The gingers are invading: an investigation of the role of humans on the dispersal of the invasive shampoo ginger (Zingiber spectabile)

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By Sofia Olivero Lora and Jeffrey Arend Cawood Mkhulu Dunnink

In 1962 Robert and Catherine Wilson founded the Wilson Botanical Garden. As botanists, they sought to establish a sanctuary for plants of all kinds, from all over the world. The botanical garden, as many others around the world, preserve endangered and extirpated species. They also serve as seed banks, provide opportunities for research and outreach for local and international communities. As great as this is, there are tradeoffs to botanical gardens in terms of conservation. They may provide an opportunity for no native plants to spread and become invasive species. This can, however, be avoided if extensive monitoring and removal programs are implemented.

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The Las Cruces forest surrounding the Wilson Botanical garden.

An invasive species of particular concern is the Zingiber spectabile. Originating in Asia, this ginger plant was introduced to Costa Rica at the Wilson Botanical garden. It spread rapidly through the country due to its high ornamental value. It also spread into many natural ecosystems, particularly the Las Cruces forests immediately surrounding the botanical garden.

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The invasive Zingiber spectabile

Invasive plant species are of grave concern globally. While all of the effects of invasive plant species are not fully understood, it is generally accepted that they outcompete indigenous species. This can lead to extirpations and even extinctions. Additionally, they can decrease ecosystem stability and reduce ecosystem function and services. Despite this lack of knowledge, the precautionary principle tells us that a lack of knowledge does not justify inaction. In this sense, it is necessary to remove invasive species despite the lack of understanding regarding the role they play in the area they have invaded.

Many of these invasive species are dispersed by human movement. In countries like Australia and South Africa invasive plants have been inadvertently carried by humans as they either walk or drive through protected wilderness areas. We set out to test if humans were having an effect on the dispersal of invasive ginger seeds in Las Cruces by running belt transects perpendicular to the Rio Java and Melissa trails as well as along the garden edge moving into the forest. If humans really are dispersing these ginger seeds, we would find that the abundance of plants decreases with distance from the trails.

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A map of the trails through Las Cruces. The garden edge (our first site) is highlighted in red, the Rio Java trail (our second site) in yellow and the Melissa trail (our third site) in purple.

We found that seeds were randomly dispersed within 50 meters of the trails. It seems that humans are not playing a massive role in dispersing these seeds. It is possible that humans originally dispersed these seeds along the various trails in Las Cruces. These plants were left to grow and spread throughout the forest allowing them to be dispersed deep into the forest by small mammals, birds and ants.

What was interesting about our findings is that abundance of invasive ginger decreased with distance from the garden (the source of ginger). This means that there is a limit to the distance that the ginger can be and is being dispersed. Additionally, it seems that the invasive ginger, Zingiber spectabile, is outcompeting the local native ginger species, Renealmia cernua.

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The native Renealmia cernua.

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Effective and continuous monitoring and removal programs need to be implemented at the Las Cruces forests. These programs need to be focused on the areas immediately surrounding the garden edge and along the trails, increasing in scale as the projects start to show success. This will allow the native ginger to flourish and thrive throughout the forest.

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A beautiful sunset seen from the balcony of the comedor at Las Cruces.

We spoke to Zack Zahawi (director of the Las Cruces biological station) to better understand the relationship between humans, botanical gardens and the spread of invasive species.

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