Differential effects of land use on ant and herbivore insect communities associated with Caryocar brasiliense (Caryocaraceae)

Simplification of natural habitats leads to a modification of the community associated with a host plant. Pequi trees (Caryocar brasiliense) are common to find in central Brazil, especially in the middle of monocultures, such as soy, corn, pasturelands or Eucalyptus plantations. On this scenario we hypothesized that habitat modification differentially affects the diversity of ants and herbivore insects associated with this species. The aim of the work was to test if C. brasiliense trees located in human modified habitats, support a lower species richness and abundance of ants, and a greater species richness and abundance of insect herbivores, compared to preserved cerrado habitats. The study was conducted in a Cerrado area located in Northern Minas Gerais State, Brazil. Ants and herbivore insects were collected monthly during 2005 using beating technique. The results showed that ant species richness was higher in pequi trees located in preserved Cerrado, followed by trees in pastureland and Eucalyptus plantation, respectively. The ant abundance was lower in the Eucalyptus plantation but no difference in ant abundance was observed between trees in pastureland and the preserved Cerrado. Moreover, herbivore insects exhibited lower number of species and individuals in trees located in the preserved Cerrado than in the pastureland and Eucalyptus plantation. We concluded that habitats simplified by human activities may result in diversity loss and may change species interactions.

La simplificación de los hábitats naturales conduce a una modificación de la comunidad asociada con una planta huésped. En la región central de Brasil es muy común encontrar árboles aislados de “pequi” (Caryocar brasiliense) en medio de monocultivos de soya, maíz, pastos o plantaciones de eucalipto. Bajo este escenario, nosotros predecimos que la modificación del hábitat afecta diferencialmente la diversidad de hormigas y herbívoros asociados a árboles de “pequi”. El objetivo de este artículo fue determinar que árboles de “pequi” localizados en hábitats modificados por actividades humanas (i.e. pastizales y plantaciones de Eucalyptus), soportan una menor abundancia y riqueza de especies de hormigas. Por el contrario, se espera una mayor abundancia y riqueza de especies de herbívoros en hábitats modificados. Este estudio fue realizado en un área de Cerrado (sabana brasileña) localizado en el Norte del estado de Minas Gerais, Brasil. Las hormigas y los herbívoros fueron muestreados usando técnicas de golpe. La riqueza de especies de hormigas fue mayor en árboles de “pequi” localizados en áreas preservadas de Cerrado, seguida por árboles de pastizales y finalmente por árboles de plantaciones de Eucalyptus. La abundancia de hormigas fue menor en plantaciones de Eucalyptus, pero no hubo diferencias en la abundancia de hormigas entre árboles de pastizales y áreas preservadas de cerrado. Por otra parte, el número de especies de insectos herbívoros fue menor en árboles de áreas preservadas de Cerrado que en pastizales y plantaciones de Eucalyptus. Concluimos que las actividades humanas reducen la complejidad del hábitat resultando en la pérdida de la diversidad de insectos y procesos ecológicos.

Ants on plants: a meta-analysis of the role of ants as plant biotic defenses.

We reviewed the evidence on the role of ants as plant biotic defenses, by conducting meta-analyses for the effects of experimental removal of ants on plant herbivory and fitness with data pooled from 81 studies. Effects reviewed were plant herbivory, herbivore abundance, hemipteran abundance, predator abundance, plant biomass and reproduction in studies where ants were experimentally removed (n = 273 independent comparisons). Ant removal exhibited strong effects on herbivory rates, as plants without ants suffered almost twice as much damage and exhibited 50% more herbivores than plants with ants. Ants also influenced several parameters of plant fitness, as plants without ants suffered a reduction in biomass (-23.7%), leaf production (-51.8%), and reproduction (-24.3%). Effects were much stronger in tropical regions compared to temperate ones. Tropical plants suffered almost threefold higher herbivore damage than plants from temperate regions and exhibited three times more herbivores. Ant removal in tropical plants resulted in a decrease in plant fitness of about 59%, whereas in temperate plants this reduction was not statistically significant. Ant removal effects were also more important in obligate ant-plants (=myrmecophytes) compared to plants exhibiting facultative relationships with hemiptera or those plants with extrafloral nectaries and food bodies. When only tropical plants were considered and the strength of the association between ants and plants taken into account, plants with obligate association with ants exhibited almost four times higher herbivory compared to plants with facultative associations with ants, but similar reductions in plant reproduction. The removal of a single ant species increased plant herbivory by almost three times compared to the removal of several ant species. Altogether, these results suggest that ants do act as plant biotic defenses, but the effects of their presence are more pronounced in tropical systems, especially in myrmecophytic plants.