Species expanding into new habitats as a result of climate change or human introductions will frequently encounter resident competitors. Theoretical models suggest that such interspecific competition can alter the speed of expansion and the shape of expanding range boundaries. However, competitive interactions are rarely considered when forecasting the success or speed of expansion, in part because there has been no direct experimental evidence that competition affects either expansion speed or boundary shape. Here we demonstrate that interspecific competition alters both expansion speed and range boundary shape. Using a two-species experimental system of the flour beetles Tribolium castaneum and Tribolium confusum, we show that interspecific competition dramatically slows expansion across a landscape over multiple generations. Using a parameterized stochastic model of expansion, we find that this slowdown can persist over the long term. We also find that the shape of the moving range boundary changes continuously over many generations of expansion, first steepening and then becoming shallower, due to the competitive effect of the resident and density-dependent dispersal of the invader. This dynamic boundary shape suggests that current forecasting approaches assuming a constant shape could be misleading. More broadly, our results demonstrate that interactions between competing species can play a large role during range expansions and thus should be included in models and studies that monitor, forecast, or manage expansions in natural systems.

由于气候变化或人类引进而扩展到新的栖息地的物种将经常遇到居民竞争者。理论模型表明，这种种间竞争可以改变扩展速度和扩展范围边界的形状。但是，在预测扩张的成功或速度时很少考虑竞争相互作用，部分原因是没有直接的实验证据表明竞争会影响扩张速度或边界形状。在这里，我们证明种间竞争改变了扩张速度和范围边界形状。使用面粉甲虫Tribolium castaneum和Tribolium confusum的两种实验系统，我们证明种间竞争极大地减缓了多代人在整个景观中的扩张。使用参数化的随机扩展模型，我们发现这种放缓可以长期持续。我们还发现，由于入侵者的驻地竞争和密度依赖性扩散的竞争效应，移动范围边界的形状在许多代扩展中连续变化，首先变陡，然后变浅。这种动态边界形状表明，假设形状不变的当前预测方法可能会产生误导。更广泛地说，我们的结果表明，竞争物种之间的相互作用可以在范围扩展过程中发挥重要作用，因此应纳入监测，预测或管理自然系统扩展的模型和研究中。