Invasive plant management is a global phenomenon in response to the negative environmental impacts of invasive species. Although monitoring of communities during and after management is essential for improving and measuring the success of management projects, such monitoring rarely occurs. In addition to improving management outcomes, examination of invasive plant management can augment ecological theory. This thesis uses management of two invasive plant species, Lythrum salicaria (purple loosestrife) and Lantana camara (lantana), to examine: the role of propagule pressure and disturbance in exotic species invasions, the role of disturbance in plant community regeneration, and the impacts of an invasive shrub on plant community re-establishment following fire.
Chapter 2 examines the influence of propagule pressure and disturbance on the “invasion” success of four insects—Galerucella calmariensis, G. pusilla, Nanophyes marmoratus and Hylobius transversovittatus—released for the control of purple loosestrife. This field survey of release and non-release sites in the Lower Columbia River Estuary, USA, uses statistical models to determine the influence of disturbance and propagule pressure on persistence at release sites, colonization of new sites, and abundance at colonized sites. The results of this study found: 1) sparse evidence for the positive influence of propagule pressure on invasion success; 2) that disturbance negatively affected invasions of these herbivorous insects; 3) the effects of disturbance and propagule pressure were species specific and varied among invasion stages, and 4) results were inconsistent across different measures of propagule pressure; therefore, single measures and proxies should be used cautiously.
Predicting community change following invasive species management and assessing this change against specific objectives is important for increasing management success. An experimental field trial in lantana-invaded wet sclerophyll forest was used to examine the independent and interactive effects of management disturbance, natural propagule supply, and pre-existing vegetation on post-management plant community regeneration (chapter 3). The field-trial compared two management treatments—low disturbance herbicide and high disturbance mechanical removal—to measure dissimilarities in abiotic environments, propagule supply, and vegetation. There were abiotic differences between both managed and reference treatments, and between management types. Management type influenced exotic tree establishment, graminoid density, and percentage ground cover; all of which were greater following mechanical removal. In addition, pre-existing tree communities influenced post-management species diversity and exotic tree establishment, and the influence of propagule supply on regeneration differed among treatments. This study demonstrates how management methods influence community restoration and how they can be manipulated to achieve management objectives. The field trial shows that methodology alone is not the sole determinant of outcomes and that pre-existing vegetation and propagule supply are also influential.
While it is generally acknowledged that long-term monitoring of communities following invasive plant management is necessary to evaluate management outcomes, optimal monitoring time is rarely examined in the literature. Monitoring using inappropriate temporal scales can result in poor adaptive management or superfluous management actions. Chapter 4 uses the field trial discussed in chapter 3 to examine plant community response time following low and high disturbance management. The understorey plant community was surveyed 6, 12 and 36 months after initial lantana management and species diversity and density of different plant groups were compared among treatments. Plant community differences, compared to the reference treatment, were detected sooner after high disturbance management (12 months). This temporal trend was consistent across multiple plant groups and for species richness. This study concluded that management disturbance can influence the temporal scale at which regeneration occurs and, therefore, when assessment is appropriate.
Determining the impacts of invasive species is important, yet assessment is problematic. Chapter 5 uses two methods of assessment, correlation and removal, to assess the impact of lantana on native tree regeneration in a forest recovering from the impacts of fire. The results of this study show that, after one year, herbicide application successfully reduced lantana density and increased mean DBH of a native early successional species. In addition, in areas not managed with herbicide, lantana density was negatively associated with the abundance of multiple native tree species and positively correlated with Acacia spp. height. This study shows that even with the presence of fire, which decreases lantana density at least in the short term and increases native tree germination, lantana was negatively associated with abundance of some, but not all, native tree species. This study demonstrates that lantana management was beneficial to regenerating trees even when assessed on a short temporal scale.
In summary, this thesis demonstrates that invasive plant management can be used to better understand the mechanisms behind invasion success, the influence of management disturbance on and temporal patterns of plant regeneration, and the impacts of invasive species. By using invasive plant management projects to examine broader ecological trends we are able to improve management success and potentially reduce invasions of unwanted species.