Plant‐mediated species networks: the modulating role of herbivore density

1. When herbivores of distinct feeding guilds, such as phloem feeders and leaf chewers, interact, the outcome of these interactions often shows facilitation. However, whether this facilitation turns into competition at stronger herbivory pressure remains unknown. 2. Using an integrative approach that links ecological processes (behavioural choices of insects) with physiological plant mechanisms (nutrient and phytohormone levels) for the wild crucifer Brassica nigra (L.) Koch., this study evaluates preferences of leaf chewers for plants previously infested with several densities of the specialist aphid Brevicoryne brassicae L. (Hemiptera, Aphididae). As leaf chewers, four species of caterpillars (Lepidoptera) were selected that differ in their degree of specialisation in crucifers. 3. These results show that, whereas at low and medium aphid densities caterpillars displayed a preference for aphid‐infested plants or no preference, at high aphid infestation density, all four species of caterpillar preferred uninfested plants, with a significant difference for Pieris rapae and Mamestra brassicae. 4. In contrast to our expectation, the consistent preference for uninfested plants at a high aphid density could not be associated with a decrease in plant nutrition. However, while jasmonate concentrations [i.e. 12‐oxo‐phytodienoic acid and jasmonic acid (JA)] at medium aphid‐density infestation decreased compared with low levels of infestation, at high infestation level, the jasmonates JA as well as JA conjugated with the amino acid isoleucine were present at higher levels compared with low‐infestation treatments. 5. This work provides evidence that positive interactions observed in herbivore communities can be transient, leading to negative interactions mediated by changes in plant defences rather than in plant nutrition.     

Growth, fledging success and post-fledging survival of juvenile Oystercatchers Haematopus ostralegus

We studied the consequences of differences in growth rate on the subsequent survival of Oystercatcher Haematopus ostralegus chicks. Fledging success increased sharply with growth rate, from zero in chicks growing at less than 6 g per day to about 85% in chicks growing at more than 10 g per day. The age at which chicks fledged varied from 27 to 52 days. Chicks which fledged at an early age displayed a much faster growth rate than later fledging chicks. Although slow growth resulted in a considerable prolongation of the period before fledging, slow-growing chicks fledged at a smaller size and with a lower body-weight than fast-growing chicks. After fledging, all chicks remained almost completely dependent on their parents up to an age of 3 months and often longer.
Almost 40% of the fledglings eventually returned to the breeding area. This figure probably reflects post-fledging survival. Age and size at fledging had no effect on a chick's probability of return. Body-weight at fledging had a small positive correlation with the return probability, but this was not statistically significant. We conclude that although slow growth severely reduces a chick's chance of fledging, it probably does not result in irreversible damage causing an increased risk of mortality during the first years after fledging. Apparently, any possible disadvantage associated with small size or low body-weight could be compensated for after fledging.     

Plant‐mediated effects of butterfly egg deposition on subsequent caterpillar and pupal development,across different species of wild Brassicaceae

1. Herbivory can change plant quality, which may have consequences for interactions between the inducing herbivore and other insect community members. 2. Studies investigating the effects of plant quality on herbivore performance often have neglected the egg stage, and instead introduced larvae onto the plant. Recently, we reported that herbivore oviposition by Pieris brassicae (Linnaeus) (Large Cabbage White Butterfly) reduced the plant quality of Brassica nigra L. (black mustard) for subsequent herbivores. 3. It remains unclear how persistent and common these plant‐mediated effects of oviposition are. Here, five species of wild Brassicaceae were used (B. nigra L., Brassica oleracea L., Sinapis arvensis L., Moricandia arvensis L., and Moricandia moricandioides Boiss). The response to oviposition by the specialist P. brassicae was determined by following the natural sequence of events: oviposition, egg, larval, and pupal development. All tested plant species are known to interact with P. brassicae in nature. Caterpillar, pupal mass, and development time on plants exposed to butterfly eggs were assessed compared with egg‐free plants. 4. It was shown that the plant‐mediated effects of oviposition are not specific for B. nigra but occur in most of the tested plant species except for M. arvensis. However, the strength of the plant‐mediated effect on caterpillar growth depended on plant species. Thus, across different members of the Brassicaceae family, oviposition can influence plant quality and has negative consequences on P. brassicae growth. Further studies are needed to assess to what extent this trait might be phylogenetically conserved.     

Évolution horaire au cours d'une journée normale de la photosynthèse, de la transpiration, de la respiration foliaire et racinaire et de la nutrition N.P.K. chez Zea mays

Hourly development during a normal day of photosynthesis, transpiration, leaf and root respiration, and of N.P.K nutrition in Zea mays. Metabolism of Zea mays L. cv. INRA F₇×F₂ can be measured hourly with the “C₂3A system”, under favourable and constant growth conditions. The photosynthesis is especially stable and is submitted only to a development linked with the leaf surface growth. During the vegetative stage the leaf surface increases regularly both in the day and in the night. The water loss does not change during the diurnal period and remains important during the night if humidity is less than 100%. The leaf respiration is nearly stable. The root respiration, measured with O₂ and CO₂, fluctuates according to a typical rhythm with two maxima. Day and night mean rates were about the same. The respiratory quotient is about one during the vegetative stage. There is no decrease in the rate of phosphate absorption during the night, and a very small decrease in the rate of nitrate absorption. Ammonium is totally consumed in the first hours after renewal of the nutrient solution. At the same time the potassium consumption is decreased, and then presents a maximum followed by a night reduction. Under our conditions, all of the observations allowed us to assume the presence of an adequate reserve of assimilates and suggest the existence of a precise regulation process, which can ensure an even day night functioning of the plant metabolism, but which does not preclude the presence of internal rhythms, as indicated by the oscillation of the root respiration.     

Testing the regeneration niche hypothesis with Gesneriaceae (tribe Sinningiae) in Brazil: Implications for the conservation of rare species

The germination requirements of sexually reproducing plants are regulated by environmental factors such as temperature. Those factors acting at the germination phase are part of the regeneration niche, which is fundamental in the processes that contribute to habitat suitability and geographic distribution. We tested the hypothesis that rarity is associated with regeneration niche in three species of plants in the family Gesneriaceae (tribe Sinningieae), Sinningia rupicola (Mart.) Wiehler, Paliavana sericiflora Benth and Sinningia allagophylla (Mart.) Wiehler, which vary in their distribution and habitat specificity but share a small zone of sympatry in rocky fields south of Belo Horizonte in Minas Gerais, Brazil. The regeneration niche was tested using a seed germination experiment under controlled light conditions at seven fixed temperatures (10–40°C at 5°C intervals). Each of the three species germinated differently at the various temperatures. The species with the smallest geographic range, S. rupicola, also had the most restricted germination: germination peaked at 15°C when relatively few seeds germinated (45%), and even fewer germinated at other temperatures. The regeneration niche was wider in P. sericiflora and wider still in S. allagophylla, with germination greater than 90% between 15–25°C and greater than 80% between 15–30°C, respectively. Our germination results provide qualified support for the hypothesis of correlation of the regeneration niche with geographic distribution of related plant taxa, with important conservation implications for rare and endangered species.     

Establishment and expansion of Lake Malawi rock fish populations after a dramatic Late Pleistocene lake level rise

Major environmental events that fragment populations among multiple island habitats have potential to drive large-scale episodes of speciation and adaptive radiation. A recent palaeolimnological study of sediment cores indicated that Lake Malawi underwent major climate-driven desiccation events 75 000–135 000 years ago that lowered the water level to at least 580 m below the present state and severely reduced surface area. After this period, lake levels rose and stabilized, creating multiple discontinuous littoral rocky habitats. Here, we present evidence supporting the hypothesis that establishment and expansion of isolated philopatric rock cichlid populations occurred after this rise and stabilization of lake level. We studied the Pseudotropheus ( Maylandia ) species complex, a group with both allopatric and sympatric populations that differ in male nuptial colour traits and tend to mate assortatively. Using coalescent analyses based on mitochondrial DNA, we found evidence that populations throughout the lake started to expand and accumulate genetic diversity after the lake level rise. Moreover, most haplotypes were geographically restricted, and the greatest genetic similarities were typically among sympatric or neighbouring populations. This is indicative of limited dispersal and establishment of assortative mating among populations following the lake level rise. Together, this evidence is compatible with a single large-scale environmental event being central to evolution of spatial patterns of genetic and species diversity in P. ( Maylandia ) and perhaps other Lake Malawi rock cichlids. Equivalent climate-driven pulses of habitat formation and fragmentation may similarly have contributed to observed rapid and punctuated cladogenesis in other adaptive radiations.     

Uncertainties in the relationship between atmospheric nitrogen deposition and forest carbon sequestration

In a recent study, Magnani et al. report how atmospheric nitrogen deposition drives stand-lifetime net ecosystem productivity (NEP_av) for midlatitude forests, with an extremely high C to N response (725 kg C kg⁻¹ wet-deposited N for their European sites). We present here a re-analysis of these data, which suggests a much smaller C : N response for total N inputs. Accounting for dry, as well as wet N deposition reduces the C : N response to 177 : 1. However, if covariance with intersite climatological differences is accounted for, the actual C : N response in this dataset may be <70 : 1. We then use a model analysis of 22 European forest stands to simulate the findings of Magnani et al. Multisite regression of simulated NEP_av vs. total N deposition reproduces a high C : N response (149 : 1). However, once the effects of intersite climatological differences are accounted for, the value is again found to be much smaller, pointing to a real C : N response of about 50–75 : 1.