Boris R. Krasnov

Nestedness versus modularity in ecological networks: two sides of the same coin?

1. Understanding the structure of ecological networks is a crucial task for interpreting community and ecosystem responses to global change. 2. Despite the recent interest in this subject, almost all studies have focused exclusively on one specific network property. The question remains as to what extent different network properties are related and how understanding this relationship can advance our comprehension of the mechanisms behind these patterns. 3. Here, we analysed the relationship between nestedness and modularity, two frequently studied network properties, for a large data set of 95 ecological communities including both plant-animal mutualistic and host-parasite networks. 4. We found that the correlation between nestedness and modularity for a population of random matrices generated from the real communities decreases significantly in magnitude and sign with increasing connectance independent of the network type. At low connectivities, networks that are highly nested also tend to be highly modular; the reverse happens at high connectivities. 5. The above result is qualitatively robust when different null models are used to infer network structure, but, at a finer scale, quantitative differences exist. We observed an important interaction between the network structure pattern and the null model used to detect it. 6. A better understanding of the relationship between nestedness and modularity is important given their potential implications on the dynamics and stability of ecological communities.

Host–habitat relations as an important determinant of spatial distribution of flea assemblages (Siphonaptera) on rodents in the Negev Desert

We studied flea assemblages on rodents in different habitats of the Ramon erosion cirque in the Negev Desert to examine whether host-habitat relations influence flea spatial distribution. Eleven flea species parasitizing 12 rodent species were recorded. There was significant positive relationship between flea species richness and body mass of the host species; no relationships were found between relative richness of flea assemblage and either the number of habitats occupied by the host species or the size of host geographical range. The differences in pattern of flea parasitism among habitat types within host species were determined by both environmental features of a habitat and the specific pattern of habitat use by rodents. There was replacement of Xenopsylla conformis by Xenopsylla ramesis on Meriones crassus and Gerbillus dasyurus among different habitats. The results of ordination of the flea collections from each individual host demonstrated that the flea assemblages were segregated mainly along 4 axes, which explained 86% of total variance. Each of the ordination axes corresponded with a change in flea species composition. The directions of these changes were (1) among-hosts within a habitat and (2) among-habitats within a host.

Host specificity in phylogenetic and geographic space

The measurement of host specificity goes well beyond counting how many host species can successfully be used by a parasite. In particular, specificity can be assessed with respect to how closely related the host species are, or whether a parasite exploits the same or different hosts across its entire geographic range. Recent developments in the measurement of biodiversity offer a new set of analytical tools that can be used to quantify the many aspects of host specificity. We describe here the multifaceted nature of host specificity, summarize the indices available to measure its different facets one at a time or in combination, and discuss their implications for parasite evolution and disease epidemiology.

Sex-biased parasitism, seasonality and sexual size dimorphism in desert rodents

We investigated seasonality of gender differences in the patterns of flea infestation in nine rodent species to test if sex-biased parasitism in terms of mean abundance, species richness, prevalence and the level of aggregation (a) varies among hosts and between seasons, and (b) is linked to sexual size dimorphism. Sexual size differences were significant in both summer and winter in Acomys cahirinus, Gerbillus pyramidum and Meriones crassus, and in winter only in Acomys russatus, Gerbillus dasyurus, Gerbillus nanus and Sekeetamys calurus. Sexual size dimorphism was male biased except for A. russatus in which it was female biased. Manifestation of sexual differences in flea infestation was different among hosts between seasons. A significant effect of sex on mean flea abundance was found in six hosts, on mean flea species richness in five hosts and on prevalence in two hosts. Male-biased parasitism was found in summer in one host only and in winter in five hosts. Female-biased parasitism occurred in winter in A. russatus. Gender differences in the slopes of the regressions of log-transformed variances against log-transformed mean abundances occurred in three hosts. No relationship was found between sexual size dimorphism and any parasitological parameter in any season using both conventional regressions and the method of independent contrasts. Our results suggest that sex-biased parasitism is a complicated phenomenon that involves several different mechanisms.

Effect of Air Temperature and Humidity on the Survival of Pre-Imaginal Stages of Two Flea Species (Siphonaptera: Pulicidae)

Abstract The survival of immature fleas at 25 and 28°C and 40, 55, 75, and 92% RH was studied to test the hypothesis that the difference in microclimatic preferences determines habitat distribution of Xenopsylla conformis Wagner, 1903 and Xenopsylla ramesis Rothschild, 1904. Survival of X. conformis eggs did not depend on either temperature or humidity or both, whereas eggs of X. ramesis survived significantly less at 40% RH than at higher humidities. No larva of either species survived at 40% RH at either temperature. Larval survival of both species at both temperature regimes was significantly lower at 55% humidity than at higher humidities. Maximal survival time of larvae that died before pupation depended on both temperature and humidity in both species. Change of humidity during early stages of the life cycle (from egg to larva) increased the maximal survival time in X. conformis larvae but decreased that in X. ramesis larvae. Pupal survival was higher at higher humidities independent of temperature. Survival of X. conformis pupae was lower than that of X. ramesis pupae when the relative humidity was low. Humidity change on later stages (from larva to cocoon) decreased X. conformis pupal survival and had no effect on X. ramesis pupal survival. The sex ratio of emerged adults was not affected by either temperature or humidity in both species. Changes in humidity between egg and larval environments significantly decreased the percentage of females in X. conformis emergence at 28°C.

THE EFFECT OF HOST DENSITY ON ECTOPARASITE DISTRIBUTION: AN EXAMPLE OF A RODENT PARASITIZED BY FLEAS

The pattern of parasitism of the flea species Xenopsylla dipodilli and Nosopsyllus iranus theodori on the desert rodent species Gerbillus dasyurus was studied to test the hypothesis that the relationships between flea abundance and host density conform to pre-existing models of R. M. Anderson and R. M. May, with the correction that the density of those host individuals that possess permanent burrows (residents) is substituted for the overall host density. It was predicted that: (1) the intensity of flea infestation would increase in curvilinear fashion with increase of host density to a plateau that would be attained at a lower level of host density than would be expected from the basic model, and (2) the prevalence of flea infestation plotted against host density would be hump-shaped. The results indicated that intensity of flea infestation increased in either curvilinear fashion to an asymptote (for X. dipodilli) or linearly (for N. i. theodori) with increase of host density. As host density increased, the prevalence of infestation changed either unimodally (X. dipodilli) or logarithmically (N. i. theodori). In addition, there was a positive relationship between the mean number of fleas per host and the percentage of hosts infested. Both basic and corrected models describing the relationships between flea burden and host density fit the observational data well. However, simulations of the fraction of resident hosts demonstrated that this parameter influences the relationship between host density and flea burden only when residents comprise ≤50% of all host individuals.

Energy cost of ectoparasitism: the flea Xenopsylla ramesis on the desert gerbil Gerbillus dasyurus

Metabolizable energy intake and changes in body mass was measured in the desert gerbil Gerbillus dasyurus when parasitized by the flea Xenopsylla ramesis. We hypothesized that energy requirements for maintenance, or average daily metabolic rate, of parasitized gerbils would be higher than those of nonparasitized gerbils. We also hypothesized that the efficiency of utilization of energy for maintenance would be the same for parasitized and non-parasitized gerbils, as this measurement is dependent mainly on dietary quality. Fifty fleas feeding on a gerbil consumed 3.68 a 1.19 mg blood, in total. This amounted to 34.3 a 1.8% of body mass of a starving flea and only about 0.17% of the blood volume of the host. The absolute amount of blood consumed by X. ramesis is the lowest reported for fleas in other studies. This suggested that the pressure of parasitism of X. ramesis on G. dasyurus in terms of blood consumed was relatively low. Both our hypotheses were confirmed. Average daily metabolic rate of the parasitized gerbils (7.75 kJ g 70.54 d 71 ) was 16% higher than that of non-parasitized gerbils (6.69 kJ g 70.54 d 71 ). In addition, at zero metabolizable energy intake, the parasitized gerbils lost body mass at a faster rate than the nonparasitized gerbils (4.34 vs 3.95% body mass d 71 ). The efficiency of utilization of energy in the parasitized and non-parasitized gerbils was similar.

Ectoparasitic “Jacks‐of‐All‐Trades”: Relationship between Abundance and Host Specificity in Fleas (Siphonaptera) Parasitic on Small Mammals

Animal species with larger local populations tend to be widespread across many localities, whereas species with smaller local populations occur in fewer localities. This pattern is well documented for free‐living species and can be explained by the resource breadth hypothesis: the attributes that enable a species to exploit a diversity of resources allow it to attain a broad distribution and high local density. In contrast, for parasitic organisms, the trade‐off hypothesis predicts that parasites exploiting many host species will achieve lower mean abundance on those hosts than more host‐specific parasites because of the costs of adaptations against multiple defense systems. We test these alternative hypotheses with data on host specificity and abundance of fleas parasitic on small mammals from 20 different regions. Our analyses controlled for phylogenetic influences, differences in host body surface area, and sampling effort. In most regions, we found significant positive relationships between flea abundance and either the number of host species they exploited or the average taxonomic distance among those host species. This was true whether we used mean flea abundance or the maximum abundance they achieved on their optimal host. Although fleas tended to exploit more host species in regions with either larger number of available hosts or more taxonomically diverse host faunas, differences in host faunas between regions had no clear effect on the abundance–host specificity relationship. Overall, the results support the resource breadth hypothesis: fleas exploiting many host species or taxonomically unrelated hosts achieve higher abundance than specialist fleas. We conclude that generalist parasites achieve higher abundance because of a combination of resource availability and stability.

Development rates of two Xenopsylla flea species in relation to air temperature and humidity

Abstract. The rate of development of immature fleas, Xenopsylla conformis Wagner and Xenopsylla ramesis Rothschild (Siphonaptera: Xenopsyllidae) was studied in the laboratory at 25°C and 28°C with 40, 55, 75 and 92% relative humidity (RH). These fleas are separately associated with the host jird Meriones crassus Sundevall in different microhabitats of the Ramon erosion cirque, Negev Highlands, Israel. This study of basic climatic factors in relation to flea bionomics provides the basis for ecological investigations to interpret reasons for paratopic local distributions of these two species of congeneric fleas on the same host. Both air temperature and RH were positively correlated with duration of egg and larval stages in both species. Change of humidity between egg and larval environments did not affect duration of larval development at any temperature. At each temperature and RH, the eggs and larvae of X. ramesis did not differ between males and females in the duration of their development, whereas female eggs and larvae of X. conformis usually developed significantly faster than those of males. For both species, male pupae developed slower than female pupae at the same air temperature and RH. Air temperature, but not RH, affected the duration of pupal development. At each humidity, duration of the pupal stage was significantly longer at 25°C than at 28°C: 15.3 ± 1.7 vs. 11.7 ± 1.2 days in X. conformis; 14.1 ± 2.0 vs. 11.5 ± 1.7 days in X. ramesis, with a significantly shorter pupal period of the latter species at 25°C. These limited interspecific bionomic contrasts in relation to basic climatic factors appear insufficient to explain the differential habitat distributions of X. conformis and X. ramesis

Immune response to fleas in a wild desert rodent: effect of parasite species, parasite burden, sex of host and host parasitological experience

SUMMARY We studied immune responses of the jird Meriones crassus to different flea species belonging to the same family. We used jirds maintained in an outdoor enclosure (enclosure; N=18) and parasitized by fleas Xenopsylla conformis mycerini and Xenopsylla ramesis, and also jirds born in the laboratory to previously parasitized mothers (laboratory animals; N=23). We asked (i) whether cross-immunity to different fleas occurs, (ii) whether there is a sex difference in immune responses to flea parasitism and (iii) whether the severity of the immune responses depends on parasite load. In the enclosure animals, immune response to antigen from the unfamiliar flea Synosternus cleopatrae pyramidis did not differ from those to antigens from the familiar fleas. In contrast, laboratory rodents demonstrated no difference in the immune response between S. c. pyramidis antigen and either the phytohemagglutinin treatment or controls, although their responses to antigens of fleas familiar to their mothers (X. c. mycerini and X. ramesis) were significantly higher than those to antigen of S. c. pyramidis and phytohemagglutinin. The results clearly demonstrated that (i) cross-reactivity in rodent responses to different flea species occurred for enclosure but not for laboratory jirds and (ii) immune-naïve animals whose mothers were parasitized by fleas had some degree of immunity against fleas. The only sex difference in immunological parameters was the higher level of circulating immune complexes in females than in males. Only phagocytic activity was affected by flea burden, decreasing with an increase in flea numbers.