Patch-leaving rules for parasitoids with imperfect host discrimination

Abstract.

• 1 The solitary parasitoid Anagrus delicatus attacks hosts (plant-hopper eggs) that exist in distinct batches and that are readily detectable to the foraging parasitoid. However, Anagrus delicatus attacks only a small fraction of the available hosts within a batch of eggs and then disperses (Cronin & Strong, 1993a). Here we address the question: Why is A.delicatus abandoning seemingly high quality patches?
• 2 A parasitoid with an imperfect ability to discriminate between unparasitized hosts and hosts that it has attacked earlier within the same patch experiences a risk of self-superparasitism when attacking multiple hosts within a single patch. Self-superparasitism can incur costs in the form of lost time and eggs. Early patch leaving can be favoured as a means of avoiding the costs of self-superparasitism.
• 3 A simple static model demonstrates that patch leaving is favoured by low costs of travelling to a new patch, high error rates in discriminating previously self-parasitized hosts, and high levels of parasitism in the currently occupied patch.
• 4 A more detailed dynamic state variable model, parameterized for A.delicatus, demonstrates that this parasitoid's seemingly enigmatic behaviour can be explained under our hypothesis. In order for this to be the case, we predicted that A.delicatus cannot recognize previously parasitized hosts. Subsequent to our prediction, Cronin & Strong (1993b) demonstrated that experienced A.delicatus do not avoid ovipositing in previously self-parasitized hosts.
• 5 Optimal patch leaving rules can be highly sensitive to even very low host discrimination error rates, which may be widespread among parasitoids.

     

El Niño Southern Oscillation and climate trends impact reservoir water quality

Low dissolved oxygen concentration in bottom layers of lakes and reservoirs usually indicates low water quality. In lakes, empirical models predicting anoxia are almost entirely based on the decay of plankton biomass, while in reservoirs recent findings suggest a prominent role of streamflow and load of organic carbon. This suggests a potential link between water quality in reservoirs and climate processes affecting streamflow. Here we support this hypothesis presenting evidence that both interannual climate variability and recent climate change, mainly consisting in a significant increase in potential evapotranspiration in the upstream basin, affected the oxygen content in a Mediterranean reservoir (Sau Reservoir, Spain). Using a 44‐year monthly record, we found strong and consistent signatures of El Niño Southern Oscillation in the inflow and reservoir oxygen content. Spectral and wavelet techniques showed that the El Niño, streamflow, and reservoir oxygen content series oscillated in common periods, which coincided with the main El Niño variability modes. An empirical model explaining the annual oxygen content in the reservoir suggested that a decreasing streamflow trend reduced the oxygen content of the reservoir by about 20%, counteracting remediation measures implemented at the basin upstream the reservoir. Our results provide the first quantitative evidence of climate change effects on reservoir water quality using long‐term instrumental data, and indicate that streamflow should be considered as a key variable in assessing climate change impact on reservoir water quality. These results are especially relevant in regions of the world where reservoirs are abundant and most climate models predict a decrease in runoff during the next decades. Both the expected trends and the sensitivity of reservoir water quality to global interannual climate variability should be considered for a correct management of water resources in the present and to design adaptation policies in the future.     

The role of light for fish–zooplankton–phytoplankton interactions during winter in shallow lakes – a climate change perspective

1. Variations in the light regime can affect the availability and quality of food for zooplankton grazers as well as their exposure to fish predation. In northern lakes light is particularly low in winter and, with increasing warming, the northern limit of some present-day plankton communities may move further north and the plankton will thus receive less winter light.
2. We followed the changes in the biomass and community structure of zooplankton and phytoplankton in a clear and a turbid shallow lake during winter (November–March) in enclosures both with and without fish and with four different light treatments (100%, 55%, 7% and <1% of incoming light).
3. In both lakes total zooplankton biomass and chlorophyll- a were influenced by light availability and the presence of fish. Presence of fish irrespective of the light level led to low crustacean biomass, high rotifer biomass and changes in the life history of copepods. The strength of the fish effect on zooplankton biomass diminished with declining light and the effect of light was strongest in the presence of fish.
4. When fish were present, reduced light led to a shift from rotifers to calanoid copepods in the clear lake and from rotifers to cyclopoid copepods in the turbid lake. Light affected the phytoplankton biomass and, to a lesser extent, the phytoplankton community composition and size. However, the fish effect on phytoplankton was overall weak.
5. Our results from typical Danish shallow eutrophic lakes suggest that major changes in winter light conditions are needed in order to have a significant effect on the plankton community. The change in light occurring when such plankton communities move northwards in response to global warming will mostly be of modest importance for this lake type, at least for the rest of this century in an IPCC A2 scenario, while stronger effects may be observed in deep lakes.     

Synergistic impacts of sediment contamination and dam presence on river functioning

1. Dam presence is commonly associated with strong accumulation of polluted sediments. In spite of this context of multiple stressors, physical effects are often solely considered in the ecological assessment of the dam impacts. 2. We studied four ‘reservoir/downstream reach’ systems differing in levels of sediment contamination in reservoirs. Using assemblages and biotrait (i.e. ecological or biological attribute) responses of macroinvertebrate communities and leaf litter breakdown, we examined the individual effects and potential interactions between sediment contamination and dam presence along the gradient of ecotoxic pressure. 3. Leaf breakdown rates ranged from 0.0044° per day in the most contaminated reservoir to 0.0120° per day in the reference reservoir. Comparisons of community trait profiles among reservoirs highlighted a gradient of trait responses to sediment contamination. 4. In the absence of toxic contamination, the dam‐induced modifications in biotraits of invertebrate assemblages were not related to a reduction of leaf litter breakdown. Conversely, contaminated sediment in reservoir induced strong functional disturbances (i.e. bioecological shifts and reduction of leaf litter breakdown) downstream of dams. 5. Key biotrait categories positively related to leaf litter breakdown rate have been identified. They corresponded mainly to shredders and/or small‐sized (<0.5 cm) insects, using aquatic (e.g. crawlers) or aerial (e.g. fliers) active dispersal strategies. In addition, trait categories positively correlated to contamination level have been considered as ‘response’ traits. They corresponded to large‐sized (>4 cm) species, having several generations per year (polyvoltin), using asexual reproduction and/or disseminating by drift (aquatic, passive). 6. In the current context of ecological continuity restoration, this study has identified the risks associated with the presence of historical contamination in the run‐of‐river reservoirs for downstream ecosystem health.     

Alternative tactics in spawning site selection by brook trout (Salvelinus fontinalis) related to incubation microhabitats in a harsh winter environment

1. Species with distributions that span a broad range of latitudes may have populations that exhibit distinct life history traits associated with environmental gradients. The majority of previous studies have indicated a strong association between spawning site selection by brook trout (Salvelinus fontinalis) and the presence of upwelling groundwater, but does this generalisation extend to the thermal regimes experienced at northern sites? 2. We investigated the role of hyporheic flow in redd site selection by brook trout in a relatively high‐latitude boreal system. Hyporheic flows through streambed substratums can be dominated by groundwater or surface water and may be influenced by the presence of morphological features. For autumn spawners such as brook trout, embryos situated in microhabitats where hyporheic flow in the shallow substratum is groundwater dominant (i.e. warmer in winter) are likely to experience accelerated development rates relative to embryos in redds where there is downwelling surface water (i.e. colder in winter). 3. We measured vertical hydraulic gradients (VHG) at the microhabitat scale and spatial and temporal variation in upwelling/downwelling flow and thermal regimes in brook trout spawning/incubation habitats. Additionally, we noted the proximity of redd sites to stream morphological features (e.g. riffle crests). 4. Our results indicate that upwelling flow was not a decisive cue in redd site selection at the microhabitat scale (10⁰ m) as an approximately equal number of redds were situated in microhabitats with upward flow as compared to microhabitats with downward flow through the redd. Redds situated on bedforms with convex longitudinal profiles (e.g. riffle crests, log steps) were associated with downward flow, whereas redds not immediately adjacent to these bedform features were associated with upward flow. Winter streambed temperatures confirmed that both sites with steady upwelling (i.e. warm incubation regime) and downwelling (i.e. cold incubation regime) were indeed selected by spawners. 5. Our observations that spawners utilised both cold‐regime and warm‐regime sites suggests the existence of distinct reproductive tactics related to hyporheic flow patterns in this boreal system. As temperature is the dominant factor controlling rates of embryonic development, the use of spawning microhabitats with distinct thermal regimes implies substantial differences in the timing of hatching and the phenology of emergence.     

Soil carbon storage potential of direct seeding mulch-based cropping systems in the Cerrados of Brazil

No‐tillage cropping systems with direct seeding into a mulch of plant residues from cover crops – the so‐called direct seeding mulch‐based cropping (DMC) systems – have been adopted widely over the last 10–15 years in the Cerrado region of Brazil. They are replacing the traditional soybean monoculture with bare fallow using conventional tillage (CT) practices. The objective of this study was to examine how DMC practices affect soil organic carbon (SOC) dynamics and to assess their potential for enhanced soil carbon (C) storage. The approach was to determine soil C stocks along a chronosequence of fields under DMC, and then to apply the generic decomposition and yield (G'DAY) plant–soil model to analyse the soil C storage potential for a number of cropping systems. Forty‐five fields were selected on a plateau of Ferralsols in the central Cerrado region to represent a chronosequence of 0–12 years under continuous DMC. Before DMC the fields had been under CT soybean monoculture following the clearing of the native savannah. An average increase in SOC stocks of 0.83 Mg C ha^?1 yr^?1 in the 0–20 cm topsoil was measured. The corresponding increase in total soil nitrogen was 79 kg N ha^?1 yr^?1. The G'DAY model predicted a net accumulation of 0.70–1.15 Mg C ha^?1 yr^?1 in the 0–40 cm topsoil for the first 12 years, depending on the type of soil and DMC system. Model predictions showed that less soil C was accumulated under DMC systems that commenced immediately after clearing the native savannah. Gains in soil C under DMC were primarily due to the introduction of a second crop that caused higher net primary productivity, leading to higher plant C inputs to soil. A rough estimation shows that the conversion of 6 million ha of CT soybean monoculture to DMC in the Cerrados would enhance soil C storage by 4.9 Tg C yr^?1 during at least the first 12 years following the conversion to DMC.     

Primary motor cortex reorganization in a long-term monkey amputee

The primary motor cortex (M1) was mapped with intracortical microstimulation (ICMS) in a 15 year-old macaque whose right upper extremity was amputated at the shoulder joint prior to 2 years of age. Movements of the right shoulder girdle and stump were evoked by ICMS throughout the left M1 upper extremity region. The size of the left M1 upper extremity region contralateral to the amputated arm was not appreciably different from the size of the right upper extremity region contralateral to the intact arm. Long stimulus trains and/or higher stimulus currents were needed to evoke detectable movements at significantly more loci in the left than in the right M1 upper extremity region. These observations would be consistent with unmasking of a high threshold representation of shoulder musculature that normally exists throughout the central core of the upper extremity region, where it underlies a lower threshold representation of the distal forelimb. Alternatively, invasion of the de-efferented distal forelimb core by surrounding shoulder representation may have occurred. Differences between the limited M1 reorganization observed in the present study and the more extensive reorganization of S1 observed in other studies may reflect fundamental differences between M1 and S1, and/or differences in the extent of de-efferentation versus deafferentation.     

Elevated methylmercury in High Arctic Daphnia and the role of productivity in controlling their distribution

Mercury is a contaminant of concern in polar regions due to long‐range atmospheric transport of this metal from southern latitudes followed by intense deposition on snow. We surveyed zooplankton in 16 lakes and ponds in the Canadian Arctic Archipelago (74–76°N) to determine methylmercury (MeHg) content and the role of environmental characteristics and taxonomic composition on accumulation processes. Zooplankton communities containing Daphnia (mainly D. middendorffiana) had on average five times the MeHg content of copepod‐dominated communities. The percent biomass of Daphnia best explained MeHg variation in bulk zooplankton compared with water chemistry and morphometric variables. Water‐column concentrations of MeHg were low at most study sites (mainly ≤0.07 ng L⁻¹), and Daphnia strongly bioaccumulated mercury through species‐specific processes. As we observed Daphnia in more productive water bodies (i.e., ponds, a eutrophied lake), we then tested the role of productivity in determining the distribution of this keystone herbivore using a broad‐scale literature dataset of 47 High Arctic lakes (65–77°N). Daphnia density was positively related to the amount of organic carbon in the water column in both dissolved and particulate fractions [dissolved organic carbon (DOC) partial , P < 0.001; particulate organic carbon (POC) partial , P=0.032]. The strong influence of DOC suggests that bacterial production is an important energy source for Arctic Daphnia. Our findings indicate that productivity influences the MeHg content of zooplankton communities through its control of species composition; specifically, low productivity limits the presence of mercury‐rich Daphnia in many copepod‐dominated lakes of the High Arctic. Aquatic productivity is expected to increase with climate warming, and we present a conceptual model that predicts how environmental drivers could extend the distribution of Daphnia in lakes and alter the movement of mercury in food webs of the Canadian High Arctic.