Priority effects and the dynamics of coral reef fish assemblages

Poulos, Davina Elise (2016) Priority effects and the dynamics of coral reef fish assemblages. PhD thesis, James Cook University.

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Understanding the processes and mechanisms by which organisms form and maintain structured assemblages is a central goal of ecology and a key component in being able to predict how environmental or ecological disruption will affect them. Individuals entering an existing population must successfully integrate to increase chances of survival, yet when resources are limited, competition for food and shelter can influence their persistence. In demographically open populations, recruits arrive somewhat stochastically from outside of local populations, meaning that the outcomes of deterministic processes at play upon arrival will be critical to securing future success. However, sequence and timing of arrival play an important role in the magnitude of these deterministic processes, suggesting that 'priority effects' play an essential part in the fate of arriving individuals, and subsequently the structure and dynamics of the assemblage. The recruitment of coral reef fishes is a perfect model in which to study these priority effects, since in many species, new settlers arrive in 'pulses' throughout the recruitment season, with sporadic inputs of fishes closely tied to lunar phases, and these settlers must rapidly integrate into the resident population or face immediate mortality.

With variation in the order and abundance of individuals arriving to habitats occupied by varying numbers of existing individuals ('prior residents'), understanding the complex behavioural mechanisms involved in who succeeds enables identification of the factors driving population structure. Chapter 2 explored, for a juvenile damselfish, the influence of prior residents on the success of newly arriving individuals and how this changed over time as subsequent 'pulses' of intruders (new settlers) were experimentally added at varying conspecific densities. Aggressive interactions occurred most strongly between individuals closest in the social hierarchy and partly explained mortality trajectories. The direction of aggression changed over time as the occupants of the habitat changed, through arrival of intruders and mortality of residents. This evidence stresses the importance of priority effects in influencing the development and composition of fish assemblages.

However, the unique set of interaction experiences possessed by an individual can also affect how they behave and their likelihood of winning future encounters, which influences their position in the social hierarchy. Chapter 3 used a field experiment to investigate the circumstances under which residency (resident or intruder), behavioural history (prior dominance or subordinance) and body size determined the direction and strength of intraspecific interactions. Results showed the hierarchical importance of body size, priority, and prior behavioural history, suggesting the latter had the least influence on the outcome of future confrontations. Aggression affected space use on a habitat patch and was itself affected by relative size difference between combatants. These findings demonstrate how an individual's physical and behavioural characteristics, and those of its competitors, interact to influence dominance status and subsequently the structure and dynamics of assemblages.

The effects of body size on the outcome of competitive interactions were pronounced in Chapter 3, thus Chapter 4 explored whether body condition also had a significant impact on post-settlement success. Body condition varies substantially in settling reef fish, as a result of their larval history and experiences. A 2 x 2 factorial field experiment was conducted that manipulated body condition (high or low, manipulated through a feeding regime), and residency (resident or intruder, where the resident arrived at the habitat three hours before the intruder) to evaluate effects on competitive ability and survival. Prior residency alleviated the disadvantage of a low body condition with respect to aggression, which was similar between low condition residents and high condition intruders. Mortality trajectories suggested that body condition modified the effect of prior residency, and intruders were more likely to suffer mortality if they had a low body condition because residents pushed them away from shelter. These results highlight that the negative effects of some traits may be compensated for by the positive effects of other traits, and that the specific ecological context an individual faces (such as the characteristics of its competitors) can have a major influence on successful establishment and persistence.

To put the role of priority effects into the context of our changing environment, Chapter 5 was designed to investigate: 1) how sequence and timing of arrival affects interactions between a habitat generalist and a specialist in healthy and degrading environments, and 2) how prior residency interacts with habitat quality and resource specialisation to affect propensity to migrate. We used the damselfishes Pomacentrus amboinensis, a habitat generalist, and Pomacentrus moluccensis, a live coral specialist. Results demonstrated that the strength of priority effects (i.e., aggression intensity) increased with increasing timing of arrival when the specialist arrived after the generalist, suggesting that as the value of the habitat increased (owing to a temporal increase in ownership duration), the motivation to defend it also increased. Propensity to migrate from dead to live coral was greater for the specialist, however arriving late (after the generalist) significantly reduced willingness to migrate to its preferred live coral habitats, indicating evidence of an inhibitory priority effect, directly affecting future persistence. The degree to which ecological versatility and priority effects combine to modify competitive outcomes in coral reef fishes has important consequences for the persistence of specialist species in the face of environmental degradation, and has substantial implications for predicting how our changing environment will affect fish community dynamics.

This study demonstrates the important role that priority effects have on individual success, and the repercussions this has on the structure and functioning of assemblages, by influencing the outcome of competitive interactions. The fate of late arrivers is not a foregone conclusion, but in fact such a disadvantage may be modified or alleviated by a series of other advantageous traits. Furthermore, evidence from this study reveals the underlying behavioural mechanisms that contribute to and are affected by priority of access to resources, suggesting that the strength and direction of competition is directly influenced by temporal variation in arrival. The effects of sequence and timing of arrival should be considered when examining the factors affecting the assembly of organisms.

Item ID: 46889
Item Type: Thesis (PhD)
Keywords: aggression, behavioural interaction, body condition, competition, coral reef fish, dominance, interference competition, Pomacentrus amboinensis, priority effect
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 3: Poulos, Davina E., and McCormick, Mark I. (2014) Who wins in the battle for space? The importance of priority, behavioural history and size. Animal Behaviour, 90. pp. 305-314.

Chapter 4: Poulos, Davina E., and McCormick, Mark I. (2015) Asymmetries in body condition and order of arrival influence competitive ability and survival in a coral reef fish. Oecologia, 179 (3). pp. 719-728.

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Date Deposited: 18 Jan 2017 01:10
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060201 Behavioural Ecology @ 100%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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