Extreme weather events may be driving replacement of native species with exotic ones

Extreme weather events are affecting land, marine and freshwater ecosystems by influencing the displacement of native species with non-native ones, a recent analysis has found. 

The increasing frequency of extreme weather events such as heatwaves, cold waves, droughts and floods due to climate change is influencing ecosystems, said researchers at Chinese Academy of Sciences in Beijing, China.

The findings were recently published in the journal Nature Ecology & Evolution. The scientists looked at 443 studies and analysed 1,852 native and 187 non-native species residing in land, marine and freshwater habitats and their responses to extreme weather events.

Marine animals overall remained insensitive to extreme weather events, irrespective of whether they were native or non-native, the analysis said. However, native molluscs, corals and anemones showed negative effects due to heatwaves. 

Heatwaves and storms affected non-native species in terrestrial and freshwater habitats, respectively. Native animals responded adversely to heatwaves, droughts and cold spells in terrestrial ecosystems and displayed more vulnerability to extreme weather events. 

On average, non-native species were less abundant in terrestrial ecosystems and their body conditions and life history traits were affected in freshwater systems. “But native animals indicated declines in body conditions, life history traits, abundance, distribution and recovery in terrestrial ecosystems and community structure in freshwater ecosystems,” the study stated.

“Overall, we found that non-native species had 24.8 per cent positive, 31.8 per cent negative and 43.4 per cent neutral responses (confidence intervals crossing zero) to extreme weather events. Native species had 12.7 per cent positive, 20.5 per cent negative and 66.8 per cent neutral responses to extreme weather events. Both non-native and native species exhibited positive, negative and neutral responses to each type of extreme weather events,” the researchers said.

Native terrestrial animals were affected harder by heatwaves, cold spells and drought. Native freshwater species, on the other hand, were vulnerable to most events except cold spells, the research showed. 

Non-native terrestrial animals, however, were affected only by heatwaves, while their counterparts in freshwater animals showed susceptibility to only storms. Non-native marine animals remained largely unaffected by most disturbances.

The scientists noted that though there are both ‘winners’ and ‘losers’ across native and non-native species ecosystems, the non-natives surpassed the native in positive responses to extreme weather events.

There are several possible explanations for why non-native animals show less sensitivity to extreme weather events compared to native species belonging to the same taxonomic class, suggested the researchers.

“Severe drought vents deceased native invertebrates and fishes by increasing water salinity, facilitating the establishment of non-native salt-tolerant counterparts,” they said.

Another reason is that the non-native or invasive species have higher growth rates, higher phenotypic plasticity, stronger competitive abilities, quicker recovery and proliferation and broader tolerance of disturbance compared to the native species.

“The abundance of most native fish in the Rio Minho estuary, Portugal declined, but the abundance of non-native fish increased after extreme droughts and floods, and thus the fish assemblage there was dominated by a few invasive fish species after extreme weather events. Non-native mesozooplankton species exhibit higher flexibility to marine heatwaves than native species in the Sevastopol Bay,” the analysis stated.

Noting another example of higher plasticity shown by non-natives to natives, the researchers said, “The abundance of the invasive South American tomato pinworm was tolerant of acute and chronic temperature stress because of high thermal plasticity in invaded ranges.”

Plasticity is the capacity of an individual organism to alter its behavior, physiology / gene expression, and / or morphology in direct response to changing environmental conditions.

A species of an invasive prawn was another example. It showed a higher plasticity of upper thermal limits than studying certain effects in native prawns and was thus less vulnerable to extreme thermal events. 

However, the researchers advised caution due to biases, as the majority of the studies examined belonged to North America or Western Europe.