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number of robins nesting in Lonicera (Fig. 3b). Woo4i 707 c thrush thrushes. which built roughly half their nests in exotics 0 60 • robin in any given year, experienced apparent competition \ with robins nesting in Lonicera (also see Schmidt & Whelan 1998). These results suggest that Lonicera are 50 unfavorable sites for thrush nests due to overlap with U nesting robins. This interaction is further exacerbated 40 by two circumstances. First, because robins often reach p large population sizes in highly fragmented landscapes, —� 30 they are perhaps most likely to overlap in breeding habi-tat with Wood Thrushes in the very landscapes most sus- 20 ceptible to exotic shrub invasion. Second, as indicated N above, the early leaf flush of Lonicera may be driving N the higher overlap with robins. z 10 LCurrently, our results are specific to a single site and pair of species; it is unknown to what extent they may 0 generalize across a greater geographic or taxonomic 92 93 94 95 96 97 range. Neither the direct nor indirect effects of exotic Year plants on nest predation need be restrictive to exotic species. Furthermore, the relationship between the inva- sion of exotics and their use by nesting birds is likely de- 0.06 b pendent on the landscape context. These caveats make COit hard to generalize from our findings without further Q) data. Nonetheless, we believe that several important V • conservation issues will benefit from a more thorough O0.05 understanding of the relationship between exotic plants J and nest predation. First, our investigation underscores • the need for accurate measures of fitness and population 0.04 performance when habitat suitability is assessed. For in- stance, van Horne (1983) emphasized that population Z • density does not necessarily correlate with habitat qual- ity. Similarly,we show that the frequency of use of a par- 0.03 • ticular plant species for nest sites does not necessarily U) correlate positively with nesting success. (e.g., Martin �- 1998). In our study system, assuming that the higher fre- • quenc}, with which available plant species were used as 0.02 ' nesting substrates is an indication of higher nesting suc- 0 10 20 30 cess would lead to the erroneous conclusion that exotic shrubs actual1v benefit native bird species. a management Robin nests in Lonicera position that is not ordv controversial (Whelan & Dilger Figure.3. (a)Percentage of annual nest attempts built 1992, 1995)but, upon inspection,wrong. in Lonicera for American Robin and Wood Thrusb; (b) Second, exotic shrubs may play an important role in regression of daily mortality rate of tbrush nests fragmented and edge habitats. There is compelling evi- placed in Lonicera against the number of robins nest- dence that various songbird species are declining, at ing in Lonicera least in some portions of their range (Askins et al. 1990; Tames et al. 1996). Although mechanisms underlying these declines are controversial (Rappoie & McDonald tions,such as higher predator activity early in the season 1994; Latta & Baltz 1997), nest predation is a leading (Schmidt 1999), may provide alternative explanations candidate (e.g., Robinson et al. 1995)• Habitat distur- for our results. We recommend experimental tests to bance and fragmentation exacerbate both nest preda- fully explore these possible interactions. tion (Robinson et al. 1995) and exotic plant invasion Second,predation among nesting Wood Thrushes was (Hobbs & Huenneke 1992; Hutchinson &Vankat 1997), higher in Lonicera than the pooled native species, but and we suggest that these phenomena are partly inter- not compared with Viburnum, a species of compara- -;wined. Insofar as exotic shrubs are associated with hab- ble height and stature. Furthermore, predation among itat fragmentation and edges, higher predation rates in thrushes nesting in Lonicera was influenced by the exotic shrubs, such as we have documented, may be Conservation,Biologq volume 13.No.6,December 1999