Decoupling habitat fragmentation from habitat loss: butterfly species mobility obscures fragmentation effects in a naturally fragmented landscape of lake islands
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Since the publication of the theory of island biogeography, ecologists have postulated that fragmentation of continuous habitat presents a prominent threat to species diversity. However, negative fragmentation effects may be artifacts; the result of species diversity declining with habitat loss, and habitat loss correlating positively with degree of fragmentation. In this study, we used butterfly assemblages on islands of Lake of the Woods, Ontario, Canada to decouple habitat fragmentation from habitat loss and test two competing hypotheses: (1) the island effect hypothesis, which predicts that decreasing fragment size and increasing fragment isolation reduces species diversity beyond the effects of habitat loss, and (2) the habitat amount hypothesis, which negates fragmentation effects and predicts that only total habitat area determines the diversity of species persisting on fragmented landscapes. Using eight independent size classes of islands (ranging from 0.1 to 8.0 ha) that varied in number of islands while holding total area constant, species diversity comparisons, species accumulation curves, and species–area relationship extrapolations demonstrated that smaller insular habitats contained at least as many butterfly species as continuous habitat. However, when highly mobile species occurring on islands without their larval food plants were excluded from analyses, island effects on potentially reproducing species became apparent. Similarily, generalized linear models suggested that effects of island isolation and vascular plant richness on insular butterfly richness were confounded by species of high mobility. We conclude that inter-fragment movements of highly mobile species may obscure important fragmentation effects on potentially reproducing populations, questioning support for the habitat amount hypothesis.
KeywordsSpecies–area relationship Habitat amount hypothesis Island biogeography Species accumulation SLOSS
We wish to extend special thanks to the staff and owners of Redwing Lodge at Lake of the Woods, Ontario for their hospitality and facilitation of field activates. We also thank the staff at J & J General Store, Morson, Ontario for their hospitality and assistance with field communications. For assistance with field preparations, study analyses, and manuscript review, we thank Meghan Dauphinee, Andreas Hamann, Sylvia Horning, Victoria Masquillier, Evelyn Merrill, Federico Riva, and Felix Sperling.
Author contribution statement
ZGM, JHA, and SEN formulated the idea, all authors contributed to sampling methods and experimental design, ZGM and IDA conducted field work (island surveys) and acquired spatial data, ZGM completed all analyses presented, and all authors contributed to the writing of this manuscript.
Compliance with ethical standards
This study was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant (Grant number 2014-04842).
Conflict of interest
The authors declare that they have no conflict of interest.
All applicable institutional and/or national guidelines for the care and use of animals were followed.
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