Correlated recovery of five lizard populations following eradication of invasive mammals

Effects of habitat perturbation on lizard assemblages in the center-west of the Arid Chaco region, Argentina

Several human disturbances contribute to the decrease of vertebrate species' richness and abundance, altering the processes of an ecosystem. We evaluate richness, diversity and relative abundance of species for lizard assemblages at sites with different degrees of perturbation in the center-west of the Arid Chaco region in Argentina. Between 2015 and 2018, six lizard assemblages were sampled monthly -using pitfall traps- in three areas of the Chaco, with a perturbed and an unperturbed (control) replica at each of the areas: (1) Chaco Mountain plain, (2) Chaco Mountain slope, and (3) Chaco Plains, and habitat characteristics of each study site were recorded. We captured 1446 lizards, belonging to 12 species. The perturbed area at the Chaco Mountain plain showed the greatest richness, diversity and abundance of species. In the perturbed Chaco Plains, species abundance decreased by about 50% with respect to the control site. Liolaemus chacoensis was the dominant species at all sites. Some species could be negatively affected by a total loss of arboreal strata, tree trunks and fallen leaves. Structural parameters of lizard assemblages were related to the habitat characteristics; therefore, these results provide information for the conservation and management of lands and lizard assemblages in the Arid Chaco.

Effects of different management strategies on long-term trends of Australian threatened and near-threatened mammals

Monitoring is critical to assess management effectiveness, but broadscale systematic assessments of monitoring to evaluate and improve recovery efforts are lacking. We compiled 1808 time series from 71 threatened and near-threatened terrestrial and volant mammal species and subspecies in Australia (48% of all threatened mammal taxa) to compare relative trends of populations subject to different management strategies. We adapted the Living Planet Index to develop the Threatened Species Index for Australian Mammals and track aggregate trends for all sampled threatened mammal populations and for small (<35 g), medium (35-5500 g), and large mammals (>5500 g) from 2000 to 2017. Unmanaged populations (42 taxa) declined by 63% on average; unmanaged small mammals exhibited the greatest declines (96%). Populations of 17 taxa in havens (islands and fenced areas that excluded or eliminated introduced red foxes [Vulpes vulpes] and domestic cats [Felis catus]) increased by 680%. Outside havens, populations undergoing sustained predator baiting initially declined by 75% but subsequently increased to 47% of their abundance in 2000. At sites where predators were not excluded or baited but other actions (e.g., fire management, introduced herbivore control) occurred, populations of small and medium mammals declined faster, but large mammals declined more slowly, than unmanaged populations. Only 13% of taxa had data for both unmanaged and managed populations; index comparisons for this subset showed that taxa with populations increasing inside havens declined outside havens but taxa with populations subject to predator baiting outside havens declined more slowly than populations with no management and then increased, whereas unmanaged populations continued to decline. More comprehensive and improved monitoring (particularly encompassing poorly represented management actions and taxonomic groups like bats and small mammals) is required to understand whether and where management has worked. Improved implementation of management for threats other than predation is critical to recover Australia's threatened mammals.

Global economic costs of mammal invasions

Invasive alien mammals cause huge adverse ecological impact on human society and natural ecosystems. Although studies have estimated economic costs of mammal invasions at regional scales, there is lacking the large-scale comprehensive assessment of currency costs for this taxon. Here, we estimated the economic cost of invasive alien mammals on a global scale using the most comprehensive global database compiling economic costs of invasive species (InvaCost). From 1960 to 2021, mammal invasions caused costs (summing damage costs and management costs) of US$ 462.49 billion to the global economy, while the total amount of robust costs reached US$ 52.49 billion. The majority of the total economic costs corresponded to damage costs (90.27 %), while only 7.43 % were related to management cost. Economic costs showed an increasing trend over time. The distribution of costs was uneven among taxonomic groups and regions, with the global total cost highly biasing toward to 5 species (European rabbit, Domestic cat, Black rat, Wild boar and Coypu), and North America reporting much higher costs (60.78 % of total economic costs) than other regions. The total costs were borne by agriculture, environment, authorities stakeholders and other sectors. Geographic and taxonomic biases suggested that total economic costs caused by invasive alien mammals were underestimated. Integrated research efforts are needed to fill in knowledge gaps in the economic costs generated by mammal invasions and to identify the drivers of the economic costs.

Changes in native small mammal populations with removal of invasive ant

Efforts to remove invasive species may benefit native species, but the effects can be complex and unpredictable. Thus, studies of invasive-species removal provide important information for guiding management and providing insight about variation in post-removal impacts within the community. Using southern pine-grassland ecosystems as a model system, we hypothesized that removal of the long-established red-imported fire ant (Solenopsis invicta, hereafter RIFA) would positively influence altricial Peromyscus species, due to increased survival of young in the nest and thus increased recruitment to the population, but would not impact semi-precocial hispid cotton rats (Sigmodon hispidus), which are mobile more quickly after birth and thus at less risk of depredation by RIFA. We compared small mammal populations on sites treated with a granular insecticide (Extinguish Plus) to remove RIFA in southwestern Georgia, United States, from April 2018 to December 2019. As expected, we detected no difference in cotton rat recruitment. However, contrary to our prediction, the same was true for cotton mice (Peromyscus gossypinus) and oldfield mice (Peromyscus polionotus). We found RIFA removal increased survival both of cotton rats and cotton mice, increasing average population rate of change (λ) on treated sites during the study period. In contrast, we observed lower survival of oldfield mice, with similar λ estimates on treated and untreated sites, but low sample sizes were problematic for this species. Our results show that removal of invasive species can have positive impacts for native species, but both the magnitude of RIFA effects on small mammals and mechanisms by which impacts occur are complex.

Indirect effects of invasive rat removal result in recovery of island rocky intertidal community structure

Eleven years after invasive Norway rats (Rattus norvegicus) were eradicated from Hawadax Island, in the Aleutian Islands, Alaska, the predicted three-level trophic cascade in the rocky intertidal, with native shorebirds as the apex predator, returned, leading to a community resembling those on rat-free islands with significant decreases in invertebrate species abundances and increases in fleshy algal cover. Rats had indirectly structured the intertidal community via their role as the apex predator in a four-level trophic cascade. Our results are an excellent example of an achievable and relatively short-term community-level recovery following removal of invasive animals. These conservation successes are especially important for islands as their disproportionately high levels of native biodiversity are excessively threatened by invasive mammals.

Invasive rat eradication strongly impacts plant recruitment on a tropical atoll

Rat eradication has become a common conservation intervention in island ecosystems and its effectiveness in protecting native vertebrates is increasingly well documented. Yet, the impacts of rat eradication on plant communities remain poorly understood. Here we compare native and non-native tree and palm seedling abundance before and after eradication of invasive rats (Rattus rattus) from Palmyra Atoll, Line Islands, Central Pacific Ocean. Overall, seedling recruitment increased for five of the six native trees species examined. While pre-eradication monitoring found no seedlings of Pisonia grandis, a dominant tree species that is important throughout the Pacific region, post-eradication monitoring documented a notable recruitment event immediately following eradication, with up to 688 individual P. grandis seedlings per 100m2 recorded one month post-eradication. Two other locally rare native trees with no observed recruitment in pre-eradication surveys had recruitment post-rat eradication. However, we also found, by five years post-eradication, a 13-fold increase in recruitment of the naturalized and range-expanding coconut palm Cocos nucifera. Our results emphasize the strong effects that a rat eradication can have on tree recruitment with expected long-term effects on canopy composition. Rat eradication released non-native C. nucifera, likely with long-term implications for community composition, potentially necessitating future management interventions. Eradication, nevertheless, greatly benefitted recruitment of native tree species. If this pattern persists over time, we expect long-term benefits for flora and fauna dependent on these native species.

Using effect size benchmarks to assess when alien impacts are actually alien

Alien predators have on average twice the impact on native prey populations than do native predators, and are a severe threat to wildlife globally. Manipulation experiments can be used to quantify the impact of an alien predator on its prey population/s, but unless the results are compared to benchmarks, it is unclear whether this impact is indeed greater than that of a native predator. Here we use the Australian garden skink Lampropholis delicata and alien black rat Rattus rattus to test if black rats are an additive source of predation for the skink, and to judge whether the effect size of rat-impact on the skink represents that of an alien or native predator. We used replicated experiments to exclude black rats at local and landscape scales to test how rats affect skink activity and trapping frequency. Both manipulations had positive effects on skinks, however, the population-level effect size was lower than that described for alien predators but similar to that expected for native predators. We suggest that Australian skinks may respond appropriately to predatory alien rats because they coevolved with endemic Rattus species. This adds novel insights into the varying levels of impact that alien predators have on native prey.

Historical Environment Is Reflected in Modern Population Genetics and Biogeography of an Island Endemic Lizard (Xantusia riversiana reticulata)

The restricted distribution and isolation of island endemics often produces unique genetic and phenotypic diversity of conservation interest to management agencies. However, these isolated species, especially those with sensitive life history traits, are at high risk for the adverse effects of genetic drift and habitat degradation by non-native wildlife. Here, we study the population genetic diversity, structure, and stability of a classic “island giant” (Xantusia riversiana, the Island Night Lizard) on San Clemente Island, California following the removal of feral goats. Using DNA microsatellites, we found that this population is reasonably genetically robust despite historical grazing, with similar effective population sizes and genetic diversity metrics across all sampling locations irrespective of habitat type and degree of degradation. However, we also found strong site-specific patterns of genetic variation and low genetic diversity compared to mainland congeners, warranting continued special management as an island endemic. We identify both high and low elevation areas that remain valuable repositories of genetic diversity and provide a case study for other low-dispersal coastal organisms in the face of future climate change.