Seed trait changes in dispersers' guts and consequences for germination and seedling growth

The effectiveness of a frugivore as a disperser of a plant is greatly determined by how fruits and seeds are handled in its mouth and its digestive tract. Although a number of studies have investigated the effect of avian ingestion on germination, we still know very little about the modifications to seeds during ingestion and the specific consequences on plant fitness. Here we investigate for the first time the different mechanisms by which germination patterns of seeds are modified following ingestion by frugivores. Specifically, we examine changes in seed mass, water content, permeability, seed coat thickness, texture, and resistance in two common Mediterranean fleshy-fruited plants, Phillyrea angustifolia and Myrtus communis, after ingestion by Eurasian Blackbirds, Turdus merula. We found a number of differences between the plant species: Phillyrea seeds lost mass, mainly due to water loss, and had thinner coats after gut passage, but Myrtus seeds did not. Seeds of both species showed increased permeability, while Myrtus seeds in particular became less resistant to breakage. No quantifiable changes in seed coat texture were detected in either species, although this trait was partly associated with differences in germination rate in Phillyrea. High intraspecific plant variation was found for most seed traits measured. Seed passage through birds' guts sped up germination in both species, especially in Myrtus. Increased permeability in seeds of both species following ingestion resulted in a higher germination rate. Moreover, seeds with thick coats (and in the case of Phillyrea, harder coats) germinated at a slower rate and produced seedlings that also grew more slowly, indicating a cost of coat thickness and/or hardness for seedling emergence. Results obtained here contribute to explaining the great heterogeneity in germination responses among and within plant species and the large variety of factors, both intrinsic and extrinsic to the plants, that influence such responses.     

Separating habitat invasibility by alien plants from the actual level of invasion

Habitats vary considerably in the level of invasion (number or proportion of alien plant species they contain), which depends on local habitat properties, propagule pressure, and climate. To determine the invasibility (susceptibility to invasions) of different habitats, it is necessary to factor out the effects of any confounding variables such as propagule pressure and climate on the level of invasion. We used 20 468 vegetation plots from 32 habitats in the Czech Republic to compare the invasibility of different habitats. Using regression trees, the proportion of alien plants, including archaeophytes (prehistoric to medieval invaders) and neophytes (recent invaders), was related to variables representing habitat properties, propagule pressure, and climate. The propagule pressure was expressed as the proportion of surrounding urban and industrial or agricultural land, human population density, distance from a river, and history of human colonization in the region. Urban and industrial land use had a positive effect on the proportion of both archaeophytes and neophytes. Agricultural land use, higher population density, and longer history of human impact positively affected the proportion of archaeophytes. Disturbed human-made habitats with herbaceous vegetation were most invaded by both groups of aliens. Neophytes were also relatively common in disturbed woody vegetation, such as broad-leaved plantations, forest clearings, and riverine scrub. These habitats also had the highest proportion of aliens after removing the effect of propagule pressure and climate, indicating that they are not only the most invaded, but also most invasible. These habitats experience recurrent disturbances and are rich, at least temporarily, in available nutrients, which supports the hypothesis that fluctuating resources are the major cause of habitat invasibility. The least invaded habitats were mires and alpine-subalpine grasslands and scrub. After removing the effect of propagule pressure and climate, some habitats actually invaded at an intermediate level had very low proportions of aliens. This indicates that these habitats (e.g., dry, wet, and saline grasslands, base-rich fens, and broad-leaved deciduous woodlands) are resistant to invasion.     

Comparative biochemical composition of ploidy groups of the lion-paw scallop (<Emphasis Type="Italic">Nodipecten subnodosus</Emphasis> Sowerby) supports the physiological hypothesis for the lack of advantage in triploid mollusc’s growth in food-rich environments

Induction of triploidy in aquatic organisms has increased worldwide in the last two decades, mostly because triploids have better growth than diploids. According to a physiological hypothesis, partial or total sterility of triploids allows the accumulation of reserves in muscle and other tissues instead of being transferred to the gonad. The present study analyzes lipid, protein, carbohydrate, and fatty acid levels in muscle and gonads of Nodipecten subnodosus triploids and diploids over 18 months from June 2001 to December 2002. An important increase in gonadosomatic index of diploids scallops was observed from May to June 2002 reaching the highest values in August. Such increase was not observed in triploid scallops. Changes in biochemical composition in female gonad were in general related to the accumulation of reserves during gonad development of diploid scallops. This accumulation was lower for triploid scallops, in accordance to their sterility, especially for carbohydrates and acylglycerides. Adductor muscle index as well as protein and carbohydrate levels in muscle increased in both ploidy groups during the reproductive period indicating no mobilization of reserves to sustain gonad development in both ploidy groups. These results partially support the physiological hypothesis on the advantage of triploids: in a rich food locality no mobilization of reserves is needed to sustain gametogenesis. This, together with a possible lower efficiency of energy assimilation at high food concentration for triploids, may be the reason for an apparent lack of superiority of N. subnodosus triploids in terms of adductor muscle growth. Only the levels of particular highly unsaturated fatty acids levels (namely 20:4n-6 and 20:5n-3) in muscle of diploid and triploid decreased during the reproductive period, indicating a possible transfer of selected fatty acids to gonads, even in triploids. The muscle of triploids has a slightly but significantly higher proportion of 22:6n-3 compared to diploids, which can have implications for the nutritional and commercial value of triploid adductor muscle.     

Osmotic and ionic concentration of the egg capsule fluid of Crepidula fornicata in relation to embryonic development

Osmotic pressure and major ions (Cl⁻, Na⁺, Mg²⁺, Ca²⁺) of the egg capsule fluid in the slipper limpet Crepidula fornicata were investigated in relation to embryonic development. Calcium permeability of the capsule wall was studied at oviposition, by dipping freshly laid egg capsules in ⁴⁵Ca as a tracer. This study also determined total calcium content of the embryos at different developmental stages. Osmolarity and major ion concentrations in egg capsule fluid were higher than seawater at uncleaved and trochophore stages, and then dropped to the same level as sea water at veliger stage. Concentrations of Cl⁻ and Na⁺ were relatively high at oviposition, peaked at trochophore stage, and finally dropped close to concentrations of seawater at hatching. In contrast, concentrations of Mg²⁺ and Ca²⁺ decreased steadily during capsular development. Radiotracer permeability experiments in freshly laid egg capsules confirmed that the capsule wall is impermeable to this ion at that stage. However, because of the dissolution of the inner layer of the wall during the final part of capsular development, the wall becomes permeable to calcium and probably to the rest of the major ions studied.     

Carotenoid-based ornamentation as a dynamic but consistent individual trait

Carotenoid-based ornaments act as signals of quality in many animal species. In contrast to feathers, which are relatively stable structures, carotenoid-pigmented integuments (e.g. bills, lores, tarsi) can change colour rapidly and may better reflect changes in physiological condition. I studied the seasonal variations in plasma carotenoids in red-legged partridges (Alectoris rufa) kept on a constant diet and free of intestinal parasites. Furthermore, I analyzed whether seasonal changes in circulating carotenoids were mirrored by the carotenoid-based coloration of eye rings and bill of this species. Plasma carotenoids showed seasonal variation, with higher levels coinciding with the end of the mating and the start of the laying season. Eye ring pigmentation was related to plasma carotenoid levels, and changes in bill hue (but not changes in UV or red bill chroma) mirrored the variation in plasma carotenoids during the breeding season. Despite the seasonal variation, individual differences in eye ring pigmentation and bill hue, UV and red chroma were consistent throughout the breeding season. Similarly, individual differences in eye ring pigmentation and bill hue and red chroma remained consistent between consecutive years. These results suggest that carotenoid based integumentary colorations act as dynamic traits that accurately reflect the carotenoid-status of individuals, thus reliably indicating consistent differences in individual quality. Furthermore, variability in signal expression appears to have a relevant genetic/phenotypic basis independently of environmental conditions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ecological implications of anti-pathogen effects of tropical fungal endophytes and mycorrhizae

We discuss studies of foliar endophytic fungi (FEF) and arbuscular mycorrhizal fungi (AMF) associated with Theobroma cacao in Panama. Direct, experimentally controlled comparisons of endophyte free (E-) and endophyte containing (E+) plant tissues in T. cacao show that foliar endophytes (FEF) that commonly occur in healthy host leaves enhance host defenses against foliar damage due to the pathogen (Phytophthora palmivora). Similarly, root inoculations with commonly occurring AMF also reduce foliar damage due to the same pathogen. These results suggest that endophytic fungi can play a potentially important mutualistic role by augmenting host defensive responses against pathogens. There are two broad classes of potential mechanisms by which endophytes could contribute to host protection: (1) inducing or increasing the expression of intrinsic host defense mechanisms and (2) providing additional sources of defense, extrinsic to those of the host (e.g., endophyte-based chemical antibiosis). The degree to which either of these mechanisms predominates holds distinct consequences for the evolutionary ecology of host-endophyte-pathogen relationships. More generally, the growing recognition that plants are composed of a mosaic of plant and fungal tissues holds a series of implications for the study of plant defense, physiology, and genetics.     

Life-history strategies of plant pathogens: distribution patterns and phylogenetic analysis

Here we analyze herbarium records, surveys, and studies of fungal plant pathogens in tropical natural systems in order to establish a framework to study plant-pathogen interactions from a life-history perspective. We looked at how life-history traits of pathogens and their host plants affect the distribution of pathogens in different tropical habitats, and the importance of host phylogeny in determining the habitat associations of obligate fungal pathogens. Our study reveals that plant-pathogen interactions are prevalent and widespread in the tropics. Phylogenetic analysis of the distribution of obligate pathogens among plant families did not suggest a strong overall pattern of higher-level host phylogeny in pathogen host range, except that smut pathogens are particularly dominant on Poaceae and Asteraceae, and rusts are most common on Fabaceae, families that dominate disturbed areas in the tropics.     

Allochthonous subsidy of periodical cicadas affects the dynamics and stability of pond communities

Periodical cicadas emerge from below ground every 13 or 17 years in North American forests, with individual broods representing the synchronous movement of trillions of individuals across geographic regions. Due to predator satiation, most individuals escape predation, die, and become deposited as detritus. Some of this emergent biomass falls into woodland aquatic habitats (small streams and woodland ponds) and serves as a high-quality allochthonous detritus pulse in early summer. We present results of a two-part study in which we (1) quantified deposition of Brood X periodical cicada detritus into woodland ponds and low-order streams in southwestern Ohio, and (2) conducted an outdoor mesocosm experiment in which we examined the effects of deposition of different amounts of cicada detritus on food webs characteristic of forest ponds. In the mesocosm experiment, we manipulated the amount of cicada detritus input to examine if food web dynamics and stability varied with the magnitude of this allochthonous resource subsidy, as predicted by numerous theoretical models. Deposition data indicate that, during years of periodical cicada emergence, cicada carcasses can represent a sizable pulse of allochthonous detritus to forest aquatic ecosystems. In the mesocosm experiment, cicada carcass deposition rapidly affected food webs, leading to substantial increases in nutrients and organism biomass, with the magnitude of increase dependent upon the amount of cicada detritus. Deposition of cicada detritus impacted the stability of organism functional groups and populations by affecting the temporal variability and biomass minima. However, contrary to theory, stability measures were not consistently related to the size of the allochthonous pulse (i.e., the amount of cicada detritus). Our study underscores the need for theory to further explore consequences of pulsed allochthonous subsidies for food web stability.     

Reproductive biology and population structure of the sea cucumber <Emphasis Type="Italic">Isostichopus fuscus</Emphasis> (Ludwig, 1875) (Holothuroidea) in Caamaño,Galápagos Islands,Ecuador

Pre-spawning reproductive biology and population structure of the sea cucumber Isostichopus fuscus were analysed at the Caamaño Island, Galápagos Islands, Ecuador on a monthly basis between January 1995 and June 1996. An average of 25 individuals of all sizes were collected each month. Using microscopic characteristics of the gonads three gonadal stages were described: (1) immature; (2) mature, and (3) spent. Mature individuals showed statistically different morphometric characteristics. I. fuscus was continuously sexually mature throughout the year independent of changes in sea water temperature. Mean oocyte diameter indicated planktotrophic larval development. Mean fecundity for this species was 567,884 (±95,528 SE) eggs per gonad. Size at first maturity was between 161.0 and 170.9 g drained weight and the smallest individual found with mature gonads had a drained weight between 121.0 and 130.9 g. The population sampled ranged between 13 and 31 cm (81.0–400.0 g) with less than 10% of the population of individuals ≥25 cm (≥300.0 g). No juveniles were recorded.