Streambank lupine (Lupinus rivularis) COSEWIC assessment and status report: chapter 6

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Biology

General

Lupine species are found in North and South America, southern Europe, the Mediterranean and North Africa, and are commonly found on dry sites, stony slopes, and calcareous and sandy loam soils (Allen and Allen 1981). The greatest number of lupine species are found in North America, where this genus can be grouped into two clades based on ITS sequencing: an eastern New World clade, and a western New World clade (Ainouche and Bayer 1999).

Estimates of the number of North American species vary from 100 to 600 taxa (Nicholls and Bohm 1983), a result of the high degree of variability in the genus that in part stems from the large amount of hybridization and introgression that occurs. Complicating this is the continued discovery of new species such as the recently discovered 30 foot tree lupine (Lupinus jaimehintoniana) found in the high plateaus of Mexico (Turner 1995). In addition, lupines are known to germinate successfully even after burial for thousands of years (Sholars 2001 pers. comm.).

While there is a great deal of literature available on the biology of legumes, and on the biology of lupines in general (e.g. the lack of nectar glands in lupines), there is very little information available on the biology of Lupinus rivularis. Some details have been found, however:

• Some lupines are considered toxic, particularly to cattle, and toxicity varies both seasonally and geographically (Allen and Allen 1981). The active toxin may be one or more of 25-30 quinolizidine alkaloids (Allen and Allen 1981). It is not known if Lupinus rivularis is toxic.
• Descriptions of lupine nodules are provided by Allen and Allen (1981), who indicate that Lupinus rivularis is a nodulated species of lupine.
• Lupinus rivularis is a perennial, primarily herbaceous species with a long taproot.  O’Dell and Trappe (1992) report that most of the lupines they examined, which included L. rivularis, had taproots that were often more than one foot deep with few fine roots, an ideal survival trait in droughty and disturbed conditions.
• Because of their ability to fix nitrogen in the soil, lupines are often species of disturbed or impoverished sites, a feature that enhances their survival and persistence (Dunn and Gillett 1966). It is noteworthy that the first plant to occur on Mount St. Helens following the eruption was a lupine (Morris and Wood 1989, Thinkquest 2000). Lupine rivularis is no exception to this, often occurring on impoverished sites that are also highly disturbed, and where competition is minimal. However, all sites are proximal to creek / riverbanks along the Pacific coast.
• Hybridization is a common feature in lupines and this is true with Lupinus rivularis.  This species is reported to hybridize with L. arboreus and L. littoralis (Rhymer and Simberloff 1996, Riggins and Sholars 1993, and others).
• Moisture levels and spring and summer temperatures play a role in lupine survival and flower persistence (Dunn 1956), although no specific information for L. rivularis was found.
• Based on reports of other lupine species (Breedlove and Ehrlich 1968, Breedlove and Erhlich 1972), mutualism should be investigated for Lupinus rivularis, however no information on this was found.

Reproduction

Lupinus rivularis is a perennial species of lupine. Little information specific to the biology of this species was found, but it is suspected that it, like many other perennial lupine species, is capable of both self-pollination and cross-pollination (Ganders 2001 pers. comm.). Cross-pollination is evident in the presence of hybrid plants. No vegetative reproduction is likely.

Although reported by Dunn and Gillett (1966) as flowering in July, BC lower mainland populations have been collected in flower in May and they continue flowering until September, thus allowing for a continuous production of seed. Dunn (1956) reports that high spring temperatures may cause some flowers to abort.

Lupines in general set prolific, heavy seed that tend to fall in the vicinity of the parent plants, resulting in the colonial appearance of many populations. This is true in the case of Lupinus rivularis. Numerous seedlings were found at most sites, all within 3 meters of the parent plant, except where clearing of vegetation and brush along the railway sites has dispersed the seed to a distance of about 100 meters. However, there is no way of knowing if seed production in our populations matches expected seed production for the species.

In spite of what seems like prolific seed set, the populations for the most part do not cover a large area. We speculate that maintenance work along the railway lines and dykes (i.e. repeated mowing, brush cutting, spraying), eventually kills off the mature plants. However, because replacement in the immediate vicinity is apparently high, the populations persist.

No information on germination in L. rivularis was found; however, Dunn (1956) studied germination in the group micranthi, and reports that seeds in this group do not necessarily germinate every year, and colonies may disappear at intervals depending on weather. Seed coats in this group require abrasion or decomposition for germination to occur.

Hybridization and introgression are significant components of lupine biology and reproduction (Kazimierski 1961a & b, Phillips 1955, and others). Hybridization may be a major factor in the reproduction of Lupinus rivularis today. Hybrids have been reported with Lupinus arboreus and L. littoralis (Rhymer and Simberloff 1996, Riggins and Sholars 1993, Wozniak 2000, Sholars 2001 pers. comm.). Hybridization in this species is a complicating factor in protection and conservation throughout its range and is clearly a factor in our Canadian populations. Where L. rivularis comes in contact with L. arboreus, it is in danger of genetic swamping and complete eradication of the species (Sholars 2001 pers. comm.). In the Fraser Valley, we observed a population of mixed Lupinus rivularis and L. arboreus. A collection made in the Sooke area by A. Ceska in 2001 may contain some hybrid genes, possibly L. rivularis x littoralis, though this is uncertain and requires verification. An interesting additional note, as mentioned above: lupine seeds are known to be viable for thousands of years, thus increasing the gene pool available in a startling way (Sholars 2001 pers. comm.).

An additional complicating factor comes from planting programs in municipalities and along roadways, where new populations of L. rivularis have been established from wildflower seed packages. Many of the lupines that are planted from seed packages appear to be hybrid stock. We have observed at least three planted L. rivularis x arboreus stands, and there are no doubt many more as dispersal of wildflower seeds has become popular, both with the public and with municipal and highway workers.

Some species of annual lupines are reported to be pollinated by bees, and lupines in general are frequently capable of self-pollinating (Dunn 1956). Further investigation of pollination in Lupinus rivularis is required, although we observed several species of Hymenoptera visiting plants at all of our sites. Presence of pollinators alone, however, may not be sufficient to allow spread of this species. Concern has been raised by some researchers for the survival of rare species, in general, when existing populations are small and isolated, even when adequate pollination is occurring (Michaels 1999). Michaels is presently studying this, and indicates that she has evidence that “lupine plants in small populations have low reproductive success, but adequate pollinator visitation” (Michaels 1999, np). Her work on lupines should be followed closely in the next few years.

Additional concerns about the long-term survival capability of small fragmented populations because of inbreeding depression have been raised by Menges (1991) and many others, and are well discussed in population genetic theory. Menges indicates that small and isolated populations may suffer several disadvantages, including greater vulnerability to outside forces. Most of this relates to an increase in inbreeding, which may or may not apply in this instance as L. rivularis clearly hybridizes. The downside to this hybridization is loss of the genetic pool for this species. Menges (1991) also discusses population bottlenecks that arise in some fragmented small populations. While some researchers indicate that small population size has been associated with reductions in seed set, Costin et al. (2001) indicate that this is not always the case as pollinators can be attracted to a site by other flowering plants, thus bringing them to the species in distress. A high diversity of other flowering species in the vicinity thus plays a role. Additionally, van Treuren et al. (1991) indicate that genetic erosion in small populations can be counteracted. In this case, it would be a simple matter to manually exchange seeds from each of our populations as part of a recovery plan.

Pollination mechanisms

Lupinus rivularis has not been studied specifically in this regard; however, detailed information on the pollination mechanisms of lupines in general is given by Dunn (1956). This species clearly cross-pollinates, and likely also self-pollinates. Individual populations are not likely close enough for gene exchange.

Survival

Several key factors affect the survival of this species. In the past, we believe that direct habitat loss has resulted in increasing rarity of this species. In addition, where it occurs in secondary floodplains, railways beds, and dykes, survival is precarious and dependent on happenstance. Finally, the very real threat of genetic swamping by the invasive and introduced Lupinus arboreus requires immediate attention. Even in natural sites, disturbance is a key element. Survival of the populations seems very tenuous.

As they exist at present, the populations are reproducing. There seems to be high germination with more than a hundred seedlings recorded at some stations we visited. In addition, there are some young plants present at most sites, as well as older plants. There is no way of knowing if populations are stable as no past work has been done on this species in BC/Canada, although its persistence in some sites since the first discovery of the stations may indicate some long-term stability.

Predation is unknown, although, in the literature, other lupine species suffer from predation by various caterpillar species. Weevils and/or bruchid bettles, and possibly other insects, are probably seed predators.

While large numbers of aphids were observed on many plants, no attendant ants were observed. At two stations, an unidentified species of Ctenucha moth was present in the adjacent grasses in great numbers.

Our observations would indicate that natural expansion of the existing populations is unlikely. However, it is possible that the number of stations could be increased through management and active intervention. The maintenance of low competition through mowing or brushing probably aids the species. However, it would be important for long term survival to ensure that brushing does not occur until seed set.

Physiology

This perennial species of lupine shows strong preference for low nutrient sites with little competition. It appears to be hardy within its restricted habitat, and exhibits a long flowering period (May to September) that would appear to be adaptive to maximizing pollination. The presence of a long taproot would indicate that it is drought hardy, although it occurs in moister areas within the sandy/gravelly riverbank habitats it prefers.

Observations in areas where it is mowed on a roadside verge indicate that it can withstand such disturbance and still flower and set seed.

Movements/dispersal

Lupines are described by Dunn (1956) as being dispersed by birds and rodents. In our case, they can also spread by mowing along railway tracks and dykes. This can disperse the seeds further from the mother plant than might otherwise have been the case.

Past use of populations as food sources by Aboriginal peoples may account for some dispersal, although there is no solid evidence of this occurring, and the very limited occurrence of the species would indicate that it has not been widely spread if such use did occur. Turner (1998) does not mention this species specifically as a food source, although she does cite “lupine species”. Only one mention of L. rivularis S. Wats. use by Aboriginals was found in Teit and Steedman (1930). However, this is not L. rivularis Dougl. ex Lindl., and it is clearly out of range.

Our field observations indicate that high numbers of seeds drop and germinate within the vicinity of the parent plant. However, we also observed two stations with single plants. These may represent dispersal or they could be plants released from the seed banks during spring flood scouring.

One observer has noted that this species can throw its seeds up to 26 feet when the pods open (Erickson 1999), while Dunn (1956) reports that, because of the highly explosive nature of the pods, lupine seeds can be thrown 15-20 feet, and the colony can therefore spread up to 20 feet in a year. The main limitation for successful dispersal is the availability of suitable habitat.

Nutrition and interspecific interactions

At this time, nutrition and interspecific interaction of Lupinus rivularis is unknown but should be investigated. However, lupines are frequently planted as cover crops to enrich soils because of their nitrogen fixing capabilities (Dunn and Gillett 1966). Lupines are well known as pioneer species that occur on impoverished ground, where they improve growing conditions for other species.

In addition, co-evolution of lupines with other species is reported (Whipple 1998, Breedlove and Erlich 1968, 1972). This includes the role of lupines (Lupinus perennis) as a food plant for the Karner Blue Butterfly in eastern North America (Balogh 1980), and suggests that other mutualistic relations between butterflies and lupines should be investigated. The butterfly Host Plant Database (XOXEARTH 2001) does not list L. rivularis as a host plant for butterflies in California, although other lupines species, such as Lupinus densiflorus, are listed.

In spite of mycorrhizal relationships in legumes, lupine species appear to be less frequently colonized by mycorrhizal fungi and some species are never colonized by fungi (O’Dell and Trappe 1992). O’Dell and Trappe (1992) could not be certain if L. rivularis was mycorrhizal or not: they examined only root portions and not the complete root systems.

Behaviour/adaptability

Our observations in 2001 indicate a few traits that this species exhibits that seem to show promise for its continued survival. It is a species that does well in disturbed situations and where competition is eliminated.

In some sites where it occurs, this adaptability to disturbance should lend it some protection from anthropogenic disturbances such as clearing brush, mowing or using a weed-eater. Indeed, many mowed plants were observed in one site and, although tiny, these flowered and set seed. However, they would still be vulnerable to eradication through radical alteration of the site.

This species flowers from May to September, and produces seed continuously from June onwards. We noted maturing seedpods on these plants in June. The species’ ability to withstand disturbance, and its obvious adaptation to riverbanks, indicate that it is probably quite capable of survival in the face of natural catastrophic disturbances such as flooding during snowmelt. Flooding would perhaps eradicate some plants, but it might also allow some seeds from the seed bank to emerge and reestablish afterwards. The perennial nature of this species, and the very deep taproot, indicate that it is likely also quite drought tolerant.

This is also a species that is being cultivated in the US. Ads for purchase of plants or seeds are common on the internet. The biggest danger in this is that the stock for these seeds/plants may contain genes from L. arboreus and thus pose a threat of hybridization and contamination of the gene pool. However, they may also contain pure species, and provide a method of continuance for the L. rivularis. This requires investigation.