Invasive species and extreme weather events (EWEs) are both major threats to native species, but their joint impacts may firstly be determined by the responses of invasive species to EWEs. Although previous studies based on limited taxa and regional scales revealed that invasive species may outperform their native counterparts due to more rapid growth rates, stronger competitive abilities, higher phenotypic plasticity, broader tolerance of disturbance, and quicker recovery and proliferation, a global synthesis across taxonomic groups at different ecosystems is still lacking.
At the beginning, I read case studies and found that both positive and negative responses of animal species to EWEs such as heatwaves, cold spells, and droughts (Fig. 1). Those responses were associated with diverse levels from physiology, life history traits, abundance, to community composition. For example, population abundance of native shellfish and corals are vulnerable to heatwaves. Many insect species have weak ability to adjust their physiological thermal limits to resist extreme high temperatures. However, specific to non-native species, I find that some invaders exhibit more tolerance and event positive responses to EWEs than natives. For example, severe drought events resulted in mass mortality of native fishes and facilitated non-native counterparts to occupy vacant niches to establish populations. Non-native arthropods, mammals, and shellfishes have more rapid growth rates, higher phenotypic plasticity, broader tolerance of disturbance, and quicker recovery than natives after experiencing EWEs. So, whether responses of non-native species to EWEs would be completely different to native species. Answering this question needs a comprehensive evaluation of non-native species responses to EWEs from gene, population, community, and ecosystem levels across multiple taxonomic groups with different adaptations, or across a global scale covering a diverse extent of EWEs types.
Under combining efforts of Prof. Liu, Prof. Jason Rohr, and master candidate Tianyi Qi, we start a comprehensive study using a meta-analysis approach to quantify positive, neutral, and negative responses of native and non-native species to EWEs. Firstly, we conducted a systematic literature search and screened a total of 147,212 unique studies. Specifically, we chose native species with the same class-level of non-native species to conduct the comparative analyses. Finally, we built a database covering 187 non-native and 1,852 native species experiencing different kinds of EWEs including heatwaves, cold spells, storms, floods, and droughts across seventeen classes in terrestrial, freshwater, and marine ecosystems in the past seventy years (Fig. 2).
Preliminary analysis supports our assumption that non-native and native animals had large differences in proportions of positive, negative, and neutral responses to EWEs. The proportion of positive responses of non-natives was 95.3% higher than natives. However, surprisingly, the proportion of negative responses of non-natives was also higher (55.1%) than natives. We then applied multilevel mixed-effects meta-regression analyses to compare the responses of non-native and native animal species across terrestrial, freshwater, and marine ecosystems to different types of EWEs. Our results show that less sensitivity of non-native animals to EWEs were commonly found in terrestrial, freshwater, and marine ecosystems. Non-native terrestrial and freshwater animals were only affected by heatwaves and storms, respectively. Native terrestrial animals negatively responded to heatwaves, cold spells, and droughts. Native freshwater animals negatively responded to most events apart from cold spells. For marine species, except for native molluscs, corals, and anemones negatively responding to heatwaves, other non-native or native marine animals were insensitive to EWEs (Fig. 3). Thus, both ‘winners’ and ‘losers’ were true for non-native and native species, but proportionally there were more positive responses of non-native than native animals to EWEs.
To further explore potential joint threats from invasive alien species and extreme weather, we calculated positive, negative, and neutral response ratios of each non-native species to EWEs, and sum of positive and neutral response ratios minus negative response ratios for each species to get a net response ratio. When the net response ratio was greater than or equal to zero, it meant the species was insensitive to EWEs. We overlapped potential habitat areas predicted by ecological niche modeling with the net response ratio of each species and obtained hotspots of the combined risks of EWEs and biological invasions (Fig. 4). Our results indicated that we may pay close attentions to Northern America, Latin America, Mediterranean, South Africa, East and Southeast Asia, southwest Australia and New Zealand, west coast and islands in the Pacific Ocean, and North Atlantic Ocean, where native species may be particularly vulnerable to their joint effects.
There is no ending work of any meta-analysis. We are conducting further empirical studies to better understand how non-native species take advantage to tolerate EWEs.