Lambir Nepenthes project

From BiodivBorneo09

Insect Yield in Carnivorous Nepenthes: Environmental and Morphological Factors

Fikty Aprilinayati, Shana Caro, and Serena Zhao

Abstract

Leaf morphology of Nepenthes gracilis is highly variable in pitcher length and capacity, and insect yield within pitchers is also variable. We surveyed three sites with varying habitats: roadside, next to a trail, and in secondary forest. We measured the effects of sunlight levels and soil type on pitcher morphology as well as insect yield. We also compared pitcher morphology to insect yield. Our results indicate a strong correlation between plant sites and morphological traits; there was likewise a strong correlation between pitcher capacity and insect yield. Overall, our data indicate that pitcher capacity and length are the mechanisms driving variation in insect yield, with environmental factors as the cause of morphological variation.

(See summary of argument)

Introduction

Environmental factors influence plant morphology: abiotic factors such as light levels, nutrient availability, soil type, and pollution constrain plant growth. Pitcher morphology is highly variable in Nepenthes gracilis: leaf color, pitcher length, peristome diameter, and capacity all vary among individuals of this species. This species, therefore, is a suitable model for an analysis of the resources a plant invests in leaf growth when constrained by different environmental influences, providing an insight into evolutionary trade-offs in plant tissues. In Nepenthes, specialized leaves form a pitcher that produces nectar to attract insects, which fall into the lure and drown in the pooled water (Osunkoya 2007). The insects are then slowly digested as an additional source of nutrition for the plants.

Question

• What are the effects of environmental variation on pitcher morphology, and, by extension, insect yield in Nepenthes gracilis?

If environmental variables have an effect on Nepenthes, which is adapted to grow on nutrient-poor soil (Osunkoya 2007), then these effects may be extrapolated to effects of environment-caused change in other types of hardy plants. Pitcher plants have been shown to react to adverse environmental conditions, but the effect of these changes on insect yield has not yet been shown (Clarke and Lee, 2004).

We hypothesize that environmental factors, such as sunlight levels and soil type, in states that are favourable to plant health will have a direct positive influence on insect yield.

Methods

Field Methods

Three sites were assessed for qualitative environmental factors: light level (direct, indirect, and shade), soil type (clay or sandy), and proximity to human activity (roadside: disturbed and polluted, trail: disturbed, stream: secondary growth). Measurements of fourteen pitchers were taken at sites varying in disturbance and light. Each pitcher was measured for length (lid hinge to bottom of pitcher), diameter of peristom, and height from the ground. Capacity was measured by filling the pitcher with water and measuring the volume with a 50ml graduated cylinder. In total, 42 pitchers were sampled.

To quantify the insect yield of each pitcher intact insects too large to fit into a pipette were removed from each sample. The remaining insect pieces were classified by visual estimates into bins of 0, 1-10, 10-25, 25-50, and >50 heads of insects. These measures were then combined into a total insect count.

Relative pitcher age was accounted for with leaf position, and variation as a result of age was accounted for by sampling only the seven newest pitchers growing from the end of the shoot.

Statistical Analysis

Our statistical hypothesis was that sunlight levels and soil type will be significantly correlated with insect yield. Our null hypothesis was that environmental factors are not significantly correlated with insect yield.

Statistical analysis was performed using ANOVA.

Results

Insect yield differed markedly between sites: roadside pitchers collected a totaly of only 177 insects, pitchers at the stream bed collected 258 insects, and trail side pitchers collected a total of 395 insects. Mean pitcher length also varied: 5.81 cm by the road, 7.84 cm by the trail, and 8.26 cm by the stream bed. Finally, mean pitcher capacity varies by location as well: 4.35 mL at the road, 9.65 mL at the trail, and 10.37 mL at the stream bed.

In a pool of data from the entire range of morphologies, insect yield as functions of sun exposure (shade, indirect, full), soil type (sandy or clay), and location (stream, trail, road) were not statistically significant (ANOVA: p = 0.2511, p = 0.0678 and p = 0.1372, respectively). In contrast, pitcher capacity differed with location (p = 0.0007656) as did pitcher length (p = 4.792 x 10^-6). Furthermore, insect yield was significantly correlated with pitcher length (p = 0.02464), and yield was strongly correlated with pitcher capacity (p = 0.0009791). Insect yield examined with respect to the combination of capacity and length remains significant for capacity (p = .001027), but not for length alone (p = 0.365858).

Discussion

Analyses of the relationships between environmental factors of location and morphology indicate a progression of influence leading to changes in insect yield in carnivorous Nepenthes. Differences in environmental factors do not affect the yields of pitchers of the same morphology, however, changes in the environment are highly correlated with morphological aspects that, in turn, result in differences in yield. Our results may have been confounded by pseudoreplication; we were unable to sample numerous sites differing in environmental conditions due to the time constraints of the study.

Changes in environmental factors, such as sun exposure, soil type, and proximity to human activities, result in variation in pitcher morphology (length and capacity). Morphological characters are strongly correlated with the three sites: roadside, trail, and stream bed. The strong correlations between environmental factors with morphology and insect yield with pitcher morphology indicate a cascade of causes: pitcher capacity and length are the mechanisms driving variation in insect yield, with environmental factors as the ultimate cause of morphological variation.

Further studies on the effects of insect yield on N. gracilis morphology are needed. A correlation between the amount of nutrition garnered from the captured insects (insect yield) and plant mortality and reproductive success would support the hypothesis that environmental factors like sunlight, soil type, and disturbance levels have a significant effect on Nepenthes.

Additionally, our study shows the effect of anthropogenic disturbance. The site in closest proximity to human disturbance- the road side- had the smallest pitcher morphs with the lowest insect yields. The site furthest from human-induced disturbance – the stream bed in secondary growth forest – had the largest pitchers with the greatest capacities. Direct measures of air, water, and soil pollution should be taken at different locations and compared to plant morphologies to determine whether our initial findings hold true.

References

• Clarke, C. and Lee, C. (2004), Pitcher Plants of Sarawak. Natural History Publications (Borneo), Kota Kinabalu.
• Osunkoya, O. O., Daud, S. D., Di-Giusto, B., Wimmer, F. L. and Holige, T. M. (2007), “Construction costs and physico-chemical properties of the assimilatory organs of Nepenthes species in Northern Borneo,” ANNALS OF BOTANY 99(5), 895-906.