<p>A weed is just a plant where <strong><em>you</em></strong> do not want it. Totally a matter of context. Tumbleweeds are non-native, introduced centuries ago. I assume you mean the invasive species of plants that have been spread by humans and are disrupting ecologies throughout most of the world</p>

<p>Until recently, these plants we consider weeds were limited in their range to home environments simply by geographic barriers and surrounding unfriendly environments. And the natural consumers, parasites and competitors in the home environments had adjusted to these plants and kept them in check.</p>

<p>When non-native plants are introduced into a new environment by humans, most of them do not thrive, but occasionally a plant is wildly successful. Eventually the potential consumers, parasites and competitors in that new environment will adjust through evolution. But the tragedy is that many or most of the original species will be destroyed before that balance is restored in a new, way more simplified form. The landscape itself may be totally changed. Removal of a key original species can cause great change too: <a href="http://voices.nationalgeographic.com/2014/02/16/this-will-shatter-your-view-of-apex-predators-how-wolves-change-rivers/" rel="nofollow">How Wolves Change Rivers</a>.</p>

<p>Not only will many ecologies be reduced to much simpler versions, even if they eventually conquer the invasive plant, those simplified ecologies will closely resemble each other, if their geography is similar, even if on the other side of the world. If humans were to totally stop transplanting invasive species (collapse of civilization?), diversity would return after millions of years. We know this from <a href="http://en.wikipedia.org/wiki/Extinction_event#Effects_and_recovery" rel="nofollow">Extinction Events</a>.</p>

<p>So to answer you question, weeds before human intervention generally did not take over in their home environments because the potential weeds and their natural consumers, parasites and competitors all evolved together in a quasi-equilibrium. Now, however, most ecologies throughout the world are out of equilibrium because of environmental change and/or invasive species. This will inevitably lead to simplified, usually less robust, weedy ecologies throughout the world. That reduction in diversity might as well be considered a permanent situation compared to the timescale of civilization.</p>

CC BY-SA 3.0
2015-11-04T20:25:41.270Z
2015-11-04T22:12:43.270Z
2015-11-04T22:12:43.270Z
Eubie Drew
<p>Given how rapidly weed plants spread and grow, choking out all other plant life, how come after millions of years we haven't ended with forests full of thistle or pokeweed, as opposed to pines or oak trees?</p>

CC BY-SA 3.0
2015-11-04T20:32:36.510Z
This question appears to be about ecological succession and probably belongs on another stack exchange.
Siv
CC BY-SA 3.0
2015-11-04T22:18:32.397Z
@Siv this Q would fit in either this SE or biology SE. zx6r apparently wanted an Earth Science perspective, more of a global and deep timescale overview.
Eubie Drew
CC BY-SA 3.0
2015-11-04T23:37:13.697Z
@Aabaakawad the scope of the Earth Science SE has been quite clear that it regards questions of palaeontology which do not have an Earth Science aspect as being off topic, I fail to see the applicability of this question to Earth Science as it is Biological/Ecological. I honestly do not see what perspective Earth Science could give to the answer.
Siv
CC BY-SA 3.0
2015-11-05T01:12:16.283Z
@siv okay, then we should eliminate a few tags from the Earth Science tag system, namely: geobiology, evolution and ecology.
Eubie Drew
CC BY-SA 3.0
2015-11-05T01:17:29.887Z
@Fred https://en.wikipedia.org/wiki/Anthropocene
Eubie Drew
CC BY-SA 3.0
2015-11-05T03:01:53.647Z
@Aabaakawad: I know what anthropocene relates to. I don't think it's relevant to the question, as asked.
Fred
CC BY-SA 3.0
2015-11-05T20:42:10.647Z
@Aabaakawad For geobiology I think it depends on the scope of the question, the same I think for evolution. I agree that the ecology tag isn't appropriate, but possibly a palaeoecology tag would be depending on the question.
Siv
CC BY-SA 3.0
2015-11-05T22:37:33.440Z
I do research in groundwater dependent ecosystems as a hydrogeologist. Thus, I prefer to take an expansive view of geoscience rather than ceding everything plant related to the ecologist. This question is a bit tenuous in the geoscience relationship but I wouldn't worry about it too much. I remember a friend getting a similar question on her geology PhD candidacy exam.
haresfur
<earth-history><mass-extinction><geobiology><evolution><ecology>
Why haven't weeds overtaken the entire planet?
zx6r
# Why haven't weeds overtaken the entire planet?

(Asked by: username_0 on 2015-11-04)

Given how rapidly weed plants spread and grow, choking out all other plant life, how come after millions of years we haven't ended with forests full of thistle or pokeweed, as opposed to pines or oak trees?

**Tags:** earth-history, mass-extinction, geobiology, evolution, ecology

> **Comments:**
> **username_1**: This question appears to be about ecological succession and probably belongs on another stack exchange.  
> **username_2**: @username_1 this Q would fit in either this SE or biology SE. username_0 apparently wanted an Earth Science perspective, more of a global and deep timescale overview.  
> **username_1**: @Aabaakawad the scope of the Earth Science SE has been quite clear that it regards questions of palaeontology which do not have an Earth Science aspect as being off topic, I fail to see the applicability of this question to Earth Science as it is Biological/Ecological. I honestly do not see what perspective Earth Science could give to the answer.  
> **username_2**: @siv okay, then we should eliminate a few tags from the Earth Science tag system, namely: geobiology, evolution and ecology.  
> **username_2**: @username_3 https://en.wikipedia.org/wiki/Anthropocene  
> **username_3**: @Aabaakawad: I know what anthropocene relates to. I don't think it's relevant to the question, as asked.  
> **username_1**: @Aabaakawad For geobiology I think it depends on the scope of the question, the same I think for evolution. I agree that the ecology tag isn't appropriate, but possibly a palaeoecology tag would be depending on the question.  
> **username_4**: I do research in groundwater dependent ecosystems as a hydrogeologist. Thus, I prefer to take an expansive view of geoscience rather than ceding everything plant related to the ecologist. This question is a bit tenuous in the geoscience relationship but I wouldn't worry about it too much. I remember a friend getting a similar question on her geology PhD candidacy exam.  

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## 1 answers

### Answer by username_2 on 2015-11-04. Score: 3 | Accepted answer

A weed is just a plant where ***you*** do not want it. Totally a matter of context. Tumbleweeds are non-native, introduced centuries ago. I assume you mean the invasive species of plants that have been spread by humans and are disrupting ecologies throughout most of the world

Until recently, these plants we consider weeds were limited in their range to home environments simply by geographic barriers and surrounding unfriendly environments. And the natural consumers, parasites and competitors in the home environments had adjusted to these plants and kept them in check.

When non-native plants are introduced into a new environment by humans, most of them do not thrive, but occasionally a plant is wildly successful. Eventually the potential consumers, parasites and competitors in that new environment will adjust through evolution. But the tragedy is that many or most of the original species will be destroyed before that balance is restored in a new, way more simplified form. The landscape itself may be totally changed. Removal of a key original species can cause great change too: [How Wolves Change Rivers](http://voices.nationalgeographic.com/2014/02/16/this-will-shatter-your-view-of-apex-predators-how-wolves-change-rivers/).

Not only will many ecologies be reduced to much simpler versions, even if they eventually conquer the invasive plant, those simplified ecologies will closely resemble each other, if their geography is similar, even if on the other side of the world. If humans were to totally stop transplanting invasive species (collapse of civilization?), diversity would return after millions of years. We know this from [Extinction Events](http://en.wikipedia.org/wiki/Extinction_event#Effects_and_recovery).

So to answer you question, weeds before human intervention generally did not take over in their home environments because the potential weeds and their natural consumers, parasites and competitors all evolved together in a quasi-equilibrium. Now, however, most ecologies throughout the world are out of equilibrium because of environmental change and/or invasive species. This will inevitably lead to simplified, usually less robust, weedy ecologies throughout the world. That reduction in diversity might as well be considered a permanent situation compared to the timescale of civilization.


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