ENERGETICS AND A N I M A L PRODUCTIVITY
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gain sufficient nutrition fsr the maintenance and reproductive tasks.
Does the efficiency of digestioii and assimilation compensate the organism for the cost of homiothermy? This question has no answer at the
present time. However, when the five situations (two homiotherms, and
three poikilotherms) are considered wherein total calories ingested are
known and if they were plotted on a graph similar to Fig.& the new
plot would suggest that the curves for poikilobherms and homiotherms
approach one another, and may even merge broadly. Thus, it would
appear that poikilothermy and homiothermy are two answers to the
same problems of existence in the environment. It would be interesting
to know where data for populations of heliotherms (such as lizards and
snakes) and the “variable homiotherms” (such as the sloths, opossums,
and the true hibernators) will fall on the graphs of maintenance metabolism - net production.
The studies seem to suggest a second point concerning the productivity of various populations. Populations in the southern latitudes
appear to be more productive than do those !n the northern latitudes.
Such a difference seems reasonable, because warmer temperatures and
longer growing seasons make possible greater productivity.
Still a third additional hypothesis suggested by these data is that
populations have greater productivity in simpler situations (such as
pioneer communities). Thus, the salt marsh populations are more productive than are those of the southern “old field” which in turn are
more productive than are those of the northern “old field”. In a like
manner, the populations studied in the northern alfalfa field far exceed
the production of the same populations found on the northern “old
field”. We must remember that here productivity of single populations
within different communities is under discurision rather than productivity of a community.
The trends suggested in these studies seem to support our present
ideas about population size within communities (i.e. that pioneer communities are made up of a few large populations while climax communities are made up of numerous relatively small populations).
Finally, the data on ecological efficiency and on efficiency of use indicate that each population utilizes only a small portion of the available
calories (the one exception being the use of seeds on the southern “old
field”). The most frequently reported figures, on food utilization were:
2% for herbivores and 30% for carnivores. When considering ecological
efficiency of a trophic level (i.e. the soil arthropods) the maximum efficiency is only 30% and the remaining 70% then goes into other energy
pathways. Thus, it is obvious that the energy fixed by the plants “cascades” into a number of energy “sumps” before it is completely dissipated through respiration.
107
gain sufficient nutrition fsr the maintenance and reproductive tasks.
Does the efficiency of digestioii and assimilation compensate the organism for the cost of homiothermy? This question has no answer at the
present time. However, when the five situations (two homiotherms, and
three poikilotherms) are considered wherein total calories ingested are
known and if they were plotted on a graph similar to Fig.& the new
plot would suggest that the curves for poikilobherms and homiotherms
approach one another, and may even merge broadly. Thus, it would
appear that poikilothermy and homiothermy are two answers to the
same problems of existence in the environment. It would be interesting
to know where data for populations of heliotherms (such as lizards and
snakes) and the “variable homiotherms” (such as the sloths, opossums,
and the true hibernators) will fall on the graphs of maintenance metabolism - net production.
The studies seem to suggest a second point concerning the productivity of various populations. Populations in the southern latitudes
appear to be more productive than do those !n the northern latitudes.
Such a difference seems reasonable, because warmer temperatures and
longer growing seasons make possible greater productivity.
Still a third additional hypothesis suggested by these data is that
populations have greater productivity in simpler situations (such as
pioneer communities). Thus, the salt marsh populations are more productive than are those of the southern “old field” which in turn are
more productive than are those of the northern “old field”. In a like
manner, the populations studied in the northern alfalfa field far exceed
the production of the same populations found on the northern “old
field”. We must remember that here productivity of single populations
within different communities is under discurision rather than productivity of a community.
The trends suggested in these studies seem to support our present
ideas about population size within communities (i.e. that pioneer communities are made up of a few large populations while climax communities are made up of numerous relatively small populations).
Finally, the data on ecological efficiency and on efficiency of use indicate that each population utilizes only a small portion of the available
calories (the one exception being the use of seeds on the southern “old
field”). The most frequently reported figures, on food utilization were:
2% for herbivores and 30% for carnivores. When considering ecological
efficiency of a trophic level (i.e. the soil arthropods) the maximum efficiency is only 30% and the remaining 70% then goes into other energy
pathways. Thus, it is obvious that the energy fixed by the plants “cascades” into a number of energy “sumps” before it is completely dissipated through respiration.