Cat and mouse

Today I came straight home, had a quick tea, and shut myself in the conservatory in an attempt to work on my final degree assignment. I have an essay to write, concerning transfer between ecosystems - is a system ever really closed, and are there examples of biotic transfer between them (movement of matter or energy by living things)?

Before I so much as opened my laptop I saw a simple example in action unfold in front of my eyes. The back garden ecosystem, though enclosed by a fence, is certainly not closed, or indeed enclosed. Insects, mice and birds feed on plants and seeds, but is there a top predator? I don't know who this cat belongs to, or where he came from, but he arrived today, from outside of this back garden, and did what cats do instinctively.

He is now doing the other thing cats do instinctively. Looking smug, licking his lips.
RIP Spidermouse. I hope there are bird-feeders in mouse heaven!