Oecologia

The aim of this study was to compare plant–soil interactions in the native range of two congeneric European species differing in their invasive success in the world: a globally invasive Cirsium vulgare and non-invasive C. oleraceum. We assessed changes in soil nutrients and soil biota following soil conditioning by each species and compared performance of plants grown in self-conditioned and unconditioned soil, from which all, some or no biota was excluded. The invasive species depleted more nutrients than the non-invasive species and coped better with altered nutrient levels. The invasive species had higher seedling establishment which benefited from the presence of unconditioned biota transferred by soil filtrate. Biomass of both species increased in soil with self-conditioned soil filtrate and decreased in soil with self-conditioned whole-soil inoculum compared to unconditioned filtrate and inoculum. However, the increase was smaller and the decrease greater for the invasive species. The invasive species allocated less biomass to roots when associated with harmful biota, reducing negative effects of the biota on its performance. The results show that in the native range the invasive species is more limited by self-conditioned pathogens and benefits more from unconditioned mutualists and thus may benefit more from loss of effectively specialized soil biota in a secondary range. Our study highlights the utility of detailed plant-soil feedback research in species native range for understanding factors regulating species performance in their native range and pinpointing the types of biota involved in their regulation.

Understanding the relative impacts of predators on prey may improve the ability to predict the effects of predator composition changes on prey assemblages. We experimentally examined the relative impact of native and introduced predatory fish on a temporary wetland amphibian assemblage to determine whether these predators exert distinct (unique or non-substitutable) or equivalent (similar) impacts on prey. Predatory fish included the eastern mosquitofish (Gambusia holbrooki), golden topminnow (Fundulus chrysotus), flagfish (Jordanella floridae), and the introduced walking catfish (Clarias batrachus). The tadpole assemblage included four common species known to co-occur in temporary wetlands in south-central Florida, USA: the oak toad (Bufo quercicus), pinewoods treefrog (Hyla femoralis), squirrel treefrog (Hyla squirella), and eastern narrowmouth toad (Gastrophryne carolinensis). Tadpoles were exposed to different predators in wading pools under conditions similar to those found in surrounding temporary wetlands (particularly in terms of substrate type, the degree of habitat complexity, and temperature). Native predators were similar with respect to predation rate and prey selectivity, suggesting similar energy requirements and foraging behavior. Conversely, native fish predators, especially G. holbrooki, were distinct from the introduced C. batrachus. In contrast to expectations, C. batrachus were less voracious predators than native fish, particularly G. holbrooki. Moreover, survival of G. carolinensis and H. femoralis were higher in the presence of C. batrachus than G. holbrooki. We suggest that C. batrachus was a less efficient predator than native fish because the foraging behavior of this species resulted in low predator-prey encounter rates, and thus predation rate. In combination with a related field study, our results suggest that native predatory fish play a stronger role than C. batrachus in influencing the spatial distribution and abundance of temporary wetland amphibians in the landscape.

We investigated genetic and environmental components of variance in avian T-cell-mediated immune response (CMI) through a cross-fostering experiment conducted on wild American kestrels (Falco sparverius). CMI was evaluated in vivo by an experimental challenge with phytohaemagglutinin, a T-cell mitogen, injected intradermally in fledglings. Additionally, we assessed two measures of nutritional condition (body mass and circulating plasma proteins) which could influence the variance components of CMI. A two-way nested ANOVA indicated that CMI of fledgling kestrels was explained more by the nest where the bird was reared (33% of the explained variance) than by the nest of origin (12%). Body mass was explained equally by familial and environmental components, while plasma proteins were only related to the rearing environment. CMI of fledglings was not related to their circulating plasma proteins, but was positively correlated with their body mass. Fledgling body mass seemed to be influenced by pre-hatching or post-hatching maternal effects prior to manipulation since resemblance in body mass of sibships at the age of manipulation was high (h 2≤0.58), and body mass at this age predicted body mass at fledging. Therefore, pre-manipulation parental effects on body mass, such as investment in egg size, could have inflated the familial effects on body mass of fledglings and then on its correlated CMI. When controlling for body mass, most of the variation in CMI of fledglings was explained by the nest where the bird was reared (36.6%), while the variance explained by the nest of origin (4%) was not significant. This means that environmental influences are major determinants of offspring CMI. The low proportion of variance explained by the familial component may have been due to the high correlation of CMI to fitness.

The haplochromine cichlid fish communities of the rocky habitats of Lake Malawi are highly diverse; however, many species live side by side with apparently very similar resource requirements. There is a long-standing debate concerning whether these species partition their resources on a finer scale than has been previously reported or if species that are truly ecologically indistinguishable can coexist. A field study of food resource use was conducted to determine whether coexisting species segregate their diet and foraging sites. Significant differences between species were found, yet considerable inter-specific resource use overlap was commonplace. The data indicate that these cichlid species coexist both with and without niche differentiation. We propose that alternatives to niche differentiation should be considered to explain how many species coexist in Lake Malawi cichlid communities.

Cimicifuga simplex, a Japanese herb, has three different pollinator guilds. At high altitudes it is pollinated by bumblebees. In lowlands, Impatiens textori, a much superior nectar-producer, outcompetes it for bumblebees. In sunny lowland habitats, flowers of a second morph produce a two-component fragrance which facilitates pollination by fritillaries. In experiments, the two components separately had limited effect on alighting rate, while application of both together elevated the attractiveness of non-fragrant flowers to the same as for naturally fragrant flowers. In heavily shaded lowland habitats, a third morph with nonfragrant flowers does not blom until after I. textori, and is pollinated by occasional bumblebees. The data strongly suggest that inferiority in competition has induced incipient speciation in this plant species.

For Bromus tectorum, an alien annual grass now widespread in western North America, we assessed the sensitivity of the phenotypic expression of populational differences in phenology and demography to variation in plant density. Plants were grown in an unheated glasshouse from seeds collected from six habitat types located along a moisture-temperature gradient. Survival to flowering was high with a mean overall survival of 91%. Survival was highest among plants from the coolest, most mesic site. Time to first flowering decreased at the higher sowing density and was dependent on seed source. Plant dry weight and average seed production was lowest within high density plots; the magnitude of this reduction was significantly dependent on seed source. Individual seed weight was also dependent on the interaction of seed source and sowing density. Seed weight did not vary as much as seed number or plant weight in response to density. Differences among sources in hierarchies of plant size and seed production were detected at low sowing densities. The degree of inequality in the distributions of plant size and seed number increased at high density.

To evaluate the adaptive value of daphnian aggregation associated with attack abatement, a combined laboratory and field experiment was performed. As the lake investigated does not contain planktivorous fish, only invertebrate predation is important. In the laboratory, newly collected Chaoborus flavicans, among the most important predators to exploit the Daphnia population in the lake, were individually placed in a spherical flask containing different densities of juvenile Daphnia prey. The number of successful attacks of Chaoborus was independent of prey abundance. Thus, in a given density regime, each individual prey gained protection from a dilution effect. In the field, dense aggregates of Daphnia were created by continuously adding food at a localized point in the littoral. Even though the food input produced aggregates of Daphnia which were maintained for 3 h, the number of invertebrate predators did not increase in the input area. It is concluded that each Daphnia in a group reduces the probability of predation through an effect of attack abatement which is approximately proportional to the number of prey in the group. However, when considering the high median density of Daphnia, together with their strong response to food input, it seems more plausible that natural aggregates are driven by a patchy food distribution, whereas attack abatement is only a secondary effect of aggregation.

The differential ability of forest herbs to colonize secondary forests on former agricultural land is generally attributed to different rates of dispersal. After propagule arrival, however, establishing individuals still have to cope with abiotic soil legacies from former agricultural land use. We focused on the plastic responses of forest herbs to increased phosphorus availability, as phosphorus is commonly found to be persistently bioavailable in post-agricultural forest soils. In a pot experiment performed under field conditions, we applied three P levels to four forest herbs with contrasting colonization capacities: Anemone nemorosa, Primula elatior, Circaea lutetiana and Geum urbanum. To test interactions with light availability, half of the replicas were covered with shade cloths. After two growing seasons, we measured aboveground P uptake as well as vegetative and regenerative performance. We hypothesized that fast-colonizing species respond the most opportunistically to increased P availability, and that a low light availability can mask the effects of P on performance. All species showed a significant increase in P uptake in the aboveground biomass. The addition of P had a positive effect on the vegetative performances of two of the species, although this was unrelated to their colonization capacities. The regenerative performance was affected by light availability (not by P addition) and was related to the species’ phenology. Forest herbs can obviously benefit from the increased availability of P in post-agricultural forests, but not all species respond in the same way. Such differential patterns of plasticity may be important in community dynamics, as they affect the interactions among species.

The response of net photosynthesis and dark respiration in eight species of Umbilicariaceae (lichenes) to temperature (-5, 0, 5, 10, 15, 20, 25, 30°C) and irradiance (55, 110, 220, 400, 620 μmol photons m-2 s-1 PAR) was studied. The samples were collected in montane and alpine localities of the Spanish Sistema Central. The species differed widely in their net photosynthetic rates. The optimal temperature for net photosynthesis in alpine species was significantly lower than in montane species. Montane species were more photophytic than alpine ones. Water saturation and water loss rate were dependent on morphology and particularly anatomy of the thallus. The physiological and structural data are useful in the interpretation of the ecology and altitudinal distribution of the Umbilicariaceae. No adaptation could be linked to particularities of the mediterranean climate.

The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m−2 year−1) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.