San Francisco State University
Geography 316: Biogeography
The Biogeography of the California Pitcher Plant (Darlingtonia californica)
by Summer Lindzey, student in Geography 316, Fall
|Figure 1: The California Pitcher Plant|
Species: (Darlingtonia californica)
copyright 1999, Charles Webber, California Academy of Sciences
Description of Species:
The California Pitcher Plant or Cobra Lily is named for the unique shape of its numerous stalks, which resemble cobra heads rearing to strike (Slack 1980). A pitcher plant colony comprises many stalks, anywhere from a few inches to a foot and a half high, and are clones of an older parent. What seems to be a stalk is actually a single, large modified leaf that folds in on itself (an epiascidate leaf; Franck 1975), forming a tube about an inch wide at the bottom (its pitcher) and flaring out at the top into a hood. Other pitcher plants develop similarly from epiascidate leaves and retain a predominantly green color, but two features set the California pitcher plant apart: its twisting stem and distinctive hood.
Unlike other pitcher plants, the California pitcher plant does not grow straight up, but turns left or right on its ascent, twisting itself and facing directly opposite from its base (Slack 1980)(see Fig. 1). Interestingly, each new pitcher grows in such a way so that its final position directs its hood as far away from the others as possible (Juniper et al. 1989). Not only may this prevent new pitchers from growing into the hoods of older pitchers, but it also maximizes the total area surveyed by the hoods, the means by which the plant traps insect prey (Juniper et al. 1989; Franck 1975). The hood is the defining feature of the pitcher plant, in both form and function.
The cobra head is the means by which the pitcher plant traps insects and thereby supplements the impoverished soil it grows on (see Habitat, below). The red and brown mottled dome is an extension of the tubular pitcher and houses the nectaries responsible for luring the insects inside. Its down-facing opening encourages insects to look elsewhere for an exit, which they are misled into thinking exists in the many windows of the dome (Juniper et al. 1989; Slack 1980). The windows are visual illusions created by thinning sections of membrane in the hood (fenestrations) that lose their chlorophyll and become transparent. Insects exhaust themselves pursuing each of the fenestrations in turn and eventually may drop down the pitcher tube into its reservoir of fluid and drown. Other insects simply may lose their footing on the slippery wax surface at the back of the hood. Once trapped, the escape of insects is impeded by the vertical ascent required to exit and the many downward pointing hairs that extend from pitcher to hood (Slack 1980).
The attraction of insects to this potentially deadly encounter is the abundant nectar the plant produces, the motherload of which lies inside the hood in a nectar roll (Juniper et al. 1989; Slack 1980; Schnell 1976). Streaks of reddish-orange guide insects to nectar producing glands along the dome (Joel 1986), where they are enticed by increasingly dense nectaries within the hood. Insects may also be drawn into the hood by a fish-tail appendage that hangs like a forked tongue from its lip and offers yet more nectar. Among juvenile pitchers which are not erect enough to catch flying insects, this forked tongue may lay prostrate along the ground and lure in crawling insects (Juniper et al. 1989).
The tongue that hangs from the lip of the hood should not be mistaken for the remnants of a blossom, which the pitcher plant disfavors producing. The California pitcher plant is a perennial that usually propagates asexually, via stolons or runners (Slack 1980; Schnell 1976). When it does blossom, between April and August, the plant sends up separate stalks terminating in a large, pendulous flower (Schnell 1976). The flower is monoecious (or hermaphroditic in animal terms): it contains both sexual parts, stamen and stigma (Miller 1999). The drooping crimson petals are surrounded by a series of pale green bracts and hang nearly upside down from the blossoms weight (see Fig.2, below).
The reproductive structure of the plant seems to encourage cross-pollination and
discourage self-pollination (Schnell 1976). First, the bell shaped ovary has twelve
to fifteen short stamens at its base which allows potential pollinators to come into
contact with pollen twice: once on the way in, and possibly on the way out, after it has
gone up and down the length of the ovary and travels back to the base again to leave.
Second, the downward slope of the ovary prevents the pollinator from crawling over the
stigma to leave, where it might deposit pollen from the same plant, and defeat the purpose
of sexual reproduction: genetic recombination. The question of who disperses the
seeds is still unanswered (Miller 1999). Nor is it clear who is the most likely
pollinator, although Rebecca Austin became convinced it was commensal spiders within the
pitcher community (Juniper et al. 1989), a speculation shared by more recent researchers.
|Figure 2: The Flower of the California Pitcher Plant|
|copyright 1999: Gladys L. Smith, California Academy of Science|
In the event that a flower blossoms and its ovary is fertilized, it will take only ten weeks for the seed to mature and drop (Schnell 1976). Small seeds about 2 mm long and once dispersed, will set by autumn. In the spring a genetically original plant, this time not a clone, may emerge and begin its own life cycle.
As mentioned above, sexual reproduction is cost intensive; it requires a lot of internal resources to develop the flower, produce sperm and egg, and nourish the growing embryo. In addition, the investment may not pay off, and the seed may never germinate, or the seedling may fail to thrive. In contrast, asexual reproduction happens regardless of availability ofmates, does not require pollination and does not take as long to produce a mature plant. The California pitcher plant persists, therefore, most often as colonies of clones (Juniper et al. 1989). They may be genetically redundant, but in stable environments this genetic constitution serves them well, and the benefit from saving internal resources outweighs the risk.
The Carnivorous Habit
Scientists suspect that carnivory among plants, the recruitment of mineral nutrients from insect or other animal prey, has evolved many times in response to very similar selective pressures (Heslop-Harrison 1976; Juniper 1986). All carnivorous plants, for example, live in nutrient-deficient soils or substrates where carnivory would be a supreme advantage (Heslop-Harrison 1976; Juniper 1986). Many have developed such a reliance on this advantage, that they lose out on nutritionally adequate soils, where their adaptive strategies confer no advantage over non carnivorous plants (Ademac 1997). Others seem to weaken or die simply from the presence of abundant nutrients in the soil (and in the absence of competitors). It is as if the source of nutrients is as important, if not more so, than their quantity.
The organic compounds derived from animals ostensibly provide the plants with nitrogen and phosphorous, the most commonly limiting nutrients for plants, but the specific destination or way in which these nutrients are used varies widely across genera and species (Adamec 1997; Chapin and Pastor 1995; Schulze et al 1997 ). Although some pitcher plants seem to derive most of their nitrogen from the soil, this is not the case with the California pitcher plant, which relies on primarily insect-derived nitrogen for its leaf tissue (Schulze et al. 1997). Studies within the family Sarraceniaceae have found animal derived nutrients to be used in cellular respiration, growth, or to boost photosynthesis (Bradshae and Creelman 1984; Adamec 1997).This would allow the California pitcher plants to meet its own nutritional needs, accummulate biomass and compete successfully for scarce resources in its habitat.
Unlike other pitcher plants, the California pitcher plant does not secrete digestive enzymes to decompose its prey. It relies on the metabolic activity of commensal bacteria in its pitcher fluid to break down organic matter into transportable molecules, which it can absorb through the pitcher lining (Heslop-Harrison 1976). The level of fluid and its acidity seem to be related to the density of organic matter in it, not the relative amount or chemical composition of precipitation. Thus the plant is not completely passive in its carnivorous metabolism.
In addition to commensal bacteria a number of microinvertebrates inhabit the pitchers and assist in the decomposition of organic matter (Naeem 1988; Heard 1994). Most of the inhabitants in the pitchers are facultative rather than obligate, but some pitcher plants may have fauna particular to their species alone (see Rango 1999; Juniper et al. 1989). Midges, mites and mosquitoes seem to show particular fidelity in pitcher plants (Naeem 1988; Heard 1994; Rango 1999). Common in the California pitcher plant are Metriocnemus edwardsi and Sarraceniopus darlingtoniae. The scavenging habit of M.edwardsi and its larvae breaks large insect-prey matter into smaller fragments, which mites (such as S. darlingtoniae) can manage and harvest, and this activity can produce finer particles which eventually make their way into the veins of the plant (Naeem 1988).
A number of other species that do not perform digestive roles inhabit pitchers in the Sarraceniaceae, including spiders, ciliated protozoans and a host of bacteria. Some occupy the pitcher for only part of their life cycles, while others spend generation after generation in the same pitcher. Naeem speculates (1988) that arthropod inhabitants rarely relocate from the pitcher they occupy or hatch in. (If they do it requires a winged adult stage.) Pitcher plants sustain this diversity of fauna within its relatively small microhabitat because the animals seem to access resources at different times and on different scales (Naeem 1988; Heard 1994).
Most of the commensal fauna of the pitcher plant do not end up as prey, but the array of victims is equally impressive. Rebecca Austin recorded a pageantry of insects (ants, bees, moths) and other invertebrates visiting or inhabiting the plants during her field observations (Juniper et al.1989). Her meticulous records bear witness to what a generalist the pitcher plant is. Unlike some carnivorous plants, the pitcher plant is not very selective and captures a wide range of insects, small and large (Gibson 1991). By being a generalist it maximizes its resources and survives the seasonal change in number and type of prey available.
For all its benefits, carnivory is an expensive enterprise for the plant. At least some of the profit gained by scavenging animal protein is lost to building and maintaining elaborate structures for luring, capturing, retaining and digesting prey (Heslop-Harrison 1976; Juniper 1986). There is also the disadvantage that the prey for carnivorous plants also include the potential pollinators of the plant. How effective is it for a plant to feed off the population it requires to perpetuate itself? Heslop-Harrison (1976) points out that if these disadvantages outweighed the benefits the habit would not have persisted because the population would have died out. The California pitcher plant has minimized the conflict between luring insects for both prey and pollination by separating these two ecological roles: it produces its flowers on stalks separate from the pitchers (Slack 1980; Schnell 1976). Therefore, the insects that may pollinate the flower do not necessarily fall into traps.
In general most insects escape pitcher traps (Gibson 1991) and effective capture and retaining of prey is rare (Cresswell 1991; Joel 1988). Because most insects approach and leave with nectar, the carnivorous habit is seen as a mutualistic rather than deceptive relationship (Joel 1988). On the one hand carnivory imparts an advantage to surviving on impoverished soils, but on the other is a metabolically exacting strategy. Moreover, its successful adaptations due it a disservice by restricting it to a narrow set of ecological conditions. Colonization and dispersal are highly unlikely, and for this reason some see carnivory as an evolutionary dead end (Heslop-Harrison 1976).
How could such an evolutionary precarious habit persist throughout time and find itself on
almost every continent? The answer lies in the fact that there is no one ancestral
origin for carnivorous plants (Albert et al. 1992), and every structural or physiological
adaptation used in carnivory has multiple origins (Albert et al. 1992; Juniper
1986). At least nine separate orders of angiosperms have carnivorous
representatives, and carnivory evolved in these orders independent of the others.
Even the most similar structures used by carnivorous plants, such as pitchers, can not be
presumed to indicate a close genetic relationship. Likewise, the most
morphologically incongruent carnivorous plants can be closely related, as is the case
between pitcher plants and fly-paper like trapping plants (Albert et al.
1992). Carnivory, then, is a product of convergent evolution, an adaptation executed
many times by many species in their attempt to cope and survive in their environments.
The California pitcher plant is a member of an evolutionary divergent and diverse group of plants we refer to as carnivorous. It is the only species in the genus Darlingtonia, and one of only ten in the pitcher plant family Sarraceniaceae (Juniper et al. 1989). All members of this family are carnivorous, or, as some prefer to call, insectivorous. Although it has been distinguished from its pitcher plant relatives because of peculiar shape and prominent hood, some researchers speculate that it is so similar to the Sarracenia genus that it should not deserve generic distinction (Juniper et al. 1989). Others suggest it may not even belong to the same family, and has been lumped together with Sarracenia because they occupy similar habitat (Naeem 1988).
The higher order taxonomic classification given above is a more modern one, which places all flowering plants or Angiosperms in one phylum, or division, the Anthophyta, and then divides them into two classes, the monocots and dicots (Campbell 1995; Munz and Keck 1959). In the past angiosperms have been defined as a class under the subkingdom Sporophyta (Jepson 1951), and more recently lumped in with all other vascular plants in the phylum Tracheophyta. Despite the differences in naming, the taxonomy is consistent in that it separates flowering plants from other vascular plants, and then divides this into two subgroups, monocots or dicots (Barnes 1998).
Until recently the California pitcher plant was one of the few carnivorous plants for which a phylogenetic history had been proposed (Juniper et al. 1989). Yet, the details are still lacking. We do know that its history continues a story that began at least 140 million years ago with the appearance of the first flowering plant. It was during this time, the Cretaceous era, that flowering plants came to dominate the landscape and supplant the gymnosperms.
The gymnosperms were not be able to keep up with the angiosperms reproductive strategies, novel to the plant kingdom at the time. Flowers used scent and color to attract pollinators, and rewarded these pollinators with nectar. The specific relationship between flower and pollinator improved the chances that an egg would be fertilized, and plants did not have to rely on wind or water to deliver the pollen. As a result, flowering plants could produce less sperm or egg, save energy, and yet successfully reproduce. Along with flowers came shelter for the ovule and developing fruit, which encouraged dispersal by animals, and the development of a doubly fertilized, doubly nutritional seed around the embryo. These collective advancements made reproduction more efficient and faster and afforded more opportunity for genetic diversification than what gymnosperms could achieve in the same period of time (Rost et al. 1998). By 65 million years ago, every flower, every fruit, and every family of flowering plants had evolved (Campbell 1995).
The hothouse climate of the Cretaceous supported the proliferation of the angiosperms, bolstered by increasing geographic differences between the continents as they migrated apart (Rost et al. 1998) . During the early Tertiary (after 65 million years ago), hot and humid climates continued, and the tropical rainforest took shape. It was during this time, 60 to 40 million years ago, that the ancestor common to the pitcher plants of the Sarraceniaceae is believed to have emerged (Juniper et al. 1989).
Where this common ancestor came from, what genera it gave rise to first, and how these may have dispersed is still debated. Previous scenarios envisioned a Heliamphora-like ancestor diverging into three genera (Darlingtonia, Sarracenia, Heliamphora) in the southeastern united states (Juniper et al. 1989). The Heliamphora somehow ended up in South America, where it has subsequently been isolated by high elevation; the Sarracenia dispersed continuously throughout the southeast and human disturbance accelerated its speciation; and Darlingtonia was able to migrate across the continent before the rise of the Rockies. This scenario assumed Heliamphora to be the closest relative to the original ancestor, based on morphological characteristics assumed to be more primitive. Newer research suggests that Darlingtonia may be the older species (Albert et al. 1992). According to this analysis, Heliamphora and Sarracenia share at least one derived genetic characteristic not present in Darlingtonia. This implies that Heliamphora and Sarracenia are more related to each other than to Darlingtonia, and that Darlingtonia is closer in genetics, and maybe time, to their collective ancestor. Perhaps Darlingtonia is an evolutionary relict whose progeny was cut off by the rise of the Rockies and migrated east and south to speciate into the Sarracenia in the southeast, and the Heliamphora in South America. As there is no fossil with intermediate characteristics to demonstrate a transition, the exact route of dispersal continues to be unanswered.
However the dispersal may have happened these vicariants have a common root in one of the oldest orders of flowering plants, the Sarraceniales. Along with the buttercup family and the Paparevales, Sarraceniaceae comprise the Ranunculiidae, a subclass of dicots closely related to the oldest known angiosperms, the Magnoliidae (Rost et al., Fig. 4).The Magnoliidae are also believed to have given rise to all monocots, the second class of angiosperms.
|Figure 3: The Phylogenetic Arrangement of Subclasses within Monocots (Liliopsida) and Dicots (Magnoliopsida) copyright Rost et al. 1998, p.434|
Distribution and Habitat
The California pitcher plant is a rare native endemic to Oregon and California (see Fig.3, below). Its range is discontinuous along coastal habitat in southern Oregon and extends into northwest California (Debuhr 1974). Additional colonies in the northern Sierras make up its southernmost distribution (Debuhr 1974). The disjunct distribution suggests tolerance of a wide variety of climates, from coastal lowlands to mountain slopes, but its ecological niches are actually quite specific. The pitcher plant is found in sphagnum bogs along the coast or inland in shady, hillside seeps and, in particular, only where there is a constant supply of cool, seeping water (Miller 1999). The availability, temperature and source of the water are the most critical elements in the plant's survival, more critical than altitude or ambient temperature (Debuhr 1974; Ziemer 1973). The colonies are mostly (but not completely) situated over serpentine rock (Juniper et al. 1989), a substratum that is toxic to most plants (Whittaker 1954; Walker 1954).
Among the pitcher plants many co-habitants are Douglas fir, Port Orford cedar, yellow pine, and a host of sclerophyllous shrubs and other forbs. There is some disagreement over whether the pitcher plant can be described as a seral species. Whittaker (1954) asserts that this serpentine community is self-sustaining and not merely a seral stage in a progression to a typical forested community; the restrictions of the serpentine soil prohibit invasion of intolerant species. By extension, the vegetative community found in association with the California pitcher plant should be evolutionarily stable. Crane (1990), however, points out that the pitcher plant also occupies coastal zones and bog habitats that are prone to disturbance and constantly undergoing succession.
Both the serpentine and bog habitats are a physiological challenge to its botanical
residents. The primary limiting factor in these habitats is not light nor water,
both of which are plenty, but is the availability of nutrients (Larsen 1982). The
high acidity of bogs, coupled with their low turnover of organic matter, results in low
concentrations of nutrients such as nitrogen, phosphorus and calcium (Larsen 1982).
Serpentine-derived soils or charged waters are also deficient in essential nutrients and,
moreover, rich in heavy metals like magnesium that most plants can not tolerate (Walker
1954, Whittaker 1954). To survive in this habitat the California pitcher plant has
had to adopt a way to supplement its nutrition, by eating insects.
Fig. 3 Distribution of the California Pitcher Plant
|Dotted lines correspond to native populations; black
identify serpentine outcrops.
copyright Juniper et al. 1989, p.37
At the present time the California pitcher plant is not considered threatened or endangered, but Crane (1990) has pointed to the encroachment of human development and the avid interest of carnivorous plant collectors as two imminent threats. Its stenotopic nature makes the pitcher plant particularly vulnerable to disturbance and limits its ability to recover from major perturbations in its environment . Fortunately, many colonies exist on either federally or state regulated lands that are protected from development and commercial intrusion.
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