Are non-native species always bad?

Many conservation and rewilding efforts aim to eradicate so-called invasive species and replace them with natives. But is this the right approach in every case?

Are non-native species always bad?
A gulf fritillary in California foraging on non-native Rubus armeniacus, or Himalayan blackberry. Photo: TJ Gehling

This article by Heather Kharouba and Stephanie A. Rivest of the University of Ottawa was originally published by The Conversation and has been reproduced here with permission.

Whether or not invasive species threaten native biodiversity and ecosystems has been a point of debate amongst researchers for years.

Invasive species have caused extinctions of native species and even altered the functioning of ecosystems. But not all species that are introduced to new areas become invasive — meaning they cause negative impacts.

Despite this, all non-native species are tagged as harmful. This way of thinking has caused conservation management actions to typically focus on the eradication of non-native species with the goal of restoring “natural” landscapes. It has led scientific research to focus on identifying the negative consequences of non-native species. And it has stripped these species of their role as biodiversity indicators used to describe the state of the environment.

Collectively, this has perpetuated the idea that native species are good and non-native species are bad. But what if we’re wrong about non-native species?

Some non-native species do good

While we should continue to be wary of non-native species, particularly those that have overwhelmingly negative impacts, the reality is that the role of most non-native species in ecological communities is uncertain or complex.

Some non-native species are a leading cause of species extinctions whereas others contribute to regional biodiversity.

Increasingly, scientists are reporting examples of positive roles that non-native species play such as providing food to native species, creating habitats or playing a role in ecosystem restoration. For example, native butterflies in Vancouver Island’s endangered Garry oak savanna ecosystem were found using non-native flowers for nectar, particularly in late summer when native flowers are scarce.

A bird seen through large blades of grass in a marsh
Today, California’s ridgeway rail species partly relies on non-native cordgrass to escape predators and build nests. Photo: Doug Greenberg.

A more complex example is the case of the California ridgeway rail, an endangered bird native to the marsh habitats in the San Francisco Bay area. Since the beginning of the 19th century, this species has been declining dramatically primarily because of the destruction of marsh habitats for agriculture and urban development.

Today, this bird relies, in part, on non-native salt-water cordgrass, introduced to California in the 1970s, to escape from predators and build nests. The non-native cordgrass enhances habitats for the birds by providing more tall plant cover and increasing the area of marsh habitat. Efforts to eradicate the cordgrass throughout the 2000s led to declines in the rail’s population size, demonstrating how a non-native species could be used to restore habitats and support some native species.

However, the cordgrass also converts mudflats into tidal wetlands, negatively impacting shorebirds that need mudflat habitats for foraging. Today, management efforts balance smaller removals of the cordgrass with monitoring of the rails.

Managing non-native species

As ecologists researching non-native species for over 10 years, we believe that the negative perception of non-native species and the one-sided aggressive approach to their management may be leading scientists, conservation practitioners and policymakers to underestimate the positive roles that these species can play in ecological communities.

Controlling non-native species is expensive, time-consuming and can be ineffective in the long term. Spending money, time and effort eradicating non-native species that have neutral or positive effects on native species is wasteful as those limited resources could be put to better use.

In some regions and cities, non-native species make up more than half of all species. Imagine the effort that would be needed to control all of the non-native species in these locations.

We need to rethink the idea that all non-native species are harmful. We need to evaluate the full range of impacts of non-native species before taking action against them. And eradication efforts should be reserved for situations where they will have the greatest benefits.

purple flowers and green grass in the foreground, a large tree and blue sky in the background
A Garry oak meadow in Victoria, B.C. Photo: Alanah Nasadyk.

Science shows the way

We suggest two ways through which science can help move us forward.

First, rigorous biological research can help prioritize which non-native species should be removed and which can be left undisturbed. In cases where non-native species need to be removed, such studies can also tell us when additional management will be needed to support the existing native community.

In the Garry oak ecosystem on Vancouver Island, non-native flowers that are highly visited by native butterflies could be left alone, whereas others that are not visited could be prioritized for removal. If all non-native flowers must be removed, then managers could plant native flowers to ensure butterflies and other pollinators are left with sufficient food resources.

Second, researching the net effects of non-native species in ecological communities can improve the effectiveness of our conservation strategies. We need more studies that explicitly consider both positive and negative impacts of the same non-native species.

We are not suggesting that we should abandon our efforts to mitigate serious problems caused by some non-native species, or that governments should stop trying to prevent potentially harmful species from entering their jurisdictions.

Rather, we urge conservation practitioners and policymakers to organize and prioritize the management of habitats around whether species are more beneficial or harmful to biodiversity.

The Conversation