Evaluating the application of island biogeography theory and
Evaluating the application of island biogeography theory and the
effects of insularity in the Boundary Waters Canoe Area Wilderness
Brian Hull, Alexandra Karosas, Brenna OGorman, and Ernest Ruiz
Faculty Advisor: Dr. Todd Wellnitz
Department of Biology, UW-Eau Claire
Island biogeography explores the factors that may
effect the species richness of islands. Few studies have
been done on the biogeography of the islands in the
Boundary Waters Canoe Area Wilderness (BWCAW), a
federally protected area on the border of Minnesota and
Canada that encompasses 1.3 million acres of pristine
forests, lakes, and islands.
Islands of various sizes and distances from shore were chosen for sampling. Area
and distance were estimated in the field using a map and measured in the lab using
Google Earth .
Woody plants were chosen as the species to sample because of their importance in
providing food and habitats for local organisms.
Our objective was to determine which of two different
theories of island biogeography better describes the
species richness of islands in the BWCAW.
A belt transect was made across the longest part of each island. The transect was
consistently 1m wide and the length of sections doubled continuously, starting with 1m.
The number of different species within each section of the transect was counted and
The Habitat Diversity Theory (HDT) states that the
number of species on an island is a function of the
number of habitats on that island (i.e. larger islands will
have more habitats).
The data from each belt transect was used to calculate species to area ratio. Total
number of species on each island was estimated by taking the largest count of species
for each transect.
The Equilibrium Theory of Island Biogeography (ETIB)
states that the species richness of an island is a function
of immigration and extinction of species, determined by
island size and distance from shore (i.e. the larger and
closer an island is, the more species it will have).
HYPOTHESIS: The HDT will better determine species
richness because more habitats will provide additional
environments for different species to live in. Islands in
the BWCAW do not vary enough in their distance from
shore for the ETIB to be plausible.
Number of habitats were determined by qualitative observation and by comparison of
canopy cover, slope, and analysis of soil samples for pH, conductivity, and % organic
Fig. 6 A view of one of the islands sampled on Ensign Lake.
As island size
increased, both number
of habitats and species
As number of habitats
Fig. 7 - Sketch of an average habitat layout of the islands.
1: Coastal, low shrubs, sometimes rocky
2: Coniferous trees, sometimes sloped (by varying degrees)
3: Grassy and open, fallen trees, new growth
Fig. 1 - Alexandra takes canopy
cover data as Brian counts species.
Fig 2 - Running a transect
through a dense island forest.
Fig 3 - Laying out a belt transect in
a rare open area.
Fig 4 The team chooses islands
Fig. 5 Path model representing species richness on 11 islands of the
Boundary Waters Canoe Area as a function of island area, number of
habitats, and distance to nearest shore. Island area positively affected the
number of habitats (r=0.81, P < 0.001) and species richness increased with
habitat availability (r=0.53, P < 0.012). Island area also had a direct effect on
species richness (r=0.44, P < .038). Distance to nearest shore had no effect
on species richness. Model fit: df = 1, GFI = 0.98, P = 0.49.
Island size may have
had an indirect effect on
species richness because
of its significant effect on
number of habitats.
Distance from shore had
no significant effect on
total number of species.
Acknowledgements: Special thanks to Bill Hintz and Dr. Evan Weiher for their help with statistical analysis.
Our data suggests that the Habitat Diversity
Theory better explains species richness of
woody plants on islands in the BWCAW than the
Equilibrium Theory of Island Biogeography.
Larger islands contained a greater amount
of habitats, providing a more diverse
environment and allowing for greater species
diversity. Distance from shore had no effect on
species richness, likely due to the limited space
between islands and the mainland in the
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