Pablo García-Díaz

Understanding the biological invasion risk posed by the global wildlife trade: propagule pressure drives the introduction and establishment of Nearctic turtles

Biological invasions are a key component of human-induced global change. The continuing increase in global wildlife trade has raised concerns about the parallel increase in the number of new invasive species. However, the factors that link the wildlife trade to the biological invasion process are still poorly understood. Moreover, there are analytical challenges in researching the role of global wildlife trade in biological invasions, particularly issues related to the under-reporting of introduced and established populations in areas with reduced sampling effort. In this work, we use high-quality data on the international trade in Nearctic turtles (1999-2009) coupled with a statistical modelling framework, which explicitly accounts for detection, to investigate the factors that influence the introduction (release, or escape into the wild) of globally traded Nearctic turtles and the establishment success (self-sustaining exotic populations) of slider turtles (Trachemys scripta), the most frequently traded turtle species. We found that the introduction of a species was influenced by the total number of turtles exported to a jurisdiction and the age at maturity of the species, while the establishment success of slider turtles was best associated with the propagule number (number of release events), and the number of native turtles in the jurisdiction of introduction. These results indicate both a direct and indirect association between the wildlife trade and the introduction of turtles and establishment success of slider turtles, respectively. Our results highlight the existence of gaps in the number of globally recorded introduction events and established populations of slider turtles, although the expected bias is low. We emphasize the importance of researching independently the factors that affect the different stages of the invasion pathway. Critically, we observe that the number of traded individuals might not always be an adequate proxy for propagule pressure and establishment success.

Integrating transport pressure data and species distribution models to estimate invasion risk for alien stowaways

The number of alien species transported as stowaways is steadily increasing, and new approaches are urgently needed to tackle this emerging invasion pathway. We introduce a general framework for identifying high-risk transport pathways and receiving sites for alien species that are unintentionally transported via goods and services. This approach combines the probability of species arrival at transport hubs with the likelihood that the environment in the new region can sustain populations of that species. We illustrate our approach using a case study of the Asian black-spined toad (Duttaphrynus melanostictus) in Australia, a species that is of significant biosecurity concern in Australasia, Indonesia, and Madagascar. A correlative model fitted to occurrence data from the native geographic range of D. melanostictus predicted high environmental suitability at locations where the species has established alien populations globally. Applying the model to Australia revealed that transport hubs with the highest numbers of border interceptions and on-shore detections of D. melanostictus were environmentally similar to locations within the species’ native range. Numbers of D. melanostictus interceptions and detections in Australia increased over time, but were unrelated to indices of air and maritime trade volume. Instead, numbers of interceptions and detections were determined by the country of origin of airplanes (Thailand) and ships (Indonesia). Thus, the common assumption that transport pressure is correlated with invasion risk does not hold in all cases. Our work builds on previous efforts to integrate transport pressure data and species distribution models, by jointly modelling the number of intercepted and detected stowaways, while incorporating imperfect detection and the environmental suitability of receiving hubs. The approach presented here can be applied to any system for which historical biosecurity data are available, and provides an efficient means to allocate quarantine and surveillance efforts to reduce the probability of alien species establishment. This article is protected by copyright. All rights reserved.

Managing the risk of wildlife disease introduction: pathway‐level biosecurity for preventing the introduction of alien ranaviruses

Summary Alien species are key vectors for the spread of globally emerging diseases, and these emerging diseases have proven to be devastating for amphibian populations world-wide. Border and post-border biosecurity activities are pivotal for preventing the introduction of new diseases, but their effectiveness has seldom been assessed. We developed and populated a model to describe transport pathways into Australia, and the biosecurity activities implemented to manage these pathways. We evaluated the capacity of Australian border and post-border biosecurity activities, frequently considered one of the best quarantine inspection services in the world, to prevent the introduction of alien ranaviruses via the unintentional transport of alien amphibians into six Australian States. High transport pressures, measured by the number of international airline passengers and ships arriving in each State, resulted in increasing total numbers of alien stowaway amphibians. Post-border detection of alien amphibians was variable across States and increased with the full-time equivalent employees devoted to post-border biosecurity in each State. The baseline probabilities of introduction, without biosecurity activities, for at least one infected amphibian into any State were very low (≤0·07 in all cases), January 2004–December 2012. The implementation of biosecurity activities reduced these introduction probabilities further, with reductions of up to 50% for some States. Synthesis and applications. We have demonstrated the efficacy of biosecurity activities in reducing the introduction risk of new diseases being transported unintentionally alongside alien amphibians. Critically, we found that not all alien amphibians had to be detected to reduce risks appreciably. We advocate the widespread adoption of border and post-border biosecurity activities to manage the risks posed by alien amphibians (and other stowaway species) as vectors of emergent diseases. We support the robust design of biosecurity activities by providing a framework to evaluate the likely outcomes of case-specific biosecurity arrangements.

Remoteness promotes biological invasions on islands worldwide

Significance Islands are hotspots of alien species invasions, and their distinct biodiversity is particularly vulnerable to invading species. While isolation has shaped natural colonization of islands for millions of years, globalization in trade and transport has led to a breakdown of biogeographical barriers and subsequent colonization of islands by alien species. Using a large dataset of 257 subtropical and tropical islands, we show that alien richness increases with increasing isolation of islands. This pattern is consistent for plants, ants, mammals, and reptiles, and it cannot simply be explained by island economics and trade alone. Geographical isolation does not protect islands from alien species, and island species richness may reach a new dynamic equilibrium at some point, likely at the expense of many endemic species. One of the best-known general patterns in island biogeography is the species–isolation relationship (SIR), a decrease in the number of native species with increasing island isolation that is linked to lower rates of natural dispersal and colonization on remote oceanic islands. However, during recent centuries, the anthropogenic introduction of alien species has increasingly gained importance and altered the composition and richness of island species pools. We analyzed a large dataset for alien and native plants, ants, reptiles, mammals, and birds on 257 (sub) tropical islands, and showed that, except for birds, the number of naturalized alien species increases with isolation for all taxa, a pattern that is opposite to the negative SIR of native species. We argue that the reversal of the SIR for alien species is driven by an increase in island invasibility due to reduced diversity and increased ecological naiveté of native biota on the more remote islands.