ASPECTS OF STRESS M THE TROPICAL URINE ENVIRONMENT
229
conditions may be expected to be exaggerated. It is naturally subdivided at several levels. The level of low water of extreme spring tides
is the upper boundary of organisms unable to tolerate even brief
exposure to aerial conditions, and the lower boundary of ones which
require such exposure. At least this statement applies to rocky shores
with tidal pools excluded. High water of extreme spring tides is a
similar boundary. Doty (1946) has pointed out that, between these
two, there are other '' critical levels," and two of these occur respectively
near high and low water of neap tides. The upper of these is the upper
limit of species unable to tolerate aerial exposure for more than one
tidal period, and the lower limit of species requiring such exposure
each tide.
Doty (1946) showed the presence of these breaks in distribution,
or ecotones, on the coast of California, and Colman (1933) showed the
two neap tide ones a t Wembury, in the English Channel. In an attempt
to see whether there is any difference between temperate and tropical
shores in these zones, we have assembled data from the surveys of
twenty rocky shores whose annual mean temperatures ranged from
9.0"C to 27.7OC. The choice was limited by the absence of adequate
tidal information in many surveys, and by the need to exclude localities
where wave action was sufficient to change the levels seriously. It is
unfortunate that no surveys were found which included adequate
sampling of the species either below low water or above high water.
The result is that the two extreme ecotones are not shown, although
they are probably much sharper than the neap tide ecotones.
In order to allow comparison of the various localities, all vertical
distributions were expressed as percentages of the local range from
extreme low to extreme high water of spring tides. This intertidal
range was then subdivided into twenty equal zones and counts were
made of the number of species occurring, and of the number terminating,
in each zone. Two statistics have proved useful in delimiting the
ecotones and the zones between them. The f i s t of these was the
number of limits in each zone ; this would be expected to be highest at
an ecotone. Because the number of species increases towards the
tropics, we have, for each zone, calculated the value:
100 x number of limits
number of species included in survey
The second statistic was based on the assumption that many species
are adapted to live only in the zone between two adjacent ecotones.
For each species whose center of vertical distribution fell in a particular
zone we recorded the extent of its vertical range (as a percentage of
A.x.B.-~O
9
229
conditions may be expected to be exaggerated. It is naturally subdivided at several levels. The level of low water of extreme spring tides
is the upper boundary of organisms unable to tolerate even brief
exposure to aerial conditions, and the lower boundary of ones which
require such exposure. At least this statement applies to rocky shores
with tidal pools excluded. High water of extreme spring tides is a
similar boundary. Doty (1946) has pointed out that, between these
two, there are other '' critical levels," and two of these occur respectively
near high and low water of neap tides. The upper of these is the upper
limit of species unable to tolerate aerial exposure for more than one
tidal period, and the lower limit of species requiring such exposure
each tide.
Doty (1946) showed the presence of these breaks in distribution,
or ecotones, on the coast of California, and Colman (1933) showed the
two neap tide ones a t Wembury, in the English Channel. In an attempt
to see whether there is any difference between temperate and tropical
shores in these zones, we have assembled data from the surveys of
twenty rocky shores whose annual mean temperatures ranged from
9.0"C to 27.7OC. The choice was limited by the absence of adequate
tidal information in many surveys, and by the need to exclude localities
where wave action was sufficient to change the levels seriously. It is
unfortunate that no surveys were found which included adequate
sampling of the species either below low water or above high water.
The result is that the two extreme ecotones are not shown, although
they are probably much sharper than the neap tide ecotones.
In order to allow comparison of the various localities, all vertical
distributions were expressed as percentages of the local range from
extreme low to extreme high water of spring tides. This intertidal
range was then subdivided into twenty equal zones and counts were
made of the number of species occurring, and of the number terminating,
in each zone. Two statistics have proved useful in delimiting the
ecotones and the zones between them. The f i s t of these was the
number of limits in each zone ; this would be expected to be highest at
an ecotone. Because the number of species increases towards the
tropics, we have, for each zone, calculated the value:
100 x number of limits
number of species included in survey
The second statistic was based on the assumption that many species
are adapted to live only in the zone between two adjacent ecotones.
For each species whose center of vertical distribution fell in a particular
zone we recorded the extent of its vertical range (as a percentage of
A.x.B.-~O
9