Niche overlap with an exotic competitor mediates the abundant niche-centre relationship for a native lady beetle

Aim: The abundant niche-centre hypothesis has been used to describe patterns of species abundance relative to position in ecological niche space. Such relationships, however, are inconsistently recovered and may be obscured due to non-equilibrium distributions, such as those caused novel interactions with exotic species. Here, we explore patterns of fitness for the nine-spotted lady (Coccinella novemnotata) following the introduction of an exotic competitor, the seven-spotted lady beetle (C. septempunctata). We examine how niche overlap between species may be causing a range reduction for C. novemnotata through a modified abundant niche-centre relationship. Location: North America. Methods: We applied a time-oriented view of the abundant niche-centre relationship for C. novemnotata, both before and after the introduction of C. septempunctata. We tested for both niche overlap and correlation between C. novemnotata body size (i.e. fitness) and the distance from its niche centroid relative to the invasion status of C. septempunctata. Results: The current niche space currently occupied by C. novemnotata is equivalent to its historic niche but has shifted to avoid overlap with C. septempunctata. A historic abundant niche-centre relationship with larger individuals closer to the niche centroid was observed for C. novemnotata. However, this pattern changed to have smaller beetles near the niche centroid when the current distribution was considered. Main Conclusions: Fitness patterns displayed by C. novemnotata are effectively described by the abundant niche-centre hypothesis. However, this relationship was modified with the introduction of C. septempunctata. Coccinella novemnotata is not excluded from previously suitable areas that correspond to niche space nearest to its niche centroid. Abundant niche-centre relationships may provide strong predictions of species' fitness across a species geographic range; however, these relationships may be obscured by novel pressures resulting in non-equilibrium states.