Some ecological factors influencing the breeding success of the Brenton Blue butterfly

The Brenton Blue butterfly, Orachrysops niobe (Trimen, 1862) (Lepidoptera: Lycaenidae), is endemic to the southern Cape and is currently listed as Endangered. This study looks at some of the key ecological factors influencing the breeding success of the species—host plant abundance and condition, nectar sources, climate/ microclimate, and vegetation management techniques. The adult butterfly population was monitored over an entire breeding season; host plants were identified and individually monitored; and egg counts were done. This enabled the effects of a number of different management techniques to be evaluated (burning, cutting, physical removal of invasive elements, and combinations thereof). A fivefold increase in the population of O. niobe was observed over the breeding season. This increase was positively correlated to a similar increase in host plant abundance in the areas where cutting and physical removal of invasive elements was practiced. Burning, by contrast, appeared to have a negative impact on host plant and butterfly abundance over the same period. Impacts of other factors such as climate, nectar sources and the natural strength of the second brood are discussed. A hypothesis, of megaherbivore activity as the principal historical disturbance mechanism promoting locally favourable conditions for O. niobe to establish and maintain colonies, is proposed. Recommendations for reserve management and future research are made.


Introduction
The Brenton Blue butterfly Orachrysops niobe (Trimen, 1862) (Lepidoptera: Lycaenidae) is one of South Africa's most threatened butterfly species, being endemic to the southern Cape and currently red-listed as Endangered (Henning & Henning 1995).It is now only known to occur at one locality -at Brenton-on-Sea near Knysna.It previously occurred in Nature's Valley 50 km to the east but has become extinct there as a result of anthropogenic activities (property development, habitat fragmentation, prevention of fires, and exclusion of larger herbivores).The colony at Brenton-on-Sea has been subject to similar pressures but has been given a reprieve from a similar fate following a protracted and highly publicised campaign to save the species (Steenkamp & Stein 1999).This has resulted in the procurement of the land on which the colony occurs by central government and the impending proclamation of a reserve which covers 1.47 ha.
Case histories from other endangered butterfly species (mostly from the northern hemi-sphere) have shown that it is essential to understand the ecology of each endangered butterfly species in detail if conservation efforts are to succeed (New 1997).
The aim of the current research is to determine some of the critical ecological factors influencing the breeding success of O. niobe.Since Edge & Pringle (1996) demonstrated by rearing adult O. niobe from ova that the larvae are essentially phytophagous the abundance and condition of the sole larval host plant Indigofera erecta Thunberg is one of the factors examined in detail.Several vegetation management techniques to promote proliferation of the host plant have been proposed by other workers (Lubke et al. 1996;Schutte-Vlok 2001), and their efficacy needs to be tested.Other factors dealt with in the present study are nectar sources and preferences; the influence of climate; and the population dynamics of O. niobe.

Study site
The study site is the Brenton Blue butterfly reserve, situated in Brenton-on-Sea near Knysna (34º04'20''S; 23º02'00''E) in the Western Cape Province of South Africa (Fig. 1).This figure is to scale and depicts areas that were treated as well as the network of paths that were cut.The reserve has a total area of 14 673 m².
The reserve is bounded to the north by a tarred road, W.K. Grobler Drive.To the north of this road is a public open space, Uitzicht 216 Portion 81, which is owned by the District Municipality.To the south, the reserve boundary is Fynbosoord, a tarred access road to the residential properties which lie south of Fynbosoord and at the south-eastern corner of the reserve.The reserve is bounded on the west by residential properties and stands, and Mountain Rose Drive.The eastern boundary abuts partly on residential property and partly onto public open space that does not presently form part of the reserve.
The study site is on a well-drained south (seaward) facing slope of average inclination of 1 in 4. It is at an altitude of 95 m to 120 m and is 450 m from the sea.The geology, climate and floristics of the site were described by Lubke et al. (1996).The vegetation can be characterised as asteraceous coastal fynbos with patches of candlewood-dominated thickets [Pterocelastrus tricuspidatus (Lamont) Sonder].

Habitat modification
Habitat modifications (treatments) were carried out in the areas indicated on the site plan (Fig. 1) and summarised in Table 1.The procedure for the burn on 20 September 2000 was described by Hiseman (2000).The "skoffeling" treatment applied to Area B involved cutting down of the bracken fern (Pteridium aquilinium (Linnaeus) Kuhn), and turning over the surface layer down to a depth of 0.2 m so that the roots of the bracken were pulled out.The intention was to simulate the activities of bush pigs, which it had been hypothesised by Schutte-Vlok (2001) played an important role in the past in controlling the bracken fern.The network of access paths (Area D) were cut during the period from 4-25 July 2001, using shears at a level of approximately 10 cm above the ground, to a width of 1-1.5 m.The paths wind to avoid larger bushes and trees and any patches of the host plant I. erecta, and have been subsequently kept open by pulling out any new bracken fern plants.The blocks contained within the paths (Area E) have not been disturbed, but difficulty of access has precluded any systematic host plant observations in this study.Area F consists of a steep embankment approximately 1.5 m high running east to west which resulted from excavations for a sewerage line in the early 1990s.

Indigofera erecta monitoring
Immediately after the fire careful watch was kept for any I. erecta seedlings.As soon as they appeared they were marked and identified with 1-m sticks and numbered tags.Thereafter, during host plant surveys, any new plant which had sprouted was similarly identified.These plants were given a letter following the number to show that they were not amongst the original cohort.
When the paths were cut all existing I. erecta plants exposed were also marked, as were the new ones that germinated and/or sprouted later on.No distinction was made as to whether the new plant had originated from seed or had resprouted from an old root stock.
ISSN 0075-6458 21 Koedoe 45/2 ( 2002) This was a subjective measure made by eye on a scale from 1 to 10, and took into account the overall size of the plant and its stolons, withering of leaves and stems, and whether the plant appeared to be spreading or shrinking.From July the size of each plant was recorded by counting the number of stolons and the number of inflorescences.A stolon, for the purpose of this study, is defined as an aerial shoot of the plant (horizontal or vertical) which originates near the root stock.Stolons vary in length from 150 mm to 500 mm.The inflorescences of I. erecta are vertical racemes that carry the flowers on sturdy peduncles.Figure 2 is a diagrammatic sketch of a typical plant (the detailed morphology of the root system is not yet known).

Adult butterfly population counts
In 2000 and 2001 experiments by the author with mark, release, recapture (MRR) techniques had shown that the trauma to the butterflies caused by this method posed an unacceptable risk to the small and fragile population.Consequently a transect method was devised, using the network of paths, to yield a comparative count of butterflies which could be used to assess population trends.This method required similar weather conditions (sunny, warm, little wind) and was carried out at the same time each day (11:30-12:30).The route through the network was the same for each count, at a fairly constant pace of 20 m/min., and took about one hour, allowing some time for stops to record significant observations such as nectar plant visitation, oviposition or mating.
During the population counts all adults seen were recorded with the place, gender and activity.Multiple sightings of a particular insect inevitably took place but each sighting was recorded.The observations for each day were recorded on a data sheet that has a schematic layout of the paths in the reserve incorporated and shows the traverse route (Fig. 3).
A second counting method was used which is referred to as the "fixed point" method.The observer was positioned at point A2 (which is on the normal male patrolling route) and recorded all O. niobe that came within 10 metres of this point during a 30 minute period (at the same time each day).
The fixed point method was "calibrated" on several separate occasions by capturing all the butterflies present and retaining them in a gauze netting cage.This enabled a relationship to be derived between the "fixed point" count and the total population of butterflies patrolling (referred to as the repetition factor for this method).The repetition factor for the transect method could only be estimated (as between 1.25 and 2.0).The butterflies on the paths were much less mobile than the patrolling males mostly seen from the fixed point and the observer moved quickly along each path, reducing the chance of repeat observations of the same insect.Observations made during the transects that fell within the "fixed point" area (defined as the three paths observable from the fixed point A2) were treated as if they were made during a fixed point count for the purpose of applying a repetition factor and calculating the actual butterflies present.

Adult butterfly behaviour
During the adult butterfly counts and the plant surveys, all noteworthy adult butterfly behaviour was recorded.Categories were defined as fluttering (typical slow female flight within 15 cm of the ground or vegetation); patrolling (more vigorous male flight up to a metre from the ground or over vegetation); mate search and acquisition (males only); nectar plant visitation (plant species and butterfly gender recorded); host plant search; oviposition (plant number recorded); copulation (time and location recorded); basking, and resting.
ISSN 0075-6458 23 Koedoe 45/2 (2002) Early stages (eggs, larvae) All the identified host plants were searched thoroughly during the period from 10-25 November 2001 and again from 28 January-1 March 2002 for eggs and larvae.The search method involved examining both sides of each stolon.In order to standardise the search about one minute was taken to search each stolon.When eggs or larvae were found the host plant number was recorded with the count and any unusual features (e.g., eggs were usually laid on the lower surface of a leaf-so if they were in another position this was recorded).Eggs were recorded as unhatched (pale blue, entire) or hatched (white ring of eggshell remnant).

Climate and microclimate
Rainfall records from the previous ten years were obtained from a weather station located at 140 Watsonia Road, Brenton-on-Sea.This station is situated at 90 m altitude, 400 m from the sea, and 500 m away from point A1 in a westerly direction and is also sited on a south-facing slope.
An additional weather station was established at the site (position near A1 as shown on Fig. 1) and daily recordings commenced in June 2001.Data recorded was mm rainfall using a conical rain gauge and the maximum and minimum temperatures using a thermometer.
The only microclimate factor recorded during the present study was a subjective estimate by eye of the

Indigofera erecta monitoring
The number of living I. erecta plants located and the number of stolons counted in each treatment area at different dates is given in Table 2, and a summary of the I. erecta microclimate and population dynamics data gathered appears in Table 3.

Adult butterfly population counts
A summary of the data obtained using the two comparative methods (transect and fixed point) is presented in Table 4.Each day's count using the two methods has been standardised to a count per hour so that the figures for each day are directly comparable.No butterflies were observed between 21 November 2001 and 10 January 2002.
Table 5 presents the total counts from the two methods taken over six days, during the peak emergence of each brood with favourable weather conditions.These results are another assessment of the relative strength of the two broods.factor of 1.25 for the transect method).The repetition factor for the fixed point method was determined to be 6.6.The overall male to female ratio in the colony falls within the range from 1.2 to 1.5.If a repetition factor for the transect method of 2.0 is used the male to female ratio falls within the range 1.6 to 1.8.

Oviposition
The I. erecta plants preferred for oviposition were mostly healthy and vigorous, on well drained sloping ground and in semi shade with bare ground in the immediate vicinity of the plant.
The eggs were usually laid on the underside of leaves on a horizontal stolon close to the ground.The eggs were therefore well concealed and in a cool, damp microclimate.
Only occasionally (3.2 % of observations) were eggs laid on the upper surface of the leaves, on stems (as opposed to leaves) or on vertical stolons (as opposed to horizontal stolons).The egg counts (H = hatched and U = unhatched) for each brood are presented in Table 7.The data from Tables 7 & 2 can be used to calculate egg densities (per plant and per stolon) for each treatment area as shown in Table 8.

Climate
The weather data in Table 10 was obtained from the 140 Watsonia Road weather station (rainfall); the weather station at the reserve (rainfall and temperatures); and the Knysna records of S.A. Weather Services (SAWS).
The Watsonia Road and reserve stations agreed for rainfall records over fourteen months to within 1.5 %.There is also good agreement between the Watsonia Road station and SAWS over the longer term.

Overall comparison of the burnt areas versus the paths
Drawing the data together from the various sources in this study the summary compiled in Table 11 was produced.

Reproduction and growth of Indigofera erecta
The genus Indigofera is large, with 78 species occurring in the Cape Floristic Region (CFR) and 720-730 worldwide (Goldblatt & Manning 2000).Schutte-Vlok ( 2001) hypothesised that the germination of I. erecta was principally fire induced, and used this as one of the main arguments for management of the reserve by regular controlled burns.Jeffrey et al. (1997) found that not all legume species tested germinated in response to dry heat treatment and hypothesised that species found in dune or forest communities, which were less fire prone, required other stimuli to break seed dormancy.Cocks & Stock (1997), in a study of germination stimulation of 16 Fabaceae species, found that response to fire varied and seemed to be correlated with certain seed characteristics.
Observations and experimentation on the host plant of Orachrysops ariadne (Butler, 1898) -which is Indigofera woodii H.Bolus var.laxa -indicated that whilst fire and heat seemed to play a role in seed germination, seedlings emerged in unburned areas as well as burnt areas.(Lu & Samways 2002b).
The evidence accumulated in the present study indicates that the germination of I. erecta is stimulated not only by fire as expected (Table 3-areas A and C-"new plant" column) but also by surface disturbance and by removal of competing plants to admit more light (access paths-area D).Effects similar to this could have been caused naturally in the past by megaherbivores creating paths for access and passage, and by their browsing and grazing on the plants along the paths.Elephants may have occasionally visited the area in the past and their partiality to many typical fynbos plants as well as P. tricuspidatus, and the rhizomes of P. aquilinium has been recorded recently in fynbos areas adjacent to the Knysna forest (Milewski 2002).It is interesting that these two species have a tendency to proliferate at the study site in the absence of any control agent(s).
At the study site the predominant growth form of I. erecta is depicted diagrammatically in Fig. 2. Most of the vegetative mass is contained in the horizontal stolons, which are ISSN 0075-6458 up to 500mm long and spread along the ground in a horizontal or downward direction, depending on the slope of the ground.Vertical stolons are much less frequent and appear to be a response to there being greater ground cover by competing plants in the vicinity.Since the stems of I. erecta are herbaceous the height of the vertical stolons is limited to 100mm by their mechanical strength unless support is gained from neighbouring plants, in which case a height of a metre or more can be attained.Leaves are sparser on these vertical stolons compared to the horizontal stolons, which are supported by the ground and therefore able to bear many more leaves.
The growth form described above is markedly different from the host plant of O. ariadne which is an upright plant growing to over a metre and which has woody stems (Lu & Samways 2001).This type of growth form can compete successfully with taller plants and does not require bare ground to grow vigorously which appears to be the case with I. erecta.
The influence of growth form can be seen by comparing the response of I. erecta to fire as opposed to cutting and clearing around the plant.The I. erecta plants that sprouted after the fire did well at first, while the ground was bare and there was not much shade.However they were out-competed later as the ground became covered with grass and sedge species and the taller more vigorous species (P.aquilinium, Helichrysum spp, Rhyncosia spp, etc) shaded out the smaller I. erecta plants.Consequently growth of both horizontal and vertical stolons was inhibited and not only did a high proportion of I. erecta plants die during the observation period (refer to Table 3), but also the overall increase in the number of stolons was low.
By contrast the I. erecta plants which were present on the paths, together with those that sprouted subsequently, had a much lower mortality and a far greater rate of increase in the number of stolons (refer to Tables 2 & 3).This shows clearly that growth of I. erecta was encouraged to a far greater extent by the activity (disturbance) of cutting the access paths rather than by the fire.
It is noteworthy that parts of the reserve area were regularly disturbed in the 1980s and early 1990s by the property owner -who used to cut down the taller plants every few years to gain access for surveying.Escom also used to keep the strip of ground along their power servitude just below the present W. K. Grobler Drive open for maintenance access.These sort of activities may have been critical sustaining factors for the butterfly at the site over the last two decades.

Orachrysops niobe adult population fluctuations
The author has been keeping records of adult population counts dating back to 1993.In that year, using a method similar to but not as rigorous as in the present study, a maximum count of 10/h was obtained for the November brood using a fixed point observation method.In November 2000 the maximum count was 16/h also using a fixed point method, and in November 2001 the maximum count was 15/h.It can therefore be stated with reasonable confidence that the November population of adult O. niobe has not shown any significant decline over the eight year period, and if anything seems to have increased.There are undoubtedly many other factors which control the population of O. niobe, such as predation and parasitism in the larval stage; the influence of the larval-ant relationship and the availability of sufficient ant colonies in proximity to the host plant concentrations.These factors are beyond the scope of the present study but will be the focus of future investigations.

Oviposition preferences of O. niobe
All members of the genus Orachrysops so far studied in any detail appear to specialise on a single larval host plant in the genus Indigofera (refer to Table 12).
In the case of the four colonies of O. ariadne studied by Lu & Samways (2001, 2002a) they found that an abundance of the host plant was an essential pre-condition for a colony to become established and to be maintained.
The abundance of I. erecta at the Brenton Blue butterfly reserve is greater than at any other place on the Brenton peninsular, and indeed greater than all other sites investigated for the presence of O. niobe over a number of years.(Ball 1997) An earlier study of oviposition sites found that the host plant and egg densities varied across the reserve (Britton & Silberbauer 1997), but were generally positively correlated.A similarly patchy distribution of host plant has been found in the present study, with the females concentrating their oviposition activities in areas containing the most host plant.
In the present study, both in the November and February broods the majority of the eggs were laid on the host plants growing on the paths, and the density of eggs per plant and per stolon were also much higher on these host plants.
The preference of the females of O. niobe for oviposition on host plants on or near the paths as opposed to plants in the burnt areas could have the following possible causes: -The very high density of host plant in some areas of the paths attracted the females by sight and smell.-Predominance on the paths of horizontal stolons close to the ground which are preferred by the females.-Accessibility of the host plants on the paths -no need to flutter between other plants.-Health and vigour of the host plants on the paths (see Table 3).-More favourable microclimate for larval survival may exist along the paths (sheltered, cool and humid).-Presence of an (as yet unknown) host ant in these areas.
It was concluded from a study of the habitat of the endangered Lycaenid Aloeides dentatis dentatis (Swierstra, 1909) that this butterfly had a preference for a disturbed community in an early stage of succession, which seemed to encourage colonisation by the host ant.(Deutschländer & Bredenkamp 1999).
Several  The dominance of forbs and thus nectar plant visitations in the burnt area is probably because the woodier shrubs had not yet gained ascendancy in the succession process.

Influence of climate and microclimate
From Table 10 it can be seen that rainfall was well below the average for previous years during the period from October 2001 to March 2002, which is the emergence time for adult O. niobe.The increase in butterfly population that has been recorded in this study took place despite the adverse dry and hot conditions.It should be noted that moisture appears to be a critical factor in all Orachrysops species studied so far (Pringle 1997;Lu & Samways 2001) From a microclimate point of view, the shade factor and the angle of slope are two of the most important factors.It can be seen from Table 3 that the butterfly larval host plant does better in intermediate shade (approx.40 %) situations.Generally the area where the paths were cut had a steeper slope than the burnt area and this made for a cooler microclimate as well as good drainage.

Overall comparison of burning versus cutting
The summary in Table 11 clearly demonstrates that the cut access paths created a superior habitat for both I. erecta to grow and for O. niobe to breed.The burnt area did however provide more male O. niobe activity, principally because it included the current patrolling path of the males.Another important attraction and contribution of the burnt area was as a nectar source because of the prevalence of nectar rich forbs in the early successional stages after a fire.This may have some significance in the overall ecology of the butterfly and needs to be taken into account.

Conclusions
The germination of the host plant Indigofera erecta is not just fire induced.There was a doubling of the number of host plants on the access paths (area D), where no fire had taken place, during the observation period.Indigofera erecta plants in the burnt areas A and C, and on the embankment (area F) had a higher mortality rate than those on the paths.The growth rate of the plants on the paths -measured by increase in the number of stolons -was an order of magnitude higher than in the burnt area.
Orachrysops niobe females had an overwhelming preference to lay eggs on the I erecta plants on or near the access paths and almost completely avoided the plants on the burnt area.There was a significant increase in the population of the butterfly between the first brood and the second brood-three to four times as many adults, and nearly five times as many eggs laid.
Orachrysops niobe uses a wide range of nectar sources, most of which are abundant at the reserve and not likely to be a limiting factor in controlling the population, although the higher abundance of nectar sources in the burnt area may have some significance.
The rainfall during this season has been significantly below the historical average so this factor could not have caused the population increase.The evidence suggests rather that both the proliferation of the host plant on the access paths and the increase in the butterfly population came about as a result of the access path cutting and maintenance activities.The areas that were burnt have produced relatively small numbers of I erecta plants on which hardly any eggs were laid.Under natural conditions the type of disturbance caused by the cutting of paths is similar to that created by large and small herbivores grazing.Burning is likely to cause a short-term negative impact on butterfly populations since it not only does not create conditions suitable for breeding, but the danger exists of larvae perishing in any fire occurring during the months from October to April.

Recommendations
Burning should only be carried out at the reserve in areas where the host plant is absent.
The network of paths should be maintained and kept open through the next breeding season in order to continue monitoring of the host plant and to re-assess the butterfly population in November 2002.
The morphology, reproductive biology (specifically the germination stimuli) and phenology of Indigofera erecta should be studied in detail.
The existence of a larval-ant association needs to be confirmed and investigated to find out if this could be a regulating factor on the populations of the butterfly.

Fig. 3 .
Fig. 3. Data sheet incorporates a schematic layout of the paths in the reserve and shows the traverse route.

Figures 4
Figures 4 and 5 present graphically the population fluctuations over a 21 weeks period from 17 October 2001 to 12 March 2002, using the two methods.

Table 1
Summary of treatment areas Surveys of all the identified I. erecta plants were conducted at regular intervals (19 March, 9 May, 18 July, 25 July, 10 October, 10-25 November 2001 and 28 January-1 March 2002).During the March and May surveys the health of each plant was recorded.

Table 2
Plants and stolons of Indigofera erecta counted in each area betweenMarch 2001 and February 2002 a Start date for areas (C), (A) and (F) was 19 March 2001, and for area (D) was 25 July 2001.

Table 3
Shade factors, population and growth records of I. erecta in each treatment area a Defined as: (Final stolon count -initial stolon count) / (Initial stolon count) x 100

Table 5
Orachrysops niobe adult population counts for six days at the peak of the November 2001 and February 2002 broods using two counting methods

Table 6
Calculation of O. niobe sex ratios during the November 2001 and February 2002 broods (male and female)

Table 7
Orachrysops niobe egg counts in each treatment area during November 2001 and January/February 2002 (H = hatched eggs and U = unhatched eggs)

Table 8
Orachrysops niobe average egg densities per plant and per stolon in each treatment area during November 2001 and January/ February 2002

Table 9
Nectar sources and visitations by O niobe adults a M = male and F = female b A = actinomorphic and Z = zygomorphic Summary of data comparing the burnt areas with the paths a from January 1992 to August 2002 b from July 2001 to August 2002 c from January 1951 to August 2002Table 11 The biology and phenology of I. erecta have not yet been studied as far as can be ascertained.Two other Indigofera species have been identified at the site (I.verrucosa and I. glaucescens Ecklon & Zeyher).

Table 12
Larval host plants of Orachrysops species