A plethora of rodents: Rattlesnake predators generate unanticipated patterns of venom resistance in a grassland ecosystem

Predation has the potential to impart strong selective pressures on organisms within their environments, resulting in adaptive changes in prey that minimize risk of predation. Pressures from venomous snakes present an exceptional challenge to prey, as venom represents a unique chemical arsenal evolutionarily tailored to incapacitate prey. In response, venom resistance has been detected in various snake prey species, and to varying degrees. This study analyzes venom resistance in an eastern Colorado grassland habitat, where the Prairie Rattlesnake (Crotalus viridis) and Desert Massasauga Rattlesnake (Sistrurus tergeminus edwardsii) co-occur with a suite of grassland rodents. We test for venom resistance across rodent and snake pairings using two geographically distant field sites to determine the role of 1) predation pressure and trophic ecology, and 2) sympatric and allopatric patterns of venom resistance. Resistance was measured using serum-based metalloproteinase inhibition assays to determine potential inhibition of proteolytic activity, augmented by median lethal dose (LD50) assays on rodent species to assess toxicity of crude venoms. Resistance is present in several rodent species, with strong resistance present in populations of Eastern Woodrat (Neotoma floridana), Ord's Kangaroo Rat (Dipodomys ordii), and Northern Grasshopper Mouse (Onychomys leucogaster). Resistance is less developed in other species, including the House Mouse (Mus musculus) and Plains Pocket Mouse (Perognathus flavescens). An unexpected differential is present, where Lincoln County Kangaroo Rats are highly resistant to venom of co-occurring Prairie Rattlesnakes yet are sensitive to an allopatric population of Prairie Rattlesnakes in Weld County. Lincoln Co. Northern Grasshopper Mice also demonstrate extremely elevated resistance to Weld Co. Prairie Rattlesnake venoms, and they may possess resistance mechanisms for myotoxin a, an abundant component of Weld Co. C. v viridis venoms. This study illustrates the complexity of venom resistance in biological communities that can exist when incorporating multiple species interactions. Future studies aimed at characterizing resistance mechanisms at the molecular level will provide a more detailed physiological context for understanding mechanisms by which resistance to venoms occurs.

Distribution and demographics of mysids (Crustacea: Mysida) as prey for gray whales (Eschrichtius robustus) in northwest Washington state

Background

The movement and distribution of gray whales (Eschrichtius robustus) during the feeding season is likely dependent on the quality of foraging opportunities and the distribution of prey species. These dynamics are especially important to understand for the Pacific Coast Feeding Group (PCFG) of gray whales which spend the feeding season along the coast from northern California, USA through northern British Columbia, Canada. In Washington state, no previous work has been done to describe available gray whale prey. The main goal of this research was to initiate studies on an important gray whale prey item in northwest Washington, mysid shrimp (Mysida), by establishing a baseline understanding of mysid swarm demographics in the area and investigating patterns in gray whale and mysid presence.

Methods

Prey samples were collected during June through November 2019 and June through September 2020 using a vertically-towed plankton net at seven sites in the Strait of Juan de Fuca and seven sites in the Pacific Ocean in areas where gray whales were known to feed. Mysids collected in the samples were counted and the sex, length, species, maturity, and gravidity were documented. Patterns in gray whale and mysid co-occurrence were explored through data visualization.

Results

Seven species of mysids were observed in the survey area. In 2019, the number of mysids per tow increased steadily through the season, the most abundant species of mysids were Holmesimysis sculpta and Neomysis rayii, and sampled mysids averaged 4.7 mm in length. In 2020, mysids were abundant in tow samples in June and July but were not abundant in the remaining months of the sampling season. The average length of mysids in 2020 was 13.3 mm, and a large portion were sexually mature males and brooded females identified as H. sculpta. Throughout the survey area, the majority of whale sightings occurred later in the season in 2019 and earlier in the season in 2020, coinciding with the trends of sampled mysids.

Discussion

This study provides the first description of mysid swarm composition and temporal variation in northwest Washington. Tows were dominated by a similar assemblage of mysid species as what is observed in other areas of the PCFG range. The differences in sampled mysid assemblages between years, and the presence of whales in the survey area in times and at sites where samples with high mysid counts were collected, suggest evidence for interesting predator-prey dynamics that warrant further investigation.

Do predator (Mystus gulio) and prey (Penaeus monodon) have differential response against heatwaves? Unveiling through oxidative stress biomarkers and thermal tolerance estimation

Extreme climatic events such as heatwaves are anticipated to intensify in future and impose additional thermal stress to aquatic animals. Knowledge regarding an organism's thermal tolerance or sensitivity is therefore important in determining the effects of fluctuating water temperature on physiological responses. Thus, thermal tolerance tests can serve as a first step in understanding the present and future effects of climate warming. Climatic variability will alter prey-predator attributes differentially and impact their subsequent interactions. The key objective of this study was to compare and decode the stress responses, resistance and vulnerability of two economically important species from Sundarbans estuarine system- Penaeus monodon (prey) and Mystus gulio (predator) subjected to acute thermal challenges such as sudden heatwaves. Both the species were subjected to an increasing thermal ramp of 1°C h^-1 from 22°C to 42°C. Organisms were observed continuously throughout the ramping period and changes in the locomotory behaviour were followed until their loss of equilibrium. The digestive tissue samples were dissected out from both M. gulio and P. monodon at every 2°C and also after a recovery period of 48 h. The SOD, CAT, GST, LPO were measured and integrated biomarker response (IBR) was analysed. The results from thermal tolerance maxima estimation, biomarker study, IBR responses indicated more intense stress response in fish M. gulio whereas recovery potential was greater in shrimp P. monodon. Our findings corroborate the 'trophic sensitivity hypothesis' which advocates predators to be less tolerant in aggravated environmental stress than their prey.

Indirect facilitation between prey promotes asymmetric apparent competition

Apparent competition is one mechanism that can contribute to the complex dynamics observed in natural systems, yet it remains understudied in empirical systems. Understanding the dynamics that shape the outcome of processes like apparent competition is vital for appreciating how they influence natural systems.

We empirically evaluated the role of indirect trophic interactions in driving apparent competition in a model laboratory system. Our experimental system was designed to let us evaluate combined direct and indirect interactions among species.

Here we describe the results of a factorial experiment using two noncompeting prey (Colpidium kleini, a heterotroph, and Chlamydomonas reinhardtii, an autotroph) consumed by a generalist predator Euplotes eurystomus to explore the dynamics of apparent competition. To gain intuition into the potential mechanism driving the asymmetry in the observed results, we further explored the system using structural equation modelling.

Our results show an important role of positive interactions and indirect effects contributing to apparent competition in this system with a marked asymmetrical outcome favouring one prey, Chlamydomonas. The selected structural equation supports a role of indirect facilitation; although Chlamydomonas (a photoautotroph) and Colpidium (a bacterivore) use different resources and therefor do not directly compete, Colpidium reduces bacteria that may compete with Chlamydomonas. In addition, formation of colonies by Chlamydomonas in response to predation by Euplotes provides an antipredator defence not available to Colpidium.

Asymmetric apparent competition may be more common in natural systems than the symmetric interaction originally proposed in classic theory, suggesting that exploration of the mechanisms driving the asymmetry of the interaction can be a fruitful area of further research to better our understanding of interspecific interactions and community dynamics.

Poacher-population dynamics when legal trade of naturally deceased organisms funds anti-poaching enforcement

Can a regulated, legal market for wildlife products protect species threatened by poaching? It is one of the most controversial ideas in biodiversity conservation. Perhaps the most convincing reason for legalizing wildlife trade is that trade revenue could fund the protection and conservation of poached species. In this paper, we examine the possible poacher-population dynamic consequences of legal trade funding conservation. The model consists of a manager scavenging carcasses for wildlife product, who then sells the product, and directs a portion of the revenue towards funding anti-poaching law enforcement. Through a global analysis of the model, we derive the critical proportion of product the manager must scavenge, and the critical proportion of trade revenue the manager must allocate towards increased enforcement, in order for legal trade to lead to abundant long-term wildlife populations. We illustrate how the model could inform management with parameter values derived from the African elephant literature, under a hypothetical scenario where a manager scavenges elephant carcasses to sell ivory. We find that there is a large region of parameter space where populations go extinct under legal trade unless a significant portion of trade revenue is directed towards protecting populations from poaching. The model is general and therefore can be used as a starting point for exploring the consequences of funding many conservation programs using wildlife trade revenue.

Ninja owl; Gerbils over-anticipate an unexpected flying predator

Foragers make decisions based on cues, information collected from their environment, processed into strategic behaviours. This information, processed in multiple regions of the brain, ultimately result in the production of stress hormones and visible changes in behaviour of animals - both reflexively to avoid depredation and strategically to avoid an encounter with the predator. In a common-garden experiment we tested how imperfect information from visual cues of a predator impacts foraging and apprehension of a desert rodent, the Egyptian gerbil (Gerbillus pyramidum). The gerbils were exposed to predation by barn owls (Tyto alba), one camouflaged on dark nights using black dye. Gerbils' response to the owls was measured using patch-use measured in giving-up densities (GUDs) and time spent in vigilance activity. Owl lethality was extrapolated from mean times spent in attacks and number of attempted strikes. Dyed owls attack-rate was lower and attack duration greater than those of the white owls. During the full moon, when dyed owls were visible, gerbils responded with extreme vigilance and minimal foraging (high GUDs). During the new moon when the owls were most stealthy, the gerbils showed low vigilance coupled with a similar high GUD. The inconsistency between gerbils' foraging and vigilance behaviours, suggest a likely mismatch between perceived risk and actual measurement of predator lethality gathered by the gerbils' observations in real time.

Increased presence of mammal-eating killer whales in the Salish Sea with implications for predator-prey dynamics

The inland waters of Washington State and southern British Columbia, collectively known as the Salish Sea, comprise key habitat for two regional populations of killer whales (Orcinus orca): the mammal-eating West Coast Transients and the endangered fish-eating Southern Residents. These two populations are genetically distinct and may avoid each other. Transient killer whale usage of the Salish Sea has been previously assessed over two seven-year time periods, showing an increase from 1987 to 2010. We documented a continued significant increase in mammal-eating killer whale presence in the Salish Sea from 2011 to 2017, with intra- and inter-annual variability and with record sightings in 2017. This continued increase, likely in response to abundant marine mammal prey, is related to both a growing population and an increase in the number of West Coast Transients visiting the area. Additionally, a negative binomial regression shows that absence of Southern Residents is correlated to transient presence. Finally, both populations of killer whales have been linked to regional harbor seal populations; harbor seals are salmonid-eating competitors of the Southern Residents and are prey for the mammal-eating transients. With Southern Residents listed as endangered, culling harbor seals has been proposed as a measure to help in their recovery. With this in mind, we developed an energetic model to assess the minimum number of harbor seals consumed by transient killer whales. Using the actual number of whales present in each age-sex class for each day of the year, we estimate that, at a minimum, transients in the Salish Sea consumed 1090 seals in 2017. This is more than 2% of the 2014 estimated harbor seal population the Salish Sea. The population controlling effects of transient killer whale predation on harbor seals should be considered when evaluating any pinniped management actions in the Salish Sea.

Predator-prey-subsidy population dynamics on stepping-stone domains with dispersal delays

We examine the role of the travel time of a predator along a spatial network on predator-prey population interactions, where the predator is able to partially or fully sustain itself on a resource subsidy. The impact of access to food resources on the stability and behaviour of the predator-prey-subsidy system is investigated, with a primary focus on how incorporating travel time changes the dynamics. The population interactions are modelled by a system of delay differential equations, where travel time is incorporated as discrete delay in the network diffusion term in order to model time taken to migrate between spatial regions. The model is motivated by the Arctic ecosystem, where the Arctic fox consumes both hunted lemming and scavenged seal carcass. The fox travels out on sea ice, in addition to quadrennially migrating over substantial distances. We model the spatial predator-prey-subsidy dynamics through a "stepping-stone" approach. We find that a temporal delay alone does not push species into extinction, but rather may stabilize or destabilize coexistence equilibria. We are able to show that delay can stabilize quasi-periodic or chaotic dynamics, and conclude that the incorporation of dispersal delay has a regularizing effect on dynamics, suggesting that dispersal delay can be proposed as a solution to the paradox of enrichment.

Evolution of mate-finding Allee effect in prey

The search for mates is often accompanied with conspicuous behaviour or morphology that can be exploited by predators. Here we explore the evolutionary consequences of a trade-off that arises naturally between mate acquisition and risk of predation and study evolution of the rate at which male prey search for mates in a population subject to a mate-finding Allee effect and exposed to either generalist or specialist predators. Since we show that the mate search rate determines the strength of the mate-finding Allee effect, we can alternatively view this as evolution of the mate-finding Allee effect in prey. We contrast two different life histories and find that, predominantly, male prey either evolve towards the maximal mate search rate yielding the weakest possible mate-finding Allee effect (thus showing no adaptive response in mating behaviour to predation risk) or evolutionary bi-stability occurs. In the latter case, males evolve a relatively low mate search rate (hence a relatively strong mate-finding Allee effect, interpreted as an adaptive response of male prey to predation) when initially slow or the maximal mate search rate when initially fast. Disruptive selection does not occur in populations exposed to generalist predators but is possible when predators are specialists. The dimorphic phase, in which fast and conspicuous male prey coexist with slow and cryptic ones, is however but a transient in evolutionary dynamics as one branch goes extinct while the other evolves towards the maximal mate search rate.

Predator-prey-subsidy population dynamics on stepping-stone domains

Predator-prey-subsidy dynamics on stepping-stone domains are examined using a variety of network configurations. Our problem is motivated by the interactions between arctic foxes (predator) and lemmings (prey) in the presence of seal carrion (subsidy) provided by polar bears. We use the n-Patch Model, which considers space explicitly as a "Stepping Stone" system. We consider the role that the carrying capacity, predator migration rate, input subsidy rate, predator mortality rate, and proportion of predators surviving migration play in the predator-prey-subsidy population dynamics. We find that for certain types of networks, added mobility will help predator populations, allowing them to survive or coexist when they would otherwise go extinct if confined to one location, while in other situations (such as when sparsely distributed nodes in the network have few resources available) the added mobility will hurt the predator population. We also find that a combination of favourable conditions for the prey and subsidy can lead to the formation of limit cycles (boom and bust dynamic) from stable equilibrium states. These modifications to the dynamics vary depending on the specific network structure employed, highlighting the fact that network structure can strongly influence the predator-prey-subsidy dynamics in stepping-stone domains.