| dc.description.abstract | Two families of brown macroalgae that occur in sympatry domínate
temperate subtidal rocky coasts: the Laminareales, and the Fucales. Both ofthese
families are habitat-forming species for a wide variety of invertebrates and fishes.
Variation in the presence, density, and composition of brown macroalgae can
have large influences on the evolution and ecology of associated organisms.
Here, using a series of observational and experimental studies, I evaluated the
effects of heterogeneity in the composition of brown macroalgal stands at the
population and community levels for reef fishes.
A central ecological challenge is the description of pattems that occur at
local scales, and how these are manifested at larger ones. I conducted further
sampling across a set of sites nested within locations over three regions, Juan
Fernández Islands (Chile), Northem New Zealand, and Tasmania (Australia), to
evaluate pattems of variation in the diversity and composition of fish
assemblages. Specifically, I explored spatial variation in fish assemblages as a
function of rocky reef habitats (dominated by brown-macroalgae) and other
sources of variation ( abiotic and biotic factors) that potentially mediate the
relationship between fishes and reef habitats. Analyses suggest that spatial
variation in diversity (e.g., species and trophic) may be explained by spatial
variation in depth, temperature, and composition of macroalgal habitats. At each
location, only 2-3 families dominated the composition of fish assemblages, but
species identities varied among locations. In a subsequent study, I assessed the
fish-habitat associations from sites within the Juan Fernández Islands, an isolated
eastem Pacific Island that lack large brown macroalgae. I found that, despite the
close proximity of these Islands to the South American continent, fish assemblages were mostly composed of endemíc representatives from families
that domínate the fish assemblages in New Zealand and Australia. Spatial
variation in depth and temperature did not contribute to the observed variation in
fish abundance. Instead, I found that benthic habitat-formíng species
(particularly foliose brown macroalgae) appeared to limit the abundance ofsome
reef fishes. These results suggest that a mixture of large-scale ( e.g., stochastic
recruitment) and small-scale processes (i.e., relating to habitat heterogeneity)
influence the diversity, compositíon and abundance offish assemblages.
Subsequently, I evaluated relationships between reef fishes and
macroalgae composition across multiple sites, surveyed repeatedly over four
seasons. I found that fishes were associated with different components of
heterogeneity in macroalgal habitats, potentially indicating interspecific
partitioning of resources that may arise from differential feeding habits and sizesusceptibility
to predation. Seasonal variation in the fish-habitat associations was
detected, and site differences in macroalgal composítion explained significant
variation in the local diversity of fishes.
Using a series of small-scale lab and field-based manipulative
experiments, I determined the demographic and behavioural responses of reefassociated
fishes to heterogeneity in the composition of brown macroalgal
habitats. I found that (i) different fish species distinguished between
monospecific macroalgae stands (macroalgal identity affected the abundance of
7 ofl5 reeffish species); (ii) there is within-species variation in the response of
fishes to macroalgal composition (suggesting ontogenetic habitat shifts); and (íii)
the abundance of 5 of 7 reef fish species, and the overall structure of the local
fish assemblage, varied with the composition ofmixed-species macroalgal stands. Lastly, I evaluated the potential for fishes to prov1s1on demographic
feedbacks to macroalgae. Specifically, I conducted a mesocosm experiment to
evaluate the effects of fishes on grazing amphipods, and therefore, the potential
indirect-effects offishes on large-brown macroalgae. I found that only one ofthe
two fish species studied reduced grazer abundance. Although the second fish
species did not consume grazing amphipods, its presence altered amphipod
behaviour to significantly reduce grazing efficiency on the macroalgal-host.
This study illustrates how density and trait-mediated indirect interactions can
have similar effects on primary producers.
Overall, my observational and experimental components of this thesis
emphasize the influence ofheterogeneity in macroalgal structures on the breadth
of habitat use for reef fishes at multiple locations. I found strong behaviourally
mediated linkages between the abundance of reef fishes and composition of
macroalgal stand. I also provide sorne evidence that mutualistic relationships
may exist between kelp and associated fishes. | |
