The other type of cacti are jungle cacti, and they do need humidity. These plants are more succulent-looking, and lack real spines, similar to 'Christmas' cactus. If this is the type you have, they like a bit more water, and during spring and summer, water plentifully to keep the potting mixture thoroughly moist.
There are about 3-4 million animaks euthanized each year by shelters. There actually are about 4-6 million animals euthanized in shelters in America each year. The above are just estimates. Shelters don't always keep accurate records, and no thorough surveys have been done. No one knows the real numbers.
I have no clue how many dogs are euthanized in the UK... but I will say that failure to spay/neuter has little to nothing to do with the plethora of dogs being euthanized in shelters. Responsible pet owners manage their dogs reproduction... and not necessarily by spaying/neutering.