Literature Review: Population Edges

by Carl Strang

Today I want to share my notes from a couple papers published last year in the open on-line scientific journal PLoS ONE (Public Library of Science). These studies looked at the geography of populations. The first considered the ecological factors that determine the edges of a species’ range [Rhainds M, Fagan WF (2010) Broad-Scale Latitudinal Variation in Female Reproductive Success Contributes to the Maintenance of a Geographic Range Boundary in Bagworms (Lepidoptera: Psychidae). PLoS ONE 5(11): e14166. doi:10.1371/journal.pone.0014166].

Here is a bagworm case, formerly the home of a female but now containing her eggs, in a crabapple tree at Mayslake Forest Preserve.

They looked at bagworm populations from Tennessee to Michigan to determine what limits their range. Caterpillars of this moth live in protective cases that they build around themselves. Wingless females mate in fall with flighted males, lay eggs in their bags, then drop to the ground and die. The eggs hatch in the spring. Female mating success dropped from near 100% to near 0% as the edge of the range was reached (apparently as the population thins out, males have a hard time finding females). Other factors were declines in fecundity, egg and pupal survivorship (all perhaps attributable to stresses resulting from the northern climate). The population density thins and local extinctions become more frequent toward the edge of the range.

Here’s another bagworm egg case, this one on a willow at Willowbrook Forest Preserve.

This study was of personal interest because I have been looking at several instances of range extensions in singing insects.

The other paper reported a study of mammal communities in forest fragments in Brazil [Pardini R, Bueno AdA, Gardner TA, Prado PI, Metzger JP (2010) Beyond the Fragmentation Threshold Hypothesis: Regime Shifts in Biodiversity Across Fragmented Landscapes. PLoS ONE 5(10): e13666. doi: 10.1371/journal.pone.0013666].

This scene is from Panama, not Brazil, but may be similar to the kind of landscape Pardini and company were studying.

Community ecologists are concerned about what happens to species diversity as large blocks of habitat become broken into smaller pieces by human activity. These scientists found a landscape-wide threshold in total forest coverage below which species went extinct throughout the system. In other words, local extinction in island-like patches of habitat no longer could be replenished through immigration from other patches because there weren’t enough of these to serve as sources. I found similar patterns in my study of periodical cicadas in DuPage County in 2007.


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