Antibiotic resistance genes (ARGs) in agricultural soils from the Yangtze River Delta, China
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Abstract Aim The ‘abundance optimum’ hypothesis predicts that species abundance peaks in the locality with the most favourable conditions and decreases with an increase of distance from that locality. We tested this prediction for 9 fleas and 13 gamasid mite species. Location We used published data on fleas and gamasid mites that are parasitic on small mammals throughout the Palaearctic. Methods For each ectoparasite, we computed the correlation between the relative abundance on its principal host species in a region and the distance from that region to the region of maximum abundance. Then, the correlation coefficients were used in a meta‐analysis. We also made a cross‐species comparison between relative abundances in localities (a) closest to and (b) furthest from the locality of maximum abundance. Results Although the relationship between the relative abundance in a region and the distance from that region to the region of maximum abundance was negative in 19 out of 22 ectoparasites, it was only statistically significant in three of them. However, a meta‐analysis of coefficients of correlations across all species revealed a significant negative effect of the distance from the region of maximum abundance on relative abundance in a particular region. A cross‐species comparison between relative abundances in the localities closest to and furthest from the locality of maximum abundance demonstrated that the former were significantly higher than the latter. Main conclusions A lack of strict host specificity in the ectoparasites studied, and the absence of any strong spatial correlations among the environmental variables affecting ectoparasite reproduction and abundance, may provide an explanation for the spatial independence in abundance values of most species. However, a preference for a particular host even in host‐opportunistic parasites combined with species‐specific environmental preferences could be the reason behind the weak, but significant, negative abundance–distance relationship across species. The contradiction between results obtained when separate species were considered and when the overall pattern was analysed across species suggests that there exists a general underlying spatial pattern that can often be masked by other factors.
Relative abundance distribution
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Abstract In South America, salmonids have been widely introduced to lakes, yet there are still many oligotrophic lakes without current abundance estimates. Catch per unit effort is a good estimator of fish abundance, but its correct implementation requires an expensive and long-term effort. Thus, there is a need to develop a rapid assessment to estimate the relative abundance of salmonids. Under well-delimited circumstances, condition indices may be a good technique to estimate fish population relative abundances. We sampled adult salmonids with gillnets from 6 oligotrophic lakes in southern Chile during the Austral summer. Our data show that as rainbow trout condition indices decrease, the abundance of salmonids increases both temporally and spatially. Because all 6 lakes are limited by food, the most likely reason for the change in the rainbow trout condition indices is change in salmonid abundance. The strong inverse relationship between condition indices of rainbow trout and the relative abundance of salmonids suggests that our model can be a useful tool for the estimation of relative fish abundance in lakes. This study provides a model that can be used to improve lake and fishery management decisions for introduced salmonids in southern South America.
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