Thorns as induced defenses: experimental evidence.

Area of Science:

• Plant biology
• Ecology
• Zoology

Background:

• Herbivory, the consumption of plants by animals, is a major selective pressure shaping plant defenses.
• Thorns and spines are common physical defenses in plants, but their effectiveness against different herbivores is not fully understood.

Purpose of the Study:

• To investigate the role of long, straight thorns in deterring herbivory by mammalian browsers.
• To determine if thorn length and density are induced responses to herbivore pressure.

Main Methods:

• Controlled experiments using cut branches of three woody species with and without thorns, exposed to tethered goats.
• Field observations on Acacia seyal plants, comparing herbivory on thornless and intact branches by free-ranging giraffes.
• Measurement of branch length changes and thorn characteristics (length, density) in relation to herbivore access.

Main Results:

• Branches with thorns removed experienced significantly higher herbivory by goats compared to intact branches.
• Acacia seyal branches accessible to giraffes showed greater herbivory when thorns were removed.
• Plants in high-herbivory areas developed longer and denser thorns, indicating an induced defense.

Conclusions:

• Long, straight thorns are an effective deterrent against mammalian browsers.
• Thorn length and density represent an inducible defense strategy in plants, increasing in response to herbivore pressure.