semiaquatic water snake preys upon small fish and frogs. The smaller snakes 

 (brown, red-bellied, ring-necked, and smooth green) eat insects, earthworms, 

 slugs, and other invertebrates. The green snake eats adult and larval insects 

 almost exclusively (Oliver and Bailey 1939). 



FACTORS OF ABUNDANCE 



Although natural factors largely determine the distribution and abundance of 

 most animals, human-induced factors increasingly alter the ecosystem and their 

 inhabitants. Some of the major factors that affect herptiles are discussed 

 below. 



Natural Factors 



The relatively long, cold winters and short, cool summers of coastal Maine are 

 probably the most influential natural limiting factor to reptiles and 

 amphibians. Other natural factors affecting abundance of herptiles are forest 

 and ground fires, beaver dams, predation, and the degree of abundance of food 

 and cover. The extent to which these factors affect populations of amphibians 

 and reptiles on the Maine coast is not known, but none appears to be 

 particularly limiting. 



Human Factors 



Agriculture . Erosion from cultivated fields may damage herptile habitats 

 seriously by causing siltation of nearby streams, rivers, and ponds (see 

 "Agricultural and Developed Land," chapter 10 and "Human Impacts On the 

 Ecosystem," chapter 3). However, farm ponds generally benefit most species of 

 herptiles, especially frogs, and salamanders, through the creation of 

 freshwater aquatic habitat. The fact that large acreages of blueberry barrens 

 are routinely burned may affect populations of herptiles living in these 

 habitats adversely, especially the blue-spotted, spotted, red-backed, and 

 dusky salamanders, and several species of snakes, including the black racer, 

 garter, and green snakes. The American toad, once abundant on Mount Desert 

 Island (region 4) was virtually eliminated during the massive fire of 1947 

 that swept the island (Davis 1959) . 



Pollution . The introduction of toxic chemicals and sediments from soil 

 erosion into coastal Maine could play major roles in reducing the abundance of 

 herptiles (Porter 1972). An average of 10,000 to 12,000 lb (4500 to 5500 kg) 

 of Guthion was sprayed on blueberry fields in Washington County between 1971 

 and 1976 (Maine Soil and Water Conservation Commission 1978). Air pollution 

 and acid rain could have an adverse effect on populations of terrestrial 

 salamanders, which respire through their skins. 



Bart and Hunter (1978) have compiled an annotated bibliography on the 

 biological impact of selected insecticides on vertebrates and invertebrates. 

 According to these authors no significant impact on populations of herptiles 

 was noted in experiments with various dilutions of the insecticides commonly 

 used in Maine (e.g., Zectran, Dylox, and Guthion) against spruce budworm or on 

 agricultural crops, but populations of aquatic insects (e.g., mayflies, 

 stoneflies, and various fly larvae) were reduced by some of these chemicals. 

 Certain insects used by aquatic herptiles as food were among these. In 

 addition, pesticide and oil films on pond surfaces may interfere with the 



18-7 



10-80 



