Luis Zambrano

Direct and indirect effects of carp (Cyprinus carpio L.) on macrophyte and benthic communities in experimental shallow ponds in central Mexico

An experiment was conducted for 11 months in five controlled small ponds (c 0.05 ha) to determine the effects of common carp on water turbidity, and macrophyte and benthic communities in subtropical shallow systems. By using exclosures in ponds, we also looked for carp direct effects (by predation disturbance) and indirect effects (by increasing turbidity, and decreasing refuge sites) on rooted macrophyte and benthic communities. Results showed that at densities equal or higher than 0.8 ind. m−2 carp suffers from intraspecific competition. At the same densities, carp produces an increment of water turbidity, but the relation is non-linear, suggesting a switch effect, from a clear-water-system to a turbid-water-system. We also found that a direct effect of carp on macrophytes depends on the susceptibility of each species. Carp affects benthic abundance directly by predation, but its indirect effects on it remain unknown.

Impact of introduced carp (Cyprinus carpio)in subtropical shallow ponds in Central Mexico

In Mexico, as in many other subtropical and tropical countries, there has been a recent trend towards stocking non-native carp (Cyprinus carpio) in lakes and ponds as a source of food in rural areas. However, the results of a study in a series of small(1–8 ha.), shallow (<2 m), semi-natural ponds in Acambay, a high altitude valley in the basin of the Lerma river in the volcanic belt in central west Mexico, illustrate that the stocking of carp over a threshold value may have a detrimental ecological impact at several trophic levels. Ponds with carp tended to be turbid with high levels of suspended solids, and with few rooted macrophytes and epibenthic invertebrates. In contrast, ponds without carp had clear water and abundant rooted macrophytes and associated invertebrates, particularly gastropod molluscs. The direct uprooting of macrophytes by benthic foraging carp appeared to be the most important mechanism in switching the ponds from a clear macrophyte-dominated to a turbid state. The subtropical study ponds thus appear to confirm the alternative stable-state hypothesis developed intemperate lakes, although the importance of benthic rather than pelagic interactions was emphasised. The implications of stocking carp for native fauna of high intrinsic conservation value and as a food supply for local people are outlined.

Chytridiomycosis survey in wild and captive mexican amphibians.

Mexico, a rich country in terms of amphibian diversity, hosts about 375 described species. Population declines have been documented for several species where it is evident that their habitat is being destroyed or modified. However, other species which inhabit pristine areas are declining as well. It has been suggested that the chytrid fungus Batrachochytrium dendrobatidis(B.d.) may be one of the causes of the enigmatic declines in Mexico. We surveyed a total of 45 localities, in 12 states across Mexico, examining a total of 360 specimens representing 14 genera and 30 species. We also examined 91 specimens of Ambystoma mexicanum from a captive population in Mexico City as well as one Pachymedusa dacnicolor obtained in a pet shop. We used a two-tiered technique to detect the pathogen. For wild-caught specimens, we utilized light microscopy to identify presence of B.d. sporangia in amphibian skin. Then, to verify the infection, we used a quantitative real-time PCR assay on collected skin sections which is specific for B.d. For captive animals, we used a nonlethal version of the real-time PCR technique. We found evidence of B.d. infection in 111 animals comprising 14 species in 13 localities. A large percentage (84%) of Ambystoma mexicanum from the colony were infected with B.d. The two most highly infected individuals were the endangered Ambystoma mexicanum, from a captive colony, and Pachymedusa dacnicolor, purchased at a pet shop.

From Aquaculture Goals to Real Social and Ecological Impacts: Carp Introduction in Rural Central Mexico

Abstract Aquaculture has been seen as a solution to food/protein availability in rural populations of poor countries. It is mainly based on exotic species, that produce changes in host system dynamics once introduced. Aquaculture not only changes the ecology of freshwater systems, but can also lead to modification of social relations. Until now, aquaculture programs have not been adequately analyzed no questioned enough. We evaluate both ecological effects and local social benefits of common carp aquaculture programs in shallow ponds of rural areas, using a municipality in Central Mexico as a case study. Using an “environmental entitlements” approach, our findings suggest that: i) carp aquaculture increases water turbidity and depletes native species reducing the poor peoples access to them; ii) aquaculture mainly benefits pond owners rather than poor peasants. This mainly results from changes in fishing rights. We conclude that aquaculture policy goals and assumptions of benefits should be reviewed, if the negative ecological effects are to be decreased and conditions for people in rural areas are to be improved.

Relationships between fish feeding guild and trophic structure in English lowland shallow lakes subject to anthropogenic influence: implications for lake restoration

The shallow lakes of Eastern England have been subject to intense anthropogenic pressures including nutrient enrichment and fish stocking. We sought to determine the relationships between fish community structure and other ecosystem characteristics in 28 of these lakes through classification of fish species into piscivorous, zooplanktivorous and benthivorous feeding guilds according to the literature. Canonical correspondence analysis produced clear associations between fish and ecosystem characteristics that generally agreed with other theoretical (e.g. the alternative stable states hypothesis) and empirical studies, but with some important differences. There was a striking lack of relationships between nutrients and other variables, indicating the importance of top-down rather than bottom-up processes as a structuring force in the generally eutrophic study lakes. The presence of submerged (and shoreline) vegetation was associated with a diverse assemblage of apparently co-existing piscivorous (principally pike Esox lucius) and zooplanktivorous species. Perch Perca fluviatilis, a significant predator in other studies, was unimportant and argued to be limited by water quality in the extremely shallow lakes. In contrast, the benthivorous fish guild (principally carp Cyprinus carpio, bream Abramis brama and tench Tinca tinca) essentially represented the inverse of the potential pelagic associations between piscivores/zooplanktivores and vegetation. The introduction of large benthivores to many study lakes could have precipitated a loss of submerged vegetation through direct uprooting during foraging, with the effect of simplifying the fish community being most acute where littoral vegetation was limited by other anthropogenic factors. It is implied that attempts to promote or restore submerged vegetation in these lakes would best target benthivorous species.

Interactions of common carp (Cyprinus carpio) with benthic crayfish decapods in shallow ponds

Introduction of common carp (Cyprinus carpio) for aquaculture increased in recent decades. This fish is now established in many new water systems creating interactions with native species. Some of these interactions have been partly understood, but most of them remain unknown. For instance, in shallow ponds of central Mexico, populations of crayfish (Cambarellus montezumae) are reduced with high carp densities, but little is known about the mechanisms that lead to this depletion. Gut analysis showed that carp ate mostly detritus, small invertebrates, plant tissues and seeds, reducing the possibility of predation as a main cause of crayfish population reduction. Field and experimental data suggest that the effect of carp on crayfish is associated with habitat depletion. Submerged macrophyte Potamogeton pectinatus and the algae Cladophoraglomerata are important components in crayfish habitat, and their coverage in the water system is affected by carp presence. A second effect of carp on crayfish populations is associated with the alteration of crayfish behaviour. Crayfish displacement speed increased significantly in the presence of carp.

Urban expansion into a protected natural area in Mexico City: alternative management scenarios

Land use change is one of the main stress factors on ecosystems near urban areas. We analysed land use dynamics within Xochimilco, a World Heritage Site area in Mexico City. We used satellite images and GIS to quantify changes in land use/land cover (LULC) from 1989 to 2006 in this area, and a Markov projection model to simulate the impact of different management scenarios through to 2057. The results show an alarming rate of urbanisation in 17 years. LULC change runs in one direction from all other land use categories towards urban land use. However, changes from wetland or agricultural LULC to urban LULC frequently occur through transitional categories, including greenhouse agriculture and abandoned agricultural land. While urbanisation of natural land is often indirect, it is also effectively permanent. Active management aimed at protecting ecologically valuable habitats is urgently needed.

Environmental and socio-economic sustainability of chinampas (raised beds) in Xochimilco, Mexico City

The chinampas (raised beds) of Xochimilco, Mexico City, are highly productive, traditional wetland agricultural systems, which were able to feed most of the population in pre-hispanic times. There is a strong trend to substitute chinampas with plastic greenhouses for flower production, which creates negative impacts in the landscape, environment and culture. This study compares the environmental and socio-economic sustainability of chinampas and greenhouses, at both the farm and regional levels, using the MESMIS framework. Even though the results show that greenhouses are more profitable, the contribution of chinampas to ecosystem services cannot be substituted by greenhouses, as tree cover is lost, canals are filled and food is not provided. Greenhouses had a higher diversity, but also a higher agrochemical use and are heavily dependent on external inputs and subsidies. Chinampas have shifted from staple crops to commercial horticulture in order to remain a technically viable and economically feasible option for local farmers. However, compensation mechanisms for the provision of ecosystem services are urgently needed if this ancient system is to be maintained. The framework allowed the integration of indicators at both farm and regional scales, combining on-farm surveys with GIS techniques, which could assist in ecosystem service valuation.

Creating Refuges for the Axolotl (Ambystoma mexicanum)

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Trophic niche overlap between native and non-native fishes

In Mexico, non-native species are established in virtually every lake and represent one of the most important factors in species diversity loss. An iconic example is Lake Patzcuaro, which used to provide one of the most abundant fisheries of native species among freshwater systems in Mexico. But in the last decades, the relative abundance of non-native species has increased together with a reduction of native species populations. In this study, we analyze the trophic niche overlap between native and non-native species by using carbon and nitrogen stable isotopes. We did not found a spatial effect between physicochemical variables and isotopic signatures. The trophic niche area showed a small overlap among native species, but a substantial overlap of native species with non-native C. carpio and O. aureus. The non-native species P. infans presented almost no trophic overlap with other species. Non-native species have a trophic niche area two times larger than natives. The trophic niche overlap between native and non-native species was higher than among natives. The narrower trophic niche area and the high overlap with non-native species may explain the decline of native species populations. Alternative but untested explanations include altered water quality stemming from pollution and indirect effects of non-natives.