Philip J. Lester

Relative roles of climatic suitability and anthropogenic influence in determining the pattern of spread in a global invader

Because invasive species threaten the integrity of natural ecosystems, a major goal in ecology is to develop predictive models to determine which species may become widespread and where they may invade. Indeed, considerable progress has been made in understanding the factors that influence the local pattern of spread for specific invaders and the factors that are correlated with the number of introduced species that have become established in a given region. However, few studies have examined the relative importance of multiple drivers of invasion success for widespread species at global scales. Here, we use a dataset of >5,000 presence/absence records to examine the interplay between climatic suitability, biotic resistance by native taxa, human-aided dispersal, and human modification of habitats, in shaping the distribution of one of the worlds most notorious invasive species, the Argentine ant (Linepithema humile). Climatic suitability and the extent of human modification of habitats are primarily responsible for the distribution of this global invader. However, we also found some evidence for biotic resistance by native communities. Somewhat surprisingly, and despite the often cited importance of propagule pressure as a crucial driver of invasions, metrics of the magnitude of international traded commodities among countries were not related to global distribution patterns. Together, our analyses on the global-scale distribution of this invasive species provide strong evidence for the interplay of biotic and abiotic determinants of spread and also highlight the challenges of limiting the spread and subsequent impact of highly invasive species.

Habitat complexity facilitates coexistence in a tropical ant community.

The role of habitat complexity in the coexistence of ant species is poorly understood. Here, we examine the influence of habitat complexity on coexistence patterns in ant communities of the remote Pacific atoll of Tokelau. The invasive yellow crazy ant, Anoplolepis gracilipes (Smith), exists in high densities on Tokelau, but still coexists with up to seven other epigeic ant species. The size-grain hypothesis (SGH) proposes that as the size of terrestrial walking organisms decreases, the perceived complexity of the environment increases and predicts that: (1) leg length increases allometrically with body size in ants, and (2) coexistence between ant species is facilitated by differential habitat use according to body size. Analysis of morphological variables revealed variation inconsistent with the morphological prediction of the SGH, as leg length increased allometrically with head length only. We also experimentally tested the ability of epigeic ants in the field to discover and dominate food resources in treatments of differing rugosity. A. gracilipes was consistently the first to discover food baits in low rugosity treatments, while smaller ant species were consistently the first to discover food baits in high rugosity treatments. In addition, A. gracilipes dominated food baits in planar treatments, while smaller ant species dominated baits in rugose treatments. We found that the normally predictable outcomes of exploitative competition between A. gracilipes and other ant species were reversed in the high rugosity treatments. Our results support the hypothesis that differential habitat use according to body size provides a mechanism for coexistence with the yellow crazy ant in Tokelau. The SGH may provide a mechanism for coexistence in other ant communities but also in communities of other terrestrial, walking insects that inhabit a complex landscape.

Demersal fish community diversity off New Zealand: Is it related to depth, latitude and regional surface phytoplankton?

Abstract Recent studies of basin-scale patterns of diversity of benthic macrofauna reported strong latitudinal gradients of diversity in the deep North Atlantic, in contrast to regionally variable patterns in the Southern Hemisphere. Here we use data from fisheries research trawl surveys to examine spatial patterns of species richness, Shannon-Wiener diversity index, and evenness of demersal fish communities in relation to latitude and depth from 80 to 898 m off south-east New Zealand. We found species richness decreased latitudinally within regions in the poleward direction, and increased with depth. Areas of high species richness were concentrated along the margins of the Chatham Rise and were associated with current intensification in regions of enhanced surface phytoplankton pigment concentration. Species richness was highest between the 500 and 1000 m contours on the Chatham Rise, where enhanced surface phytoplankton pigment is associated with a major oceanographic feature, the Subtropical convergence. A predominance of species-rich locations was found on the more steeply shelving northern margin of the Chatham Rise. The regional latitudinal pattern of diversity appears to be correlated with regional production, and to be influenced by mesoscale oceanographic features constrained by the bathymetry, although the proximal causes for high diversity remain speculative.

Long-legged ants, Anoplolepis gracilipes (Hymenoptera: Formicidae), have invaded Tokelau, changing composition and dynamics of ant and invertebrate communities.

This report documents the ongoing invasion of the Tokelau atolls by the long-legged ant, Anoplolepis gracilipes ( Jerdon). These ants were collected from two of the three Tokelau atolls. On the island of Fenua Fala of Fakaofo Atoll, long-legged ants appear to be a recent arrival and occur in only a small area around one of the two ports. Most of the inhabited islands of Vao and Motuhuga on Nukunonu Atoll have been invaded, in addition to several of the uninhabited, forested islands. Despite this ant having been previously recorded from at least one island of Fakaofo and Nukunonu, these appear to be new invasions. Densities of up to 3,603 A. gracilipes per pitfall trap were caught per 24 hr. A significant reduction in ant species diversity was observed with increasing A. gracilipes densities. Densities of this ant were not uniformly high, perhaps due to variation in food availability. Prey such as crabs, ant colonies, and other insects were directly observed being attacked, and long-legged ants were observed to feed on honeydew produced by high densities of aphids, mealybugs, and scale insects on a variety of plants. Interspecific competition was investigated as an additional mechanism for the successful invasion. Long-legged ants found and removed bait faster than the dominant resident ant species, Paratrechina longicornis (Latreille), in forested areas of Nukunonu Island, though needing smaller numbers of recruits to achieve this result. This A. gracilipes invasion is of serious concern for the biodiversity of Tokelau and probably many of the other Pacific islands where these ants have invaded.

Temperature-dependent development of the Argentine ant, Linepithema humile (Mayr) (Hymenoptera: Formicidae): a degree-day model with implications for range limits in New Zealand.

The Argentine ant, Linepithema humile, is an invasive species with the potential to cause significant economic and ecological damage in New Zealand. Using published information on rates of development, we induced parameters for a cumulative degree-day model for each life-stage of the Argentine ant.A summary model suggested that complete development, from egg to worker, requires approximately 445 degree-days above a threshold of 15.9°C. Meteorological records of air and soil temperatures indicated a number of sites in New Zealand that fulfil this minimum temperature requirement on an annual basis. Maps based on soil temperature data indicated suitable conditions as far south as Central Otago, while air temperature data predicted a more limited northerly distribution in Northland and Hawkes Bay. Additional factors, such as microclimatic variation, thermoregulatory behaviour, biotic interactions and dispersal opportunities will also be important in determining the precise range limits of the Argentine ant in New Zealand.

Effects of riparian willow trees (Salix fragilis) on macroinvertebrate densities in two small Central Otago, New Zealand, streams

Abstract The effects of an introduced willow tree species (Salix fragilis) on the densities of macroinvertebrates were examined in two Central Otago, New Zealand, streams during 1991. Significantly lower invertebrate densities and biomass were observed in willow‐lined sections of the streams than in nearby open sections in summer, autumn, and winter. This result was observed in riffles and pools, for most dominant species and nearly all functional feeding groups. The effect was not associated with differences in the amount of fine paniculate organic matter ( 5 mm) were significantly higher in willow‐shaded riffles, but this did not result in increased abundance or biomass of shredders. Willow trees reduced incident stream illumination by as much as 80%, but did not appear to influence water chemistry between open and willow‐shaded s...

The widespread collapse of an invasive species: Argentine ants (Linepithema humile) in New Zealand.

Synergies between invasive species and climate change are widely considered to be a major biodiversity threat. However, invasive species are also hypothesized to be susceptible to population collapse, as we demonstrate for a globally important invasive species in New Zealand. We observed Argentine ant populations to have collapsed in 40 per cent of surveyed sites. Populations had a mean survival time of 14.1 years (95% CI = 12.9–15.3 years). Resident ant communities had recovered or partly recovered after their collapse. Our models suggest that climate change will delay colony collapse, as increasing temperature and decreasing rainfall significantly increased their longevity, but only by a few years. Economic and environmental costs of invasive species may be small if populations collapse on their own accord.

Some effects of pre-release host-plant on the biological control of Panonychus ulmi by the predatory mite Amblyseius fallacis.

Amblyseius fallacis Garman has been selected for pyrethroid resistance and mass reared for experimental release as a biological control agent for tetranychid mites on a number of crops in Canada. Several releases of this predator onto apple and peach trees have failed to result in the establishment of A. fallacis, or in the biological control of Panonychus ulmi Koch. Here, we test the hypothesis that the change of host-plant at the time of release is a critical factor in the establishment of A. fallacis for biological control of P. ulmi. Functional and numerical response studies were undertaken on two populations of A. fallacis: a wild strain collected from the canopy foliage of an apple orchard near Vineland, Ontario; and a second strain reared on bean plants in a commercial insectary with Tetranychus urticae as prey. Each population consumed significantly more P. ulmi and produced significantly more eggs when on leaf disks from the plant species they were reared on, than on leaf disks from the novel host plant. A further experiment was conducted to determine if establishment and biological control of mass-reared A. fallacis could be affected by rearing a population for a short term on apple leaves prior to release on apple trees. Three release treatments were made into potted apple trees in a glasshouse, using predators commercially mass-reared on bean and T. urticae: A. fallacis released directly; A. fallacis reared in the laboratory for four weeks on bean and T. urticae; A. fallacis reared on apple leaves and T. urticae for four weeks. They were compared with a control treatment lacking predator release. Contrary to results of the functional and numerical response studies, no difference was observed between release treatments. All release treatments adding A. fallacis resulted in a similar, if limited, degree of biological control of P. ulmi. These results indicated that there may be short-term effects of host plant on the establishment of A. fallacis and biological control of P. ulmi, which in our study were observed as an initial reduction of the predatory response. However, in a test, the predators appeared to overcome these short-term effects and successfully established on the new host-plant to control P. ulmi.

Anthropogenic Landscape Change and Vectors in New Zealand: Effects of Shade and Nutrient Levels on Mosquito Productivity

Anthropogenic environmental changes, such as deforestation, agriculture, and introduced exotic species, have often coincided with an increase in mortality and morbidity from mosquito-borne diseases worldwide. Deforestation and agricultural development are likely to regulate immature mosquito populations through the addition of nutrients from livestock waste, decreased shade resulting in increased insolation (solar radiation), and the proliferation of artificial container habitats. We conducted a field experiment in Waikanae, New Zealand, to tease apart the relative effects of shade and nutrient levels on aquatic immature populations of two generalist mosquito species. Container habitats were subjected to five levels of detrital input (0–500-g sheep manure/liter of water) and three shade treatments (open, artificial shade, and forest canopy) in a factorial design. The native species Culex pervigilans constituted 98.9% of all late-instar larvae; the remainder being the exotic Ochlerotatus notoscriptus. We observed higher overall immature mosquito abundance and pupal productivity in open containers with medium detrital loads (5 g/L). Exotic mosquito abundance was low in all treatments and was excluded from containers in unshaded, or deforested, areas. No native or exotic mosquito abundance was observed in containers with extremely high detrital loads (500 g/L). As many exotic species thrive in similar high nutrient conditions, these containers and other larval habitats of similarly high nutrient levels represent potentially vacant niches for exotic mosquito invasion. These results indicate the importance of shade and nutrient level as central determinants of mosquito productivity in temperate climates, such as New Zealand, as well as show that anthropogenic environmental change can have flow-on effects on the ecology of disease-vector mosquitoes.

A neurotoxic pesticide changes the outcome of aggressive interactions between native and invasive ants

Neurotoxic pesticides, such as neonicotinoids, negatively affect the cognitive capacity and fitness of non-target species, and could also modify interspecific interactions. We tested whether sublethal contamination with neonicotinoid could affect foraging, colony fitness and the outcome of behavioural interactions between a native (Monomorium antarcticum) and an invasive ant species (Linepithema humile). The foraging behaviour of both ants was not affected by neonicotinoid exposure. Colonies of the invasive species exposed to the neonicotinoid produced significantly fewer brood. In interspecific confrontations, individuals of the native species exposed to the neonicotinoid lowered their aggression towards the invasive species, although their survival probability was not affected. Exposed individuals of the invasive species interacting with non-exposed native ants displayed increased aggression and had their survival probability reduced. Non-exposed individuals of the invasive species were less aggressive but more likely to survive when interacting with exposed native ants. These results suggest that non-target exposure of invaders to neonicotinoids could either increase or decrease the probability of survival according to the exposure status of the native species. Given that, in any community, different species have different food preferences, and thus different exposure to pesticides, non-target exposure could potentially change the dynamics of communities and influence invasion success.