Editing Eastern Prickly Pear (section)

=== Why ''aren't'' they there? ===

Another question that is sometimes puzzling is why, within fairly large stretches of potentially excellent habitat, there seems to be no trace of ''O. humifusa''.  The answer to this question is three-fold.

To begin with, even though birds offer the most promising mode of dispersal for ''O. humifusa'', a substantial amount of luck is required for this dispersal mechanism to succeed.  Distribution by birds occurs when they defecate, depositing the seeds in their waste.  But birds are in no way selective about where they choose to defecate, generally doing so whenever the urge strikes, either while they are flying or while they are perched.  When you consider the very specific habitat requirements of ''O. humifusa'', it is no surprise that the vast majority of ''O. humifusa'' seeds are deposited in bird droppings in habitat areas that entirely incompatible with the needs of the cactus.  Only by sheer luck will a bird that has been feeding on cactus pears happen to defecate in a spot which satisfies all of the criteria for those seeds to give rise to a mature plant (well-drained soil, full sun, minimal competing plants).

But let's say that a given seed, despite the overwhelming odds, is deposited by a bird on a perfect traprock ledge in Connecticut, replete with sandy soil and full sunlight.  Even still, there is only about a 25% chance that the seed will prove viable.  A number of factors can influence the relatively low likelihood of O. humifusa being dispersed to a wholly new habitat area, such as the following:

<ol>
<li>
50% of the seeds produced by ''O. humifusa'' aren't eaten in the first place
</li>
<li>
95% of the seeds consumed by birds are dispersed in unsuitable habitat
</li>
<li>
75% of the seeds that are deposited on ideal habitat will not actually germinate
</li>
</ol>

Considering all of these factors, there would only be a 0.75% chance that any given seed will be eaten by a bird, deposited on good habitat and actually germinate.  And, in truth, everyone of the above estimated percentages is probably far too optimistic in favor of ''O. humifusa''.  If we were more skeptical in all three of the cases above, then the adjusted chance of successful dispersal and germination for any given seed would probably be significantly lower.

Furthermore, we must consider that even those seeds which have successfully germinated are then subject any number of disturbances, for the resilience attributed to mature ''O. humifusa'' plants does not necessarily extend to small, immature specimens.  A newly-sprouted cactus will spend much of its first growing season barely larger than a thimble; it might incidentally be trampled by a hiker, crushed beneath a fallen tree branch, consumed by insects, or freeze to death during an unusually extreme cold snap during its first winter.  When we factor this into the odds, the likelihood that a given ''O. humifusa'' plant will produce a seed which will be consumed by a bird, dispersed to a new habitat area, germinate and ultimately mature into a large, healthy, resilient plant are probably disparagingly low... somewhere in the range of 1:10,000 and 1:100,000).  Thus, it is understandable that it could take decades, perhaps even centuries, for ''O. humifusa'' to become established in new territory.  It is partially for this very reason that the cactus receives recognition as a Connecticut Species of Special Concern, for the destruction of a given colony in a certain town may very well mean that ''O. humifusa'' will be gone from that place for an extremely long time before it can be re-colonized.  In fact, the undeveloped landscapes of many towns have become so thickly reforested in the last 150 years that the remaining available habitat for ''O. humifusa'' is incredibly small; in these cases, if the existing cactus population was lost, the likelihood of re-colonization from a coastal colony is so low that, for all intents and purposes, it would be appropriate to declare them permanently extirpated from those towns.

In many cases, areas devoid of cacti that would seem to present excellent habitat have likely experienced considerable changes to the face of the landscape over the last 150 years, meaning that suitable habitat may not have existed even in very recent history.  Powerline cuts, for example, oftentimes offer what would seem to be remarkably good habitat the cactus, but despite all of my research and a few field explorations, I have found no evidence that ''O. humifusa'' has been capable of colonizing these areas yet.  It may simply be that these features of the landscape were too recently constructed and that the long-range dispersal characteristics of ''O. humifusa'' prevent it from quickly taking advantage of these places.