Processes underlying the fine-scale partitioning and niche diversification in a guild of coral reef damselfishes

Eurich, Jacob G. (2018) Processes underlying the fine-scale partitioning and niche diversification in a guild of coral reef damselfishes. PhD thesis, James Cook University.

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Abstract

A major goal of ecology is to explain the mechanisms that drive species distributions and ecological partitioning along gradients in the natural environment. The distributions and coexistence of ecologically similar animals may depend on the degree of niche diversification and competitive interactions within and among species. The extent of ecological partitioning in guilds of coral reef fishes was hotly debated in the 1980s, and despite 4 decades of research, the issue remains unresolved. In particular, the link between niche partitioning and agonistic interactions together have received little attention. In the thesis I investigated finescale species distributions, resource use (e.g., habitat and food), and competition in a guild of 7 territorial damselfish species in Kimbe Bay, Papua New Guinea. Common generalisations about the ecological function of territorial damselfish and associated interactions with important roving herbivorous fishes were also investigated. Using ecological surveys, laboratory-based analytical methods, observational studies, and manipulative field experiments, this thesis addresses novel questions about the ecology and functional role of territorial damselfish and the resulting community effects.

Competition over resources is recognised to play a primary role in the structure of coral reef fish communities. The distribution of ecologically similar species may depend on the degree to which traditional niche mechanisms operate alongside competitive dynamics. In Chapter 2 these effects were examined by investigating fine-scale species distributions, microhabitat use, and aggression among territorial damselfishes. I documented patterns of habitat partitioning across the 3 reef zones – reef flat, reef crest, and reef slope – with distinct patterns of distribution within these zones at extremely fine scales (1 − 2 m). Distinct differences between neighbouring species in the microhabitat use selected were also observed. Furthermore, aggression elicited by neighbouring species was significantly higher for all species, compared with non-adjacent species. This chapter revealed a fine level of spatial partitioning among reef zones and microhabitats in this guild of damselfish, which was likely maintained by agonistic interactions among neighbouring species.

While Chapter 2 found that neighbouring damselfish constrain their microhabitat use to facilitate the co-habitation of reef zones, microhabitat selectivity alone was insufficient to explain the distinct zonation and limited distributional overlap. I hypothesised that in this highly partitioned ecological community, where there are intense agonistic interactions for resources, ecologically neighbouring territorial damselfish may also partition food resources. Other studies had previously quantified diet contributions for the study species, but no isotopic analysis had been conducted for territorial damselfish to date to specifically target pelagic-based food sources. In Chapter 3 I quantified the trophic niches of the territorial damselfish guild through the use of stable isotopes (δ¹³C and δ¹⁵N). Adjacent species on the reef flat, reef crest, and reef slope exhibited high to intermediate trophic niche partitioning when examining pelagic versus reef-based dietary sources, with two species previously described as benthic herbivores actually exhibiting pelagic feeding. Findings of Chapter 3 indicate that diet choice reinforces the patterns of spatial partitioning and coexistence among ecologically similar species. In addition, evidence of planktivorous pelagic feeding adds to the growing view that interspecific differences among similar species are lost when categorizing species into broad functional classifications.

In Chapter 4 the direct and indirect effects of interference competition on resource partitioning were measured. The previous chapters found that microhabitat selectivity and dietary diversification facilitated the co-habitation of reef zones among the territorial damselfish guild. Additionally, evidence suggested that interspecific aggression helped maintain the distributional boundaries between neighbouring species. However, an experimental removal of a dominant competitor was necessary to understand if interference competition is present and if subordinate distributional shifts would occur. I employed an observational experiment and a large-scale removal experiment (220 m²) to examine the intensity of agonistic interactions among species and the extent to which the most abundant species influenced the distribution and abundance of neighbouring and non-neighbouring species in the guild. The findings indicated that the distinct distribution patterns among the reef crest species were linked to levels of interspecific agonistic behaviour. The competitive release following the removal of a superior competitor resulted in comprehensive direct and indirect effects, with the subordinate neighbour shifting into the newly available space, followed by successive shifts in species responding to the change in the distributions of their immediate neighbours. Through a novel multi-species large-scale experiment, Chapter 4 provides the conclusive evidence that distributions and the coexistence of the territorial damselfish guild are a result of niche diversification and competitive interactions within and among species.

Chapter 5 broadens the thesis by investigating the common perception that all territorial damselfish negatively influence overall reef function and roving herbivorous fishes. Similar to Chapter 2 where generalisations about the guild's diet were questioned, here I challenged the paradigm that intermediate-sized territorial damselfishes have a negative influence on surgeonfish, parrotfish, and rabbitfish abundance and foraging behaviour. To test this, I conducted experimental removals (220 m²) of the most abundant territorial damselfish to examine its impact on roving herbivores and the benthic community structure. The overall relative abundance (MaxN) of roving herbivores was not influenced by the removal. No changes in foraging patterns were observed for parrotfish, the family that received the highest rate of agonistic interactions, and rabbitfish. Instead, the removal resulted in a significant decrease in surgeonfish feeding, suggesting the territorial damselfish species altered foraging patterns indirectly through territorial maintenance and not aggression. The results indicate that all territorial damselfishes do not have a negative impact on all roving herbivores and instead may enhance surgeonfish foraging indirectly through the removal of sediment. The generalisation that territorial damselfish reduce foraging rates of roving herbivores may not be applicable in all systems or for all species.

In summary, this thesis investigated the mechanisms that drive species distributions and ecological partitioning along gradients in the natural environment as a precursor to the long-term ecological changes on coral reefs. It first established fine-scale partitioning in a guild of competing fishes on a high-diversity coral reef. Second, the research showed that microhabitat selectivity and dietary diversification facilitates the co-habitation of reef zones, and that interspecific aggression maintains the distributional boundaries between neighbouring species. In order to demonstrate how competitive interactions and resource partitioning influence species coexistence in a complex ecosystem a large-scale field experiment was conducted. The results demonstrate that when exploring coexistence in reef fish communities, the more traditional niche mechanisms operate alongside direct and indirect competitive dynamics, and within highly diverse systems these ecological processes are magnified. Moreover, the thesis highlights the importance of challenging common generalisations and paradigms. By examining the functional role of territorial damselfishes this research provides evidence of novel dietary diversification and demonstrates the complexity of territorial damselfish and roving herbivore interactions.

Item ID: 55992
Item Type: Thesis (PhD)
Keywords: aggression, agonistic behavior, competition, coral reef fish, coral reef, damselfish, depth gradient, distribution gradient, distribution, ecological versatility, interspecific competition, intruder experiment, multi-species, niche, Papua New Guinea, parrotfish, Pomacentridae, rabbitfish, removal experiment, resource partitioning, resource use, sediment dynamics, species loss, surgeonfish
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Copyright Information: Copyright © 2018 Jacob G. Eurich.
Additional Information:

Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 2: Eurich, J.G., McCormick, M.I., and Jones, G.P. (2018) Habitat selection and aggression as determinants of fine-scale partitioning of coral reef zones in a guild of territorial damselfishes. Marine Ecology Progress Series, 587. pp. 201-215.

Chapter 3: Eurich, J.G., Matley, J.K., Baker, R., McCormick, M.I., and Jones, G.P. (2019) Stable isotope analysis reveals trophic diversity and partitioning in territorial damselfishes on a low-latitude coral reef. Marine Biology, 166 (2). 17.

Chapter 4: Eurich, Jacob G., McCormick, Mark I., and Jones, Geoffrey P. (2018) Direct and indirect effects of interspecific competition in a highly partitioned guild of reef fishes. Ecosphere, 9 (8). e02389.

Chapter 5: Eurich, Jacob G., Shomaker, Simone M., McCormick, Mark I., and Jones, Geoffrey P. (2018) Experimental evaluation of the effect of a territorial damselfish on foraging behaviour of roving herbivores on coral reefs. Journal of Experimental Marine Biology and Ecology, 506. pp. 155-162.

Date Deposited: 21 Jul 2019 23:04
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970106 Expanding Knowledge in the Biological Sciences @ 100%
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