Amplification of Habitat
In order to estimate habitat amplification, we must to assess abundance. The sponges are very fragile so we needed to devise a way to assess numbers without laying out transect lines, or measuring strips.
We designed a 2 meter long rod with vertical laser pointers at each end, and a calibrated flow meter on one side. The first diver traveled for an estimated 100 meters. The second diver followed above and behind counting all sponges which were at least 50% inside the 2 meters wide transect. The population in one 88 meter long transect was 64 sponges or about a 1/3 of a sponge per meter2.
To measure the area of the sponge population on the reef, divers flew propulsion vehicles along and across the sponge bed. Distances were calculated from elapsed time to travel known distances at a set speed.
The area of the west reef is 0.17 hectares. If it had a comparable sponge density 1/3 of a sponge per meter2, it would contain approximately 635 sponges. This many sponges, typically ½ to 1m in size constitute a major hard habitat amplification. The density is not atypical. It is less than the maximum densities in Jervis Inlet equivalent to 2.4 per meter2. But significantly more than the maximum of approximately 0.07 per meter2 previously reported for Saanich Inlet(Leys et al. 2004). If we consider the surface area determined for ourtypical sponge to be representative (Austin 2007) then this surface area of 3.68 meter2 X the average density of0.36 sponges per meter2 = 1.3 meter2 or asurface habitat amplification of 130%
As noted earlier, living sponges provide available habitat to only appropriately armoured species. In a preliminary survey of biota on one moderate size dead sponge we found 27 species, including 10 higher taxa.
These results signify that the A. vastus here can be considered a Foundation species. That is, it amplifies habitat for many species when dead or when living but with a dead base.