Multiple stage hydra effect in a stage-structured prey-predator model

In this work, we show by means of numerical bifurcation that two alternative stable states exhibit a hydra effect in a continuous-time stage-structured predator-prey model. We denote this behavior as a stage multiple hydra effect. This concomitant effect can have significant implications in population dynamics as well as in population management.

Parasite-driven cascades or hydra effects: susceptibility and foraging depression shape parasite-host-resource interactions

When epidemics kill hosts and increase their resources, should the density of hosts decrease (with a resource increase, this constitutes a trophic cascade) or increase (a hydra effect)? Seeking answers, we integrate trait measurements, a resource-host-parasite model, and experimental epidemics with plankton. This combination reveals how a spectrum from cascades to hydra effects can arise. It reflects tension between parasite-driven mortality (a density-mediated effect) and foraging depression upon contact with parasite propagules (a trait-mediated one). In the model, mortality rises when higher susceptibility to infection increases infection prevalence. Epidemics release resources while suppressing hosts (creating a cascade). In contrast, when hosts are less susceptible and parasites depress their foraging, a resource feedback can elevate host density during epidemics (creating a hydra effect), particularly at higher carrying capacity of resources. This combination elevates primary production relative to per-host consumption of resources (two key determinants of host density). We test these predictions of the qualitative effects of host traits and resource carrying capacity with trait measurements and a mesocosm experiment. Trait measurements show clonal lines of zooplankton hosts differ in their foraging depression and susceptibility. We seeded resource-host-parasite mesocosms with different host genotypes and provided different nutrient supplies to test model predictions. Hydra effects and trophic cascades arose under different conditions, as predicted by the model. Hence, tension between trait-mediated and density-mediated effects of parasites governs the fate of host density during epidemics, from cascades to hydra effects, via feedbacks with resources.

Stage-specific overcompensation, the hydra effect, and the failure to eradicate an invasive predator

As biological invasions continue to increase globally, eradication programs have been undertaken at significant cost, often without consideration of relevant ecological theory. Theoretical fisheries models have shown that harvest can actually increase the equilibrium size of a population, and uncontrolled studies and anecdotal reports have documented population increases in response to invasive species removal (akin to fisheries harvest). Both findings may be driven by high levels of juvenile survival associated with low adult abundance, often referred to as overcompensation. Here we show that in a coastal marine ecosystem, an eradication program resulted in stage-specific overcompensation and a 30-fold, single-year increase in the population of an introduced predator. Data collected concurrently from four adjacent regional bays without eradication efforts showed no similar population increase, indicating a local and not a regional increase. Specifically, the eradication program had inadvertently reduced the control of recruitment by adults via cannibalism, thereby facilitating the population explosion. Mesocosm experiments confirmed that adult cannibalism of recruits was size-dependent and could control recruitment. Genomic data show substantial isolation of this population and implicate internal population dynamics for the increase, rather than recruitment from other locations. More broadly, this controlled experimental demonstration of stage-specific overcompensation in an aquatic system provides an important cautionary message for eradication efforts of species with limited connectivity and similar life histories.

Virulent disease epidemics can increase host density by depressing foraging of hosts

All else equal, parasites that harm host fitness should depress densities of their hosts. However, parasites that alter host traits may increase host density via indirect ecological interactions. Here, we show how depression of infected host foraging rate can produce such a hydra effect. Using a foraging assay, we quantified reduced foraging rates of a zooplankton host infected with a virulent fungal parasite. We then parameterized a dynamical model of hosts, parasites, and resources with this foraging function, showing how foraging depression can create a hydra effect. Mathematically, the hydra arose when increased resource productivity exceeded any increase in resource consumption per host. Therefore, the foraging-mediated hydra effect more likely emerged (1) for hosts which strongly control logistic-like resources and (2) during larger epidemics of moderately virulent parasites. We then analyzed epidemics from 13 fungal epidemics in nature. We found evidence for a foraging-mediated hydra effect: large outbreaks depressed foraging rate and correlated with increased densities of both algae and hosts. Therefore, depression of foraging rate of infected hosts can produce higher host densities even during epidemics of parasites that increase host mortality. Such hydras might prevent collapse of host populations but also could produce higher densities of infected hosts.

Disrupting the ethnographic imaginarium

This paper is a contemplation of a digital ethnography with the community surrounding Silk Road, the first widely used cryptomarket for drugs on the Dark Web. To position the study within the broader field of illegal anthropology, it provides links between the existing literature on the study of cryptomarkets with relevant anthropological scholarship. A theory of piracy is interrogated for its explanatory capacity of the digital pirates of the Dark Web. The start of the study unexpectedly coincided with the FBI seizure of Silk Road in October 2013. The field site disappearance provoked a practice-based and conceptual rewiring. The paper unpacks how the ‘hydra effect’ introduced to conceptualise resilient innovation within cryptomarkets can also apply to the multiplicity of identities linked to research practice. This effect also raises how the knowledge production within digital ethnographic practice may be reconfigured through notions of opportunism, replication, obsolescence, regeneration, iteration, adaptation and proliferation.