PubMed Health⌕ Search

Biomedical subjects

David R Schiel

Publications and source records attributed to David R Schiel.

5 recordsLinked to original sources
PubMed2006

Community effects following the deletion of a habitat-forming alga from rocky marine shores.

Habitat-forming species increase spatial complexity and alter local environmental conditions, often facilitating a diversified assemblage of plants and animals. Removal of dominant species, therefore, can potentially lead to pronounced changes in diversity and community structure through a series of negative and positive interactions involving several components of the community. Here we test community responses to the deletion of the dominant, canopy-forming alga Hormosira banksii from the mid-intertidal zone of wave-protected rocky shores in southern New Zealand. This species was removed in winter (July) from three 3x3-m areas at each of two platforms (Kaikoura and Moeraki) on the east coast of the South Island. Initially, 59 taxa occurred in stands, but there were only four algal species with greater than 5% cover and three mobile invertebrate species with more than five individuals per 0.25 m(2). By 6 months after Hormosira removal, most fucoid and coralline algae had burned off, and there were blooms of ephemeral algae in the removal plots, but almost no change within controls. After 2 years, diversity declined by 44% relative to controls at Kaikoura and 36% at Moeraki, and the amount of bare space had increased by tenfold at Kaikoura and twofold at Moeraki. Few sessile or mobile invertebrates were present. Recruitment of Hormosira occurred after 14 months in the removal plots. At this time, a "press" disturbance was initiated into one half of each removal plot to test the effects of continued removal of Hormosira on diversity. Similar "end-points" of the control and "press" removal plots were not reached after 2 years, and even after Hormosira recruitment into the original "pulse" experiment there was little recovery of the community. In this mid-intertidal system with considerable thermal stress, and perhaps in others with few perennial species, diversity and community structure can critically depend on positive associations with a single dominant species.

Animals↗
PubMed2006

Comparative phylogeography of coastal limpets across a marine disjunction in New Zealand.

Cook Strait, which separates the North and South Island of New Zealand, has been a transient, but re-occurring feature of the New Zealand land mass throughout the Pleistocene, maintaining its current width and depth for the past 5000 years. Historic land fragmentation coupled with the complex hydrography of the Greater Cook Strait region has created both biogeographic and phylogeographic disjunctions between the North and South Island in several marine species. Here we use mitochondrial cytochrome b DNA sequences of three endemic intertidal limpets, Cellana ornata, Cellana radians and Cellana flava to assess intraspecific phylogeographic patterns across Cook Strait and to look for interspecific concordance of ecological and evolutionary processes among closely related taxa. We sequenced 328-359 bp in 85-321 individuals from 8-31 populations spanning the biogeographic range of the three species. Intraspecific phylogeographic analyses show moderate to strong genetic discontinuity among North and South Island populations due to allopatric fragmentation. This pattern was broadly concordant across the three species and the observed divergence among this group of intertidal limpets (0.3-2.0%) is similar to that of previously studied subtidal organisms. For each species, divergence time calculations suggest contemporary North and South Island lineages diverged from their respective most recent common ancestor approximately 200 000 to 300 000 years before present (bp), significantly earlier than previous estimates in other coastal marine taxa that arose from a miscalculation of divergence time.

Animals↗
PubMed2006

Seascape-dependent subtidal-intertidal trophic linkages.

In this study, we test in southern New Zealand a conceptual model of food web linkage that is seascape dependent, which can explain some of the variability in rocky shore community structure among sites and coasts. Using a comparative-experimental approach at local and distant sites we demonstrate that mobile subtidal predators (fish and crabs) can exert strong predation pressure on small mussels in the low tidal zone, but only in sites where the seascape includes subtidal reefs. On intertidal benches with adjacent subtidal reefs (+SR), 60-100% of small (5-15 mm) transplanted mussels were removed within a day from experimental tiles on the low shore when unprotected from predation, compared to fully caged controls that had approximately 100% survival over several months. In partial cages that exclude fish but not crabs, survivorship was intermediate. In contrast, on benches without subtidal reefs (-SR) 40-100% of mussels survived for months, even if unprotected. This difference is expressed in lower cover (0-60%) of mussels on rocks at +SR benches compared to -SR benches (70-99%). The central to northern west coast of the South Island is composed mostly of -SR benches, and predation on small mussels there was low and similar to the -SR benches on the east coast, whereas the +SR benches on the east coast had much greater predation. This contrasts to other studies in New Zealand that examined only predation on larger mussels by seastars and concluded that predation is strong on the west coast and weak on the east coast. Excluding large predators from low-shore areas with new recruits for a year in one +SR site showed longer-term predation effects on their abundance and cover. Short-term sampling at the east coast sites showed that mussel settlement was greater in -SR compared to +SR sites, providing some evidence that seascapes may also affect settlement. Overall, predation depended on the local seascape and ultimately affected community structure via suppression of effective recruitment rates. This study emphasizes the importance of predation on early life stages of basal species and the influence of seascapes on top-down interactions between subtidal predators and their intertidal prey.

Animals↗