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Intraspecific spatial variation in bird abundance: patterns and processes

 

  Contact Info:
  Patrick J. Doran, The Nature Conservancy in Michigan
    (pdoran@tnc.org)

Richard T. Holmes, Dartmouth College (Primary Contact)
    (richard.t.holmes@dartmouth.edu)

Scott J. Goetz, Ralph Dubayah, Matthew Betts and Others


 

WITHIN A SPECIES, the abundance of individuals varies from place to place, even in what may appear to be fairly homogenous habitat, such as the northern hardwoods of the 3160-hectare Hubbard Brook Experimental Forest in the White Mountains of New Hampshire (Holmes 2011). Beginning in the summer of 1999 (and ongoing), we began a multi-year study to examine the causes and consequences of variation in the abundance of individuals within the entire Hubbard Brook watershed (Figure 1).

Sample location map 
  Figure 1. General physiographic features of the Hubbard Brook Experimental Forest, New Hampshire, USA, showing elevations and drainage patterns. Paired dots represent sample points (n=371) at which birds abundances are being determined each year..
 

Using a valley-wide grid of over 370 plots (Figure 1) established in the mid-1990s by Paul Schwarz (See Schwarz et al. 2002), we have been conducting intensive bird surveys, using the point-count method. We found that most bird species exhibited intraspecific spatial variation in abundance (Figure 2). Furthermore, these patterns remained stable across years despite variation in total abundance across years (Figure 3). For five species, spatial variation in abundance was fully explained by spatially varying habitat factors. However, the remaining species continued to exhibit spatial variation in abundance after removing habitat effects, indicating that alternative ecological mechanisms (e.g., conspecific attraction; Stamps 1988) may be operating to increase local abundance beyond that expected based on habitat associations (Doran 2003).

For the black-throated blue warbler (Dendroica caerulescens) one consequence of spatial variation in abundance is that specific locations on the landscape are occupied consistently from year-to-year, while others are occupied intermittently. Consistently occupied sites had more deciduous vegetation and a well-developed dense understory. Warblers were more abundant and arrived earlier in the breeding season at these sites (Geotz et al. 2010). While there were no differences in average reproductive output across sites with different occupancy rates, these results indicate that the majority of individuals are found, and, subsequently, the majority of young are produced, within relatively few locations on the landscape (Doran and Holmes 2005, Goetz et al. 2010). Combined, these results indicate that, even within relatively undisturbed, homogenous landscapes, there is significant intraspecific temporal and spatial variation in bird abundance, which results from subtle variation in environmental and behavioral factors.

 Maps of bird distrubution and abundance
Figure 2. Distribution and abundance of three relatively common species at the Hubbard Brook Experimental Forest . The Black-throated Blue Warbler is a species of primarily deciduous forest; the Blackpoll Warbler found exclusively in high elevation conifer stands; and the Yellow-rumped Warbler found in mixed deciduous-coniferous stands. The size of the green circle represents an index of abundance, with larger dots representing a higher number of singing males detected at a point. In general, points with greater abundance tend to be clumped within the valley. 
Maps of warbler distribution and abundance 1999-2001
Figure 3. Distribution and abundance of the Black-throated Blue Warbler from 1999-2001 as well as an overall count of the number of years that the Black-throated Blue Warbler was detected at each survey point across those three years. Symbols are as in Figure 2. Note that areas of high abundance, as well as areas of low abundance, remain in the same general location within the valley across years.

 

References

Doran, P.J. 2003. Intraspecific spatial variation in bird abundance: patterns and processes. Ph.D. thesis, Dartmouth College, Hanover, NH.

Doran, P.J. and R.T. Holmes. 2005. Habitat occupancy patterns of a forest dwelling songbird: causes and consequences. Canadian Journal of Zoology 83:1297-1305.

Goetz, S.J., S. Steinberg, M. Betts, R.T. Holmes, P. Doran, R. Dubayah, and M. Hofton. 2010. Lidar remote sensing variables predict breeding habitat of a Neotropical migrant bird. Ecology 91: 1569-1576.

Holmes, R.T. 2011. Avian population and community processes in forest ecosystems: long-term studies in the Hubbard Brook Experimental Forest. Forest Ecology and Management 262:20-32.

Schwarz, P.A., T.J. Fahey and C.E. McCulloch. 2002. Factors controlling spatial variation of tree species abundance in a forested landscape. Ecology 84: 1862-1878.

Stamps, J. A. 1988. Conspecific attraction and aggregation in territorial species. American Naturalist 131:329-347.

Swatantran, A., R. Dubayah, S. Goetz, M. Hofton, M.G. Betts, M. Sun, M. Simard, and R.T. Holmes. 2012. Mapping migratory bird prevalence using remote sensing data fusion. PLoS One 7(1): e28922.doi:10.1371/journal.pone.0028922.

Date Updated: May 2013