Native plant extinctions caused by invasive plants occur over a long period of time and are difficult to detect. Research indicates a need for tracking plant decline and instating measures to prevent extinctions.

A new research paper by Paul O. Downey and David M. Richardson, published by Oxford University Press, tackles the problem of measuring threats that invasive plants pose to native plants. It is well known that invasive plants are problematic on many levels–they compete with native plants for space and nutrients, and in pushing out the natives, often destroy important habitat and food sources for native animals.

The researchers point out that “Plants contribute substantially to the global problem of biological invasions, both in terms of the number of species and their influences on ecosystems, especially on fire regimes, nutrient cycling, ecosystem services, and geomorphology. They also alter successional rates and trajectories. Islands have been particularly severely affected.”

In New Zealand, 72% of the threatened plant species with the highest priority for conservation were threatened by alien plant species, highlighting the significant impact alien plant species post to such native plant species.

While it’s well understood that invasive plants can be highly destructive, it is difficult to confirm that plant species extinctions are solely caused by invasives. One reason for this is that plant extinction usually happens very slowly–the process is difficult to monitor and document.

Very few species (either native or alien) are monitored systematically over sufficiently long periods.

Another important consideration is that plant propagules (i.e. seeds) can survive for hundreds of years out of sight, making it extremely difficult to confirm total extinction of any plant species.

The researchers emphasize that the difficulty of attributing plant extinction to invasives is not grounds for dismissal of the problem: “The lack of evidence for extinctions attributable to plant invasions does not mean we should disregard the broader threat.” Dave Richardson of the Centre for Invasion Biology at Stellenbosch University, South Africa explains that plants may be well on their way to extinction by the time biologists notice anything is wrong:

If we wait until we have sufficient evidence to show that extinctions are occurring, it will be too late to save a great number of species.

This is why the researchers created a conceptual framework that identifies the steps along the way to a plant species’ extinction. This framework offers researchers and conservationists a novel way to track and interpret native plant decline, and will guide and inform conservation intervention decisions and methods.

The six thresholds on the extinction trajectory are:

1. Decline in number of individual plants in a local population.
2. Loss of all plant individuals in one or more populations, but propagules (seeds) remain.
3. Loss of all individual plants and propagules in one or more populations.
4. Loss of all individual plants in all populations, but propagules remain in some.
5. Loss of all individual plants and all propagules in the wild, but some individuals and propagules may be kept in storage or cultivation
6. Complete loss of all individual plants and propagules.

This framework will help conservationists determine whether invasive plants are pushing native plants across extinction thresholds, enabling specific and meaningful assessments of changes in plant populations. Be referring to the framework, conservationists can organize actions on a time scale that fits their lifetimes but is also compatible with nature’s pace (it can take centuries for threatened plants to go extinct). Thanks to the context created by the framework, conservationists can plan logical actions from day to day, year to year, and decade to decade.

The researchers emphasize that if a plant crosses any of the six thresholds, it can be a cause for concern. Each step along the framework indicates potential trouble for the ecosystem the plant is part of, whether damage is visible or not.

The critical issue for conservation managers is the trend, because interventions must be implemented before extinctions occur…It is the direction of change that is fundamentally important, not whether a native plant species has actually been documented as going extinct due to alien plant species based on a snapshot view.

Recognizing a plant’s position along this extinction staircase will help conservationists implement plans and programs to prevent further loss of native plants and ultimately, extinction.

There is absolutely no doubt that alien plant invasions are eating away at native plant biodiversity.

The researchers also encourage conservationists to consider the effects of alien plant invasions in conjunction with other threats, noting that “alien plants can contribute to the further decline of native species previously affected by other threatening processes.” They encourage conservationists to focus on actions that will prevent further plant decline, and call for improved monitoring of ecosystem responses to control alien plants. Finally, they call for emphasis on the types of plants that are mostly likely to go extinct instead of on the cause of the decline.

Invasive plants and animals threaten ecosystems around the world. Many of their effects are already felt, but sometimes their consequences are unseen. Removing invasive species from the habitats they invade is an effective strategy for preventing extinctions and protecting native species. The researchers hope this framework will assist conservationists in doing just that.

Featured photo: Flower on Isla de Plata. Rory Stansbury/Island Conservation
Access the paper at Oxford Univeristy Press