ESE Editor’s Choice 1:1 – Understanding invasive plant phenology to better protect native island species

Laysan albatrosses amongst a field of golden crownbeard © David Dow

Taylor et al.’s recent From Practice article details a case study on Midway Atoll that demonstrates the importance of understanding plant phenology to better control and eradicate non-native species. Lead Editor Carolyn Kurle highlights this article as the inaugural Ecological Solutions and Evidence Editor’s Choice.

Invasive species on islands are an enormous problem; largely because they threaten native species, especially nesting seabirds. My experiences working with invasive species on islands have focused exclusively on animal species, but invasive plants are also problematic across multiple ecosystems, worldwide. And eradicating them is especially difficult, as anyone who has repeatedly pulled weeds in a home garden is aware.

The authors of our Editor’s Choice article present an elegant and relatively straight-forward recommendation for better management and control of invasive plant species with applications for restoring islands and any ecosystem in which invasive plants are problematic.

Midway Atoll, a series of three small islands, is part of the Northwestern Hawaiian Islands and the field site for the article’s case study. The first botanist at Midway described 13 plant species in 1902, but with the influx of humans and development over the past century there has now been up to 389 observed species of plants on the islands; most of which are invasive and introduced over time by accident, or on purpose by humans trying to transform the relatively desolate island into a tropical plant paradise.

Laysan Albatrosses on Midway © Tim Clark, US Fish and Wildlife Service

An especially pervasive invasive species on Midway is the golden crownbeard (Verbesina encilioides), a chest-high plant that can create cover so dense so as to prevent seabirds from accessing their nests, taking off (as their wings catch on the branches), and staying cool (the density of the plant’s growth blocks the island breezes, creating overly warm conditions for chicks). As late as the 1990s, the plant covered at least 60% of Midway but efforts to eradicate the species – applying herbicide that can suppress seed germination for a year – have been hugely successful, with less than 1% of land area currently covered by golden crownbeard.

A single flower head of Golden Crownbeard can yield 300-350 seeds © Forest & Kim Starr/Starr Environmental

However, the plants have an extensive seed bank in the Midway soil from decades of reproducing and spreading seeds, so they will continue to rebound unless they are persistently treated with the herbicide. For full eradication, any newly grown plants must not be allowed to develop to the seed production and dispersal stages of their life cycle.

To prevent the pest’s regrowth, Taylor et al.’s research details the life cycle of the golden crownbeard on Midway Atoll, making the important point that understanding a plant’s phenology – the timing of the important events in its life cycle – is critical for understanding the best timing for application of control measures.

The authors’ work addresses this by detailing the number of days required for V. encelioides seedlings to mature and produce seeds, and they demonstrated that variations in the days to maturity depends on the season in which the plant sprouts. As golden crownbeard sprouts throughout the year , the discovery of seasonal variability in its timing to seed production has motivated managers on Midway to implement herbicide application schedules following the seasonal patterns of the plant’s phenological traits.

Golden crownbeard © Forest & Kim Starr/Starr Environmental

The authors also stress that plant phenologies not only change seasonally, but can also vary from year to year depending on multiple climate and other circumstances. They emphasise that ongoing monitoring of the timing to maturity for V. encelioides is crucial for the continued success of using phenological data to guide herbicide application and for lasting suppression of this harmful plant.

Controlling invasive plants is an ongoing and seemingly intractable problem across all ecosystems and Taylor et al.’s research underscores the critical importance of studying phenologies of invasive plants with conservation and restoration applications that extend far beyond Midway Atoll.

Midway Atoll is now part of the Papahānaumokuākea Marine National Monument and is one of the biggest marine conservation areas in the world. Dr Tim Clark, currently a biologist with US Fish and Wildlife Service working on Midway Atoll, shares some of his pictures below to provide inspiration that this wild, remote, beautiful place continues to recover with the help of researchers and field crews dedicated to restoring ecosystems throughout the Northwest Hawaiian Islands.