Effect of land cover, habitat fragmentation and ant colonies on the distribution and abundance of shrews in southern California

Abstract

Summary1. Because effects of habitat fragmentation and anthropogenic disturbance on native animals have been relatively little studied in arid areas and in insectivores, we investigated the roles of different land covers, habitat fragmentation and ant colonies on the distribution and abundance of shrews, Notiosorex crawfordi and Sorex ornatus , in southern California. 2. Notiosorex crawfordi was the numerically dominant species (trap-success rate 0·52) occurring in 21 of the 22 study sites in 85% of the 286 pitfall arrays used in this study. Sorex ornatus was captured in 14 of the sites, in 52% of the arrays with a total trap-success rate of 0·2. Neither of the species was found in one of the sites. 3. The population dynamics of the two shrew species were relatively synchronous during the 4-5-year study; the peak densities usually occurred during the spring. Precipitation had a significant positive effect, and maximum temperature a significant negative effect on the trap-success rate of S. ornatus . 4. Occurrence and abundance of shrews varied significantly between sites and years but the size of the landscape or the study site had no effect on the abundance of shrews. The amount of urban edge had no significant effect on the captures of shrews but increased edge allows invasion of the Argentine ants, which had a highly significant negative impact on the abundance of N. crawfordi . 5. At the trap array level, the percentage of coastal sage scrub flora had a significant positive, and the percentage of other flora had a significant negative effect on the abundance of N. crawfordi . The mean canopy height and the abundance of N. crawfordi had a significant positive effect on the occurrence of S. ornatus . 6. Our study suggests that the loss of native coastal sage scrub flora and increasing presence of Argentine ant colonies may significantly effect the distribution and abundance of N. crawfordi . The very low overall population densities of both shrew species in most study sites make both species susceptible to extinction in isolated habitat fragments due to environmental stochasticity, and anthropogenic disturbance.

Seawalls do not sustain viable populations of limpets

“…For some habitats, it is not known whether fragmentation by building something is more or less of an influence on sustainability of species than would be the case for other types of fragmentation. On the other hand, where there have been studies of artificial structures, they tend to note whether abundances of animals are similar to those in natural habitats (Dickman & Doncaster 1987, Madejczyk et al 1998, Laakkonen et al 2001. When separated from demographic functionality, density may, however, lead to wrong conclusions about the value of artificial structures as alternative habitats, because they may just act as ecological sinks in which organisms fail to breed successfully (Smallwood 2001).…”

Factors influencing the variation in capture rates of shrews in southern California, USA

“…The data was collected from 20 sites used in our earlier study from 1995 (1996 in some cases) until 1999 (for details of the trapping periods see Laakkonen et al 2001). Additional trapping was done in some areas in 2000 and 2001.…”

“…In this study we further analyzed the effect of precipitation on the capture rates by studying the effect of short term (during the trapping period), monthly, and annual precipitation on the capture rates of shrews at 2 sites differing in the amount of annual rainfall. Although precipitation had no significant effect on the short-term capture-rates (Laakkonen et al 2001), we predicted that high levels of monthly and annual rainfall would increase the long-term capture rates of N. crawfordi due to the increase in their prey populations (Lindstedt 1980), as has been documented for other shrew species (Holling 1959, Butterfield et al 1981, Henttonen et al 1989). …”

“…The recent studies on shrews occupying arid habitats of western North America indicate that some of these habitats can support relatively species-rich assemblages (Williams 1984, 1991, Kirkland et al 1997, Laakkonen et al 2001) but their abundance is consistently low. In an attempt to determine factors influencing such low estimates of density of shrew populations, we examined the temporal variations in capture rates of the desert shrew Notiosorex crawfordi (Coues, 1877), and the ornate shrew Sorex ornatus (Merriam, 1895), from southern California, USA, in relation to the overall abundance, area of habitat fragmention, location (coastal vs inland), altitude and disturbance (edge effect) of the study sites.…”

“…Our previous study (Laakkonen et al 2001) showed that unlike in S. ornatus, the capture rate of N. crawfordi was not affected by rainfall during the trapping period (or during the previous trapping period). In this study we further analyzed the effect of precipitation on the capture rates by studying the effect of short term (during the trapping period), monthly, and annual precipitation on the capture rates of shrews at 2 sites differing in the amount of annual rainfall.…”