Mangroves are ecologically important and extensive in the Neotropics, but they are visibly threatened by selective logging and conversion to pastures in the Southern Caribbean. The objective of this paper was to summarize the impacts of both threats on forest structure, species composition, aboveground biomass and carbon reservoir, species introgressions, and benthic fauna populations by collating past and current data and by using an interdisciplinary approach in the Urabá Gulf (Colombia) as a case study. Mangroves in the Eastern Coast have been decimated and have produced unskewed tree-diameter (DBH) distributions due to the overexploitation of Rhizophora mangle for poles (DBH range: 7-17 cm) and of Avicennia germinans for planks and pilings (DBH > 40 cm). Selective logging increased the importance value of the light-tolerant white mangrove Laguncularia racemosa, also increasing biomass and carbon storage in this species, thus offsetting reductions in other species. Introgressions (cryptic ecological degradation) by L. racemosa and Acrostichum aureum (mangrove fern) and low densities of otherwise dominant detritivore snails (Neritina virginea) were observed in periurban basin mangroves. Finally, basin mangroves were more threatened than fringing mangroves due to their proximity to expanding pastures, villages, and a coastal city.

Diadromous faunas dominate most tropical coastal streams and rivers, but the factors controlling their distribution are not well understood. Our study documents abiotic variables controlling the distribution and abundance of the diadromous snail Neritina virginea (Gastropoda:Neritidae) in the Caribbean island of Puerto Rico. An intensive survey of N. virginea density and shell size, and channel substrate, velocity, and depth was conducted at microhabitat, habitat, and reach scales of a coastal plain reach of the Río Mameyes between August and December 2000. In addition, the inland extent of distribution (stream-network scale) and presence (regional scale) of N. virginea were surveyed in 32 coastal rivers around the island during summer 2001 and 2003. At the microhabitat scale, snail density and microhabitat electivity were greater in patches consisting of a mix of boulders and cobbles than in other types of substrate. At the habitat scale, snail density increased with depth. At the reach scale, snail density increased with fast and turbulent flows (riffle. pools. pond), whereas snail size showed the opposite pattern. At the regional scale, populations were present in 13 of 32 streams. Populations of N. virginea were not found in rivers that were disconnected from the ocean for most of the year because of channel dewatering, formation of sediment bars at their mouths, and low mean monthly discharge (Q ¼ 0.69 m 3 /s). In contrast, rivers with N. virginea populations had a permanent (Q ¼ 4.04 m 3 /s) or seasonal (Q ¼ 2.88 m 3 /s) connection to the ocean over the year. At the regional scale, the inland distribution of populations was not correlated with stream gradient, but was negatively correlated with concentrations of SiO 2 , P, and acid neutralizing capacity of the water. Populations colonized montane reaches in only 5 rivers, all of which were forested and protected. Our study highlights the importance of taking a hierarchical approach in managing tropical coastal rivers, and the usefulness of neritid snails as biological indicators of the physical and chemical integrity of rivers.

1. Overlapping river and road networks provide a framework for studying the complex interactions between natural and human systems, with river-road intersections as focal areas of study. Roads can alter the morphology of stream channels, pose barriers to freshwater fauna, provide easy access to streams for humans and non-native species and accelerate the expansion of urban development. 2. We determined what variables control the structure of diadromous fish and shrimp communities and assessed whether particular road crossings altered community structure in north-eastern Puerto Rico. We identified 24 sites that represented a range of river and road sizes across two catchments that drain El Yunque National Forest in Puerto Rico. 3. The location of natural barriers and the size of stream pools were the most important variables for predicting six of fifteen fish and shrimp distributions. Predatory fishes were predicted to be limited to areas in the river network below large, steep waterfalls, whereas adult shrimp Atya lanipes (Atyidae) were predicted to be present above these waterfalls. The fish Awaous banana was predicted to be present in pools >11.6 m wide, whereas the shrimp Xiphocaris elongata was predicted to be present in pools <10.4 m wide. The distributions of nine species were predicted poorly, but three of these species were common and three were rare. 4. Although urban and agricultural land covers were among the top three predictors of five species distributions, they were probably good predictors because they were correlated with the natural gradient. Further study is necessary to disentangle natural and anthropogenic gradients. 5. Road crossings, 10 of which were culverts, were not dispersal barriers for fishes or shrimps. On average, species were present both upstream and downstream from road crossings at 68% of sites where they occurred. Absences upstream or downstream from road crossings occurred at 16% of sites each and likely resulted from a failure to detect species.

This paper relates differences in flow hydraulics between a main channel (MC) and a side channel (SC) of a river to patterns of upstream migration by Neritina virginea (Neritidae: Gastropoda), a dominant diadromous snail in streams of Puerto Rico (Greater Antilles). Near-bed water velocity, snail density and shell size were measured on a weekly basis between August and December 2000 along cross-sections in a main channel (MC) and an adjacent channel (SC) under a bridge crossing of the Río Mameyes of Northeastern Puerto Rico. Near-bed velocity and water depth were used to compute Reynolds (Re) and Froude (Fr) numbers, and to classify flows within each channel. During base flow conditions (<2 m 3 s À1), flow was chaotic and supercritical (Fr > 1) in the MC, and non-chaotic and subcritical (Fr < 1) in the SC. Higher mean densities (>100 ind m À2 ) of relatively small snails (mean AE s.d., 6.3 AE 2.8 mm) were consistently recorded in the MC. Conversely, the SC had lower mean densities (<20 ind m À2 ) and significantly larger snails (7.6 AE 2.4 mm). Within the MC, migratory groups preferred near-bed velocities > 0.8 m s À1. Within the SC, they preferred the channel thalweg and depths > 30 cm. The spatial arrangement that was observed between and within the channels may be related to food resources, predation pressure or biomechanics. Characteristics of preferred upstream migration pathways of N. virginea must be accounted when building road crossings in coastal streams with diadromous fauna. Published in 2007 by John Wiley & Sons, Ltd.