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What if kangaroos (specifically eastern grey kangaroos) became invasive in the eastern part of North America?

How would they effect native flora and fauna? What would they compete for resources with? What might eat them? How fast would their population grow? Stuff like that.

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    $\begingroup$ A quick Google Search turns up numerous yummy-looking kangaroo-based recipes. If humans live there, it won't be a problem for long. $\endgroup$ – user535733 Jul 5 at 23:08
  • $\begingroup$ Well, in Australia their numbers are rapidly growing, actually. Though they can’t make a very good estimate because of Australia’s size etc., there were estimated to be around ~27 million in 2009, now there are estimated to be about 50 million! And that’s in spite of all of the kangaroo recipes, and the (at least thousands) of unrecorded kangaroo deaths occurring due to how they’re viewed as ‘pests’. $\endgroup$ – Fivesideddice Jul 5 at 23:43
  • $\begingroup$ So if you gave them time to get going in an uninhabited area, it would take a cull to get rid of them, not just ‘incidental’ (as it were) deaths. $\endgroup$ – Fivesideddice Jul 5 at 23:45
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    $\begingroup$ Boxing gloves would be mandatory walking to work every morning. $\endgroup$ – DKNguyen Jul 6 at 0:14
  • $\begingroup$ Well, in North America, see here for useful hunting numbers you can work with. If the goal is to exterminate, the season can be widened. Plenty of hunters, as those numbers show. $\endgroup$ – user535733 Jul 6 at 1:03
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Kangaroos are basically (for a broad definition of "basically") just upright deer. Indeed, a side-by-side comparison of the eastern grey kangaroo and the ubiquitous white-tailed deer suggests they share much the same ecological niche:

  • They are ruminants with similar diets, mainly consisting of grass and low-lying plants of various descriptions. Deer in the eastern US mostly consume scrub and brush because that's what's most available, but in the west they're perfectly happy to graze on grasses. Conversely grey kangaroos are mostly grass-eaters but there are other species of kangaroo that prefer brush.
  • They have broadly similar social habits, revolving around small family units. Kangaroos are more inclined to former larger groups, in the ~10 member range, though this appears to vary with geography.
  • Their natural predators are similar... to an extent. Coyotes and dingos are very much akin to one another, being predatory canids of about the same size. However, North American predators can get quite a bit bigger than (extant) Australian ones. In the western part of the continent, kangaroos would have to contend with wolves and bears, but in the east they are less common.
  • Their interactions with humans are similar. They are relatively skittish and typically pose no threat to humans (except on roadways, where they are both menaces to careless drivers).
  • What kangaroos have, and deer lack, are a wide variety of coping mechanisms for extreme aridity that will virtually never be useful in the eastern US. (They might find some use for it in the west, but never to the same extent as in the Outback.) This suggests that they would be outperformed by native species until they manage to adapt. Wikipedia suggests they require a particular habitat to reproduce but doesn't say what; if this is taken to mean that they need access to the covered part of their range (with trees and brush) as opposed to the open grassland then they're fine. If their needs are more specific, it could be a problem.
  • Conversely, kangaroos are probably less adept at surviving winter. They can readily survive temperatures down to the single digits Celsius, but rarely have to encounter multiple months below freezing as occurs in the northern part of their proposed range. They might fare better in the south.
  • As for how fast they can grow: in theory, very fast. Finland has a population of about a hundred thousand deer descended from a handful of animals shipped over in the mid-30s, meaning a tenfold increase in population every 20 years or so. Your kangaroos will probably not be so fortunate, because of the different climate than what they're used to, but if they thrive in North America they could grow quite rapidly.

Tl;dr: the main problems would be adapting to a wetter, colder environment and potentially large predators such as bears. Their main competition would be medium-sized grazers like deer, and their main predators would be coyotes and the like.

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    $\begingroup$ Kangaroos are also very susceptible to toxoplasmosis, so feral cats would also be a larger problem in the US (50 million+) than in Australia (~6 million) $\endgroup$ – jdunlop Jul 6 at 1:39
  • $\begingroup$ What they need is water. Kangaroos won't breed unless there is a wet season. They can survive for years without a good rain but without a wet period in which they can raise young their populations will never increase. This is why provisioning cattle with water troughs is a big deal in the Outback. $\endgroup$ – user2352714 Jul 6 at 2:41
  • $\begingroup$ The cat problem cannot be overstated, feral cats are ubiquitous in the eastern US. $\endgroup$ – John Jul 23 at 14:17
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Eastern North America is heavily forested with high-density forests. Eastern Grey Kangaroos live in grasslands and low-density forests. The main advantage of being a kangaroo - the ability to jump - would be completely negated. They couldn't possibly compete with deer and would soon die off completely.

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