Skip booklet index and go to page content

Recovery strategy for the Eastern ribbonsnake (Thamnophis sauritus), Atlantic population in Canada

2. Threats

2.1 Classification of known and potential threats

Like other aspects of eastern ribbonsnake biology, threats are not well understood. Known and potential threats have been identified in Table 3 and ranked according to the level of concern in each threat category. The occurrence, frequency, certainty, and severity of most of these threats remain unknown and there may be additional threats to ribbonsnake populations that have not yet been identified. The assessment of threat information presented in Table 3 is based on documented research (see References Cited) or expert opinions from members of the Eastern Ribbonsnake Recovery Team.

 

Table 3 (A. Threat category: Changes in ecological dynamics or natural processes): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Lack of informationInaction or inappropriate action (inability to recognize threats; delayed implementation of recovery actions; initiation of actions with unexpected detrimental effects)mortality (Increase);
Degradation of habitat
WSCNAHUH
2. Small, isolated concentrations of snakes within the NS rangeSmall population effects (inbreeding; genetic drift; increased susceptibility to stochastic events) due to isolation of populations resulting from human influencesmortality (Increase);
fecundity (Decrease)
UUULUH
3. Cottage and residential developmentAlteration of natural disturbance regimes and functional processesAlteration of movement patterns;
Alteration of genetic structure
UUULUM

 

Table 3 continued (B. Threat category: Habitat loss or degradation): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Dam construction or removal (human or beaver)Footnote aAlteration of water level and seasonal water flow regimes; long-term cumulative habitat lossChange in habitat availability;
Winter mortality;
Change in prey availability;
Local extinction
LCULUH
2. Cottage and residential developmentHabitat degradation, fragmentation, and loss; alteration of water level and seasonal water flow regimes; alteration of shorelines; long-term cumulative habitat lossHabitat availability (Decrease);
mortality (Increase);
movement between concentrations (Decrease);
local extinction
WSCRMUH
3. Road density and new road constructionHabitat fragmentation; long-term cumulative habitat lossmortality (Increase);
isolation of concentrations;
WSCUUUH
4. Climate changeFootnote aAlteration of water level and seasonal water flow regimesChange in habitat availabilityWSACLUM
5. Forest harvesting practices (i.e., clear cutting, harvesting in the riparian zone, rotation times)Alteration of water level and seasonal water flow; decrease in water quality; habitat degradation, fragmentation, and loss; long-term cumulative habitat loss; potential disruption of overwintering siteshabitat availability (Decrease);
mortality (Increase);
movement between concentrations (Decrease)
WSCULUM
6. Agricultural practices (i.e., tilling, crop production, cranberry growing)Alteration of water level and seasonal water flow; decrease in water quality; habitat degradation, fragmentation, and loss; long-term cumulative habitat losshabitat availability (Decrease);
mortality (Increase);
movement between concentrations (Decrease)
WSUULUM
7. Infilling/ draining wetlandsHabitat degradation, fragmentation, and loss; long-term cumulative habitat loss; loss of wetland functionmortality (Increase);
habitat availability (Decrease);
local extinction
LCMMHH
8. Peat miningHabitat degradation, fragmentation, and loss; long-term cumulative habitat lossmortality (Increase);
habitat availability (Decrease);
local extinction
LAUMUL

 

Table 3 continued (C. Threat category: Accidental mortality): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. On and off road vehiclesVehicular mortality; increased encounters with peoplemortality (Increase)WSCCMUH
2. Cottage, residential and road developmentVehicular mortality; increased encounters with people; increased predation by petsmortality (Increase)WSCUMUH
3. Agricultural and forestry equipmentVehicular mortalitymortality (Increase)LCULUL

 

Table 3 continued (D. Threat category: Disturbance and persecution): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Negative perception of snakesIntentional killing by humans resulting from hatred or fear of snakesmortality (Increase)UUULUL
2. Cottage, residential and road developmentIncreased encounters with people, pets and machinery; disturbance to snakes and habitat featuresmortality (Increase);
fecundity (Decrease)
WSUULUL

 

Table 3 continued (E. Threat category: Exotic or invasive species): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Exotic predatory fishIntroduction of small mouth bass and chain pickerel; changes to ecosystem functionmortality (Increase);
competition for prey (Increase);
altered prey composition
UCULUH

 

Table 3 continued (F. Threat category: Climate change): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Climate changeFootnote aShifts in seasonal temperatures (reduction of heat units for development of young; increased winter mortality); alteration of water level and seasonal water flowChange in fecundity;
Change in prey availability;
Winter mortalityFootnote a
WSACLUM

 

Table 3 continued (G. Threat category: Natural Processes or activities): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Human activities and developmentIncrease in natural predator populationsmortality (Increase)UUULUL

 

Table 3 continued (H. Threat category: Pollution): Classification of known and suspected threats to eastern ribbonsnake recovery in Nova Scotia
General ThreatSpecific ThreatPotential stressThreat Information
Ext.Footnote bOcc.Footnote cFrq.Footnote dC. C.Footnote eSev.Footnote fL. of C.Footnote g
1. Pesticide use (i.e., landscaping, crop and agricultural production, forestry)Direct and long-term exposuremortality (Increase);
prey availability (Decrease);
fecundity (Decrease)
UCULUL
2. Air pollutionAccelerated effect of methylated mercury impacting the food chainmortality (Increase);
prey availability (Decrease)
UCULUL

Footnotes

Footnote A

These factors in particular could have a positive or negative effect on ribbonsnake populations.

Return to footnote a

Footnote B

Extent: WS = widespread; L = localized; U= unknown

Return to footnote b

Footnote C

Occurrence: C = current; A = anticipated; U = unknown

Return to footnote c

Footnote D

Frequency: C = continuous; R = recurrent; U = unknown; NA = not applicable

Return to footnote d

Footnote E

Causal Certainty: H = high; M = medium; L = low

Return to footnote e

Footnote F

Severity: U = unknown; H = High

Return to footnote f

Footnote G

Level of Concern: H= high; M= medium; L=low

Return to footnote g

 

2.2 Description of “specific” threats

The following descriptions highlight the “specific” threats as outlined in Table 3, emphasizing key points and providing additional information on the threats. As there is little published literature outlining threats to ribbonsnakes, most information presented here deals with potential threats and effects, as assessed by the Recovery Team.

While the threats below and in the table are listed separately, an important concern is the long-term cumulative effect of a variety of threats on the eastern ribbonsnake population. As human development increases, the overall impact of activities on the habitat becomes more serious at both the small and larger scales. At the landscape scale, incremental losses and multiple threats often complicate recovery efforts and confound understanding of population trends (Jensen et al. 1993).

Lack of information for decision making

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) status report identified lack of information as the greatest current threat to ribbonsnake recovery (Smith 2002). Little is reported on eastern ribbonsnake ecology or threats anywhere in its range. What is reported elsewhere is not necessarily applicable to the Nova Scotia population. Up until now, there has not been an inclusion of Aboriginal Traditional Knowledge and community knowledge in recovery. At this point, it is not known what factors limit eastern ribbonsnakes in the province (geographically or demographically), how healthy the population is, or what level of population fluctuations are normal. This could prevent recovery managers from recognizing population decline or identifying significant threats. In fact, it is not known if the effect of many of the potential threats listed below will be positive or negative (especially water level and climate change). Lack of information could also result in delay of recovery actions because of uncertainty of potential detrimental effects, or risk of initiating actions that may have unanticipated negative effect(s).

Small population effects

The Nova Scotia eastern ribbonsnake population is geographically isolated from the species’ main range, which prevents any possibility of in-migration or a natural rescue effect. Although the full extent of the range within Nova Scotia remains unknown, it does appear to be restricted to a small number of watersheds in the southwest region of the province. This places the snakes at risk of small population effects such as inbreeding and genetic drift, and increases the population’s susceptibility to stochastic events. These risks are further exacerbated if the Nova Scotia population actually consists of numerous isolated sub-populations with little or no movement between them.

Alteration of water level and seasonal water flow

Alterations to water level and seasonal water flow are anticipated to be among the most significant threats to ribbonsnake habitat. Hydrological changes can result from many activities, including installing or removing human-made dams, removing beaver dams, forest harvesting practices, agricultural practices, infilling of wetlands, altering shorelines and climate change. Even a small change in flooding regimes could result in vegetation succession in previously wet meadows, or in preventing vegetation from establishing. While these changes to the terrestrial and aquatic habitat could affect all aspects of ribbonsnake life history, the impacts on feeding and overwintering sites are of particular concern.

Effects from either stabilizing or destabilizing water levels could be positive or negative for ribbonsnakes. We do not yet have sufficient understanding to be able to predict the type or scale of impact resulting from particular activities. Two of the sites containing the highest known densities in the province (Grafton Lake and Cobrielle Brook) have had human-made dams removed within the last decade. The destabilizing impact of natural beaver dam cycles may also be important for the local ribbonsnake population.

Habitat degradation, fragmentation, and loss

Habitat degradation, fragmentation, and loss can occur from many activities, including cottage and residential development, road construction, agricultural practices and forest harvesting practices, and infilling wetlands. Cottage development is of particular concern as it could result in alteration of shorelines, destruction of habitat, and fragmentation. Expansion of the road network could also result in fragmentation as well as direct mortality from vehicles and increased encounters with people and pets. In turn, fragmentation could result in isolation of individual concentrations or increased risk to individuals as they travel between concentrations or between seasonal habitats.

Changes to water quality (e.g., pollution, eutrophication, siltation) may also pose a threat to ribbonsnakes, particularly those that affect prey abundance. Based on limited data, adult ribbonsnakes in Nova Scotia appear to prey primarily on amphibians and small fish (Bell et al. 2007, NS ribbonsnake database 2010); the diet of neonate ribbonsnakes, however, is largely unknown. The extent to which prey availability limits population numbers, as well as the degree of dependence of ribbonsnakes on specific prey items, are also still unknown.

Vehicular mortality

Vehicular mortality is one of the few direct sources of mortality that has been documented in Nova Scotia. This includes mortality from both automobiles and all terrain vehicles. It could also include farm or forestry equipment and lawn mowers, although these are probably a lesser threat at the present time. Vehicular mortality may be particularly significant if snakes must cross roads to travel between overwintering and summering sites, or if they are drawn to roads to bask. Since 2004, nine incidences of vehicular mortality have been recorded in Nova Scotia (8 on roads, 1 on an all-terrain vehicles (ATV) trail). Most of these reports have been incidental and not the result of targeted road surveys, and predators are likely to pick up any road kill snakes very quickly. The true incidence of vehicular mortality is likely to be considerably higher. This is particularly disturbing given the relatively low volume of traffic in the area. As development increases, vehicular mortality is also expected to increase.

Introduction of exotic predatory fish

The introduction of exotic predatory fish into areas containing ribbonsnakes is of particular concern. As a result of introductions, the ranges of both smallmouth bass and chain pickerel have been expanding in Nova Scotia, including the southwestern region of the province. These species can alter trophic dynamics, which could negatively impact ribbonsnakes through either direct predation or competition. Predatory fish have been shown to dramatically reduce populations of small fish and affect amphibian abundance (Vander Zanden et al. 2004, Jackson 2002). Both of these exotic fish species are more tolerant of higher water temperatures than many of the native fish, thus increasing their potential effect on ribbonsnake prey populations (Vander Zanden et al. 2004).

Shifts in seasonal temperatures associated with climate change

Climate change may affect ribbonsnakes in Nova Scotia, and the impact may vary according to the different aspects of their life history. It is not known whether the overall effect on ribbonsnake populations, if any, will be positive, negative, or neutral. Having said this, if the effects are negative they could be very significant.

In addition to affecting water level, prey abundance, and habitat as described above, climate change may also directly impact ribbonsnake physiology. Ribbonsnakes in Nova Scotia occur near the northern limit of the species range (Bleakney 1951). Each year they spend 4-6 months in overwintering sites and have only 6-7 active months to feed and reproduce. While it might appear that an increase in annual temperatures will benefit this southern species, this may not necessarily be the case. Changes in seasonal temperature and precipitation conditions may affect growth and development of young, particularly if it results in cooler, wetter weather during the active season.

Winterkill can be a significant source of mortality for other species of Thamnophis (Shine and Mason 2004). The significance of winterkill has not been documented in ribbonsnakes, although one dead eastern ribbonsnake was found at the top of a hibernaculum in Michigan (Carpenter 1953). Snakes are at risk of freezing and dehydration over the winter and may be dependent on specific conditions inside the hibernacula. Altered winter climates may directly impact the severity of winterkill.

Increased predation associated with human activities

Snakes are at risk of predation from both natural predators and domestic pets. Predation by domestic pets has been documented in Nova Scotia (one documented cat kill and several other anecdotal reports) and is expected to increase as cottage, residential, and road development increase. Natural predators and predation rates remain unknown and until these are determined, it is difficult to identify potential anthropogenic effects on these predators and their subsequent effect on ribbonsnake predation.

Intentional killing and disturbance resulting from increased encounters between humans and snakes

Many people fear or dislike snakes and some people will kill any that they see, particularly if the snakes are found near or in their homes While intentional killing has been a significant threat in other snake species at risk (Eastern Massasauga Recovery Team 2002), and may be the cause of death of one ribbonsnake found with extensive wounds in Nova Scotia in 2007, it is believed that the overall risk is relatively low for Nova Scotia ribbonsnakes at present. People rarely venture into wetlands where ribbonsnakes are typically found, and even when they do, these snakes are cryptic and difficult to catch. Because ribbonsnakes are strongly associated with wetlands, they are not as commonly encountered on lawns or in houses as some other species in Nova Scotia. However, with increasing development near wetlands, encounters by people will become more common and the threat may increase. It is not known how effective education will be in mitigating this threat.