Jon M. Yearsley

Weak interactions, omnivory and emergent food-web properties

Empirical studies have shown that, in real ecosystems, species–interaction strengths are generally skewed in their distribution towards weak interactions. Some theoretical work also suggests that weak interactions, especially in omnivorous links, are important for the local stability of a community at equilibrium. However, the majority of theoretical studies use uniform distributions of interaction strengths to generate artificial communities for study. We investigate the effects of the underlying interaction–strength distribution upon the return time, permanence and feasibility of simple Lotka–Volterra equilibrium communities. We show that a skew towards weak interactions promotes local and global stability only when omnivory is present. It is found that skewed interaction strengths are an emergent property of stable omnivorous communities, and that this skew towards weak interactions creates a dynamic constraint maintaining omnivory. Omnivory is more likely to occur when omnivorous interactions are skewed towards weak interactions. However, a skew towards weak interactions increases the return time to equilibrium, delays the recovery of ecosystems and hence decreases the stability of a community. When no skew is imposed, the set of stable omnivorous communities shows an emergent distribution of skewed interaction strengths. Our results apply to both local and global concepts of stability and are robust to the definition of a feasible community. These results are discussed in the light of empirical data and other theoretical studies, in conjunction with their broader implications for community assembly.

Does the activity budget hypothesis explain sexual segregation in ungulates

The activity budget hypothesis proposes that the main force driving sexual segregation is the difference in activity between males and females. Recently, a model was developed to demonstrate explicitly that such differences in activity could, in theory, produce sexual segregation (Ruckstuhl & Kokko 2002, Animal Behaviour, 64, 909–914). The question remains whether realistic parameter values can also generate significant sexual segregation. Using this model and data on the body size dimorphism for 144 ungulate species, we compared the sexual segregation predicted by the model with expectations based on field observations. The results do not support the activity budget hypothesis as the main factor explaining sexual segregation. We investigated activity synchronization, the animal movement rule and transient spatial distributions in further detail. Introducing activity synchronization into the model slightly increases the ability of the activity budget to generate sexual segregation. Changing an animals movement rule has a strong effect; movement rules that are independent of activity generate no long-term segregation. Finally, changes in a populations home range allow activity budget differences to generate transient sexual segregation. This method of generating sexual segregation is not sensitive to the animal movement rule and is potentially an important mechanism by which activity budgets can generate sexual segregation.

Loss of functionally unique species may gradually undermine ecosystems

Functionally unique species contribute to the functional diversity of natural systems, often enhancing ecosystem functioning. An abundance of weakly interacting species increases stability in natural systems, suggesting that loss of weakly linked species may reduce stability. Any link between the functional uniqueness of a species and the strength of its interactions in a food web could therefore have simultaneous effects on ecosystem functioning and stability. Here, we analyse patterns in 213 real food webs and show that highly unique species consistently tend to have the weakest mean interaction strength per unit biomass in the system. This relationship is not a simple consequence of the interdependence of both measures on body size and appears to be driven by the empirical pattern of size structuring in aquatic systems and the trophic position of each species in the web. Food web resolution also has an important effect, with aggregation of species into higher taxonomic groups producing a much weaker relationship. Food webs with fewer unique and less weakly interacting species also show significantly greater variability in their levels of primary production. Thus, the loss of highly unique, weakly interacting species may eventually lead to dramatic state changes and unpredictable levels of ecosystem functioning.

Current hypotheses to explain genetic chaos under the sea

Chaotic genetic patchiness (CGP) refers to surprising patterns of spatial and temporal genetic structure observed in some marine species at a scale where genetic variation should be efficiently homogenized by gene flow via larval dispersal. Here we review and discuss 4 mechanisms that could generate such unexpected patterns: selection, sweepstakes reproductive success, collective dispersal, and temporal shifts in local population dynamics. First, we review examples where genetic differentiation at specific loci was driven by diversifying selection, which was historically the first process invoked to explain CGP. Second, we turn to neutral demographic processes that may drive genome-wide effects, and whose effects on CGP may be enhanced when they act together. We discuss how sweepstakes reproductive success accelerates genetic drift and can thus generate genetic structure, provided that gene flow is not too strong. Collective dispersal is another mechanism whereby genetic structure can be maintained regardless of dispersal intensity, because it may prevent larval cohorts from becoming entirely mixed. Theoretical analyses of both the sweepstakes and the collective dispersal ideas are presented. Finally, we discuss an idea that has received less attention than the other ones just mentioned, namely temporal shifts in local population dynamics.

Sensitivity analysis of equilibrium population size in a density-dependent model for Short-tailed Shearwaters

Population models are often used to guide conservation management decisions. Sensitivity analysis of such models can be useful in setting research priorities, by highlighting those parameters that have the most influence on population growth rate. Much of the work on sensitivity analysis in this context has been for density-independent models. We present a sensitivity analysis of a density-dependent model for a population of Short-tailed Shearwaters (Puffinus tenuirostris ), in which the output of interest is the equilibrium population size, Ne. We calculate the sensitivity and elasticity of Ne to both the equilibrium parameter value and the strength of density-dependence associated with each input parameter. The rankings of the sensitivities and elasticities associated with the strength of density-dependence are of particular interest, as they cannot be predicted from a sensitivity analysis for the corresponding density-independent model. In calculating sensitivities we make use of the characteristic equation of the model, rather than the left and right eigenvectors of the projection matrix. In order to check the robustness of our conclusions to the strength of density-dependence specified for each input parameter, we consider a range of relative strengths. Within this range there are no major effects on the rankings. The largest sensitivities of Ne to the strength of density-dependence were for breeder survival, emigration and immigration; the largest corresponding elasticities were for emigration, immigration and breeder skipping rate.

Does size matter? Atmospheric CO2 may be a stronger driver of stomatal closing rate than stomatal size in taxa that diversified under low CO2

One strategy for plants to optimize stomatal function is to open and close their stomata quickly in response to environmental signals. It is generally assumed that small stomata can alter aperture faster than large stomata. We tested the hypothesis that species with small stomata close faster than species with larger stomata in response to darkness by comparing rate of stomatal closure across an evolutionary range of species including ferns, cycads, conifers, and angiosperms under controlled ambient conditions (380 ppm CO2; 20.9% O2). The two species with fastest half-closure time and the two species with slowest half-closure time had large stomata while the remaining three species had small stomata, implying that closing rate was not correlated with stomatal size in these species. Neither was response time correlated with stomatal density, phylogeny, functional group, or life strategy. Our results suggest that past atmospheric CO2 concentration during time of taxa diversification may influence stomatal response time. We show that species which last diversified under low or declining atmospheric CO2 concentration close stomata faster than species that last diversified in a high CO2 world. Low atmospheric [CO2] during taxa diversification may have placed a selection pressure on plants to accelerate stomatal closing to maintain adequate internal CO2 and optimize water use efficiency.

Invading and expanding : range dynamics and ecological consequences of the Greater White-Toothed Shrew (Crocidura russula) invasion in Ireland

Establishing how invasive species impact upon pre-existing species is a fundamental question in ecology and conservation biology. The greater white-toothed shrew (Crocidura russula) is an invasive species in Ireland that was first recorded in 2007 and which, according to initial data, may be limiting the abundance/distribution of the pygmy shrew (Sorex minutus), previously Irelands only shrew species. Because of these concerns, we undertook an intensive live-trapping survey (and used other data from live-trapping, sightings and bird of prey pellets/nest inspections collected between 2006 and 2013) to model the distribution and expansion of C. russula in Ireland and its impacts on Irelands small mammal community. The main distribution range of C. russula was found to be approximately 7,600 km2 in 2013, with established outlier populations suggesting that the species is dispersing with human assistance within the island. The species is expanding rapidly for a small mammal, with a radial expansion rate of 5.5 km/yr overall (2008–2013), and independent estimates from live-trapping in 2012–2013 showing rates of 2.4–14.1 km/yr, 0.5–7.1 km/yr and 0–5.6 km/yr depending on the landscape features present. S. minutus is negatively associated with C. russula. S. minutus is completely absent at sites where C. russula is established and is only present at sites at the edge of and beyond the invasion range of C. russula. The speed of this invasion and the homogenous nature of the Irish landscape may mean that S. minutus has not had sufficient time to adapt to the sudden appearance of C. russula. This may mean the continued decline/disappearance of S. minutus as C. russula spreads throughout the island.

Modelling the impact of microbial grazers on soluble rhizodeposit turnover

Rhizodeposition represents a relatively large carbon flow from a plant’s root into the surrounding soil. This carbon flow may have important implications for nitrogen mineralisation and carbon sequestration, but is still poorly understood. In this paper we use a simple compartment model of carbon flow in the rhizosphere to investigate the proposed benefits of rhizodeposition and the effect of microbial grazers. Model parameters were fitted to published, experimental data. Analysis of the model showed that dead organic matter (necromass) had a much longer time-scale than the other carbon pools (soluble, microbial and grazer carbon), which allowed an approximate, mathematical solution of the model to be derived. This solution shows that the level of necromass in the soil is an important factor in many processes of interest. The short-term carbon and nitrogen turnover increases with the level of necromass. Microbial grazers decrease carbon turnover at high levels of necromass, whilst at lower, and possibly more realistic, levels of necromass grazers increase turnover. However, the largest effect of grazers was to increase carbon turnover by 10%, suggesting that grazers are relatively unimportant in larger scale models of soil organic matter turnover. The marginal benefits of rhizodeposition increase with the level of necromass. The model suggests that the short-term benefits of rhizodeposition to a plant are marginal, but long-term benefits may still occur.

Emerging Infectious Disease Implications of Invasive Mammalian Species: The Greater White-Toothed Shrew (Crocidura russula) Is Associated With a Novel Serovar of Pathogenic Leptospira in Ireland.

The greater white-toothed shrew (Crocidura russula) is an invasive mammalian species that was first recorded in Ireland in 2007. It currently occupies an area of approximately 7,600 km2 on the island. C. russula is normally distributed in Northern Africa and Western Europe, and was previously absent from the British Isles. Whilst invasive species can have dramatic and rapid impacts on faunal and floral communities, they may also be carriers of pathogens facilitating disease transmission in potentially naive populations. Pathogenic leptospires are endemic in Ireland and a significant cause of human and animal disease. From 18 trapped C. russula, 3 isolates of Leptospira were cultured. However, typing of these isolates by standard serological reference methods was negative, and suggested an, as yet, unidentified serovar. Sequence analysis of 16S ribosomal RNA and secY indicated that these novel isolates belong to Leptospira alstonii, a unique pathogenic species of which only 7 isolates have been described to date. Earlier isolations were limited geographically to China, Japan and Malaysia, and this leptospiral species had not previously been cultured from mammals. Restriction enzyme analysis (REA) further confirms the novelty of these strains since no similar patterns were observed with a reference database of leptospires. As with other pathogenic Leptospira species, these isolates contain lipL32 and do not grow in the presence of 8-azagunaine; however no evidence of disease was apparent after experimental infection of hamsters. These isolates are genetically related to L. alstonii but have a novel REA pattern; they represent a new serovar which we designate as serovar Room22. This study demonstrates that invasive mammalian species act as bridge vectors of novel zoonotic pathogens such as Leptospira.

Molecular and morphological insights into the origin of the invasive greater white-toothed shrew (Crocidura russula) in Ireland

Identifying routes of invasion is a critical management strategy in controlling the spread of invasive species. This is challenging however in the absence of direct evidence. Therefore, indirect methodologies are used to infer possible invasion sources and routes, such as comparisons of genetic and morphological data from populations from invasive ranges and putative source areas. The greater white-toothed shrew (Crocidura russula) was first discovered in Ireland from skeletal remains in the pellets of birds of prey collected in 2007 and is it is now sufficiently established that the species has a detrimental impact on Ireland’s small mammal community. In this study, we address the uncertain origin(s) of the Irish population of C. russula. The cytochrome b gene of mitochondrial DNA was analysed from 143 individuals from throughout its range within a phylogenetic and approximate Bayesian computation framework. These analyses revealed that the Irish population stemmed from Europe as opposed to North Africa. Additionally, mandibles from 523 individuals from Ireland and 28 other European populations were subjected to multivariate and distance-based analyses, which demonstrated an association between the Irish population and those in France, Switzerland and Belgium. When the genetic and morphological analyses were considered together, an origin stemming from France was deemed the most likely scenario for the source of the invasive Irish population. This study has demonstrated the importance of utilising a multidisciplinary approach when attempting to identify the origins and invasion routes of invasive species.