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Recovery Strategy for the Eastern Prickly Pear Cactus (Opuntia humifusa) in Canada

4. Threat Identification

The three extant native populations of the species at two locations (two populations at Point Pelee National Park [PPNP] and one at Fish Point Provincial Nature Reserve [FPPNR]) face three significant threats: loss of habitat through succession, loss of suitable habitat through the alteration of natural disturbance regimes and collection (Table 1).

Table 1: Threat classification table.

ThreatExtentCausal CertaintyOccurrenceFrequencySeverityOverall Level of Concern
Vegetation successionWidespreadHighCurrentContinuousHighHigh
Alteration of natural disturbance regimesWidespreadHigh (shoreline populations)Current (shoreline populations)ContinuousHighMedium
CollectionWidespreadHigh (FPPNR), Medium (PPNP)CurrentRecurrentMediumMedium

4.1 Loss and Degradation of Suitable Habitat

The loss of suitable habitat detailed below places the few, small, Canadian Eastern Prickly Pear Cactus populations, already living in a dynamic and harsh environment, in danger of complete loss due to stochastic (random) events.

4.1.1 Vegetation Succession

Suitable habitat for Eastern Prickly Pear Cactus has been lost or degraded due to vegetation succession. Alteration of the natural disturbance regimes can result in direct competition with other plants as succession proceeds. Eastern Prickly Pear Cactus is intolerant of such competition. This is the most imminent threat posed to the species and of the highest concern. This threat is considered current and severe at all three extant, native populations.

4.1.2 Alteration of Natural Disturbance Regimes

Shoreline erosion is leading to the loss of individuals and habitat, caused in part by interference with Lake Erie sediment transport dynamics (COSEWIC 2000). Eastern Prickly Pear Cactus individuals have been relocated away from the shoreline at Point Pelee National Park in the past to prevent their loss from storm action (Klinkenberg and Klinkenberg 1984).

4.2 Collection

The collection of whole specimens representing genetically unique individuals, or their parts, for horticultural purposes has, and continues to pose a threat to native Canadian populations at both sites, particularly the small population at Fish Point Provincial Nature Reserve (Canadian Parks Service 1991, COSEWIC 2000). Two instances of collection or attempted collection were reported in 2009 at Point Pelee National Park (V. L. McKay pers. obs., L. Ritchie pers. comm. 2009).

4.3 Other Threats

The following threats are considered to be potential or historical and are thought to be of low concern in terms of population-level effects to the Eastern Prickly Pear Cactus at this time.

  • Disease: Monitoring of the Point Pelee National Park population demonstrated that clusters of cactus with more than 75 stems having 70 percent or more shading suffered from an unspecified blight that decreased the health and survival of the cactus (Chiarot 1992).
  • Non-Indigenous, invasive species:Non-indigenous, invasive plants represent an important threat to the habitats in which Eastern Prickly Pear Cactus is found. Daylily (Hemerocallis species), Spotted Knapweed (Centaurea maculosa) and White Sweet-clover (Melilotus alba) have been noted to competitively exclude Eastern Prickly Pear Cactus (Canadian Park Service 1991). Other invasive, woody species advance rates of natural succession.
  • Loss of genetic integrity through the introduction of non-native genes: Should cacti that are not of local provenance be planted in areas immediately adjacent to the native populations, cross-pollination could potentially result in the alteration of the native genetic makeup.
  • Loss of genetic integrity through genetic isolation occurring in small populations: Genetic isolation can lead to random fluctuations in the frequency of the appearance of a gene in a small population, presumably owing more to chance rather than natural selection. This can affect the genetic makeup of the population, leading to differences from the main population (founder effect). The effect of this genetic drift is more pronounced in small isolated populations and can result in reduced fitness.
  • Off-trail traffic: Point Pelee National Park experiences high rates of visitor traffic during peak seasons. These visitors do not always remain on trails. Soil compaction, the introduction of exotic species and trampling of species at risk result (Geomatics International Inc. 1994). The effect of this perceived threat remains to be clarified and quantified. While increased pedestrian traffic may have its impacts, such disturbances may also contribute to the maintenance of the open nature of the Lake Erie Sand Spit Savannas ecosystem upon which Eastern Prickly Pear Cactus depends.
  • Introduction of non-indigenous, invasive species, including exotic pests and diseases: This concern relates primarily to a potential, future invasion of the Cactus Moth (Cactoblastis cactorum), a South American species native to Argentina, Paraguay, Uruguay and southern Brazil. The larvae feed on Opuntia species' cactus pads and resulting wounds and secondary pathogens are often fatal to the plant (Zimmermann et al. 2000). Although it has recently appeared in Florida, the Canada Food Inspection Agency does not consider this tropically adapted species to be a risk as it is felt that it would be unable to survive the harsh Canadian winter. However, impacts could be catastrophic for Eastern Prickly Pear Cactus if this is not the case.