Enhancing lepidoptera diversity in Hong Kong urban parks : a caterpillar survey of urban green spaces

Abstract

Urban Green Space (UGS) has been described as wildlife refuge as the habitat offers the ability to support native fauna population. However, the struggle between human preference and conservation needs often prevents UGS from achieving its potential ecological functions. Conventional park management practice, including monocultural horticulture, frequent pruning and trimming, application of broadspectrum insecticide, and the introduction of exotic plant species, result in a simplified understorey complexity in urban parks, and subsequently make it less ideal for native wildlife to inhabit. This is the first study to investigate the impacts vegetation attributes have on immature-stage Lepidoptera in UGS. In the urban food web, caterpillars are crucial to insectivorous birds because of their unique nutritious values. Without a reliable population of caterpillars in the city, the reproductive success of song birds may be compromised. To compare the extent urban parks are able to achieve their potential ecological functions in terms of supporting immature-stage Lepidoptera, I conducted plant and caterpillar surveys in five pairs of urban parks and their adjacent non-park UGS. In each site, a 100m transect comprising 10 equaldistanced sections was designed. A plot sample was randomly selected and surveyed in each section on the data collection days. I conducted a monthly survey for four months, during which the understorey vegetation structure, number of plant species, and caterpillar abundance, were measured in each plot. Caterpillars were collected with a combination method of branch-beating, sheeting, and visual search. I also obtained information about insecticide use in the urban parks. A total of 77 live caterpillars and 2 pupae were recorded in 20 survey days. Using the generalized linear mixed model, I demonstrated that lower native plant diversity (p < 0.01) had a negative effect on caterpillar abundance, regardless of site, insecticide use, temperature, total plant diversity in the site, total plant diversity in the transect, total plant diversity in the section, and the average plant height in the section. By controlling the same factors, I reported that both insecticide use (p < 0.01) and lower diversity of native plants (p < 0.0001) negatively affect the size of caterpillars. In other words, caterpillars were less abundant, and tend to be smaller, where plant composition was comprised of lower native plant richness. In places where chemical pest management were more intense, caterpillar size was found to be smaller. Based on the above findings, I made several suggestions for park management to enhance the Lepidoptera abundance and diversity. These suggestions include the adoption of taxon-specific insecticide, plantation of understorey vegetation with native species, and address the fear factors in pests among the general public.