Biological Dynamics of Forest Fragments Project - Example Studies

Example Studies

There has been a broad scope of studies performed on the BDFFP site focusing on many elements of fragmentation effects on organisms and habitat conditions. Subjects from a range of biota are studied, including trees and other flora, microorganisms, and a variety vertebrate and invertebrates. Soil chemistry and development, as well as human and environmental factors in fragments have also been researched. Some notable studies performed on the BDFFP site are summarized below.

Edge Effects

Edge effects are a general term for the impacts of different habitats at or near the barrier where they meet, the “edge.” They are an important factor in many habitats, especially when it comes to fragmented habitats, and are studied extensively in the field. One major study performed on this subject was “Edge-related changes in environment and plant responses due to forest fragmentation in central Amazonia“ by Valerie Kapos, Elisa Wandelli, Jose Luis Camargo, and Gislene Ganade. The following was summarized from pages 33 to 44 of Tropical Forest Remnants: Ecology, Management and Conservation of Fragmented Communities by W.F. Laurance and R.O. Bierregaard.

A major change brought about by habitat fragmentation is an increase in the proportion of edge exposed to other habitats, and the importance of this change depends to a degree on the contrast between the fragmented habitat and the matrix in which it occurs. Edge effects play an important role in the regional environment as well, with the Amazon forests helping maintain hydrological cycles through their roles in evapotranspiration and soil protection. Depending on the extent to which the influence of desiccating edge-effect conditions from the clearings (matrix) penetrates into the forest and on how plants respond to it, forest fragments might be expected to evapotranspire more than equivalent areas of continuous forest. Therefore this research focused on assessing the edge related gradients of factors that affect evapotranspiration in forest fragments, and plant responses to them. Changes in edge effects with time were also studied.

Temperature, vapor pressure deficit (VPD), and soil moisture were the factors surveyed. The study compared measurements made at different distances along transects from the western isolated edge toward the center of the reserve with measurements made in control areas more than 500m from the forest edge. Understory plant/water relations were monitored in dry seasons, soil moisture measurements were made over ten months, understory plant distributions were measured, microclimatic and leaf expansion studies were done, as well as vegetation structure studies, all between 1988 and 1990.

In both wet and dry seasons, soil moisture in the edge transects was similar to that in the control areas except at the edge itself and in the region between 40 to 80 m from edge. Soil water potentials below wilting point occurred in the forest during dry season, but the driest points were not necessarily near the edge and no evidence of prolonged drought was found. Thus, any edge drought effects on plants must be due to combined effects of reduced soil moisture and higher atmospheric demand exceeding the supplying power of vascular systems, rather ecosystem-level water shortage.

Plant responses: For leaf expansion in Duguetia, there was no difference in the rate of leaf expansion between plants in edge zones and those in control areas. When looking at whether canopy trees close their stomata to reduce water loss as a response to the changed environment, they found no difference between these two locations. They looked at the same response in understory plants, and found the results might suggest that the understory species did have greater water use efficiencies near the forest edge. This pattern could be explained by either greater mixing of air from outside the forest with the understory air or lower decomposition rates, or both, near the edge. Vegetation structure was also analyzed. They found clear edge-related gradients in environmental factors gave way to more complex patterns, which suggested some influence of the edge. Although there was little evidence of edge effecting plant water status, distribution of at least one understory species suggests proximity to edge is disadvantageous.

Some general implications suggested by this study are edge effects on environmental variables become more complex as edge ages. These complex patterns and plant responses to them are likely strongly influenced by frequent gaps near edges, changing vegetation structure and continuing to alter the nature and extent of edge effects. Management decisions based on edge effects should incorporate understanding of the mechanisms behind those effects, and long term studies are necessary to determine those mechanisms and their changes over time.

Effects on Fauna

Many studies have been conducted about the fragmentation effects on vertebrates and invertebrates, including amphibians, insects, mammals and birds. This study, “Understory birds and dynamic habitat mosaics in Amazonian rainforests” by Richard Bierregaard and Philip C. Stouffer is a long term experiment studying birds in a dynamic system of small forests remnants surrounded by pasture or abandoned pasture undergoing secondary succession. The following was summarized from pages 138 to 155 of Tropical Forest Remnants: Ecology, Management and Conservation of Fragmented Communities by W.F. Laurance and R.O. Bierregaard.

Within a series of 1 and 10 hectare and one 100 hectare fragments, mark-recapture program was conducted focusing on understory birds to reveal changes in species composition and activity level. Analysis of broadly defined ecological guilds was performed, including nectar feeders, insectivores, and frugivores. They related changes in the fragment avifauna to remnant size, time since isolation, and the nature of the surrounding vegetation.

For insectivores, abundance and species richness of most frequently captured birds declined significantly in post-isolation reserves. The obligate army ant-following species disappeared completely from 1 and 10 ha isolates within 2 years of isolation. As with insectivores, frugivores showed significant declines in capture rates after isolation and four species showed effects of time since isolation. Nectarivores, such as understory hummingbirds, proved to be less vulnerable to fragmentation than insectivores and frugivores.

Birds play an integral role in tropical rainforest ecosystems and are likely the best studied group of organisms in these forests, so they provide an excellent opportunity to understand faunal responses to habitat fragmentation. This study identified both particularly sensitive and insensitive groups of species. These are first steps in developing understanding needed to minimize effects humans are having on tropical rainforest ecosystems.