Alien species are also known as invasive species. They can be any kind of organism that is not native to an ecosystem, which causes harm to the environment, the economy, human health or conservation and biodiversity. When an alien species is introduced to an ecosystem it may not have any natural predators to control its population and it may spread and take over an area quickly. Alien species may out-compete native species which may remove them from the area or even cause them to go extinct. This can have larger ecosystem impacts such: changing food webs by removing or replacing native food sources, decreasing biodiversity by altering the abundance and diversity of species, or altering ecosystem conditions such as soil chemistry.

Climate change and human population pressures are expected to impact alien species invasions. Climate change has already caused the Earths temperature to increase by 0.85°C and temperatures are continuing to rise, this increase in temperature will enable some alien species to survive in areas where they weren’t able to before. No ecosystem on earth is free from the influence of humans and many ecosystems are completely dominated by humanity; this has been intensified by the exploding human population size and the increase in individual resource demands. Human activities such as global trade, transport and tourism have expanded movements of organisms globally, which means the introduction of species into new areas is rapidly accelerating, which has caused a rearrangement of earths biotic systems, through the mixing of species, which in the past have been isolated from each-other. Which means, it is becoming increasingly important to understand how climate change and human pressures will interact to shape patterns of alien and native species richness and distribution.

Alien plant species share similar characteristics which help them invade, such as: fast growth and maturity, high seed production, successful seed dispersal; germination and colonization, rampant vegetative spread and the ability to out-compete native species. Whereas native plant species have different characteristics which may make them less competitive. Therefore alien and native plant species are expected to respond differently to global change.

Marini et al. (2012), including Phil Hulme from Lincoln University, conducted a study in the European Alps in Italy which investigated whether differences in native and alien species richness was dependent on climate or human pressures. In this study they used elevational gradients because they are relatively short distances in which there are varying degrees of human and climate pressures. Information about native and alien plant species richness was obtained from existing datasets of over 495,000 records of plant species distributions. Human population and climate data was also obtained from existing datasets.

The main results from this study showed that native and alien plant species do respond differently to human and climatic drivers of global change. Alien species were more affected by human and climate pressures than native species were. Native plant species richness was found to be the highest at mid elevations, but this was not found to be related to climate or human pressures, which suggests that other factors must determine native species distributions. Alien plant species richness was found to be highest at lower elevations where it is warmer and there are more humans. This result supports that human pressures, such as the introduction of the species to the area and the availability of new niches created by human-caused environmental conditions are the dominant drivers of alien species distribution patterns, rather than climatic pressures. High elevations have a harsher climate and difficult access, which means generally there are less humans in these areas and therefore alien plant species are less likely to be introduced to high elevation areas. Although this may change with global warming as human activities are expected to move upwards with with an increase in temperature, and this is likely to lead to an increase in the introduction of alien plant species and a decline in the abundance of native plant species.

It is well known that alien plant species are expected to become more problematic with future climate warming, but as Marini et al. (2012) showed human pressures may play an even larger part, especially at high elevations which currently are less affected by alien species. Global change will also continue to affect plant species distributions at low elevations which will impact everyday situations. Climate change may make a more favorable environment for many species either already in the area or by allowing them to survive in an area where they couldn’t before. It has been estimated that between 30 and 80% of invasive plant species originated as garden plants. This means some exotic plant species already in your garden may not be causing any problems now, but in the future could become weeds which could invade your garden or interfere with other species that you do want to grow. Human pressures will also continue to increase with the steady population growth already taking place and also due to rapid development in technology which will allow for even more transportation of species around the world.

Simple ways you personally can help control alien species:

• Remove introduced species from your garden and replace them with natives.
• Volunteer for organisations which work to remove alien species from natural areas.
• Learn to identify alien species, or if you see any species that look suspicious, report them.
• When traveling internationally, make sure you don’t pack any whole or parts of plants, soil or animals and clean any equipment or clothing (e.g. hiking boots) that has been used for outdoor activities.
• The author Georgie Stevenson is a postgraduate student at Lincoln University. She wrote this article as part of her assessment for ECOL 608 Research Methods in Ecology.