Dear Marmamers, A PDF copy of the following Ph.D. thesis is available on request. If you are interested in receiving a copy, please email c.d.macleod@abdn.ac.uk. The file size is just over 6mb. Thanks. Colin MacLeod, C.D. 2005. Niche partitioning, distribution and competition in North Atlantic beaked whales. Ph.D. Thesis, University of Aberdeen, Aberdeen, UK. SUMMARY The beaked whales remain the least known family of large mammals. Little is known about much of their ecology and there has been little investigation of ecological differences between species within this family. The primary reason for this lack of knowledge is that studying living beaked whales in the wild is technically and logistically difficult due to their occurrence in deep, oceanic waters, often far from shore. This study aimed to investigate the ecology of the six species of beaked whales that occur in the North Atlantic, and define and compare the niches that each species occupies in this area in relation to a number of factors. To counteract the difficulties of studying beaked whales in the wild, a variety of different approaches were used to investigate their ecology. These included the analysis of stomach contents, biochemical analysis bone collagen, the creation of a Geographic Information System (GIS) and ecological modelling. The ecological aspects that were investigated where prey type preferences, prey size preferences, trophic level, geographic distribution, preferred temperature and latitude ranges, water depth, seabed gradient and seabed aspect.
From these analyses, a general beaked whale niche could be identified, covering the niches of all members of the family Ziphiidae in the North Atlantic, as deep-diving oceanic predators of deep-water squid, fish and to a much lesser extent other organisms, in areas with a sloping seabed and in all latitude and temperature ranges. However, no one species occupies this entire general beaked whale niche and each species occupies its own niche partition defined by individual species preferences for specific ranges of different niche factors. The primary partitioning of the general beaked whale niche is based on prey size, with one group of species, the large prey consumers (LPCs), consisting of two species that consume all prey sizes but preferentially take larger prey, and a second group, the small prey consumers (SPCs), consisting of four species that specialise in consuming smaller prey items. Within the LPCs, the two species are segregated geographically, probably by water temperature preferences, with Cuvier's beaked whale (Ziphius cavirostris) only occurring in warmer, more southern waters and northern bottlenose whale (Hyperoodon ampullatus), only occurring in colder, more northern waters. Within the SPCs, there is a similar geographic partitioning, with Sowerby's beaked whale (Mesoplodon bidens) only occurring in the coldest, most northern waters, Gervais' beaked whale (Mesoplodon europaeus) in warmest, most southern waters and True's beaked whale (Mesoplodon mirus) in intermediate waters between these two species. The last beaked whale species in the North Atlantic, Blainville's beaked whale (Mesoplodon densirostris) differs from all other species in the habitat it occupies, preferentially occurring in over steeper areas of seabed and in relatively shallow waters, especially around oceanic islands, while all other species preferentially occur over areas with more gentle gradients and deeper water depths.
This partitioning of the general beaked whale niche may allow all six species to occur in the North Atlantic without undue levels of competition. For example, LPCs and SPCs can co-exist at the same location because of differences in their diet. However, it is also possible that the observed distributions of each species in the North Atlantic is, in part, determined by competitive interactions between species, with some species being competitively excluded from certain locations. For example, competitive exclusion may explain why the two different LPC species are rarely recorded in the same place at the same time. Differences in adaptations to different water temperature ranges may mean that Cuvier's beaked whales can out compete and exclude northern bottlenose whales from warmer waters, while the opposite occurs in colder waters. Therefore, the distribution of individual species of beaked whale in the North Atlantic may be determined both by the niche each species occupies and by competition with other species with similar niche preferences. The actual niche occupied by a species will define its 'fundamental' distribution (all locations where the conditions are sufficient for the species to survive), while competition with other beaked whale species will, in part, define the species 'realised' distribution where the species can exclude potentially competing species and so where it actually occurs. ================================================== Dr. C.D. MacLeod, Ph.D. School of Biological Sciences (Zoology), University of Aberdeen, Tillydrone Avenue, Aberdeen, AB24 2TZ, UK Tel: 01224 272648 Fax: 01224 272396 Email: c.d.macleod@abdn.ac.uk ===================================================