Читать книгу Ecology - Michael Begon - Страница 114

Species threatened with extinction often persist as a series of small, fragmented populations. Conservation programmes must then tread a fine line between being focused on the particular needs of specific threatened fragments and retaining a commonality of approach that makes the programme as a whole affordable and practicable. With these ideas in mind, three small populations of the northern wheatear, Oenanthe oenanthe, one of the most rapidly declining breeding birds in Europe, were studied in the Netherlands (Van Oosten et al., 2015), where numbers have dropped by at least 80% since 1990. Of the three populations, numbers in the 1990s (females holding territories) increased from five to 30 at Aekingerzand (A) but decreased from 165 to 34 at Castricum (C) and fluctuated between 45 and 69 at Den Helder (D). Field data were therefore collected between 2007 and 2011 both to estimate the sizes of the breeding populations (Figure 4.18a) and to monitor the demographic processes (vital rates) determining the population sizes: fecundity, juvenile (first year) and adult survival, and in this case immigration.

Figure 4.18 Analyses of life table response experiments (LTREs) can guide customised conservation. (a) Population sizes of northern wheatears, Oenanthe oenanthe (numbers of breeding territorial females) 2007–11 at three study sites in the Netherlands, as observed and as estimated from a matrix model integrating data on survival and fecundity. The variation in the estimates (box‐and‐whisker plots) arises because those estimates are taken regularly from chains generated by Monte Carlo Markov Chain (MCMC) procedures. The IPM does a good job at capturing the dynamics of the populations. (b) Contributions of the four vital rates (fecundity, f, juvenile and adult survival, ϕ_j and ϕ_a, and immigration, I) to the population growth rates observed in the Aekingerzand (purple) and Castricum (green) populations compared with that at Den Helder.

Source: After Van Oosten et al. (2015).

A matrix model integrating data on survival and fecundity was fitted to these demographic data and proved successful in recreating the observed population dynamics (Figure 4.18a), which in turn provides us with confidence that the IPM can usefully be used in an LTRE analysis. The results of this are shown in Figure 4.18b, comparing populations A and C to population D, as a benchmark, in terms of the contributions of the differences in vital rates to the overall differences in R. The relatively high rate of increase at site C (albeit from a low base) appears to be driven largely by immigration with some contribution from juvenile survival. The clear decrease at site A (R negative) resulted from low fecundity and adult survival.

The LTRE analysis, therefore, argues, most fundamentally, for customised approaches to be taken to local populations, since the forces driving their dynamics can be so different. Indeed, Van Oosten et al. (2015) conclude their study by explaining how they have responded to the analysis. To counteract the problems at site A, they introduced wire mesh covers to protect the nests and nesting adults, which led to an immediate more‐than‐doubling of fecundity and no predation by foxes, previously the main culprits. To support site C, so reliant on immigration, their strategy has been to protect site D, the main source of those immigrants. Fortunately, site D itself supports the most stable population of the three, and the strategy therefore is simply to protect the habitat there for the sake of that population and of others, like C, that rely on it.