While many animals respond to seasonal variation in their environment, animals such as pocket gophers Thomomys bottae that live below ground might seem to be buffered against such variation. In some ...areas, however, the patterns of burrowing activity by pocket gophers are tied to the seasons, one factor of which is rainfall. Variation in activity patterns may result from the ease of digging in moist soil or increased food availability during the wet season. Previous simulation modelling work suggests that food availability influences burrowing patterns, while soil conditions do not. Thus, field experiments were used to investigate how soil conditions and food availability influence seasonal burrowing activity. Results indicate that an increase in soil moisture initiates activity. After this initial increase in activity, mound production declines and reaches a steady rate, which can be supported by vegetation availability, in agreement with previous model results. Our findings support the idea that moist soil promotes a burst of digging activity, potentially for burrow maintenance when soil becomes easily workable, and the eventual growth of vegetation provides the food necessary to support continued activity.
Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of ...nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature – herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local‐scale herbivory, and its interaction with nutrient enrichment and climate, within global‐scale models to better predict land–atmosphere interactions under future climate change.
In this study, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under unfertilized and fertilized conditions. We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization; the highest soil C and N pools were found in grazed (+H) and fertilized plots (+F). Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide and incorporating local‐scale herbivory within global‐scale models to better predict land–atmosphere interactions under future climate change.
One way for animals to decrease energy expenditures is to minimize the cost of movement. For animals dwelling on slopes, gravity can impart a large energetic cost to movement. For this reason, ...animals traveling aboveground alter their movement patterns in response to the steepness of terrain (specifically hillslope angle) so as to minimize their energetic costs. Subterranean animals should also benefit from choosing optimum movement paths in relation to hillslopes but concurrently must factor the cost of excavation into their movement decisions. In cases where the excavation costs are much higher than the costs of working against gravity, excavation costs may override the consideration of gravitational costs and movement of subterranean animals may be independent of hillslope angle. To determine the response of a subterranean animal to hillslope angle, we excavated tunnels in the burrow systems of 19 pocket gophers in southern California that occupied hillslopes ranging from 2 to 3°. At each excavation we measured several characteristics of burrow geometry and used these data in a model of pocket gopher energetics to calculate the cost of tunnel construction at the various hillslope angles. We found that the cost of tunnel construction was independent of hillslope angle, and that the costs of shearing soil and pushing soil horizontally through the tunnels were 3 orders of magnitude greater than the costs of lifting the soil against the force of gravity. Accordingly, pocket gopher foraging tunnels were oriented independently of the hillslope. The decoupling of the movement patterns of subterranean animals from the effects of gravity is a distinctive feature of the subterranean habit compared to the movement of above-ground animals. Because of the important effects of tunnel construction on soil processes, this unique biological feature of subterranean animals has implications for basic physical processes, such as soil erosion. We found that the rate of soil flux generated by pocket gopher activity was invariant to hillslope. This relationship is in contrast to the most common model of soil movement generated by purely physical processes.
The pathogen and parasite community that inhabits every free-living organism can control host vital rates including lifespan and reproductive output. To date, however, there have been few experiments ...examining pathogen community assembly replicated at large-enough spatial scales to inform our understanding of pathogen dynamics in natural systems. Pathogen community assembly may be driven by neutral stochastic colonization and extinction events or by niche differentiation that constrains pathogen distributions to particular environmental conditions, hosts, or vectors. Here, we present results from a regionally-replicated experiment investigating the community of barley and cereal yellow dwarf viruses (B/CYDV's) in over 5000 experimentally planted individuals of six grass species along a 700 km latitudinal gradient along the Pacific coast of North America (USA) in response to experimentally manipulated nitrogen and phosphorus supplies. The composition of the virus community varied predictably among hosts and across nutrient-addition treatments, indicating niche differentiation among virus species. There were some concordant responses among the viral species. For example, the prevalence of most viral species increased consistently with perennial grass cover, leading to a 60% increase in the richness of the viral community within individual hosts (i.e., coinfection) in perennial-dominated plots. Furthermore, infection rates of the six host species in the field were highly correlated with vector preferences assessed in laboratory trials. Our results reveal the importance of niche differentiation in structuring virus assemblages. Virus species distributions reflected a combination of local host community composition, host species-specific vector preferences, and virus responses to host nutrition. In addition, our results suggest that heterogeneity among host species in their capacity to attract vectors or support pathogens between growing seasons can lead to positive covariation among virus species.
Animals can attain fitness benefits by maintaining a positive net energy balance, including costs of movement during resource acquisition and the profits from foraging. Subterranean rodent burrowing ...provides an excellent system in which to examine the effects of movement costs on foraging behavior because it is energetically expensive to excavate burrows. We used an individual-based modeling approach to study pocket gopher foraging and its relationship to digging cost, food abundance, and food distribution. We used a unique combination of an individual-based foraging-behavior model and an energetic model to assess survival, body mass dynamics, and burrow configurations. Our model revealed that even the extreme cost of digging is not as costly as it appears when compared to metabolic costs. Concentrating digging in the area where food was found, or area-restricted search (ARS), was the most energetically efficient digging strategy compared to a random strategy. Field data show that natural burrow configurations were more closely approximated by the animals we modeled using ARS compared to random diggers. By using behavior and simple physiological principles in our model, we were able to observe realistic body mass dynamics and recreate natural movement patterns.
Photoautotroph nitrogen (N) and phosphorus (P) tissue concentrations can influence ecosystem function via processes including growth, decomposition, and consumption, and may reflect traits ...maintaining coexistence. Studies in terrestrial systems have led to hypotheses that latitudinal trends in the N and P content of leaves may be driven by soil substrate age, environmental temperature, or season lenght; however, terrestrial patterns alone cannot differentiate these mechanisms. Here, we demonstrate that broad geographical patterns of N and P in freshwater and marine multicellular photoautotrophs are concordant with those in terrestrial ecosystems. Our > 6800 record database reveals that mean tissue N and P increase with latitude in all ecosystems, but P increases more rapidly, causing N:P to decline; mean N:P scaling within individuals also is identical among systems, despite very different evolutionary environments. A partitioning of the variance in these data suggests that species composition and local environmental context likely lead to the variation observed within a latitudinal band. However, the consistency of trends in photosynthetic tissue chemistry across Earth's ecosystems suggests that biogeographical gradients in insolation and growing season length may constrain tissue N and P, whereas global trends in temperature, nutrient supply, and soil substrate age are unlikely to generate the consistent latitudinal trends among ecosystems. Thus, this cross-ecosystem comparison suggests a new hypothesis, global patterns of insolation, while also providing a new perspective on other mechanisms that have been hypothesized to underlie latitudinal trends in photosynthetic tissue chemistry.
Host nutrient supply can mediate host–pathogen and pathogen–pathogen interactions. In terrestrial systems, plant nutrient supply is mediated by soil microbes, suggesting a potential role of soil ...microbes in plant diseases beyond soil‐borne pathogens and induced plant defenses. Long‐term nitrogen (N) enrichment can shift pathogenic and nonpathogenic soil microbial community composition and function, but it is unclear if these shifts affect plant–pathogen and pathogen–pathogen interactions. In a growth chamber experiment, we tested the effect of long‐term N enrichment on infection by Barley Yellow Dwarf Virus (BYDV‐PAV) and Cereal Yellow Dwarf Virus (CYDV‐RPV), aphid‐vectored RNA viruses, in a grass host. We inoculated sterilized growing medium with soil collected from a long‐term N enrichment experiment (ambient, low, and high N soil treatments) to isolate effects mediated by the soil microbial community. We crossed soil treatments with a N supply treatment (low, high) and virus inoculation treatment (mock‐, singly‐, and co‐inoculated) to evaluate the effects of long‐term N enrichment on plant–pathogen and pathogen–pathogen interactions, as mediated by N availability. We measured the proportion of plants infected (i.e., incidence), plant biomass, and leaf chlorophyll content. BYDV‐PAV incidence (0.96) declined with low N soil (to 0.46), high N supply (to 0.61), and co‐inoculation (to 0.32). Low N soil mediated the effect of N supply on BYDV‐PAV: instead of N supply reducing BYDV‐PAV incidence, the incidence increased. Additionally, ambient and low N soil ameliorated the negative effect of co‐inoculation on BYDV‐PAV incidence. BYDV‐PAV infection only reduced chlorophyll when plants were grown with low N supply and ambient N soil. There were no significant effects of long‐term N soil on CYDV‐RPV incidence. Soil inoculant with different levels of long‐term N enrichment had different effects on host–pathogen and pathogen–pathogen interactions, suggesting that shifts in soil microbial communities with long‐term N enrichment may mediate disease dynamics.
Soil microbes alter plant nutrient supply and plant defenses, both of which can modify plant‐pathogen and pathogen‐pathogen interactions. It is unclear if changes in soil microbial communities exposed to long‐term nitrogen (N) enrichment affect their mediation of plant diseases. In a laboratory experiment, soil exposed to long‐term N enrichment neutralized the negative effects of nitrogen and co‐inoculation on infection incidence of a grass virus.
To evaluate how increased anthropogenic nutrient inputs alter carbon cycling in grasslands, we conducted a litter decomposition study across 20 temperate grasslands on three continents within the ...Nutrient Network, a globally distributed nutrient enrichment experiment
We determined the effects of addition of experimental nitrogen (N), phosphorus (P) and potassium plus micronutrient (Kμ) on decomposition of a common tree leaf litter in a long‐term study (maximum of 7 years; exact deployment period varied across sites). The use of higher order decomposition models allowed us to distinguish between the effects of nutrients on early‐ versus late‐stage decomposition.
Across continents, the addition of N (but not other nutrients) accelerated early‐stage decomposition and slowed late‐stage decomposition, increasing the slowly decomposing fraction by 28% and the overall litter mean residence time by 58%.
Synthesis. Using a novel, long‐term cross‐site experiment, we found widespread evidence that N enhances the early stages of above‐ground plant litter decomposition across diverse and widespread temperate grassland sites but slows late‐stage decomposition. These findings were corroborated by fitting the data to multiple decomposition models and have implications for N effects on soil organic matter formation. For example, following N enrichment, increased microbial processing of litter substrates early in decomposition could promote the production and transfer of low molecular weight compounds to soils and potentially enhance the stabilization of mineral‐associated organic matter. By contrast, by slowing late‐stage decomposition, N enrichment could promote particulate organic matter (POM) accumulation. Such hypotheses deserve further testing.
In a long‐term experiment across 20 diverse and widespread temperate grassland sites, nitrogen enhanced the early stages of aboveground plant litter decomposition, accelerating the initial decay rate, ka, and reducing the time to 10% mass loss, t1/10, but slowed late‐stage decomposition, increasing the mean residence time, MRT, and fraction of slowly decomposing litter, asymptotic A.
Disease dilution (reduced disease prevalence with increasing biodiversity) has been described for many different pathogens. Although the mechanisms causing this phenomenon remain unclear, the ...disassembly of communities to predictable subsets of species, which can be caused by changing climate, land use or invasive species, underlies one important hypothesis. In this case, infection prevalence could reflect the competence of the remaining hosts. To test this hypothesis, we measured local host species abundance and prevalence of four generalist aphid-vectored pathogens (barley and cereal yellow dwarf viruses) in a ubiquitous annual grass host at 10 sites spanning 2000 km along the North American West Coast. In laboratory and field trials, we measured viral infection as well as aphid fecundity and feeding preference on several host species. Virus prevalence increased as local host richness declined. Community disassembly was non-random: ubiquitous hosts dominating species-poor assemblages were among the most competent for vector production and virus transmission. This suggests that non-random biodiversity loss led to increased virus prevalence. Because diversity loss is occurring globally in response to anthropogenic changes, such work can inform medical, agricultural and veterinary disease research by providing insights into the dynamics of pathogens nested within a complex web of environmental forces.
Interactions among co-infecting pathogens are common across host taxa and can affect infectious disease dynamics. Host nutrition can mediate these among-pathogen interactions, altering the ...establishment and growth of pathogens within hosts. It is unclear, however, how nutrition-mediated among-pathogen interactions affect transmission and the spread of disease through populations. We manipulated the nitrogen (N) and phosphorus (P) supplies to oat plants in growth chambers and evaluated interactions between two aphid-vectored Barley and Cereal Yellow Dwarf Viruses: PAV and RPV. We quantified the effect of each virus on the other’s establishment, within-plant density, and transmission. Co-inoculation significantly increased PAV density when N and P supplies were low and tended to increase RPV density when N supply was high. Co-infection increased PAV transmission when N and P supplies were low and tended to increase RPV transmission when N supply was high. Despite the parallels between the effects of among-pathogen interactions on density and transmission, changes in virus density only partially explained changes in transmission, suggesting that virus density-independent processes contribute to transmission. A mathematical model describing the spread of two viruses through a plant population, parameterized with empirically derived transmission values, demonstrated that nutrition-mediated among-pathogen interactions could affect disease spread. Interactions that altered transmission through virus density-independent processes determined overall disease dynamics. Our work suggests that host nutrition alters disease spread through among-pathogen interactions that modify transmission.
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