A bimodal pollination system enhances reproductive potential of translocated populations of an endangered grassland succulent

Abstract

A major concern during the translocation of higher plant species is related to habitat suitability and the availability of pollination services. Should these not meet the requirements of the plant, then successful reproduction and establishment cannot occur. We studied an endangered succulent, Frithia humilis , which had previously been translocated to typical and atypical habitats, to assess the occurrence of potential pollinators at these sites. Insects visiting F. humilis flowers and showing signs of pollen were captured, preserved and studied using a scanning electron microscope. Pollen of F. humilis was searched for. Abundance and diversity patterns of these pollen carriers across edaphic habitats of translocated populations were compared with those in a natural occurring population. Pollination success of guilds was compared amongst translocated F. humilis populations by considering the number of seedlings in a new season. Across F. humilis populations, Hymenopteran species had the largest pollen loads, making this a Melittophilous pollination system, typical for the Aizoaceae. Additionally, Anemophilous syndrome was described for translocation sites which provide reserve pollinators. Fruit formation was more favourable in suitable edaphic habitat on Ecca sandstones. Presence of seedlings at both translocation sites was indicative of successful pollination events of the self-incompatible species, but recruitment was proportionally higher in suitable habitat. Habitat suitability, in the case of this threatened species, is more of a limiting factor than pollination services after a translocation event.

Conservation implications: Translocations are fast becoming an attractive alternative for developers. This study cautions that the presence of pollinators and successful reproduction in translocated populations are only effective if the populations were translocated to an ideal habitat.

Introduction

Frithia humilis Burgoyne is an endangered, cryptic, dwarf succulent in the Aizoaceae (Burgoyne & Krynauw 2005; Burgoyne, Smith & Du Plessis 2000). It is endemic to the Rand Highveld Grassland of Gauteng and Mpumalanga in South Africa, specifically the area between Bronkhortspruit, Ogies and Middelburg. Here it is restricted to flat sandstone plates of the Dwyka and Ecca formations in microhabitats comprising aggregates of weathered rock and organic materials (Burgoyne & Hoffman 2011). The succulent leaves of the species generally grow from beneath the soil surface (a typical window plant), seldom protruding more than 20 mm above ground level. The leaves are contractile, allowing the plant to retract into the soil where it is protected from desiccation and frost in the winter months (Burgoyne et al. 2000). Consequently, leaves of this species are only visible during active growth, specifically when flowering during the summer months ( Figure 1a).

FIGURE 1:

Frithia humilis plants in flower (a) and displaying a fresh (left) and an expired flower (right) on the same plant (b).

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In 2008, a population of F. humilis was discovered at a coal mine in the north of eMalahleni after a mining license had already been granted (Harris et al. 2014). In situ conservation was impossible because of the destructive nature of open-cast mining practices. Considering the species’ endangered status (Burgoyne & Krynauw 2005), translocation was regarded as a last resort to save the population (Godefroid et al. 2011; Gordon 1994). Translocation is the process whereby a population of living organisms is deliberately moved from one area to another suitable habitat within its existing distribution range (IUCN 2013). Consequently, conservation agencies translocated the population to pre-selected receptor sites (Burgoyne & Hoffmann 2011).

A rapid pre-translocation habitat assessment was conducted to assess the suitability of receptor sites in terms of substrate, ecological integrity, genetic impacts and long-term protection (Burgoyne & Hoffmann 2011). A major critique of such rapid assessments is that in-depth assessments of pollinators are generally lacking (Forup et al. 2008), putting the translocated population at risk, especially in the case of a self-incompatible species such as F. humilis (Harris, Van den Berg & Siebert 2016). However, it could be argued that by the very nature of a translocation the number of plant individuals should be high enough to attract pollinators from elsewhere in the distribution range and provide for their dietary requirements (Menz et al. 2011).

The translocation of F. humilis provided an ideal opportunity to test whether the pollination guild of the translocated populations is similar to that of a naturally occurring population. Knowledge of this provides a measure to gauge whether a population is only reproducing because of chance pollination, or it persists because of effective pollination (Montalvo et al. 1997). A poorly functioning pollination system can have several genetic consequences (Armstrong & Seddon 2008; Montalvo et al. 1997; Moritz 1999) and may sooner or later affect population viability, eventually leading to population extinction (Kearns & Inouye 1997).

Johnson (2010) has called for pollination research in South Africa to improve our understanding of the role of pollination niches for the maintenance of plant species diversity in a rapidly changing environment. This study heeds the call to supplement existing knowledge concerning the pollinators of F. humilis (Harris et al. 2016). It highlights the importance of pollination studies in translocation initiatives and provides insight into the status of a biotic factor that could limit reproductive success after a founder population was established in a harsh environment.

Flowers of Frithia humilis

The flowers of F. humilis are 15 mm–20 mm in diameter and white with a yellow centre ( Figure 1a, b). The petals, especially towards the tip, are occasionally tinged pink. Flowers are either carried on very short stalks or are entirely stalkless. The pollen is yellow and tricolpate ( Figure 2). The sepals resemble the leaves and form a short tube (Burgoyne et al. 2000). Frithia flowers are self-incompatible (Burgoyne et al. 2000), a common characteristic of South African Aizoaceae (Mayer & Pufal 2007). After pollination the flowers turn yellow or pink before expiring ( Figure 1b), which may serve as an indication to pollinators that the flower is pollinated and without reward (Burgoyne et al. 2000).

FIGURE 2:

Scanning electron micrographs of Frithia humilis pollen grains in its distal polar-equatorial view (a) and polar view (b).

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Based on familial pollination syndromes (Hartmann 1991), F. humilis flowers could be Melittophilous because they are wide open, petals are shiny, whitish and open diurnally (from mid-morning to mid-afternoon) for several days. Pollen is less abundant than in most Melittophilous species but easily accessible. Further support for this syndrome is the self-incompatible sexual phase, with flowers reported to have a distinct male phase (protandrous) followed by a female phase approximately 4 days later (Hammer 1995; Hartmann 1991). Stigmata are initially shorter than stamens, elongating and emerging only at the beginning of the female phase when the male phase ends.

Research method and design

Study area and site selection

Two sites were chosen as receptor sites based on pre-selected criteria (Burgoyne & Hoffmann 2011) for plants translocated from a coal mine ( Figure 3). Half of the affected population was translocated to typical Ecca Group sandstone which corresponded with the geology of the donor site. The other half of the population was translocated to less ideal outcrops of the sedimentary Wilge River Formation of the Waterberg Group to test whether other related rock types could serve as an alternative should mining activities warrant future translocations (Harris et al. 2014).

FIGURE 3:

Localities of the receptor sites (Ecca sandstone at Goedvertrouwdt Farm and sedimentary rock at Eagle’s Rock Private Estate) and control population at Ezemvelo Nature Reserve on Ecca and Dwyka sandstone in South Africa. The donor population at Inyanda Coal Mine is indicated and the shaded grey triangle represents the approximate endemic distribution of Frithia humilis. Distances between study populations are indicated.

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Insect observations

Qualitative observations for pollinators were made at the receptor sites of Goedvertrouwdt and Eagle’s Rock, and a large, natural population at Ezemvelo Nature Reserve. As the flowers open only on sunny days, observations were made on non-overcast days of March 2016 and insects that visited the flowers were collected by hand. One observation session (of 3.5 h) was performed per day to coincide with the flowering time of the species for each population, on 3 suitable days ( Table 1). Three 1-m² patches with most flowers were visually estimated and demarcated at each site for observation. One person per patch observed for pollinators for the entire period of anthesis from 11:30 to 15:00. Thus 10.5 h of observation was conducted per population. Insects alighting or foraging on flowers were captured using an insect net and those species observed for the first time were euthanised in a killing jar with ethyl acetate. These specimens and a set of duplicates were then stored in individually marked containers for later identification and pollen assessments. Repeat visitors of the same species were only counted and released.

TABLE 1

Overview of time spent and flower visitors recorded during the survey.

Locality	Mean number of plants per 1 m²	Total hours of observation	Number of flower visitors	Number of species
Eagle’s Rock	62	10.5	6	4
Goedvertrouwdt	54	10.5	14	6
Ezemvelo Nature Reserve	429	10.5	26	4
Total	-	31.5	46	9

Note: Daily observations were made between 11:30 and 15:00. Three observations were made per site.

Verification of pollen and insect identification

All euthanised specimens were photographed with a Nikon AZ1000 stereomicroscope to assist in identification and scanned for pollen grains. Particular attention was paid to the head and legs, where pollen was most likely to be found. Those specimens carrying pollen were sputter-coated with a thin gold-palladium layer and inspected under an FEI Quanta FEG 250 scanning electron microscope (SEM) and micrographs of relevant pollen loads were taken. Duplicate insect specimens were pinned and labelled as prescribed by Uys and Urban (2006) and submitted to the Biosystematics Division of the Plant Protection Research Institute of the Agricultural Research Council (ARC) for identification ( Appendix 1). Identification to species level was hampered by the taxonomic impediment whereby large numbers of species still require classification or have gone undiscovered (Eardley, Gikungu & Schwarz 2009). Verification of F. humilis pollen was based on pollen micrographs in Burgoyne et al. (2000).

Plant counts

In each of the three subplots per population, all the adult plants and flowers were counted according to the technique described by Harris et al. (2014). The fruits were counted per subplot 30 days later. To estimate seed set per capsule in each subplot, 20 capsules were removed from plants bordering on the subplot (after 6 months) and the seeds were counted to determine the mean number of seeds per capsule. The sites were revisited after the first follow-up rains in October 2017 to count the seedlings per subplot.

Ethical considerations

Ethical clearance was obtained from the North-West University. This article followed all ethical standards for research without direct contact with human or animal subjects.

Results

Pollinator diversity

A total of 46 insects were captured whilst alighting on F. humilis flowers. These comprised nine different species of which six were hymenopterans and three were dipterans ( Table 2). No lepidopterans were captured during this survey although species were observed making darting visits to Frithia flowers. Harris et al. (2016) did record two butterfly species alighting on Frithia flowers but these were void of pollen.

TABLE 2

Frithia humilis pollen carriers with collection locality and pollen load.

Order/Family	Species	Common name	Pollen placement	Pollen load	Study sites	Source
Diptera
Bombyliidae	Exoprosopa eluta (Loew)	Bee fly	Eyes and head cavity	Low	E(1), G(5), ENR(8)	Harris et al. (2016)
	Notolomatia (Greathead) sp.	Bee fly	Eyes and proboscis	Low	G(1)	Reported here
Syrphidae	Paragus (Latreille) sp.	Hover fly	Body and anus	Low	E(1)	Reported here
Hymenoptera
Apidae	Amegilla fallax (Smith)	Banded bee	Head, legs and eyes	Medium	G(2), ENR(9)	Harris et al. (2016)
Halictidae	Lipotriches (Gerstaecker) sp.	Sweat bee	Pollen baskets and legs	High	E(2), ENR(4)	Reported here
	Seladonea (Robertson) sp.	Sweat bee	Pollen baskets and legs	High	G(2)	Reported here
Megachilidae	Megachile niveofasciata (Friese)	Dauber bee	Abdomen	High	G(3), ENR(5)	Harris et al. (2016)
Sphecidae	Ammophila (W.Kirby) sp.	Thread-waist wasp	Mouth parts	Medium	G(1)	Reported here
Vespidae	Quartinia (André) sp.	Pollen wasp	Head, body and legs	Low	E(2)	Reported here

E, Eagle’s Rock Private Estate; G, Goedvertrouwdt Farm; ENR, Ezemvelo Nature Reserve.

Note: Numbers in brackets per study site indicate the number of pollen carriers successfully captured and identified per site in March 2016. Low, <10; medium, 10–100; high, >100 pollen grains.

At Goedvertrouwdt, 14 individuals belonging to six species were recorded, with Exoprosopa eluta (Loew) (Hymenoptera: Bombyliidae) being the most frequent visitor (five records) of Frithia flowers ( Table 2). Species of Lipotriches (Gerstaecker) (Hymenoptera: Bombyliidae) and Quartinia (André) (Hymenoptera: Vespidae) were recorded the most at Eagle’s Rock Private Estate ( Table 2). At Ezemvelo Nature Reserve, 26 individuals (four species) were captured, with Amegilla fallax (Smith) (Hymenoptera: Apidae) and E. eluta accounting for 65% of the visitations ( Table 2).

Pollen carriers

Harris et al. (2016) already provided evidence that the bees Amegilla fallax and Megachile niveofasciata (Friese) (Hymenoptera: Megachilidae), together with the bee fly E. eluta, carry F. humilis pollen. Scanning electron microscope evidence reported here was, therefore, specifically aimed at the five newly captured flower visitors and also on recaptured Lipotriches sp. previously identified as a potential pollen carrier by Harris et al. (2016).

Frithia pollen was found in varying amounts on the selected insect specimens and these species are therefore considered pollen carriers ( Table 2). Some pollen grains were visible on Notolomatia sp. ( Figure 4), on and immediately adjacent to the mouth parts. Although only three pollen grains were found on the abdomen of Paragus sp. ( Figure 5), a significant clump of Frithia pollen was found directly on the anus of the specimen. Pollen found on the Quartinia sp. ( Figure 6) was sparsely distributed over its entire body. Copious amounts of pollen were found beneath the mouth parts of the Ammophila sp. ( Figure 7). Halictidae specimens (Seladonea sp. and Lipotriches sp.) ( Figures 8 and 9) were carrying copious amounts of Frithia pollen, particularly in their pollen baskets.

FIGURE 4:

(a) Stereomicrograph of Notolomatia sp. (Bombyliidae). Scanning electron micrographs of several Frithia humilis pollen grains on the eyes (b) and proboscis (c).

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FIGURE 5:

(a) Stereomicrograph of Paragus sp. (Syrphidae). Scanning electron micrographs of Frithia humilis pollen grains above the anus (b), on the body (c) and on the anus (d).

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FIGURE 6:

(a, b) Stereomicrographs of Quartinia sp. (Vespidae). Scanning electron micrographs of several Frithia humilis pollen grains on the body (c) and the head and antenna (d).

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FIGURE 7:

(a, b) Stereomicrographs of Ammophila sp. (Sphecidae). Scanning electron micrographs of Frithia humilis pollen grains on the underside of the mouth parts (c) and on a maxillary palp (d).

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FIGURE 8:

(a) Stereomicrograph of Seladonea sp. (Halictidae). (b, c) Scanning electron microscope micrographs of Frithia humilis pollen grains in the pollen baskets.

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FIGURE 9:

(a, b) Stereomicrograph of Lipotriches sp. (Halictidae). (c, d) Scanning electron micrographs of Frithia humilis pollen grains in the pollen baskets.

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Pollination system

The two translocation sites and the control populations were situated in an area of approximately 200 km². Within this area, three bee species (A. fallax, Lipotriches sp. and M. niveofasciata) were recorded from at least two of the sites ( Figure 10) and a bee fly species, E. eluta, was recorded from all three sites ( Table 2). The 39 flower visits by these four species accounted for 85% of all recorded visitations. This indicates that the pollination system of F. humilis is dominated by generalist bee pollinators that carry medium to high loads of pollen. Although the bee fly was a more regular flower visitor (14 visits), its pollen load was low ( Table 2). Reserve pollinators were observed at Goedvertrouwdt and Eagle Rock Private Estate ( Figure 10). These pollinators were mostly flies and wasps and generally had medium to low pollen loads. A sweat bee (Seladonea sp.) was only recorded from Goedvertrouwdt and had a high pollen load. These five reserve pollinators accounted for 15% of flower visitations.

FIGURE 10:

Pollination system of translocated Frithia humilis populations (G; E) in relation to a control population (ENR).

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Pollination success

When mature plants, flowers, fruits and seedlings were compared across sites per 1 m², Ezemvelo Nature Reserve had significantly higher numbers, as would be expected for a natural population ( Table 3). However, no significant differences were recorded between the typical and atypical habitats. Plants from Ezemvelo Nature Reserve had the highest number of flowers as a percentage of total number of mature plants, with fruit set as a percentage of flowers the highest for the sites at Goedvertrouwdt and Ezemvelo Nature Reserve ( Table 3). As this study was conducted 7 years after translocation, it may be assumed that the seed bank would make a minor contribution to recruitment and that recent seed production is a consequence of recent flowering seasons. Based on this premise, seedling emergence at Goedvertrouwdt receptor site as a percentage of seed numbers was considerably higher than that of the control population and is indicative of viable seed ( Table 3). Considering the 7-year time span since translocation, the presence of six pollinators ( Figure 10) and seedlings, it can be deduced that effective pollination is taking place.

TABLE 3

Mean number of mature plants, flowers, fruit, seed and seedlings per 1 m² for receptor sites and the control, as well as the flowering, fruiting and seeding percentage in brackets.

Locality	Mature plants	Flowers	Capsules	Seed	Seedlings
G	52 ± 18 a	11 ± 8 a (21%)	3 ± 1 a (27%)	438	62 ± 39 a (14%)
E	78 ± 33 a	25 ± 9 a (32%)	2 ± 1 a (8%)	292	18 ± 14 a (6.2%)
ENR	398 ± 163 b **	184 ± 82 b ** (46%)	42 ± 27 b * (26%)	6132	185 ± 112 b * (3.1%)

Note: Mean seed set per capsule: 146 ± 12.. One-way analysis of variance; Unequal N Tukey test: **p < 0.005; *p < 0.05.

Superscripts that differ denote significant variation

E, Eagle’s Rock Private Estate; G, Goedvertrouwdt Farm; ENR, Ezemvelo Nature Reserve.

Discussion

Flower visitation and pollen carriers

Exoprosopa and Notolomatia of the Bombyliidae are primarily Afrotropical in occurrence (Greathead & Evenhuis 2001). Larvae are either predators or parasitoids, whilst adult flies in most genera are adapted for nectar feeding. Female flies also feed on pollen for reproductive purposes and many of them have special adaptations for the collection of pollen (Greathead et al. 2006). Bombyliidae are most active during sunny middays that coincide with the anthesis of F. humilis. Little is known about their role as pollinators; it is generally believed that most of them are generalists (Szucsich & Krenn 2002) although some specialised relationships are known (Johnson & Steiner 1997). The small number of pollen grains on the Notolomatia specimen could be a consequence of the species or individuals (the inherent interested in pollen), the gender of the specimen (male flies may not be as interested in pollen, if at all) and the fact that the specimen was captured as soon as it landed on the flower, thereby shortening its potential visitation time.

Larvae of the hover fly genus Paragus (Diptera: Syrphidae) are recognised as aphid predators (Hayat & Claussen 1997), whilst adult flies are nectar feeders. Female flies consume pollen because it is a rich protein source necessary for reproduction (Haslett 1989). Even if pollen is consumed and digested by all syrphids, it still retains its shape after passing through the gut, making identification possible (Holloway 1976). This explains the large cluster of pollen grains from F. humilis and other plant species on the anus of the specimen. The specimen only foraged for a short period before it was captured. Although the pollen load was low on the insect itself, the pollen cluster on the anus suggested that it had recently fed on F. humilis pollen.

The Halictidae, represented by the species Lipotriches and Seladonea in this study, is the second largest family of bees with over 3500 members. Many of them are pollen specialists with distinctive adaptations for collecting nectar and/or pollen from a small number of closely related plant species (Danforth et al. 2008). In southern Africa, these non-apis bees visit numerous plant families but the most predominant is the Aizoaceae, along with Asteraceae, Fabaceae and Zygophyllaceae (Gess & Gess 2004a). Gess and Gess ( 2004a) reported that as many as 13 different plant families may be visited by a single species but visitation records pointed to preferences for certain families. Seladonea sp., for example, seemed to prefer Asteraceae compared with other families. However, when considering the species richness and diversity of the Halictidae, along with their polylectic manner of feeding, it can be concluded with a reasonable degree of certainty that they are important pollinators for many seed-bearing plants (Dikmen 2007). All of the collected Halictidae specimens displayed a typical pollen collection behaviour, which explains the copious amounts of pollen observed on their bodies and in their pollen baskets.

Ammophila is a genus of thread-waisted wasps in the Sphecidae well known to be parasitic and predatory in nature. Preferences for specific flowers are known amongst the genera of Sphecidae but such relationships are not as strong as in bees or masarids (Bohart & Menke 1976). Spechids, in general, feed on a variety of food including nectar, honeydew and bodily fluids of their prey. Genera with short tongues seek nectar from flowers with short corollas such as the Apiaceae, Asteraceae, Euphorbiaceae and Polygonaceae (Gess & Gess 1991). Ammophila has elongated mouthparts that enable it to feed on a greater variety of flowers. Flowers known to be visited by Ammophila include Acanthaceae, Apiaceae and Lamiaceae (Gess & Gess 1991; Herrera 1989; Weaving 1989). The captured Ammophila specimen was observed to make precise efforts to probe the nectaries of the F. humilis flower by circumnavigating the flower in such a way to ensure contact between the anthers and the underside of its mouth parts where pollen grains were found.

All species of pollen wasps (Vespidae), including those belonging to the genus Quartinia, are nectar feeders and pollen collectors (Gess & Gess 2010). Of the 40 species recorded to visit flowers, 75% preferred the Aizoaceae. Quartinia is also generally regarded as an effective pollinator for many Aizoaceae species and numerous specialised relationships have been noted (Gess & Gess 2004b , 2010). During the survey, one of the Quartinia specimens continued to forage in a F. humilis flower even after the hand net was placed over it and the flower. Even though little pollen was found on the specimen, the fact that all species of Quartinia are pollen feeders and collectors indicate that this genus may be an efficient reserve pollinator of F. humilis (Gess & Gess 2010).

Pollination system

Although few specimens (46 individuals) of pollen carriers were captured during this study, our observations provided enough evidence to support the hypothesis presented by Hartmann (1991) that F. humilis belongs to the Melittophilous syndrome, thus being pollinated mostly by bee and bee-like pollinators. This was especially the case for the natural F. humilis population at Ezemvelo Nature Reserve, where bee pollinators were predominant. At translocated sites, additional observations were made of flies and the Ammophila wasps which support the presence of Anemophilous syndrome and imply a bimodal pollination system. These ‘reserve’ pollinators (Johnson, Harris & Procheş 2009) associated with the Anemophilous syndrome provide plausible alternatives for the translocated populations.

However, it must be cautioned that even though specimens might carry F. humilis pollen, it does not conclusively prove that any of these species are pollinators. In addition, the amount of pollen on a pollinator’s body is regarded as an unreliable determinant of efficiency (Adler & Irwin 2006). Identifying a primary pollinator depends on the pollination efficiency of the species, which is defined as the amount of con-specific pollen transferred to a stigma in any one visit (Inouye et al. 1994). However, determining pollination efficiency is problematic because pollination is a highly variable ecological interaction and can be influenced by factors such as pollinator abundance across years, flower visit duration and frequency, contact with flower parts as well as pollen removal and deposition (Ivey, Martinez & Wyatt 2003).

With this in mind, the bees Amegilla fallax, Megachile niveofasciata and Lipotriches sp. that form part of the Melittophilous syndrome are cautiously proposed as primary pollinators of F. humilis because of their higher visitation frequencies and larger pollen loads. The sweat bee, Seladonea sp., could also be a primary pollinator because of its higher observed pollen loads, despite only being observed twice. Whilst pollen load may not be a direct indication of pollination efficiency, the larger pollen loads of bees compared with other F. humilis visitors were particularly evident on SEM micrographs. The number of observations recorded as visitation frequency is also in favour of bees compared with the various other species. One generalist pollinator, E. eluta, was observed at all sites but is likely an inefficient pollinator because of the limited amount of pollen found only on the eyes (Harris et al. 2016).

Further evidence for bee pollination is supported by the pollination system of a closely related species, Bergeranthus multiceps (Aizoaceae), from spring and autumn rainfall areas of the Eastern Cape Province, South Africa. This plant is also self-incompatible and flowers open between 15:30 and 18:30. Bees were determined to be the most important pollinators to this species, placing it in the Melittophilous syndrome (Peter et al. 2004). Other potential pollinators included butterflies, bees and hover flies belonging to the families Bombyliidae, Tachinidae and Syrphidae (Peter et al. 2004). This study and Harris et al. (2016) made similar observations for F. humilis.

Pollination syndromes may maximise successful pollination by a specific species whilst still reserving the option for chance pollination by a less suitable species. The non-Melittophilous Frithia pollen carriers are suggested to be reserve pollinators because of their frequent visits and exclusive occurrence in translocated populations. They are primarily wasp and fly species that form part of an Anemophilous syndrome. Whilst bees may be primary pollinators, the small size and isolation of F. humilis populations supports the idea that F. humilis is a generalist rather than a specialist, and it can rely on reserve pollinators as its flower structure allows pollination by a different syndrome. A bimodal pollination system has also been identified previously for a member of the Ruschioideae in the Aizoaceae (Zietsman 2013).

Pollination efficacy

Pollination was equally successful at the natural and translocated Goedvertrouwdt populations although flower density differed (Harris et al. 2014). This is in accordance with the flower density hypothesis (Pufal et al. 2008). Flower density is known to have a strong influence on pollinator visits (Kunin 1997) but the size of the population had little effect on pollination efficacy. In other words, more flowers do not imply higher proportional frequencies of pollination.

Seedling recruitment per seed quantity was the highest for Goedvertrouwdt, but this could be ascribed to lower plant densities and more suitable niches (Eriksson & Ehrlen 1992). Eagle’s Rock has a different geology and as expected did not have ideal conditions for seedling germination and establishment (Harris et al. 2014). The presence of seedlings, therefore, is no indication of pollination efficacy but only that successful pollination takes place. This indicates that at least some of the pollen carriers recorded in this study are also pollinators.

Implications for translocation science

This study showed that plant populations translocated to atypical habitats and geology had lower levels of fruit formation despite having higher levels of flowering compared to translocated populations of typical habitat. Site selection is important and should be performed carefully to avoid plant stressors and nutrient deficiencies which may affect plant and flower health, and the availability of pollen (Mayer 2004). Reduced flowering can affect the number of pollinators that may visit a population and might limit outcrossing in small populations resulting in pollen transfer between relatives or not being pollinated at all (Kearns & Inouye 1997).

The sparsely populated translocated sites had higher levels of fruit set and seedling occurrence per individual than the natural population. This shows that it is important to consider transplant density for xenogamous species, as too high densities influence the available pollinators and could result in many unpollinated flowers, which could restrict geneflow in the newly established population. Pufal et al. (2008) suggested that decreased density of flowers increases pollination success and fruit set per plant as pollinators are known to be scarce, with a few taxa accounting for most of the flowers at a site (Herrera 1989).

Translocation requires relocating a species to areas within its existing distribution range (IUCN 2013). This study has shown that generalist pollinators of F. humilis occur throughout its natural distribution range and are common and mobile enough to locate newly established populations. Translocation is therefore a low risk conservation option in terms of generalist plant species. Pollination is further enhanced by the bimodal pollination system that is characteristic for some of members of the Aizoaceae (Zietsman 2013), which makes them resilient to shifts within their distribution range.

Conclusion

Nine arthropod species have now been confirmed as F. humilis pollen carriers. Hymenopteran species had the largest pollen loads in pollen baskets and on front legs. This positioning of pollen ensures contact with the stigmata of the open flowers of F. humilis and could result in effective pollination. The pollination system is Melittophilous as would be expected for the family, with an Anemophilous syndrome providing reserve pollinators at translocation sites. The presence of seedlings at both translocation sites was indicative of successful pollination by insects of this self-incompatible species.

Pollination of flowers was proportionally similar between the varying population densities of natural and translocated populations on Ecca sandstone. This indicated that flower density did not influence fruit formation in this species. When the two translocated populations were compared, it was evident that fruit formation was more effective on Ecca sandstone. This indicated that reproductive success of this edaphic specialist decreases on non-typical rock habitats.

Micro-habitat conditions, planting density and locality within the species’ natural distribution range require an in-depth understanding before sites are selected to enhance pollination probability. However, even if predictions based on this are not accurate, it can be assumed that generalist and ‘reserve’ pollinators are widespread and abundant enough to locate translocated populations of F. humilis within the natural distribution range of the species. It can be concluded that pollinators are not limiting factors when this species is translocated.

Acknowledgements

The authors thank Dr Duncan MacFayden (Manager: Research and Conservation at E. Oppenheimer & Son) for providing access to the Frithia population at Ezemvelo Nature Reserve. They are grateful to the landowners for providing access to translocated populations at Eagle’s Rock Private Estate and Goedvertrouwdt Farm and giving permission to collect insect specimens on their properties. Mr Scott van Eeden prepared the locality map and Mr Koos Smit photographed the species in habitat.

Competing interests

The authors declare that they have no financial or personal relationships that may have inappropriately influenced them in writing this article.

Authors’ contributions

P.G.J. conducted fieldwork and species identification and wrote part of the manuscript. S.J.S. designed the study, conducted initial surveys and wrote part of the manuscript. F.S. contributed to the writing of the manuscript and data analyses. J.v.d.B. performed sorting of arthropod material and coordinated the identification of pollinators. A.J. conducted scanning electron microscopy.

Funding information

Financial assistance for this study was provided by the National Research Foundation (Grant UID: 103370).

Data availability statement

Data sharing is applicable to this article as new data were created or analysed in this study.

Disclaimer

Opinions expressed and conclusions arrived at in the article are those of the authors and are not necessarily to be attributed to the National Research Foundation (NRF).

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