Host Relative Abundance Research Articles

The role of small‐scale environmental gradients on trematode infection

Abstract Parasitism rates can vary dramatically across large and small spatial scales. Such heterogeneity in infection is driven by variation in environmental factors that influence the probability of host and parasite encountering each other (encounter filters) and of genetic and physiological factors that alter whether that encounter results in a successful infection (compatibility filters). Here, we investigated how infection in first‐intermediate hosts (snails) by trematode parasites is influenced by small‐scale variation in environmental variables (final host relative abundance, nutrients, and temperature) and population origin across multiple ponds within an artificial flow‐through wetland system in Atlanta, GA, U.S.A. We placed individually marked, lab‐reared F1 generation snails of the same age from three different populations into field enclosures at nine ponds along a 1‐km stretch of connected, flow‐through ponds for 86 days and allowed them to accrue infections naturally. This highly controlled field experiment allowed us to disentangle the underlying factors influencing small‐scale variation in infection risk, which can be masked when looking only at natural patterns of infection prevalence across space. We found high heterogeneity in infection risk. The probability of infection was highest in ponds downstream and with high levels of bird activity; population origin was not important. This work provides experimental evidence that infection risk varies across small spatial scales (tens of metres), driven by steep gradients in influential environmental drivers, emphasising the importance of controlled field manipulations to understand infection risk. Identifying small‐scale drivers of infection risk can help mitigate infection in hosts of interest (e.g., humans, fish), and should be acknowledged in organismal field studies, where infection and infection risk may influence the ecology, physiology, or behaviour of animals.

Evidence of host switching: Sea lampreys disproportionately attack Chinook salmon when lake trout abundance is low in Lake Ontario

Lake trout (Salvelinus namaycush) is the presumed preferred host of the invasive sea lamprey (Petromyzon marinus) in the Great Lakes, but little is understood about this preference outside of laboratory experiments. By preference we mean sea lamprey attacks on hosts are disproportionate to host relative abundance. The purpose of this study was to quantify host preference of sea lampreys in the field for the first time. We focused our analysis on Lake Ontario, where the two dominant host species for sea lampreys are lake trout and Chinook salmon (Oncorhynchus tshawytscha). Sea lampreys exhibited a strong preference for lake trout when host abundance was ≥ 32% lake trout, but sea lamprey preference switched to Chinook salmon when host abundance was ≤ 13% lake trout. Model results were equivocal with respect to determining whether the primary predictor of sea lamprey preference was relative or absolute abundance of lake trout. Other evidence, particularly the different spatial distributions of the two hosts, suggests that sea lamprey preference for lake trout is based on a higher encounter probability, driven by absolute abundance of lake trout. This study confirms a widely held concern that observed sea lamprey-induced marking rates on lake trout, used to assess the status of sea lamprey control in the Great Lakes, can be influenced by the abundance of alternative hosts. As an alternative host for sea lamprey parasitism, Chinook salmon may contribute to the persistence of lake trout in Lake Ontario.

Host Abundance and Identity Determine the Epidemiology and Evolution of a Generalist Plant Virus in a Wild Ecosystem.

Increasing evidence indicates that in wild ecosystems plant viruses are important ecological agents, and with potential to jump into crops, but only recently have the diversity and population dynamics of wild plant viruses begun to be explored. Theory proposes that biotic factors (e.g., ecosystem biodiversity, host abundance, and host density) and climatic conditions would determine the epidemiology and evolution of wild plant viruses. However, these predictions seldom have been empirically tested. For 3 years, we analyzed the prevalence and genetic diversity of Potyvirus species in preserved riparian forests of Spain. Results indicated that potyviruses were always present in riparian forests, with a novel generalist potyvirus species provisionally named Iberian hop mosaic virus (IbHMV), explaining the largest fraction of infected plants. Focusing on this potyvirus, we analyzed the biotic and climatic factors affecting virus infection risk and population genetic diversity in its native ecosystem. The main predictors of IbHMV infection risk were host relative abundance and species richness. Virus prevalence and host relative abundance were the major factors determining the genetic diversity and selection pressures in the virus population. These observations support theoretical predictions assigning these ecological factors a key role in parasite epidemiology and evolution. Finally, our phylogenetic analysis indicated that the viral population was genetically structured according to host and location of origin, as expected if speciation is largely sympatric. Thus, this work contributes to characterizing viral diversity and provides novel information on the determinants of plant virus epidemiology and evolution in wild ecosystems.

Foliar pathogens are unlikely to stabilize coexistence of competing species in a California grassland

Pathogen infection is common in wild plants and animals, and may regulate their populations. If pathogens have narrow host ranges and increase with the density of their favored hosts, they may promote host species diversity by suppressing common species to the benefit of rare species. Yet, because many pathogens infect multiple co-occurring hosts, they may not strongly respond to the relative abundance of a single host species. Are natural communities dominated by specialized pathogens that respond to the relative abundance of a specific host or by pathogens with broad host ranges and limited responses to the relative abundance of single host? The answer determines the potential for pathogens to promote host coexistence, as often hypothesized, or to have negligible or even negative effects on host coexistence. We lack a systematic understanding of the impacts, identities, and host ranges of pathogens in natural communities. Here we characterize a community of foliar fungal pathogens and evaluate their host specificity and fitness impacts in a California grassland community of native and exotic species. We found that most of the commonly isolated fungal pathogens were multi-host, with intermediate to low specialization. The amount of pathogen damage each host experienced was independent of host species local relative abundance. Despite pathogen sharing among the host species, fungal communities slightly differed in composition across host species. Plants with high pathogen damage tended to have lower seed production but the relationship was weak, suggesting limited fitness impacts. Moreover, seed production was not dependent on the local relative abundance of each plant species, suggesting that stabilizing coexistence mechanisms may operate at larger spatial scales in this community. Because foliar pathogens in this grassland community are multi-host and have small fitness impacts, they are unlikely to promote negative frequency dependence or plant species coexistence in this system. Still, given that pathogen community composition differentiates across host species, some more subtle feedbacks between host relative abundance and pathogen community composition, damage, and fitness impacts are possible, which could, in turn, promote either coexistence or competitive exclusion.

Correction: Prevalence Patterns of Avian Plasmodium and Haemoproteus Parasites and the Influence of Host Relative Abundance in Southern China

Correction: Prevalence Patterns of Avian Plasmodium and Haemoproteus Parasites and the Influence of Host Relative Abundance in Southern China

Prevalence patterns of avian Plasmodium and Haemoproteus parasites and the influence of host relative abundance in southern China.

Infectious diseases threaten the health and survival of wildlife populations. Consequently, relationships between host diversity, host abundance, and parasite infection are important aspects of disease ecology and conservation research. Here, we report on the prevalence patterns of avian Plasmodium and Haemoproteus infections and host relative abundance influence based on sampling 728 wild-caught birds representing 124 species at seven geographically widespread sites in southern China. The overall prevalence of two haemoprotozoan parasites, Plasmodium and Haemoproteus, was 29.5%, with 22.0% attributable to Haemoproteus and 7.8% to Plasmodium. Haemoproteus prevalence differed significantly among different avian host families, with the highest prevalence in Nectariniidae, Pycnonotidae and Muscicapidae, whereas Plasmodium prevalence varied significantly among host species. Seventy-nine mitochondrial lineages including 25 from Plasmodium and 54 from Haemoproteus were identified, 80% of which were described here for the first time. The phylogenetic relationships among these parasites indicated stronger host-species specificity for Haemoproteus than Plasmodium. Well-supported host-family (Timaliidae) specific clades were found in both Plasmodium and Haemoproteus. The Haemoproteus tree shows regional subclades, whereas the Plasmodium clades are “scattered” among different geographical regions. Interestingly, there were statistically significant variations in the prevalence of Plasmodium and Haemoproteus among the geographical regions. Furthermore, the prevalence of Plasmodium and Haemoproteus were not significantly correlated with host relative abundance. Further efforts will focus on exploring the relationships between parasite prevalence and sex, age, and immune defense of the host.

THE PATCHINESS OF EPIFOLIAR FUNGI IN TROPICAL FORESTS: HOST RANGE, HOST ABUNDANCE, AND ENVIRONMENT

Fungal symbioses affect the diversity, dynamics, and spatial patterns of trees in tropical forests. Their ecological importance is partly driven by their inherent patchiness. We used epifoliar fungi, a guild of common, benign, obligate, fungal symbionts of plants, as a model system to evaluate the relative importance of host phylogeny, host relative abundance, and microclimate on the three-dimensional distribution of plant-fungus symbioses. In parallel studies in rainforests in Panama and Australia, most epifoliar fungi were able to colonize several plant lineages but showed significant host preferences within the local plant community. More closely related plant species were not more likely to share fungal symbionts. Instead, fungal species were more likely to be shared by more abundant hosts, which supported a greater number and diversity of fungi. Environmental conditions strongly affected spatial distributions, with sites in the dark understory 2.5- to fourfold more likely to have epifoliar fungi than in the exposed forest canopy. In the understory, fungal incidence increased with canopy openness. Canopy trees supported only a subset of the fungal symbionts found in the understory, suggesting that adult trees are not reservoirs of these fungal symbionts for understory juveniles.

Variation in mistletoe seed deposition: effects of intra‐ and interspecific host characteristics

We investigated differences in host infection by a desert mistletoe, Phoradendron californicum, and examined one of the processes that contributes to these differences: variation in seed deposition among host individuals and species. In the Sonoran Desert, P. californicum parasitizes the sympatric leguminous trees Olneya tesota, Cercidium microphyllum, Prosopis velutina, Acacia constricta, and Acacia greggii. We hypothesized that seed deposition depends on host height and crown architecture. At a site in Arizona, frequency of infection did not reflect host relative abundance. Olneya tesota was parasitized at a higher frequency than expected from its abundance and maintained the highest mistletoe loads per individual host. In contrast, P. velutina was infected less frequently than expected. Infection frequency increased with host tree height for all hosts. Mistletoe seed deposition by avian dispersers differed among host species and was disproportionately high in O. tesota and P. velutina. Seed deposition was higher in infected than in non‐infected host trees, and increased with tree height in O. tesota but not in C. microphyllum. We suspect that increased seed deposition with height in O. tesota may be due to the preference of seed‐dispersing birds for higher perches. Some host tree species, such as C. microphyllum and A. constricta, probably received fewer mistletoe seeds because birds avoid hosts with dense and spiny crowns. Mistletoe populations are plant metapopulations in which host trees are patches and the frequency of infection in each host species/patch type is the result of interspecific differences in the balance between mistletoe colonization and extinction. From this perspective, our study of host use and seed dispersal is a metapopulation study of patch occupancy and propagule distribution among available patch types. Our seed‐dispersal study demonstrates that the mechanisms that create pattern in patchy plant populations can be investigated in mistletoe systems.

Cuterebrid parasitism (Rogenhofera bonaerensis) on the shrubland mouse (Akodon molinae), in Argentina.

We evaluated the influence of temperature, rainfall, and host relative abundance on Rogenhofera bonaerensis (Diptera, Cuterebridae) parasitism prevalence in shrubland mouse (Akodon molinae) populations in central Argentina, from February 1983 to December 1987. Parasitism did not vary significantly with host age: juvenile-subadults (32%), adults (26%), and old adults (29%). Females were more frequently parasitized (36%) than were males (20%). There was no correlation between parasitism and reproductive activity. Infested hosts were recorded most commonly in summer (January to March, 19%), and in fall (April to June, 30%). During the dry season, July through November, cuterebrid parasitism averaged only 3%. The monthly prevalence of parasitism throughout the year was not associated either with monthly precipitation nor with mean monthly temperature at the time of sampling. But a 2 to 3 month time-lag effect of both climatic variables on parasitism was recorded. Bot fly prevalence was correlated to an index of host density. We propose climate and host availability as important factors affecting bot fly parasitism in the semiarid shrubland of central Argentina.