The relationship between eDNA particle concentration and organism abundance in nature is strengthened by allometric scaling.

Organism abundance is a critical parameter in ecology, but its estimation is often challenging. Approaches utilizing eDNA to indirectly estimate abundance have recently generated substantial interest. However, preliminary correlations observed between eDNA concentration and abundance in nature are typically moderate in strength with significant unexplained variation. Here, we apply a novel approach to integrate allometric scaling coefficients into models of eDNA concentration and organism abundance. We hypothesize that eDNA particle production scales nonlinearly with mass, with scaling coefficients < 1. Wild populations often exhibit substantial variation in individual body size distributions; we therefore predict that the distribution of mass across individuals within a population will influence population-level eDNA production rates. To test our hypothesis, we collected standardized body size distribution and mark-recapture abundance data using whole-lake experiments involving nine populations of brook trout. We correlated eDNA concentration with three metrics of abundance: density (individuals/ha), biomass (kg/ha) and allometrically scaled mass (ASM) (∑(individual mass0.73 )/ha). Density and biomass were both significantly positively correlated with eDNA concentration (adj. r2  = 0.59 and 0.63, respectively), but ASM exhibited improved model fit (adj. r2  = 0.78). We also demonstrate how estimates of ASM derived from eDNA samples in "unknown" systems can be converted to biomass or density estimates with additional size-structure data. Future experiments should empirically validate allometric scaling coefficients for eDNA production, particularly where substantial intraspecific size distribution variation exists. Incorporating allometric scaling may improve predictive models to the extent that eDNA concentration may become a reliable indicator of abundance in nature.

Looking at both sides of the invasion: patterns of colonization in the violet tunicate Botrylloides violaceus.

Understanding the ecological and evolutionary forces that shape the genetic structure of invasive populations and facilitate their expansion across a large spectrum of environments is critical for the prediction of spread and management of ongoing invasions. Here, we study the dynamics of postestablishment colonization in the colonial ascidian Botrylloides violaceus, a notorious marine invader. After its initial introduction from the Northwest Pacific, B. violaceus spread rapidly along the Pacific and Atlantic coasts of North America, impacting both aquaculture facilities and natural ecosystems. We compare genetic diversity and patterns of gene flow among 25 populations (N=679) from the West and East coasts, and evaluate the contribution of sexual vs. asexual reproduction to this species' invasion success using data from the mitochondrial cytochrome c oxidase subunit I (COI) gene and 13 nuclear polymorphic microsatellite loci. Our results reveal contrasting patterns of spread in the coastal waters of North America. While the West coast was colonized by noncontiguous (long-distance) dispersal, the East coast invasion appears to have occurred through contiguous (stepping-stone) spread. Molecular data further indicate that although dispersal in colonial ascidians is predominantly achieved through sexually produced propagules, aquaculture practices such as high-pressure washing can facilitate fragmentation and potentially exacerbate infestations and spread via asexual propagules. The results presented here suggest that caution should be used against the general assumption that all invasions, even within a single species, exhibit similar patterns of colonization, as highly contrasting dynamics may transpire in different invaded ranges.

The noncosmopolitanism paradigm of freshwater zooplankton: insights from the global phylogeography of the predatory cladoceran Polyphemus pediculus (Linnaeus, 1761) (Crustacea, Onychopoda).

A major question in our understanding of eukaryotic biodiversity is whether small bodied taxa have cosmopolitan distributions or consist of geographically localized cryptic taxa. Here, we explore the global phylogeography of the freshwater cladoceran Polyphemus pediculus (Linnaeus, 1761) (Crustacea, Onychopoda) using two mitochondrial genes, cytochrome c oxidase subunit I and 16s ribosomal RNA, and one nuclear marker, 18s ribosomal RNA. The results of neighbour-joining and Bayesian phylogenetic analyses reveal an exceptionally pronounced genetic structure at both inter- and intra-continental scales. The presence of well-supported, deeply divergent phylogroups across the Holarctic suggests that P. pediculus represents an assemblage of at least nine, largely allopatric cryptic species. Interestingly, all phylogenetic analyses support the reciprocal paraphyly of Nearctic and Palaearctic clades. Bayesian inference of ancestral distributions suggests that P. pediculus originated in North America or East Asia and that European lineages of Polyphemus were established by subsequent intercontinental dispersal events from North America. Japan and the Russian Far East harbour exceptionally high levels of genetic diversity at both regional and local scales. In contrast, little genetic subdivision is apparent across the formerly glaciated regions of Europe and North America, areas that historical demographic analyses suggest that were recolonized just 5500-24 000 years ago.

Dispersal of the Ponto-Caspian amphipod Echinogammarus ischnus: invasion waves from the Pleistocene to the present.

The geographical range of the amphipod crustacean Echinogammarus ischnus has expanded over the past century from the Ponto-Caspian region to Western Europe, the Baltic Sea, and the Great Lakes of North America. The present study explores the phylogeographic patterns of this amphipod across its current distribution, based on an examination of nucleotide diversity in the mitochondrial cytochrome c oxidase subunit I (COI) gene. Marked genetic divergence exists among populations of E. ischnus from the Black and Caspian Seas, as well as those from the drainage system of the Black Sea. This divergence suggests the prolonged geographic isolation of these native populations, reflecting the limited dispersal capability of E. ischnus. By contrast, invading populations are characterized by a lack of genetic variation; a single mitochondrial genotype of Black Sea origin has colonized sites from the Rhine River to North America. The dispersal pattern in E. ischnus is very similar to that in the Ponto-Caspian cladoceran Cercopagis pengoi. Despite their contrasting life history strategies, these invading species followed the same route of invasion from the northern Black Sea to the Baltic Sea region, and subsequently to North America.