ECOLOGY OF NORTHERN MICHIGAN. 23 



strata form the bottom of a wave cut terrace off shore and determine the 

 breaker line, Fig. 7. The submerged edges of these strata continue to be 

 worn off by the abrasion of the material carried by the undertow until the 

 surface of the terrace is more or less uniform. 



On account of the exposure and the difficulty of obtaining a foothold, 

 the environmental conditions are very severe, and the biota is limited both 

 in individuals and species. The beach has been divided by physiographers 

 into three parts, the Upper, Middle, and Lower Beaches, according to the 

 influence of the hydro-dynamic factors. These factors, as Cowles ('99, pp. 

 112-175, and '01, pp. 56-57) has shown, have an important influence upon 

 the environment of plants, so that the beach may be divided into the same 

 divisions for our present purpose. 



The Lower Beach is that portion of the shore exposed to the action of the 

 waves during the summer months. At the point studied, Station I, owing 

 to the dip of the strata, the water deepens rapidly off shore, and the breaker 

 line is brought near to the beach, the lower part of which is thus exposed 

 to the full force of the waves, Fig. 5. Exposed as it is alternately to the 

 pounding of the waves and dessication in the sun the conditions are so severe 

 that very little life can exist. The only form that is found here is the snail 

 Limnaea decollata that occurs occasionally behind projecting outcrops, 

 where it is to some extent protected from the direct force of the waves. 

 This snail seems to be adapted to habitats of this nature, for it was found 

 elsewhere only in rapid water in the larger rivers (Ruthven, '04, p. 192). 



The Middle Beach lies above the Lower Beach and differs from it in not 

 being exposed to the summer waves. It is, however, exposed to the action 

 of the waves during the early winter months, while during the latter part 

 of the winter the ice is shoved upon it in great ridges. It is thus, only during 

 the growing season free from those conditions which make the Lower Beach 

 practically uninhabitated, but certain conditions still make the habitat unfavor- 

 rable. Owing to the difficulty in gaining and retaining a foothold, the vegetation 

 consists principally of lichens, Lecidea lactea and Parmelia conspersa, that 

 are not dependent upon the substratum for nourishment. Along the ex- 

 posed edges of joint and bedding planes, where disintegration is more rapid, 

 a slight soil accumulates, Fig. 6. These crevices, as may be seen from the 

 photograph, are taken possession of by several of the higher plant forms, 

 but, owing to the short period that this beach is not exposed to the pounding 

 of the waves, only the hardier plants can obtain a foothold, such as the 

 Beach Pea,* Silvery Cinquefoil, Goldenrod, Pale Spiked Lobelia, Harebell 

 and a few grasses, Fig. 6. 



The fauna, probably owing in part to the lack of food, is also limited. 

 The insects are represented by the butterflies, Argynnis ailantis, Phyciodes 

 tharos, Grapta gracilis, Grapta progne, Basilarchia arthemis, and Colias philo- 

 dice, and the dragonflies, Aeschna clepsydra and Sympetrum costiferum; 

 forms that also occur on the Upper Beach. The spiders are more character- 

 istic ; Epeira trifolium and patagiata and Linyphia phrygiana spin their webs 

 behind the outcropping strata, while Pardosa lapidicina runs about over 

 the rocks. The destruction of this rock beach will probably destroy the 

 habitats of these spiders, especially of those that spin webs, as it would be 

 impossible for them to attach their webs on a sandy beach. The butterflies 

 on the other hand are dependent upon the flowering plants, which are essen- 

 tially crevice forms, so that their habitats would be increased by the com- 

 minution of the beach. 



*All plant names in this report follow Britton's Manual of (he Flora of the Northern Stntes and 

 Canada. 1901. 



