BioLeaf: A professional mobile application to measure foliar damage caused by insect herbivory

In coastal China, the exotic invasive Spartina alterniflora is preventing the establishment of native mangroves. The use of exotic species, control of exotic plant invasion, and restoration of native plant communities are timely research issues. We used exotic Sonneratia apetala Buch.-Ham and S. caseolaris (L.) Engl. to control invasive Spartina alterniflora Loisel through replacement control for five years, which concurrently promoted the restoration of native mangroves. This process includes three stages. I: In a mangrove area invaded by S. alterniflora, exotic S. apetala and S. caseolaris grew rapidly due to their relatively fast-growing character and an allelopathic effect. II: Fast-growing S. apetala and S. caseolaris eradicate S. alterniflora through shading and allelopathy. III: The growth of native mangrove was promoted because exotic plant seedlings cannot regenerate in the understory shade, whereas native mesophytic mangrove plants seedlings can grow; when the area experiences extreme low temperatures in winter or at other times, S. apetala dies, and native mangrove species grow to restore the communities. This model has important implications for addressing the worldwide problems of "how to implement the ecological control of invasion using exotic species" and "how to concurrently promote native community restoration during the control of exotic invasion".

Use of exotic plants to control Spartina alterniflora invasion and promote mangrove restoration

By altering the strength of intra- and interspecific competition, droughts may reshape plant communities. Furthermore, species may respond differently to drought when other influences, such as herbivory, are considered. To explore this relationship, we conducted a greenhouse experiment measuring responses to inter- and intraspecific competition for two grasses, Schedonorus arundinaceus and Paspalum dilatatum, while varying water availability and simulating herbivory via clipping. We then parameterized population growth models to examine the long-term outcome of competition under these conditions. Under drought, S. arundinaceus was less water stressed than P. dilatatum, which exhibited severe water stress; clipping alleviated this stress, increasing the competitive ability of P. dilatatum relative to S. arundinaceus. Although P. dilatatum competed weakly under drought, clipping reduced water stress in P. dilatatum, thereby enhancing its ability to compete with S. arundinaceus under drought. Supporting these observations, population growth models predicted that P. dilatatum would exclude S. arundinaceus when clipped under drought, while S. arundinaceus would exclude P. dilatatum when unclipped under drought. When the modeled environment varied temporally, environmental variation promoted niche differences that, though insufficient to maintain stable coexistence, prevented unconditional competitive exclusion by promoting priority effects. Our results suggest that it is important to consider how species respond not just to stable, but also to variable, environments. When species differ in their responses to drought, competition, and simulated herbivory, stable environments may promote competitive exclusion, while fluctuating environments may promote coexistence. These interactions are critical to understanding how species will respond to global change.

The role of drought- and disturbance-mediated competition in shaping community responses to varied environments

Biotic resistance to alien plant invasions is mainly determined by ecological interactions in two layers of the food web: competition with native plant species and herbivory by native herbivores. While the direct effect of native plants on alien plant performance via competition has been well documented across ecosystems, less is known about the direct and indirect effects of herbivores in providing biotic resistance. Our main aims were to determine whether temperate native aquatic plants and herbivores can provide biotic resistance to plant invasions, understand the underlying mechanisms and search for potential interactive effects of competition and herbivory on invader performance (i.e. growth). We mimicked natural temperate mesotrophic and eutrophic freshwater lakes in mesoscosms, by growing three native submerged plant species in monocultures (Ceratophyllum demersum, Myriophyllum spicatum and Potamogeton perfoliatus) at three competition levels (no, low and high) without and with the native aquatic generalist snail Lymnaea stagnalis. We subsequently simulated an early stage of establishment of the South American highly invasive alien plant species Egeria densa. We found that competition by native plant biomass significantly reduced invader performance but depended on native species identity. Herbivory had no direct negative effect on invader performance as the snails fed mainly on the available filamentous algae, which are commonly found in meso‐ and eutrophic systems, instead of on the plants. However, the consumption of filamentous algae by herbivores indirectly had positive effects on the invader total biomass, thus facilitating the invasion by E. densa. Nonetheless, these indirect effects worked through different pathways depending on the native plant identity. Synthesis. We found evidence for biotic resistance through competition by native plant species. However, we show that herbivores can indirectly facilitate South American plant E. densa invasion promoting its growth through selective feeding on filamentous algae, but this effect depends on the native plant species involved. Our experiment illustrates the important role of indirect interactions to understand the potential of biotic resistance in natural ecosystems.

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Direct and indirect effects of native plants and herbivores on biotic resistance to alien aquatic plant invasions

In this study, Mimosa pigra dieback is described as plants or population of plants which appeared to suffer from ill health, death of their growing points and the emergence of green reshoots coming from the base. Dieback is an important phenomenon, not only in many crops and natural forests worldwide, but also in many invasive trees and shrubs in Australia. The dieback phenomenon is thought to be caused by complex and interacting factors and causes are often difficult to disentangle. In some cases it is a result of an interaction between both biotic and abiotic factors. Since 1988, dieback has been widely observed in Mimosa pigra L. (Fabaceae), a woody shrub that is one of the most serious exotic invaders of floodplains and riparian areas in the Top End of the Northern Territory. However, there has been no conclusive evidence of links between M. pigra dieback and specific causal factors. In addition, the dieback phenomenon has not been previously quantified. This thesis aimed to describe M. pigra dieback phenomenon at plant and population levels and its effect on plant growth, survival and demography. Specifically, the aim of this thesis was to evaluate the role of biotic factors (insect borers and fungal pathogens), and abiotic factors (soil pH and salinity), in relation to the development of M. pigra dieback. In addition, the thesis examines the pathogenicity of the fungal endophytes/pathogens associated with dieback-affected and healthy M. pigra. M. pigra has been a focus of biological control programs for more than 30 years, but it still remains a serious threat in the wetlands and conservation areas in the Top End of the Northern Territory. While extensive studies have been conducted to evaluate the impact of biological control programs, little is actually known about dieback and its causal factors. To evaluate the causal factors of M. pigra dieback, repeated ground surveys, plant health assessment and specimen collection of stems and soils, were conducted in the period 2011 to 2013 in established M. pigra sites on Melaleuca Station, Mary River, Northern Territory. Insect borers were quantified and putative fungal pathogen abundance were surveyed. A combined morphological and phylogenetic approach was used for fungal pathogens/endophyte identification. Using a subset of isolates, trials were performed to examine differences in the pathogenicity of the recovered fungal pathogens/endophytes. Results of field research showed that dieback in M. pigra is highly dynamic and fluctuates in its severity. Different sites showed increases and/or decreases in plant health over time. Dieback affects plants in all age classes, from seedlings, juveniles to adult plants, resulting in partial mortality of plants in all age categories over the trial period. Contrary to expectations, none of the biotic and abiotic factors examined showed a direct relationship to the changing population dynamics of M. pigra dieback over time. Nevertheless, fungal isolations demonstrated that M. pigra is a host to a diverse range of endophytes, with the most frequently isolated taxa belonging to the Botryosphaeriales, Xylariales and Diaporthales. Results of the fungal identification studies showed that M. pigra harbours diverse endophyte communities. The use of morphological characters was crude and unreliable. However, species identification using the ITS sequence data was also insufficiently discriminatory while inclusion of sequence data based on EF1-a allowed the discrimination between sibling species, especially among the confusing taxa within the Botryosphaeriaceae. The pathogenicity trials undertaken revealed the potential of fungal species from Botryosphaeriaceae to kill seedlings and cause lesion development in juvenile and adult plants in both controlled and field conditions. Overall, the research findings highlight the challenges in attributing a single factor as being responsible for the dieback phenomenon. This is the first study to investigate M. pigra dieback and the results reveal that M. pigra dieback has potential dramatic plant population-level effects. The results also provide evidence of the ubiquity of the phytophagous insects and fungal endophytes across all sites and seasons. Furthermore, among the 1,352 fungal isolations, it is likely that new fungal species will be identified. However, a number of factors were able to be ruled out in relation to potential causes of M. pigra dieback. Continued efforts are needed to focus on interaction studies of biotic and abiotic factors in relation to M. pigra dieback. The fact that population (stand) dieback in M. pigra occurs in multiple catchments in the Northern Territory, and over large areas, makes the dieback phenomenon to be of considerable ecological importance.

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Mimosa pigra dieback in the Northern Territory, Australia: Investigation into possible causes

A considerable number of host-specific biological control agents fail to control invasive plants in the field, and exploring the mechanism underlying this phenomenon is important and helpful for the management of invasive plants. Herbivory and competition are two of the most common biotic stressors encountered by invasive plants in their recipient communities. We predicted that the antagonistic interactive effect between herbivory and competition would weaken the effect of herbivory on invasive plants and result in the failure of herbivory to control invasive plants. To examine this prediction, thus, we conducted an experiment in which both invasive Mikania micrantha and native Coix lacryma-jobi were grown together and subjected to herbivory-mimicking defoliation. Both defoliation and competition had significantly negative effects on the growth of the invader. However, the negative effect of 75% respective defoliation on the above- and below-ground biomass of Mikania micrantha was alleviated by presence of Coix lacryma-jobi. The negative effect of competition on the above- and below-ground biomass was equally compensated at 25%, 50% and 100% defoliation and overcompensated at 75% defoliation. The interactive effect was antagonistic and dependent on the defoliation intensity, with the maximum effect at 75% defoliation. The antagonistic interaction between defoliation and competition appears to be able to release the invader from competition, thus facilitating the invasiveness of Mikania, a situation that might make herbivory fail to inhibit the growth of invasive Mikania in the invaded community.

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Junmin Li

Papers

Interactive effect of herbivory and competition on the invasive plant Mikania micrantha.