Chemical signals and habitat selection by three zooplankters in Andean Patagonian ponds

1. Zooplankton may react differently to chemical signals produced by macrophytes in shallow systems. They may be attracted by macrophytes, as the plants may be used as a refuge against predators, or the plants may have a repellent effect (e.g. when the plants are a habitat for numerous invertebrate predators or fish). In fishless Patagonian ponds, the structural complexity provided by macrophytes modulates the rate of predation on zooplankton by the invertebrate predator Mesostoma ehrenbergii (Turbellaria). 2. We performed a field study to analyse the coexistence of M. ehrenbergii and three of its prey (two copepods, the calanoid Boeckella gracilis and the cyclopoid Acanthocyclops robustus, and the cladoceran Ceriodaphnia dubia) in four ponds. In two of the ponds, we carried out day and night sampling to evaluate the influence of macrophytes on the distribution of these zooplankters. 3. In laboratory experiments, we analysed the response of the zooplankters to the chemical signals produced by macrophytes (the emergent Juncus pallescens and the submerged Myriophyllum quitense), the predator M. ehrenbergii and the ‘alarm signal’ provided by a homogenate of conspecifics. 4. Our field studies demonstrated the coexistence of M. ehrenbergii and the selected prey in different seasons and that A. robustus and C. dubia choose the vegetated area (a mixed bed of J. pallescens and M. quitense) over the non‐vegetated area. The habitat choice experiments indicated that the presence of M. ehrenbergii may directly affect the habitat selection of B. gracilis, because this zooplankter swam away from the predator. In addition, Mesostoma may indirectly affect the habitat selection of the cyclopoid copepod A. robustus and the cladoceran C. dubia as both zooplankters exhibited a negative response to the alarm signal produced by crushed conspecifics. 5. The presence of the submerged M. quitense did not affect the horizontal movements of any of the zooplankters studied. In contrast, the emergent macrophyte J. pallescens elicited a positive response of B. gracilis, suggesting that this aquatic plant may act as a predation refuge. 6. Our results suggest that predator avoidance behaviour can occur in fishless environments in response to a tactile invertebrate predator like Mesostoma. In addition, the refuge effect of emergent macrophytes, enhancing the survival of pelagic zooplankters, may act as a key factor in stabilizing predator–prey interactions in fishless Patagonian ponds, as has been widely recorded in northern temperate lakes with fish.     

Does urbanization affect selective pressures and life‐history strategies in the common blackbird (Turdus merula L.)?

Urbanization, one of the most extreme land‐use alterations, is currently spreading, and the number of species confronting these changes is increasing. However, contradictory results of previous studies impede a clear interpretation of which selective pressure (nest predation or food limitation) is more important in urban habitats compared with natural situations, and whether birds can confront them by adjusting their life‐history strategies. We investigated life‐history syndromes of three common blackbird (Turdus merula) populations differing in their human influence (urban, rural, and woodland). We analysed daily nest predation and nestling starvation rates to assess the relative importance of these selection pressures in each habitat. Simultaneously, several life‐history traits were investigated to determine if T. merula seem adapted to their main source of selection. Food limitation was more important in the city, whereas nest predation was the most important selective force in the forest. The rural habitat was characterized by an intermediate influence of these two factors. Life‐history syndromes, as the covariation of a suite of traits, confirmed these results because T. merula seem well adapted to the main cause of selection in each habitat. Our results are consistent with urbanization imposing new challenges on birds, and that they adaptively respond to them. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 759–766.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.     

Positive interaction between invasive plants: The influence of Pyracantha angustifolia on the recruitment of native and exotic woody species

Abstract: Positive interactions between species are known to play an important role in the dynamics of plant communities, including the enhancement of invasions by exotics. We studied the influence of the invasive shrub Pyracantha angustifolia (Rosaceae) on the recruitment of native and exotic woody species in a secondary shrubland in central Argentina mountains. We recorded woody sapling recruitment and micro‐environmental conditions under the canopies of Pyracantha and the dominant native shrub Condalia montana (Rhamnaceae), and in the absence of shrub cover, considering these situations as three treatments. We found that native and exotic species richness were higher under Pyracantha than under the other treatments. Ligustrum lucidum (Oleaceae), an exotic bird‐dispersed shade‐tolerant tree, was the most abundant species recruiting in the area, and its density was four times higher under the canopy of Pyracantha. This positive interaction may be related to Pyracantha's denser shading, to the mechanical protection of its canopy against ungulates, and/or to the simultaneous fruit ripening of both woody invaders.     

Linking plant functional traits with post‐fire sprouting vigour in woody species in central Argentina

Abstract Sprouting vigour is determined by the plant amount of reserves and intrinsic growth rate of plants. While the first factor has been well studied, the second is far less understood. Although a higher growth rate would imply a higher sprouting vigour, fast‐growing species may have less below‐ground reserves, and thus, a lower sprouting potential. The relative importance of both opposite effects was little explored in the literature. To analyse the influence of growth rate on sprouting vigour, one growth season after a fire we measured plant height of the old (pre‐fire) and new (post‐fire) tissue in 194 individuals of 14 woody species from a woodland in central Argentina. We calculated a mean value of pre‐ and post‐fire height for each species, and obtained from a data‐base potential height at maturity, wood density (WD) and specific leaf area (SLA), as surrogates of intrinsic growth rate. We performed a forward stepwise multiple regression using WD and SLA, together with mean pre‐fire height or potential height as independent variables, and mean post‐fire height (as an indicator of resprout vigour) as the dependent variable. Interactions were also tested. Pre‐fire height, WD and their interaction term were the variables that best explained post‐fire height. We also analysed the relationship between pre‐ and post‐fire size for each species independently by fitting hyperbolic functions. Then we correlated both parameters of the functions to species characteristics (WD, SLA, potential height and mean pre‐fire height). Both parameters of the hyperbolic functions were significantly correlated only with WD, but not with the other species characteristics. All results together indicate that species with low WD (i.e. high potential growth rate) regrow more vigorously than species with high WD when pre‐fire individuals were tall. In contrast, when pre‐fire individuals were small, WD had no influence on sprout vigour. A trade‐off between allocation of biomass to underground reserves and shoot growth seems to be responsible for the patterns obtained. For small individuals, below‐ground reserves seem to play a more important role than inherent growth rate (here measured through WD) in determining the sprouting vigour, while for large individuals, growth rate seems more important than reserves.     

Foliar resistance to simulated extreme temperature events in contrasting plant functional and chorological types

We analysed leaf resistance of 41 Angiosperms belonging to a wide range of plant functional (PFTs) and chorological types (PCTs) to simulated frost and high‐temperature extreme events (EE). Leaf resistance was estimated as percentage of membrane electrolyte leakage under heating and freezing treatments in the lab. Leaf resistance to heating or freezing was not significantly correlated with the main resource‐use characteristics that defined PFTs, such as leaf specific area, toughness, N concentration or thickness. Leaf resistance to heating differed among PFTs (graminoids and bromeliads were the more resistant groups), but not among PCTs. In contrast, leaf resistance to freezing significantly differed among PCTs. Along a steep regional climatic gradient, climate variables (annual mean temperature, mean minimum temperature, mean maximum temperature and number of frost‐free months) at the locations where the given species were most abundant were also significantly correlated with freezing resistance. Species from colder habitats both at the sub‐continental and regional scales showed the highest leaf resistance to freezing. Our work indicates that leaf resistance to climatic EE and resource‐use strategy (assessed in previous studies) represent two different, partially decoupled axes of plant specialisation. It also suggests that changes in the frequency of very low temperature events might have regional‐scale impacts on vegetation, whereas changes in the frequency of very high temperature events might have more influence at the local scale.     

A rare case of interspecific hybridization in the tracheophone suboscines: Chestnut-naped Antpitta Grallaria nuchalis × Chestnut-crowned Antpitta G. ruficapilla in a fragmented Andean landscape

Based on analyses of variation in plumage, morphometrics, vocalizations, and mitochondrial and nuclear DNA sequences, we document the occurrence of interspecific hybridization between a Chestnut-naped Antpitta Grallaria nuchalis and a Chestnut-crowned Antpitta G. ruficapilla in a high-elevation forest fragment in the Cordillera Central of the Andes of Colombia. One hybrid individual was collected and at least two were recorded singing. The hybrid specimen exhibits a combination of phenotypic traits that exclude other species of antpittas as potential parents, and its vocalizations combine elements of songs of both of its parental species. Genetic analyses demonstrate that the hybrid has G. nuchalis mitochondrial DNA and mixed nuclear DNA with copies corresponding to G. nuchalis and G. ruficapilla alleles, demonstrating that the female parent was G. nuchalis. Hybridization is considered extremely rare in most groups of suboscine passerines, but it may be facilitated when populations are decimated as a consequence of deforestation and habitat fragmentation.     

Fecundity Effects of Dichogamy in an Asynchronically Flowering Population: a Genetic Model

Timing of pollen presentation and ovule maturation at the levelof the flower, individual and population may exert a profoundinfluence on individual fitness and population structure. Weexplored the evolutionary consequences of asynchrony in floweringin a population of dichogamous plants, making use of a geneticmodel to represent pollen and ovule availability over time.When the model was run for 18 generations, starting from a mixedpopulation of both early- and late flowering, protandric andprotoginous genotypes, the system evolved differently accordingto whether genetic dominance for precocity (or lateness) waspresent or absent. Under the assumption of dominance, the populationevolved towards a structure dominated by protandrous, late floweringgenotypes. In the absence of dominance, the system seemed toattain steady state equilibrium in which both dichogamous modescoexisted in predominantly intermediate-flowering genotypes.Copyright1998 Annals of Botany Company Dichogamy, protandry, protogyny, asynchronic flowering, genetic model.     

Different pathways are involved in the early development of the transient oral apparatus in anuran tadpoles (Anura: Leiuperidae)

The oral apparatus of anuran tadpoles is a unique structure composed of soft and keratinized parts surrounding the mouth. Among the many variations, a common oral apparatus involves a dorsal gap in the marginal papillae, keratinized jaw sheaths, and two upper and three lower rows of labial teeth. In Leiuperidae, besides this generalized morphology, four configurations are distinguished by the arrangement of the lower marginal papillae and the number of lower tooth rows. Study of the early oral ontogeny in 12 species representing these five configurations shows variations in the development of the lower marginal papillae and the third lower labial tooth row. Similar configurations may result from similar pathways (e.g. Physalaemus cuvieri group and Pseudopaludicola falcipes) or different pathways (e.g. generalized oral discs of Pleurodema and Physalaemus). Different oral configurations may result from overlapping trajectories ending at different stages (e.g. Physalaemus riograndensis and Ph. biligonigerus) or different trajectories (e.g. Ph. henselii and Ph. gracilis). Further studies are needed to interpret the role that heterochrony has played in evolutionary change within this family. The unsuspected variation occurring in this transient structure highlights its evolutionary potential and might be insightful in studies of anuran phylogenies that are largely based on adult characters. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 330–345.