The success of mergers and acquisitions (M&As), particularly in knowledge-intensive industries, depends heavily on human capital dynamics. This study examines how acquirer inventors collaborate with the incoming target workforce, focusing on joint invention―a critical but often overlooked aspect of post-acquisition integration. Drawing on niche theory, we investigate how acquirer inventors respond to niche crowding caused by the addition of the target workforce. We explore how they manage the tension between cooperation and competition in joint invention efforts. Using data from inventors in the U.S. high-tech sector involved in acquisitions between 2002 and 2015, we find that acquirer inventors are more likely to collaborate when their competitiveness is relatively low―marked by declining performance and peripheral positions in the inventor network. In contrast, when both groups operate in similar technological domains or within related acquisitions, competitive pressure intensifies. This often shifts the dynamic toward rivalry, reducing joint invention engagement. These findings contribute to the literature on post-acquisition integration and extend niche theory by showing how niche crowding shapes the balance between cooperation and competition among merged workforces.