Determinants of home range and territory size in coral reef fishes

Most animals limit their daily movements to a familiar space known as a home range. However, some animals may also defend part or all of the home range, with the defended portion termed a territory. The twin concepts of home range and territory define an individual's use of space. The determinants of home range and territory size are known to be complex. Body size, diet and food availability can set the minimum area required to meet metabolic demands, for both home ranges and territories. However, population density and interactions with other species may constrain the size of defended areas. Coral reef fishes typically have relatively small home ranges and are also considered to be highly territorial. However, patterns in the use of space are highly variable and the differing determinants of home range and territory size are poorly understood. An understanding of the ecological factors that govern an individual's use of space is critical, not only to understanding the evolution of behaviour, but also provide information on how best to manage species in changing environments. The overall aim of this thesis was to increase our empirical and theoretical understanding of the determinants of home range and territory size in coral reef fishes. I applied both comparative and experimental approaches to generate and test hypotheses concerning a range of factors affecting both home range and territory size in reef fishes. In the comparative studies (Chapters 2 and 3), the goal was to extend the taxonomic data-base to explore the correlates between both home range and territory size, and the factors theory predicts to be important, including body size and trophic status. In addition, I focussed on factors that may specifically apply to coral reef environments such as depth, coral cover and habitat complexity. The experimental studies focussed on the potential roles of food availability and local population density as determinants of territory size in an herbivorous damselfish (Chapter 4) and butterflyfishes (Chapter 5). In Chapter 2, I compared home ranges sizes within and among 24 species from 10 commonly present families of coral reef fishes, representing different body sizes, trophic groups and local habitats at Lizard Island (Great Barrier Reef). Home range sizes varied from 0.5 m² in the territorial damselfish Pomacentrus chrysurus to 350 m² in the butterflyfish Chaetodon melannotus, with an overall average of approximately 62 m². Home range sizes differed among the families studied, being highest in the butterflyfish (Chaetodontidae), emperors (Lethrinidae) and rabbitfishes (Siganidae), and lowest in the damselfishes (Pomacentridae). This appeared to reflect both trophic position (with home ranges largest in corallivores, macro-carnivores and roving herbivores, and smallest in planktivores), and body size (with home ranges larger for typically larger species). Within species, home ranges were commonly found to increase with body size and proximity to the substratum and to decrease with topographic complexity. However, there were no general rules and much of the variation in home range size among species and individuals was unexplained. The results confirm that home range sizes of reef fishes are an order of magnitude smaller than terrestrial vertebrates of similar size. The small home ranges of most reef fishes closely associated with complex reef habitats makes them particularly sensitive to habitat degradation, as they are likely to have limited ability to relocate following local disturbances. The next chapter (Chapter 3) explored the determinants of homing and territorial behaviour in damselfish (family Pomacentridae), a group recognized for having small home ranges, strong territoriality and a close association with coral reef habitats. This comparative behavioural study examines the correlates of variation in home range and territory size in nine different damselfish species that are common at Lizard Island (Great Barrier Reef), and that encompass a range of trophic and social systems. All species exhibited relatively small home ranges, ranging from an average of only approximately 0.2 m² for the coral-associated Chromis viridis to 6.5 m² for the omnivorous Abudefduf sexfasciatus. All species except two small schooling planktivores exhibited some level of aggressive defence of core areas in their home range, but the proportion of the home range defended varied among species. Farming herbivorous species and A. sexfasciatus defended the largest proportion of their home range, while planktivores generally had the smallest home ranges and were the least aggressive. Both home range and territory sizes were weakly correlated with body size and average distance above the substratum, both within and among species. These results confirm that damselfishes commonly defend core areas within their home ranges. Population density and food availability are two key ecological factors known to influence the size of defended areas in territorial animals. Territories are predicted to be smaller at high population densities and with increasing food availability. However, these two factors potentially interact, with high population densities constraining territories and preventing them from expanding when food declines, which may lead to territory abandonment. The aim of Chapter 4 was to examine the correlates of natural variation in territory size in low and high density populations of the herbivorous damselfish Dischistodus melanotus at Lizard Island (Great Barrier Reef). The roles of food availability were then tested by experimentally decreasing and increasing food levels in the low-density population (experiment one) and high density populations (experiment two). In experiment two, the effects of local density were tested by experimentally removing conspecific neighbours. Average territory size was two times larger in low-density populations, compared with high-density populations. Unexpectedly, there was no significant change in territory size in response to experimental increases or decreases in food in the low-density population, but several individuals abandoned territories when food was decreased. Also unexpectedly, individuals in the high-density population did not expand territories when neighbours were removed, but many relocated to occupy new territories. Individuals showed a significant increase in territory size when food was increased, both when neighbours were removed and in the controls. These results suggest that both population density and food availability do influence territory size, but the effects are context specific and do not conform to common theoretical predictions. Chapter 5 focussed on specialised coral-feeding butterflyfishes (family Chaetodontidae). It examined whether patterns of home range use and territorial behaviour relate to coral cover, and experimentally investigated whether individuals expand home range size when coral cover is reduced. Behavioural observations on five species showed that individuals concentrate activity in multiple core areas within their home ranges, which between 80 and 380 m². Core areas appeared to be determined by the presence of preferred coral species. When access to core areas was experimentally reduced for two species by placing cages over the corals in core areas within home ranges, two different patterns were observed. A specialised corallivore (Chaetodon lunulatus) tended to show no response or abandon home ranges, while a more generalised coral-feeder (C. auriga) expanded its home range by over 60%. For both species, individual variation in home range size was positively correlated with body size and rugosity, and negatively correlated with coral cover. These results suggest that the localised availability of corals is critical in determining the location and size of butterflyfish home ranges and territories, which may explain the dramatic population consequences of reef degradation. Overall, this study has shown that the spatial behaviour of coral reef fishes, including both home range and territorial behaviour is complex, and influenced by a range of factors, most notably body size, degree of association with benthic substrata, reef rugosity, population density and food availability. It confirms the general perception that most small reef fish have small home ranges, relative to similar-size terrestrial animals. It confirms that territorial behaviour is commonly associated with small home range size in coral reef damselfishes. These spatial behaviours suggest that most small reef fishes will be amenable to management inside relatively small marine protected areas. However, additional actions to protection reef health will be essential to maintain natural patterns in the behavioural ecology of this diverse group of vertebrates