Relationships Between Landscape Structure and Breeding Birds in the Oregon Coast Range

Abstract

Human—caused fragmentation of forests is increasing, yet the consequences of these landscape changes to vertebrate communities are poorly understood. Although bird community response to forest fragmentation caused by agricultural or urban development has been well studied, we have little understanding of these dynamics in landscapes undergoing intensive forest management, where late—seral forest stands are separated by younger forest stands of varying ages and are part of a spatially and temporally dynamic forest landscape. We investigated the relationship between landscape structure and breeding bird abundance in the central Oregon Coast Range. We sampled vegetation and birds in 30 landscapes (250—300 ha) distributed equally among three basins. Landscapes represented a range in structure based on the proportion of the landscape in a late—seral forest condition and the spatial configuration of that forest condition within the landscape. We computed a variety of landscape metrics from digital vegetation cover maps for each landscape. Using analysis of variance and regression procedures, we quantified the independent effects of habitat area and configuration on 15 bird species associated with late—seral forest. Species varied dramatically in the strength and nature of the relationship between abundance and several gradients in habitat area and configuration at the landscape scale. Landscape structure (composition and configuration) typically explained <50% of the variation in each species' abundance among the landscapes. Species' abundances were generally greater in the more heterogenous landscapes; that is, they were associated with the more fragmented distribution of habitat. Only Winter Wrens showed evidence of association with the least fragmented landscapes. These results must be interpreted within the scope and limitations of our study. In particular, the scale of our analysis was constrained by the lower and upper limits of resolution in our landscapes, as set by minimum patch size and landscape extent, respectively. Thus, our results do not preclude much stronger and different relationships at finer and/or coarser scales. In addition, our community—centered habitat classification scheme and artificially discrete representation of patch boundaries may not have captured the functionally meaningful heterogeneity for each species. Finally, our analysis was limited to relatively common and widespread diurnal breeding bird species. Species sensitive to habitat fragmentation at the scale of our analysis may have been rare already and therefore not subject to the parametrical statistical approach that we employed.

Movements of Marine Fish and Decapod Crustaceans: Process, Theory and Application

“…The selection of an appropriate minimum mapping unit (MMU) is also an important consideration, since this constrains the lower limits of resolution (Li and Reynolds, 1994;McGarigal and McComb, 1995;Gustafson, 1998) and influences the behaviour of spatial pattern metrics (Saura, 2002). Furthermore, delineating boundaries requires careful consideration, since the production of a categorical map using statistical clustering algorithms typically results in an abrupt division between groupings.…”

“…For studies in terrestrial systems, an extensive set of indices or metrics has been developed and applied to measure the composition and spatial pattern of mosaic structure (e.g. O'Neill et al, 1988;Turner, 1989;Gustafson et al, 1994;McGarigal and Marks, 1994;McGarigal and McComb, 1995;Haines-Young and Chopping, 1996;Gustafson, 1998;Hargis et al, 1998). Mosaic composition encompasses the variety and abundance of patch types, without being spatially explicit and includes the proportion of each class (patch type), as well as richness, evenness and diversity indices.…”

“…In animal ecology, however, the process must also ensure that the chosen metrics capture all dimensions of spatial pattern relevant to an animal. For exploratory studies of the influence of mosaic pattern on animals, a suite of metrics is typically used (26 metrics by McGarigal and McComb (1995) for bird assemblages; 17 metrics by McAlpine et al (1999) for kangaroos and eight metrics by Hansen et al (2001) for caribou). For marine systems, however, this type of approach remains largely theoretical (Robbins and Bell, 1994), with few attempts to quantify spatial pattern (Garrabou et al, 1998 for rocky shores; Robbins and Bell, 2000 for seagrasses; Teixidó et al, 2002, for Antarctic benthic communities).…”

“…In addition, exploratory studies undertaken on a wide range of highly mobile animals including hoverflies (Haslett, 1994), birds (Pearson, 1993;McGarigal and McComb, 1995), kangaroos (McAlpine et al, 1999) and Florida panther (Kerkhoff et al, 2000) have recognised that investigation of a species response to spatial pattern must take place at multiple scales. These previous studies have quantified mosaic structure within a range of spatial extents, delineated around an observation area for a particular species or group of species.…”

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“…McGarigal and McComb (1995) found in a study of birds in Oregon that forest cover explained more variance in species abundance than landscape structure, and most of the significant effects of fragmentation on abundance were positive. Meyer and Irwin (1998) found that the main influences of landscape structure on spotted owls were due to the amount of habitat, not configuration.…”