Abstract:

Aims: Biological invasions are a vital component of research in invasion biology, biogeography, ecology and other disciplines. Better understanding drivers of the invasion process is essential to predict and prevent future invasions. Controversy persists over whether the niche of invaded range has shifted compared to native range. The diffusion process of invasive species is complex. In addition to directly transferring from a native to an invaded range, species can transfer from the initial invaded range to other invaded ranges. This phenomenon is known as the bridgehead effect and is commonly present in the diffusion process of invasive species. Therefore, the bridgehead effect must be considered when assessing the biogeography, intercontinental mobility, and climatic niche changes of invasive species. Our study compared the climatic niche progression of populations in native, bridgehead, and secondary invaded ranges of Solenopsis invicta. We uncovered the ecological mechanisms underlying the successful invasion of S. invicta to provide new insights into the niche conservatism hypothesis.

Methods: We used global distribution data and environmental variables of S. invicta obtained from http://www.moa.gov.cn. Based on pre-analysis results of the ecological niche model, we selected the environmental variables with a greater contribution to the distribution of the species. A two-by-two comparative analysis of the climatic niche of populations in native range (South America), bridgehead range (United States), and secondary invaded range (China) was done by using the “ecospat” package. The test of equivalency and similarity of the niche was performed to assess the degree of niche overlap. Additionally, the environmental space occupied by the climatic niche of S. invicta was categorized into stability, unfilling, and expansion. Further, the size of the expansion was used to assess the extent of niche displacement in the invaded range.

Results: The climatic niche overlap between South American and United States populations of S. invicta was relatively low (Schoener’s D = 0.06, Hellinger’s I = 0.20), with high niche stability (93.78%) and low expansion (6.22%). The climatic niche overlap between South American and Chinese populations of S. invicta was also low (Schoener’s D = 0.01, Hellinger’s I = 0.05), with high niche stability (97.60%) and low expansion (2.40%). However, considering the bridgehead effect, the climatic niche overlap between the bridgehead range (United States) and the secondary invaded range (China) populations was relatively high (Schoener’s D = 0.34, Hellinger’s I = 0.51), with low niche stability (28.08%) and high expansion (71.92%). The degree of ecological niche conservancy was highest, and the degree of deviation lowest, between the native range and secondary invaded range, which was consistent with the result of ecological niche overlap between the two regions. Although the ecological niche conservancy degree between the native range and bridgehead range was relatively high and the degree of deviation low, the ecological niche conservancy degree between the bridgehead range and secondary invasion interval was extremely low. There was also a significant deviation in the ecological niche. During the invasion process, S. invicta first spread to adjacent United States, which served as a center for further invasion, leading to subsequent invasions into regions such as China. This process is known as the bridgehead effect. The degree of climate niche overlap between South America and the United States and China was relatively low, while the degree of climate niche overlap between the United States and China was relatively high. Further, the current process of trade globalization centered on the United States has broken the limitations of long-distance geographic space and led to the rapid adaptation of S. invicta from the United States to China, resulting in ecological niche shift.

Conclusion: The current trade globalization process, with United States as the center, is breaking down long-distance geospatial limitations. Thus, the invasion of populations from this bridgehead range to China has rapidly adapted to similar climatic environments. On this basis, the adaptability of populations in the bridgehead range implies the importance of the bridgehead effect in facilitating the invasion of S. invicta globally.

Key words: climatic niche, Solenopsis invicta, biological invasion, bridgehead effect, niche shift