Saturday, June 25, 2011

ANNUAL REPORT: Quantitive study of the ecology and distribution of Tasmanian snake population at Wesley Vale

Annual Report, as submitted to the Animal Ethics Committee, Hobart, Sept. 2010

Project Title:   Quantitive study of the ecology and distribution of Tasmanian snake population at Wesley Vale, and their relationship to prey species   
  •  Chief investigators:  Ian Norton, Underwood, Jane Guy, Perth, TAS.
  •  Approval date of initial application:   9th Sept 2008
Status of project  Multi-year project 
  •  Period covered by this report: From: 2009   To: 2010
  •  Specific aims of the project
This is the only long-term, detailed population study of elapid snakes being undertaken anywhere in the world and is therefore of international significance. 
Apart from research conducted by Terry Schwaner on island populations of tiger snakes off South Australia and Tasmania in the 1980’s and 90’s, this is the only longitudinal study of elapids in Australia’s history. 

To date 300 snakes (254 Copperheads, 43 tiger snakes) have been PIT-tagged at the Wesley Vale study site. This represents 60 field trips, and many thousands of dollars investment in the project. 
The snake population at Wesley Vale is highly clumped around the perimeter of ephemeral wetlands. This is greatly significant to the research as it means the population can be treated as closed. As predicted for such a population, we are recording a high level of multiple recaptures of individual snakes through time. This demonstrates high individual fidelity to the site and allows the gathering of remarkable data that is shedding light on the ecology and population dynamics of snake populations on the Tasmanian mainland for the first time.

At approximately the half way mark of our proposed 10year study, our mark recapture data indicates a total population size of up to 1200 copperheads at our study site. If this trend continues it would represent one of the densest population of snakes recorded anywhere in the world.  

For the first time ever, our data is shedding light on the survival of individual wild snakes. The probability of survival of our marked individuals (Jolly-Seber analysis) during the sampling period ranged from 1.0 to 1.7 (average 1.3) indicating a high probability that an individual marked at the beginning of the study was still alive at the end of the study. For the first time ever, we are able to determine the growth rates of free-living snakes in Tasmania. So far our data demonstrates a clear trend in growth rates of individuals grouped into 50mm size classes rom 650mm to 1150mm SVL (snout to vent length).

Smaller (younger) individuals grow more rapidly than larger ones. The regression of growth rate on SVL for all recaptured individuals indicated that SVL accounted for 60% of the variance in growth rate (F= 108.2, df = 72, P= 0.0000). Smaller (650mm SVL), and presumably younger snakes are growing at 0.5213mm/day (190mm per year). Larger (1150mm SVL), and presumably older snakes are
growing at only 0.0758mm/day (27mm per year).

Continuing the field research as well as future plans of rearing neonates under laboratory conditions to determine growth of juveniles, will allow us to quantify the entire growth and longevity history of copperheads and tiger snakes in this population.

Copperheads at the study site display a diet composed entirely of frogs of three species with spotted marsh frogs comprising the bulk of these. Tiger snakes are eating frogs but also introduced rodents (rats/mice).

The reproductive cycle of females is, at best, biennial. Presumably the ‘costs’ of reproduction in females,(loss of fat bodies/condition) results in them having to ‘take a year off’ to recoup body fat.

The copperhead population appears to be highly skewed with males outnumbering females 7 to 1. We as yet have no firm explanation for this, but doubt that sampling error is responsible as both sexes consume the same prey and are highly clumped around the lagoon perimeter where the bulk of their amphibian prey is present.

The copperheads at the study site are strongly sexually dimorphic with males growing much larger (30%) than females. So great is the difference in body size that it is one of the most male favoured sexually dimorphic snake populations ever examined. Male copperheads are known to engage in combat bouts for access to reproductive females. Presumably intense competition between males for a limited number of sexually receptive females in any given year has resulted in selection for greater male length and muscle mass, as the dominant males engaged in combat bouts would be expected to obtain more matings.

Snake body temperatures (Tb) are independent of ambient temperatures (Ta). Snakes are actively foraging with Tb’s more than twice that of Ta (eg. 14°C Ta, 32°C Tb). Snake Tb’s are more closely correlated with substrate temperature (Ts). When Ts attains or exceeds 34°C snakes cease foraging in the open and retire to shade (usually dense button grass clumps) where they remain until Ts subsides in the late afternoon/early evening.

At the halfway mark of the study the amount of data coming in is truly remarkable and it would be very desirable to continue the research. This has become particularly pertinent this season, as it is the second year of our study that there has been a drought. The lagoons are dry for the second time and it will be extremely important to record how this impacts on the snake population in terms of activity
patterns, feeding patterns, growth, reproduction and survival. Such research may eventually become increasingly valuable in providing base line data on aspects of a wetland ecosystem during the projected global warming event that data from around the world is continuing to indicate.   We propose to:
  1. Continue to PIT tag and collect data from individual snakes as we have done for the past seven years                           
  2. Seek funding for future radio tracking of individual snakes of both sexes to more precisely quantify the day-to-day movements, activity patterns and temperature relations of the snakes.                         
  3. Raise newborn neonates of both sexes under laboratory conditions to quantify growth rates from birth to 650mm SVL. (snout vent length) There is no way of quantifying juvenile growth rates under field conditions as snakes in that size class are extremely difficult to find and cannot be PIT- tagged.                          

Modifications to the original project that have been approved by the AEC

  1. 11/3/2003, Blood sampling to resolve possible Rickettsial effects on snakes; this has been the only additional request since the project was first approved. No additional modifications since 2003.

Progress  achieved
This research project has turned up surprising results and may well be one of the most important indicators of the impact of climate fluctuations/change on coastal marshland snake populations. The sudden decline of the population following three years of unseasonal drought will help determine the sustainability of what for many years has been considered a secure and vibrant population. Expected increased rainfall, may well see a resurgence in population and if so will answer questions as to their resilience. 

We need to determine whether this has been a seasonally induced cycle, and the resilience of the snake population to either reestablish or rebound. Whether the attrition is part of a natural cycle, or whether this shift marks a serious long-term decline. Only by continuing to monitor this location can we ever hope to determine its ecological sustainability. It’s a bit like the canary in the coal mine and may well demonstrate the impact of long-term drought or a shift in unusual climate events. 

Summary of the next steps of the project

The intent over the next period of approval is to monitor only, with no addition to the numbers marked to date. We wish to capture and scan for recaptures and record the relevant data to determine whether the attrition was terminal or migratory. There are other habitats located on the property that have not been overly affected by drought. It’s intended that we investigate these locations as refuges, including the initial research site, to determine an outcome one way or the other. Is the population resilient? Do the snakes translocate when under extreme duress?  Evidence suggests that the large copperhead snakes were unable to maintain their body mass as the resource declined. This is a predictable outcome, reflected in the skeletal remains of large snakes found towards the end of summer. Smaller body mass offers the best chance of sustainability where resources aren’t infinite.  

Problems which interfered with the progress of the project

A three-year drought that dried the marshland. Snake capture rates barely register, due to a massive decline in the population. Therefore little can be surmised as to what has happened to the population. Natural attrition, or migration? Subsequent rains have revitalized the area with a return of marsh weed.

Changes envisaged for this project,
Animal welfare is a prerequisite; no animals are harmed during this research. In future there would be a decreased impact on individual snakes as they will be captured and data only taken. This will only apply to micro-chipped snakes; new snakes will not be recruited but released immediately with no further imposition.
Number of animals used during the period covered by this report
Species & Strain (common and scientific names)   Number used    (in last 12 months)   Number killed  (in last 12 months)   Accumulated total number used for the project to date   Total number approved for the project,   Source of Animals,  Location of Project Activities Undertaken

  • Tiger Snake Notechis scutatus   # Used/ 1 # Killed / nil  # Caught/ 52       Wild caught – Wesley Vale 
  • Copperhead Ausrelaps superbus   # Used/ 3 # Killed / nil    # Caught/ 261   Wild caught – Wesley Vale
  • White-lipped Snake Drysdalia coronoides  # Used/ 0  # Killed / nil   # Caught/ 3 Wild caught – Wesley Vale
Additional Information/ Comments
  • The period that you report on here should be the most recent 12 month Discrepancies between the number of animals approved and the number used – Not applicable
  • Unplanned/unintentional deaths/euthanasias or unexpected effects on the welfare of the animals used in this project – Nil 
  • Did the AEC (Animal Ethics Committee) receive notification of these deaths or occurrences?     No
  •  Was this project monitored by the AEC in the period covered by this report?     Yes 
  • This project has been running since 2001 with AEC approvals and permits.

Chief Investigators: Ian Norton & Jany Guy     21 November 2010

1 comment:

TDschaner said...

Jane and Ian,

Great work in looking at pop size and growth rates. The results are similar to what I'm finding for Mt. Chappell snakes - what's taken so long is that I've analyzed the data there using hand calculations for Jolly-Seber and survivability. However just this past semester a student and I started learning and using the computer program MARK so we could publish the results with greater precision. Let me know if you want us to run the results using that program. TDS