@foosyerdoos Said
I don't believe other animals are sentient! Or at least, I don't believe other animals have a sense of self. Or an awareness of self. I believe all living things wish to survive and breed. To insure survival they will react to pain and dangerous situations. That doesn't make them sentient. It mearly make them alive. What it does, is evoke pity(a human emotion) from us because we can see that pain. And we know pain hurts. I maintain that animal rights advocates are anthromorphizing different species by applying human emotions onto animals.
Other animals do have s sense of self as determined by the Mirror Self Recognition Test (MSR). Here is a paper reporting on the tests involving bottlenose dolphins from 2001: https://www.pnas.org/content/98/10/5937.full
There have been prior and subsequent tests involving several other species, including even some birds, and some of them also pass the MSR, providing evidence of their self-awareness (chimpanzees, elephants, magpies and others). There has also been considerable interest in using other sense-awareness markers to determine self-awareness - since we know that a human being born without sight is self-aware, there must be ways to test for self-awareness that do not involve sight. Recognizing one's self in a mirror is one way, but it cannot be the only way, to determine self-awareness.
Sentience is not best understood as this kind of higher order self-awareness though. Sentience is a more basic form of consciousness, which has been described as the ability of a being to know, for example, what it is like for it to be a bat. We may not be able to describe what it is like for a bat to be a bat, but there is something that it is like to be a bat, and bats know what it is.
In that sense, there is not something it is like to be a tree. Trees do not possess sense organs or a central nervous system and although plants generally respond to stimuli, stimulus response is condition of being alive, not of being sentient. Sentience implies some level of decision making as a result of sensation. Plants cannot choose to not grow towards the sun, but a snake could choose not to eat the frog (and choose instead to eat the grasshopper he also sees).
In the end, it is an age-old and possibly intractable problem to resolve the questions surrounding consciousness and the problem of "other minds". Philosophy has yet to provide a definitive way for me to say with absolute confidence that you have a mind, although I can say with confidence that I have one. What we all do, in our day-to-day lives, is assume that those others we see around us, who look as we look, and who act as we act, are similarly situated as ourselves. We go with the best evidence we have, and govern ourselves accordingly.
The best evidence we have to date suggests that most animals are like us in relevant ways - they are alive, they know that it is they and not some other who is alive, and they knowingly act in ways to keep themselves alive. They are sentient beings who have an interest in their own lives, just like you and me. We should respect and protect their interest in staying alive.