Review: Using physiologically based models to predict population responses to phytochemicals by wild vertebrate herbivores

Small rodent population cycles and plants - after 70 years, where do we go?

Small rodent population cycles characterise northern ecosystems, and the cause of these cycles has been a long-lasting central topic in ecology, with trophic interactions currently considered the most plausible cause. While some researchers have rejected plant-herbivore interactions as a cause of rodent cycles, others have continued to research their potential roles. Here, we present an overview of whether plants can cause rodent population cycles, dividing this idea into four different hypotheses with different pathways of plant impacts and related assumptions. Our systematic review of the existing literature identified 238 studies from 150 publications. This evidence base covered studies from the temperate biome to the tundra, but the studies were scattered across study systems and only a few specific topics were addressed in a replicated manner. Quantitative effects of rodents on vegetation was the best studied topic, and our evidence base suggests such that such effects may be most pronounced in winter. However, the regrowth of vegetation appears to take place too rapidly to maintain low rodent population densities over several years. The lack of studies prevented assessment of time lags in the qualitative responses of vegetation to rodent herbivory. We conclude that the literature is currently insufficient to discard with confidence any of the four potential hypotheses for plant-rodent cycles discussed herein. While new methods allow analyses of plant quality across more herbivore-relevant spatial scales than previously possible, we argue that the best way forward to rejecting any of the rodent-plant hypotheses is testing specific predictions of dietary variation. Indeed, all identified hypotheses make explicit assumptions on how rodent diet taxonomic composition and quality will change across the cycle. Passing this bottleneck could help pinpoint where, when, and how plant-herbivore interactions have - or do not have - plausible effects on rodent population dynamics.

Increased availability of preferred food and decreased foraging costs from degraded grasslands lead to rodent pests in the Qinghai-Tibet Plateau

The increased population density of rodent species during ongoing grassland degradation further deteriorates its conditions. Understanding the effects of grassland degradation on rodent feeding habits is of great value for optimizing grassland management strategies. In this study, lightly degraded (LD), moderately degraded (MD), severely degraded (SD), and reseeded grassland (RG) were selected and their plant resources and soil physical properties were investigated. In addition, the study used ITS2 barcode combined with the Illumina MiSeq sequencing method to analyze the food composition and proportion of plateau zokors in different grassland conditions. The results showed that, with grassland degradation, plant biomass decreased, but the relative proportion of forbs increased (LD: 32.05 ± 3.89%; MD: 28.97 ± 2.78%; SD: 49.16 ± 4.67% and RG: 10.93 ± 1.53%). Forbs were the main food of the plateau zokor, accounting for more than 90% of their diet, and the animal had a clear preference for Potentilla species; the soil compaction of feeding habits showed a decreasing trend in the 10–25 cm soil layer, suggesting a decreased foraging cost. Nutritional analysis showed that the stomach content of crude protein in zokors feeding on MD grassland was significantly higher than that of animals feeding on the other grassland types. Structural equation modeling showed that soil physical properties and the relative biomass of forbs had significant (P < 0.05) and extremely significant (P < 0.001) impacts on the population density of plateau zokors, with direct impact contribution rates of 0.20 and 0.63. As the severity of grassland degradation increased, although the aboveground and underground biomass of the plants decreased, the proportion of food preferred by the plateau zokor increased, and the corresponding changes in the feeding environment resulted in decreased foraging energy expenditure, thereby increasing the suitability of the degraded grassland for the plateau zokor. Compared with degraded grassland, the food diversity and evenness of zokors increased, the food niche width enlarged, and the proportion of weeds decreased in RG, which increased the difficulty of obtaining food. Reseeding in grassland management is therefore an effective way to control plateau zokors.

Warmer Ambient Temperatures Depress Detoxification and Food Intake by Marsupial Folivores

Ambient temperature is an underappreciated determinant of foraging behaviour in wild endotherms, and the requirement to thermoregulate likely influences food intake through multiple interacting mechanisms. We investigated relationships between ambient temperature and hepatic detoxification capacity in two herbivorous marsupials, the common ringtail possum (Pseudocheirus peregrinus) and common brushtail possum (Trichosurus vulpecula) that regularly feed on diets rich in plant toxins. As an indicator of hepatic detoxification capacity, we determined the functional clearance rate of an anaesthetic agent, Alfaxalone, after possums were acclimated to 10°C [below the thermoneutral zone (TNZ)], 18°C [approximately lower critical temperature (LCT)], and 26°C [approximately upper critical temperature (UCT)] for either 7 days or less than 24 h. We then measured intake of foods with high or low plant secondary metabolite (PSM) concentrations under the same temperature regimes. After 7 days of acclimation, we found a positive correlation between the functional clearance rate of Alfaxalone and ambient temperature, and a negative relationship between ambient temperature and intake of foods with high or low PSM concentrations for both species. The effect of ambient temperature on intake of diets rich in PSMs was absent or reduced when possums were kept at temperatures for less than 24 h. Our results underscore the effects of ambient temperature in hepatic metabolism particularly with respect intake of diets containing PSMs. Given that the planet is warming, it is vital that effects of ambient temperature on metabolism, nutrition and foraging by mammalian herbivores is taken into account to predict range changes of species and their impact on ecosystems.

Differences in dietary composition and preference maintained despite gene flow across a woodrat hybrid zone

Ecotones, characterized by adjacent yet distinct biotic communities, provide natural laboratories in which to investigate how environmental selection influences the ecology and evolution of organisms. For wild herbivores, differential plant availability across sharp ecotones may be an important source of dietary-based selection.We studied small herbivore diet composition across a sharp ecotone where two species of woodrat, Neotoma bryanti and N. lepida, come into secondary contact with one another and hybridize. We quantified woodrat dietary preference through trnL metabarcoding of field-collected fecal pellets and experimental choice trials. Despite gene flow, parental N. bryanti and N. lepida maintain distinct diets across this fine spatial scale, and across temporal scales that span both wet and dry conditions. Neotoma bryanti maintained a more diverse diet, with Frangula californica (California coffeeberry) making up a large portion of its diet. Neotoma lepida maintains a less diverse diet, with Prunus fasciculata (desert almond) comprising more than half of its diet. Both F. californica and P. fasciculata are known to produce potentially toxic plant secondary compounds (PSCs), which should deter herbivory, yet these plants have relatively high nutritional value as measured by crude protein content. Neotoma bryanti and N. lepida consumed F. californica and P. fasciculata, respectively, in greater abundance than these plants are available on the landscape-indicating dietary selection. Finally, experimental preference trials revealed that N. bryanti exhibited a preference for F. californica, while N. lepida exhibited a relatively stronger preference for P. fasciculata. We find that N. bryanti exhibit a generalist herbivore strategy relative to N. lepida, which exhibit a more specialized feeding strategy in this study system.Our results suggest that woodrats respond to fine-scale environmental differences in plant availability that may require different metabolic strategies in order to balance nutrient acquisition while minimizing exposure to potentially toxic PSCs.

Parasitic development in intestines and oocyst shedding patterns for infection by Eimeria uekii and Eimeria raichoi in Japanese rock ptarmigans, Lagopus muta japonica, protected by cages in the Southern Japanese Alps

The population of Japanese rock ptarmigan (Lagopus muta japonica), an endangered species with a habitat above the timberline of the southern Japanese Alps, has declined. As one of the recent conservation strategies for this species, cage protection for broods (hens and chicks) has been introduced in their habitats. Two species of Eimeria have frequently been detected in these birds, but little is known about the parasitic circulation in the region, including among birds and in the environment. Here, we conducted histopathology examinations of dead chicks collected under cage protection in 2018, and examined the feces of the hens and chicks of three broods and environmental soils for parasites in 2019 in order to assess the potential sources of infection and pathogenicity. Developmental zoites were found in the epithelial mucosa and/or the submucosa from the duodenum to the colon of all dead chicks. Fecal examination revealed oocysts of E. uekii and/or E. raichoi in all hens and chicks. Oocysts of Eimeria spp. per gram of feces in chicks increased within 2 weeks after hatching and then gradually deceased. Following infection of the chicks, oocysts could accumulate within the cage areas, and oocyst density exceeded more than 1000 oocysts per gram of cage soils. Based on having sporulated morphologies, oocysts could be infective and therefore, be direct or indirect potential sources of infection. However, based on our findings that not all chicks were clinically affected by the infections, other factors such as microbial flora in the chicks established by coprophagy or from the habitat environment, including climate, might be associated with the pathogenicity of Eimeria spp., although further studies are needed to assess these correlations.

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Cage protection is effective on conservation of Japanese rock ptarmigans.

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Hens caring chicks were highly infected with Eimeria spp. and shedded the oocysts.

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Potential sources of the infection could be contaminated soils but not coprophagy behaviors of chicks.

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Pathogenicity of Eimeria might be also associated with factors like establishment of gut microbiota or habitat conditions.

Role of coprophagy in the cecal microbiome development of an herbivorous bird Japanese rock ptarmigan

The transgenerational maintenance of symbiotic microbes that benefit host nutrition and health is evolutionarily advantageous. In some vertebrate lineages, coprophagy is used as a strategy for effectively transmitting microbes across generations. However, this strategy has still not been studied in birds. Accordingly, the aim of the present study was to evaluate the role of maternal cecal feces consumption by Japanese rock ptarmigan (Lagopus muta japonica) chicks as a strategy for acquiring essential gut microbes. Both the duration of coprophagy behavior by the chicks and the development process of the chick cecal microbiome (n=20 one- to three-week-old chicks, from three broods) were investigated. In all three broods, coprophagy behavior was only observed from 3 to 18 days of age. Furthermore, there was no significant difference in the number of bacterial operational taxonomic units (OTUs) in 1-week-old chicks (n=651) and adults (n=609), and most of the main OTUs observed in the adults were already present in the 1-week-old chicks. These results indicate that, in this precocial bird species, coprophagy may contribute to the early establishment of cecal bacteria that are essential for food digestion and, thus, chick survival. In fact, Japanese rock ptarmigan chicks consume the same food as their hens from the time of hatching. This behavior may have applications to ex-situ conservation.