Mechanical plant adaptations to prevent damage
Infection and response • Plant disease (biology only)
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Definition and role of mechanical adaptations
Mechanical adaptations are structural traits that directly prevent damage or reduce the likelihood of attack. Cause → effect: a physical barrier or deterrent reduces contact or feeding by herbivores, which in turn reduces tissue loss and the chance of pathogen entry. The main limiting factor is that mechanical defences impose resource costs (materials and growth) and can reduce access to mutualists such as pollinators or seed dispersers.
Thorns, spines and prickles
Thorns, spines and prickles are hardened, sharp structures on stems, leaves or modified branches that cause discomfort or injury on contact. Cause → effect: contact with sharp structures deters large herbivores and prevents browsing, which preserves leaf area and photosynthetic capacity. Limiting factors include vulnerability where animals learn to feed around the defence or specialist herbivores that tolerate or avoid the thorns. Many plants with such structures gain a net fitness advantage in habitats with high grazing pressure .
Hairs (trichomes) on surfaces
Hairs or trichomes form dense surface layers on stems and leaves that trap small insects, slow movement of pests, and reduce water loss by creating a microclimate. Cause → effect: hairs physically block or entangle small herbivores and can reduce the rate of pathogen entry through epidermal breaks. Some hairs are glandular and secrete chemicals that are sticky or toxic, increasing the deterrent effect. Limiting factors include reduced gas exchange if hairs are excessive and lower light interception on very hairy surfaces, affecting photosynthesis.
Leaves that droop, fold or curl (rapid movement)
Some plants fold, droop or curl leaves quickly when touched or shaken. Cause → effect: rapid leaf movement dislodges small insects and makes the plant less attractive to grazing, lowering the probability of sustained attack and limiting reproductive competition from herbivore-damaged tissues. Examples of this tactile response include species that fold on contact; the movement often provides immediate mechanical removal of pests and reduces further feeding opportunities . Limiting factors include energy costs for movement and slower responses against larger herbivores.
Mimicry as a mechanical/structural defence
Mimicry in plants uses appearance to change other organisms’ behaviour without direct force. Cause → effect: spots or markings that resemble insect eggs cause egg-laying insects to avoid those leaves, preventing future herbivore larvae from hatching and feeding on the plant. Other forms of mimicry include resemblance to less-palatable species or to inedible objects, producing reduced attack rates. Limiting factors include specificity: mimicry works only if the target animal recognises and responds to the mimic, and it may be ineffective where different herbivores use alternate cues . Structural mimicry also appears across ecosystems, where some animals mimic dangerous species for protection .
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