Mapping invasive hybrid swarms and swarms of hybrids

Background/Question/Methods

Alien species generally enter their new ecosystem as a small founder population, presumably with low genetic diversity causing a bottleneck for population growth. However, a species may be able to overcome bottlenecks by adapting to novel environments, or hybridizing with similar native or alien species. Genetic diversity might be further augmented by multiple invasions from a single location over time, or multiple invasions from several locations, and further cross-breeding and hybridization creating a ‘hybrid swarm’ to previous and new habitats. Classic examples include the hybrids of native California cordgrass, Spartina foliosa, with Spartina alterniflora from the Atlantic coast, and the hybrids of the escaped ornamental shrub, Rosa rugosa, with the native R. blanda in eastern North America.  Our objectives were to quantify the geographic range and environmental ranges of several hybrid taxa compared to parent genotypes or species using species-environment matching (SEM) models (MaxEnt in this case). We hypothesize that many hybrids will have broader potential geographic and environmental distributions than their parents. We test the hypothesis on the hybrids of Tamarix ramosissima and T. chinensis throughout the western U.S. We also assess the potential hybrid zone of Phragmites australis in North America.

Results/Conclusions

Our preliminary analysis of the hybrids of Tamarix ramosissima and T. chinensis showed broad distributions of both parent species throughout the western U.S. The SEM models for parent taxa resulted in high predictability (AUC >0.90).  However, the hybrids of T. ramosissima X T. chinensis may become more broadly distributed, indicating the hybrids may be a generalist invader (AUC = 0.80).  In all models, predictor variables included seasonal patterns of temperature and precipitation, and distance to water.  Preliminary analysis of Phragmites genotypes showed highly predictable suitable habitat. The potential hybrid zone (AUC = 0.95) appeared to be broader than the native parent (AUC = 0.95), with potentially greater tolerance to warmer and colder temperatures. We review other case studies from plants and animals (e.g., Africanized Honeybees) showing an alarming number of hybrids may be spreading more broadly than parent genotypes forming a ‘swarm of hybrids.’ We conclude that hybrids of invasive species may become more commonplace in many ecosystems.