Typically, at least one student per group eventually came up with
“mammoth,” which (corrected to “mastodon”) was established in the
discussion as a hypothesis that could not be rejected on the basis of
students’ observations and our current background knowledge in
the group. If no student suggested that loss of this defense structure
may only be a matter of time, I pointed to the fact that the megafaunal
extinctions (and thus the loss of the “evolutionary pressure” for being
thorned) in North America date back only a comparatively short time,
equaling relatively few tree generations. I also informed them that
thornless specimens indeed appear at times in the wild, from which
the unarmed “inermis” form was selected – a thornless mutation that
would surely struggle to form stem old enough to form fruit in the
presence of browsing and debarking megaherbivores.
In evolutionary terms, the tracks left in the coevolved plant’s
morphology are still fresh and have not yet been blurred by a more
recent evolutionary history. Similarly, in other regions of the world
morphologies of plants have been interpreted as defense mechanisms against browsing by past faunas (e.g., New Zealand’s moas
or Madagascar’s elephant birds; Bond & Silander, 2007; Dempewolf & Rieseberg, 2007).
The fruit was discussed as the second central element. Asked
what it reminded them of, students typically referred to the fruits
of other Fabaceae (beans, mange tout, pea pods, etc.), and there were
a few “joke” answers such as “worms.” Pointing out the pods’ large
size and how they pile up at the foot of the stem, I asked how the
seed might be dispersed. Pleistocene megafauna having already
appeared in our discussion, the suggestion that this fruit might be
eaten and the seed dispersed by an extinct large animal came forward
readily. The conjecture that this trait was not lost yet because of the
relatively recent extinction of the Pleistocene megafauna was easily
established. Several other leguminous species of the New World
are believed to have been dispersed by now-extinct large herbivores
(Janzen & Martin, 1982). Also, the question of why the loss of the
dispersal agent did not lead to extinction of the plant was answered
easily with reference to the short time that has passed since.
A final question put the focus on what looks at first like a contradiction: the tree, on the one hand, attracts large herbivores; on
the other hand, it defends itself against them. Suggestions
included, among others, that the tree may show adaptations to
different animals, that the fruit may be available only once fallen
off the tree, and that the animal disperser may be forced to carefully angle with its trunk for the fruit dangling at the branches,
avoiding the thorns and thus causing no damage to either the
bark or its own skin.
In Gleditsia, the thorny adaptation may be understood as a
mechanism that channels the movement of the mastodon attracted
by the fruit, keeping the animal at bay while getting it to disperse
the seed and not allowing it to destroy vital structures of the plant’s
body. The tree thus lures, restrains, and ushers the energetically
browsing, branch-breaking and debarking animal on its way, directing the mastodon’s energy into channels favorable to the preservation
of its own species.
“Strange Trees All Over . . . ”
Gleditsia triacanthos combines in its morphology – namely that of
flowers, fruit, and armature – several unusual features that may help
attract student interest and improve retention. Indeed, students
showed a sound interest in the tree, and natural history and cultural
dendrology facts and “stories” were well remembered in the group:
one student referred to the species when encountering another
specimen elsewhere (on an excursion weeks later) as “that tree of
the mammoth,” while another dubbed it “the voodoo tree.” One stu-
dent later reported he had now seen the tree in another part of town,
upon which his classmate added that he was now often reencounter-
ing this and other “strange trees” covered in this class now “all the
time, all over.”
The student interest I observed is noteworthy and can be consid-
ered a learning success in view of the waning of natural history from
curricula (Cheesman & Key, 2007; Leather & Quicke, 2010) and the
dwindling of plant knowledge in the Western world (Hershey, 1992;
Wandersee & Schussler, 2001; Bebbington, 2005). Working with
“storied” plants that provide unusual models for ecological and bio-
logical principles not only may offer a rich resource of illustrative
case studies but also may help to bring natural history back into
biological and environmental education.
This exercise is not predominantly about the acquisition of factual
knowledge, but is more about exercising basic scientific reasoning in
an ecology-and-evolution context based on easily observable mor-
phologies. My students exhibited innovative thinking and engage-
ment with the living object in the various explanations they brought
forward. Once the potential role of a large herbivorous animal was
touched upon, students were able to develop the ecological context.
The discussion of the ambiguous role of the animal, as “
destructive” browser or “helpful” disperser, typically brought the groups to
the point of considering that there was no simple answer and
acknowledging the complexity of the interaction. In the grander picture of evolutionary biology, work with this living object was thus a
first step toward challenging a habit of thought based on the “
gladiatorial” view of Huxleyan Darwinism. In Gleditsia, the complexity
and multidimensionality of the plant–animal relation becomes visible
in striking structures that allow the interpretation that plants do not
simply defend themselves against a “damaging” herbivore, but redirect and channel the energy of the browsing animal from being
destructive of its tissue to being a useful disperser of its seed. Discussing such cases as the “liaison dangereuse” of a thorned plant with a
toothed herbivore can be expected to help students move away from
all-too-simplified “friend” and “foe” imagery in ecology and evolution
and toward complex systems thinking.
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