The wide open prairie
that is characteristic of the American heartland is, these days, increasingly
narrow and fractured, found only in the interstices of expansive fields of corn
and soybeans. Habitat loss from the conversion of prairie to cropland, of course,
has serious implications for the flora and fauna that make up the prairie
ecosystem. But grassland species in small remnants or reconstructions may also
face additional pressures - at the boundaries of these patches of prairie,
where one type of habitat meets another, organisms often exhibit interesting
variations in population dynamics that differ from those found in the middle of
a patch. In ecology, the concept of edge effects attempts explain and
quantify this differentiation. The ratio of area to perimeter contributes to
how vulnerable an area is to invasion from predators, diseases, or other
threats that exist outside of its borders.
Figure 1: Because of the
ratio of perimeter to area, the size and shape of a patch of habitat influences
how much of it is affected by edge effects. http://www.bio.utexas.edu/faculty/sjasper/Bio301M/biodiversity.html
In a study on population
responses to different types of edge habitat, William Jensen and Elmer Finck
examined nest densities of dicksissels (Spiza americana) in
several patches of tallgrass prairie in Eastern Kansas, in the Osage Cuestas
and Flint Hills regions. The dickcissel is a songbird species that is often used
to gauge the success of prairie restorations due to its dependence on prairie
habitat. Dicksissels are, of course, vulnerable to nest predation. They are
also vulnerable to parasitism from cowbirds, which lay their eggs in other
songbirds’ nests, either outright killing the nestlings that are present or
greatly decreasing their probability of survival due to competition with the
cowbird hatchling for heat, space, and food. Jensen and Finck measured the
density of nests and the percentage that had been predated and parasitized in
three different settings: the edge of grassland abutting woodland, the edge of
grassland abutting cropland, and the interior of grassland. They hypothesized
that cropland edges would be the more favorable edge type for dicksissel nests
(lower rates of predation and parasitism and higher density), since parasitic
cowbirds and some predators favor grassland-woodland edges. They found that
parasitization is higher the closer the nests are to woodland edges, but not in
those near croplands, most likely because cowbirds cannot find perches on which
to survey nests to parasitize along grassland-cropland border. However,
dicksissel success along cropland edges was more variable, probably due to
human disturbances. Overall, their results suggest that prairie fragments with
cropland edges may be more suitable for grassland birds than those with
woodland edges, an important consideration in conservation efforts.
Fig. 2. Distance from
edge compared with success rates of dicksissel nests. Dark bars represent study
sites with woodland edges, while white represents those with cropland edges.
Some results show a gradient with distance, like increased parasitization with
closeness to a woodland edge and increased predation in the interior of grasslands.
Jensen and Finck’s
research indicates that the type of habitat found at the edge of a prairie
remnant or reconstruction can have a significant effect on its inhabitants. Our
study addresses the plant community in small urban prairie plantings, which are
increasingly popular for their aesthetics, their ecosystem services, like erosion and storm water
reduction, pollutant filtration and habitat for pollinators, and their educational value. Our research,
conducted in a small, narrow prairie planting (1-20m in width and 270m in
length) adjacent to Grinnell College’s athletic center, examines abundance of
invasive or weedy species and of a few common forbs, compared to the distance
from the edge of the planting as well as type of edge. Like the study of
dicksissels, we examined two different types of edges, fertilized lawn and
herbicide-sprayed railroad, as well as the interior of the habitat, and saw
differences in the two treatments. Insofar, we have observed that compass
plants and ragweed, which we are measuring the abundances of, as well as
pigweed and some non-native grasses, are prevalent along the railroad, while
sweet clover tends to be found along the lawn’s edge. The interior of the
planting appears to have fewer undesirable weeds as it becomes wider. This
supports the concept of edge effects, suggesting that there is protection from
invasive species further into the interior of a planting and that the shape and
size of a planting, as well as the type of edge, should be considered when
creating a prairie reconstruction.
Works cited:
Jensen, William and
Elmer Finck. 2004. Edge effects on nesting dickcissels (Spiza americana) in
relation to edge type of remnant tallgrass prairie in Kansas. American
Midland Naturalist 151:192-199.
See also:
Another example of an
urban prairie planting: http://www.winonadailynews.com/news/wsu-garden-goes-back-to-its-roots/article_75ffd9ea-f348-11e2-b2ca-001a4bcf887a.html
I found your addition of sites for further research really helpful.
ReplyDeleteJensen and Finck's article was very interesting because it suggests that we can encourage healthy prairie reconstructions by modifying edges. Without large areas of available land for prairie reconstructions, almost all conservation efforts will have to deal with the impact of edge effects on all species, both plant and animal. Quantifying such effects is an important step in understanding how best to conserve smaller reconstructions. This article, though, brings up the interesting question of how to make conservation decisions because different species have different requirements. While cropland edges may be better for the dicksissels, I would guess that woodland edges may benefit other species.
It is nice how both your study and the Jensen and Finck study tie into each other. Your study takes the perspective of the plants that are in an area whereas the other study takes on that of the birds. It is a reminder that edge effects can cause changes in many different forms of life and that edge effects should be taken into account when trying to recreate or preserve habitats.
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