How dangerous is dispersal across habitat boundaries?

In fragmented landscapes, where habitat patches are surrounded by an unsuitable matrix, the matrix often acts as a strong barrier to dispersal. Crossing the boundary between patch and matrix can be a dangerous but necessary step. How can a species within the patch successfully navigate this patchiness? One way may be through dormancy, which is usually considered separately from dispersal, but can potentially be a successful stage of dispersal.

Researchers studied serpentine grassland patches in California to tackle the question of how dormancy aids dispersal across habitat boundaries by comparing seed dispersal in two ways. First, they surveyed the matrix for both the abundance of seeds in the seed bank and the abundance of adults of patch specialists, which included multiple native and endemic species. Then they flipped things around, and surveyed patches for both seed abundance and adult abundance of a matrix specialist – in this case, the invasive wild oat. They were then able to compare the consequences of dispersal across the habitat boundary between patch specialists and matrix specialists.

They found that there were many seeds of patch specialists in the seed bank in the matrix, but comparatively fewer adults. This discrepancy between the number of seeds and the number of adults supports the idea that remaining dormant may be a key strategy to traversing unsuitable habitats, at least for patch specialists. Seeds with no mechanism for dispersal accumulated downslope from patches, suggesting that even seeds with very limited dispersal abilities still have a chance of reaching other patches if they remain dormant long enough.

In contrast, nearly all the seeds of the matrix specialist became adults within a patch. This indicates that dormancy may not be as important for species crossing from the matrix to the patches, and that the boundary between habitats has different impacts depending on direction. In addition, there was a high abundance of adults that invaded the patch perimeter, but very few in the patch center, leading to contrasting source-sink dynamics within the patch.

These results show that crossing the habitat boundary can be a two way street, but it’s more beneficial if heading in a certain direction: out into the matrix. In other words, crossing from the patch to the matrix does not have the same demographic consequences as crossing from the matrix to the patch. This makes sense given that matrix habitat is often more abundant and connected, so that matrix specialists have less need to invade patches and patch specialists have more incentive to survive crossing the matrix.

Incorporating this information into dispersal models can give a more realistic view of how species move through fragmented habitats and give a more accurate assessment of connectivity. Dormancy as a strategy may be particularly helpful for species with short dispersal distances that need to cross long distances to get to another patch. This could give native species an advantage in variable environments, and the means with which to overcome patchiness and preserve biodiversity in fragmented ecosystems.

Related Posts