Disturbances that influence patterns of benthic assemblages.
Date
2008
Authors
Roberts, Bethany K.
Editors
Advisors
Connell, Sean Duncan
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Thesis
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Abstract
Understanding the influence of disturbance, both natural and human-induced, is a
persistent challenge in ecology. Recently, attempts to predict future environments
have focused on the consequences of broad scale disturbances. In this thesis I
focus on environmental and trophic disturbances as shapers of benthic
assemblages. There is growing recognition of the need for greater scientific
investment in understanding environmental disturbances to balance the continuing
focus of research assessing trophic theories (e.g. herbivory). Historically, it is
these theories that have provided a cornerstone to describe and manage subtidal
rocky coasts worldwide. In this thesis, therefore, I first assess how our ecological
perception of such disturbances (i.e. water pollution and harvesting grazers) may
vary as a consequence of the choice of taxonomic classifications used to observe
benthic patterns (Chapter 2). I then assess how mechanical disturbance (i.e. wave
exposure) may affect the morphology of benthic habitat (Chapter 3) and how
temperature disturbances (i.e. oceanographic, cold water pulsing) may affect the
consumers of these habitats (Chapter 4).
The critical first finding centred on the effectiveness of alternate scales and
metrics of taxonomic classification to detect the effects of water pollution (i.e.
nutrient enhancement) as the largest disturbing agent on the benthos, and that this
effect may be exacerbated by loss of grazers. While observations of the benthos
as morphological groups detected the effects of enhanced nutrients, species
diversity (as a measure of phylogenetic relatedness) was the only one of the
chosen measures sensitive enough to detect the interaction of both top-down and bottom-up stressors. This chapter highlights the importance of choice of
classification (e.g. morphology v. species) and indices (e.g. Shannon index v.
ABC curves and phylogenetic diversity) in their potential to predetermine our
perception of ecological change and thereby predict future environments.
Mechanical disturbance (i.e. wave exposure) has been widely studied as a
mechanism that creates new space for colonisation by alternate species, but is less
well studied as a force that can change the shape of communities by mediating
their morphology. Macroalgal morphology varies in response to wave exposure
such that individuals at high exposures are often smaller than individuals in more
sheltered environments. Observations not only confirmed these patterns for a
general assemblage on a wave exposed southern coast, but reciprocal transplants
of assemblages between exposures also revealed that morphological differences
were likely to be a product of flexibility in morphological response of algae to
local environments (Chapter 3). In contrast to the often multi-directional
responses of a complex suite of morphological characters (e.g. smoothness, stipe
length, frond width), overall size has the potential to be used as a broad and
predictive tool to identify hydrodynamic stressors across an entire exposure
gradient or geographic range.
Strong trophic interactions are often considered characteristic of aquatic systems
and due to their perceived ubiquity on temperate rocky coasts, there has been an
emphasis in the literature on the influence of herbivores in determining
assemblages. Given the importance of the link between herbivores and
assemblage structure, in my final chapter I investigated the potential for disturbance to act indirectly on benthos by affecting the survivorship of an
herbivorous urchin (Chapter 4). I used observations of variable temperature
regimes in a region of upwelling to design an experiment that tested whether this
temperature variation could negatively affect the survivorship of settling sea
urchins (Heliocidairs erythrogramma). When exposed to cold water, mortality
increased by up to 70 %, within 12 h of settlement, representing a massive loss of
benthic consumers within a very short time scale. This result was used to assess
the potential of temperature to indirectly influence benthic habitats across several
spatial scales, a process that may have been profoundly underestimated.
In summary, this thesis provides insight into environmental and trophic
disturbances as shapers of benthic assemblage patterns, both as natural and
human-induced phenomena. I show that our perception of ecological response to
the combination of such disturbances can be contingent on the organisational
scales and metrics used. Subtle differences in initial choice of such observational
units may not only have large affects on the kinds of benthic patterns and
disturbances ecologists detect, but also those that they pursue. I recognise that
while some physical disturbances can appear subtle (e.g. morphological variation)
or strong (e.g. high mortality rate of herbivores), their relative impacts on the
broader assemblage (e.g. understorey flora and fauna) will often be dependent on
biogeography. Integrating local-scale biological interactions with regional-scale
physical processes, therefore, appears to be a potentially progressive line of future
enquiry. Indeed, consideration of responses from the physiological through to
physical and biogeographical scales will not only strengthen our understanding of the effects of alternate disturbance regimes, but also our predictive power to
anticipate future change.
School/Discipline
School of Earth and Environmental Sciences : Ecology and Evolutionary Biology
Dissertation Note
Thesis (Ph.D.) - University of Adelaide, School of Earth and Environmental Sciences, 2008
Provenance
Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.