For many agencies and community groups interested in protecting biodiversity, a primary goal has been to protect native species and to control introduced alien species, such as feral animals and weeds.
The reality is that for many millennia flora and fauna have moved around of their own accord with changing climates, in response to the addition or removal of geographic barriers and with changing habitats.
So in the world we are facing – one of rapid global change – what should be considered native?
Essentially humans have pressed the fast-forward button on factors that strongly influence where species are distributed. Aspects of global change, such as human-induced climate change and landscape modification, are driving major shifts in species distributions.
Global biodiversity is reassembling in new combinations in both natural ecosystems and managed environments. For example, many mobile species are moving of their own accord to find suitable habitat and climates.
On top of this natural relocation, humans are moving organisms around the world, either deliberately or accidentally. We are doing this far more rapidly than species could achieve through natural means.
Land managers are also implementing “managed relocation” strategies to adapt to climate and other pressures. They are relocating species to new locations well beyond their current native range, but still within areas with environmental conditions where they can thrive.
Globally significant biodiversity guidelines, like the Convention on Biological Diversity and the International Union for the Conservation of Nature guidelines for the prevention of biodiversity loss, include definitions of natives and aliens. Managers around the world rely on these as part of their decision making tools.
The widely applied tags of “in a place due to human assistance” or “in a place before a particular date” are commonly used to define introduced aliens or natives, respectively.
But these tags are becoming increasingly inappropriate, conflicting with many management strategies proposed for conserving species. For example, with managed relocation many existing definitions don’t allow for organisms in their new ecosystems to be classed as native.
If we don’t change the way species are classified as native or alien, and we continue to rely on those tags to guide management and policy for controlling aliens and conserving natives, then the long-term conservation of global biodiversity is likely to be compromised. This increasingly outdated approach could also waste significant resources.
Can we do things better?
How we define “native” might seem like an esoteric issue. But this question of classification underpins diverse research fields including climate change adaptation, conservation biology and invasion ecology, as well as the policy frameworks informed by this work.
Scientists at the CSIRO have recently proposed a solution to this definitions conundrum. They have developed a framework that uses ecological theory to work out if a population could have moved naturally to where it is in the time available, not how or when it moved there.
To achieve this goal the scientists propose the concept of a “projected dispersal envelope”. This defines the region where a species is or could be native. The definition says that human involvement or an arbitrarily defined time frame are irrelevant.
This envelope is based on the increasingly large body of research on biogeography, dispersal and niche theory. It takes into account native species’ ability to move to better habitats, and identifies alien populations for appropriate management.
A shift in the way we view natives and aliens will also need to be accompanied by a shift in our approach to managing species in a world of rapid global change.
More and more, we will need to assess species management case by case, regardless of whether the species are native or alien. For example, rare and threatened plants that have been moved well beyond their native range due to large-scale clearing may be managed for protection rather than control, despite their alien status in the new location.
This decision would mirror the current approach to conserving rather than controlling dingoes in certain areas of Australia.
Research suggests dingoes arrived in Australia by boat with humans approximately 5000 years ago. Therefore, with no natural means to cross the Torres Strait, dingoes would be classified as aliens in Australia.
In contrast to dingoes, a range-expanding plant native to parts of eastern Australia, sweet pittosporum, is currently controlled as an environmental weed in Victoria. While the reasons for it inhabiting new areas are complex (including garden plantings, changes in fire regimes and introduced birds now dispersing its seeds), the new framework suggests that in many regions, this plant should be viewed as a range-expanding native rather than a threatening alien.
A new framework for defining natives and aliens may well bring clarity to those who are trying to understand how best to deal with this challenge. While this is a significant issue for managing species with rapid global change, it has never previously been addressed.
If we remember that all species can potentially be invasive depending on the context, in the future we may need to reconsider our decision to conserve or control even more often that we do today. Both management choices will have significant impacts for maximising global biodiversity in a rapidly changing world.