Richard F. Piola

The Role of Propagule Pressure in Invasion Success

One of the core goals of invasion biology is the identification of factors that increase the risk of establishment success of non-native species. Historically, marine invasions have been investigated through observational studies and surveys (Cohen and Carlton 1998; Ruiz et al. 2000). These have guided ecologists towards the processes most relevant to invasion, but researchers are becoming increasingly aware of the limitations of observational studies alone. It is clear that different factors may influence invasion success at different stages of the invasion process (Kolar and Lodge 2001) and a major challenge is to quantify the relative importance of these factors. Understanding the intricacies of invasion dynamics requires a rigorous approach, in which potentially important factors can be controlled, manipulated and tested (Ruiz et al. 2000). Particularly strong calls have been made for the inclusion of propagule pressure or invader supply into our models, experiments and surveys, and this chapter reviews recent progress in elucidating the role of propagule pressure on invasion success in marine ecosystems.

Comparing differential tolerance of native and non-indigenous marine species to metal pollution using novel assay techniques.

Recent research suggests anthropogenic disturbance may disproportionately advantage non-indigenous species (NIS), aiding their establishment within impacted environments. This study used novel laboratory- and field-based toxicity testing to determine whether non-indigenous and native bryozoans (common within marine epibenthic communities worldwide) displayed differential tolerance to the common marine pollutant copper (Cu). In laboratory assays on adult colonies, NIS showed remarkable tolerance to Cu, with strong post-exposure recovery and growth. In contrast, native species displayed negative growth and reduced feeding efficiency across most exposure levels. Field transplant experiments supported laboratory findings, with NIS growing faster under Cu conditions. In field-based larval assays, NIS showed strong recruitment and growth in the presence of Cu relative to the native species. We suggest that strong selective pressures exerted by the toxic antifouling paints used on transport vectors (vessels), combined with metal contamination in estuarine environments, may result in metal tolerant NIS advantaged by anthropogenically modified selection regimes.

The potential for translocation of marine species via small-scale disruptions to antifouling surfaces

Vessel hull fouling is a major vector for the translocation of nonindigenous species (NIS). Antifouling (AF) paints are the primary method for preventing the establishment and translocation of fouling species. However, factors such as paint age, condition and method of application can all reduce the effectiveness of these coatings. Areas of hull that escape AF treatment (through limited application or damage) constitute key areas that may be expected to receive high levels of fouling. The investigation focused on whether small-scale (mm2 to cm2) areas of unprotected surface or experimental ‘scrapes’ provided sufficient area for the formation of fouling assemblages within otherwise undamaged AF surfaces. Recruitment of fouling taxa such as algae, spirorbids and hydroids was recorded on scrapes as narrow as 0.5 cm wide. The abundance and species richness of fouling assemblages developing on scrapes ≥1 cm often equalled or surpassed levels observed in reference assemblages totally unprotected by AF coatings. Experiments were conducted at three sites within the highly protected and isolated marine park surrounding Lady Elliott Island at the southernmost tip of the Great Barrier Reef, Australia. Several NIS were recorded on scrapes of AF coated surfaces at this location, with 1-cm scrapes showing the greatest species richness and abundance of NIS relative to all other treatments (including controls) at two of the three sites investigated. Slight disruptions to newly antifouled surfaces may be all that is necessary for the establishment of fouling organisms and the translocation of a wide range of invasive taxa to otherwise highly protected marine areas.

Effect of vessel voyage speed on survival of biofouling organisms: implications for translocation of non-indigenous marine species

This study experimentally determined the effect of different vessel voyage speeds (5, 10 and 18 knots = 2.6, 5.1 and 9.3 ms−1, respectively) and morphological characteristics including growth form (solitary or colonial), profile (erect or encrusting) and structure (soft, hard or flexible) on the survival of a range of common biofouling organisms. A custom built hydrodynamic keel attached to the bottom of a 6 m aluminium powerboat was used to subject pre-fouled settlement plates for this purpose. Vessel speeds of 5 and 10 knots had little effect on the species richness of biofouling assemblages tested, however richness decreased by 50% following 18 knots treatments. Species percentage cover decreased with increasing speed across all speed treatments and this decrease was most pronounced at 10 and 18 knots, with cover reduced by 24 and 85% respectively. Survival was greatest for organisms with colonial, encrusting, hard and/or flexible morphological characteristics, and this effect increased with increasing speed. This study suggests that there is predictive power in forecasting future introductions if we can understand the extent to which such traits explain the world-wide distributions of non-indigenous species. Future introductions are a certainty and can only provide an increasing source of new information on which to test the validity of these predications.

Assessing the efficacy of spray-delivered ‘eco-friendly’ chemicals for the control and eradication of marine fouling pests

Despite its frequent use in terrestrial and freshwater systems, there is a lack of published experimental research examining the effectiveness of spray-delivered chemicals for the management of non-indigenous and/or unwanted pests in marine habitats. This study tested the efficacy of spraying acetic acid, hydrated lime and sodium hypochlorite for the control of marine fouling assemblages. The chemicals are considered relatively ‘eco-friendly’ due to their low toxicity and reduced environmental persistence compared to synthetic biocides, and they were effective in controlling a wide range of organisms. Pilot trials highlighted acetic acid as the most effective chemical at removing fouling cover, therefore it was selected for more comprehensive full-scale trials. A single spray of 5% acetic acid with an exposure time of 1 min effectively removed up to 55% of the invertebrate species present and 65% of the cover on fouled experimental plates, while one application of 10% acetic acid over 30 min removed up to 78% of species present and 95% of cover. Single-spray treatments of 5% acetic acid reduced cover of the tunicate pest species Didemnum vexillum by up to 100% depending on the exposure duration, while repeat-spraying ensured that even short exposure times (1 min) achieved ∼99% mortality. Both 5 and 10% acetic acid solutions appeared equally effective at removing the majority of algal species. This technique could be used for controlling the introduction of unwanted species or preventing the spread of pests, and is applicable to use on a variety of natural and artificial substrata, or for the treatment of structures that can be removed from the water.

Ontogeny of behaviour relevant to dispersal and connectivity in the larvae of two non-reef demersal, tropical fish species

In demersal marine fishes, the dispersal of larvae determines the geographical scale of population connectivity, and larval behaviour may influence dispersal. Yet, little is known of the ontogeny of behaviours that can influence dispersal. The present study examined the development of these behaviours in pelagic larvae of tropical marine fishes (4–21 mm) that occupy non-reef habitats as adults: Eleutheronema tetradactylum (Polynemidae) and Leiognathus equulus (Leiognathidae). In the laboratory, critical speed (Ucrit) increased from 3 to 34 cm s–1 at 1.3–1.7 cm s–1 per mm of size, with the fastest larvae up to 50% faster. In situ speed increased from 4 to 25 cm s–1 at 0.7–2.2 cm s–1 per mm, and was 10–14 body length s–1 (60–90% of Ucrit). Endurance increased from 0 to >40 km at 2.4–4.7 km per mm. In the sea, orientation precision did not change ontogenetically, both species tended to swim in loops, and neither significant overall directionality nor ontogenetic change in orientation was present. Larval orientation of these non-reef species was less precise than that of reef fishes. The two species differed in depth distribution, and one ascended ontogenetically. These behaviours can potentially influence dispersal outcomes over the full size range of these larvae.

The effect of vessel speed on the survivorship of biofouling organisms at different hull locations.

This study used a specially designed MAGPLATE system to quantify the en route survivorship and post-voyage recovery of biofouling assemblages subjected to short voyages (<12 h) across a range of vessel speeds (slow, medium, fast; in the range 4.0–21.5 knots). The effect of hull location (bow, amidships and stern) was also examined. While no significant differences were evident in en route survivorship of biofouling organisms amongst hull locations, biofouling cover and richness were markedly reduced on faster vessels relative to slower craft. Therefore, the potential inoculum size of non-indigenous marine species and richness is likely to be reduced for vessels that travel at faster speeds (>14 knots), which is likely to also reduce the chances of successful introductions. Despite this, the magnitude of introductions from biofouling on fast vessels can be considered minor, especially for species richness where 90% of source-port species were recorded at destinations.

Modeling biofouling from boat and source characteristics: a comparative study between Canada and New Zealand

In marine systems subject to vessel traffic, the likelihood of an invasion by an exotic species typically increases with the arrival of each infested boat. In this paper, recreational boating activity patterns and boat fouling by tunicates were compared between eastern Canada and central New Zealand, and the relative importance of boat characteristics on fouling was evaluated using Boosted Regression Tree analysis. For Canada, we also determined the relative importance of boat characteristics and propagule exposure (i.e., the interaction between tunicate density in source region and time in water) on patterns of boat fouling. Approximately half of boats examined during the fall in eastern Canada and during the summer in central New Zealand were fouled by tunicates. Although there was a greater richness of tunicate species on New Zealand boats, the two countries had several species or genera in common, including Botryllus schlosseri, Ciona spp. and Botrylloides spp. The time since last boat maintenance was longer in New Zealand than in Canada. However, boat fouling and boat-mediated spread may be facilitated in Canada by boating activity patterns there, as the movement of boats among multiple marinas is considerably greater than in New Zealand. Among the boat characteristics, voyage type, the time that boats spent in water (Canada) and time since last application of antifouling paint (New Zealand) were among the best predictors of boat fouling. However, our results from Canada showed that propagule exposure was more important than boat characteristics in predicting the presence of the invasive colonial tunicate, B. schlosseri. This study shows the importance of small boats as potential vectors for tunicates and demonstrates that predictive models for the spread of biofouling species should be based on regional boating patterns, boating characteristics, and local propagule exposure.

Carbon and nitrogen stable isotope analysis indicates freshwater shrimp Paratya australiensis Kemp, 1917 (Atyidae) assimilate cyanobacterial accumulations

Large areas of uncompacted cyanobacterial accumulations (or “gyttja”) have been observed in Myall Lake, New South Wales, Australia. To determine whether the cyanobacterial accumulations were assimilated into the local food web, carbon and nitrogen stable isotopes were used to identify the primary food sources of a primary consumer in Myall Lake, the freshwater atyid shrimp Paratya australiensis. Suspended particulate organic matter (POM) and the macrophyte Myriophyllum salsugineum were identified as major dietary sources of P. australiensis. Enriched stable isotope signatures (δ13C and δ15N) of shrimp from gyttja-affected sites, relative to shrimp from unaffected locations, also indicated that P. australiensis were deriving a considerable portion of their dietary carbon and nitrogen requirements from gyttja. Stable isotope mixing models estimated that cyanobacterial accumulations might constitute up to 69% of P. australiensis biomass carbon and nitrogen requirements at gyttja-affected locations. To our knowledge, this is the first study to use stable isotope analysis to trace the assimilation of potentially toxic cyanobacterial accumulations into the trophic pathways of an affected system.

Thermal treatment as a method to control transfers of invasive biofouling species via vessel sea chests

This study examined the efficacy of heated seawater for the treatment and remediation of fouled vessel sea chest habitats. In laboratory trials, three temperature regimes (37.5°C for 60 min, 40°C for 30min and 42.5°C for 20 min) were tested on a range of temperate taxa commonly found in sea chests. Field validation trials further assessed the efficacy of heat treatment within a replica sea chest environment. During laboratory trials, 100% mortality was achieved across all three treatments for the majority of taxa; the exceptions being the barnacle Elminius modestus and the oyster Crassostrea gigas. Temperature tolerance limits observed in the laboratory were successfully replicated under simulated sea chest conditions; however, a failure to achieve even heat distribution was an obstacle to achieving uniform mortality. This study provides guidance on the temperature/exposure parameters required for vessels plying temperate latitudes, and demonstrates that heated seawater has potential for controlling biofouling in vessel sea chests.