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Ocybadistes knightorum 

Scope: Global
Language: English
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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Arthropoda Insecta Lepidoptera Hesperiidae

Scientific Name: Ocybadistes knightorum Lambkin & Donaldson, 1994
Common Name(s):
English Black Grass-dart Butterfly, Knight's Dart

Assessment Information [top]

Red List Category & Criteria: Endangered B1ab(iii,v)+2ab(iii,v) ver 3.1
Year Published: 2014
Date Assessed: 2014-06-02
Assessor(s): Andren, M. & Cameron, M.A.
Reviewer(s): Black, S.
Justification:
The Black Grass-dart Butterfly is assessed as Endangered. It has an extent of occurrence (EOO) of 312 km2, an area of occupancy (AOO) of 76 km2 and is known from three locations. There is continuing decline in the area, extent and quality of habitat and the number of mature individuals due to invasion of introduced weeds and this is predicated to continue in the future as climate change results in sea level rise. Fire, residential and commercial development and agricultural development are also threats.

Geographic Range [top]

Range Description:The Black Grass-dart Butterfly is a subtropical narrow-range endemic species of the New South Wales (NSW) north coast, Australia. The habitat is located in two regions, Sawtell (containing 94% of the habitat) and Warrell Creek. The habitat is generally very low-lying, with the exception of two headland occurrences. Most of the habitat (over 90%) is 1-2 m asl.

A simple minimum convex polygon that encompassed all the potential habitat mapped by Andren and Cameron (2012) was used to calculate the extent of occurrence (EOO). The EOO is 312 km2. This area contains some ocean and very extensive unsuitable areas such as those that have been highly modified by urban and rural development.

The Andren and Cameron (2012) mapping was also used as the basis for calculating area of occupancy (AOO). While potential habitat was mapped, since over 97% of it was found to be occupied by O. knightorum, all potential habitat was included in this assessment. The area was scaled up in order to ensure that  the assessment against the criteria thresholds was evaluated at the appropriate scale, as recommended in the IUCN guidelines (IUCN 2011). This was achieved by overlaying the habitat map with a 2 km grid and then counting each occupied 4 km2 grid cell. The  total area of habitat mapped by Andren and Cameron (2012) is 0.324 km2. When the 4 km2 grid was overlain, habitat was found to occur in 19 grid cells, equating to an AOO of 76 km2.

The mapping of Andren and Cameron (2012) identifies 293 habitat patches. Most of the patches are very small and poor or zero butterfly habitat value; O. knightorum was absent from 48% of them at the time of sampling. The total area (0.324 km2) is still minor when compared with most other species. Despite the discovery of a second population at Warrell Creek, the habitat remains highly concentrated, with about 75% of it located along a single 7 km stretch of Pine Creek. Sea-level rise is a clear threatening event that will impact O. knightorum. It will affect virtually all the low-lying patches by 2100. Only the two headland populations will not be impacted by this threat. However, each small headland population (in total only 0.4 ha in size or 1% of the total habitat) could easily be affected by a single threatening  event, such as fire, weed invasion or competition from native plants. Therefore, there are three locations.
Countries occurrence:
Native:
Australia (New South Wales)
Additional data:
Estimated area of occupancy (AOO) - km2:76
Number of Locations:3
Lower elevation limit (metres):1
Upper elevation limit (metres):2
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:Historical decline has not been documented, but can be inferred from the observed extent of weed invasion of habitat patches and the extent of urban and rural development throughout the range of the species (Andren and Cameron 2014).
Current Population Trend:Decreasing
Additional data:
Continuing decline of mature individuals:Yes

Habitat and Ecology [top]

Habitat and Ecology:Floyd's Grass (Alexfloydia repens) is the food plant of this species. At Sawtell, high tide is often 0.6 m to 0.8 m asl, and occasionally about 1.0 m asl during the largest king tides or High Higher Water Solstices Springs (HHWSS). Only 7% of  the O. knightorum habitat is within this zone of occasional HHWSS tidal inundation, strongly indicating a lack of tolerance by the food plant A. repens for highly saline conditions. A. repens is suspected not be able to easily migrate to higher elevations as the sea level rises. Firstly, the rise is very rapid in geomorphological terms and the alluvial terraces that the species currently occupies will not  have time to be reformed at higher elevations. Secondly, in many areas the fact that A. repens does not currently occupy the higher sites demonstrates that there are natural restrictions to its spread. Finally, there are many instances where higher elevations are occupied by weeds that are currently invading and would restrict migration (Andren and Cameron 2014).
Systems:Terrestrial
Continuing decline in area, extent and/or quality of habitat:Yes

Threats [top]

Major Threat(s): A clear threat to this species is from rising sea levels (New 2011), since the habitat of the species sits directly above the king tide mark. On the north coast of NSW, there is predicted to be a rise in sea level of 0.9 m above the 1990 level by 2100 (DECCW 2009). Although the science on which this estimate is based is rapidly-evolving, it was recently reviewed and still found to be adequate (NSW Chief Scientist and Engineer 2012). Assuming the assumptions hold (see Andren and Cameron 2014), 85% of the current habitat would be inundated or too saline for A. repens by 2100. Furthermore, the remaining 15% would be located in thin strips of  isolated marginal habitat that would be unlikely to be high quality for the butterfly. O. knightorum may struggle to survive such extreme habitat loss, as it has not demonstrated a capacity to migrate quickly to higher elevations.

There is also predicted to be an increase in the frequency, height and extent of flood events under the currently accepted climate change scenario (DECCW 2010). This is likely to impact the low-lying, riparian habitat.

Fire regimes may also be modified by climate change. Increased evaporation and drier conditions in winter are predicted to occur on the NSW north coast (DECCW 2010), which will lead to seasonal alterations in fire frequency and intensity. Immature stages of O. knightorum may be unable to escape intense fires (D. Sands pers. comm. 2010). Therefore, fires that occur early in the season (e.g. August or September) may result in severe impact if there are few butterflies in the adult stage that are able to escape. Some of the best habitat is associated with peat formations (Sands 1997), increasing susceptibility to fire. At least one patch has experienced a protracted burn consistent with a peat fire (M. Smith pers. comm. 2010, NPWS).

Butterflies may be affected by higher temperatures. Two woodland species have been shown to have lower longevity and fecundity in high temperature regimes (Karlsson and Wiklund 2005). The hotter predicted temperatures on the NSW north coast under climate change (DECCW 2010) may therefore also have a detrimental impact on O. knightorum.

The crucially important Pine Creek and Bonville Creek catchments (containing 79% of the known habitat) are subject to impoundment, although this has rarely occurred in the recent past. The creek mouth closed in 2012 for the first time in over 50 years and increased water levels in both catchments.

Invasion by introduced  weeds, particularly L. camara and P. mandiocanum, has previously been identified as a major threat to the habitat quality of O. knightorum (Braby 2000, Sands and New 2002) and in developed catchments there is a significant threat posed by anthropogenic disturbance (Andren and Cameron 2012). These threats continue to operate. In the Sawtell  population, L. camara occurred in 63% of patches, with significant infestations in 32% of patches, while P.  mandiocanum occurred in 50% of patches, with significant infestations in 23% of patches. Habitat quality is of fundamental importance for the maintenance of butterfly populations (Thomas et al. 2001, Wood and Pullin 2002, Dover and Settele 2009).

Some preliminary data was collected on the magnitude of the threat due to weed invasion of the two most serious weeds, Lantana Lantana camara and Broad-leaved Paspalum Paspalum mandiocanum. Anecdotal observations suggest that these weeds have actively invaded habitat patches (M. Smith pers. comm. 2010, NPWS, M. Andren pers. obs. 2012). Other weeds recorded less frequently included Ochna Ochna serrulata, Senna Senna pendula, Trad Tradescantia fluminensis, Plantain Plantago lanceolata and Rhodes Grass Chloris gayana.

Conservation Actions [top]

Conservation Actions: In 2000, Braby assessed  the species as Vulnerable at a national level,  largely due to its restricted distribution and the threat posed by introduced weeds (Braby 2000). Sands and New (2002) evaluated the conservation status in the Action Plan for Australian Butterflies. They also found  the species to be nationally Vulnerable, based on: (i) few breeding populations in protected areas; (ii) small number of populations (five known at the time) and small range of occurrence; and (ill) management that was  inadequate for reducing the risks of extinction in the longer term (Sands and New 2002). O. knightorum was not, however, listed under the national Environment Protection and Biodiversity Conservation (EPBC) Act 1999. Finally, it was assessed by the NSW Scientific Committee in 2002 and as a result listed as an Endangered species in NSW in that year. They found that the butterfly occurred in only a few small discrete patches (all threatened by weed invasion) and likely to become extinct unless the factors threatening its survival cease to operate (NSW Scientific Committee 2002). Conservation planning by the New South Wales State Government is under way for this species under their Save Our Species Program.

As a first step in developing a sea-level rise adaptation strategy, potential sea¬≠ level rise refugia need to be identified for the species. Improving the quality of habitat can be more important for butterfly conservation than, for example, increasing habitat size or connectivity (WallisDeVries 2004). To this end, it is vital that weeding programs are maintained and expanded. Research is also needed on the genetic variation within and between the two major and apparently isolated populations.

On  a more positive note, O. knightorum is a species where active management can play a successful role in securing its long-term persistence. The degree to which the habitat is contained  in protected areas is high (Andren and Cameron 2012) and the threat from weed invasion is being managed in some reserves where key patches have responded well to weeding, providing a number of luxuriant examples of successful habitat  rehabilitation.The existence of the headland  populations demonstrates that the typical swamp forest habitat is not obligatory for either A. repens or O. knightorum. This suggests that the fundamental niche of both species is considerably broader than   that realised across most of their current distribution. Translocation of A. repens has been successful on four occasions and O. knightorum occupies all four translocated patches (Andren and Cameron 2012). It is not known whether the butterfly was inadvertently translocated with its food plant, or later colonised the translocated patches. Regardless of the dispersal mechanism, the translocations are highly successful.

Citation: Andren, M. & Cameron, M.A. 2014. Ocybadistes knightorum. The IUCN Red List of Threatened Species 2014: e.T64004480A64004482. . Downloaded on 26 September 2017.
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