58
RICHARD S. MILLER
and a total of 28 foods with a mean of 20.84 per stomach in the P.
vehiculum sample. The greater number of individual items in the P.
vehiculum stomachs is explained by the fact that the somewhat smaller
salamander ( P . vehiculum, snout-vent length 46 to 57 mm) consumed
far more small-sized forms such as mites than did the larger species
(P. dunni, snout-vent length 48 to 65 mm). When the foods were
separated by frequency index into “primary” and “secondary” foods,
it was found that of 16 primary foods 10 were shared by both species
but P. vehiculum had fewer secondary sources (14) than P. dunni (27).
The similarity of primary food sources might suggest rather strong
competition for these items, but P. dunni has more secondary sources
and would be better able to survive periods of food scarcity than P.
vehiculum. Dumas (1956) concluded that foods acceptable to both
species are sufficiently abundant that competition for this factor will
rarely be critical, but that there is rather high mortality from adverse
physical conditions and there is definite competition for sites with high
humidities, with P. dunni excluding P. vehiculum.
D. CRUSTACEA
Connell’s (1961) study of competition between the barnacles Chthamalus stellatus and Balanus balanoides defines both the fundamental
and realized niches of these species. The center of distribution for C.
stellatus is in the Mediterranean and it reaches its northern limit in the
Shetland Islands. Balanus balanoides is a boreal-arctic species which
reaches its southern limit in northern Spain. At Millport, Scotland
where this study was done, the larvae of C. stellatus settle in the marine
intertidal zone between the levels of mean high water of spring tides
and the mean tide level, but few survive below the mean high water of
neap tide (see Fig. 11). The larvae of B. balanoides settle throughout
the range of all intertidal levels from mean low water to mean high
water of spring tide, but poor survival between the mean high water of
spring and neap tides restricts the adult distribution in this region.
Connell showed that the upper limit of distribution of Balanus is
determined by mortality from desiccation, competition for space and
predation by the snail Thais lapillus, but C. stellatus can survive at all
levels and increased time of submergence was not a factor in the
elimination of this species at low shore levels. Thus, in view of its greater
ability to withstand alternate submergence and desiccation at the higher
shore levels, C. stellatus has a larger niche than B. balanoides.
Intraspecies competition for space is rarely observed in Chtharnalus
but is common among individuals of Balanus. Comparison of the survival of Chthamalus in the presence and absence of Balanus showed
that Balanus could cause considerable mortality among the Chtha.m,alus;
RICHARD S. MILLER
and a total of 28 foods with a mean of 20.84 per stomach in the P.
vehiculum sample. The greater number of individual items in the P.
vehiculum stomachs is explained by the fact that the somewhat smaller
salamander ( P . vehiculum, snout-vent length 46 to 57 mm) consumed
far more small-sized forms such as mites than did the larger species
(P. dunni, snout-vent length 48 to 65 mm). When the foods were
separated by frequency index into “primary” and “secondary” foods,
it was found that of 16 primary foods 10 were shared by both species
but P. vehiculum had fewer secondary sources (14) than P. dunni (27).
The similarity of primary food sources might suggest rather strong
competition for these items, but P. dunni has more secondary sources
and would be better able to survive periods of food scarcity than P.
vehiculum. Dumas (1956) concluded that foods acceptable to both
species are sufficiently abundant that competition for this factor will
rarely be critical, but that there is rather high mortality from adverse
physical conditions and there is definite competition for sites with high
humidities, with P. dunni excluding P. vehiculum.
D. CRUSTACEA
Connell’s (1961) study of competition between the barnacles Chthamalus stellatus and Balanus balanoides defines both the fundamental
and realized niches of these species. The center of distribution for C.
stellatus is in the Mediterranean and it reaches its northern limit in the
Shetland Islands. Balanus balanoides is a boreal-arctic species which
reaches its southern limit in northern Spain. At Millport, Scotland
where this study was done, the larvae of C. stellatus settle in the marine
intertidal zone between the levels of mean high water of spring tides
and the mean tide level, but few survive below the mean high water of
neap tide (see Fig. 11). The larvae of B. balanoides settle throughout
the range of all intertidal levels from mean low water to mean high
water of spring tide, but poor survival between the mean high water of
spring and neap tides restricts the adult distribution in this region.
Connell showed that the upper limit of distribution of Balanus is
determined by mortality from desiccation, competition for space and
predation by the snail Thais lapillus, but C. stellatus can survive at all
levels and increased time of submergence was not a factor in the
elimination of this species at low shore levels. Thus, in view of its greater
ability to withstand alternate submergence and desiccation at the higher
shore levels, C. stellatus has a larger niche than B. balanoides.
Intraspecies competition for space is rarely observed in Chtharnalus
but is common among individuals of Balanus. Comparison of the survival of Chthamalus in the presence and absence of Balanus showed
that Balanus could cause considerable mortality among the Chtha.m,alus;