Soil, Growth, Arthropods, and Competition: Native parasitic vine Cassytha pubescens and its indirect ecological effects on native and invasive host species.

Abstract

In spite of negative perception, parasites are keystone organisms because of their indirect benefits on biotic and abiotic environments. In spite of their abundance, parasites have not been fully incorporated into current understanding of ecological communities; this represents a massive gap in current understanding of community ecology. This project focused on the indirect effects of an Australian-native parasitic plant, Cassytha pubescens. This project identified whether these effects differed when the parasite infected native or invasive plants, and whether these effects benefit natives or invasives. Cassytha pubescens has strong negative effects on invasive host species, particularly on one of the most invasive plant species in the world, Ulex europaeus. In contrast, C. pubescens has negligible impacts when infecting native species, therefore C. pubescens has biocontrol applications. I assessed whether C. pubescens modified litterfall of native and invasive hosts, and whether this influenced soil nutrient returns. Using plastic pots capturing litterfall in the field and using plant and soil nutrient analyses, I found that invasive host U. europaeus had decreased litterfall, but infection had no effect on native host litterfall. Infected plants had minor differences in soil composition compared with uninfected plants. This demonstrated that C. pubescens may have little impact on soil nutrient returns, unlike other parasitic plants, because it is leafless. I assessed how C. pubescens litter and soil under infected shrubs influenced seedling emergence and growth. In a glasshouse study, seeds of native and invasive species were sown in soil taken beneath infected or uninfected shrubs, adding C. pubescens litter or not. Native and invasive species had decreased emergence under parasite litter – with stronger effects on invasive U. europaeus. Only U. europaeus grew larger in soil from under infected shrubs, probably because of its greater resource use efficiency. This study demonstrated that C. pubescens litter, which can be substantial in mass upon the death of host, can decrease invasive species recruitment around hosts. I assessed whether C. pubescens influenced abundance and composition of arthropod communities. I used pitfall traps in the field to trap arthropods over one year, beneath infected and uninfected native and invasive shrubs. I found slight differences for beetles, being less abundant under infected shrubs than uninfected shrubs. Cassytha pubescens did not increase arthropod abundance, unlike other parasitic plants, but we found no negative effect on arthropod communities. These results suggest C. pubescens has negligible effects on arthropod communities. I assessed whether C. pubescens modified competitive interactions between native and invasive host species. Using glasshouse studies I assessed whether growth of native and invasive plants differed when grown alone vs. grown with a competitor. I also assessed how the native and invasive grew, in a single large pot, when both were uninfected, only invasive infected, and only native infected. Invasive U. europaeus had less growth when infected, whether or not A. paradoxa was infected. However, native A. paradoxa grew equally well competing with an infected invasive. These results suggest that C. pubescens will decrease the competitive ability of invasive U. europaeus, but not that of natives.