Assessing threats of non-native species to native freshwater biodiversity: Conservation priorities for the United States

Abstract

Non-native species pose one of the greatest threats to native biodiversity, and can have severe negative impacts in freshwater ecosystems. Identifying regions of spatial overlap between high freshwater biodiversity and high invasion pressure may thus better inform the prioritization of freshwater conservation efforts. We employ geospatial analysis of species distribution data to investigate the potential threat of non-native species to aquatic animal taxa across the continental United States. We mapped non-native aquatic plant and animal species richness and cumulative invasion pressure to estimate overall negative impact associated with species introductions. These distributions were compared to distributions of native aquatic animal taxa derived from the International Union for the Conservation of Nature (IUCN) database. To identify hotspots of native biodiversity we mapped total species richness, number of threatened and endangered species, and a community index of species rarity calculated at the watershed scale. An overall priority index allowed identification of watersheds experiencing high pressure from non-native species and also exhibiting high native biodiversity conservation value. While priority regions are roughly consistent with previously reported prioritization maps for the US, we also recognize novel priority areas characterized by moderate-to-high native diversity but extremely high invasion pressure. We further compared priority areas with existing conservation protections as well as projected future threats associated with land use change. Our findings suggest that many regions of elevated freshwater biodiversity value are compromised by high invasion pressure, and are poorly safeguarded by existing conservation mechanisms and are likely to experience significant additional stresses in the future.

Keywords: Freshwater biodiversity; Invasive species; Non-native species; Priority mapping; Threatened and endangered species.

Fig. 1.

Native biodiversity metrics, mapped at HUC8. Metrics were calculated for major taxonomic groups, including fish (A, F, K), amphibians (B, G, L), invertebrates (crustaceans and mollusks; C, H, M), and turtles (D, I, N) and also aggregated across all taxa (E, J, O). Shown are the total richness (species count; A–E), the count of IUCN threatened and endangered species (F–J), and the summed rarity index across species (K–O). Note that the color legend varies across maps.

Fig. 2.

Non-native species richness (A) and Cumulative Invasion Pressure (CIP; B) per HUC8. CIP is the log transformed sum of years present for all non-native species in the watershed.

Fig. 3.

Priority mapping of HUC8 watersheds. The water-sheds with an index value 1.5 standard deviations above the mean or greater (priority score ≥ 0.49) are considered to be our priority areas. A total of 58 watersheds fall into 11 priority areas; region numbering corresponds to Table 1. HUC2 watershed boundaries are shown in gray.

Fig. 4.

The boundaries of 58 watersheds in 11 priority areas displayed over protected areas (A) and future change in development (B). The protected areas fall into three categories of protection according to the U.S. Geological Survey Gap Analysis Program (GAP). Change in percent developed per HUC8 was derived from FORE-SCE models of future land use and land cover change. The rates shown are the difference from year 2000 to 2060.

Fig. 5.

Downscaled map of priority watersheds within the Tennessee River drainage, comprising the majority of priority area 1. A) Political boundaries and major geographic features; B) land cover map showing overlay with priority watersheds (red line fill). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)