Effects of arthropod inquilines on growth and reproductive effort among metacommunities of the purple pitcher plant (Sarracenia purpurea var. montana)

Many plant species harbor communities of symbionts that release nutrients used by their host plants. However, the importance of these nutrients to plant growth and reproductive effort is not well understood. Here, we evaluate the relationship between the communities that colonize pitcher plant phytotelmata and the pitcher plants’ vegetative growth and flower production to better understand the symbiotic role played by phytotelma communities. We focus on the mountain variety purple pitcher plant (Sarracenia purpurea var. montana), which occurs in small and isolated populations in Western North Carolina. We found that greater symbiont community diversity is associated with higher flower production the following season. We then examined geographic variation in communities and found that smaller plant populations supported less diverse symbiont communities. We relate our observations to patterns of community diversity predicted by community ecology theory.

Inter- and intraspecific competition and shade avoidance in the carnivorous pale pitcher plant in a nutrient-poor savanna

PREMISE OF THE STUDY

Ecologists generally agree that weak interspecific competition for light contributes to high plant species diversity in ecosystems with nutrient-poor soils. However, the role of competition for light in such ecosystems that are also maintained by fire is poorly understood. I quantified intra- and interspecific competition for light in a fire-maintained nutrient-poor pine savanna by contrasting the effects of conspecific and heterospecific neighbors of the pale pitcher plant, Sarracenia alata.

METHODS

Accounting for initial neighbor abundance/aboveground production and initial transplant size, I measured growth and survival of small and large pitcher plant ramets of Sarracenia alata transplanted to the vicinity of natural, undisturbed mixtures of large pitcher plants and their heterospecific neighbors in the field. I tested competition for light and nutrients by clipping conspecific neighbors and by excluding prey from unclipped neighbors of transplants. I tested interspecific competition by uprooting heterospecific neighbors.

KEY RESULTS

Plant survivorship increased when conspecific neighbors were clipped and/or starved but not when heterospecific neighbors were uprooted. Small pitcher plants benefited from clipping large conspecific neighbors, suggesting that competition for light was important. Large pitcher plants benefited from excluding prey from their neighbors, with no additional benefit of clipping, suggesting that competition for prey limited their survival. Transplants produced new pitchers that were taller with narrower openings (i.e., shade avoidance) when heterospecific neighbors were left intact but not when conspecifics were unclipped.

CONCLUSIONS

Results demonstrate size-dependent intraspecific competition for light and nutrients and interspecific shade avoidance in Sarracenia alata, which could be important to understanding species coexistence in fire-maintained nutrient-poor ecosystems.

Effects of Exyra ridingsii (Lepidoptera: Noctuidae) on Sarracenia flava (Nepenthales: Sarraceniaceae)

Exyra ridingsii (Riley) (Lepidoptera: Noctuidae) is a moth whose obligate host is the pitcher plant Sarracenia flava (L.) (Nepenthales: Sarraceniaceae). The entire life cycle of the moth is completed in the trumpets of this fire-dependent plant that is found throughout the southeastern United States in bogs, long-leaf pine savannas, and pocosins. The purpose of this study was to determine the effects of E. ridingsii on S. flava, including the effect of herbivory on trumpet height in the year subsequent to feeding and whether moths select trumpets for oviposition based on height. Although most forms of herbivory by insects might be expected to have negative effects on plants by reducing photosynthetic abilities, it would be counterproductive for herbivory by E. ridingsii to negatively affect S. flava as this plant is the only possible habitat for E. ridingsii. At each site in selected quadrats, the number of trumpets, trumpet height, trumpet status, number of trumpets in a clump, and number of clumps were recorded. The relationship between height and herbivory was analyzed using a linear model, and a positive correlation was found between height and herbivory. E. ridingsii herbivory had no effect on the next year's growth of S. flava based on a Spearman's correlation. Therefore, we concluded that E. ridingsii has little effect on S. flava populations and has likely evolved to selectively avoid herbivory on more vulnerable, smaller plants.

The cobra's tongue: Rethinking the function of the "fishtail appendage" on the pitcher plant Darlingtonia californica

PREMISE OF STUDY

Carnivorous pitcher plants employ a variety of putative adaptations for prey attraction and capture. One example is the peculiar forked "fishtail appendage", a foliar structure widely presumed to function as a prey attractant on adult leaves of Darlingtonia californica (Sarraceniaceae). This study tests the prediction that the presence of the appendage facilitates prey capture and can be considered an example of an adaptation to the carnivorous syndrome.

METHODS

In a field experiment following a cohort of Darlingtonia leaves over their growing season, before the pitcher traps opened, the fishtail appendages from half of the leaves were removed. Additionally, all appendages were removed from every plant at two small, isolated populations. After 54 and 104 d, prey items were collected to determine whether differences in prey composition and biomass existed between experimental and unmanipulated control leaves.

KEY RESULTS

Removal of the fishtail appendage did not reduce pitcher leaves' prey biomass nor alter their prey composition at either the level of individual leaves or entire populations. Fishtail appendages on plants growing in shaded habitats contained significantly greater chlorophyll concentrations than those on plants growing in full sun.

CONCLUSIONS

These results call into question the longstanding assumption that the fishtail appendage on Darlingtonia is an adaptation critical for the attraction and capture of prey. I suggest alternative evolutionary explanations for the role of the fishtail structure and repropose a hypothesis on the mutualistic nature of pitcher plant-arthropod trophic interactions.

Adaptive significance and ontogenetic variability of the waxy zone in Nepenthes rafflesiana

BACKGROUND AND AIMS

The slippery waxy zone in the upper part of pitchers has long been considered the key trapping structure of the Nepenthes carnivorous plants; however, the presence of wax is reported to be variable within and between species of this species-rich genus. This study raises the question of the adaptive significance of the waxy zone and investigates the basis for an ontogenetic cause of its variability and correlation with pitcher shape.

METHODS

In Brunei (Borneo) the expression of the waxy zone throughout plant ontogeny was studied in two taxa of the Nepenthes rafflesiana complex, typica and elongata, which differ in pitcher shape and size. We also tested the adaptive significance of this zone by comparing the trapping efficiency and the number of prey captured of wax-bearing and wax-lacking plants.

KEY RESULTS

In elongata, the waxy zone is always well expanded and the elongated pitchers change little in form during plant development. Wax efficiently traps experimental ants but the number of captured prey in pitchers is low. In contrast, in typica, the waxy zone is reduced in successively produced pitchers until it is lost at the end of the plant's juvenile stage. The form of pitchers thus changes continuously throughout plant ontogeny, from elongated to ovoid. In typica, the number of captured prey is greater, but the role of wax in trapping is minor compared with that of the digestive liquid, and waxy plants do not show a higher insect retention and prey abundance as compared with non-waxy plants.

CONCLUSIONS

The waxy zone is not always a key trapping structure in Nepenthes and can be lost when supplanted by more efficient features. This study points out how pitcher structure is submitted to selection, and that evolutionary changes in developmental mechanisms could play a role in the morphological diversity of Nepenthes.

Longevity, lignin content and construction cost of the assimilatory organs of Nepenthes species

BACKGROUND AND AIMS

This study examined level of causal relationships amongst functional traits in leaves and conjoint pitcher cups of the carnivorous Nepenthes species.

METHODS

Physico-chemical properties, especially lignin content, construction costs, and longevity of the assimilatory organs (leaf and pitcher) of a guild of lowland Nepenthes species inhabiting heath and/or peat swamp forests of Brunei, northern Borneo were determined.

KEY RESULTS

Longevity of these assimilatory organs was linked significantly to construction cost, lignin content and structural trait of tissue density, but these effects are non-additive. Nitrogen and phosphorus contents (indicators of Rubisco and other photosynthetic proteins), were poor predictors of organ longevity and construction cost, suggesting that a substantial allocation of biomass of the assimilatory organs in Nepenthes is to structural material optimized for prey capture, rigidity and escape from biotic and abiotic stresses rather than to light interception. Leaf payback time - a measure of net carbon revenue - was estimated to be 48-60 d. This is in line with the onset of substantial mortality by 2-3 months of tagged leaves in many of the Nepenthes species examined. However, this is a high ratio (i.e. a longer minimum payback time) compared with what is known for terrestrial, non-carnivorous plants in general (5-30 d).

CONCLUSIONS

It is concluded that the leaf trait bivariate relationships within the Nepenthes genus, as in other carnivorous species (e.g. Sarraceniaceae), is substantially different from the global relationship documented in the Global Plant Trait Network.