Richard P. Duncan

A proposed unified framework for biological invasions

There has been a dramatic growth in research on biological invasions over the past 20 years, but a mature understanding of the field has been hampered because invasion biologists concerned with different taxa and different environments have largely adopted different model frameworks for the invasion process, resulting in a confusing range of concepts, terms and definitions. In this review, we propose a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions. The unified framework combines previous stage-based and barrier models, and provides a terminology and categorisation for populations at different points in the invasion process.

Are treelines advancing? A global meta‐analysis of treeline response to climate warming

Treelines are temperature sensitive transition zones that are expected to respond to climate warming by advancing beyond their current position. Response to climate warming over the last century, however, has been mixed, with some treelines showing evidence of recruitment at higher altitudes and/or latitudes (advance) whereas others reveal no marked change in the upper limit of tree establishment. To explore this variation, we analysed a global dataset of 166 sites for which treeline dynamics had been recorded since 1900 AD. Advance was recorded at 52% of sites with only 1% reporting treeline recession. Treelines that experienced strong winter warming were more likely to have advanced, and treelines with a diffuse form were more likely to have advanced than those with an abrupt or krummholz form. Diffuse treelines may be more responsive to warming because they are more strongly growth limited, whereas other treeline forms may be subject to additional constraints.

Determinants of establishment success in introduced birds.

A major component of human-induced global change is the deliberate or accidental translocation of species from their native ranges to alien environments, where they may cause substantial environmental and economic damage. Thus we need to understand why some introductions succeed while others fail. Successful introductions tend to be concentrated in certain regions, especially islands and the temperate zone, suggesting that species-rich mainland and tropical locations are harder to invade because of greater biotic resistance. However, this pattern could also reflect variation in the suitability of the abiotic environment at introduction locations for the species introduced, coupled with known confounding effects of nonrandom selection of species and locations for introduction. Here, we test these alternative hypotheses using a global data set of historical bird introductions, employing a statistical framework that accounts for differences among species and regions in terms of introduction success. By removing these confounding effects, we show that the pattern of avian introduction success is not consistent with the biotic resistance hypothesis. Instead, success depends on the suitability of the abiotic environment for the exotic species at the introduction site.

Global patterns of introduction effort and establishment success in birds

Theory suggests that introduction effort (propagule size or number) should be a key determinant of establishment success for exotic species. Unfortunately, however, propagule pressure is not recorded for most introductions. Studies must therefore either use proxies whose efficacy must be largely assumed, or ignore effort altogether. The results of such studies will be flawed if effort is not distributed at random with respect to other characteristics that are predicted to influence success. We use global data for more than 600 introduction events for birds to show that introduction effort is both the strongest correlate of introduction success, and correlated with a large number of variables previously thought to influence success. Apart from effort, only habitat generalism relates to establishment success in birds.

Ecology: Darwin's naturalization hypothesis challenged

Naturalized plants can have a significant ecological and economic impact, yet they comprise only a fraction of the plant species introduced into new areas by humans. Darwin proposed that introduced plant species will be less likely to establish a self-sustaining wild population in places with congeneric native species because the introduced plants have to compete with their close native relatives, or are more likely to be attacked by native herbivores or pathogens, a theory known as Darwins naturalization hypothesis. Here we analyse a complete list of seed-plant species that have been introduced to New Zealand and find that those with congeneric relatives are significantly more, not less, likely to naturalize — perhaps because they share with their native relatives traits that pre-adapt them to their new environment.

COMPETITION AND THE COEXISTENCE OF SPECIES IN A MIXED PODOCARP STAND

The importance of intra- and interspecific competition in affecting the growth and survival of Dacrycarpus dacrydioides and Dacrydium cupressinum, the two dominant trees in a mixed podocarp stand in Ohinemaka Forest, south Westland, New Zealand, was investigated. Two lines of evidence (spatial patterning of trees and correlates of tree performance with neighbourhood density) were used to infer the importance of competition. Intraspecific competition among D. dacrydioides trees appeared to be intense. Mean radial growth rate (MGR) was significantly negatively correlated with the number of conspecific neighbours, and patterns of non-random mortality resulted in a shift in spatial pattern from a significantly clumped to a significantly regular distribution (...)

Flood disturbance and the coexistence of species in a lowland podocarp forest, south Westland, New Zealand.

The structure and composition of four forest stands affected by catastrophic floods on a lowland floodplain, south Westland, New Zealand, was investigated using stand history reconstruction. Age and size structures, and tree locations were used to identify cohorts of trees that were initiated by flood events. This, along with information on species microsite preferences and spatial patterning, was used to reconstruct the patterns of establishment of the four dominant canopy trees (Dacrycarpus dacrydioides, Dacrydium cupressinum, Prumnopitys ferruginea and Weinmannia racemosa) in response to flood disturbance. The four species coexisted by partitioning establishment sites with respect to the amount of overhead cover and the type of forest floor microsite (...)

Darwin’s naturalization conundrum: dissecting taxonomic patterns of species invasions

Darwin acknowledged contrasting, plausible arguments for how species invasions are influenced by phylogenetic relatedness to the native community. These contrasting arguments persist today without clear resolution. Using data on the naturalization and abundance of exotic plants in the Auckland region, we show how different expectations can be accommodated through attention to scale, assumptions about niche overlap, and stage of invasion. Probability of naturalization was positively related to the number of native species in a genus but negatively related to native congener abundance, suggesting the importance of both niche availability and biotic resistance. Once naturalized, however, exotic abundance was not related to the number of native congeners, but positively related to native congener abundance. Changing the scale of analysis altered this outcome: within habitats exotic abundance was negatively related to native congener abundance, implying that native and exotic species respond similarly to broad scale environmental variation across habitats, with biotic resistance occurring within habitats.

A global synthesis of plant extinction rates in urban areas

Plant extinctions from urban areas are a growing threat to biodiversity worldwide. To minimize this threat, it is critical to understand what factors are influencing plant extinction rates. We compiled plant extinction rate data for 22 cities around the world. Two-thirds of the variation in plant extinction rates was explained by a combination of the citys historical development and the current proportion of native vegetation, with the former explaining the greatest variability. As a single variable, the amount of native vegetation remaining also influenced extinction rates, particularly in cities > 200 years old. Our study demonstrates that the legacies of landscape transformations by agrarian and urban development last for hundreds of years, and modern cities potentially carry a large extinction debt. This finding highlights the importance of preserving native vegetation in urban areas and the need for mitigation to minimize potential plant extinctions in the future.

Negative soil feedbacks accumulate over time for non‐native plant species

The enemy release hypothesis is a common explanation for species invasions, suggesting that introduced species benefit from leaving behind natural enemies in the native range. However, any such advantage may attenuate over time. In this study, we test a prediction of this more dynamic enemy release hypothesis: that non-native plant species that became established longer ago exhibit stronger negative feedbacks with the soil. Consistent with declining enemy release over time, we found increasingly negative soil feedbacks for species established longer ago in New Zealand. Negative soil feedbacks were also stronger for more widespread species, but weaker for more locally abundant species, suggesting that species accumulate negative interactions as they spread and can be locally regulated by these interactions. We also present data to support the common assumption that relatives have similar impacts on and responses to soil communities. Together, these data highlight the dynamic nature of novel interactions arising from species introductions.