Cost of reproduction as reduced growth in genotypes of two congeneric species with contrasting life histories

Summary We examined the effect of reproduction on growth in 33 genotypes of Plantago major and 14 genotypes of P. rugelii. These two herbaceous perennials have contrasting life histories; P. major reproduces at a smaller size, and allocates a larger proportion of its biomass to reproduction, than P. rugelii. The effect of reproduction on frowth was determined experimentally using photoperiod manipulations to control level of reproduction. The difference in growth between reproductive treatments was divided by the difference in capsule weight to produce a measure of reproductive cost per g of capsule for genotypes of the two species. In both species there was substantial variation among genotypes in the effect of reproduction on growth. Much of this variation could be correlated with differences among genotypes in the extent of reproductive investment and plant size. Cost in terms of reduction in growth per g of capsule increased with reproductive investment in P. rugelii, and with plant size in P. major. We suggest the differences between species in timing and extent of reproduction are related to the differences between species in effect of reproduction on growth. Plantago rugelii may reproduce to a lesser extent than P. major because cost per g of capsule in terms of reduced vegetative biomass, increases with reproductive output in the former species, but not in the latter. Similarly, P. major may reproduce earlier than P. rugelii because cost per g of capsule increases with plant size in P. major, but not in P. rugelii.     

Spatial extent of bird species response to landscape changes: colonisation/extinction dynamics at the community-level in two contrasting habitats

Animal community dynamics in changing landscapes are primarily driven by changes in vegetation structure and ultimately by how species respond to these changes and at which spatial scale. We consider two major components of local community dynamics, species colonisation and extinction. We hypothesise that (1) the optimal spatial extent needed to accurately predict them will differ between these two processes; (2) it will also likely differ from species to species as a result of life history traits differences related to differences in habitat selection and (3) that a species' primary habitat will determine the spatial extent at which it perceives change in vegetation structure. We used data collected over 25 yr in a changing Mediterranean landscape to study bird species local colonisation and extinction patterns in two groups of species typical from two habitats: open farmland and woodland. Vegetation changes were measured at spatial extents ranging from 0.2 to 79 ha. Local species colonisation and extinction estimates were computed using a method accounting for heterogeneity in detection probability among species. We built linear models between local species colonisation/extinction estimates and vegetation changes and examined variations in model quality with respect to the spatial extent at which vegetation changes had been measured. Models for open habitat species showed that colonisation processes operated at the landscape scale (79 ha), while extinction was more tightly linked to local habitat requirements (0.2 ha). Models for woodland species presented a low and constant model quality whatever the spatial extent considered. Our results suggest that the dynamics of the woodland species considered responded to a combination of vegetation changes at several scales and, in particular, to changes in the vertical structure of the vegetation. We highlight the need to explicitly consider spatial extent in studies of habitat selection and of habitat and population dynamics to improve our understanding of the biological consequences of land use changes and guide more effective conservation efforts.     

Spatial genetic structure of two sympatric neotropical palms with contrasting life histories

The spatial genetic structure within sympatric populations of two neotropical dioecious palm species with contrasting life histories was characterized to evaluate the influence of life history traits on the extent of genetic isolation by distance. Chamaedorea tepejilote is a common wind-pollinated arboreal understory palm. Chamaedorea elatior is an uncommon climbing subcanopy palm with entomophilous pollination syndrome. A total of 59 allozyme alleles for C. tepejilote and 53 alleles for C. elatior was analyzed using both unweighted (Iu) and weighted (Iw) Moran's I spatial autocorrelation statistics. The spatial genetic structure detected within these populations is consistent with those reported for highly dispersed plants. A significance test for differences between mean Moran's I-coefficients revealed less spatial genetic structure within the C. tepejilote population than that in the C. elatior population. Adjacent individuals of C. elatior exhibited significant spatial genetic autocorrelation (Iu=0.039, Iw=0.034), indicating a Wright's neighborhood size of about 100 individuals. For C. tepejilote, nonrandom genetic distribution among nearest neighbors was detected, even from small spatial autocorrelation values (Iu=0.008, Iw=0.009), consistent with a neighborhood size of about 300 individuals. For both species, seed dispersal, mortality among life cycle stages, overlapping generations, and contrasting traits of mating and reproduction influence the standing spatial genetic structure within populations.     

Driving forces shaping phytoplankton assemblages in two subtropical plateau lakes with contrasting trophic status

1. We studied driving forces shaping phytoplankton assemblages in two subtropical plateau lakes with contrasting trophic status, the oligotrophic deep Lake Fuxian and the eutrophic shallow Lake Xingyun. 2. Phytoplankton samples were taken monthly for a year and phytoplankton species were sorted into the main taxonomic groups and associations proposed by Reynolds. A canonical correspondence analysis (CCA) was used to test the occurrence of these classification schemes and to determine their discriminatory power. 3. The results suggest that the major driving forces in Lake Fuxian were physical variables, and particularly the underwater light climate, whereas, nutrients were the important driving force in Lake Xingyun. 4. Top–down control through zooplankton grazing in Lake Fuxian was hardly ever a significant determinant itself, because of the scarcity of zooplankton and their low grazing efficiency of predation while a dominance of inedible cyanobacteria throughout the year rendered top–down controls ineffective failing in Lake Xingyun. Hence phytoplankton communities in both lakes appear to be regulated primarily by bottom–up controls.     

Nutrient foraging traits in 10 co-occurring plant species of contrasting life forms

1 Responses to spatial heterogeneity of soil nutrients were tested in 10 plant species that differ in life form and successional status, but which co-occur in the South Carolina coastal plain. The morphological responses of the root system were tested by assessing scale (represented by root mass and root length densities), precision (preferential proliferation of roots in nutrient-rich patches compared with less fertile patches) and discrimination (ability to detect and proliferate within the richest patches when patches vary in nutrient concentration). We also investigated sensitivity (growth benefits gained as spatial heterogeneity of nutrients increases, measured as total biomass).
2 Ten individuals of each species were grown in pots under four treatments that had differing nutrient distribution but the same overall nutrient addition. Plants were harvested when roots reached pot edge.
3 We observed high variation between species in scale, precision and sensitivity. No significant discrimination responses were observed, although greatest root mass density occurred at intermediate fertility levels for all species.
4 We rejected the hypothesis that scale and precision are negatively correlated. Indeed, in herbaceous species alone, scale and precision were positively correlated.
5 Sensitivity was not closely related to precision, indicating that proliferation of roots in fertile patches does not always yield growth benefits in heterogeneous soils. Further, some sensitive species had very low precision, suggesting that a positive growth response in heterogeneous environments may be related to plasticity in physiology or root life span, rather than morphology.
6 Plant life form was not correlated with precision or sensitivity. However, scale of response was greater in herbs than in woody plants, possibly because the two life forms develop root systems at different rates.     

Inbreeding depression in two species of Mimulus (Scrophulariaceae) with contrasting mating systems

We examined the effect of self- and cross-pollination on germination success, flowering probability, pollen and ovule production, survivorship, and adult aboveground biomass in two species of Mimulus with contrasting mating systems: the highly seifing M. micranthus and an outcrossing population of M. guttatus. Cross-pollinations were performed both within and between populations in order to examine the scale at which the genetic load is distributed. We found significant inbreeding depression in M. guttatus in four of the six traits, with the highest inbreeding depression observed in biomass (68% and 69% based on within- and between-population crosses, respectively) and lowest in ovule production (21% based on between-population crosses only). M. micranthus displayed significant inbreeding depression in only two of the six traits examined. Again, we observed the highest inbreeding depression in biomass (47–60% based on within- and between-population crosses, respectively), but both traits showing significant differences between self and outcross progeny expressed lower inbreeding depression than in M. guttatus. We detected no significant inbreeding depression for either pollen or ovule production in M. micranthus. An estimate of total inbreeding depression based on the multiplicative effects of all traits was also lower in M. micanthus than∗∗∗ in M. guttatus. Our results are consistent with the expected purging of genetic load in populations with high selfing rates. The absence of inbreeding depression in M. micranthus pollen and ovule production, two traits with strong links to fitness in a selfing annual, further suggests the important role of directional selection in determining the population's genetic load. Comparison of cross-pollinations made within and between populations revealed little evidence of divergence of genetic load among the M. micranthus and M. guttatus populations examined.     

Responses to natural climatic variation and experimental warming in two tundra plant species with contrasting life forms: Cassiope tetragona and Ranunculus nivalis

Two circumpolar tundra plant species, the evergreen dwarfshrub Cassiope tetragona and the perennial herb Ranunculus nivalis , were studied at Latnjajaure in northern Swedish Lapland during three consecutive growing seasons (1993–95) as a contribution to the ITEX programme. Open-top chambers (OTCs) were used in a passive heating experiment, and the performance of the plants in unmanipulated controls was correlated with climatic fluctuations among the years. Phenological, vegetative, and reproductive variables were measured. In both species phenological responses were controlled mainly by ambient air temperature. In the evergreen C. tetragona vegetative growth was controlled mainly by the influx of global solar radiation and was not temperature-dependent, whereas the opposite applied in the herbaceous R. nivalis . Vegetative growth in C. tetragona was rather stable among years as well as between treatments, whereas it was strongly influenced by annual climate in R. nivalis . Both species increased their reproductive success with increasing temperature, but R. nivalis was also radiation-dependent in this case, probably because of its green, photosynthetic nutlets. Ovule number in R. nivalis increased steadily in the experimentally heated plots during the study in response to the constant temperature amelioration above the ambient. At the community level, evergreen C. tetragona seems to have low competitive ability under warmer conditions. The situation for vernal low-growing herbs like R. nivalis is more complex; despite a strong positive response to increased temperature, they may exhibit decreased reproductive success if overgrown by a vigorous graminoid canopy.     

Comparing life expectancy of three deer species between captive and wild populations

Life in zoological gardens provides a number of benefits to captive animals, resulting in an artificial reduction of the “struggle for life” compared to their free-ranging counterparts. These advantages should result in a higher chance of surviving from 1 year to the next, and thus in longer average life expectancies for captive animals, given that the biological requirements of the species are adequately met. Here, we compare the life expectancy of captive and free-ranging populations of three deer species (reindeer Rangifer tarandus, red deer Cervus elaphus, and roe deer Capreolus capreolus). Whereas captive reindeer and red deer had life expectancies equal to or longer than free-ranging individuals, the life expectancy of captive roe deer was shorter than that of free-ranging animals. These results support the impression that roe deer are difficult to keep in zoos, whereas reindeer and red deer perform well under human care. We suggest that the mean life expectancy of captive populations relative to that of corresponding free-ranging populations is a reliable indicator to evaluate the husbandry success of a species in captivity.     

Population dynamics of two Scottish Ultramafic (serpentine) rarities with contrasting life histories

Summary

The paper reports demographic studies of the endemic Cerastium nigrescens and the very rare Arenaria norvegica ssp. norvegica on the Keen of Hamar ultramafic outcrop on Unst, Shetland from June 1994 to November 1996. Plants of Cerastium nigrescens showed a Deevey type II curve and mature plants had a half-life of 3.8 years. Plants of Arenaria norvegica ssp. norvegica showed a Deevey type I curve with high mortality after flowering in the second year. There was some evidence of increased mortality during droughts but this had little impact on the total population. Seed production and seedling recruitment for both species was good throughout the study period. Seed bank measurements ranged from 12–13 m² for Cerastium nigrescens and 24–43 m^-2 for Arenaria norvegica ssp. norvegica. The study showed that there was no immediate threat to the populations of the two species but because of their isolated occurrence they are susceptible to extinction and should be carefully monitored.     

Contrasting responses of two passerine bird species to moose browsing

Large herbivores may modify the ecosystem in a way that affects habitat quality and resource availability for other fauna. The increase in wild ungulate abundance in many areas may therefore lead to ecosystem changes, affecting distribution and reproduction of other species. Moose (Alces alces) in Scandinavia is a good example of a herbivore that has recently increased in abundance and has the potential to affect the ecosystem. In this study, we investigated how different levels of moose winter activity around supplementary feeding stations for moose affect reproduction in two insectivorous passerines: great tits (Parus major) and pied flycatchers (Ficedula hypoleuca). The two bird species showed contrasting responses to high moose activity at feeding stations. Great tits avoided habitats with high moose activity, where fledging success and feeding frequency was lower than at low moose activity habitats. Flycatchers nested more often at high moose activity habitats where fledging weight and feeding frequency were higher than at low moose activity habitats. Filming of nest boxes with great tits showed an increase in adult Lepidoptera in the diet at supplementary feeding stations for moose, and a smaller size of caterpillar prey at intermediate moose activity. The results support the hypothesis that herbivores may affect insectivorous passerines through changed arthropod food availability.     

Pollen tube development in two species of Trithuria (Hydatellaceae) with contrasting breeding systems

Trithuria (Hydatellaceae; Nymphaeales) is unique among early-divergent angiosperms in that its species are extremely small and most have exceptionally short, annual life histories. Given the evolution of these extremes of size and development, we sought to understand whether post-pollination processes still varied predictably with breeding system in Trithuria. To address this question, we studied two Western Australian species, Trithuria austinensis (dioecious, obligately outcrossing) and Trithuria submersa (bisexual, highly selfing). To document developmental timing, carpels were hand-pollinated, collected at sequential time points, and examined with light and fluorescence microscopy. In both species, pollen tubes first entered ovules<1 h after pollination, but the pollen tube pathway of outcrossing T. austinensis was almost four times longer and its pollen tube growth rates were up to six times faster (≤2,166 vs. 321 μm/h) than those of T. submersa. T. austinensis also exhibited greater male investment, slower pollen germination, and greater pollen tube attrition. These differences in male gametophyte development are predicted for outcrossers versus selfers in phylogenetically derived angiosperms. These new data for Hydatellaceae reinforce the idea that an acceleration of pollen tube development occurred in the Nymphaeales stem lineage, before the origin of Hydatellaceae. We infer that a recent evolutionary transition to selfing in T. submersa has been accompanied by predictable modifications to reproductive development, which, because of the ancient relationship between Hydatellaceae and all other angiosperms, suggests that traits underlying the lability of flowering plant post-pollination biology were present early in their history.     

Differential salt tolerance of two Artemisia species growing in contrasting coastal habitats

To investigate factors determining the differences in their salt tolerance, growth and germination, experiments were conducted on two plant species belonging to genus Artemisia: Artemisia fukudo Makino, a biennial salt marsh plant and Artemisia stelleriana Bess, a perennial coastal hind dune plant. Growth experiments revealed that salinity (100 and 300 m m NaCl) inhibited the relative growth rate (RGR) in A. stelleriana significantly but not in A. fukudo. These specific differences in salt tolerance were mainly attributed to differential responses of net assimilation rate (NAR). That is, the reduction in RGR in A. stelleriana was mainly due to the reduction in NAR, whereas no significant reduction in NAR was observed in A. fukudo. The reduction in RGR in A. stelleriana in the salt treatment was also attributable to a reduced leaf area ratio (LAR). Specific leaf area (SLA) in the two species decreased in the 300 m m treatment. The decrease in SLA in A. fukudo was, however, compensated for partly by an increase in leaf weight ratio (LWR). Germination experiments also showed that A. fukudo has a higher salt tolerance than does A. stelleriana. These results are consistent with the differences in the salinity conditions between the native habitats of the two species.     

Plant species distribution across two contrasting treeline ecotones in the Spanish Pyrenees

We describe the structure of two contrasting (elevation, topography,climate, vegetation, soil) alpine forest–pasture ecotones located in theCentral Pyrenees (sites Ordesa, O, and Tessó, T). We define ecotonestructure as the spatial distribution of trees of different size classes andgrowth-forms and the relationship between these aspects and the spatialdistribution of understory vegetation and substrate. The studied ecotones aredominated by Pinus uncinata Ram. and have been littleaffected by anthropogenic disturbances (logging, grazing) during this century.One rectangular plot (30 × 140 m) was located within eachsite with its longest side parallel to the slope and encompassing treeline andtimberline. The distribution of size and growth-form classes at site O followeda clear sequence of increasing size downslope from shrubby multistemmedkrummholz individuals to bigger arborescent trees. At site O, regeneration wasconcentrated near the krummholz area and over rocky substrates. This suggeststhat krummholz may modify microenvironment conditions and increase seedlingsurvival. At site T, regeneration was abundant above the treeline where thecover of the dominant understory shrub (Rhododendronferrugineum) decreased. In both ecotones the diversity of plants washigher above the treeline than in the forest and decreased going downslopecoinciding with the increase of P. uncinata cover. Thereduction of plant diversity appeared above the current timberline. At site O,the decrease was steep and spatially heterogeneous what may be due in part tothe edaphic heterogeneity. At site T the change was abrupt though smaller. Therelationships between the plant community and tree regeneration should be takeninto account in future ecological studies of treeline pattern.     

High levels of genetic variability and inbreeding in two Neotropical dioecious palms with contrasting life histories

We characterized the population genetics of two Neotropical dioecious palm species of Chamaedorea with contrasting life strategies from the region that is both the northernmost extent and most species rich of the genus. Chamaedorea tepejilote is a common, wind-pollinated arboreal understory palm. Although most adult plants reproduce each year, only a few individuals produce the majority of flowers and seeds. Chamaedorea elatior, conversely, is an uncommon climbing subcanopy palm with entomophilous flowers (insect-pollinated characteristics). Most of the mature palms do not reproduce in consecutive years and fruiting is episodic. Isozymes with a total of 107 alleles for 27 loci of 17 enzymes from six populations were assessed. For both species, co-occurrence of high levels of genetic variation and homozygosity was observed (C. tepejilote: He: 0.385-0.442, f: 0.431-0.486; C. elatior: He: 0.278-0.342, f: 0.466-0.535). Genetic differentiation of C. elatior was much lower (theta=0.0315) than that for C. tepejilote (theta=0.152). The contrast in differentiation may be influenced by differences in the spatial scale of the genetic neighborhoods of the two species. The simultaneous maintenance of inbreeding and of a large number of alleles within the populations is attributable to the low and variable number of mating pairs. Demographic studies indicate that this pattern could be explained by low reproductive frequency among individuals and over years in C. elatior and by reproductive dominance in C. tepejilote.     

Nutrient supply and bird predation additively control insect herbivory and tree growth in two contrasting forest habitats

It has been suggested that bottom–up and top–down forces interactively control food web dynamics. While top–down effects would increase with resource availability to plants, bottom–up effects would be stronger under low predator abundance. These predictions, however, have rarely been tested at contrasting sites while keeping the dominant plant species unchanged. Furthermore, few studies have factorially manipulated both types of forces in forest communities. For two years, we evaluated the effects of fertiliser (NPK) addition and bird exclusion on tree growth, leaf traits, insect abundance, and folivory rates in a dry/warm and a wet/cold Nothofagus pumilio forest in Patagonia, Argentina. Overall, we found no interaction between nutrient supply and bird predation, although the strength of bottom–up and top–down forces differed markedly between forest sites. Treatment effects were generally weak in the wet forest, where tree growth rates and insect herbivory were low relative to the dry forest. In the dry forest, fertilisation increased sapling growth, insect abundance and folivory, whereas bird exclusion increased leaf damage and reduced tree growth. In the wet forest, fertilisation enhanced leaf nutrient contents and folivore abundance but not sapling growth, while bird exclusion had little impact on insects or trees. These results imply that factors other than nutrients and birds were important in controlling tree growth and folivore activity in the wet forest. While treatment effect sizes varied widely among feeding guilds, in general, nutrient effects on folivores were stronger than predator effects. We conclude that, within the time‐frame of this study, tree growth and herbivory were additively affected by soil nutrients and predator presence, as bird exclusion effects did not change with elevated folivore activity on fertilised trees. We also show that both top–down and bottom–up cascades were weaker in a forest site characterised by slow‐growing juvenile trees subjected to low folivore pressure.