The co-occurrence of three species of Diorchis Clerc, 1903 (Cestoda: Hymenolepididae) in the European coot, Fulica atra L

Potential interactions between the intestinal helminths of the cichlid fish Cichlasoma synspilum from southeastern Mexico

This study presents and analyzes positional data on 18 helminth species forming the intestinal infracommunities of 59 Cichlasoma synspilum from southeastern Mexico. Interspecific interactions were found between 4 pairs of host specialists: Crassicutis cichlasomae (Digenea) with Neoechinorhynchus golvani (Acanthocephala), Spirocamallanus rebecae (Nematoda) with N. golvani, and Raillietnema kritscheri (Nematoda) with N. golvani. The interactions were expressed as negative correlations, positional shifts, and reductions in realized distribution. It is suggested that N. golvani produces unsuitable habitats for C. cichlasomae, S. rebecae, and R. kristscheri. All interactions were found among phylogenetically unrelated helminth specialists of cichlids.

Patterns of macroparasite abundance and aggregation in wildlife populations: a quantitative review

In this paper we review the published literature on patterns of abundance and aggregation of macroparasites in wildlife host populations. We base this survey on quantitative analyses of mean burden and a number of measures of the degree of aggregation of parasite burdens between hosts. All major parasite and vertebrate host taxa were represented in the database. Mean parasite burden was found to be log-normally distributed, indicating that all parasite burdens are regulated to some degree. In addition, all but one of the parasitic infections were aggregated with respect to their hosts, and the relationship between log mean parasite burden and log variance was found to be very strong (R2= 0·87). That is, for a given mean parasite burden there are constraints on the degree of variation in individual host burdens. The aggregated nature of the parasitic infections is also apparent from other measures of the degree of aggregation: prevalence – mean relationships, and the negative binomial parameter,k. Using a relatively new technique for parasitological infection data – tree-based models, as well as traditional linear models – a number of the parasitic infections was found to be associated with systematically lower or higher parasite burdens. Possible biological explanations for these and other patterns are proposed.

The population dynamics of competition between parasites

A number of published studies of competition between parasite species are examined and compared. It is suggested that two general levels of interaction are discernible: these correspond to the two levels of competition recognized by workers studying free-living animals and plants: 'exploitation' and 'interference' competition. The former may be defined as the joint utilization of a host species by two or more parasite species, while the latter occurs when antagonistic mechanisms are utilized by one species either to reduce the survival or fecundity of a second species or to displace it from a preferred site of attachment. Data illustrating both levels of interaction are collated from a survey of the published literature and these suggest that interference competition invariably operates asymmetrically. The data are also used to estimate a number of population parameters which are important in determining the impact of competition at the population level. Theoretical models of host-parasite associations for both classes of competition are used to examine the expected patterns of population dynamics that will be exhibited by simple two-species communities of parasites that utilize the same host population. The analysis suggests that the most important factor allowing competing species of parasites to coexist is the statistical distribution of the parasites within the host population. A joint stable equilibrium should be possible if both species are aggregated in their distribution. The size of the parasite burdens at equilibrium is then determined by other life-history parameters such as pathogenicity, rates of resource utilization and antagonistic ability. Comparison of these theoretical expectations with a variety of sets of empirical data forms the basis for a discussion about the importance of competition in natural parasite populations. The models are used to assess quantitatively the potential for using competing parasite species as biological control agents for pathogens of economic or medical importance. The most important criterion for identifying a successful control agent is an ability to infect a high proportion of the host population. If such a parasite species also exhibits an intermediate level of pathology or an efficient ability to utilize shared common resources, antagonistic interactions between the parasite species contribute only secondarily to the success of the control. Competition in parasites is compared with competition in free-living animals and plants. The comparison suggests further experimental tests which may help to assess the importance of competition in determining the structure of more complex parasite-host communities.