Anne Bourne

Modelling non-equilibrium distributions of invasive species: a tale of two modelling paradigms

Invasive species, biological control and climate change are driving demand for tools to estimate species’ potential ranges in new environments. Flawed results from some tools are being used to inform policy and management in these fields. Independent validation of models is urgently needed so we compare the performance of the ubiquitous, logistic regression and the CLIMEX model in predicting recent range extensions of the livestock tick, Rhipicephalus (Boophilus) microplus, in Africa. Both models have been applied to the tick so new, independent data can be used to test their ability to model non-equilibrium distributions. Logistical regression described the spatial data well but failed to predict the range extensions. CLIMEX correctly predicted the extensions without fitting the non-equilibrium data accurately. Our results question the validity of using descriptive, statistical models to predict changes in species ranges with translocation and climate change. More test cases that include independent validation are needed.

Host Plant and Biotype Density Interactions – Their Role in the Establishment of the Invasive B Biotype of Bemisia tabaci

Bemisia tabaciis a complex of closely related genetic types of whiteflies, few of which are invasive. One of these, B biotype, has proven to be particularly adapted to invading new areas, but the underlying reasons as to why it has a well-developed capacity to invade is not known. To develop an understanding of factors that may be contributing to B’s invasive capacity, inter-biotype mating interactions and host plant suitability for the exotic B (B. tabaci Mediterranean/Asia Minor/Africa) and the indigenous Australian (AN) biotype (B. tabaci Australia) were examined. The results suggest that when confined to a mutually acceptable host, B cannot establish when the ratio of AN : B exceeds 20 : 1. However, when simultaneously provided with a host that only it prefers, B is able to establish even at 50 : 1 (AN : B). Further, when both biotypes occur together the number of progeny per female increases (relative to the number produced when only one biotype is present). The response is observed for both biotypes, but is considerably greater in the case of B. In addition, B performs better in the presence of the AN biotype B. tabaci Australia while AN perform worse in coexistence with B, but only if the demographics allow B to mate without significant interference. This leads to the prediction that B will invade in circumstances where its unique hosts are of sufficient number to escape the full negative impact of inter-biotype mating interactions and reduced competitiveness in terms of reproductive rate, while exposing the indigenous biotype to the full effects of the interaction.

Prerelease Assessment of Impact on Biomass Production of an Invasive Weed, Lygodium microphyllum (Lygodiaceae: Pteridophyta), by a Potential Biological Control Agent, Floracarus perrepae (Acariformes: Eriophyidae)

Abstract A prerelease assessment of impact by a potential biological control agent, Floracarus perrepae Knihinicki and Boczek, on the invasive weed, Lygodium microphyllum (Cav.) R. Br., was conducted in a 2-yr field study in their native range—Australia. Thirty-two pairs of test plants were planted in a field plot with two levels of shade, with one plant in each pair treated biweekly with the miticide abamectin. The mite caused a significant reduction in biomass of above ground stems and leaves and below ground roots and rhizomes. The mean leaf longevity was significantly longer for the treated versus the mite infested untreated plants. Populations of native predator mites were low throughout the study; however, the mite pathogen Hirsutella thompsonii Fisher was common in the second year of the study, but neither reduced the impact of F. perrepae. Based on its potential to cause significant damage to L. microphyllum under field conditions in the native range and extremely narrow field host range, F. perrepae is an excellent candidate for biological control of this invasive fern in Florida.

Compensatory branching and changes in nitrogen content in the aquatic weed Salvinia molesta in response to disbudding

SummarySecondary side branching in Salvinia molesta plants grown in nutrient solution was dependent on the availability of nitrogen. The compensatory response of S. molesta to damage by complete manual disbudding was the production of extensive secondary side branching at nitrogen levels below those required for this type of branching in undamaged plants. After 28 days damaged plants had the same number of ramets as undamaged plants but the dry weight was considerably reduced.After 7 days there was no change in dry weight with partial or complete disbudding. However completely disbudded plants had no new ramets and plants with all terminal buds removed had numbers of ramets significantly reduced. The concentration of nitrogen in the tissue of new growth was higher in damaged than undamaged plants. The results are discussed in relation to the biological control of S. molesta by the bud-eating weevil, Cyrtobagous salviniae.

Ovipositional host choice by an invader accelerates displacement of its indigenous competitor

While competition for resources leading to invasion success is well recognized, avoidance of competition is much less so. Changes in behaviour that lead to avoidance are usually displayed by the weaker competitor. In our case though it was the stronger competitor that changed its behaviour by choosing a host for oviposition that was unacceptable to its competitor and avoiding one that was mutually acceptable; this accelerated the displacement of the competitor. We showed this by enclosing both invader (Middle East Asia Minor 1, B biotype) and indigenous (Australia, AN biotype) members of Bemisia tabaci species complex (sap sucking insects known as whiteflies) into field cages with either two plants that were a mutually acceptable oviposition host or one host that was acceptable to both and the other acceptable to the invader only. When only the mutually acceptable host was available, invader and indigenous females oviposited equally across the two plants. However, when given the choice, adult invaders still distributed themselves evenly across both hosts, but shifted their oviposition away from the mutually acceptable host and instead laid mostly on the host poorly utilized by the indigenous competitor. This indicates that the invader can change ovipositional choice to escape into competition free space.

Temporal Resource Partitioning and Interspecific Correlations in a Warm, Temperate Climate Assemblage of Dung Beetles (Coleoptera: Scarabaeidae)

Guilds of dung dwelling and tunneling dung beetles coexist in local assemblages in warm temperate regions, despite the tendency of dwellers to be inferior competitors. A field experiment on the Black Sea coast of Turkey examined the role of temporal resource partitioning in their coexistence. Standardized dung pads deposited at 4 h intervals through a 24 h period in summer were collected 12, 24, or 48 h later. Adults from 10 tunneling and seven dung dwelling species were collected. The tunnelers contributed a high proportion of both total abundance and biomass. There was a significant effect of dung deposition time and exposure period on mean tunneler abundance. Mean tunneler abundance was nearly seven times higher in dung deposited at 06:00 than at 18:00. The dwellers reduced the potential for competitive interactions with tunnelers by relatively uniform dispersal across the six dung deposition times. The distinctly different dung use patterns by dwellers and tunnelers demonstrated temporal resource partitioning. Interspecific correlation coefficients were also determined because interspecific relationships are at the core of resource partitioning. Total tunneler and dweller abundances were not correlated. Overall, there were strong positive correlations between tunneling species and low correlations between tunneling and dwelling species, and between dwelling species. The five most abundant tunnelers, from two tribes and three genera, were strongly positively correlated. There were substantial size differences among the four most abundant tunnelers that probably facilitate their coexistence.

Age-based differential host acceptability and human mediated disturbance prevent establishment of an invasive species and displacement of a native competitor.

Establishment failure by immigrant species, while believed to be a common occurrence, is difficult to observe and so factors contributing to failure are poorly explored. Human mediated disturbance is a key element in facilitating biological invasions, but its role in preventing establishment is rarely considered. In Australia, the whitefly, Bemisia tabaci biotype B was first detected in 1994 and has since spread widely and displaced an indigenous competitor. However, in one location it has failed to establish permitting an indigenous competitor to persist. In this area, the dominant plant host is Euphorbia cyathophora. Through a combination of laboratory and field experimentation we show that while both can utilise the young, upper leaves only the indigenous competitor can use the older lower leaves. We then demonstrate that the removal of the upper leaves as part of foreshore vegetation management is the key factor in preventing the establishment of the invader and subsequent displacement of its competitor. While human mediated disturbance coupled with complex biological interactions may promote invasion they may equally well retard rather than facilitate invasion, a factor that appears to have been largely ignored. Complexities of this kind must be conceived in our thinking to achieve a comprehensive understanding of biological invasion.

Effects of leaf-feeding by larvae of the moth Samea multiplicalis Guen. (Lep., Pyralidae) on the floating weed Salvinia molesta

Damage sustained by, and growth of, Salvinia molesta in the presence of zero and three population densities of Samea multiplicalis larvae were measured over 15 experiments between spring and autumn using three levels of plant nutrition. At densities of 0.8 and 1.6 per plant larvae severely damaged S. molesta destroying approximately half the leaf area and reducing plant weight and number of ramets. The leaf damage did not induce compensatory growth. Roots and rhizomes remained undamaged and no buds were destroyed, leaving plants with the capacity to continue to grow.

Oviposition by the weevil Cyrtobagous salviniae Calder and Sands when its host plant, Salvinia molesta is damaged

Female Cyrtobagous salviniae Calder and Sands laid fewer eggs on Salvinia molesta plants damaged by the weevil, C. salviniae or by larvae of the moth, Samea multiplicalis Guenée than on undamaged plants. The rate of oviposition was dependent on the number of buds available for adult feeding and the nitrogen content of the buds. Females did not discriminate between damaged and undamaged portions of the same plant when ovipositing. However, in undamaged regrowth which had thin rhizomes, most eggs were laid in the buds whereas in damaged growth, most eggs were laid in the thicker rhizomes where larval development occurs.

How Guilds Build Success; Aspects of Temporal Resource Partitioning in a Warm, Temperate Climate Assemblage of Dung Beetles (Coleoptera: Scarabaeidae)

Abstract Succession in local dung beetle assemblages influences their delivery of ecological functions in natural and modified environments globally. Short-term changes in dung beetle (Coleoptera: Scarabaeidae) species richness, abundance, and biomass were investigated in standardized dung pads in northern, coastal Turkey. For mean tunneling guild abundance, dung deposition time, dung exposure period, and their interaction were significant, and for mean dung dwelling guild abundance, dung exposure period was significant, as was the interaction with dung deposition time, which collectively evidenced temporal resource partitioning, based principally on differences in diel activity. Succession was highly compressed, with maximum abundance at 12 h and maximum species richness at 24 h. A large ball roller and small- to medium-sized tunnelers dominated different periods in the first 24 h but were superseded by dwellers. Regression analysis demonstrated a significant, positive relationship between species richness and the evenness of abundance for both dung deposition times. Correlation analysis generally showed strong, positive correlations between tunneling species, low correlations between tunneling and dwelling species, and low correlations between dwelling species. Niche partitioning based on size difference appears to have acted on the environmental filtering of tunneling species along the temporal gradient of declining moisture, thereby limiting the number of abundant, concurrent species. The aggregation of tunneling species provided opportunities for the less competitive dwelling species to occupy less densely populated zones termed probability refuges. The network of strong, positive correlations between tunneling species may indicate that their collective functionality is vulnerable to loss of efficiency if species are lost.