Understanding the mechanisms that regulate local species diversity and community structure is a
perennial goal of ecology. Local community structure can be viewed as the result of numerous
local and regional processes; these processes act as filters that reduce the regional species pool
down to the observed local community. In stream ecosystems, the natural flow regime (including
the timing, magnitude, and duration of high and low flow events) is widely recognized as a
primary regulator of local diversity and community composition. This is especially true in aridland
streams, where low- and zero-flow events can occur frequently and for extended periods of
time (months to years). Additionally, wetted habitat patches in arid-land stream networks are
often fragmented within and among stream networks. Thus dispersal between isolated aquatic
patches may also play a large role in regulating local communities. In my dissertation, I explored
the roles that drought, dispersal, and local habitat factors play in structuring arid-land stream
communities. I examined the impact of flow permanence and seasonal variation in flow and
other abiotic factors on aquatic communities at both fine spatial scales over a long time period (8
years; Chapter 2) and at a broad spatial scale over a shorter time period (1-2 years; Chapter 4).
Additionally, I quantified aquatic invertebrate aerial dispersal over moderate spatial scales (��� 0.5
km) by conducting a colonization experiment using artificial stream pools placed along and
inland from two arid-land streams (Chapter 4). Finally, I examined the roles of spatial isolation,
microhabitat type, and local abiotic and biotic factors in structuring aquatic communities in
freshwater oases scattered across one of the most arid regions of North America, the southern
Sonoran Desert (Chapter 5). In Chapter 2, I found that severe drought caused an unprecedented
drying event in isolated perennial stream pools, and that several additional drying events occurred
over the following four years. This transition to intermittent flow caused the extirpation of
several large, long-lived species with low dispersal abilities (including the top predator) and
drove the local community into an alternative state. In the colonization experiment described in
Chapter 3, I found that several arid-land stream invertebrate taxa disperse widely and frequently.
The widespread dispersers identified by this experiment included several of the earliest colonist
taxa observed following the severe drought described in Chapter 2. Other taxa, though, only
dispersed overland after receiving an environmental cue (rainfall) or preferentially dispersed
along stream corridors. In Chapter 4, where I examined invertebrate community structure across
a large network of well-connected intermittent and perennial reaches, I found low diversity in
intermittent reaches, regardless of their connectivity to diverse upstream perennial reaches. These
species-poor, intermittent communities were composed of a unique suite of species with lifehistory
adaptations that conferred desiccation resistance, including extended egg and larval
diapause stages. The short flow duration of intermittent reaches (<100 days) likely precluded
upstream perennial taxa from establishing populations in downstream intermittent reaches before
drying occurred, while the relative predictability of flow timing (Dec-Apr) likely allowed for a
small number of species to develop appropriate life-history traits (e.g., diapause stage, rapid
development time) to exploit these temporally-fleeting habitats. In Chapter 5, I found over 220
species of aquatic animals (including ��� 5 undescribed species) in the 19 desert oases that were
sampled across the southern Sonoran Desert. Local community composition in these oases was
strongly driven by microhabitat type. Additionally, native aquatic species richness and
abundance in these oases were significantly reduced by the introduction of tilapia, an exotic fish
species. The threats to arid-land streams presented by increased drought severity, anthropogenic
water withdrawals, and local habitat degradation (e.g., introduced species, unmanaged
recreational use) are grave across the southwestern US and northwestern Mexico. I hope that in
addition to furthering our understanding of ecological processes in arid-land streams, this
dissertation makes a small contribution towards the efforts to preserve these habitats. / Graduation date: 2013 / Access restricted to the OSU Community at author's request from July 18, 2012 - July 18, July 2013
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/31292 |
| Date | 18 July 2013 |
| Creators | Bogan, Michael T. |
| Contributors | Lytle, David A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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