Killer Cactus?

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IanW
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Re: Killer Cactus?

Post by IanW »

Thermoman wrote:If 'No' is your choice, is this based on field work? Have you, or some other trusted authority, actually counted animal bones around hook-spined cacti and compared the result with that around straight-spined plants? Were the results published?
So you're not after a yes/no answer then are you? You're after a detailed description of what we know to date. You're both simultaneously demanding detail, whilst also demanding a simplistic yes/no answer. Which is it?

My "No" is based on the fact that searching and complete lack of reference to any such thing brings up no evidence of any such study. I could have instead said "Yes" to also fit your binary criteria, but that would be based on even less evidence - I'd be responding in the affirmative to something for which there is no evidence of. I could say I don't know, but that both is and isn't an acceptable answer based on your contradictory posts about wanting a binary answer but also being happy with a don't know, but then not being happy with it because it's not binary but in that case you might as well just have a thread with zero replies or a lot of don't knows.

You seem to desperately want to be told yes, cacti are evolved as vicious predators, and there's lots of evidence everywhere, but you're not going to get that, because there isn't, and they most probably aren't. You want to be told that spines have evolved to catch creatures that die and provide nutrients, whilst seemingly being evasive of the idea that those spines most likely simply exist to protect the nutrients they're already effective at obtaining and storing from elsewhere.

If we look at other actual carnivorous plants, they have additional mechanisms to aid their predatory aspects - they have an inherent ability to break down and absorb nutrients from living things, which cacti do not, and they have a clear and succesful lure, which cacti do not. There are many documented lures to increase pollination however, on flowers that are exposed - i.e. Pilosocereus azureus with it's cabbage scented flowers that attract bats, but these flowers are big and extend out well beyond the spines, explicitly allowing pollinators to avoid the danger of the spines when pollinated.

It's not even really clear that Puya chilensis is genuinely predatory, the original source seems to be mostly local hearsay and folklore rather than any evidence of it being a widespread trait. Are the sorts of sheep it catches even native to where it grows in the first place as opposed to imported livestock or descendants of?
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Re: Killer Cactus?

Post by graham »

KarlR wrote:What I would suggest though, is that cacti won't have developed hooked spines for no reason.
But, if you'll forgive me, the curved spines did develop for absolutely no reason - they arose either from some simple mutation in a gene (less likely a mutation in more than one gene at the same time) or from the interaction of genes. Having arisen the point is that they give the plant no disadvantage and may well spread. If there is a difference to the effect then it could be that they are as effective, and possibly more so, with very small herbivores who can get their muzzles very close to the plant by carefully avoiding straight spines, having done so the recurved spines would then be in a position to be felt by the muzzle as it moved around close to the plant - but this is conjecture.

The other point is that having spines what can then happen to them ? I believe it is safe to start from the position of a generally straight spine (from a modified stem as also seen in various thorns) and having arrived there why not a bend at the end ? I've just had a look at my small collection of cacti at the moment and the vast majority do not have completely straight spines, the Gymnocalycium, Rebutia e.g. have a gentle curve which results in the spines while not laying flat against the body they are certainly not sticking straight out but they certainly do offer a barrier with pointy ends to any small muzzle trying to gain access to the body. I have a Coryphantha maiz-tablensis that offers an interesting form of spines: while not having a central one the four or so major spines are quite straight for 80% of their length and show a pronounced curve at the end. My point really is that curvature in spines is quite common and the example of Mammillaria bocasana is simply one, perhaps extreme, of many.

But to move away from curved spines - what about Leuchtenbergia principis - spines, call those spines ?

And while I do not own sheep I have had them grazing my fields regularly; having seen these examples of selective breeding in action I'm sure that I could make a position that thorns on brambles have evolved to trap sheep. When I had some smaller lambs here I did go out once a day with a Stanley (there are other makes of DIY tools) knife to cut them out of the brambles; from 50 or so lambs I would say that there was one or two per week who were well caught and at least two who may not have escaped by themselves (one had several stems with a pair actually wrapped around its neck and that did take me a couple of minutes to cut through them all).

graham
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Re: Killer Cactus?

Post by Thermoman »

IanW wrote:You seem to desperately want to be told yes, cacti are evolved as vicious predators, and there's lots of evidence everywhere.
No! No! You completely misrepresent me. I am not desperate for, nor do I even prefer, either answer. I am merely making the point that if hooked-spined cacti are associated with disproportionately large numbers of small animal bones - the 'Yes' option - then this is worthy of further investigation. Maybe cacti are using their hooked spines to trap small animals. On the other hand it could simply be an accumulation of bones from Jane O's cactus wrens, resulting from fledglings falling out of their nests, over the years.

If the 'No' answer prevails then that's that. There is no need to add complexity. My wren died from blundering into a mammillaria bocasana which was no more responsible for its death than was Graham's Saguaro when it fell on the gun-slinger.

I am not prejudging the issue in any way when I ask for a Yes/No answer as an essential prerequisite. In the mean time I would offer you the well-known caveat that 'Absence of evidence does not constitute evidence of absence'.
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Re: Killer Cactus?

Post by IanW »

graham wrote:But, if you'll forgive me, the curved spines did develop for absolutely no reason - they arose either from some simple mutation in a gene (less likely a mutation in more than one gene at the same time) or from the interaction of genes. Having arisen the point is that they give the plant no disadvantage and may well spread.
I don't think that's what he meant, I think he meant "developed as a predominant trait", not simply developed. You're still then conflating "no disadvantage" with may well spread. Again, no they wont, how do you propose a single mutated plant in a population can predominantly spread it's genes through reproduction unless it's more fit to reproduce, because it has an evolutionary advantage?

So yes, you can have mutations that happen, offer no advantage, and the plant survives, but that mutation wont become a predominant trait unless it allows the plant to be more successful at interbreeding and spreading that trait. It's a simple numbers game - 1 plant cannot breed more widely and successfully with it's distinct trait than 10,000, or even millions of plants unless there is an evolutionary benefit to that trait that allows it to do so.

You're technically right therefore that such a trait may arise, give no disadvantage and may well spread, but if it does well spread that's because it's not simply giving no disadvantage, but also giving an advantage.
graham wrote:And while I do not own sheep I have had them grazing my fields regularly; having seen these examples of selective breeding in action I'm sure that I could make a position that thorns on brambles have evolved to trap sheep. When I had some smaller lambs here I did go out once a day with a Stanley (there are other makes of DIY tools) knife to cut them out of the brambles; from 50 or so lambs I would say that there was one or two per week who were well caught and at least two who may not have escaped by themselves (one had several stems with a pair actually wrapped around its neck and that did take me a couple of minutes to cut through them all).
Unfortunately sheep often sometimes also fall to their death from high cliff faces, but unfortunately this is not evidence that cliffs have evolved to hunt sheep.
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KarlR
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Re: Killer Cactus?

Post by KarlR »

Hooked spines in any given species (or any other trait for that matter) have probably arisen from a chance mutation, but when the entire population of a species has adopted the trait, it is not a matter of said trait merely not being disadvantageous. When an entire population of a species has adopted a certain trait it must infer some advantage or benefit to that species.

If we go back to Mammillaria bocasana which was the plant that set off the discussion, there is a reason why this plant is globular, why it has fairly small flowers, why it has spines etc. It follows that there will also be a reason why it has hooked centrals. Maybe the hooked centrals allow the plant to spend less energy on the rest of the spines, thus maximising protection with minimum effort, maybe (as you suggest) the hooked centrals work better at keeping a certain kind of animal away,or maybe they have some other function. But such a shape doesn't simply appear one day and then becomes dominant over time to the point where all members of the species show the trait, unless it gives the plant some advantage or benefit.

As for Leuchtenbergia, I assume the spines are a way of camouflaging the plant (particularly when young), similar to Sclerocactus (Toumeya) papyracantha.

I agree that a great many cacti have spines that curve somewhat, perhaps even the majority do. However, there is a big difference between this and having one hooked central spine. Having spines that curve somewhat allows for more efficient coverage of the plant body. If the spines are completely straight they'll probably need to be longer or else there must likely be many more of them to offer the same level of protection. In my opinion there must be a reason why some species have hooked centrals and others do not.

As to what that reason might be I do not know. Maybe it's another way of simply deterring predators, maybe it's a more efficient way than going for either denser spine coverage or stronger spines, or maybe they serve another purpose. I have never read anything about cacti trapping animals with their hooked spines to supplement their diet but, considering that animals can and do get trapped by the spines (as we have two examples of in this thread only), I don't think it is out of the question that one reason for having hooked spines (perhaps among many others) may be that on occasion an animal will be caught and serve as a nutrient supplement.
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KarlR
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Re: Killer Cactus?

Post by KarlR »

Didn't see Ian's post there, but yes, that's what I intended when using the word "develop". Perhaps it was worded poorly.

As for the sheep, they aren't really good indicators I think. Perhaps the Puya kills the odd sheep, but if it does I think it's more likely to be a case of the sheep simply getting entangled by misfortune than the plant actively seeking to attract sheep in order to catch them. That the plant might benefit from nutrients seeping into the ground from the rotting carcass I think is likely however, so maybe some time in the future a Puya specimen might appear which is a bit more active in seeking out sheep. But domestic sheep haven't really been given the chance to develop proper sensibilities when it comes to things that might kill them. Whether it's falling off cliffs, drowning in bogs or lakes, eating poisonous plants, getting their wool entangled by spines and so on, domestic sheep aren't really bred to cope with such things.
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Re: Killer Cactus?

Post by rodsmith »

Jane O wrote:Maybe Thermoman's victim was trying to build a nest but got caught out by hooked spines. I don't think the cactus is the killer - more a question of poor nesting spot by very inquisitive bird.
It's the wrong time of year, Jane. Birds in the UK nest in the spring.
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Growing a mixed collection of cacti & other succulents; mainly smaller species with a current emphasis on lithops & conophytum.
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Re: Killer Cactus?

Post by JaneO »

Point taken Rod. There are some birds that have a go at a second brood in July. Not sure about the wren but we had a brood of very noisy chaffinches eating hearts and suet fat a few weeks back. That was late but some at least survived. I know the much loved house sparrow is not so nice when it comes to nests too. He happily ejects chicks so he can take possession of nest and ex Mrs sparrow!
Thanks
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Re: Killer Cactus?

Post by PerG »

A very interesting discussion
I found an alternative explanation to hooked spines. It may help the plant move to a more favourable position as it gets carried with debris after heavy rain. I have no idea whether this is a good explanation, I just read it here:https://en.wikipedia.org/wiki/Sclerocactus (Sclerocactus wiki). (of course this is not about Mammillaria bocasana, don't know how relevant it is).
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Re: Killer Cactus?

Post by Phil_SK »

I'd meant to come back to this topic but forgot! I'd been daydreaming and found myself wondering whether there was any evolutionary advantage in first having large carcasses attached over the photosynthetic areas and then being physically damaged by scavenging animals. :oops:
Phil Crewe, BCSS 38143. Mostly S. American cacti, esp. Lobivia, Sulcorebutia and little Opuntia
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