5.Summary of “Natural” Gannet Vegetation and Comparison with that in Bass Strait
Most of the gannet colonies visited were higher above sea level than were the tern colonies (notable exceptions being the Black Reef gannetry and White Island ternery). They were thus more remote from sea spray on the whole, but the expected increase in diversity of the flora was prevented by the heavier deposition of guano. It was the rule rather than the exception for them to be on bare ground instead of on the Disphyma mats so characteristic of the tern colonies.
Eleven species were recorded in seven colonies (some of the latter multiple colonies and collectively covering more ground than the 9 multiple tern colonies listed in Table I under 15 headings). Seven of these species were recorded in both tern and gannet colonies.
As with the terns, Disphyma australe was the dominant species with 100% frequency, Coprosma repens the next most abundant (71%) and Senecio lautus third in importance (43%) with, in the case of the gannets and the Bass Strait terns, Bromus unioloides
The only annuals recorded were again aliens—opportunist invaders of the open, well manured habitats created by the birds (Table II)
Only one gannet colony was visited in the Bass Strait, Tasmania this being on Cat Island, several hundred metres inland from the deserted crested tern colony. The gannet colony had previously been much larger and the vacated area was being colonised by Bromus unioloides and Tetragonia implexicoma with an advancing
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|(Annual spp. marked with an asterisk are aliens)||Halophyte||Annual||Percentage||†Three Kings||Black Reef||White I||Double I||Bush I||Gannet I||Horuhoru||Frequency Perennial|
|Poa anceps v condensata||H||P||x||14|
margin of Spergulamia rubra and Bulbine semibarbata. Bromus occupied 39% of the ground, Tetragonia 29%. The surrounding territory was burrowed by thousands of short-tailed shearwaters and showed no Bromus and little Bulbine. Poa poaeformis tussock, which occupied only 15% of the old gannetry, here occupied 59%; Tetragonia only 18%. (Soil around the surviving gannets was bare.)
Thus, in the Australian tussock grassland, as in the New Zealand bush, the presence of gannets had led to replacement of the indigenous vegetation by maritime succulents and the large alien Bromus. The presence of the succulent Bulbine with the Tetragonia instead of Disphyma and Salicornia as on the tern reef off Little Dog to the W, reflected the slighter exposure to sea spray.
[Footnote] † Spp recorded on Three Kings Islands by Oliver (1948)