The Importance of Body Size in Determining the Ecological Niche of the Black Rat Snake (Elaphe obsoleta)

Denise M. Roth

So…Why study snakes??

http://www.he-man.org/cartoon/cmotu-pop/snakemen1.jpg

http://img114.imageshack.us/img114/8529/snake2tp9.jpg

http://www.larry-adams.com/snake-check.jpg

Snake populations are declining!

Habitat changes

Pet trade

Habitat fragmentation

Senseless killing

“Never wound a snake; kill it.” – Harriet Tubman

“If you see a snake, just kill it - don't appoint a committee on snakes.” – Ross Perot

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http://magickcanoe.com/snakes/baby-gartersnake-large.jpg

Changes in Habitat Use and Movement Patterns with Body Size in Black Ratsnakes (Elaphe obsoleta)

Gabriel Blouin-Demers, Laura P. G. Bjorgan and Patrick J. Weatherhead

Question Addressed

Do black ratsnakes exhibit an ontogenetic niche shift?

In other words: Does body size influence movement patterns and use of habitat in black ratsnakes?

Hypothesis:

Juvenile black ratsnakes should move more often, move over longer distances and travel further from hibernacula.

Why does body size matter?

It influences thermoregulatory requirements

Habitat use

Susceptibility to predation

Diet

Black Ratsnake(Elaphe obsoleta)

Adult length 3.5-8 ft

Hatchlings 11-16 in

Live in or near woodlands (near water)

Good climbers: competition for tree cavities

Diet: Rodents and bird eggs

Reproduce in early summer

Oviparous- 12-20 eggs

Protected in neighboring states

Threatened in Canada

http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12201-61209--,00.html

Geographic Distribution

http://www.dlia.org/atbi/species/Animalia/Chordata/Reptilia/Squamata/Colubridae/Elaphe_obsoleta.shtml

Queen’s University Biological Station

50 km north of Kingston, Ontario, Canada

100 km south of Ottawa, Ontario, Canada

http://www.queensu.ca/biology/qubs/directions.html

http://www.dlia.org/atbi/species/Animalia/Chordata/Reptilia/Squamata/Colubridae/Elaphe_obsoleta.shtml

Study area: 30 km² ~36 Snakes/ 1 km²

Specimen Capture

Funnel traps Measured for Snout-to-vent length (SVL)

Weighed

Sexed

Marked with a passive integrated transponder taghttp://icwdm.org/Images/amph-reptile/snake-nonpoison/Nonpoi10.jpg

Distinguishing between adult and juvenile

JuvenileAdult

http://animaldiversity.ummz.umich.edu/site/resources/usfws/juvratsnake.jpg/medium.jpg

http://www.tamssunshinehouse.com/Snakes/blackrat.jpg

< 1050 mm SVL

MethodsImplanted specimens with radio-transmitters

Used movement data of adults from previous study (n = 82)

Additional 10 adults tracked

35 juveniles

Located snakes on foot and recorded location using GPS

Also recorded behavior observed

153 known juvenile locations

153 random locationshttp://www.sciencedaily.com/releases/2007/07/070721200335.htm

Classification of Variables and Statistical Analysis

Habitat Classification

Forest

Forest edge

Open habitat

MANOVA

Habitat selection based on the 23 structural variables

Analysis of Movement Patterns:

Mean distance from hibernacula during active season

Total distance travelled during active season

Number of times individual moved when relocated

Results: Habitat Use

Juveniles utilized all available macrohabitats but preferred forest

No significant difference in microhabitat use between chosen and random locations

Key note:

If juveniles are in dispersal stage, they are potentially less selective for previously visited locations

n = 306

P = 0.11

Results: Movement PatternsDistance Travelled vs. SVL

All individuals followed at least 3 months during active period

ANCOVA:

Relationship between the total distance travelled and SVL

Significant with SVL

P = 0.002

No significant relationship with sex

P = 0.99

Results: Movement Frequency vs. SVL

Do smaller snakes move more often?

Relationship between SVL and number of times snake had moved

Significant relationship with sex

P < 0.001

Significant relationship with size

P = 0.04

Conclusion

Movement patterns and behavior varied with body size

Larger snakes moved further distances

Juveniles moved more often

Juveniles and adults differed in habitat use

Juveniles visited sites at random

Adults seemed to have preferred habitats and locations

Discussion

Possible explanations for habitat differences

Thermoregulation strategies

Juveniles escaping predation

Juvenile dispersal

Future Research

Ontogenetic niche shifts in reptiles

Juvenile dispersal stages?

Is it related to body size of the individual?

Study different land configuration

Where do the neonates go?

Does habitat fragmentation actually benefit the adults?

What this means for the black

ratsnake

Ontogenetic shifts in habitat use do not seem to play a major role in species survival at this time and

conservation strategies should be focused elsewhere to have an impact in sustaining

populations in Ontario, Canada.

ReferencesBlouin-Demers, Gabriel, Bjorgan, Laura P. G. & Weatherhead, Patrick J. (2007). Changes in Habitat Use and Movement Patterns with Body Size in Black Ratsnakes (Elaphe obsoleta). Herpetologica, 63(4), 421-429.

Arendt, J. D., &  Wilson, S. (1997). Optimistic growth: Competition and an ontogenetic niche-shift select for rapid growth in pumpkinseed sunfish (Lepomis gibbosus). Evolution, 51(6), 1946-1954.

Harding, James H. (1997). Amphibians and Reptiles of the Great Lakes Region. 308-312.

Michigan Department of Natural Resources. Black Rat Snake (Elaphe obsoleta obsoleta). Accessed from http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12201-61209--,00.html

http://thinkexist.com/quotes/with/keyword/snake/

Ohio Department of Natural Resources: Division of Wildlife. Eastern Ratsnake or Black Ratsnake. Accessed from http://www.dnr.state.oh.us/Home/species_a_to_z/SpeciesGuideIndex/blackeasternratsnake/tabid/6556/Default.aspx

http://www.washjeff.edu/Chartiers/Chartier/KEY/Reptiles/Snakes/brat.htm

http://www.ext.colostate.edu/PUBS/natres/06501.html

http://www.statistics.com/resources/glossary/a/ancova.php

Questions

Description of Microhabitats

Distance from

Hibernacula vs.

Snout-to-Vent

Length

Area of Home Range vs. SVL

1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 19980

20

40

60

80

100

120

Average Number of Black Ratsnakes Found in Queen's Biological Station over a Period of 17 Years

Average ~ 65 snakes/ 1.8 km² (Blouin-Demers et. al 2002) This study (Blouin-Demers et. al 2007) was 30 km²

Based on previous data: Average in this study~1087 snakes/ 30 km ² or 36 snakes/1 km²