Problema presentado

3 Chapter 3: Fish habitat use within and across tropical and temperate reefs

3.1 Introduction

Previously, the effects of changing reef habitat on the entire reef – associated fish community were discussed in a tropical ecosystem (Chapter 2). The current chapter focused on a number of fish species, commonly occurring in these tropical ecosystems, to examine the effect of behavioural variation in habitat use though niche partitioning. At the same time, the effect of behavioural variation in niche partitioning was also observed for a number of species from a temperate ecosystem. The mechanisms of fish habitat use were subsequently compared within, as well as across the ecosystems studied. As a result, this chapter increases the understanding of the role the habitat plays in niche partitioning processes responsible for sculpturing fish communities.

The species’ niche can be described as an n-dimensional hyperspace partitioned into environmental and trophic (resource) components, representing the ecosystem (Hutchinson, 1957). The fundamental niche is defined as the space resulting from the combination of physiological and behavioural characteristics in the absence of competition, when the species potentially occupies the space along all the axes of the ecosystem (Begon, Harper and Townsend, 2006). The presence of physiologically intolerable environmental conditions, dominant competitors and predation pressure, along the ecosystem axes, prevents the species to fully exploit the entire ecosystem, so the constrained space actually occupied by the species under these effects represents the realised niche (Hutchinson, 1957;

Whittaker, Levin and Root, 1973; Devictor et al., 2010). As a result of many different simultaneous interactions shaping and defining its size, realised niche space is n-dimensional. In order to measure any dimension of the realised niche, an interaction of the

specific acting agents needs to be considered. Empirical studies encompassing in situ niche measurements are extremely scarce and the relative roles of the interactions which shape species’ realised niches, other than those based on species’ phenotypic traits, remain poorly understood (Hooper et al., 2005; Tingley et al., 2014).

How habitat influences the distribution of organisms is of central importance to ecology (Nanami et al., 2005), with a number of studies reporting species-specific habitat associations (Jones, 1991). To date however, in order to understand communities and ecosystems, studies generally do not take into account the organisms’ evolutionary history for evaluating the species’ ecosystem function and subsequently quantifying the species’

niches (Petchey and Gaston, 2006; Villéger, Novack-Gottshall and Mouillot, 2011). In 2014, Bellwood and Brandl pioneered a study, by measuring in situ, part of the realised niche of herbivorous fish species on a tropical marine coral reef (Brandl and Bellwood, 2014). The niche sizes of the species studied were calculated and were subsequently used to infer the degrees of the redundancy and complementarity between the species.

To describe a realised niche, currently most interest is based around the measurement of the niche space, delineated by a number of individuals with the most extreme positions within the sample, represented by the total area of the convex hull (TA) encompassing the data points (Jackson et al., 2011; Brandl and Bellwood, 2014). The niche sizes obtained using this method are generally characterised by a smaller number of divergent individuals within the population and thus measuring the TA takes into consideration individual variability within a species. More recently, a different method based on the measurement of the standard ellipses (SEA) has been proposed. While SEA provide ecologically relevant information about the individual, population or community they represent (Bearhop et al., 2004), the SEA metric has only been discussed for the calculations of the isotopic niche overlaps

(Jackson et al., 2011; Syväranta et al., 2013). The use of SEA has been argued mainly because this method may not be as strongly influenced by the sample size as the convex hull method. The SEA essentially represents the niche mean values of the community in question (Jackson et al., 2011). Ecologically speaking this method does not emphasise the importance of the within-species individual variability and likely represents the niche space delimited by the most frequent and abundant traits characterising the individuals sampled. As such, the SEA niche measurement method is representative of the niche of the majority of the individuals within the population, but not of the few divergent individuals.

Using individual based, continuous behavioural data, estimations were made of realised niches sizes, the niche overlaps and the interactions between the TA and SEA metrics, for the most commonly occurring species of the fish families representative of distinct feeding guilds within a tropical coral- and temperate algal- dominated reef habitat. Patterns of fish habitat partitioning were observed and compared for species, within each and across different ecosystems, as a function of the space occupied at any given moment, while considering behaviour traits, based on what the fish do while moving, as well as the total time dedicated to each habitat occupancy and related behaviour. By carrying out the continuous individual behavioural observation, documenting all behavioural units and habitats used, a new way of assessing realised niches of species in the habitats studied is offered. Firstly, realised niche overlaps were compared by measuring the TA of each species, but as an alternative, a metric based on SEA was also used. Subsequently an attempt was made to give a comprehensive comparison of the differences in the realised niche sizes between the two metrics used. In order to address the issue of the adequate sample size encountered in ecological studies, an alternative sample size correction was presented for the both metrics and the pros and cons of using either was discussed. The following hypotheses were tested: i) the time budget for occurrence in a specific habitat and expressing

certain behaviour associated with a substrate type will be distinct for fish species within and across ecosystems; ii) realised niche sizes will differ between species and ecosystems; iii) the optimum sample size for measuring the realised niche will be independent of the ecosystem studied but dependent on the measurement metric used.

3.2 Materials and methods

Field-based observations were conducted using behavioural time budgets to measure part of the realised niche and their overlaps and subsequently test for sample size dependency for a number of fish species on tropical coral- and temperate algal-dominated rocky reefs.