Impacts of man-made structures on marine biodiversity and species status - native & non-native species
Coastal environments are exposed to anthropogenic activities such as frequent marine traffic and restructuring, i.e., addition, removal or replacing with man-made structures. Although maritime shipping and coastal infrastructures provide socio-economic benefits, they both cause varied perturbations to marine ecosystems. The ports and marinas receiving a high frequency of international vessels, act as ‘hot-spots’ for marine invasions. The disturbed and modified habitats found in harbours and ports provide opportunities for non-native species to settle due to their competitive traits. Once established, the non-native species may spread to neighbouring habitats, thereby modifying the adjacent natural environment, its biodiversity, ecosystem structure and functioning. Up to 70% of coastlines around the world have now been modified and is expected to rise in future. New bioinvasions are still being reported even with various biosecurity and management approaches across the globe. It is essential to understand the potential factors influencing the bioinvasions to have effective biosecurity measures and management plans. The overall aim of this thesis is to determine the influence of man-made structures on the marine biodiversity and presumptive fitness of native and non-native species on these structures. This thesis investigates ports and harbours as man-made environments, their impacts on marine biodiversity and the species status – native, non-native and cryptogenic, and the factors facilitating the spread of non-native species. Chapter 2 focussed on two large national-scale baseline port surveys; a) Australian Port Survey (APS), and b) New Zealand Port Survey (NZPS). The two datasets were analysed to determine the community structure and species status, i.e., native, non-native and cryptogenic as a function of the surveyed ports, port type (major vs minor ports) (based on the volume of vessels) and latitudinal groups. A) APS: The results for community composition indicated significant effects as a function of surveyed ports, port type and latitudinal group. The community composition was relatively more abundant at major ports than at minor ports. The factor, the latitudinal group indicated significant results, and a distinct separation in community composition was observed between low (15, 20oS) and high (35, 40oS) latitudes. The species status showed a significant and positive relationship between native vs non-native, indicating with an increase in the number of native species there was an increase in the number of non-native species. The species status indicated significant results for the factors; surveyed ports, port type and latitudinal group. The native species were abundant throughout the study. However, the non-native species were relatively abundant at major ports compared to minor ports. Regarding the latitudinal groups, the abundance of non-native species was observed to increase at higher latitudes (latitudinal gradients). B) NZPS: The community composition and species status showed significance among the 27 surveyed ports; however, no significant results were observed for the factor port type (major vs minor). The community composition significantly varied as a function of latitudinal groups, with species at higher latitudes (45oS) being better discriminator explaining the differences. Latitudinal groups, however, highlighted sub-groupings of ports with similar community composition (e.g. Bluff and Dunedin; Nelson, Wellington and Picton; Lyttelton and Timaru; Whangarei, Tauranga and Taranaki; Auckland, Gulf Harbour Marina and Opua Marina). The ports in question are within close proximity of each other (distance). This suggests the possibility of natural dispersal of species between ports on top of the human-mediated dispersal. The responses in Australia were very different from those in New Zealand, which suggests that the responses are regional or country-specific and not global. Chapter 3 describes fieldwork using settlement tile arrays to examine the effects of man-made built structures and natural rocky reefs on marine biological community composition and successional patterns over two years. The work also tests the preference of native and non-native species in terms of habitat type (natural reef vs man-made habitat) and substratum type (PVC vs slate tile). The results showed a rapid increase in species settlement on bare tiles as the available bare space was 30% just after 3 months of submersion. The community composition significantly differed as a function of the interaction of factors, habitat × substratum × sample interval. However, differences between the habitat types and substratum types, respectively, were explained by the difference in abundance of the same suite of species. The species were abundant at marina sites compared to reef sites; however, in terms of substrata, the species were abundant on slate (natural) tiles than on PVC tiles. The succession patterns of species over time (8 sample intervals) showed a similar trend on both the habitat type and substratum type, with differences in the average abundances of each species. The differences in abundances highlight the influence of species dispersal patterns, recruitment patterns and post-settlement processes of species between habitat type and substratum type, respectively. Subsequently, the species status indicated significance as a function of habitat type, substratum type and sample intervals. The cryptogenic species were abundant throughout the study. The cryptogenic species, however, decreased in abundance over time, with an increase in abundance of native and non-native species. Subsequently, the non-native species significantly varied between habitat type, with relatively higher abundance at marina (man-made) sites compared to reef (natural) sites. However, the non-native species did not show significant variation as a function of substratum type (PVC vs slate). The results are discussed in the context of the recruitment of species on a new barren substrate, and the preference of habitat type and substratum type by native, non-native and cryptogenic species. In Chapter 4, the reproduction output (gonadosomatic index, GSI) of the Southern hemisphere, native (SHMg) and Northern hemisphere, non-native (NHMg) lineages of the blue mussel, Mytilus galloprovincialis were measured. The GSI and shell length of NHMg and SHMg were compared between habitat type; reef (natural) vs marina (man-made) sites. This study aimed to identify reproductive patterns (i.e., timing and magnitude of spawning events) and differences in performance (presumptive fitness) of the native and non-native blue mussel lineages at the natural and man-made habitats. The results for shell length indicated significance for habitat type and no significance as a function of lineage. The mussels were relatively bigger mussels at marina sites compared to reef sites; however, the differences were trivial. The GSI values as a function of habitat type, lineage and sampling time showed a significant difference between habitat type, with high GSI values at reef sites than at marina sites. However, this indicates that the blue mussels at marina sites had comparatively higher spawning activity than at reef sites. The temporal variation of GSI of NHMg and SHMg showed a similar reproductive trend (i.e., spawning and gametogenesis) at both habitats. However, significant spawning activity was observed in July and November when compared between reef and marina habitats. The results are discussed in the context of management implications and strategies regarding the establishment and success of non-native M. galloprovincialis lineage and whether their eradication is necessary or even possible. The findings of this research are summarised and discussed in relation to our understanding of biological community composition and diversity on man-made habitats and the subsequent invasion in the neighbouring natural habitats. This study, from an eco-engineering perspective, highlights the importance of complex habitats and surfaces, and not just material type. However, from a biosecurity and management approach, even though Australia and New Zealand have one of the strong international biosecurity country-specific legislation; the continuous arrival of non-native species in these countries indicates that such marine legislation is not sufficiently compelling on its own. This study highlights the interaction of non-native species at proximity ports, and it provides recommendations towards regional-scale management measures concentrating on intra-coastal transfer of invaders through domestic maritime traffic or natural dispersal. The life-history traits, recruitment timing and post-settlement processes, plays an essential role in determining long term patterns. Lastly, this research indicated that native and non-native species with ecologically similar responses lead to limited management options to some extent. Therefore, from a manager’s perspective, the eradication of non-native species may not be necessary if it does not cause any negative impacts to the biodiversity or the environment.