Science Center Objects

Restoration of ecological systems in wildland areas often involves restoring species to habitats degraded by invasive plant and animal species.  Often, such invasive species exert community level impacts, such as direct competition, but may also alter ecosystem function. For example, invasive plants have been documented to alter fire regimes, soil nutrients and microbes, food webs, and/or pollinator visitation in ways that hinder native plant re-establishment and survival.  As such, restoration cannot be achieved without understanding invasive species dynamics, and their impacts.  It is also crucial to understand how to best re-introduce native species to the ecosystem in an efficient, cost-effective manner.

USGS volunteer measures DBH of koa tree

USGS volunteer measures the diameter breast height (DBH) of koa (Acacia koa) trees in Hakalau National Forest Wildlife Refuge. The thick layer of kikuyu grass (Pennisetum clandestinum) in the understory hinders natural regeneration of native woody understory species, even after grazers have been removed for over 2 decades. Photo: S. Yelenik

Overview:

Restoration of ecological systems in wildland areas often involves restoring species to habitats degraded by invasive plant and animal species.  Often, such invasive species exert community level impacts, such as direct competition, but may also alter ecosystem function. For example, invasive plants have been documented to alter fire regimes, soil nutrients and microbes, food webs, and/or pollinator visitation in ways that hinder native plant re-establishment and survival.  As such, restoration cannot be achieved without understanding invasive species dynamics, and their impacts.  It is also crucial to understand how to best re-introduce native species to the ecosystem in an efficient, cost-effective manner.

Project Objectives:

The objectives of this task are multifaceted. First, to gain an understanding of why certain species are so invasive in Hawaiʻi, which will help managers better predict which newly introduced species are likely to become problematic over time.  Second, to understand which traits or ecosystem interactions are important to invasive plant spread, in order to better target which lifestage, or dispersal syndrome is best targeted to slow spread into more pristine areas.  Third, to understand the impacts of invasive species, and their relative importance, such that managers know which impacts are important to mitigate before attempting native plant re-establishment.  Fourth, to understand which native species are reestablishing naturally, and the life stages most limiting to the reestablishment of those native species that are not returning naturally.  Finally, to develop best practices for successfully re-establishing native plant species to ecosystems.

Native understory plants at Hawaii Volcanoes National Park

 ‘Ie‘ie (Freycinetia arborea) and hapu‘u (Cibotium glaucum) in mesic forest understory in Hawai‘i Volcanoes National park. The functional and species diversity of forest understory may play an important role in the invasibility of forest communities. Photo: S. Yelenik

Highlights and Key Findings:

Traits and Invasion:

The question of why some non-native plants become invasive while others do not is a key question in ecology. Recently, functional traits have gained traction as a tool  to investigate this question and propose invasion mechanisms by examining the similarities and differences between resident species and invaders. Generally, differences suggest that invaders succeed by occupying different niches than resident species while similarities suggest that invaders possess fitness differences that allow them to successfully compete for resources. We studied the differences in functional traits of invasive species and the resident species in the communities that they occupy in Hawai’i Volcanoes National Park, USA across a temperature and precipitation gradient. We found that invasion mechanisms vary widely among invasive plant species. Some common invaders, including Morella faya and Psidium cattleianum occupy a similar trait space as resident communities across the environmental gradient while others, including Hedychium gardnerianum and Melinis minutiflora occupy very different trait spaces than the resident communities which they invade. Notably, the invaders who occupy the same trait space as resident species are found across a greater length of the environmental gradient than other, more specialized, invasive species. Additionally, the generalist invaders displayed greater intraspecific trait plasticity, suggesting that invaders with a high degree of trait plasticity rely on fitness differences to invade more than the specialized invaders, which rely more on niche differences. Overall, these results suggest that invasion mechanism varies across species.   Because of this, simply comparing traits of potential invaders and resident species may not necessarily predict whether a species is likely to invade, although case studies such as these can offer insight into species selection for restoration projects.  Further research on how general this pattern of invasion mechanism of specialist vs generalist invasive species would be useful in further understanding this question.

This research was submitted in abstract form for the British Ecological Society Annual Meeting in 2014: Henn, J., Yelenik, S.G., and Warman, L. 'Similarities and differences between native and invasive plant functional traits across environmental gradients in Hawai’i Volcanoes National Park, USA'

2015 Progress:

Traits and Invasion:

We implemented a project to better understand invasion mechanisms of different exotic plant species across environmental gradients.  We used a trait based approach, comparing traits between invaders and resident species, to inform whether or not highly invasive species initially invade communities by utilizing empty niches (different traits than residents, suggesting use of different resources) or if successful invaders tend to rely on fitness differences (e.g., higher growth rate or seed set) to invade communities (similar traits to residents, suggesting use of similar resources, but more efficently).  We chose four highly invasive species in Hawaii Volcanoes Park, and returned to sites where they were mapped by Inventory and Monitoring crews in 2009.  We measured traits of invaders and dominant resident species, as informed by percent cover estimates from the original I&M dataset. Traits included leaf area, wetness, and nutrient content, as well as maximum height, seed size and wood density.