Predator diversity, intraguild predation, and indirect effects drive parasite transmission

Abstract

Humans are altering biodiversity globally and infectious diseases are on the rise; thus, there is interest in understanding how changes to biodiversity affect disease. Here, we explore how predator diversity shapes parasite transmission. In a mesocosm experiment that manipulated predator (larval dragonflies and damselflies) density and diversity, non-intraguild (non-IG) predators that only consume free-living cercariae (parasitic trematodes) reduced metacercarial infections in tadpoles, whereas intraguild (IG) predators that consume both parasites and tadpole hosts did not. This likely occurred because IG predators reduced tadpole densities and anticercarial behaviors, increasing per capita exposure rates of the surviving tadpoles (i.e., via density-and trait-mediated effects) despite the consumption of parasites. A mathematical model demonstrated that non-IG predators reduce macroparasite infections, but IG predation weakens this "dilution effect" and can even amplify parasite burdens. Consistent with the experiment and model, a wetland survey revealed that the diversity of IG predators was unrelated to metacercarial burdens in amphibians, but the diversity of non-IG predators was negatively correlated with infections. These results are strikingly similar to generalities that have emerged from the predator diversity-pest biocontrol literature, suggesting that there may be general mechanisms for pest control and that biocontrol research might inform disease management and vice versa. In summary, we identified a general trait of predators-where they fall on an IG predation continuum-that predicts their ability to reduce infections and possibly pests in general. Consequently, managing assemblages of predators represents an underused tool for the management of human and wildlife diseases and pest populations.biodiversity-ecosystem function | dilution effect | schistosomiasis | snail | trophic cascade

Disease ecology across soil boundaries: effects of below-ground fungi on above-ground host–parasite interactions

“…Specifically, the relationship between a protozoan parasite and its herbivore host above ground depends upon the presence and the abundance of soil fungi that influence plant nutritional and defensive traits. Our study also adds to the growing body of literature showing that community composition has profound effects on host-parasite interactions [1][2][3], and that trait-mediated indirect effects can connect below-and aboveground ecosystems [9,10]. As with other studies of AMF, we also found that these effects were dependent on both the abundance of mycorrhizal fungi and the identity of plant species that they colonize [34,35].…”

“…For example, dilution effects on disease transmission are essentially density-mediated effects in which numerical changes in the relative abundance of competent hosts affect disease transmission [4]. Likewise, trait-mediated indirect effects on host quality, size and behaviour can be equally important [3,[5][6][7][8]45]. Among the factors that influence host-parasite interactions, host diet quality can play a particularly significant role because it can affect host growth rate, immune function and foraging behaviour [7,24,26,27].…”

Biodiversity inhibits parasites: Broad evidence for the dilution effect

“…For example, during succession or restoration, host species must establish in a community before their parasites can invade (26). More broadly, theoretical models suggest that the effect of biodiversity on disease depends on the traits of hosts, parasites, and additional species (27,28). However, our meta-analysis detected dilution effects robustly across variation in study design, resistance traits of diluting hosts, host range of parasites (human vs. wildlife), and parasite lifecycle and type (macro-vs. microparasite; Fig.…”

“…For example, nonfocal host species can inhibit disease spread by regulating host populations via competition or predation, interfering with the transmission process, or altering host behavior (6,8). Theoretical models can generally delineate which mechanisms can promote or inhibit disease (27,28), but experimental tests that assess the relative importance of these mechanisms, their generality across disease systems, and their dependence on temporal and spatial scales remain scarce. These open issues represent increasingly important intersections among ecology, conservation science, and epidemiology.…”

“…We analyzed these data with a multilevel randomeffects model (28). Our compilation of effect sizes showed a clear hierarchical structure; there were multiple effect sizes for some experiments and some parasite species.…”