The energetics of reproduction in endotherms and its implication for their conservation.

The energy expenditure of endotherms, through its impact on the rate of reproduction, affects their ability to withstand competition, to tolerate environmental disturbances, and to endure predation. The fecundity of eutherian mammals increases with rate of metabolism because the post-natal growth rate increases and the gestation and conception-to-weaning periods decrease with a mass-independent increase in basal rate of metabolism. These correlations account for the observation that species that have large population fluctuations have high rates of metabolism and reproduction. Species with high rates of metabolism out-compete species with low rates when using resources that permit consumers to have high rates of metabolism, which explains why eutherian carnivores replace marsupial carnivores, none of which have high basal rates as a result of their form of reproduction. Fecundity in birds also appears to correlate with energy expenditure, which may account for the huge die-off of birds endemic to oceanic islands after the invasion of humans: island endemics, many of which have low rates of metabolism, are unable to increase fecundity in response to a human-based increase in mortality. The long-term protection for endotherms characterized by low rates of energy expenditure requires their isolation from high levels of predation and competition, conditions that are likely to occur only on islands free from eutherian predators and with low species diversity. Such endotherms may survive on continents if they are ecologically isolated from the general fauna.