Urbanization drives partner switching and loss of mutualism in an ant-plant symbiosis.

Mutualistic interactions between species underpin biodiversity and ecosystem function, but may be lost when partners respond differently to abiotic conditions. Except for a few prominent examples, effects of global anthropogenic change on mutualisms are poorly understood. Here we assess the effects of urbanization on a symbiosis in which the plant Cordia nodosa house ants in hollow structures (domatia) in exchange for defense against herbivores. We expected to find that mutualist ants would be replaced in the city by heat-tolerant opportunists, leaving urban plants vulnerable to herbivory. In five protected forest sites and five urban forest fragments in southeast Perú, we recorded the identity and heat tolerance (CTmax) of ant residents of C. nodosa. We also assayed their plant-defensive behaviors and their effects on herbivory. We characterized the urban heat-island effect in ambient temperatures and within domatia. Forest plants housed a consistent ant community dominated by three specialized plant ants, whereas urban plants housed a suite of 10 opportunistic taxa that were, collectively, about 13 times less likely than forest ants to respond defensively to plant disturbance. In the forest, ant exclusion had the expected effect of increasing herbivory, but in urban sites, exclusion reduced herbivory. Despite poor ant defense in urban sites, we detected no difference in total standing herbivory, perhaps because herbivores themselves also declined in the city. Urban sites were warmer than forest sites (daily maxima in urban domatia averaged 1.6°C hotter), and the urban ant community as a whole was slightly more heat tolerant. These results illustrate a case of mutualism loss associated with anthropogenic disturbance. If urbanization is representative of increasing anthropogenic stressors more broadly, we might expect to see destabilization of myrmecophytic mutualisms in forest ecosystems in the future.

Las relaciones de mutualismo entre especies sustentan la biodiversidad y la función de los ecosistemas, pero pueden ser perturbadas si las especies que interactúan responden de manera diferente a los cambios en los factores abióticos. Sin embargo, los efectos del cambio antropogénico global sobre los mutualismos no son bien conocidos. Evaluamos los efectos de la urbanización en la simbiosis entre la planta tropical Cordia nodosa que alberga colonias de hormigas, en estructuras específicas en las plantas (domacios), a cambio de defensa contra herbívoros. Esperábamos encontrar que las hormigas oportunistas, no defensoras, y tolerantes al calor urbano reemplazarían a las mutualistas. En cinco sitios de bosque protegido y cinco fragmentos de bosque urbano en el sureste de Perú, registramos la identidad y la tolerancia al calor (CTmax) de las hormigas residentes en C. nodosa. Analizamos sus comportamientos defensivos y sus efectos sobre la herbivoría. Medimos el efecto de isla de calor urbano en la temperatura del ambiente y dentro de los domacios. Las plantas del bosque albergaban una comunidad estable de hormigas, dominada por tres taxones de hormigas especializadas, mientras que las plantas urbanas albergaban diez taxones, mayormente con especies oportunistas. En general, fue 13 veces más probable que las hormigas del bosque respondan a la perturbación de las plantas, y lo hacían más rápidamente que las hormigas urbanas. La exclusión de las hormigas del bosque tuvo el efecto anticipado de aumentar la herbivoría, contrariamente en los sitios urbanos, la exclusión redujo la herbivoría. Aún con hormigas menos defensivas, no detectamos diferencias en la herbivoría total en sitios urbanos, tal vez porque los herbívoros también fueron impactados en la ciudad. Los sitios urbanos eran más cálidos que los sitios forestales, y las hormigas urbanas eran ligeramente más tolerantes al calor. Si la urbanización es representativa de los factores estresantes creados por el cambio global antropogénico, anticiparíamos ver una desestabilización de los mutualismos mirmecofíticos en los ecosistemas forestales en el futuro.

Correlations between physical and chemical defences in plants: tradeoffs, syndromes, or just many different ways to skin a herbivorous cat?

Most plant species have a range of traits that deter herbivores. However, understanding of how different defences are related to one another is surprisingly weak. Many authors argue that defence traits trade off against one another, while others argue that they form coordinated defence syndromes. We collected a dataset of unprecedented taxonomic and geographic scope (261 species spanning 80 families, from 75 sites across the globe) to investigate relationships among four chemical and six physical defences. Five of the 45 pairwise correlations between defence traits were significant and three of these were tradeoffs. The relationship between species' overall chemical and physical defence levels was marginally nonsignificant (P = 0.08), and remained nonsignificant after accounting for phylogeny, growth form and abundance. Neither categorical principal component analysis (PCA) nor hierarchical cluster analysis supported the idea that species displayed defence syndromes. Our results do not support arguments for tradeoffs or for coordinated defence syndromes. Rather, plants display a range of combinations of defence traits. We suggest this lack of consistent defence syndromes may be adaptive, resulting from selective pressure to deploy a different combination of defences to coexisting species.

Putting plant resistance traits on the map: a test of the idea that plants are better defended at lower latitudes.

• It has long been believed that plant species from the tropics have higher levels of traits associated with resistance to herbivores than do species from higher latitudes. A meta-analysis recently showed that the published literature does not support this theory. However, the idea has never been tested using data gathered with consistent methods from a wide range of latitudes. • We quantified the relationship between latitude and a broad range of chemical and physical traits across 301 species from 75 sites world-wide. • Six putative resistance traits, including tannins, the concentration of lipids (an indicator of oils, waxes and resins), and leaf toughness were greater in high-latitude species. Six traits, including cyanide production and the presence of spines, were unrelated to latitude. Only ash content (an indicator of inorganic substances such as calcium oxalates and phytoliths) and the properties of species with delayed greening were higher in the tropics. • Our results do not support the hypothesis that tropical plants have higher levels of resistance traits than do plants from higher latitudes. If anything, plants have higher resistance toward the poles. The greater resistance traits of high-latitude species might be explained by the greater cost of losing a given amount of leaf tissue in low-productivity environments.