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Bowls are shallow hillslope concavities linked by moist drainageways (draws) that deepen into ravines. This setting provides some of the most fertile settings in the Northern Appalachian / Acadian region. These concave areas on moderate slopes (technically head slopes or locally covesor hollows) are places were rain water converges and slopewash sediments collect. Like streams, they form multifaceted networks connected by toe-slopes. With over 200,000 in the ecoregion, it can be hard to determine where one occurrence ends and another begins.. Most are small, averaging 18 acres in size, with 88% of them being less than 25 acres. In landscapes conducive to their development, extensive intertwined networks occur. These large examples reflect the extent and complexity of the system more than an increase in the size of the individual components (Figure 1). Although widespread in the region, they are concentrated on harder bedrock at moderate elevation.

Bowl, Hollow and Ravine Portfolio Summary

The portfolio identifies the set of critical sites most important to the conservation of enriched bowl/ravine ecosystems. The screening criteria used to identify crucial portfolio occurrences required that each example:

 Was large and contiguous: over 25 acres

 Was in good landscape settings (Land Cover Index < 20)

 Was in good condition based on ground surveys and expert opinion (corroboration by at least one source)

 Contained other confirmed biodiversity features (element occurrences) appropriate to the system type.

Size criteria were determined by an analysis of over a thousand survey records for species and communities occurring on bowl/ravine settings. As with other network forming features, the size of an occurrence did not appear to directly effect its function as a coarse filter for associated species. Although larger occurrences typically had more species represented, we did not find any species restricted only to small examples. A direct relationship was apparent in the ground inventoried information between the condition of the inventoried example and the size of the occurrence to which it corresponded (Figure 4). The 25 acre size criterion, slightly above the features mean size in the region, increases the likelihood of selecting high condition examples with complete biodiversity.

Results

Insuring that the portfolio would make a lasting impact on the conservation of biodiversity requires having adequate replication and redundancy of bowl/ravine features across all important environmental gradients. Thus, we set an initial minimum number of critical occurrences to protect as 20 examples per 24 bedrock/elevation combinations. This goal of at least 480 individual occurrences totals to less than 1 percent of all the bowl/ravine features in the ecoregion or an estimated 1% of all bowls/ravines by area (using the mean size of features over 25 acres -e.g. 107 acres). After examining the distribution of the occurrences across gradients, we redistributed the minimum goal of 480 across the bedrock/elevation classes in proportion with the number of possible occurrences, adding a minimum of two for rarer environments. This totaled to a goal of 499 (Table 5).

Our results identified 1269 critical occurrences, 770 more than the number needed to meet our minimum goals. We met or surpassed the specific minimum goals for each bedrock/elevation setting except in a few uncommon settings. Measured by area, the critical sites account for 13% percent of all bowl/ravine features, more than the expected estimate of 1% because the critical sites were consistently larger than the average size.

Occurrences, Sites and Goals

In our analysis a site could consist of either an exemplary individual bowl/ravine feature or a natural complex comprised of many bowls/ravines and other features in close proximity. In some of the latter cases, not all the individual examples met our selection criteria, but as an aggregate the area did. The surplus of occurrences, a rarity in the overall portfolio, reflects the tangled and intertwined nature of these networks in regions of high bowl/ravine densities. From a practical stand point they will need to be conserved as a complex in order to conserve the processes that form and maintain these fertile drainageways. Thus, although we are able to count them as individual features they are not independent at a landscape scale.

Table 5. Goals and Distribution for critical occurrences of bowls and draws. This table gives detail on the goals set for critical sites and the adequacy of the portfolio in meeting those goals.