The rapid deforestation of tropical forests around the world is one of the main anthropogenic drivers of species extinction. Understanding the factors causing this wave of biodiversity loss is of paramount importance. But it is just as crucial to determine the effects of anthropogenic disturbances on species interactions such as pollination and seed dispersal. Species interactions are as fundamental as species richness (number of species in an area) in terms of biodiversity dynamics, and therefore the responses of ecosystems to anthropogenic impacts. This is succinctly expressed by Daniel Janzen, a pioneering evolutionary ecologist: “What escapes the eye, however, is a much more insidious kind of extinction: the extinction of ecological interactions”.
To evaluate the effects of deforestation on plant-animal interactions (at a landscape scale), a research team from the Applied Ecology and Conservation Lab of the State University of Santa Cruz, Brazil has been studying several forest remnants in the threatened Atlantic Forest of Brazil.
Our team surveyed fruiting trees for a full year and counted all of the fruits produced in 100 plots established in 20 forest remnants, totalling to a sampled-area of 100 acres. We also recorded traits like fruit and seed size, colour, and nutritional contents. Together, this data composing ‘functional traits’ allowed us to infer how deforestation affected fruit attributes that are directly related to frugivore attraction. We also estimated the total biomass of the fruits (kg/ha) in all of the forest remnants.
We recorded an impressive diversity of zoochoric (animal-dispersed) trees – 415 species in total in all of the forest remnants, representing about 80% of total tree assemblage. Large fruits (>1.2 cm) were produced by 32% of these zoochoric species. We found that deforestation affected the diversity of fruit traits, resulting in the loss of specific characteristics that are important for attracting frugivores. For instance, in deforested landscapes, fruits from pioneer tree species such as Melastomataceae, a plant family found mostly in the tropics, were nutrient-poor. These fruits contained comparatively less lipid and protein contents. We also found that deforestation negatively affected fruit biomass, mainly due to the greater loss of species that are tolerant of low light levels and consequently are able to grow under shade conditions.
Our team also surveyed frugivorous birds, and as expected, the diversity of these birds has decreased in deforested landscapes. Habitat simplification, characterised by a decrease in the density of large trees and an increase in the canopy openness, and fruit scarcity were negatively affecting forest-dependent frugivorous birds. Interestingly, a higher diversity of non-dependent frugivorous birds was found in landscapes with higher amounts of forest edges, which constitute an abrupt transition between the forest and the adjacent habitat. Probably because these species are capable of using resources from those adjacent habitats and consequently thrive due to edge creation.
Most forest-dependent frugivores are obligate frugivores, which means more than 60% of their diet is dependent on fruits and have larger body sizes than habitat-generalist frugivores. Obligate frugivores have higher seed dispersal effectiveness, and large-body frugivores are important because they can consume and disperse large seeds. Thus, seed dispersal services offered by forest-dependent species cannot be ecologically compensated for by the proliferation of generalist species. Preventing deforestation is imperative to guaranteeing the productivity and quality of food resources for frugivores and, consequently, the ecological functionality of these threatened tropical forests.
Morante-Filho, J. C.; Arroyo-Rodríguez, V; Pessoa, M.S.; Cazetta, E; Faria, D. 2018. Direct and cascading effects of landscape structure on tropical forest and non-forest frugivorous birds. Ecological Applications 28: 2024-2032.
Pessoa, M. S.; Hambuckers, A.; Benchimol, M.; Santos, L. R.; Bomfim, J.; Faria D.; Cazetta, E. 2017. Deforestation drives functional diversity and fruit quality changes in a tropical tree assemblage. Perspectives in Plant Ecology Evolution and Systematic 28:78-86.
Pessoa, M.S; Santos, L. R.; Talora D. C.; Faria, D.; Mariano E.; Hambuckers, A.; Cazetta, E. 2017. Fruit biomass availability along a forest cover gradient. Biotropica 49: 45-55.