Why do conservation biologists often focus their attention on saving "umbrella species" from extinction?

Saving umbrella species helps in saving other species present in that ecosystem.

Explanation:

Umbrella species is the species which is important in an ecosystem because the presence and conservation of umbrella species helps in the conservation of other species present in the ecosystem. So the presence of umbrella species signifies that the ecosystem is healthy.

For example, the tiger is an umbrella species because the presence of a tiger helps in the thriving of other species like deer, monkeys, etc. Presence of tiger protects the habitat for many different species. Therefore scientists often focus their attention on saving umbrella species from extinction.  

tsd is temerature dependent sex determination.

tsd: it is a type of environmental sex determination in which the temperature experienced during embryonic/ larval development (ambient temperature for development) determine the sex of the offspring.

crocodile displays the tsd where the ambient temperature of the developing egg determines the individual's sex. for example in the american alligator's eggs, incubation at 33 ºc produces mostly males, while incubation at 30 ºc produces mostly females. so if the american alligator's egg will get the ambient temperature (33 degree celcius)during embryonic development from egg, the offspring will be male and same for the female sex determination.

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.