Naturally evolved unknown or little studied species are a rather different proposition to 100 new varieties of well studied apple and wheat etc, created by and for industrial agriculture.
Who knows what chemical and medicinal discoveries we shut ourselves away from as species are dying out. Consider a single poppy species that gave medicine 4 or 5 chemicals that turn out to be the basis of the vast majority of medicine's strong pain relief repertoire.
1) Many of the bred/GMO cultivars are sterile and/or unable to survive in the wild. They actively work against biodiversity.
2) Many of the bred/GMO strains are used in monocultures either in agriculture or gardens. These both displace the native plants, steal resources, and are also far more vulnerable to pests, disease, etc. Again, not improving, but destroying biodiversity.
3) Much of human activity in addition to breeding new species/cultivars is also deliberately or inadvertently moving species into new ecosystems. Some of these become rapidly spreading invasives that damage or destroy the local ecosystem, both plants and insects, animals, and birds that feed on the now damaged ecosystem (& no, they usually cannot just eat the new plant, nevermind if it's not toxic).
4) Many of the bred/GMO variants, even if planted in their native habitat, are incapable of providing sustenance to native insects & birds. While they may still have the external characteristics to attract them (odor, UV spectrum colors, etc.), they lack the actual nutrients, e.g., because they are sterile. So, not only do the human breeding/GMO activities damage biodiversity in plants, they are also damaging the other ecosystems.
So, if you are going to count human bred/GMO'd cultivars, they need to be counted against biodiversity.
Between these disruptions of the plant ecosystem, pesticides, habitat destruction, climate change, and other effects, we are much more dangerously close to breaking the food chain than people imagine.
This is invisible because of ignorance, normalcy bias, and the fact that these are enormously complex ecological webs, and they are very resilient until they aren't -- at which point the system collapses and it is too late to repair.
Numerous civilizations and indeed ecosystems have collapsed because of this sort of problem (see e.g., Permian Extinction which nearly extinguished life on the planet).
We are now causing this on a global scale and, except for the asteroid ~65MM years ago, at a pace unprecedented in geologic history.
Yes, and that's the problem. We breed new subtypes of plants for human consumption.
The problem from a mass extinction of plants is that we don't know what the reverberating effects of those eradications can mean. There are many animal, insect etc species which may be reliant on said plants for primary nutrition which then percolates all the way up the food chain and potentially decimating other animal populations.
It is not hard to imagine the potential side effects of say, the resulting plant extinction resulting in massive bee population losses for example. But that's just one prominent example of what lost biodiversity can potentially cause. This lost biodiversity can then further contribute to the rise of invasive species and the result is a destabilization of multiple biomes which have been relatively stable for a long period of time.
I wouldn't say "no bearing". But if current trends continued, there wouldn't be much left except people, livestock, pets, crop plants (including trees), insect pests, and microorganisms. And stuff in zoos and botanical gardens.
However, that's unlikely. Well before that, human society and population will likely collapse, due to climate change.
GP likely isn't talking about lab-grown experimental GMOs, but regular farming. Many if not most of the food we eat is completely unlike it was before humans invented agriculture.
GMOs and cultivars grown in labs not only have no positive effect on biodiversity, they have significant negative effects. See more detailed post above.
