Is it sufficient to have one connection or are multiple paths between locations required? This later concept of connectivity redundancy has been of increasing interest to quantify in landscapes because theory predicts that it can affect movement rates, and in turn increase foraging and reduce extinctions. Furthermore, recent studies suggest that connectivity measures that incorporate redundancy (circuit theory) can explain gene flow better than connectivity measures that don’t incorporate redundancy (least-cost path). However, experiments that test the role of redundancy on movement are very rare.
Fletcher et al. tested the role of path redundancies on movement of habitat specialist cactus bugs (Chelinidea vittiger) using mark-recapture in experimental arenas of cactus habitat patches (Opuntia humifusa) with various stepping stone configurations of path redundancies. Because the matrix can mediate the role of habitat linkages, they tested connectivity redundancy across a gradient of matrix resistance.
Experimental arenas (n = 10) had a center release patch for cactus bugs and four surrounding target patches 2.5m away. Target patches had four connection treatments between them and the release patch: 1) no patches (i.e., unconnected), 2) a single path configuration with two patches, 3) a parallel path configuration with four patches, and 4) a funnel path configuration with three perpendicular patches nearest the release patch and a single patch nearest to the target patch. The last two configurations differ in circuit theory resistance but not least-cost path distance (ignoring matrix resistance). Spacing of patches was chosen based on perceptual range (> 1m but < 2m) and movement abilities (1.5-2.5 m per week) of cactus bugs from previous experiments. Matrix resistance was quantified by measuring vegetation height and density.
They found that redundancy greatly increased colonization rates – the parallel treatment resulted in approximately two times greater colonization probability than the single path or unconnected treatments. The funnel path treatment resulted in approximately two times greater colonization than the parallel path treatment, likely through increased perception of habitat amount. The best model for colonization probability was one of the matrix resistance and redundancy treatment interaction, indicating that the matrix is important in mediating the role of redundancy on colonization rates.
These findings emphasize the importance of incorporating connectivity redundancy for estimating functional connectivity. Moreover, they show how the structure of the matrix can interact with connectivity configuration. This has important implications for the ways we measure connectivity in landscapes, design corridors, and think about processes that mediate movement patterns.