Distribution, Abundance, Microhabitat Use and Interspecific Relationships Among Terrestrial Salamanders on Vancouver, Island, British Columbia ty Theodore M. Davis B.S., Portland State University, 1968 M.Sc., University of Victoria, 1991 A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY in the Department of Biology We accept this dissertation as conforming m the required standard Dr. P. T. Gregory, sjiipervi^r (Department of Biology) Dr. G. A. Allen, Departmental Member (Department of Biology) Dr. N_LiMneston,, Departmental Member (Department of Biology) Dr. E. A. Roth, Outside Member ^Department ofAnthropology) Dr. N. L. Staub, External Examiner (Biology Department, Gonzaga University) © Theodore M. Davis, 1996 University of Victoria All rights reserved, This dissertation may not be reproduced in whole or in part, by photocopying or other means, without the permission of the author, ii Supervisor: Dr. Patrick T. Gregory Abstract A fundamental aim of ecology is the study of patterns of distribution and abundance of organisms. These patterns can be influenced by intrinsic responses to environmental conditions, interspecific interactions, or both. If individuals of similar co-existing species use the same limited resources, competition can result in resource partitioning, but this pattern can also be the result of intrinsic differences. On Vancouver Island, British Columbia, two ecologically similar plethodontid salamanders, Plethodon vehiculum and Aneides ferreus, are each common only where the other species is uncommon. I described their distribution and abundance, investigated differential microhabitat use, and evaluated interspecific interactions between them. At each of nine sites I established arrays of six 0.3 x 2 m artificial cover objects (ACOs). Each ACO consisted of three boards arranged to create multiple microhabitats. ACOs allow sampling without disturbance of natural cover, provide a standard sampling unit, and minimize observer bias. In 1992 and 1993,1 checked 228 ACOs every other week, but less often in 1994. I also searched natural microhabitats and investigated distribution and abundance with time-constrained searches at 16 additional sites. I collected data on 2790 salamanders. At the northern sites, A. fe rre u s was relatively abundant compared to P. vehiculum, but the iii situation was reversed in the south. I found no differences in site characteristics that would explain this pattern. Salamander abundance was reduced in clearcuts, but there was no difference among old-growth, mature, and immature sites. The density of P. vehiculum in Coldstream Provincial Park was exceptional. In one area, surface density was 1.8 individuals/m2, but 200 m away there was <0.03 individuals/m2. From censuses in fenced plots elsewhere, I estimated that <24% of the P. vehiculum population was on the surface at any particular time. Thus, by extrapolation, there are at least 75,000 P. vehiculum/ha in one area of Goldstream Park. The density of P. veh icu lu m was <0.1 individuals/m2 across wide areas of forest habitat with occasional patches of higher density. Surface abundance wras correlated with the area of ground covered by coarse woody debris (CWD) and with surface moisture, and abundance varied by a factor of 12 over a distance of 50 m. I collected microhabitat data o.i 1306 salamanders. Of the A. ferreus, 95% were under the bark on logs or within logs. In. contrast, 67% of the P. v e h ic u lu m were under CWD on the soil and 20% were within logs. Aneides ferreus used logs in an early stage of decay, and P. vehiculum , when under bark on logs oi within logs, used logs in a late stage of decay. Under ACOs, 98% of the A . ferreus were found between boards, whereas 85% of the P. veh icu lu m were found on the soil under boards. Microhabitat use by Ensatina eschscholtzii and Taricha granulosa was iv similar to that of P. vehiculum, except that 23% of the T. granulosa were on the surface. In staged encounters, there was no aggression or predation between A . ferreu s and P. veh icu lu m . In laboratory and outdoor enclosures, microhabitat selection was not influenced by the presence of the other species. Thus, differential microhabitat use is due to intrinsic differences, and is not the result of interspecific interactions. The distribution and abundance of these species is not explained by interspecific interactions or site characteristics as measured in this study. Examination of habitat features at a finer scale might explain differences in distribution and abundance, but the requirements of these species could be so similar or correlated that differences might not be found. Additional sites need to be investigated to determine the detailed pattern of distribution of these species on Vancouver Island and adjacent islands. Examiners: Dr. P. T. Gregory, Su^brvisor (Department of Biology) Dr. G. A. Allen, Departmental Member (Department of Biology) Dr. N. Lfy/ngston, ^Dep^tmental Member (Department of Biology) Dr. E. A. Roth, Outside Member (Department of Anthropology) Dr. N. L. Staub| External Examiner (Biology Department, Gonzaga University) vi Table of Contents Abstract ..... b Table of Contents ..................................................................................................... vi List of Tables .......................... - ....................................x List of Figures............................................................................................................. xi Acknowledgments xi v Chapter 1: Introduction ...................................................... 1 Objectives..........................................................................................................7 Interspecific interactions among salamanders....................................... 11 N atural History....................................................................... 18 Clouded Salamanders (Aneides ferreus)............................................ 19 Western Red-backed Salamander (Plethodon vehiculum) 20 Ensatina Salamanoer (Ensatina eschscholtzii)................................... 21 Rough-skinned Newt (Taricha granulosa).......................................22 Chapter 2: Methods ............................................................. —25 Methodological Overview.........................................................................25 Artificial cover objects (ACO s)............................................................25 General methodology ...................................... 27 Detailed Methodology...............................................................................32 Study sites................................................................................................32 Artificial cover objects (ACOs)............................................................. 33 A C O P lots ................................................................................................37 Fenced Plots.............................................................................................40 Sampling Frequency ..............................................................................41 Area constrained searches(ACSs) of ACO plots ............................... 42. Handling of salamanders .....................................................................42 Statistical methods ...............................................................................,.43 vii Chapter 3: Population Ecology..............................................................................44 Methods.................................................................................... 45 Area-constrained searches (ACSs) of natural cover in ACO plots.............................................. 45 Body condition ..................................................................................47 Statistical methods ............................................................................48 Results...........................................................................................................49 Seasonal variation in surface abundance........................................ 49 Variation in abundance among sites...............................................59 Variation in abundance among species..........................................59 Population structure............... .......................................................72 Body condition ..................................................................................79 Movement across fences ..................................................................88 M o v e m e n t s................................................. 88 Population size in fenced and unfenced ACO plots....................... 89 D e n s ity ............................................................................................... 92 Discussion....................................................................................................... 95 Seasonal variation in surface abundance .........................................95 Variation in abundance among sites............................................... 97 Variation in abundance among species ..........................................98 Metapopulation effects .....................................................................99 Population structure .......................................................................101 Population size and density........................................................... 104 Chapter