Invasive knotweed has greater nitrogen-use efficiency than native plants: evidence from a 15 N pulse-chasing experiment Abstract Habitats with fluctuating resource conditions pose specific challenges to plants, and they often favor a small subset of species that includes exotic invaders. These species must possess a superior ability to capitalize on resource pulses through faster resource uptake or greater resource-use efficiency. We addressed this question in an experiment with invasive knotweed, a noxious invader of temperate ecosystems that is known to benefit from nutrient fluctuations. We used stable isotopes to track the uptake and use efficiency of a nitrogen pulse in competition pairs between knotweed and five native competitors. We found that nitrogen pulses indeed promoted knotweed invasion and that this is explained by a superior efficiency in turning the taken-up extra nitrogen into biomass, rather than capturing an overproportional share of the nitrogen. Thus, temporary increases in nutrient availability might help knotweed to invade natural environments, such as river banks or nitrogen-polluted margins and wastelands, where nutrient fluctuations occur. Our experiment shows that resource-use efficiency can drive invasion under fluctuating resource conditions, and that stable isotopes help to understand these processes.

Effects of flight and food stress on energetics, reproduction, and lifespan in the butterfly Melitaea cinxia Abstract Environmental change can have drastic effects on natural populations. To successfully predict such effects, we need to understand how species that follow different life-history strategies respond to stressful conditions. Here I focus on two stressors, increased flight and dietary restriction, and their effects on bioenergetics and life-history. Using the Glanville fritillary butterfly (Melitaea cinxia), I subjected mated females to three treatments: (1) control conditions, (2) repeated forced flight with unlimited food, and (3) repeated forced flight coupled with food restriction. Interestingly, flight increased fecundity: females in both flight treatments initiated oviposition earlier, laid more egg clutches, and had higher total fecundity than control females. However, food-restriction by 50% reduced clutch size and resulted in an approximately 25% decrease in total fecundity compared to flown females with unlimited food. There were no differences in egg wet mass, water content or hatching success. Flown females with unlimited food appeared to exhibit a trade-off between reproduction and lifespan: they had higher mass-independent resting metabolic rate and shorter lifespan than females in the other treatments. Mass-independent flight metabolic rate, reflecting flight capacity, did not differ among the treatments. There were no differences in the rate of metabolic senescence across the treatments. The current findings suggest a mechanistic link between flight and reproduction, potentially mediated by juvenile hormone signalling. It appears that this wing-monomorphic butterfly does not show an oogenesis-flight trade-off often found in wing-dimorphic insects. Nevertheless, nectar-feeding is needed for achieving maximum reproductive output, suggesting that diminishing nectar resources may negatively impact natural populations.

Reproductive consequences of climate variability in migratory birds: evidence for species-specific responses to spring phenology and cross-seasonal effects Abstract Climate change is altering global temperature and precipitation regimes, and the ability of species to respond to these changes could have serious implications for population dynamics. Flexible species may adjust breeding dates in response to advances in spring phenology. Furthermore, in migratory bird species, conditions experienced during the non-breeding season may have cross-seasonal effects during the subsequent breeding season. We evaluated species-specific responses to antecedent non-breeding (winter) and current breeding (spring) conditions. We used a data set composed of 21,230 duck nests from 164 sites in the Canadian Prairie Pothole Region, 1993–2011, to determine how environmental conditions influenced timing of nesting and subsequent nest survival in eight duck species representing varying life-histories. We tested how species responded in timing of nesting and nest survival, respectively, to El Niño Southern Oscillation (ENSO) conditions experienced during the preceding non-breeding season (winter; Dec–Feb), and spring (Mar–Jun) temperature and moisture conditions on the breeding grounds. Ducks tended to nest earlier in warmer springs; however, in El Niño winters, with warmer spring temperatures, nesting tended to be later. We did not find evidence for direct effects of environmental variables on nest survival; however, evidence of indirect effects of winter conditions on nest survival for some species via strong direct effects on timing of nesting provides new insights into mechanisms for cross-seasonal effects on reproductive success.

Ecophysiological determinants of sexual size dimorphism: integrating growth trajectories, environmental conditions, and metabolic rates Abstract Sexual size dimorphism (SSD) often results in dramatic differences in body size between females and males. Despite its ecological importance, little is known about the relationship between developmental, physiological, and energetic mechanisms underlying SSD. We take an integrative approach to understand the relationship between developmental trajectories, metabolism, and environmental conditions resulting in extreme female-biased SSD in the crab spider Mecaphesa celer (Thomisidae). We tested for sexual differences in growth trajectories, as well as in the energetics of growth, hypothesizing that female M. celer have lower metabolic rates than males or higher energy assimilation. We also hypothesized that the environment in which spiderlings develop influences the degree of SSD of a population. We tracked growth and resting metabolic rates of female and male spiderlings throughout their ontogeny and quantified the adult size of individuals raised in a combination of two diet and two temperature treatments. We show that M. celer’s SSD results from differences in the shape of female and male growth trajectories. While female and male resting metabolic rates did not differ, diet, temperature, and their interaction influenced body size through an interactive effect with sex, with females being more sensitive to the environment than males. We demonstrate that the shape of the growth curve is an important but often overlooked determinant of SSD and that females may achieve larger sizes through a combination of high food ingestion and low activity levels. Our results highlight the need for new models of SSD based on ontogeny, ecology, and behavior.

Does differential iron supply to algae affect Daphnia life history? An ionome-wide study Abstract The availability of iron (Fe) varies considerably among diet items, as well as ecosystems. Availability of Fe has also changed due to anthropogenic environmental changes in oceanic as well as inland ecosystems. We know little about its role in the nutrition of ecologically important consumers, particularly in inland ecosystems. Physiological studies in several taxa indicate marked effects of dietary Fe on oogenesis. We predicted that differential Fe supply to algae will impact algal Fe concentration with consequences on the life history of the freshwater grazer, Daphnia magna. We found that algal Fe concentration increased with Fe supply, but did not affect algal growth, indicating that the majority of experimental Fe additions were likely adsorbed to, or stored in algal cells. Regardless, data indicate that algal Fe impacted the reproductive traits (age and size at maturity) but not juvenile growth rate of Daphnia. A subsequent experiment revealed that Fe concentration in eggs was significantly higher than the rest of Daphnia. These results indicate that the concentration of Fe in or on algal cells may vary considerably among ecosystems overlying distinct geological formations differing in Fe, possibly with important implications for zooplankton life histories. Understanding the mechanisms underlying this response is unlikely to be accomplished by a strict focus on Fe because we found correlated shifts in the algal ionome, with concomitant ionome-wide adjustments in Daphnia. Information on ionome-wide responses may be useful in better understanding the responses of biota to changes in the supply of any one element.

Sex and the cost of reproduction through the life course of an extremely long-lived herb Abstract Despite being central concepts for life history theory, little is known about how reproductive effort and costs vary with individual age once plants have started to reproduce. We conducted a 5-year field study and estimated age-dependent reproductive effort for both sexes in the extraordinarily long-lived dioecious plant Borderea pyrenaica. We also evaluated costs of reproduction on vital rates for male and female plants, both by examining effects of differences in individual reproductive effort under natural conditions, and by conducting a flower removal experiment, aimed at decreasing reproductive effort. Reproductive effort was fairly constant and independent of age for males, which may reflect a strategy of adjusting overall reproductive output by spreading reproduction over the life course. Females had a higher total effort, which first increased and then decreased with age. The latter may be a response to an increasing reproductive value—an inverse of a terminal investment—or a sign of reproductive senescence due to an age-related physiological decline. Seed production was lower in plants with higher previous reproductive effort and this effect increased with age. We found no evidence for costs of reproduction on other vital rates for either sex. Experimental flower removal only resulted in progressively more negative effects on flower production in older male plants, whereas female vital rates were unaffected. Overall, this study demonstrates that not only sex, but also age influences resource allocation trade-offs and, thus, plant life history evolution.

Avian and rodent responses to the olfactory landscape in a Mediterranean cavity community Abstract Animals rely on cues informing about future predation risk when selecting habitats to breed in. Olfactory information may play a fundamental role in the assessment of predation threats, because predators produce characteristic body odours, but the role of odours in habitat selection has seldom been considered. Here, we test whether fear of predation induced by odour cues may affect the settlement pattern of a Mediterranean cavity-dependent community of rodents and non-excavator hole-nesting birds. To test this hypothesis, we experimentally manipulated the perception of predation risk on a scale of patch by applying either odours of a carnivore predator (risky odour treatment), lemon essence (non-risky odour treatment) and a control non-odorous treatment and studied bird and rodent settlement patterns. Nest-box occupation probability differed across treatments so that species in the community settled in more numbers in control than in non-risky and than in risky odour-treated nest boxes. Concerning settlement patterns, control nest boxes were occupied more rapidly than nest boxes with odour information. Birds and rodents settled earlier in control than in risky odour-treated nest boxes, but their settlement pattern did not significantly vary between risky odour and non-risky odour-treated nest boxes. Our findings demonstrate that olfactory cues may be used to assess habitat quality by settling species in this community, but we cannot pinpoint the exact mechanism that has given rise to the pattern of preference by nest boxes.

Kin-dependent dispersal influences relatedness and genetic structuring in a lek system Abstract Kin selection and dispersal play a critical role in the evolution of cooperative breeding systems. Limited dispersal increases relatedness in spatially structured populations (population viscosity), with the result that neighbours tend to be genealogical relatives. Yet the increase in neighbours’ fitness-related performance through altruistic interaction may also result in habitat saturation and thus exacerbate local competition between kin. Our goal was to detect the footprint of kin selection and competition by examining the spatial structure of relatedness and by comparing non-effective and effective dispersal in a population of a lekking bird, Tetrao urogallus. For this purpose, we analysed capture–recapture and genetic data collected over a 6-year period on a spatially structured population of T. urogallus in France. Our findings revealed a strong spatial structure of relatedness in males. They also indicated that the population viscosity could allow male cooperation through two non-exclusive mechanisms. First, at their first lek attendance, males aggregate in a lek composed of relatives. Second, the distance corresponding to non-effective dispersal dramatically outweighed effective dispersal distance, which suggests that dispersers incur high post-settlement costs. These two mechanisms result in strong population genetic structuring in males. In females, our findings revealed a lower level of spatial structure of relatedness and genetic structure in respect to males. Additionally, non-effective dispersal and effective dispersal distances in females were highly similar, which suggests limited post-settlement costs. These results indicate that kin-dependent dispersal decisions and costs have a genetic footprint in wild populations and are factors that may be involved in the evolution of cooperative courtship.

The costs of keeping cool: behavioural trade-offs between foraging and thermoregulation are associated with significant mass losses in an arid-zone bird Abstract Avian responses to high environmental temperatures include retreating to cooler microsites and/or increasing rates of evaporative heat dissipation via panting, both of which may affect foraging success. We hypothesized that behavioural trade-offs constrain the maintenance of avian body condition in hot environments, and tested predictions arising from this hypothesis for male Southern Yellow-billed Hornbills (Tockus leucomelas) breeding in the Kalahari Desert. Operative temperatures experienced by the hornbills varied by up to 13 °C among four microsite categories used by foraging males. Lower prey capture rates while panting and reductions associated with the occupancy of off-ground microsites, resulted in sharp declines in foraging efficiency during hot weather. Consequently, male body mass (Mb) gain between sunrise and sunset decreased with increasing daily maximum air temperature (Tmax), from ~ 5% when Tmax < 25 °C to zero when Tmax = 38.4 °C. Overnight Mb loss averaged ~ 4.5% irrespective of Tmax, creating a situation where nett 24-h Mb loss approached 5% on extremely hot days. These findings support the notion that temperature is a major determinant of body condition for arid-zone birds. Moreover, the strong temperature dependence of foraging success and body condition among male hornbills provisioning nests raises the possibility that male behavioural trade-offs translate into equally strong effects of hot weather on female condition and nest success. Our results also reveal how rapid anthropogenic climate change is likely to substantially decrease the probability of arid-zone birds like hornbills being able to successfully provision nests while maintaining their own condition.

Effects of extreme rainfall events are independent of plant species richness in an experimental grassland community Abstract Global climate models predict more frequent periods of drought stress alternated by heavier, but fewer rainfall events in the future. Biodiversity studies have shown that such changed drought stress may be mitigated by plant species richness. Here, we investigate if grassland communities, differing in species richness, respond differently to climatic extremes within the growing season. In a 3-year outdoor mesocosm experiment, four grassland species in both monoculture and mixture were subjected to a rainfall distribution regime with two levels: periods of severe drought in the summer intermitted by extreme rainfall events versus regular rainfall over time. Both treatments received the same amount of water over the season. Extreme rainfall combined with drought periods resulted in a 15% decrease in aboveground biomass in the second and third year, compared to the regular rainfall regime. Root biomass was also reduced in the extreme rainfall treatment, particularly in the top soil layer (− 40%). All species developed higher water use efficiencies (less negative leaf δ13C) in extreme rainfall than in regular rainfall. These responses to the rainfall/drought treatment were independent of species richness, although the mixtures were on an average more productive in terms of biomass than the monocultures. Our experimental results suggest that mixtures are similarly able to buffer these within-season rainfall extremes than monocultures, which contrasts with findings in the studies on natural droughts. Our work demonstrates the importance of investigating the interactions between rainfall distribution and drought periods for understanding effects of climate change on plant community performance.