A framework for adapting survey design through time for wildlife population assessment

Sampling strategies for monitoring the status and trends in wildlife populations are often determined before the first survey is undertaken. However, there may be little information about the distribution of the population and so the sample design may be inefficient. Through time, as data are collected, more information about the distribution of animals in the survey region is obtained but it can be difficult to incorporate this information in the survey design. This paper introduces a framework for monitoring motile wildlife populations within which the design of future surveys can be adapted using data from past surveys whilst ensuring consistency in design-based estimates of status and trends through time. In each survey, part of the sample is selected from the previous survey sample using simple random sampling. The rest is selected with inclusion probability proportional to predicted abundance. Abundance is predicted using a model constructed from previous survey data and covariates for the whole survey region. Unbiased design-based estimators of status and trends and their variances are derived from two-phase sampling theory. Simulations over the short and long-term indicate that in general more precise estimates of status and trends are obtained using this mixed strategy than a strategy in which all of the sample is retained or all selected with probability proportional to predicted abundance. Furthermore the mixed strategy is robust to poor predictions of abundance. Estimates of status are more precise than those obtained from a rotating panel design.     

Spawning,larval development and settlement behaviour of Gibbula cineraria (Gastropoda: Prosobranchia) with a reappraisal of torsion in gastropods

Cultures of developing larvae of Gibbula cineraria (L.) were obtained from adults spawning in the laboratory, and these cultures were reared to settlement of the larvae at 9 days. Dispersal of the outer jelly coat of the egg appeared to be delayed in the presence of spermatozoa. Early cleavage followed the typical spiral pattern, and gastrulation was by epiboly. The trochophore larvae hatched at about 28 h, before shell-formation began. The first 90° of torsion was completed between 48 and 56 h. The second part of torsion was completed within 4 days, and preliminary attempts to retract into the shell were made at 76h. After 96 h, although torsion was complete, the larvae were still unable to retract fully into the shell. The larvae remained in a stage of swimming-attempted creeping until the 4th or 5th day after torsion was complete, but were unable to pull the shell upright until the end of this time. Throughout this stage, the velum was reduced and was finally shed. The animals all died after metamorphosis, which occurred at about 9 days, when the larval shell comprised 1 1/2 whorls, with a breadth of 250 to 300m. Settlement appeared to be passive, i.e., governed by wave action and tides. In the light of these observations, theories concerning torsion are reappraised. The advantage to the larva of torsion is not clear, as the larvae are not able to retract into the shell until after metamorphosis, yet it is clear from previous authors' observations that the first part of torsion in most gastropods involves only larval structures. This makes it difficult to suggest that torsion is entirely of advantage to the adult snails. It is thus proposed that the two components of torsion may have evolved independently. The first, relatively rapid component of torsion, resulting in 90° of displacement of the mantle cavity in relation to the visceral mass, may be of advantage to the swimming larva. The final, slower component of torsion may be of advantage to the newly-metamorphosed benthic snail, and is the only component of torsion found in those gastropods which have no free-swimming larva.     

Contributions of Private Landowners to the Conservation of the Gopher Tortoise (<Emphasis Type="Italic">Gopherus polyphemus</Emphasis>)

Private landowners play a pivotal role in determining whether or not rare species persist in regions where privately owned land is extensive. The range of the gopher tortoise (Gopherus polyphemus) is confined to the Southeastern U.S., a region predominantly under private ownership, and thus the status of this species is largely dependent upon land management decisions made by private landowners. We sent an anonymous mail survey to 2,584 individuals to examine factors affecting gopher tortoise occurrence on private lands in Mississippi (adjusted response rate of 23%). Few respondents (19%) reported currently having tortoises on their property, although many had them in the past (30%). Tortoises were persisting primarily on larger properties with longleaf pine that were not managed chiefly for timber production. In general, respondents were largely unaware of habitat requirements of tortoises or effects of various land management practices on them, and few reported using management techniques that benefit tortoises, such as prescribed burning. Most respondents (57%) knew of wildlife incentive programs, but were hesitant to enroll because they did not want to commit to managing their property in a particular manner (34%). We suggest actions that could improve the likelihood of tortoise persistence in this region, as well as changes that could be made to incentive programs to increase landowner participation. These suggestions should be relevant to the conservation of other rare species on private lands in other regions.