04 reading3 ant acacia mutualism breakdown ·

[Reading 3] Citation

Palmer et al. 2008, Science 319:192, Breakdown of an ant-plant mutualism follows the loss of large herbivores from an African savanna

[Reading 3] Research question

Do large mammalian herbivores maintain the ant-Acacia mutualism?

[Reading 3] Study tree

Whistling-thorn Acacia drepanolobium (Laikipia, Kenya)

[Reading 3] Domatia

Hollow swollen thorns that serve as ant housing

[Reading 3] Extrafloral nectaries

Glands near leaf bases secreting carbohydrate-rich nectar for ants

[Reading 3] Experiment name

Kenya Long-term Exclosure Experiment (KLEE)

[Reading 3] KLEE design

3 blocks, 6 replicates per treatment; all wild herbivores >15 kg excluded since 1995 (10 years at sampling); trees 1.8-3.0 m

[Reading 3] Four ant species

Crematogaster mimosae (Cm), C. sjostedti (Cs), C. nigriceps (Cn), Tetraponera penzigi (Tp)

[Reading 3] C. mimosae (Cm)

~52% of trees; aggressive defender; relies on swollen thorns and nectar; the true mutualist

[Reading 3] C. sjostedti (Cs)

~16% of trees; weak defender; nests in beetle-bored stem cavities, not domatia; needs little nectar; more predatory diet; the antagonist

[Reading 3] C. nigriceps (Cn)

~15% of trees; effective defender; prunes axillary buds and kills apical meristems

[Reading 3] Tetraponera penzigi (Tp)

~17% of trees; intermediate protector; destroys host nectaries ("scorched earth")

[Reading 3] Why does C. nigriceps prune its host?

Reduces lateral canopy spread and contact with hostile colonies; pruning mimics browsing so the tree keeps producing rewards

[Reading 3] Why does T. penzigi destroy nectaries?

Reduces the chance of takeover by neighboring nectar-dependent Crematogaster colonies

[Reading 3] Result 1: plant rewards

Without browsers, trees reduced nectaries and swollen thorns (likely a plastic response); Cn trees changed little

[Reading 3] Why were Cn trees little changed by exclusion?

Cn pruning mimics browsing and maintains reward production

[Reading 3] Result 2: ant community shift

Cs occupancy doubled and became most abundant; Cm fell by more than 30%; Cn and Tp unchanged

[Reading 3] Result 3: colony size

Cm colony size dropped 47% in exclosures

[Reading 3] Result 4: Cm behavior

Cm tended scale insects on 19.4% vs 9.7% of branches (about 2x) and aggressive recruitment fell 50%

[Reading 3] Costs and benefits for Cm-occupied trees without herbivores

Increased costs (more scale insect tending) and decreased benefits (reduced defense aggression)

[Reading 3] Was the ant shift due to more arthropod prey?

No; arthropod abundance and biomass did not differ between treatments

[Reading 3] Result 5: tree growth and survival

Over 8 years (>1,750 trees), Cs-occupied trees grew slower and had about twice the mortality

[Reading 3] Mechanism of Cs harm

Cs facilitates wood-boring cerambycid (long-horned) beetles

[Reading 3] Ant removal experiment

Removing Cs reduced beetle attack by 77%; Cm and Cn actively protect trees from the beetles

[Reading 3] Why is Cs less dependent on host rewards?

It nests in beetle cavities rather than domatia and has low nectary attendance and a more arthropod-based diet (lower d15N)

[Reading 3] Why is this an ecological cascade?

Herbivore loss -> fewer plant rewards -> ant shift Cm to Cs -> more beetle attack -> slower growth and higher tree mortality

[Reading 3] Main conclusion

Large herbivores maintain the ant-acacia mutualism; their loss tips it toward antagonism

[Reading 3] Broader implication

Anthropogenic loss of megafauna may have strong, unanticipated consequences for whole communities