of detachment - in combination with the host's space use - affects dispersal and transmission success of the parasites and of the pathogens they vector. Before reaching the adult reproductive stage, ticks need to go through multiple immature developmental stages (larva and nymph), each feeding on host blood. In between the feeding bouts, they often remain in the off-host environment for considerable periods of time. With this study, we aimed to obtain more insight in Ixodes frontalis' off-host habitat use by comparing its detachment pattern in different life stages with that of two habitat-specialized ticks also found on birds: the endophilic tree-hole tick (Ixodes arboricola) and the exophilic sheep tick (Ixodes ricinus), the latter living in humid understory vegetation of forests. For this, we artificially infested hole-roosting (great tits, Parus major) and open-roosting (blackbirds, Turdus merula) birds with ticks under laboratory conditions, and recorded whether detachment occurred during the day or the night. We hypothesize that nocturnal detachment improves off-host mating opportunities and host localization, whereas diurnal detachment optimizes tick dispersal. Ixodes frontalis nymphs detached during the night, especially when feeding on blackbirds. This behaviour was very similar to that of I. arboricola (larva and nymph) feeding on great tits. In contrast, I. frontalis larvae detached during the day, especially when feeding on great tits, which resembles that of I. ricinus' feeding behaviour (larva and nymph). Ixodes frontalis left the host within seven days, immediately after completion of the blood meal. This is similar to both developmental stages of I. ricinus but contrasts with the very long (up to 20 days) feeding duration in I. arboricola. Thus I. frontalis shows strong plasticity, switching from dispersal-centered (larvae) to host-centered (nymphs) detachment behaviour. Findings are discussed with regard to the ticks' habitat use, dispersal, life history and host specificity.

Mating strategies and multiple paternity, assessed by microsatellites, of the dispersal-limited, ectoparasitic tree-hole tick, Ixodes arboricola.

Multiple mating is common among ticks, a large group of haematophagous ectoparasites, but multiple paternity has rarely been investigated. Multiple paternity may be common because the resultant increased genetic diversity allows ticks to rapidly evolve in relation to host responses and increases colonisation potential in novel habitats. Knowledge concerning mating systems is important because ticks may have profound effects on their hosts and are the principal transmitters of many pathogenic agents. In the current study, we investigated the mating system of the nidicolous tick Ixodes arboricola. These ticks attach to their bird hosts in the nest, which restricts gene flow but facilitates finding a partner off-host. Having genetically variable offspring may be beneficial for ticks which may encounter very different conditions when dispersed to the nest of another host type. We conducted an experiment in which female ticks fed on great tit nestlings and mated with two males in three treatments of the females: mating with both males before feeding, mating with one male before and the other male after feeding, or mating with both males after feeding. We investigated paternity with microsatellites. In a complementary experiment we investigated male preference for unfed or engorged females, and measured mating duration. We predicted (i) there would be multiple mating by I. arboricola males and females, leading to multiple paternity, and (ii) males would prefer to mate with engorged females and those matings would last longer because engorged females present a higher probability of successful reproduction. We found multiple paternity within clutches but no indications of sperm precedence. Males preferred to mate with engorged females and those matings lasted significantly longer, even including attachment beyond egg deposition. We suggest such mate guarding and male preference for mating after feeding is adaptive because there is no first male precedence. Male preference for mating after feeding may also be adaptive because dispersal is low and females are available after the blood meal.

The more the merrier--experimental evidence for density-dependent feeding facilitation in the bird-specialised tick Ixodes arboricola.

Similar to many other parasites, the distribution of ticks among hosts is strongly skewed, with few hosts harbouring the majority of parasites. Because parasite-induced impairment of host health, parasite population growth and pathogen transmission are density-dependent, understanding why tick distributions are skewed is important for the population and evolutionary dynamics of both parasite and host. However, there is currently no knowledge concerning parasites that strongly depend on individual hosts. Here, we investigated the effects of tick density on feeding performance in the nidicolous tree-hole tick, Ixodes arboricola, which feeds on cavity-nesting birds and is the carrier of several tick-borne pathogens. Nidicolous ticks reside in or close to their hosts' nests and therefore depend strongly on individual hosts and their offspring. Increased feeding success at higher densities (facilitation) may therefore be detrimental to the ticks themselves. We investigated the effects of tick density on feeding performance of I. arboricola by infesting great tit nestlings with one to five adult ticks, which is within the natural range. There was no effect of tick density on initial attachment success, attachment after 48 h or engorgement weight, but tick recovery rates increased significantly with tick density. We also found a modest increase in nestling body mass with tick density, suggesting that birds over-compensate resource drainage by the ticks and, by doing this, anticipate the costs of a tick-rich environment. Our results indicate that nidicolous ticks perform better when feeding in aggregation. This may have important consequences for population dynamics and consequently pathogen transmission.