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Matrix models and population viability analysis (PVA) have become useful tools to understand population attributes and dynamics. Demography analysis gives valuable information for the management of threatened species, and can be used to create action plans for their conservation. PVA is particularly useful in those species with small population sizes difficult to sample. By calculating the individual fate of each member of the population, PVA simulates temporal population changes and estimates extinction risk over a time period. Here we use these models to analyse the population of axolotl Ambystoma mexicanum, which has decreased more than six times within only 5 years. Its natural environment (the Xochimilco aquatic system within Mexico City) has deteriorated significantly in the last decades. The matrix analysis showed large oscillations in the axolotl population growth rate (damping behaviour), which could explain the fast density reduction in only few generations. Younger ages (eggs and larvae) showed the highest sensitivity and elasticity values, suggesting that the lack of food sources such as zooplankton or the increased predation by exotic carp and tilapia are capable to reduce axolotl density. PVA shows low extinction probabilities using laboratory data for younger ages. However, a small reduction in egg or larvae survival rate is capable to increase extinction probabilities to 100% in 20 years. Based on these results, we found that the best strategy to restore the axolotl population is to increase the survival rate of eggs and larvae by restoring the habitat, eradicating introduced fish and improving water quality, rather than implementing a reintroduction programme.

We evaluated the nesting by Crocodylus moreletii in Lago de Catemaco, Veracruz, southeastern, Mexico. During the nesting and hatching seasons, we searched for nests along the northern margins of the lake and small associated streams. We investigated egg mortality by weekly monitoring each of the nests found, recording sign of predation (tracks and holes dug into the nest) and the effect of water level fluctuations. We not found differences to nest between inland or flooded zones. However, we found that egg size varied among nests. In nests built inland, predation was the major cause of egg mortality whereas flooding resulted in more deaths of eggs in the flooding zone. Flooding killed 25% of eggs monitored in this study. We suggest that to increase nest success in the Morelet’s crocodile it is necessary to promote conservation of nesting areas around the lake, recently occupied by urban or tourist developments. Resumo Nós avaliamos a nidificação numa população de Crocodylus moreletii no Lago de Catemaco, Veracruz, sudeste, México. Durante as temporadas de nidificação e eclosão buscamos ninhos ao longo das margens situadas ao norte do lago e pequenos riachos associados. Estabelecemos a mortalidade de ovos por semana monitorando cada ninho achado, registrando qualquer sinal de predação (rastros e buracos cavados no ninho) e o efeito das flutuações no nível da agua. Não achamos nenhuma preferência para aninhar na terra ou em áreas inundadas. Por outro lado, achamos que o tamanho de ovo foi significativamente diferente entre os tipos de ninho. Em ninhos construídos na terra, a predação foi a principal causa da mortalidade de ovos enquanto que nas áreas inundadas, inundação foi um fator importante na morte dos ovos. Em comparação com depredação, a ruptura acidental e colonização fúngica, as inundações mataram 25% de ovos monitorados. Nós sugerimos que para aumentar o sucesso de aninhamento do crocodilo do Morelet é preciso promover a conservação das áreas de nidificação ao redor do lago, recentemente ocupadas pelo desenvolvimento urbano ou turístico.