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According to Dooge (1986) intermediate-scale catchments are systems of organized complexity, being too organized and yet too small to be characterized on a statistical/conceptual basis, but too large and too heterogeneous to be characterized in a deterministic manner. A key requirement for building structurally adequate models precisely for this intermediate scale is a better understanding of how different forms of spatial organization affect storage and release of water and energy. Here, we propose that a combination of the concept of hydrological response units (HRUs) and thermodynamics offers several helpful and partly novel perspectives for gaining this improved understanding. Our key idea is to define functional similarity based on similarity of the terrestrial controls of gradients and resistance terms controlling the land surface energy balance, rainfall runoff transformation, and groundwater storage and release. This might imply that functional similarity with respect to these specific forms of water release emerges at different scales, namely the small field scale, the hillslope, and the catchment scale. We thus propose three different types of "functional units" - specialized HRUs, so to speak - which behave similarly with respect to one specific form of water release and with a characteristic extent equal to one of those three scale levels. We furthermore discuss an experimental strategy based on exemplary learning and replicate experiments to identify and delineate these functional units, and as a promising strategy for characterizing the interplay and organization of water and energy fluxes across scales. We believe the thermodynamic perspective to be well suited to unmask equifinality as inherent in the equations governing water, momentum, and energy fluxes: this is because several combinations of gradients and resistance terms yield the same mass or energy flux and the terrestrial controls of gradients and resistance terms are largely independent. We propose that structurally adequate models at this scale should consequently disentangle driving gradients and resistance terms, because this optionally allow sequifinality to be partly reduced by including available observations, e. g., on driving gradients. Most importantly, the thermodynamic perspective yields an energy-centered perspective on rainfall-runoff transformation and evapotranspiration, including fundamental limits for energy fluxes associated with these processes. This might additionally reduce equifinality and opens up opportunities for testing thermodynamic optimality principles within independent predictions of rainfall-runoff or land surface energy exchange. This is pivotal to finding out whether or not spatial organization in catchments is in accordance with a fundamental organizing principle.
Seed traits matter
(2021)
Although many plants are dispersed by wind and seeds can travel long distances across unsuitable matrix areas, a large proportion relies on co-evolved zoochorous seed dispersal to connect populations in isolated habitat islands. Particularly in agricultural landscapes, where remaining habitat patches are often very small and highly isolated, mobile linkers as zoochorous seed dispersers are critical for the population dynamics of numerous plant species. However, knowledge about the quali- or quantification of such mobile link processes, especially in agricultural landscapes, is still limited. In a controlled feeding experiment, we recorded the seed intake and germination success after complete digestion by the European brown hare (Lepus europaeus) and explored its mobile link potential as an endozoochoric seed disperser. Utilizing a suite of common, rare, and potentially invasive plant species, we disentangled the effects of seed morphological traits on germination success while controlling for phylogenetic relatedness. Further, we measured the landscape connectivity via hares in two contrasting agricultural landscapes (simple: few natural and semi-natural structures, large fields; complex: high amount of natural and semi-natural structures, small fields) using GPS-based movement data. With 34,710 seeds of 44 plant species fed, one of 200 seeds (0.51%) with seedlings of 33 species germinated from feces. Germination after complete digestion was positively related to denser seeds with comparatively small surface area and a relatively slender and elongated shape, suggesting that, for hares, the most critical seed characteristics for successful endozoochorous seed dispersal minimize exposure of the seed to the stomach and the associated digestive system. Furthermore, we could show that a hare's retention time is long enough to interconnect different habitats, especially grasslands and fields. Thus, besides other seed dispersal mechanisms, this most likely allows hares to act as effective mobile linkers contributing to ecosystem stability in times of agricultural intensification, not only in complex but also in simple landscapes.
Seed traits matter
(2021)
Although many plants are dispersed by wind and seeds can travel long distances across unsuitable matrix areas, a large proportion relies on co-evolved zoochorous seed dispersal to connect populations in isolated habitat islands. Particularly in agricultural landscapes, where remaining habitat patches are often very small and highly isolated, mobile linkers as zoochorous seed dispersers are critical for the population dynamics of numerous plant species. However, knowledge about the quali- or quantification of such mobile link processes, especially in agricultural landscapes, is still limited. In a controlled feeding experiment, we recorded the seed intake and germination success after complete digestion by the European brown hare (Lepus europaeus) and explored its mobile link potential as an endozoochoric seed disperser. Utilizing a suite of common, rare, and potentially invasive plant species, we disentangled the effects of seed morphological traits on germination success while controlling for phylogenetic relatedness. Further, we measured the landscape connectivity via hares in two contrasting agricultural landscapes (simple: few natural and semi-natural structures, large fields; complex: high amount of natural and semi-natural structures, small fields) using GPS-based movement data. With 34,710 seeds of 44 plant species fed, one of 200 seeds (0.51%) with seedlings of 33 species germinated from feces. Germination after complete digestion was positively related to denser seeds with comparatively small surface area and a relatively slender and elongated shape, suggesting that, for hares, the most critical seed characteristics for successful endozoochorous seed dispersal minimize exposure of the seed to the stomach and the associated digestive system. Furthermore, we could show that a hare's retention time is long enough to interconnect different habitats, especially grasslands and fields. Thus, besides other seed dispersal mechanisms, this most likely allows hares to act as effective mobile linkers contributing to ecosystem stability in times of agricultural intensification, not only in complex but also in simple landscapes.
Strong spatiotemporal variation in population size often leads to reduced genetic diversity limiting the adaptive potential of individual populations. Key genes of adaptive variation are encoded by the immune genes of the major histocompatibility complex (MHC) playing an essential role in parasite resistance. How MHC variation persists in rodent populations that regularly experience population bottlenecks remains an important topic in evolutionary genetics. We analysed the consequences of strong population fluctuations on MHC class II DRB exon 2 diversity in two distant common vole (Microtus arvalis) populations in three consecutive years using a high-throughput sequencing approach. In 143 individuals, we detected 25 nucleotide alleles translating into 14 unique amino acid MHC alleles belonging to at least three loci. Thus, the overall allelic diversity and amino acid distance among the remaining MHC alleles, used as a surrogate for the range of pathogenic antigens that can be presented to T-cells, are still remarkably high. Both study populations did not show significant population differentiation between years, but significant differences were found between sites. We concluded that selection processes seem to be strong enough to maintain moderate levels of MHC diversity in our study populations outcompeting genetic drift, as the same MHC alleles were conserved between years. Differences in allele frequencies between populations might be the outcome of different local parasite pressures and/or genetic drift. Further understanding of how pathogens vary across space and time will be crucial to further elucidate the mechanisms maintaining MHC diversity in cyclic populations.
Earthworms affect various soil ecosystem processes in their role as ecosystem engineers. The spatial distribution of earthworms determines the spatial distribution of their functional effects. In particular, earthworm-induced macropore networks may act as preferential flow pathways. In this research we aimed to determine earthworm distributions at the catchment scale with species distribution models (SDMs). We used land-use types, temporally invariant topography-related variables and plot-scale soil characteristics such as pH and organic matter content. We used data from spring 2013 to estimate probability distributions of the occurrence of ten earthworm species. To assess the robustness of these models, we tested temporal transferability by evaluating the accuracy of predictions from the models derived for the spring data with the predictions from data of two other field surveys in autumn 2012 and 2013. In addition, we compared the performance of SDMs based (i) on temporally varying plot-scale predictor variables with (ii) those based on temporally invariant catchment-scale predictors. Models based on catchment-scale predictors, especially land use and slope, experience a small loss of predictive performance only compared with plot-scale SDMs but have greater temporal transferability. Earthworm distribution maps derived from this kind of SDM are a prerequisite for understanding the spatial distribution patterns of functional effects related to earthworms.
Social organisation in species with fluctuating population sizes can change with density. Therefore, information on (future) density obtained during early life stages may be associated with social behaviour. Olfactory cues may carry important social information. We investigated whether early life experience of different experimental densities was subsequently associated with differences in attraction to adult conspecific odours. We used common voles (Microtus arvalis), a rodent species undergoing extreme density fluctuations. We found that individuals originating from high experimental density populations kept in large outdoor enclosures invested more time in inspecting conspecific olfactory cues than individuals from low-density populations. Generally, voles from both treatments spent more time with the olfactory cues than expected by chance and did not differ in their latency to approach the odour samples. Our findings indicate either that early experience affects odour sensitivity or that animals evaluate the social information contained in conspecific odours differently, depending on their early life experience of conspecific density.
Tula virus (TULV) is a vole-associated hantavirus with low or no pathogenicity to humans. In the present study, 686 common voles (Microtus arvalis), 249 field voles (Microtus agrestis) and 30 water voles (Arvicola spec.) were collected at 79 sites in Germany, Luxembourg and France and screened by RT-PCR and TULV-IgG ELISA. TULV-specific RNA and/or antibodies were detected at 43 of the sites, demonstrating a geographically widespread distribution of the virus in the studied area. The TULV prevalence in common voles (16.7 %) was higher than that in field voles (9.2 %) and water voles (10.0 %). Time series data at ten trapping sites showed evidence of a lasting presence of TULV RNA within common vole populations for up to 34 months, although usually at low prevalence. Phylogenetic analysis demonstrated a strong genetic structuring of TULV sequences according to geography and independent of the rodent species, confirming the common vole as the preferential host, with spillover infections to co-occurring field and water voles. TULV phylogenetic clades showed a general association with evolutionary lineages in the common vole as assessed by mitochondrial DNA sequences on a large geographical scale, but with local-scale discrepancies in the contact areas.
Animal movements arise from complex interactions of individuals with their environment, including both conspecific and heterospecific individuals. Animals may be attracted to each other for mating, social foraging, or information gain, or may keep at a distance from others to avoid aggressive encounters related to, e.g., interference competition, territoriality, or predation. With modern tracking technology, more datasets are emerging that allow to investigate fine‐scale interactions between free‐ranging individuals from movement data, however, few methods exist to disentangle fine‐scale behavioural responses of interacting individuals when these are highly individual‐specific.
In a framework of step‐selection functions, we related movements decisions of individuals to dynamic occurrence distributions of other individuals obtained through kriging of their movement paths. Using simulated data, we tested the method's ability to identify various combinations of attraction, avoidance, and neutrality between individuals, including asymmetric (i.e. non‐mutual) behaviours. Additionally, we analysed radio‐telemetry data from concurrently tracked small rodents (bank vole, Myodes glareolus) to test whether our method could detect biologically plausible behaviours.
We found that our method was able to successfully detect and distinguish between fine‐scale interactions (attraction, avoidance, neutrality), even when these were asymmetric between individuals. The method worked best when confounding factors were taken into account in the step‐selection function. However, even when failing to do so (e.g. due to missing information), interactions could be reasonably identified. In bank voles, responses depended strongly on the sexes of the involved individuals and matched expectations.
Our approach can be combined with conventional uses of step‐selection functions to tease apart the various drivers of movement, e.g. the influence of the physical and the social environment. In addition, the method is particularly useful in studying interactions when responses are highly individual‐specific, i.e. vary between and towards different individuals, making our method suitable for both single‐species and multi‐species analyses (e.g. in the context of predation or competition).
My niche
(2020)
Intraspecific trait variation is an important determinant of fundamental ecological interactions. Many of these interactions are mediated by behaviour. Therefore, interindividual differences in behaviour should contribute to individual niche specialization. Comparable with variation in morphological traits, behavioural differentiation between individuals should limit similarity among competitors and thus act as a mechanism maintaining within-species variation in ecological niches and facilitating species coexistence. Here, we aimed to test whether interindividual differences in boldness covary with spatial interactions within and between two ecologically similar, co-occurring rodent species (Myodes glareolus, Apodemus agrarius). In five subpopulations in northeast Germany, we quantified individual differences in boldness via repeated standardized tests and spatial interaction patterns via capture-mark- recapture (n = 126) and automated VHF telemetry (n = 36). We found that boldness varied with space use in both species. Individuals of the same population occupied different spatial niches, which resulted in non-random patterns of within- and between-species spatial interactions. Behavioural types mainly differed in the relative importance of intra- versus interspecific competition. Within-species variation along this competition gradient could contribute to maintaining individual niche specialization. Moreover, behavioural differentiation between individuals limits similarity among competitors, which might facilitate the coexistence of functionally equivalent species and, thus, affect community dynamics and local biodiversity.
Personality-dependent space use and movement might be crucially influencing ecological interactions, giving way to individual niche specialization. This new approach challenges classical niche theory with potentially great ecological consequences, but so far has only scarce empirical support. Here, we investigated if and how consistent inter-individual differences in behavior predict space use and movement patterns in free-ranging bank voles (Myodes glareolus) and thereby contribute to individual niche specialization. Individuals were captured and marked from three different subpopulations in North-East Germany. Inter-individual differences in boldness and exploration were quantified via repeated standardized tests directly in the field after capture. Subsequently, space use and movement patterns of a representative sample of the behavioral variation (n=21 individuals) were monitored via automated VHF telemetry for a period of four days, yielding on average 384 locations per individual. Bolder individuals occupied larger home ranges and core areas (estimated via kernel density analyses), moved longer distances, spatially overlapped with fewer conspecifics and preferred different microhabitats based on vegetation cover compared to shyer individuals. We found evidence for personality-dependent space use, movement, and occupation of individual spatial niches in bank voles. Thus, besides dietary niche specialization also spatial dimensions of ecological niches vary among individuals within populations, which may have important consequences for ecological interactions within- and between species.