Tri-trophic interactions of a predator-parasite-host assemblage in New Zealand

<p>Parasites are ubiquitous and the antagonistic relationships between parasites and their hosts shape populations and ecosystems. However, our understanding of complex parasitic interactions is lacking. New Zealand’s largest endemic moth, Aenetus virescens (Lepidoptera: Hepialidae) is a long-lived arboreal parasite. Larvae grow to 100mm, living ~6 years in solitary tunnels in host trees. Larvae cover their tunnel entrance with silk and frass webbing, behind which they feed on host tree phloem. Webbing looks much like the tree background, potentially concealing larvae from predatory parrots who consume larvae by tearing wood from trees. Yet, the ecological and evolutionary relationships between the host tree, the parasitic larvae, and the avian predator remain unresolved. In this thesis, I use a system-based approach to investigate complex parasite-host interactions using A. virescens (hereafter “larvae”) as a model system. First, I investigate the mechanisms driving intraspecific parasite aggregation (Chapter 2). Overall, many hosts had few parasites and few hosts had many, with larvae consistently more abundant in larger hosts. I found no evidence for density-dependent competition as infrapopulation size had no effect on long-term larval growth. Host specificity, the number of species utilised from the larger pool available, reflects parasite niche breadth, risk of extinction and ability to colonise new locations. In Chapter 3, I investigate larvae host specificity in relation to host nutritional rewards (phloem turnover and phloem sugar content) and host defences (bark thickness and wood density). The number of species parasitized was not explained by tree abundance, nutritional rewards or wood density. However, the number of trees parasitised declined significantly with increasing bark thickness indicating host external defences are an important driver of host specificity. Camouflage in animals has traditionally been considered an anti-predator adaptation. Yet the adaptive consequences of camouflage, i.e. increased survivability via predator avoidance, has rarely been tested. In Chapter 4, I show that larvae webbing is visually cryptic to predating kaka, yet did not protect larvae from attack. Instead, cryptic webbing aids larvae thermoregulation suggesting crypsis is non-adaptive. These results support an exciting newly emerging paradigm shift that indicates the evolution of camouflage in animals may be more to do with abiotic conditions than biotic signalling. Males are often the “sicker sex”, experiencing higher pathogen and parasite loads than females. In Chapter 5, I construct the largest host-parasite database to date, spanning 70 animal and 22 plant families, from which I conduct a meta-analysis testing for male biased susceptibility (MBS). Then, I develop a theoretical model that explain MBS as a result of parasite-offspring competition for female resources. I present the first, unified model that explains male-biased susceptibility in animals and plants and provide parameters for model replication, applicable to almost all host-parasite pairings on Earth. This thesis presents the first investigations of the natural history of Aenetus virescens larvae, their relationships with host trees, and the interactions with their avian predator. The results herein support existing theories of parasite aggregation and host specificity from a novel perspective. Furthermore, results support a newly emerging paradigm shift in animal camouflage evolution, and suggest a unified explanation for male biased susceptibility in animals and plants. The results herein help further our understanding of complex antagonistic relationships between parasites and their hosts, presenting novel theories on which future research can be built.</p>

Tri-trophic interactions of a predator-parasite-host assemblage in New Zealand

<p>Parasites are ubiquitous and the antagonistic relationships between parasites and their hosts shape populations and ecosystems. However, our understanding of complex parasitic interactions is lacking. New Zealand’s largest endemic moth, Aenetus virescens (Lepidoptera: Hepialidae) is a long-lived arboreal parasite. Larvae grow to 100mm, living ~6 years in solitary tunnels in host trees. Larvae cover their tunnel entrance with silk and frass webbing, behind which they feed on host tree phloem. Webbing looks much like the tree background, potentially concealing larvae from predatory parrots who consume larvae by tearing wood from trees. Yet, the ecological and evolutionary relationships between the host tree, the parasitic larvae, and the avian predator remain unresolved. In this thesis, I use a system-based approach to investigate complex parasite-host interactions using A. virescens (hereafter “larvae”) as a model system. First, I investigate the mechanisms driving intraspecific parasite aggregation (Chapter 2). Overall, many hosts had few parasites and few hosts had many, with larvae consistently more abundant in larger hosts. I found no evidence for density-dependent competition as infrapopulation size had no effect on long-term larval growth. Host specificity, the number of species utilised from the larger pool available, reflects parasite niche breadth, risk of extinction and ability to colonise new locations. In Chapter 3, I investigate larvae host specificity in relation to host nutritional rewards (phloem turnover and phloem sugar content) and host defences (bark thickness and wood density). The number of species parasitized was not explained by tree abundance, nutritional rewards or wood density. However, the number of trees parasitised declined significantly with increasing bark thickness indicating host external defences are an important driver of host specificity. Camouflage in animals has traditionally been considered an anti-predator adaptation. Yet the adaptive consequences of camouflage, i.e. increased survivability via predator avoidance, has rarely been tested. In Chapter 4, I show that larvae webbing is visually cryptic to predating kaka, yet did not protect larvae from attack. Instead, cryptic webbing aids larvae thermoregulation suggesting crypsis is non-adaptive. These results support an exciting newly emerging paradigm shift that indicates the evolution of camouflage in animals may be more to do with abiotic conditions than biotic signalling. Males are often the “sicker sex”, experiencing higher pathogen and parasite loads than females. In Chapter 5, I construct the largest host-parasite database to date, spanning 70 animal and 22 plant families, from which I conduct a meta-analysis testing for male biased susceptibility (MBS). Then, I develop a theoretical model that explain MBS as a result of parasite-offspring competition for female resources. I present the first, unified model that explains male-biased susceptibility in animals and plants and provide parameters for model replication, applicable to almost all host-parasite pairings on Earth. This thesis presents the first investigations of the natural history of Aenetus virescens larvae, their relationships with host trees, and the interactions with their avian predator. The results herein support existing theories of parasite aggregation and host specificity from a novel perspective. Furthermore, results support a newly emerging paradigm shift in animal camouflage evolution, and suggest a unified explanation for male biased susceptibility in animals and plants. The results herein help further our understanding of complex antagonistic relationships between parasites and their hosts, presenting novel theories on which future research can be built.</p>

Vertical Variability in Bark Hydrology for Two Coniferous Tree Species

As the outermost layer of stems and branches, bark is exposed to the influence of atmospheric conditions, i.e., to changes in the air’s relative humidity and wetting during storms. The bark is involved in water interception by tree canopies and stemflow generation, but bark–water relations are often overlooked in ecohydrological research and insufficiently understood. Relative to other canopy ecohydrological processes, little is known about vertical variation in bark properties and their effect on bark hydrology. Thus, the objective of this study was to analyze changes in physical properties (thickness, outer to total bark thickness ratio, density, and porosity) and hydrology (bark absorbability, bark water storage capacity, and hygroscopicity) vertically along stems of Norway spruce [Picea abies (L.) Karst.] and silver fir (Abies alba Mill.) trees. Our null hypotheses were that bark hydrology is constant both with tree height and across measured physical bark properties. We found that bark thickness and the ratio of outer-to-total bark thickness decreased with tree height for both species, and this was accompanied by an increase in the bark water storage capacity. In contrast, the bark’s density, porosity, and hygroscopicity remained relatively constant along stems. These results inform ecohydrological theory on water storage capacity, stemflow initiation, and the connection between the canopy water balance and organisms that colonize bark surfaces.

Bark Thickness and Heights of the Bark Transition Area of Scots Pine

The estimation of forest biomass is gaining interest not only for calculating harvesting volumes but also for carbon storage estimation. However, bark (and carbon) compounds are not distributed equally along the stem. Particularly when looking at Scots pine, a radical change in the structure of the bark along the stem can be noted. At the bark transition area, the bark changes from thick and rough to thin and smooth. The aim of our study was (1) to analyze the height of the bark transition area where the bark structure changes and (2) to analyze the effect of cardinal direction on the bark thickness. Regression analyses and forward selection were performed including measured tree height, DBH, bark thickness, crown base height and upper and lower heights of the bark transition areas of 375 trees. While the cardinal direction had no effect on bark thickness, DBH was found to have a significant effect on the heights of the bark transition areas, with stand density and tree height having a minor additional effect. These variables can be used to estimate timber volume (without bark) with higher accuracy and to predict the carbon storage potential of forest biomass according to different tree compartments and compounds.

A Newly Built Model of an Additive Stem Taper System with Total Disaggregation Model Structure for Dahurian Larch in Northeast China

Stem taper function is an important concept in forest growth and yield modeling, and forest management. However, the additivity of the function and the inherent correlations between stem components (diameter outside bark—dob, diameter inside bark—dib, and double-bark thickness—dbt) are seldom considered. In this paper, a total disaggregation model (TDM) structure was developed based on the well-known Kozak (2004) model to ensure the additivity of the stem components. The reconstructed model was fitted with the data of 1281 felled Dahurian larch trees from three regions of Daxing’anling Mountains in Northeast China. The results from TDM were compared with other additive model structures including adjustment in proportion (AP), non-additive taper models (NAM), and three logical structures of NSUR (AMO, SMI, SMB). The results showed that the difference was significant among the three regions. The performance of TDM was slightly better than those of other model structures. Therefore, TDM was considered as another optimal additive system to estimate stem, bark thickness, and volume predicting for Dahurian larch in Northeast China besides NSUR, a method widely used in calculating additive volume or biomass throughout the world. We believe this work is cutting-edge, and that this methodology can be applied to other tree species.

Variation in Bark Allocation and Rugosity Across Seven Co-occurring Southeastern US Tree Species

Bark is a complex multifunctional structure of woody plants that varies widely among species. Thick bark is a primary trait that can protect trees from heat generated in surface fires. Outer bark on species that allocate resources to thick bark also tends to be rugose, with bark being thickest at the ridges and thinnest in the furrows. Tree diameter or wood diameter is often used as a predictor for bark thickness but little attention has been made on other factors that might affect bark development and allocation. Here we test multiple mixed effect models to evaluate additional factors (height growth rate, measure height) that correlate with bark allocation and present a method to quantify bark rugosity. We focused on seven co-occurring native tree species in the Tallahatchie Experimental Forest in north Mississippi. Approximately ten saplings of Carya tomentosa, Nyssa sylvatica, Prunus serotina, Pinus echinata, Pinus taeda, Quercus marilandica, and Quercus falcata were destructively sampled for stem analyses. Outer bark thickness (OBT) ranged from 0.01 to 0.77 cm with the thickest maximum outer bark occurring on P. taeda (0.77 cm) and the thinnest maximum outer bark occurring on P. serotina (0.17 cm). Our outer bark allocation models suggest that some individuals with rapid height growth allocate less to outer bark in C. tomentosa, N. sylvatica, P. taeda, and P. serotina, but not for P. echinata or either oak species. All species except for C. tomentosa and N. sylvatica showed evidence for outer bark taper, allocating more outer bark at the base of the bole. Inner bark also was tapered in Carya and the oaks. Bark rugosity varied among species from 0.00 (very smooth) to 0.17 (very rugose) with P. Serotina and C. tomentosa having the smoothest bark. OBT was the best fixed effect for all species. Aside from providing data for several important yet understudied species, our rugosity measures offer promise for incorporating into fluid dynamics fire behavior models.

Comparison of different approaches to estimate bark volume of industrial wood at disc and log scale

Within the wood supply chain, the measurement of roundwood plays a key role due to its high economic impact. While wood industry mainly processes the solid wood, the bark mostly remains as an industrial by-product. In Central Europe, it is common that the wood is sold over bark but that the price is calculated on a timber volume under bark. However, logs are often measured as stacks and, thus, the volume includes not only the solid wood content but also the bark portion. Mostly, the deduction factors used to estimate the solid wood content are based on bark thickness. The aim of this study was to compare the estimation of bark volume from scaling formulae with the real bark volume, obtained by xylometric technique. Moreover, the measurements were performed using logs under practice conditions and using discs under laboratory conditions. The mean bark volume was 6.9 dm3 and 26.4 cm3 for the Norway spruce logs and the Scots pine discs respectively. Whereas the results showed good performances regarding the root mean square error, the coefficient of determination (R2) and the mean absolute error for the volume estimation of the total volume of discs and logs (over bark), the performances were much lower for the bark volume estimations only.