Invasive Bromus tectorum alters natural selection in arid systems

While much research has documented the impact of invaders on native communities and ecosystem services, there has been less work quantifying how invasion affects the genetic composition of native populations. That is, when invaders dominate a community, can they shift selection regimes and impact the evolutionary trajectory of native populations?
The invasion of the annual grass Bromus tectorum in the Intermountain West provides an opportunity to quantify the effects of invasion on natural selection in wild populations. The shift from a perennial-dominated native community to one dominated by a highly competitive annual species alters the timing and intensity of competitive pressure, which has the potential to strongly shift selection regimes for native plants.
To quantify traits under selection in contrasting environments, we planted seeds of two native perennial grasses, Elymus multisetus and Poa secunda, into three invaded, invaded but weeded and relatively uninvaded sagebrush systems. We quantified phenotypic traits of seedlings from separate maternal plants, describing differences in phenotypes among individuals. We then asked which traits were associated with survival and plant size in adjacent invaded and uninvaded sagebrush systems, following individual seed performance for 3 years.
We found evidence for divergent selection between invaded and uninvaded sagebrush systems, with contrasting phenotypic traits associated with greater survival or plant size in these different growing conditions at all three field sites. For example, at one field site, P. secunda families with higher root tip production were more likely to survive in invaded and weeded environments, but this was not the case in uninvaded environments. Similarly, for E. multisetus, root mass fraction, seed mass and allocation to coarse or fine roots affected survival and plant size, again with contrasting relationships across invaded, weeded or uninvaded environments.
Synthesis. Impacts of invasive species extend beyond ecosystem and community composition changes and can affect the evolutionary trajectory of native populations. By quantifying natural selection in invaded landscapes, we identified phenotypic traits that are potentially adaptive in these invaded systems. Importantly, these traits differed from traits associated with success in uninvaded communities. This insight into adaptive, contemporary evolution in native species can guide restoration and conservation efforts.

尽管已有大量研究阐明了外来入侵物种对本地群落及生态系统服务的影响，但针对入侵如何改变本地种群遗传组成的定量研究仍较为匮乏。换言之，当外来入侵物种主导群落时，它们能否改变本地种群所面临的选择格局，并进而影响其进化轨迹？
北美西部内陆山地的一年生草本植物旱雀麦（Bromus tectorum）入侵，为定量研究入侵对野生种群自然选择的影响提供了理想研究体系。原本以多年生植物为主的本地群落被竞争力极强的一年生物种替代后，种间竞争的发生时序与强度均发生改变，这极有可能大幅重塑本地植物所面临的选择格局。
为定量分析异质环境下受选择作用的功能性状，我们将两种本地多年生草本——多花偃麦草（Elymus multisetus）和加拿大早熟禾（Poa secunda）的种子，分别种植于三类蒿灌丛生境中：完全入侵生境、入侵但已清除入侵物种的生境，以及相对未入侵的生境，共设置3个重复样地。我们对来自不同母株的幼苗表型性状进行了定量测定，以刻画个体间的表型差异。随后，我们对单粒种子的定植表现进行了为期3年的跟踪监测，以此探究在相邻的入侵与未入侵蒿灌丛生境中，哪些性状与幼苗存活及植株体量存在显著关联。
我们在所有3个样地中均发现，入侵与未入侵的蒿灌丛生境间存在显著的歧化选择：在这两类不同的生境中，与更高存活率或更大植株体量相关的表型性状存在显著差异。例如，在其中一个样地中，加拿大早熟禾的家系若根尖产生量更高，则在入侵及清除入侵的生境中存活率更高，但这一关联在未入侵生境中并不存在。类似地，对于多花偃麦草而言，根质量分数、种子质量以及粗/细根分配比例均会影响其存活与植株体量，且在入侵、清除入侵及未入侵生境中，这些性状与适合度指标的关联模式同样存在显著差异。
研究综合与结论：外来入侵物种的影响并非仅局限于改变生态系统与群落组成，其还可对本地种群的进化轨迹产生作用。通过定量测定入侵生境中的自然选择作用，我们鉴定出了在入侵生境中具有潜在适应性的表型性状。值得注意的是，这些性状与未入侵群落中与物种成功定植相关的性状并不相同。此项针对本地物种适应性当代进化的研究结果，可为物种恢复与保护工作提供科学指导。