| Popis: |
Dugongs are large primary consumers of seagrass communities, and thereby shape the diversity, structure, and dynamics of these extensive ecosystems. The dugong is listed as vulnerable to extinction at a global scale. Because dugongs are seagrass specialists, understanding the interaction between dugongs and their seagrass habitats is crucial to their conservation. Habitat use by dugongs is beginning to receive greater attention by managers and ecologists, but a spatially-explicit model capable of predicting usage by dugongs based on attributes of those habitats is lacking. Studying the interaction between dugongs and their seagrass food requires knowledge of the movements and diving behaviour of dugongs at scales relevant to both dugongs and managers. Information is needed on dugong spatial patterns, including movement behaviours and habitat use, across domains of scale. Multi-scale approaches to dugong research have not been possible in the past because of the difficulties in observing dugongs directly and the low resolution of telemetric equipment. My project capitalised on recent technology incorporating accurate GPS technology into tracking equipment to monitor the habitat use of wild animals at very high resolution ( 0.1 dugongs/km2) of dugongs over 20 years along the urban coast of Queensland was 77 km2 (± 4 s.e.). Hence, at regional and landscape scales (> 100 km2) dugongs select habitat at the level of individual bays along the coast. The tracked dugongs were followed for periods ranging from 15 to 551 days and exhibited a large range of individualistic movement behaviours; 26 individuals were relatively sedentary (moving < 15 km) while 44 made large-scale movements (> 15 km) of up to 560 km from their capture sites. Male and female animals, including cows with calves, undertook large-scale movements (LSM; > 15 km). At least some of these movements were return movements to the capture location, suggesting that such movements were ranging rather than dispersal movements. Large-scale movements included macro-scale regional movements (> 100 km) and meso-scale inter-patch local movements (15 ≤ 100 km) and were qualitatively different from tidally-driven micro-scale commuting movements between and within seagrass beds (< 15 km). Large-scale movements were rapid and apparently directed. Tracked dugongs rarely travelled far from the coast (mean max distance = 12.8 ± s.e. 1.3 km). Dive profiles from the timedepth recorders suggest that dugongs make repeated deep dives while travelling rather than remaining at the surface. Some animals caught in the high latitude limits of the dugongs’ range on the Australian east coast in winter apparently undertook long distance movements in response to low water temperatures, similar to the seasonal movements of Florida manatees. A 24 km2 seagrass meadow in Hervey Bay, Queensland, Australia was confirmed as important dugong habitat on the basis of the tracking data. Marine videography, Near-infrared Spectroscopy (NIRS) and Geographic Information Systems (GIS) were used to survey, analyse and map seagrass species composition, nutrient profile and patch structure of the meadow at high resolution (200 m). Five species of seagrass covered 91 % of the total habitat area. The total above and below-ground seagrass biomass was estimated to be 222.7 ± s.e. 19.6 t dry-weight. Halodule uninervis dominated the pasture (81.8 %, 162.2 t), followed by Halophila ovalis (35.3 %, 16.5 t), Zostera capricorni (15.9 %, 22.2 t), Halophila spinulosa (14.5 %, 21.9 t), and traces of Halodule pinifolia. Because the distributions of the various seagrass species overlapped, their combined percentage totalled > 100 % of the survey area. The seagrass formed a continuous meadow of varying density. For all seagrass species, the above-ground component (shoots and leaves) possessed greater total nitrogen than the below-ground component (roots and rhizomes), which possessed greater total starch. Because of the relatively low intraspecific variation in nutrient composition, nutrients were concentrated according to seagrass biomass density. H. uninervis was the most nutritious seagrass species because of its higher whole-plant nitrogen (1.28 ± s.e. 0.05 % DW) and starch (6.42 ± s.e. 0.50 DW %) content. H. uninervis formed large, clustered patches of dense biomass across the pasture and thus nitrogen and starch were concentrated where H. uninervis was prevalent. These survey and analytical techniques enabled me to rapidly, economically and accurately quantify and characterise seagrass habitat at scales relevant to a large forager. I used GIS and spatial statistics to identify the role of physical environmental characteristics in determining the activity patterns and fine-scale space-use of dugongs tracked in coastal and deepwater seagrass habitats using GPS telemetry. A seagrass meadow was defined as a core dugong habitat if more than 10 days of satellite location fixes were obtained from an individual animal occupying an area 10 000 km2) dugongs select habitat at the level of individual bays along the Queensland coast; (2) at a landscape-scale (< 10 000 km2), dugongs select seagrass pastures within bays along the Queensland coast comprised of nutritious plant species; (3) at a local-scale (< 10 km2) within seagrass pastures that are within bays along the Queensland coast, dugongs select seagrass patches on the basis of their nutrient concentrations. I recommend that the appropriate scales at which to manage dugong populations and their seagrass habitats be co-ordinated within and across the hierarchical scales of habitat use indicated by my analysis. My finding that dugongs frequently undertake large-scale moves has implications for management at a range of scales, and strengthens the aerial survey and genetic evidence for management and monitoring at ecological scales that cross jurisdictions. The capacity of large-scale monitoring programs to detect trends in dugong numbers at scales of even thousands of km2 is confounded by the dugongs' tendency to undertake large-scale moves. With movement between bays a common occurrence, estimates of population size and trends can only be meaningfully made at regional scales. The tendency for dugongs to track the bottom on large-scale movements may increase their vulnerability to incidental capture in bottom set gill nets. In addition, if dugongs transfer their spatial knowledge of the location of quality food resource patches to their offspring, then local depletions will lead to loss of this knowledge. Areas of high quality seagrass may thus become unknown to dugongs. In the absence of grazing pressure such areas may become less valuable as dugong habitat if the early seral stage species of seagrass preferred by dugongs convert to more fibrous species. My research suggests that dugongs actively select seagrass habitats comprised primarily of H. ovalis and H. uninervis, based on the high starch and nitrogen content of these species. Bays containing these quality food resources comprise an interlinked network of core habitats between which dugongs frequently move. Accordingly, bays along the Queensland coast with seagrass meadows dominated by H. ovalis and H. uninervis should be afforded a high level of protection as potential quality dugong habitat. Bays with extensive intertidal meadows of H. uninervis should also receive enhanced protection, even if the seagrass biomass is low. Even though they have low seagrass biomass, thermoregulatory habitats play an important role in maintaining dugong populations and should be included in dugong habitat protection strategies. |
