The sicker the better: nematode-infected passalus beetles provide enhanced ecosystem services

Abstract

There is growing appreciation for the role that parasites have in ecosystems and food webs, though the possibility that they could improve an ecosystem service has never been considered. In forest ecosystems, fallen trees naturally decay over time and slowly return their nutrients to the soil. Beetles in the family Passalidae play a key role by excavating tunnels and consuming wood from these logs, thereby breaking down the wood into smaller debris. In the eastern United States, the horned passalus (Odontotaenius disjunctus) is host to a naturally occurring nematode, Chondronema passali, which appears to cause little harm to the beetles. We suspected this was due to compensatory food consumption by parasitized individuals, which we tested here. We collected and housed 113 adult beetles in individual containers with wood for three months, then determined the amount of wood each beetle had processed into fine debris and frass. We then assessed beetles for C. passali and compared wood processing rates between parasitized and non-parasitized groups. Results showed the average daily processing rate of parasitized beetles ( x ¼ 0.77 g d 21 ) was 15% greater than that of unparasitized ones ( x ¼ 0.67 g d 21 ). Parasitized beetles were 6% larger, and this may explain some of this pattern, though the effect of parasitism was still significant in our analysis. By extrapolating the daily rates, we estimate that 10 adult beetles without nematodes would break down approximately 2.4 kg of wood in a single year, while a group of 10 parasitized beetles would break down 2.8 kg. While our data are consistent with the idea of compensatory feeding, because these results are based on natural infections, we cannot rule out the possibility that beetles with heightened wood consumption are simply more likely to acquire the parasite. At an ecosystem level, it may not matter which is the case; parasitized beetles provide a more effective ecosystem service.

Sex-specific effects of a parasite on stress-induced freezing behavior in a natural beetle-nematode system

“…Most of the physiological and biological functions evaluated thus far also appear to show that males and females do not differ substantially, especially in terms of energy use; baseline metabolic rate [37] and heart rate [38,45] are both similar between the sexes. The overall feeding rate of males and females (indexed by how much wood is broken down) is also similar between males and females [17], consistent with their similar metabolism. There is no indication that the immune system of males and females differ either, based on hemocyte density [38].…”

“…Nematode prevalence Females = males [34] Parasite Biology Host nematode burden Females = males [44] Host Biology Body size Females>males [32,44] Host Biology Walking/exploration behavior Females>males [34] Host Biology Wood breakdown rate Females = males [17] Host Biology Physical lifting/pulling strength Females = males [39,44,46] Host Biology Fighting ability Females = males [36] �…”

“…In some scientific disciplines this is referred to as thanatosis [5,6] or tonic immobility [7,8]. There can be many forms of this behavior, such as curling the abdomen Importantly, one recent study also demonstrated that nematode-parasitized beetles appear to break down more wood during their lives [17], which could be construed as evidence that they are more motivated to eat, perhaps as a compensatory mechanism against the energy drain of the nematodes. Given this finding, an interesting follow-up question relates to the host behavior when dealing with perceived threats.…”

Healing while parasitized: impact of a naturally‐occurring nematode during energy‐intensive wound‐healing in a beetle

“…Nor can we say for certain that a lack of significance implies a lack of an effect (of the parasite). For example, O. disjunctus beetles harbouring C. passali nematodes tend to consume more wood (Davis & Prouty, ), perhaps as a compensatory mechanism. This could explain why baseline respiration rates are not elevated in parasitized beetles (Fig.…”

Benefits of Parasitism in Polluted Environments: A Review and Perspectives

“…Evaluating potential ecotoxicological consequences on parasites' transmission, reproduction, or survival is thus a complex issue (Khan and Thulin, 1991;Sures et al, 2017). It is noteworthy that adverse impacts of pollutants on parasites could have cascading effects on ecological processes, ecosystem functioning, and services (Gómez and Nichols, 2013;Davis and Prouty, 2019).…”