Mutualistic stability in the arbuscular mycorrhizal symbiosis: exploring hypotheses of evolutionary cooperation

The 450-million-year-old symbiosis between the majority of land plants and arbuscular mycorrhizal fungi (AMF) is one of the most ancient, abundant, and ecologically important mutualisms on Earth. Yet, the evolutionary stability of mycorrhizal associations is still poorly understood, as it follows none of the constraints thought to stabilize cooperation in other well-known mutualisms. The capacity of both host and symbiont to simultaneously interact with several partners introduces a unique dilemma; detecting and punishing those exploiting the mutualism becomes increasingly difficult if these individuals can continue to access resources from alternative sources. Here, we explore four hypotheses to explain evolutionary cooperation in the arbuscular mycorrhizal symbiosis: (1) pseudo-vertical transmission and spatial structuring of plant and fungal populations leading to local adaptation of partners; (2) luxury resource exchange in which plants trade surplus carbon for excess fungal nutrients; (3) partner choice allowing partners to associate with better cooperators; and (4) host and symbiont sanctions which actively reward good partners and punish less cooperative ones. We propose that mycorrhizal cooperation is promoted by an exchange of surplus resources between partners and enforced through sanctions by one or both partners. These mechanisms may allow plant and fungal genotypes to discriminate against individuals employing exploitative strategies, promoting patterns of partner choice. Together these selection pressures provide a framework for understanding the stabilization of mycorrhizal cooperation over evolutionary time.     

Transient population dynamics in periodic matrix models: methodology and effects of cyclic permutations

Many biological populations are subject to periodically changing environments such as years with or without fire, or rotation of crop types. The dynamics and management options for such populations are frequently investigated using periodic matrix models. However the analysis is usually limited to long-term results (asymptotic population growth rate and its sensitivity to perturbations of vital rates). In non-periodic matrix models it has been shown that long-term results may be misleading as populations are rarely in their stable structure. We therefore develop methods to analyze transient dynamics of periodic matrix models. In particular, we show how to calculate the effects of perturbations on population size within and at the end of environmental cycles. Using a model of a weed population subject to a crop rotation, we show that different cyclic permutations produce different patterns of sensitivity of population size and different population sizes. By examining how the starting environment interacts with the initial conditions, we explain how different patterns arise. Such understanding is critical to developing effective management and monitoring strategies for populations subject to periodically recurring environments.     

Caribbean mangroves and seagrass beds as daytime feeding habitats for juvenile French grunts, Haemulon flavolineatum

Caribbean seagrass beds are important feeding habitats for so-called nocturnally active zoobenthivorous fish, but the extent to which these fishes use mangroves and seagrass beds as feeding habitats during daytime remains unclear. We hypothesised three feeding strategies: (1) fishes feed opportunistically in mangroves or seagrass beds throughout the day and feed predominantly in seagrass beds during night-time; (2) fishes start feeding in mangroves or seagrass beds during daytime just prior to nocturnal feeding in seagrass beds; (3) after nocturnal feeding in seagrass beds, fishes complete feeding in mangroves or seagrass beds during the morning. We studied the effect of habitat type, fish size, social mode and time of day on resting and feeding behaviour of large juvenile (5–10 cm) and sub-adult (10–15 cm) Haemulon flavolineatum in mangroves and seagrass beds during daytime. Sub-adults occurred in mangroves only, spent most time on resting, and showed rare opportunistic feeding events (concordant with strategy 1), regardless of their social mode (solitary or schooling). In contrast, large juveniles were present in both habitat types and solitary fishes mainly foraged, while schooling fishes mainly rested. Exceptions were small juveniles (±5 cm) in seagrass beds which foraged intensively while schooling. Large juveniles showed more feeding activity in seagrass beds than in mangroves. In both habitat types, they showed benthic feeding, whereas pelagic feeding was observed almost exclusively in the seagrass beds. In both habitat types, their feeding activity was highest during 8:00–10:30 hours (concordant with strategy 3), and for seagrass fishes, it was also high during 17:30–18:30 hours (concordant with strategy 2). The study shows that both mangroves and seagrass beds provide daytime feeding habitats for some life-stages of H. flavolineatum, which is generally considered a nocturnal feeder.     

Effects of glucosinolate and myrosinase levels in Brassica juncea on a glucosinolate-sequestering herbivore – and vice versa

Summary. Larvae of the turnip sawfly, Athalia rosae L. (Hymenoptera: Tenthredinidae), sequester glucosinolates of their host plants, namely members of the Brassicaceae family, in their haemolymph. Therefore, they need to circumvent myrosinase activities of the plant tissue which normally hydrolyse the glucosinolates after plant damage. Effects of varying levels of glucosinolates and myrosinases on the performance of A. rosae were investigated using homozygous lines of Brassica juncea (L.) with either (1) low glucosinolate (lowGS) and low myrosinase (lowMR), (2) high glucosinolate (highGS) and high myrosinase (highMR), or (3) high glucosinolate (highGS) and low myrosinase (lowMR) levels. To insure that the given quantities remained as constant as possible, newly hatched larvae were enclosed on the second-youngest leaf of a plant, and were offered a new plant of comparable physiological age (6-leaf-stage) every day. The performance of A. rosae was little affected by leaf quality. Body masses of eonymphs and adults were on average lowest on the highGS/highMR-line, but these differences were rarely significant. The pupal developmental times of females and males were longest on the highGS/lowMR-line in only one of two replicate experiments. All other performance traits (developmental times of larvae, egg numbers, adult longevity) were not significantly different. Glucosinolates, sequestered by the larvae, are carried through the pupal stage. The glucosinolate concentration measured in adult insects reflected the level of the host plant line, without showing any obvious costs for sequestration. Obviously, A. rosae is highly tolerant to variation in the glucosinolate-myrosinase system of its host. In addition, induced changes of glucosinolate concentrations and myrosinase activities caused by 24 h-feeding of groups of three small larvae were analysed in the second-youngest leaves. In contrast to the patterns most herbivores evoke on Brassicaceae, namely an increase of both glucosinolate concentration and myrosinase activity, we detected a significant decrease of both traits in all three lines where the respective trait was originally high in the plants. Although glucosinolate levels dropped in the highGS lines about 50%, these still contained higher concentrations than the lowGS line. Whereas the activity of soluble myrosinases remained highest in the highMR line, even after a decrease to almost 30% due to feeding, the levels of insoluble myrosinases converged after feeding in lowMR and highMR lines. Levels of the signalling molecule salicylic acid slightly decreased on average after feeding, whereas jasmonic acid was below the detection threshold in almost all samples. The concentration of several molecules varies strongly in plant tissue with age and can change due to induction by herbivore feeding. Therefore, if performance of an insect species is measured on plants with specific traits, the variability in these traits needs to be carefully controlled in experiments.     

Arsenic-speciation in arsenate-resistant and non-resistant populations of the earthworm,Lumbricus rubellus

Arsenic speciation was determined in Lumbricus rubellus Hoffmeister from arsenic-contaminated mine spoil sites and an uncontaminated site using HPLC-MS, HPLC-ICP-MS and XAS. It was previously demonstrated that L. rubellus from mine soils were more arsenate resistant than from the uncontaminated site and we wished to investigate if arsenic speciation had a role in this resistance. Earthworms from contaminated sites had considerably higher arsenic body burdens (maximum 1,358 mg As kg-1) compared to the uncontaminated site (maximum 13 mg As kg-1). The only organo-arsenic species found in methanol/water extracts for all earthworm populations was arsenobetaine, quantified using both HPLC-MS and HPLC-ICP-MS. Arsenobetaine concentrations were high in L. rubellus from the uncontaminated site when concentrations were expressed as a percentage of the total arsenic burden (23% mean), but earthworms from the contaminated sites with relatively low arsenic burdens also had these high levels of arsenobetaine (17% mean). As arsenic body burden increased, the percentage of arsenobetaine present decreased in a dose dependent manner, although its absolute concentration rose with increasing arsenic burden. The origin of this arsenobetaine is discussed. XAS analysis of arsenic mine L. rubellus showed that arsenic was primarily present as As(III) co-ordinated with sulfur (30% approx.), with some As(v) with oxygen (5%). Spectra for As(III) complexed with glutathione gave a very good fit to the spectra obtained for the earthworms, suggesting a role for sulfur co-ordination in arsenic metabolism at higher earthworm arsenic burdens. It is also possible that the disintegration of As(III)-S complexes may have taken place due to (a) processing of the sample, (b) storage of the extract or (c) HPLC anion exchange. HPLC-ICP-MS analysis of methanol extracts showed the presence of arsenite and arsenate, suggesting that these sulfur complexes disintegrate on extraction. The role of arsenic speciation in the resistance of L. rubellus to arsenate is considered.     

Genetic variation in defense chemistry in wild cabbages affects herbivores and their endoparasitoids

Populations of wild Brassica oleracea L. grow naturally along the Atlantic coastlines of the United Kingdom and France. Over a very small spatial scale (i.e., <15 km) these populations differ in the expression of the defensive compounds, glucosinolates (GS). Thus far, very few studies have examined interactions between genetically distinct populations of a wild plant species and associated consumers in a multitrophic framework. Here, we compared the development of a specialist (Pieris rapae) and a generalist (Mamestra brassicae) insect herbivore and their endoparasitoids (Cotesia rubecula and Microplitis mediator, respectively) on three wild populations and one cultivar of B. oleracea under controlled greenhouse conditions. Herbivore performance was differentially affected by the plant population on which they were reared. Plant population influenced only development time and pupal mass in P. rapae, whereas plant population also had a dramatic effect on survival of M. brassicae. Prolonged development time in P. rapae corresponded with high levels of the indole GS, neoglucobrassicin, whereas reduced survival in M. brassicae coincided with high levels of the aliphatic GS, gluconapin and sinigrin. The difference between the two species can be explained by the fact that the specialist P. rapae is adapted to feed on plants containing GS and has evolved an effective detoxification system against aliphatic GS. The different B. oleracea populations also affected development of the endoparasitoids. Differences in food-plant quality for the hosts were reflected in adult size in C. rubecula and survival in M. mediator, and further showed that parasitoid performance is also affected by herbivore diet.     

Population feedback after successful invasion leads to ecological suicide in seasonal environments

For most consumer species, winter represents a period of harsh food conditions in addition to the physiological strain that results from the low ambient temperatures. In size-structured populations, larger-bodied individuals do better during winter as they have larger energy reserves to buffer starvation periods. In contrast, smaller-bodied individuals do better under growing conditions, as they have lower maintenance costs. We study how the interplay between size-dependent life-history processes and seasonal changes in temperature and food availability shape the long-term dynamics of a size-structured consumer population and its unstructured resource. We show that the size dependence of maintenance requirements translates into a minimum body size that is needed for surviving starvation when consumers can adapt only to a limited extent to the low food densities in winter. This size threshold can lead to population extinction because adult individuals suffer only a little during winter and hence produce large numbers of offspring. Due to population feedback on the resource and intense intra-cohort competition, newborn consumers then fail to reach the size threshold for survival. Under these conditions, small numbers of individuals can survive, increase in density, and build up a population, which will subsequently go extinct due to its feedback on the resource. High juvenile mortality may prevent this ecological suicide from occurring, as it releases resource competition among newborns and speeds up their growth. In size-structured populations, annual fluctuations in temperature and food availability may thus lead to a conflict between individual fitness and population persistence.     

UVR-induced photoinhibition of summer marine phytoplankton communities from Patagonia

During austral summer 2006, experiments were carried out to evaluate the effects of ultraviolet radiation (UVR, 280–400 nm) on carbon fixation of natural phytoplankton assemblages from Patagonia (Argentina). Surface water samples were collected (ca. 100 m offshore) at mid morning using an acid-cleaned (1 N HCl) dark container. The short-term impact of UVR (measured as radiocarbon incorporation) was immediately assessed by exposing samples to three artificial illumination treatments: PAR (400–700 nm), PAR + UV-A (320–700 nm), and PAR + UV-A + UV-B (280–700 nm). Pico-nanoplankton characterized the assemblages, and taxon-specific pigment fingerprinting combined with CHEMTAX and supplemented with microscopic observations showed varied proportions of diatoms, chlorophytes, and cyanobacteria throughout January–February 2006. Photosynthetic efficiency, as assessed through assimilation numbers, was high [between 4.4 and 10.4 μg C (μg chl-a)⁻¹ h⁻¹], and it was probably favored by the supply of inorganic nutrients from the Chubut River. UVR-induced photoinhibition appeared to be related to the taxonomic composition: in general, higher photoinhibition was observed when diatoms dominated, whereas this was lower when samples were dominated by chlorophytes. Our data suggest that xanthophyll pigments might have provided only limited protection in these already highlighted acclimated assemblages.     

Qualitative time trend analysis of ground water monitoring networks

The detection of significant (short-term) time trends is one of the major goals of ground water monitoring networks. These trends can be used to recognize active geochemical processes and potential environmental threats. This paper presents a case history of time trend analysis on macrochemical parameters of ground water quality. It shows the difficulties and traps that are generally encountered in such studies. The data used originated from the Dutch National Groundwater Quality Monitoring Network. This network is operative since 1979, and keeps track of the ground water composition at 350 locations at two depths (ca. 10 and 25 m below surface; general density, one location per 100 km²). Prior to the trend analysis the data set was divided into geochemically homogeneous groups using fuzzy c-means clustering. Each group represents a specific ground water type, characterized by a distinct source (seawater, surface water or precipitation) and a unique combination of dominant geochemical processes (e.g. mineralization of organic matter, carbonate dissolution and cation exchange).To study trends qualitatively, the concentrations of the various macro-constituents in ground water are correlated with time of sampling. The nonparametric and outlier insensitive Spearman rank correlation coefficient is computed per well screen. A frequency distribution of correlation coefficients is formed by combining the Spearman correlation coefficients of all individual wells within a homogeneous group. This distribution is tested for trends against the appropriate theoretical distribution of zero correlation by use of the Kolmogorov-Smirnov one-sample test. The type of trend is derived from the shape of the distribution.Most ground water types show statistically significant qualitative trends, of which many, however, are caused by changes in the sampling and analytical procedures over the monitoring period. After elimination of differences in limits of detection for NO₃, total-P, and NH₄, most trends in these compounds disappeared. In some water types trends for alkalinity, apparent trends for pH, Ec, and total-P are caused by variations in the laboratory practice, e.g. varying storage procedures, leading to erroneous analyses. Other parameters showed statistically significant trends, related to geochemical processes.The most interesting and most substantial trends are observed in the water type characterized by infiltrating rainwater with agricultural pollutants. In this water type the lowering ground water table induces lower rates of evapotranspiration, giving lower concentrations in time of conservative parameters (Cl, Na, Ca). The aerated zone is enlarged, resulting in increased oxidation of organic material, less efficient nutrient (NO₃, K) uptake by plant roots, leading to increased ground water concentrations of nutrients. In other water types trends are quantitatively small. However, trends are not necessarily linear, and all should be closely monitored in future.     

Defining disaster resilience: comparisons from key stakeholders involved in emergency management in Victoria,Australia

Three years after the introduction of the National Strategy for Disaster Resilience there remains no unanimously adopted definition of disaster resilience within Australia's emergency management sector. The aim of this study is to determine what the concept means to key stakeholders in the emergency management sector in the Australian State of Victoria, and how these conceptualisations overlap and diverge. Via an online survey, 113 people were asked how they define disaster resilience in their work in the emergency management sector. A data mining software tool, Leximancer, was employed to uncover the relationships between the definitions provided. The findings show that stakeholders see resilience as an ‘ability’ that encompasses emergency management activities and personal responsibility. However, the findings also highlight some possible points of conflict between stakeholders. In addition, the paper outlines and discusses a number of potential consequences for the implementation and the success of the resilience‐based approach in Australia.