Altlunigli different species reach peaks of abun- 

 dance at different points in the habitat gradient pro- 

 ceeding from open sand to dense forest, the nature 

 of the substratum divides the species into two major 

 grou])s: tliose tliat tolerate and reacli tlieir greatest 

 abundance in the sandy areas where vegetation is 

 scattered, and tliose that are limited by sand and re- 

 quire humus in the soil or the microhabitat of decay- 

 ing logs. The transition or ecotone between these 

 two ant communities comes at the pine and black oak 

 stages. F.xperimental studies of six species in the 

 genus Formica indicate that physiological differences 

 occur, and that some s])ecies are able to invade places 

 of low relative humidity that others cannot. 



Spiders 



In the sand dunes on the south shore of Lake 

 Michigan and in adjacent areas, 228 species of spiders 

 are to be found (I.owrie 1948). The number of fami- 

 lies represented, the number of species involved, and 

 the abundance of individuals per unit area increase 

 as plant stages in the sere succeed one another (Table 

 8.3). Probably because of the greater diversification 

 of the vegetation, the availability of logs, the increase 

 in number of strata, and the consequent greater va- 

 riety of niches, spiders, like ants, are represented by 

 a larger number of species in the oak communities 

 than in the earlier stages of the sere or in the climax. 

 It is of significance that up through the black oak 

 stage new species appear in each succeeding stage with 

 very few dropping out. In the beech-maple climax, 

 however, 51 per cent of the spider fauna occurring in 

 preceding stages are no longer found, while 79 per 

 cent of the species are either new with this stage or 

 came in at the black oak stage and remained. Up to 

 the black oak stage the species composition of the 

 spider population shows ecesis, but with the advent 

 of deciduous forest, the change in the fauna composi- 

 tion is sufficiently extensive to indicate succession of 

 distinct communities. 



There is also a change in the mores of spiders 

 as the sere advances. Small lycosids that hide during 

 the day under driftwood or other debris and run over 

 the sand at night hunting for insect prey washed up 

 by the waves are most characteristic of the beach. 

 The permanent population is small. A burrowing 

 spider, Gcolycosa zvrightii. is usually common. The 

 burrows in which the spiders stay during the day may 

 be easily spotted on the beach and through the grass 

 and Cottonwood stages. Web-building species are at 

 a disadvantage in the early stages of the sere, how- 

 ever, because of the general lack of vegetation to 

 which their webs may be anchored and because of 

 the destructive effect of unchecked wind. With the 

 appearance of grasses, a substratum in which spiders 



can l)uil(l webs becomes available. In later stages, the 

 percentage of web-buiUlcrs increases considerably as 

 stratification jirogresses and the forest furnishes a 

 scaffold. 



Other animal life 



Strong offsliore winds often blow insects out 

 over the water where they are forced down onto the 

 surface and washed ashore. Windrows of such in- 

 sects many thousands of individuals representing a 

 wide variety of species, are sometimes to be seen. 

 Dead fish washed up on shore are fed upon by flesh- 

 flies and histerid, dermestid, and rove beetles. The 

 tiger beetles Cicindcla hirticollis, and C. ciiprascens, 

 a white ground beetle and other carabids, shore bugs, 

 digger-wasps, robber flies, and other insects and 

 spiders come down from higher ground to feed on 

 tiie scavenger species and those washed up bv the 

 waves (Shelford 1913, Park 1930). The tiger beetles, 

 ground beetles, digger-wasps, and sand spiders build 

 their burrows and larval stages far enough back to 

 escape the summer waves. Termites feed on buried 

 wood that is decaying or on the undersides of logs 

 that have drifted ashore. The piping plover and 

 spotted sandpiper place their nests in the middle and 

 upper beaches. At night, the toad, opossum, raccoon, 

 and the deer mouse come down to scavenge 

 whatever is available. The light coloration of many 

 of the insects and spiders that occur on sand is doubt- 

 less an adaptation for concealment (Hart 1907). 



The kinds of animals occurring in the grass, 

 shrub, and cottonwood communities are similar ex- 

 cept that new species invade with each successive 

 plant stage. The white tiger beetle Cicindela lepida 

 first appears on the upper beach and reaches maxi- 

 mum populations in the cottonwood stage, as do the 

 digger-wasps, robber flies, and sand spiders. Another 

 tiger beetle, Cicindcla jormosa. occurs in the ecotone 

 between the cottonwood and the pine stages. Snout 

 beetles, spittle bugs, and miscellaneous other insects 

 are occasionally very numerous. Some 592 species 

 and varieties of beetles have been taken from various 

 stages of this sere (Park 1930). Fifty species were 

 found to occur in the cottonwood stage, 23 in the 

 conifer stage, and about 200 in each succeeding forest 

 stage. The occurrence of bees is dependent to a large 

 extent on the variety and abundance of flowers, but 

 the number of species in each plant stage increases 

 up to the black oak and then declines to the climax 

 (Pearson 1933). 



Vertebrates are not usually numerous on sandy 

 flats or dunes away from the water's edge. The 

 vesper and lark sparrows occur among the grasses, 

 the prairie warbler and chipping sparrow are found 

 among the shrubs, and the kingbird is conspicuous 



Rock, sand, and clay 109 



