Estimates of the percent of Earth’s land surface that has either been transformed or degraded by human activity range between 39 and 50 percent, with agriculture accounting for the vast majority of these changes. Although much of the focus of research on land use and cover change in the tropics has been on deforestation, ongoing socioeconomic changes both locally and globally have made land transitions in the tropics extremely fluid. In addition, feedbacks between land cover change and human behavior constrain the extent and trajectories of land transitions. The sustainability of land use systems in the tropics depends on an understanding of coupled human–natural systems that can lead to general frameworks for management and prediction. The unprecedented availability of land use/cover data together with ecological data collected at large spatial scales offer exciting opportunities for advancing our understanding of socioecological systems. We rely on six studies of land transitions in the tropics to illustrate some promising approaches and pose critical questions to guide this body of research.

The threat imposed by increased sediment loading rates ranks among the most important stressors affecting coral reef ecosystems worldwide. This study represents an effort to quantify the effects of unpaved roads on erosion rates in a dry sub-tropical area of Puerto Rico and is intended to aid in developing scientifically-based erosion mitigation strategies. Hence, the specific objectives of this study were to: (1) measure sediment production rates from unpaved roads; (2) evaluate the effect of precipitation, rainfall erosivity, slope, plot length, and vegetation cover on sediment production rates; and (3) compare measured sediment production rates to published surface erosion data from roaded and natural sites in the Eastern Caribbean. Sediment production from nine abandoned road segments with varying slopes and plot lengths were measured with sediment traps in southwestern Puerto Rico from August 2003 to September 2005. The overall average sediment production rate was 0.84 Mg ha−1 yr−1, and the range of observed values was 15–50 times higher than locally-measured natural erosion rates. Only four of the nine study sites had a statistically significant correlation between sediment production and total rainfall and this is attributed to progressive changes in some of the conditions controlling erosion rates. Sediment production rates were dependent on slope raised to the 1.6th power, as well as to the product of plot length times slope1.6. Average erosion rates were inversely but poorly related to vegetation cover. An observed decline in sediment production rates was observed for all nine study segments, and this amounted to a statistically significant difference between observations made during the early stages of monitoring (Period 1: August 2003–April 2004) relative to those during the latter parts of the study (Period 2: May 2004– September 2005). Annual erosion rates during Period 1 amounted to 0.18 to 4.0 Mg ha−1 yr−1 for road segments with 1% and 22% slopes, respectively; rates during Period 2 were between 0.024 and 0.52Mg ha−1 yr−1, or only 13% of those during Period 1. Differences in sediment production rates between the two periods are attributed to more intense rainfall during Period 1 and to a notably higher vegetation cover during Period 2. Rainfall appears to play a paradoxical role in controlling surface erosion rates on abandoned road surfaces in a sub-tropical dry region.While ample rainfall is needed to generate erosion by rainsplash and overland flow, once rainfall satisfies soil moisture requirements for sustaining vegetation colonization it may also contribute to declining sediment production rates. Therefore, any model that attempts to properly address the temporal variation in erosion rates occurring on abandoned roads in a climatic setting where moisture availability is a limiting factor must not only follow the more traditional surface armoring-based approach but must also integrate the effects of re-vegetation. Such types of modelswill eventually become useful tools to properly assess the effects of past, current, and future land use practices on erosion rates, and to improve mitigation and land development strategies to lessen the impact on vital marine habitats

The extensive recovery from agricultural clearing of Puerto Rican forests over the past half-century provides a good opportunity to study tropical forest recovery on a landscape scale. Using ordination and regression techniques, we analyzed forest inventory data from across Puerto Rico’s moist and wet secondary forests to evaluate their species composition and whether the landscape structure of older forest affected tree species composition of recovering forests at this scale. Our results support conclusions from studies conducted in Puerto Rico at smaller scales and temperate forests at larger scales that timing of abandonment and land use history are of overwhelming importance in determining the species composition of recovering forests. Forest recovery is recent enough in Puerto Rico that previous land use is clearly evident in current species composition, and creates new forest communities. As demonstrated in other work, physical factors such as elevation and substrate co-vary with land use history, so that the species composition of the forest landscape results from the interplay between biophysical and socioeconomic forces over time. Our results also indicate that increasing the distance to the largest forest patches occurring in the landscape 12 years previous had a small negative impact on species richness but not species diversity or community composition.We conclude that land use history has as much influence in species composition as biophysical variables and that, at the scale of this study, there is no large influence of forest landscape structure on species diversity or composition.

Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where experimental data are available. In tropical developing countries, however, experimental studies are lacking. This paper summarizes available data and assesses the impacts of human activities on N fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape transformations over the last century associated with rapid socioeconomic changes. N yield calculations conducted in several watersheds of different anthropogenic influences revealed that disturbed watersheds export more N per unit area than undisturbed forested watersheds. Export of N from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to 32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan Area receives primary treatment before it is discharged into the Atlantic Ocean. These discharges are N-rich and export large amounts of N to the ocean at a rate of about 140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4 þ NO
3 ) suggest that rates of atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively characterize the local N cycle. More research is required to assess rates of atmospheric N deposition, N fixation in natural and human-dominated landscapes, N-balance associated with food and feed trade, and denitrification.