“Biogeographic affinity” describes the similarities in the flora and fauna of one location compared to those of another location, resulting from their common evolutionary history and/or their contemporary connectivity (Loveland and Merchant 2004). The existence of the biogeographic affinity hypothesis allows the hierarchical classification of regions into faunal, floristic or other biogeographic provinces. Additionally, the latitudinal biodiversity gradient (LBG; Brown and Lomolino 1998) is normally illustrated by a diverse tropical fauna at low latitudes and decreasing species richness with increasing latitude. Here, I compare the relative influences of biogeographic affinity and the primary ecological gradients of the southern Colorado Plateau in the Grand Canyon ecoregion (GCE, Table 2; elevation, litter accumulation, and anthropogenic influences) on the distribution and abundance of land mollusks. I test four specific null hypotheses: (1) Biogeographic affinity does not influence the distribution of GCE land mollusks, (2) GCE land mollusk species and assemblages are uniformly distributed across elevation, analogously reflecting the absence of biogeographic affinity, (3) Habitat patch size (spring area) does not influence GCE land mollusk species richness and (4) The GCE land mollusk assemblage is not strongly influenced by microhabitat factors that are independent of elevation impacts.
To test my hypotheses, I used classic and contemporary literature and museum collections to build a comprehensive species list for the Grand Canyon ecoregion. This list included spatial (location, elevation) and ecological (biogeographic affinity) data to test hypothesis 1 (proportional representation) and hypothesis 2 (neotropical to boreal distribution. across elevation). I also used randomly sampled terrestrial mollusks in three elevation zones at 19 paired spring and matrix habitat sites, while also recording environmental data on leaf litter depth, isopod presence or absence, grazing presence or absence at each sample, canopy cover and spring area at each site. These data were used to test hypothesis 3 (spring area-species diversity relationships) and 4 (measured site- specific independent variables will predict species diversity).
The results of this analyses show: 1. The GCE land mollusk fauna is not equally distributed across 7 North American Biogeographic Provinces. Instead, the GCE is overrepresented by boreal, alpine and temperate species. 2. This two-dimensional pattern of overrepresentation was preserved across (3-dimensional) elevational gradients within the GCE. Communities were at least 84% dissimilar between elevation zones, with New- World temperate and wide-ranging circumpolar species being the dominant drivers the mollusk community composition even at low elevation sites. 3. Shannon-Weiner diversity (supporting the Latitudinal Biodiversity Hypothesis) and leaf litter depth (productivity) within springs decreased with increasing elevation. 4. Among the measured physical (spring/matrix, litter depth, canopy cover, species-area) and anthropogenic (isopods, grazing) factors, elevation was the only one found to predict species community composition.
These results indicate that while springs in the southwest USA are presumed to be relictual habitats, temperate land mollusks still possess an apparent advantage over neotropical species even at low elevation springs, where they are more abundant and diverse. These data support the Latitudinal Biodiversity Gradient (LBG) in a novel and unusual way, where low elevation sites are the most diverse, but are characterized by wide-ranging temperate and boreal species, with significant community compositional change across elevation.
|Commitee:||Stevens, Larry, Whitham, Thomas|
|School:||Northern Arizona University|
|School Location:||United States -- Arizona|
|Source:||MAI 56/05M(E), Masters Abstracts International|
|Subjects:||Ecology, Conservation, Natural Resource Management|
|Keywords:||Affinity, Arizona, Biodiversity, Biogeography, Elevation gradient, Grand Canyon, Land mollusca|
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