Invasive plants as drivers and passengers of community change in a disturbed urban forest.

Abstract

Invasive species can impact local ecosystems by decreasing biodiversity and local abundances of native species. Invasive species also frequently establish in disturbed habitats. An invasive species may dominate a habitat because the introduced species is a superior competitor (driver model) for resources or because the introduced species is more tolerant of noncompetitive processes such as anthropogenic disturbance that reduces the diversity and abundance of native species (passenger model).Ranunculus ficaria (Ranunculaceae) is an invasive plant species in the northeastern United States, and can be especially dense in urban riparian habitats. It emerges early and forms thick mats of vegetation that may outcompete other plants for resources. It also produces an abundance of showy flowers that may impact local pollination service. The urban riparian habitats where this species occurs are also subject to intense flooding events that can alter the riparian corridor and eliminate plants not adapted to this disturbance. This work examines R. ficaria's role in the community changes associated with a highly disturbed urban riparian habitat. I studied R. ficaria tolerance of leaf litter disturbance associated with flooding, as well as its direct competitive effects on other species. I also examined how R. ficaria alters pollinator v services in invaded areas. To gain insight into how removal of an invasive species affects arthropod assemblages and associated ecosystem services, I also compared arthropod diversity, abundance, and decomposer and nectarivore functional groups in plots invaded with Lonicera maackii (bush honeysuckle) and removal plots.I found that R. ficaria is tolerant of changes in leaf litter depth caused by flooding.It can also negatively impact the sprouting and growth of native species, probably through competition for space. I generally found positive impacts on pollination services in invaded areas, but I found low seed set in one native species, possibly due to the degraded habitat. Removal of L. maackii produced only small changes in arthropod abundance and diversity, suggesting either little initial impact of invasion, quick recovery of arthropod groups after biomass removal of L. maackii, or a shift to an alternative stable state following L. maackii invasion. Our results suggest that R. ficaria can act as a passenger, tolerating aspects of hydrological disturbance that other species cannot.However, this species can also drive low abundance and diversity through resource competition. Lonicera maackii appears to be acting as a passenger in this system with little impacts on native arthropod communities. These results have ecosystem management implications for both of these invasive species. While removal of L. maackii would likely have little impact on arthropod ecosystem services, removal of R. ficaria would negatively impact pollination services in an already degraded habitat.vi

Effects of invasive shrub honeysuckle (Lonicera maackii) and forest composition on bird communities in woodland stands.

“…Sticky traps were set one and three years post honeysuckle removal across all seasons except winter. Results of sticky traps one year after removal indicated a 53% increase in arthropod abundance and a 12.3% increase in species richness in the removal plots, but no difference in diversity was noted in the third year (Masters 2014). The outcomes of the first year could be a result of increased openness in the plots following removal of the dense shrub layer of honeysuckle and the close proximity of the paired plots could have influenced study results; the long-term impact of honeysuckle on the arthropod community seems negligible based on this study (Masters 2014).…”

“…Results of sticky traps one year after removal indicated a 53% increase in arthropod abundance and a 12.3% increase in species richness in the removal plots, but no difference in diversity was noted in the third year (Masters 2014). The outcomes of the first year could be a result of increased openness in the plots following removal of the dense shrub layer of honeysuckle and the close proximity of the paired plots could have influenced study results; the long-term impact of honeysuckle on the arthropod community seems negligible based on this study (Masters 2014). However, a similar study of urban and suburban forests in Cincinnati, OH utilizing paired plots with and without L. maackii but separated by at least 30 meters found an increase in species diversity in Psocoptera (barklice) and Coleoptera (beetles) in plots with honeysuckle, compared to plots without honeysuckle (Loomis and Cameron 2013).…”

Plant community responses to the removal of Lonicera maackii from an urban woodland park.

“…Likewise, the exotic Ranunculus ficaria, was also more abundant in the removal than in the control plots (2.1% mean cover in removal plots vs. 0.06% mean cover in control plots). Considered an early emergent herb, it is possible that R. ficaria is also well positioned to take advantage of greater light once shrub cover is removed (Masters 2014;Masters and Emery 2016). Why native spring ephemerals did not respond with a greater cover response under the same circumstances as these particular exotics may be related to potential contrasting means of dispersal and potentially low abundance in the seed bank.…”