Killer Cactus?

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Thermoman
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Killer Cactus?

Post by Thermoman » Thu Sep 10, 2015 12:48 pm

A recent upsetting incident led me to wonder whether there is a dark side to our spiky charges.

A wren got into one of my greenhouses. I found it, entangled and struggling, on a mature Mammillaria bocasana. It took twenty minutes or so to free it. All the entrapping spines had to be individually detached with a pair of fine tweezers and it proved very difficult to prevent the creature from re-hooking itself as each part of its anatomy was released. Sadly, the bird died, probably from shock, within a few minutes of eventual freedom.

Hooked spines can be a bit of a puzzle. What is their purpose? Up until now I have assumed that they facilitate the dissemination of species that carry them. If a cactus pup gets hooked-up in the wool/hair/feathers or even the bare skin of some passing creature then it could well be pulled off the parent plant and subsequently transported perhaps miles away where, when it is eventually released, it could re-root. However, an examination of the Mammillaria bocasana suggested that its hooked spines are poorly suited to this purpose. They point downward, making them ideal for holding and further ensnaring a struggling creature like a small bird or mammal. The plant itself is quite robust. It would take some force to detach a pup and even more to uproot the whole plant – certainly well beyond the capability of a wren. In contrast, the pads, or segments, of, say, Cylindropuntia tunicata are easily pulled from the parent plant by even quite modest force exerted via one or more of its barbed spines embedded in the flesh of some hapless passer-by.

If Mam. Bocasana attaches itself to an animal large enough and strong enough to pull off a pup, it seems more likely that the attaching spine, or spines, would be ripped out of the plant. Inviting damage appears to have little survival value whereas entrapping small creatures could be rewarding. Post mortem 'juices' could make a valuable contribution in a parched environment. Is there any field evidence of dead creatures being found in close association with cacti?
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Re: Killer Cactus?

Post by IanW » Thu Sep 10, 2015 2:22 pm

I think a defensive explanation is still adequate in most cases:

- Any creature dying on such a plant will inherently be a case of losing to survival of the fittest. The members of said species that die on the plant die, and the ones that know to keep away survive, eliminating a predator of that plant through such selection.

- Putting your mouth around a Mammillaria bosacana probably isn't pleasant. In fact, if you put that in your mouth you may find yourself unable to eat, again, eliminating such members of a species attempting this from the ecosystem, they'll instead have to go for something more likely to go down your throat than the hooked spines of this sort of plant.

- Even if a creature breaks free and damages a plant spelling the end for that plant, it doesn't mean that the creature doesn't later die, or isn't simply capable of learning never to go near any such plant ever again. It's probably worth noting though that many cactus can re-grow their spines, so losing them isn't the end of the world. In fact, because I was grafting on Trichocereus pasacana a few years back I first yank the whole woolly areole including spines and all off the plant with pliers to make it easier to work with. Some I did this too and haven't yet subsequently used despite doing so in an incredibly blunt, messy, and unclean manner have completely regrown those areoles, spines, wool, and all such that you wouldn't know they'd ever been pulled off bar a bit of damage to the surrounding epidermis in some cases.

I'm sure in some cases like the easily detached opuntia that cling on, spreading yourself around is also a useful adaptation - in fact we also know that some spines help catch airborne moisture in places like Chile, but I think survival is still a major factor for the most part.
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Re: Killer Cactus?

Post by KarlR » Thu Sep 10, 2015 3:02 pm

Puya chilensis (of Chile) traps and kills sheep by hooking onto their wool, thus gaining a nutrient boost from their decaying bodies.

I've never considered this in terms of cacti, having always thought the hooked spines were there for protection. However, long and straight spines should serve just as well as a deterrent as hooked spines, one would think. Perhaps there is something in this. Apart from small birds, there are also such animals as bats, rodents, insects and lizards that might all get entangled by the hooked spines. If they did I suppose it might contribute some nutrients to the plant. I suppose the hooked spines might also serve to catch leaves and maybe small branches blown on the wind or similar which could also add a wee bit of nutrients.

I'd tend to agree with Ian, though I don't think a cactus would say no to an extra snack as it were.
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DaveW
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Re: Killer Cactus?

Post by DaveW » Thu Sep 10, 2015 3:30 pm

We always assume that features such as hooked spines need to be advantageous for a species, Darwin's survival of the fittest, when all that evolution requires is they are not disadvantageous and prevent the plant flowering and setting seed, therefore passing on it's genes to further generations. Any mutations or alterations of features that do prevent reproduction, or make them more attractive to predation automatically die out.
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Re: Killer Cactus?

Post by IanW » Thu Sep 10, 2015 4:09 pm

DaveW wrote:We always assume that features such as hooked spines need to be advantageous for a species, Darwin's survival of the fittest, when all that evolution requires is they are not disadvantageous and prevent the plant flowering and setting seed
No. A change in a species (such as the development of hooked spines) has to come about as the result of survival of the fittest. The way in which a new change like this develops for the first time in a species is through an event such as mutation or cross-breeding, and events like mutation and cross-breeding are typically relatively rare events. As such, if it offers no survival advantage then the other 10,000 members (or whatever) of the species that don't have this trait will simply overwhelm it when being pollinated and whilst it may exist as a recessive gene upon breeding that comes true every once in a while, it will still relatively rarely occur.

So for it to become a predominant trait, one which we see in nearly every member of the species as we do with hooked spines on M. bosacana it has to offer some kind of survival advantage that allows plants with that initial trait, and any subsequently growing with that recessive gene being realised, to be more likely to survive and produce seed than the original members of the species without that trait such that other time the original trait all but disappears and the new trait (e.g. hooked spines) becomes predominant.

Something could not simply happen, and stick, without any relevance to survivability unless it was a genetic change effecting the majority of a member of a species in a short period because otherwise the simple statistical odds of the original trait outnumbering the new one would ensure that all things being equal, the original trait would out breed it.
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Re: Killer Cactus?

Post by Diane » Thu Sep 10, 2015 4:26 pm

A similar upsetting incident happened to me too, a couple of weeks ago. I found a robin (one of "my" robins) wrapped around my prized Mammillaria tetrancistra - completely entrapped. Fortunately it was no longer alive (probably only a few hours), as I don't know what I would have done had it still been alive. It was really difficult to remove too, being well hooked-up, and I didn't want to damage the plant any more than necessary. Such a shame, usually robins are pretty savvy about flying in and out of the greenhouses, unlike other birds which get in a panic.
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Re: Killer Cactus?

Post by JaneO » Thu Sep 10, 2015 4:56 pm

Campylorhynchus brunneicapillus is commonly known as the Cactus Wren! So presumably a distant relative from South West to central Mexico. This bird often nests in cactus plants, using the spines for protection from predators. 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 always sad to lose something of beauty though.
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Re: Killer Cactus?

Post by Thermoman » Thu Sep 10, 2015 5:28 pm

I was hoping that someone would have picked up my concluding question – namely, “Is there any field evidence of predation?”

We can ask, “What is the purpose of a spider's web? Is its evolutionary significance neutral? Did spiders just learn to make webs in order to while away the time before television?” The answer is easily found. We go into our greenhouse where we find spider's webs containing 'mummified' flies, wrapped up for later consumption. Underneath are piles of empty 'husks' – the remains of earlier meals. No such concentrations of insect remains are found elsewhere in the greenhouse. Conclusion – Spiders use their webs to catch flies and other insects which they then eat.

If I rephrase my earlier question, my meaning might be clearer. “Are disproportionately large concentrations of mammal/bird bones found in the immediate vicinity of hook-spined cacti, in habitat?” If so, I suggest that this could be prima facie evidence for predation. If not then Mammillaria bocasana is innocent.
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Re: Killer Cactus?

Post by Thermoman » Thu Sep 10, 2015 5:44 pm

Jane O wrote:Campylorhynchus brunneicapillus is commonly known as the Cactus Wren! So presumably a distant relative from South West to central Mexico. This bird often nests in cactus plants, using the spines for protection from predators.
That could well be but remember the remora. It associates with sharks but, as Wikipedia points out, occasionally becomes their prey. Little Red Riding Hood was wise to be suspicious of her grandmother.
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DaveW
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Re: Killer Cactus?

Post by DaveW » Thu Sep 10, 2015 7:36 pm

Not intending to hijack the thread but to clarify why I believe not every feature needs a useful function to survive.

It is notable for places like coral reefs in warmer waters there is a greater bizarre range of variation between the fish species so the survival of the fittest is not so obvious compared to colder more stressful waters where variation in species becomes far less extreme and only the fittest shapes survive.

If we take Darwin's survival of the fittest to it's logical conclusion all habitats by now should be only populated by a single species, or at least all species having a similar form. That is patently not true since evolution only weeds out the unfittest which don't survive long enough to breed. It is surviving long enough to breed and passing on their genes that determines survival, anything non detrimental to that evolution ignores.

If only the fittest were left and climate or conditions changed, as it has throughout the earths history, all life would be wiped out. It is having a range of survivable forms in any habitat that allows those less specialised to survive the new conditions. Man is never the fittest for any environment he resides in but is able to tolerate a great variation in them and breed successfully.

If you take the male birds of paradise for instance, producing long ornate plumes to attract the female prejudice's their flying ability and so makes them more vulnerable to predator's, but as long as the males last long enough to breed and pass on their genes the species survives.

Evidently Wallace agreed with my emphasis above that evolution works by the elimination of the unfittest, leaving all those that can survive long enough to breed, not simply the single most fit, as the following quote shows:-

""Darwin's response is, on the face of it, rather puzzling. Why did he not protest Wallace's assertion that selection works principally through the elimination of unfavourable variants"

Other quotes from the following link:-

"The answer probably lies in Darwin's frustration with misunderstandings attributable to the connotations of "selection." As early as 1860 he wrote to Lyell:" Talking of 'natural selection'; if I had to commence de novo, I would have used 'natural preservation.'"

"Alfred Russel Wallace was even more troubled by difficulties arising from the term. Indeed, in his personal copy of the first edition of the Origin, Wallace frequently crossed out "natural selection" and substituted Spencer's phrase."

"It is in any case clear, both from their correspondence and from Darwin's autobiography (where he writes that he is "'not conscious of having profited in my own work by Spencer's writing"), that neither Darwin nor Wallace believed himself to be taking a step of great significance in adopting "'survival of the fittest" as a synonym for "natural selection."

"Waddington, for example, asserted that "the meaning of natural selection can be epigrammatically summarized as 'the survival of the fittest.' . . . to speak of an animal as 'fittest' does not imply that it is strongest or most healthy, or would win a beauty contest. Essentially it denotes nothing more than leaving more offspring"

"To most of the nineteenth-century evolutionists, natural selection meant the "survival of the fittest" in the "struggle for existence." These emotionally loaded phrases have been often misused for political propaganda purposes. A less spectacular but more accurate statement is that carriers of different genotypes transmit their genes to the succeeding generations at different rates ... . The "fittest" is nothing more remarkable than the producer of the greatest number of children and grandchildren"

http://www.dianebpaul.com/uploads/2/3/2 ... ittest.pdf
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