226
EIILARY B. MOORE
extreme conditions for the species to extend into. The analysis was
therefore repeated including only species whose limit stopped one
degree short of the available extreme, and which therefore had room for
further expansion of range. The results in this case were essentially
the same.
An alternative analysis avoids comparisons with a predicted value.
Localities, instead of species, were grouped by their mean temperature,
and for each group the mean temperature range determined for all
species occurring within the limits of the group (Fig. 7). Here again the
range is greatest at medium temperatures and low at polar and tropical
ones. As in the first analysis, exclusion of species whose limits were
within one degree of the limits of available temperature did not change
the general trend. The association of small temperature tolerance with
the tropics cannot be explained by the small seasonal range they
normally encounter there. If tropical species had a greater tolerance
range, and if temperature is their limiting factor, then they could
extend into cooler waters.
B . Intertidal zonation
Another and independent estimate of stress in the tropics is available
in intertidal zonation. The intertidal zone is, in any case, a region of
stress, so effects in it may well be amplified. It would be expected that
under conditions of increasing stress, the decreased tolerance range of a
species would limit it to a smaller fraction of the intertidal zone. It is
unfortunate that so many of the excellent accounts of intertidal zonation cannot be used because of inadequate tidal data. Those sources
which we have used are listed in the bibliography. We have excluded
estuarine localities and ones where it appeared that wave action was
too strong for the levels to be significant.
Many species extend only slightly into the intertidal zone and there
are hardly any surveys of the immediately sublittoral and supralittoral
zones. We have therefore included only those species which extend
above mean low water of neap tides or below mean high water of neap
tides as the case may be. Because tide ranges vary greatly, zonation is
expressed as a percentage of the distance from extreme low water of
spring tides to extreme high water of spring tides.
The results, grouped in terms of the mean seasonal temperature of
the locality, are shown in Fig. 8. Since the localities are widely spread
it has been necessary to use ocean temperatures, but this should not
cause any serious error in the general trends. The Arctic data are of
questionable significance because there are so few observations and also
EIILARY B. MOORE
extreme conditions for the species to extend into. The analysis was
therefore repeated including only species whose limit stopped one
degree short of the available extreme, and which therefore had room for
further expansion of range. The results in this case were essentially
the same.
An alternative analysis avoids comparisons with a predicted value.
Localities, instead of species, were grouped by their mean temperature,
and for each group the mean temperature range determined for all
species occurring within the limits of the group (Fig. 7). Here again the
range is greatest at medium temperatures and low at polar and tropical
ones. As in the first analysis, exclusion of species whose limits were
within one degree of the limits of available temperature did not change
the general trend. The association of small temperature tolerance with
the tropics cannot be explained by the small seasonal range they
normally encounter there. If tropical species had a greater tolerance
range, and if temperature is their limiting factor, then they could
extend into cooler waters.
B . Intertidal zonation
Another and independent estimate of stress in the tropics is available
in intertidal zonation. The intertidal zone is, in any case, a region of
stress, so effects in it may well be amplified. It would be expected that
under conditions of increasing stress, the decreased tolerance range of a
species would limit it to a smaller fraction of the intertidal zone. It is
unfortunate that so many of the excellent accounts of intertidal zonation cannot be used because of inadequate tidal data. Those sources
which we have used are listed in the bibliography. We have excluded
estuarine localities and ones where it appeared that wave action was
too strong for the levels to be significant.
Many species extend only slightly into the intertidal zone and there
are hardly any surveys of the immediately sublittoral and supralittoral
zones. We have therefore included only those species which extend
above mean low water of neap tides or below mean high water of neap
tides as the case may be. Because tide ranges vary greatly, zonation is
expressed as a percentage of the distance from extreme low water of
spring tides to extreme high water of spring tides.
The results, grouped in terms of the mean seasonal temperature of
the locality, are shown in Fig. 8. Since the localities are widely spread
it has been necessary to use ocean temperatures, but this should not
cause any serious error in the general trends. The Arctic data are of
questionable significance because there are so few observations and also