I used to talk about how to me, the scariest part of Jurassic Park had nothing to do with the dinosaurs. I mean, let's be honest here: Homo sapiens is the scariest killing machine on the planet. We don't have big teeth or claws, but we are pretty clever with the use of tools and terrain, and we have language and can use it to plan and execute coordinated operations with terrifying effectiveness. This deceptively wimpy bipedal primate hunted mammoths, aurochs and moas to extinction. We went after sperm whales, fercryinoutloud! A tyrannosaur gets loose from a zoo? Bah. It might eat a few people before we can mobilize an attack helicopter to take it out, but we probably wouldn't even need to do that; a resourceful bunch of humans on the ground would probably figure something out by the time the pilot finished his preflight checks. (Yes, I know the velociraptors were supposed to be the really scary, smart ones, but seriously, they'd be hunted to extinction in a matter of months, and more people would die in car accidents driving to the store to buy anti-velociraptor gadgets than would be eaten by velociraptors.) Sure, scary to the people on the ground actually being chased by dinosaurs, but not especially more so than most of the countless other ways each and every one of us will face death.
No, to me, the scariest part was the scene just before we actually saw the dinosaurs, when the paleobotanist played by Laura Dern is marvelling over a fern she identifies as having been extinct for millions of years. I thought that fern was the most dangerous thing in the whole movie.
Why? Because over time, invasive plant species can do way more harm to a lot more people than a couple of hungry carnivores, and while we humans are unchallenged apex predators who excel at killing other predators to the point that we have to rein ourselves in so we can still have things to kill, we totally suck at controlling little things like weeds. The economic damage caused by kudzu, water hyacinth, and "harmless" little critters like zebra mussels and carp and pine beetles adds up year by year by year, and there's very little we seem to be able to do about it.
So the Jurassic Park geneticists revive a fern that's been extinct for millions of years, and whose natural predators are, presumably, still extinct. Oh crap. What's going to eat it? What's going to keep it from growing everywhere, including places where we're trying to grow corn or rice or potatoes?
But then I happened to see this video, "Why aren't all plants poisonous?" in which the answer to the question is actually that all plants are poisonous, but we creatures who feed on plants keep evolving counters to the toxins the plants come up with. I agree with the analysis, subject to some qualifications: some plants benefit from the action of hungry animals, and thus make delicious fruit or nectar to encourage them. Also, some plants, like thistles and cacti, may rely more on physical deterrents against predation than chemical ones. (I always suspected that thistles must be tasty, or they wouldn't devote so much metabolic effort to growing those prickles. I first checked a couple of books to confirm they weren't toxic, and then picked some thistle from my weedy backyard, and trimmed the prickles from the leaves. Turns out, they're actually quite palatable!)
Anyway, it got me to thinking: this arms race of plant toxins vs. evolved herbivore resistance to those toxins has kept on going ever since that fictional fern in Jurassic Park went extinct. That fern may have had state-of-the-art chemical defenses against the herbivores large and small of the late Cretaceous, but the art has evolved considerably since then. It may well be that many of the genetic tricks that herbivores developed to feed on the extinct plant have been retained and refined in the genome of modern animals (including us), particularly if that plant's surviving relatives have continued to use and refine those defenses.
So maybe, I was wrong, and rather than deathweed juggernauts blacking out the sun, the reanimation of extinct plant species would instead lead to delicious and healthy "new" foods for us to eat. Either that, or they'll be completely ungrowable because every agricultural pest and microbe will eagerly devour them before we can.