Left out in the cold: temperature‐dependence of defense in an African ant–plant mutualism

Abstract

Many tropical plants are defended by ants, and the costs and benefits of these mutualisms can vary across gradients of herbivory, soil fertility, latitude, and other environmental factors. Yet despite an abundant literature documenting thermal constraints on ant activity and behavior, we know little about whether temperature variation can influence the benefits conferred by ants to plants. We evaluated the effects of dawn‐to‐dusk fluctuations in temperature on patrolling and aggressive behavior in four arboreal ant mutualists of Acacia drepanolobium trees in central Kenya. We found that ant aggressive behavior significantly increased with branch surface temperature, primarily in the two most aggressive ant species: Crematogaster mimosae and C. nigriceps workers attacked a simulated herbivore at higher rates as surface temperature rose. In a browsing experiment, we found that goats browsed more frequently and for longer durations on C. mimosae‐defended trees during cooler times of day, while goat browsing on plants from which ants had been removed was not affected by temperature. Our study demonstrates temperature‐dependence in the efficacy of ant defense against herbivory and suggests that these ant‐plants may be more vulnerable to herbivory during cooler hours of the day, when many native browsers are most active.

The response of ants to climate change

“…Climate change research in this field is rare, particularly in relation to temperature effects, although generally it is thought that stress conditions may mean plants invest more in ant defences (Coley et al, 1985) and climate change may enhance the sensitivity of plants with EFNs to anthropogenic disturbance (Arnan et al, 2022). With elevated temperatures in East Africa, the activity of ants defending Acacia plants increased, increasing the degree of protection the plant received (Tamashiro et al, 2019); therefore, greater protection from herbivory may result under climate change. Elsewhere this relationship might not be as evident because higher temperatures may shrink the window available for foraging, ultimately decreasing ant activity (e.g., in tropical canopies), reducing ant access to EFNs with consequences for the plant via increased herbivory.…”

Phenotypic plasticity in size of ant-domatia

“…Tetraponera associates of Barteria may not be able to withstand temperatures reached under the high insolation typical of open tropical savannas, as suggested by the observation that when the tropical rain forest specialist tree Barteria fistulosa occurs in open, sunny clearings it is occupied by Crematogaster instead of Tetraponera ants (Doyle McKey, personal communication), although size of domatia is suitable for Tetraponera ants. The thermal optimal range of symbiotic ants was shown to constrain protection efficiency of the ant-plant Acacia (Vachellia) drepanolobium 28 , although in this case, protection efficiency decreased with temperature.…”

Climate and ant richness explain the global distribution of ant-plant mutualisms

“…Temperature and precipitation may directly influence the ability of plants to produce sugary nectar, which attracts ants to establish mutualistic interactions (34). Likewise, ants are thermally sensitive and activity of ants against herbivores can be temperature-dependent (24, 35), leading to indirect effects via ants on the diversity of plants with domatia and EFNs. These may partly explain the low presence of ant-defended plants in temperate regions.…”

“…Productive habitats and regions may promote ant defenses, where the energetic costs of producing domatia and secreting sugary nectar are relatively inexpensive while the protection of fresh young leaves is essential (“resource limitation hypothesis”) (17, 23). The efficiency of ant defenses also depends on daily fluctuations in temperature, as ants behave more aggressively in warm periods (24). In contrast to ant-defended plants, plants with elaiosomes attached to their seeds typically occur in dry and low-nutrient soils (e.g., Mediterranean climate in Australia and South Africa).…”