Editing Eastern Prickly Pear (section)

== Theories & Unanswered Questions ==

Perhaps the most surprising discovery throughout the course of my explorations for Connecticut's ''Opuntia humifusa'' (aside from the cacti themselves) has been the realization that remarkably little is known about this cactus.  Thus, one of the goals of my research has been to try and better understand the ecology of these cacti in Connecticut through observing them directly.

Below are a series of questions and tentative answers that I've assembled.  In most cases, I can offer only observations, suggestions and theories.

=== How did it get there? ===

'''What accounts for the ability of a colony of ''O. humifusa'' to exist as far as 20 miles from the nearest neighboring colony?  How did the cactus make such a huge leap without any intermediate colonies?'''  

Granted, this questions seems rather simple.  But, the fact of the matter is that surprisingly minimal research has been invested in answering this question.  Furthermore, the answer is exceptionally important in understanding the fragmented and sometimes boggling distribution of ''O. humifusa'' in Connecticut.

Before I begin, I'll briefly clarify that there are only two basic ways that ''O. humifusa'' can produce independent off-spring.  The first is vegetative, meaning that whenever a cladode becomes detached from the parent plant and hits the ground, it stands a fair chance of sinking its own roots and becoming an independent plant.  The second method is by seed.  The fruits, or "pears", of ''O. humifusa'' contain seeds and wherever the fruits go, the seeds accompany.

First off, we know that the wind is not strong enough to carry a detached cladode or fruit more than perhaps a foot or two from the parent plant.  Under extremely windy conditions, with the right topography, perhaps the wind could move plant pieces even further; I'll say a maximum of 10 to 15 feet, though that would truly exceptional.  This mechanical force may be sufficient to push a detached cladode or fruit over a nearby cliff, at which point it might land on a rocky terrace and begin a new colony.  But certainly, the wind cannot carry a cladode for any significant distance across a landscape.  So, we can say with certainty that ''O. humifusa'' does not disperse long distances of its own ingenuity, in the sense that a dandelion can potentially fly its kite-like seeds for several miles.

Second, I have frequently observed droppings amongst ''O. humifusa'' colonies which appear to be from small mammals, most likely rabbits.  And, indeed, studies in Kansas in the 1940s revealed that rabbits were known to feed upon both the fruits and cladodes of ''O. humifusa''<ref name="kansas">Riegel, Andrew. "Some Coactions of Rabbits and Rodents with Cactus." ''Transactions of the Kansas Academy of Science''. 44. (1941): 96-103. Print. <http://www.jstor.org/stable/3624871>.</ref>.  However, the study observed that rabbits tended to limit their consumption of the cactus to those times when food was particularly scarce, when perhaps the hardy, evergreen ''O. humifusa'' might be the only green plant matter available.  Furthermore, I cannot say that I have noticed any extensive evidence that Connecticut ''O. humifusa'' are subject to consumption by small mammals.  In most cases, the cladodes of ''O. humifusa'' that I've observed are in remarkably pristine condition.  Even if we were to entertain that at some point in the past there was a serious enough food shortage in Connecticut for cottontail rabbits to begin gnawing on ''O. humifusa'' plants, the potential for dispersal by this method is not exceptionally great.  Rabbits aren't likely to detach cladodes and haul them any significant distance, and even if they did, it is further unlikely that they would lose interest in it before consuming enough to kill the vegetative potential of the broken stem.  But then there is the possibility that the seeds of the ''O. humifusa'' pears, after being consumed, might pass through the digestive system of rabbits and be deposited elsewhere.  This is a plausible means of dispersal to which I am open, but it still cannot explain how ''O. humifusa'' might travel a distance of several miles over rough terrain.  The home range of any rabbit is typically restricted to a relatively small area, and no species living in Connecticut are known to migrate long distances.

The most promising theory is that ''O. humifusa'' is distributed long distances chiefly by birds.  Birds have long been observed to consume the pears of ''O. humifusa'' and other species of ''Opuntia'' found elsewhere in the world.  In fact, studies have shown that seeds which first pass through the digestive system of an animal will germinate more reliably and more quickly than those that simply fall from the cactus in an unscathed pear<ref name="us-forest-service"></ref>.  Since birds can potentially travel enormous distances over terrain that would be challenging for terrestrial animals, as well as deposit their droppings from the air in relatively random locations, it becomes clear that birds must have played a decisive role in distributing ''O. humifusa'' across Connecticut.

When we add birds to the equation, there ceases to be any mystery behind how ''O. humifusa'' might have appeared on a high ledge along a ridge or in an isolated glade deep within the forest.  While all of these locations seem like relatively inaccessible locations to man and other mammals, they are simply another place for traveling birds to land or "lighten the load", so to speak, while flying high above.

=== 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.

=== How can colonies at the coastline become so well-dispersed? ===

If we consider the two questions and answers above, it is clear that ''O. humifusa'' colonizes new habitat areas very, very slowly.  But if we should believe the above assertions to be accurate, then how can a stronghold such as the Long Beach Complex possibly exist?  To summarize, the Long Beach Complex includes literally hundreds of cacti scattered over upon a 1.5-mile stretch of the Long Beach Peninsula.  If ''O. humifusa'' has so aggressively colonized Long Beach, why can't it do the same elsewhere?  The answer to this question rests solely upon a series of beneficial conditions which converge on the Long Beach Peninsula to truly favor the expansion of ''O. humifusa''.

For starters, we have already established that birds are the primary vector for ''O. humifusa'' seed dispersal.  In the case of inland colonies, the species of birds that feed upon pears, as well as the frequency with which the plants are visited, is probably extremely variable.  Coastal areas, however, have served as nesting sites of various seabirds since prehistory.  These coastal nesting sites, which include Long Beach Peninsula, offer unusually high numbers of nesting birds which are tethered to the immediate landscape for several months at a time.  This means that, wherever nesting sites host a population of ''O. humifusa'', the cacti are literally at the dead-center of a dispersal hotspot where pears are routinely foraged by resident seabirds.

Furthermore, unlike inland sites that are typically surrounded by thick woodlands, coastal sites such as Long Beach offer an extremely high ratio of suitable habitat per acre.  Long Beach Peninsula is ripe with wide swaths of open dunes and scrubland, all of which are steeped in direct sunlight and enjoy wintertime low temperatures that are slightly warmer than those experienced at inland locations.  In short, the majority of the landscape on coastal sites tends to be accommodating of the needs of ''O. humifusa'', while a comparatively minute percentage of the inland landscape can provide for the complex needs of the cactus.

Let's consider these more optimal conditions in the same format that we considered them above:

<ol>
<li>
25% of the seeds produced by ''O. humifusa'' aren't eaten in the first place
</li>
<li>
40% 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>

As we can see here, the higher population and longer residence of birds, when coupled with the abundance of suitable cactus habitat, means that the rate of successful short-distance dispersal and germination increases to roughly 11% on coastal sites like Long Beach Peninsula.  While not staggeringly high, this represents a drastic increase over dispersal rates experienced inland.  Thus, the abundance of ''O. humifusa'' on Long Beach Peninsula can be explained as an example of a series of ecological conditions which conspire to help ''O. humifusa'' thrive to a degree that is otherwise unthinkable in most other environments.

=== 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.