When are alien species good or bad for native plants? Assessing positive and negative effects of interactions with alien species that vary through wet and dry seasons in a native plant population

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Alexander Loomis, Duke University

Background/Question/Methods
Invasive species can simultaneously have negative and positive effects on native species. Effects of alien species on natives can be direct (e.g., through consumption or competition for abiotic resources) or indirect (e.g., supporting populations of pollinators or making natives less apparent to herbivores or pollinators). Indirect effects could be mediated through interactions with other invasive or other native species. Furthermore, abiotic conditions could modify these indirect and direct effects, changing their magnitudes—including within individual populations as abiotic conditions change seasonally and over different years.
Hawaii’s diverse but highly invaded terrestrial ecosystems provide excellent opportunities for understanding how native and alien species interact. This study examines the impact of interactions with non-native species in a population of the endemic Schiedea globosa that experiences a cooler, wet winter and a hotter, dry summer. I used a demographic approach, conducting bi-annual censuses, to evaluate positive or negative effects of species interactions on individual vital rates. Alien neighbor removal,and supplemental watering experiments were implemented to increase the variation of interactions with alien species
Results/Conclusions
Preliminary results show that Schiedea globosa in this population had higher growth during the wet season when alien cover was high (p<0.001) and higher fecundity when either native or alien cover was high (p=0.03, but not when cover of both alien and native cover was high). These results indicate that alien or native neighbors could facilitate growth and fecundity of S. globosa, at least under certain conditions. There was no effect of neighbors on growth during the (more abiotically stressful) dry season, suggesting that positive effects are not due to habitat amelioration but rather more likely to be through another interaction with alien species, such as non-native slug herbivory. There were nearly significant negative relationships between survival and native (P=0.061) and alien (P=0.059) neighbor cover, indicating that there is likely a negative effect on survival under certain conditions. These relationships also all had a significant effect of year, indicating the the relationship between natives and alien species varied across years with different abiotic condtions. Examining the demography of this species across the two seasons allows for analysis of whether the relationship with non-native species is cnstant across vastly different abiotic conditions or changes between seasons, indicating a potential trade-off between negative and positive interactions with potential implications for the net-effect of non-native species on population growth.