134 Papers from the Department of Marine Biology. 



Syntherisrna and Cenchrus echinatus, with several Poinsettias, On 

 Garden Key portions of the parade-ground, possibly through long 

 years of cultivation and some attempts at fertihzation and artificial 

 watering, show a difference in the soil from that of the rest of the key. 

 The accumulation of some humus here also makes a change in the soil's 

 character. This portion of the parade-ground is thickly covered with. 

 a mat of Lippia, various examples of Sida, Dolicholus, etc., plants 

 which need slightly more nutriment in their substratum than is afforded 

 by the coarse, limy gravel and sand of other parts of the key. 



Human and animal influence on vegetation. — Man's influence on the 

 plants in the Tortugas may be designated as consti-uctive and desti*uc- 

 tive. The introduction of plants for use or ornament on Loggerhead 

 and Garden Keys and their introduction unwittingly ^^dth ballast, 

 etc. (such as Leptilon, Sonclms, Syntherisma, Portulaca), together wdth 

 coconuts, papaws, Casuarinas, and Salvia serotina (used in earlier 

 years as a febrifuge), may all be classed as constructive influences. 

 The cutting down of the white buttonwood trees by the old fishermen, 

 according to tradition, the burning off of the Suriana on Loggerhead, 

 and the burning over of the parade-ground in the fort on Garden Key 

 are destructive activities. On Bird Key the presence of the warden 

 does not seem to have produced any change in the vegetation, while 

 the terns which breed on the island have very little effect, as they do 

 not feed on any plant substances and it is only the noddy terns which 

 make nests of a few diy sticks ; the other two species of terns lay their 

 eggs on the bare sand. 



Millspaugh ascribes to birds a large influence in the distribution of 

 plants in this region, but in the vrriter's opinion this has been over- 

 estimated; the only birds in the region are the terns and a few frigate- 

 birds which prey on the terns. None of these birds are waders, nor do 

 they spend much time on land, according to the author's observation. 

 Millspaugh mentions 8 plant species as avevectant by the feet of sea 

 birds, six of these occurring in the Tortugas, but as there are few 

 birds in these islands and those are of such habits as scarcely to permit 

 of carrying seeds in the webbed-toe expansion, the hypothesis is hardly 

 tenable. A more reasonable factor for distribution in the Tortugas is 

 the sea alone. The undoubtedly aquavectant plants (as Cakile) appear 

 on the islands no sooner than such supposedly bird-borne plants as 

 Cenchrus and Sccevola. 



Other animals associated with plants are the red land-crab, Gecar- 

 cinus lateralis, which lives on various islands, but was observed espe- 

 cially on Loggerhead, where it fed on decaying leaf-mold accumulating 

 in the grove of Sehesten trees, but it probably has no effect on this tree's 

 economy. A small black beetle also Uves in this tree's deep salver-form 

 corolla and perhaps aids in fertilization. Another insect is the tiny 

 butterfly Thecla, which lives on the nectar of Melanthera and incident- 

 ally pollinates its stigmas. 



