Monarch butterflies choose
where they want to live
Monarch butterflies, Danaus plexippus, have sturdy wings which allow them to fly many miles and choose
their own habitat. In the spring, monarchs choose to fly north in order to lay
their eggs on suitable milkweed plants. In the fall, monarchs sense the cooling
temperatures, and respond by flying south to sunnier climates, aggregating in
southern Florida, Mexico, and Southern California. In order to fuel these long
flights, the butterflies must find flowers with sugary nectar along the way. The
monarchs have migrated for thousands of years, but lately their population has
been shrinking. Habitat
fragmentation is one explanation for their shrinking population; their
habitat has become increasingly fragmented as humans build houses, roads,
cities, and farm large portions of the United States. A fragmented habitat may be more challenging to navigate than an unfragmented habitat.
Experimental Habitat
Fragmentation in Ohio
In the summer of 1999, Summerville
and Crist investigated the impact of habitat fragmentation on butterfly abundance
and species diversity near Oxford, Ohio. In June of 1999, they divided the land
into 25 separate plots and mowed created habitat fragmentation by mowing
various spaces (Figure 2). On six different sampling days, from June to August,
they visually monitored each plot for butterflies. They found that butterflies preferred the patches
with the most flowers. Additionally, they found that butterflies preferred the
patches with the largest habitat area.
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Figure 2: Summerville and Crist used 25 plots of
land, measuring 15mx15m for each plot. The mowed area is represented in white,
while the intact habitat areas are shaded gray.
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Habitat Fragmentation at Grinnell College
In October of 2016, a group of three Ecology students at
Grinnell College set out with the question, do monarch butterflies travel between fragmented habitats? To answer
their question they observed the flight and landing patterns of monarch butterflies on planted prairie patches on the Grinnell College campus (Figure 3). They hope that their experiment will help them better understand where monarch butterflies like to spend their time. Based on Summerville and Crist’s findings, they
expect to find more monarchs in the larger patches and patches with the most
flowers. However, they should consider that the fall timing of their study may impact their results, as the monarchs have already started migrating south for the winter.
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Figure 3: Layout
of the Grinnell College project. The prairie patches are divided by a sidewalk
and mowed grass. To left of the picture are railroad tracks, and to the right
of the picture are athletic fields.
References
Summerville, K. S., &
Crist, T. O. (2001). Effects of experimental habitat fragmentation on patch use
by butterflies and skippers (Lepidoptera).Ecology, 82(5), 1360-1370.
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