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Woodland Caribou

Limiting Factors

A Caribou TrailLimiting factors can be discussed in a strict scientific sense, or in more general terms. From the scientific perspective of population ecology, a limiting factor is anything that has a measurable negative effect on the population's rate of change. In a broader context, limiting factors may be seen as anything that negatively affects either population dynamics or habitat suitability. Changes in habitat quality or quantity may indirectly affect survival or reproduction of an animal and, as such, the two views of limiting factors are interrelated. Woodland Caribou, which naturally exist at low density and have low reproductive output, cannot recover from the effects of an array of limiting factors as quickly as species such as deer, elk or moose, which naturally maintain higher population densities and are more productive. Unfortunately, there are many limiting factors that may be affecting Woodland Caribou including: Predation, Habitat Loss and Alteration, Linear Corridors and Human Activity, and Weather and Climate.

Predation:  Predation is an important limiting factor for caribou populations. In order to reduce the impact of predation, Woodland Caribou remove themselves spatially from other ungulate prey. Caribou accomplish these spacing strategies through seasonal migrations and/or by being sparsely distributed in very large range areas that contain lower densities of alternate prey. Various factors, such as human intervention, can affect the vulnerability of caribou to predation by affecting caribou condition/behaviour or predator abundance/behaviour. Caribou have coexisted with wolves and other predators for thousands of years; however, human alterations of ecological relationships have important implications for the persistence of Woodland Caribou in Alberta. Interactions between predation and other factors such as habitat alteration (timber harvesting, linear corridors, alternate prey, etc.), human activity (recreation, road use, etc.), or weather are complex. Although survival and population growth of ungulates may respond to predator control, the results are not consistent, especially in multiple predator-prey systems. Furthermore, justification for predator control is increasingly difficult to achieve given changing societal values. 

Habitat Loss and Alteration:  Woodland Caribou require large tracts of old forest that contain lichens. Natural or human-caused disturbances can alter such habitat features significantly. Many biologists have postulated that habitat loss is a major limiting factor for caribou populations. True loss of caribou habitat probably only occurs as a result of permanent modifications of the habitat associated with land-use conversion. If forests altered by fires or logging were allowed to follow a natural successional path, most habitat "loss" would be better defined along a gradient of habitat alteration. However, if regenerating forests are subsequently scheduled for harvesting before they reach an age at which they can sustain lichens (and caribou), then habitat alteration effectively becomes habitat loss. 

Forest Fire Fire is the dominant force shaping the boreal forest of Alberta and has important implications for caribou populations. In the short term, fire is detrimental to caribou habitat, however, in the long term fire may be required to alter landscape vegetation characteristics, allowing lichen biomass to be maintained or increased. Conversely, forestry activity on caribou winter range in west-central Alberta is thought to have negatively affected mountain caribou populations. There is also concern that commercial logging may not be equivalent to wildfire in creating optimal conditions for the renewal of lichen growth.

Extensive oil and gas deposits underlie most Caribou ranges in Alberta. Very high levels of petroleum and natural gas exploration and development have taken place in most of Alberta's caribou management zones during the last 10 to 20 years, and the extent and intensity of this work has dramatically accelerated in recent years. Subsurface mineral rights are currently leased throughout most of the area of all caribou ranges in the province. In west central caribou ranges in particular, a large number of land parcels have recently been leased under the mineral sales process. Habitat supply is an obvious challenge for caribou conservation in west central Alberta, and applies to the boreal region if the direct or indirect effects of all industrial activity cause caribou to avoid heavily developed areas. Resource extraction in the form of forestry, petroleum and natural gas exploration and production, mining (coal, peat and potentially diamonds), and agricultural expansion all have the potential to negatively affect caribou in Alberta. The challenge for caribou conservation is to maintain sufficient quantities of suitable habitat through time within each caribou range, and not unduly increase predation pressure, in order to avoid local extirpation of caribou populations.

Linear Corridors and Human Activity:  Fragmentation of habitat by linear corridors such as pipelines, roads, seismic lines, transmission corridors or forestry cutblocks may have a number of effects on caribou movements, distribution, and survival. Corridors provide access for humans and predators to penetrate vast tracts of wilderness caribou range that formerly was not easily accessible. Licensed harvest of caribou has not been allowed in Alberta since 1981. Some small caribou groups northwest of Manning are known to have been recently eliminated because of hunting activity. Improved access into caribou range as a result of an expanding network of linear corridors in addition to expanding use of all-terrain vehicles could lead to increased legal and illegal hunting of caribou. Careful planning and regulation of access development within caribou range is needed to minimize these risks. 

Linear corridors or cutblocks may also affect caribou population dynamics by altering the movements and distribution of both predators and prey and increasing predation pressure on caribou. Caribou, other ungulates, and omnivorous predators such as bears may be attracted to the vegetation surrounding linear corridors and well sites. Corridors penetrating caribou habitat may thus allow for a significant increase in encounter rates between predators and caribou. 

Weather and Climate:  Weather may be considered a limiting factor through a complex set of interactions with caribou movements, habitat use, energetics, reproduction and survival, and as it may affect the abundance or distribution of other ungulates and predators. While caribou are well adapted to winter, conditions can develop that alter their behaviour, reproduction and survival.  Winter conditions in Alberta are not likely to negatively affect caribou condition, survival or reproduction. However, in winters with above average snowfall and/or severe crusting, caribou condition, reproduction and survival may be compromised. In addition to normal winter stresses, caribou body condition may be further reduced through movements to avoid extensive human activity or through reduced food intake. Variations in weather patterns may also exist over longer time and broader spatial scales. Warming associated with global climate change may alter habitat and caribou population dynamics through increased frequency/severity of forest fires, changes in snow conditions, changes to forage type/ quality/abundance, and altered predator-prey dynamics. Loss and/or change of Woodland Caribou habitat resulting from altered climatic regimes could potentially overshadow or exacerbate changes associated with industrial development. 

Reprinted from Alberta Wildlife Status Report No. 30 (2001), with permission from Alberta Sustainable Resource Development

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