August 2005 CA Grasslands Newsletter
This month’s reading - pop quiz next week!
Welsh, Hodgson, and Lind. 2005. Ecogeography of the herpetofauna of a northern California watershed: linking species patterns to landscape processes. Ecography 28: 521-536.
Abstract: Ecosystems are rapidly being altered and destabilized on a global scale, threatening native biota and compromising vital services provided to human society. We need to better understand the processes that can undermine ecosystem integrity (resistance-resilience) in order to devise strategies to ameliorate this trend. We used a herpetofaunal assemblage to first assess spatial patterns of biodiversity and then to discover the underlying landscape processes likely responsible for these patterns. Reptiles and amphibians are a phylogenetically diverse set of species with documented sensitivity to environmental perturbations. We examined ecogeographic patterns of these taxa in aquatic and riparian environments across the landscape mosaic of the Mattole River watershed of northern California, USA. We analyzed species distributions relative to three primary vegetation types (grassland, second-growth forest, late-seral forest) and two hydrologic regimes (perennial vs intermittent). We sought evidence for the processes behind these patterns by modeling animal distributions relative to multi-scale compositional, structural, and physical attributes of the vegetation or hydrologic type. Total herpetofaunal diversity was higher along perennial streams, with reptile diversity higher in mixed grassland. Amphibian and reptile richness, and reptile evenness, varied significantly among the three vegetations. Evidence indicated that distinct assemblages were associated with each end of a seral continuum. Four amphibians were more abundant in late-seral forest, while two amphibians and two reptiles were more abundant in second-growth forest, or mixed grassland, or both. Two amphibians were more abundant along intermittent streams. Models for predicting reptile richness, or abundances of the two amphibian assemblages, indicated water temperature was the best predictor variable. Based on these results and the physiological limits of several sensitive species, we determined the primary processes influencing faunal assemblage patterns on this landscape have been vegetation changes resulting from the harvesting of late-seral forests and the clearing of forest for pasture. Comparing past with present landscape mosaics indicated that these changes have transformed the dominant amphibian and reptile species assemblage from a mostly cold-water and cool forest-associated assemblage to one now dominated by warm-water and mixed grassland/woodland species.
Welsh, Hodgson, and Karraker. 2005. Influences of the vegetation mosaic on riparian and stream environments in a mixed forest-grassland landscape in "Mediterranean" northwestern California. Ecography 28: 537-551.
Abstract: We examined differences in riparian and aquatic environments within the three dominant vegetation patch types of the Mattole River watershed, a 789-km(2) mixed conifer-deciduous (hardwood) forest and grassland-dominated landscape in northwestern California, USA. Riparian and aquatic environments, and particularly microclimates therein, influence the distributions of many vertebrate species, particularly the physiologically-restricted ectotherms - reptiles and amphibians (herpetofauna), and fishes. In addition to being a significant portion of the native biodiversity of a landscape, the presence and relative numbers of these more tractable small vertebrates can serve as useful metrics of its "ecological health." Our primary objective was to determine the range of available riparian and aquatic microclimatic regimes, and discern how these regimes relate to the dominant vegetations that comprise the landscape mosaic. A second objective, reported in a companion paper, was to examine relationships between available microclimatic regimes and herpetofaunal distributions. Here we examined differences in the composition, structure, and related environmental attributes of the three dominant vegetation types, both adjacent to and within the riparian corridors along 49 tributaries. Using automated dataloggers, we recorded hourly water and air temperatures and relative humidity throughout the summer at a representative subset of streams; providing us with daily means and amplitudes for these variables within riparian environments during the hottest period. Although the three vegetation types that dominate this landscape each had unique structural attributes, the overlap in plant species composition indicates that they represent a seral continuum. None-the-less, we found distinct microclimates in each type. Only riparian within late-seral forests contained summer water temperatures that could support cold-water-adapted species. We evaluated landscape-level variables to determine the best predictors of water temperature as represented by the maximum weekly maximum temperature (MWMT). The best model for predicting MWMT (adj. R-2=0.69) consisted of catchment area, aspect, and the proportion of non-forested (grassland) patches. Our model provides a useful tool for management of cold-water fauna (e.g. salmonids, stream amphibians) throughout California's "Mediterranean" climate zone.
Condon, Kershner, Sullivan, Cooper, and Garcelon. 2005. Spotlight surveys for grassland owls on San Clemente Island, California. Wilson Bulletin 117: 177-184.
Abstract: According to Breeding Bird Survey data, grassland birds are among the most imperiled species in North America. Within this group, grassland owls show steep population declines across the United States. Despite these declines, questions still remain regarding the seasonal and geographic distribution of grassland owls. On San Clemente Island (SCI), California, grassland owls are known to occur, but nothing is known about their distribution or abundance. To increase our understanding of owl populations on SCI, we used night-time spotlighting to survey for grassland owls from October 2001 to October 2002. We recorded 733 detections of three species of owls: Barn Owl (Tyto alba), Burrowing Owl (Athene cunicularia), and Short-eared Owl (Asio flammeus). Barn (8.3 +/- 0.8 owls/hr) and Burrowing owls (2.2 +/- 0.7 owls/hr) were the most frequently detected species, whereas Short-eared Owls were rarely detected (0.2 +/- 0.1 owls/hr). We detected owls during all night-time hours surveyed and detected Barn Owls in every month of the study. We detected Burrowing Owls only from October to March and Short-eared Owls from December to April, suggesting that they are winter visitors. Despite the bias of increased detectability using roadside surveys, spotlighting from a vehicle enabled us to efficiently cover a large proportion of the island (compared to walking surveys) and survey multiple grassland species using one survey technique.
Keeley and Fotheringham. 2005. Plot shape effects on plant species diversity measurements. Journal of Vegetation Science 16: 249-256.
Abstract: Question: Do rectangular sample plots record more plant species than square plots as suggested by both empirical and theoretical studies?
Location: Grasslands, shrublands and forests in the Mediterranean-climate region of California, USA.
Methods: We compared three 0.1-ha sampling designs that differed in the shape and dispersion of 1-m(2) and 100-m(2) nested subplots. We duplicated an earlier study that compared the Whittaker sample design, which had square clustered subplots, with the modified Whittaker design, which had dispersed rectangular subplots. To sort out effects of dispersion from shape we used a third design that overlaid square subplots on the modified Whittaker design. Also, using data from published studies we extracted species richness values for 400-m(2) subplots that were either square or 1:4 rectangles partially overlaid on each other from desert scrub in high and low rainfall years, chaparral, sage scrub, oak savanna and coniferous forests with and without fire.
Results: We found that earlier empirical reports of more than 30% greater richness with rectangles were due to the confusion of shape effects with spatial effects, coupled with the use of cumulative number of species as the metric for comparison. Average species richness was not significantly different between square and 1:4 rectangular sample plots at either I or 100-m2. Pairwise comparisons showed no significant difference between square and rectangular samples in all but one vegetation type, and that one exhibited significantly greater richness with squares. Our three intensive study sites appear to exhibit some level of self-similarity at the scale of 400 m(2), but, contrary to theoretical expectations, we could not detect plot shape effects on species richness at this scale.
Conclusions: At the 0.1-ha scale or lower there is no evidence that plot shape has predictable effects on number of species recorded from sample plots. We hypothesize that for the mediterranean-climate vegetation types studied here, the primary reason that 1:4 rectangles do not sample greater species richness than squares is because species turnover varies along complex environmental gradients that are both parallel and perpendicular to the long axis of rectangular plots. Reports in the literature of much greater species richness recorded for highly elongated rectangular strips than for squares of the same area are not likely to be fair comparisons because of the dramatically different periphery/area ratio. which includes a much greater proportion of species that are using both above and below-ground niche space outside the sample area.
Amme, David 2005. UC Richmond Field Station's remnant coastal terrace grassland. Grasslands. 15: 3-5.