Editing Eastern Prickly Pear (section)

=== Why are some inland colonies so isolated despite abundant habitat very nearby? ===

This may seem to be a repetition of a question that has already been discussed.  However, while our previous question focused on reasons why some large area of habitat may not have ''any'' cacti, this question focuses on why seemingly suitable habitat areas that ''are in very close proximity to'' a known colony would still be oddly devoid cacti.  For example, if an established colony of ''O. humifusa'' lives upon a south-facing ledge on a traprock ridge, why would the seemingly identical ledge only 60 feet away be devoid of the cactus?

The simplest explanation, which I suspect contributes only partially to this phenomenon, relates to all of the aforementioned difficulties in dispersal which I've already discussed.  Even if a suitable ledge is only 40 feet away, the random and rather unreliable nature of dispersal by birds mean that the ledge might as well be a mile away... or several miles away... and still possess a nearly equal likelihood of being colonized.  However, I would submit that in many of these cases, the supposedly "identical" nearby ledges aren't actually as identical as they might seem to an ordinary observer.

While it is true that the majority of inland cacti colonies are found upon traprock ledges, we must temper this finding with the observation that an overwhelming majority of traprock ledges in Connecticut do not, and never have, supported colonies of ''O. humifusa''.  I interpret this as an indication that traprock ridges generally '''do not offer suitable habitat for the cactus'''.

As we know, ''O. humifusa'' has a series of requirements that must be met in order thrive in a given habitat.  They are as follows:

<ol>
<li>'''Temperatures should not drop below 0&deg;F.'''  If they do, it should be an exceptionally rare occurrence which takes place only once every few years.  Otherwise, young cacti would probably die before becoming firmly-established enough to weather these rare cold spells.</li>
<li>'''At least six hours of unobstructed sunlight must be available every day.'''  Established cacti can tolerate as little as perhaps four hours, but young plants would suffer considerable stress and growth deformities with such reduced light exposure.</li>
<li>'''Neighboring trees and competing plants must not be capable of obstructing sunlight.'''  See #2 above.</li>
<li>'''Soil must be very well-drained.'''  Any places where water dwells for even 24 hours after a heavy rainfall are entirely unsuitable for ''O. humifusa''.  Such conditions lead to rotting of the roots, a phenomenon that will quickly kill even large, mature cacti.</li>
<li>'''Soil must be voluminous enough for cacti to firmly take root.''' Although I've observed ''O. humifusa'' growing on a 1/4"-thick mat of dried lichen upon an otherwise bare rock face, such plants are unlikely to ever reach an advanced state of development.  Bare rock is NOT a suitable habitat for ''O. humifusa''.  All colonies found on rocky outcrops inhabit an area of the outcrop where at least some soil has accumulated.</li>
<li>'''Soil must be stable.'''  In habitats where the dirt or sand can easily be moved about by wind and water, the cacti will be severely stressed, either by being periodically buried or by having their roots periodically exposed.</li>
</ol>

Given these strict criteria, it's almost amazing that '''any''' traprock ridges are capable of supporting ''O. humifusa''.  And, indeed, most of Connecticut's inland ridges and rocky outcrops are decidedly unsuitable.  These are habitat areas in Connecticut where ''O. humifusa'' is truly braving the fringes of its range and battling conditions which routinely conspire to destroy its presence.  A scant few ledges scattered throughout the state possess the delicate balance of factors that produce a micro-climate that is conducive to the cactus.  Even if a certain ledge meets 5 of the 6 criteria listed above, it still will probably prove unsuitable for ''O. humifusa''.  In such harsh, borderline environments, every aspect of the habitat area must fall within an extremely narrow range of acceptable tolerances.

So, while nearby ledges may seem to be practically identical to the plain sight, there could be any number of minute variations in temperature, drainage, available sunlight and available soil which might make them unsuitable in comparison... even if they are only 20 to 100 feet away from a firmly-established colony.

When we examine the local isolation of inland colonies in light of the slow dispersal rate and complex habitat criteria of ''O. humifusa'', it is clear that "jumping" to a nearby ledge is no easy task.  In many cases, it may well be impossible since the plant cannot live on the nearby ledge because the temperature drops slightly lower, because the soil is too unstable, the sunlight is limited by neighboring trees, etc, etc.