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REVIEW
Year : 2013  |  Volume : 11  |  Issue : 3  |  Page : 233-246

Pistia stratiotes L. in the Florida Peninsula: Biogeographic Evidence and Conservation Implications of Native Tenure for an 'Invasive' Aquatic Plant


Environmental Policy Program, Carl Vinson Institute of Government, University of Georgia, Athens, GA, USA

Correspondence Address:
Jason M Evans
Environmental Policy Program, Carl Vinson Institute of Government, University of Georgia, Athens, GA
USA
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DOI: 10.4103/0972-4923.121026


Date of Web Publication6-Nov-2013
 

   Abstract 

Pistia stratiotes L. (water lettuce) is a floating aquatic plant with wide pantropical distribution and the sole extant species of the Pistia genus. Fragmented knowledge about pre-modern Holocene dispersal and widespread observations of ecological invasiveness have made Pistia a classic example of a 'cryptogenic' species (i.e., indeterminately native or non-native) in much of its contemporary range. Questions about Pistia biogeography have likely received the most attention in North America's Florida peninsula, where the species is currently listed and aggressively controlled as an invasive non-native. However, emergent conservation concerns have prompted interest in resolving persistent uncertainties about this designation and associated management strategies. Towards this purpose, this paper develops a comprehensive and critical review of scientific literature pertaining to the Florida biogeography of Pistia. Remarkably, all claims in support of the non-native designation can be dismissed as scientifically and/or logically insufficient through hypothetico-deductive analysis. Conversely, a holistic synthesis of paleo-botanical, historical, and ecological evidence overwhelmingly points toward a native Florida presence for Pistia. Observations and associated ecological inferences further suggest that intensive Pistia control programs, which for many years have been implemented on an assumption of non-native status, may pose some conservation concerns for rare native biota in Florida's spring-fed streams. This case study joins several other studies indicating that due caution and research diligence should be employed when managing cryptogenic taxa in this time of global change.

Keywords: cryptogenic species, invasive species, native species, Pistia stratiotes L., water lettuce, Florida biogeography


How to cite this article:
Evans JM. Pistia stratiotes L. in the Florida Peninsula: Biogeographic Evidence and Conservation Implications of Native Tenure for an 'Invasive' Aquatic Plant. Conservat Soc 2013;11:233-46

How to cite this URL:
Evans JM. Pistia stratiotes L. in the Florida Peninsula: Biogeographic Evidence and Conservation Implications of Native Tenure for an 'Invasive' Aquatic Plant. Conservat Soc [serial online] 2013 [cited 2014 Sep 16];11:233-46. Available from: http://www.conservationandsociety.org/text.asp?2013/11/3/233/121026


   Introduction Top


The identification and control of invasive non-native species is widely regarded as a primary conservation imperative. One of the more recent developments and ongoing research challenges within the invasive species literature is associated with the phenomena of 'cryptogenic' species (sensu Carlton 1996). Generally speaking, cryptogenic species are characterised by two biogeographic traits: 1) very wide to cosmopolitan distribution; and 2) an ambiguous or indeterminate place of origin within that distribution. Following the seminal work of Carlton (1996), it is widely suspected that the cosmopolitan ranges of most cryptogenic species likely reflect anthropogenic transport and dispersal mechanisms. Correspondingly, there is significant worry that recent increases in the local dominance (i.e., "invasiveness") of cryptogenic species may pose a severe threat to native biodiversity as such invasive behaviour, may often be the latent consequence of previously unrecognised (i.e., 'cryptic') non-native introductions (Boudoresque and Verlaque 2002; Hewitt et al. 2004; Kerchof et al. 2007).

Consistent with such worries, a number of high-profile studies have indeed confirmed non-native status for species that were previously regarded as cryptogenic (e.g., Saltonstall 2002; Dawson et al. 2005; Blakeslee 2008). However, several other studies have indicated quite the opposite: unequivocal evidence of local native status (i.e., paleo-dispersal into a range area independent of human influence) for cryptogenic species previously regarded as likely non-native invaders (e.g., van Leeuwen et al. 2008; Scott et al. 2009; Coffey et al. 2011). This mixed literature broadly suggests that detailed biogeographic research into cryptogenic species, particularly those currently showing major population increases and other invasion ecology characteristics, should be regarded as a key conservation and management priority (Saltonstall 2002; Froyd and Willis 2008; Thomsen et al. 2010).

Pistia stratiotes L. (water lettuce; [Figure 1] is a floating aquatic plant of the family Araceae and the lone extant species of the Pistia genus (and, thus, is hereafter referred to simply as Pistia). Widely distributed through the humid tropics and sub-tropics, Pistia's propensity for 'weedy' overgrowth has long placed it among the world's list of most potentially invasive species (Holm et al. 1977). However, determination of Pistia's native range has long posed a difficult biogeographic challenge due to the plant's wide-ranging dispersal throughout the fossil and historic records (e.g., Dray et al. 1988; Stoddard 1989; Habeck and Thompson 1997). This weedy habit and biogeographic uncertainty have together made Pistia a classic example of a cryptogenic species in many areas of the world where it is currently found (see, e.g., Rana and Ranade 2009).
Figure 1: Scientific illustration of Pistia stratiotes L.

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Perhaps the most notable and longstanding example of Pistia's ambiguous biogeography comes from the Florida peninsula of the southeastern United States (SE US). On the one hand, Pistia is officially listed by the State of Florida as a Class I invasive non-native species (FDEP 2008; USACE 2010; USDA 2010), a designation that refers to a small subset of non-native species that cause a very high degree of damage to native ecological communities (FLEPPC 2009). On the other hand, numerous Florida reports of Pistia by eighteenth century botanists John and William Bartram (Bartram 1955) are widely acknowledged as a persistent source of uncertainty about whether or not the plant is native to Florida (e.g., Stuckey and Les 1984; Dray et al. 1988; FDEP 2008; USACE 2010; USDA 2010).

The Bartram accounts are widely regarded as a primary resource for identifying the composition and extent of native plant communities in Florida and other areas of the SE US (see, e.g., Ward and Minno 2002; Egerton 2007). Presumably based on this authority, a number of historic naturalists conspicuously listed Pistia among Florida's native flora (e.g., Besse 1980; Sykes 1987; Carr 1994). Over the past three decades, however, a variety of hypotheses that the Florida Pistia observed by the Bartram explorers was likely of non-native origin (i.e., introduced into Florida during the post-Columbian era) have been published within scientific literature (Stuckey and Les 1984; Dray et al. 1993; Dray and Center 2002; Neuenschwander et al. 2009) and, as noted above, officially accepted by management agencies. Although some recent authors have cited the Bartram records to suggest possible native status for the plant (e.g., Evans 2008; Belleville 2011), most recent research has proceeded from the assumption that Pistia is unambiguously non-native to Florida (e.g., Gordon 1998; Adams and Lee 2007; Neuenschwander et al. 2009).

This situation provides for an interesting case study simply due to the fact that there are very few, if any, similar examples of a long-term non-native invasive control programme being implemented on a species in which there remains such a large degree of uncertainty about the veracity of the non-native designation.

Given this backdrop, the primary purpose of this paper is to develop an up-to-date synthesis and critical discussion of literature pertaining to Pistia biogeography in the Florida peninsula.

At least three sets of issues indicate the need for such a synthesis at this time. First, a number of observations suggest that Pistia control programmes may in some cases have serious non-target effects on aquatic snails and other native fauna of conservation concern in Florida, particularly in the context of the state's many spring-fed stream systems (Bryan 1990; Corrao et al. 2006; Evans 2008). Because the non-native designation is quite central in determining how such management interventions are legally implemented and otherwise justified to the public (e.g., FDEP 2008), accurate biogeographic information is highly relevant to future assessments of non-target damage tolerances and other evaluations of Pistia control programmes. Second, several recent management and research studies have recommended possible use of Pistia for remediating nutrients (SJRWMD 2008; Liu et al. 2010), toxic contaminants (Liu et al. 2011), and/or undesirable algal overgrowth (Evans 2008) in degraded Florida waters. Accurate information about biogeographic tenure is crucial for appropriate evaluation of such proposals as remediation with invasive non-native species very clearly implies a need for increased caution and concern as compared to remediation with even the most weedy native species (see, e.g., Susarla et al. 2002; Gifford et al. 2007). Finally, a synthesis of Pistia's Florida biogeography has broader importance in a basic scientific sense as key clarifications provided through this review may be expected to help guide future evolutionary and biogeographic studies (e.g., Renner and Zhang 2004; Rana and Ranade 2009) into this wide-ranging monotypic plant species.


   Global Overview of Pistia Phytogeography Top


Comprehensive genetic studies suggest that the Pistia clade first originated during the late-Cretaceous at a location near the present day Seychelles Islands (Renner and Zhang 2004; [Figure 2]. Fossil records of Pistia forms similar to P. stratiotes range from the SE US during the Eocene (Berry 1910; Berry 1920) and into much of Europe and western Russia through the Miocene (Stoddard 1989; Renner and Zhang 2004). Due to a noted intolerance of today's P. stratiotes to severe freezing temperatures, it is generally presumed that the Pistia genus experienced major range contractions away from temperate latitudes and into tropical refugia with the advent of Pleistocene glaciations (Stoddard 1989; Dray and Center 2002; Renner and Zhang 2004). Specific paleo-botanical details about the Pleistocene retreat and subsequent Holocene expansion of Pistia are somewhat sparse, but have prompted considerable speculation over time (e.g., Dray et al. 1988; Stoddard 1989). For many years it was hypothesised that the Nile River watershed in northeastern Africa likely served as the source of a modern, anthropogenic dispersal of Pistia into its present pan-tropical range (Holm et al. 1977; Dray et al. 1988). Detailed observations of Pistia by Greek and Roman scholars along the Nile River that date to approximately 2,000 BP (Sculthorpe 1967; Stoddard 1989) apparently served as a primary basis for this suggestion. A common belief that Pistia's non-African populations did not set seed, presumably due to a lack of appropriate pollinators in these other range areas, was also once widely interpreted as further evidence of a Nile River origin (Holm et al. 1977; Stuckey and Les 1984; Dray et al. 1988). However, serious doubts about the modern Nile dispersal hypothesis emerged with research indicating that, contrary to previous assumptions, copious seed production does occur among Pistia populations outside of Africa (Dray and Center 1989; Harley 1990). Further doubt on the Nile dispersal hypothesis was cast by a series of entomological studies in the 1980s and 1990s that resulted in the discovery of several unique herbivore species with apparent Pistia specialisation and dependency. These discoveries included unique species in South America (Cordo et al. 1981; Dray et al. 1993) and completely different species in radically disjunct areas of southeastern Asia (Habeck and Thompson 1997). Taken together, these entomological discoveries more generally implied that the species persisted in multiple tropical refugia during the Pleistocene glacial periods. More recent literature provide a greater degree of support for the hypothesis that Pistia maintained a cross-continental distribution across much of the humid tropics and near tropics well into the pre-modern Holocene. Ancient tenure in the Western hemisphere is, for example, strongly indicated by both high endemism among herbivorous Pistia specialists in Chaco, Argentina (Cordo et al. 1981; Dray et al. 1993) and a fossilised Pistia seed dated at ~2,240 BP from lake sediments in Oaxaca, Mexico (Goman et al. 2010). An equally far-ranging paleo-tenure in the Eastern hemisphere is suggested by the ancient Nile River reports that indicate presence of several Pistia specialist herbivores in southeast Asia (Habeck and Thompson 1997), and use of the plant as a traditional medicine by people in India and southern Asia (Stoddard 1989; Tripathi et al. 2010). Documented historical introductions and, by extension, unequivocal non-native status for Pistia are confirmed for southern Australia and New Zealand (Waterhouse 1997), remote islands of the tropical Pacific (Fosberg et al. 1987), and isolated thermal streams in Slovenia (Sajna et al. 2007) and southern Idaho (Howard 2010). A map summary of Pistia's paleo-botany and present global distribution is provided in [Figure 2] (map constructed from Berry 1910; Berry 1920; Sculthorpe 1967; Cordo et al. 1981; Fosberg et al. 1987; Stoddard 1989; Dray et al. 1993; Habeck and Thompson 1997; Waterhouse 1997; Renner and Zhang 2004; Goman et al. 2010).
Figure 2: Global phytogeographic highlights of Pistia sp.

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   Evaluation Of Non-Native Hypothesis for Pistia in Florida Top


The non-native designation for Florida's Pistia has been historically justified by three interrelated claims: 1) historical mechanisms provide a plausible explanation for a pre-Bartram introduction of the species into Florida during the Spanish colonial period (circa 1565-1763); 2) observations of invasive overgrowth by the plant in Florida waters are broadly suggestive of non-native origins; and 3) the plant's invasiveness is associated with a local condition of ecological release, which further implies a non-native origin. Each of these claims is respectively explored and critically evaluated in the following sub-sections.

Spanish introduction hypothesis

The claim that Florida's Pistia was introduced during the colonial Spanish period is advanced in several scientific publications (e.g., Stuckey and Les 1984; Schmitz et al. 1993; Cordo and Sosa 2000; Dray and Center 2002; Neuenschwander et al. 2009) and species descriptions by various state and federal agencies (e.g., FDEP 2008; USACE 2010; USDA 2010). Although management agencies typically cite accidental discharge of Pistia fragments from the ballast of colonial ships as the most likely introduction pathway (e.g., FDEP 2008; USACE 2010; USDA 2010), intentional introduction by colonial Spanish settlers has been suggested in at least one publication (Stuckey and Les 1984).

One version of the accidental discharge hypothesis suggests that Pistia may have been originally introduced into Florida through the discharge of ballast water from Spanish ships (USACE 2010; USDA 2010). If taken at face value, this specific suggestion is quite remarkable in that it can be readily dismissed as an historical impossibility, as no Spanish colonial ships before the historical Bartram sightings (pre-1765) used ballast water systems that characterise modern ocean-faring ships. Instead, all colonial era ships exclusively relied upon so-called 'dry ballast' systems that were filled with stones, sand, and other solid materials (see, e.g., Carlton 1989).

Other accounts more generally describe Pistia's historic pathway into Florida as discharge from "ship's ballast" (FDEP 2008) or from "Spanish ships" (Neuenschwander et al. 2009), both of which may be taken to imply a dry ballast introduction. On the one hand, this mechanism has initial plausibility in the sense that dry ballast disposal during the colonial period is known as a primary invasion vector for several non-native maritime plants now naturalised in North America (Carlton 1989; Chambers et al. 1999). On the other hand, at least four factors, together, suggest the unlikelihood of this pathway in the case of Florida's Pistia: 1) none of Florida's extant non-native plant species, which have been researched in great detail relative to many other locations throughout the world (e.g., Austin 1978; Gordon 1998), are known to have been established through dry ballast disposal (Baker et al. 2004); 2) there is little reason to suspect that typical dry ballast materials such as coastal rocks and sand (see, e.g., Carlton 1989) would contain fragments of an obligate freshwater plant such as Pistia; 3) because Pistia is notably intolerant to saltwater (Haller et al. 1974), it would not be expected to survive in the leaky ballast conditions of sea-faring colonial ships (see, e.g., Carlton and Hodder 1995), nor establish populations in typical dry ballast disposal sites such as saltwater ports and intertidal zones; and 4) historical records indicate that treacherous navigational conditions along Florida's inlets and estuaries precluded oceanic ships of the Spanish period from venturing into upstream areas of coastal rivers (e.g., Chatelain 1941), suggesting that there is very little reason to suspect that such ships would have discharged ballast materials into freshwater zones conducive for successful Pistia establishment. It is also notable that the St. Johns River Pistia sightings by the Bartram explorers extended to remote areas approximately 300 km upstream (river distance) of the inlets where sea-faring ships would enter and typically deposit dry ballast materials [Figure 3]. Even if trans-oceanic and discharge survival is assumed, a hypothesised dry ballast introduction still begs the question of historical dispersal for a plant that utilises downstream currents as the predominant mode of naturalisation and spread across watersheds (see, e.g., Howard and Harley 1998). While none of these objections absolutely rule out the possibility of a dry ballast introduction, they do suggest that the plausibility is quite remote.
Figure 3: Phytogeographic highlights of Pistia stratiotes L. in the Florida peninsula

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In a summary review of the Pistia sightings by the Bartram explorers, Stuckey and Les (1984) speculate that Spanish settlers at the colonial capital of St. Augustine (noted in [Figure 3] may have originally introduced the plant into Florida. A relative strength of this hypothesis is that the invocation of direct human care in disbursing the plant makes the assumed trans-oceanic conditions for Pistia survival much more ecologically plausible than with dry ballast. However, Stuckey and Les (1984) justify this hypothesis solely on the casual observation that many non-native plants are known to have been introduced into Florida in the Spanish colonial period. Considered more closely, the reasoning behind this suggestion is significantly weakened by the fact that confirmed intentional plant introductions into Florida during the Spanish colonial period were almost exclusively composed of agricultural commodities such as fruits and grains (see, e.g., Austin 1978; Ruhl 1997). In the absence of any archaeological or historical evidence of Pistia use by the colonial Spanish in Florida (or elsewhere), there appears little reason to suspect, much less assume, that an intentional introduction for this species was likely during the sixteenth to eighteenth centuries. The geography of the Bartram sightings provides additional difficulties for the Stuckey and Les (1984) hypothesis, as it does not explain the apparent dispersal of Pistia into upstream areas of the St. Johns River approximately 200 km from St. Augustine [Figure 3].

Invasiveness implies non-native hypothesis

From a hypothetico-deductive standpoint, the previous section indicates that there is no apparent scientific or historical basis for accepting claim of a Spanish introduction of Pistia into Florida. However, a complementary argument that has been advanced in favour of the non-native hypothesis stems directly from the historical observation of severe overgrowth by Pistia in a number of Florida waterways (e.g., USACE 1973; Dray et al. 1988; Schmitz et al. 1993). Since other aquatic plants of Florida that show invasive overgrowth such as Eichhornia crassipes (water hyacinth), Hydrilla verticillata (hydrilla), and Alternanthera philoxeroides (alligator weed) are non-native, the inference is that similar behavior by Pistia is likewise associated with a non- native origin.

Although such an inference is intuitively appealing and may be generally appropriate for hypothesis building, a number of commentators (Schrader-Frechette 2001; Colautti and MacIsaac 2004; Larson 2007) have noted the formal fallacy associated with simplistic conflation of a term describing an ecological behaviour (i.e., invasiveness) with a term describing anthropogenic dispersal history (i.e., non-native). Intertwined historical and theoretical ecology considerations further suggest that the grounds for such a conflation are particularly weak when applied to Pistia.

To flesh out this criticism, it is crucial to first note that Pistia's recent ecological history (~ past 100 years) in Florida is closely connected to that of E. crassipes (see, e.g., Schmitz et al. 1993), a non-native floating aquatic plant first introduced to North America in the late nineteenth century. Throughout the early to mid-twentieth century, many naturalists and managers observed that Pistia was almost always competitively displaced in Florida ecosystems where it co-occurred with unmanaged E. crassipes (Dray et al. 1988; Schmitz et al. 1993; Carr 1994). Upon the advent of E. crassipes control programmes in Florida in the 1950s and 1960s, it was frequently noted that rapid growth of Pistia into nuisance populations would often rapidly follow in the aftermath of successful E. crassipes control (e.g., USACE 1973; Dray et al. 1988; Habeck and Thompson 1997). This phenomenon was generally attributed to Pistia's relative resistance to 2,4-D, the herbicide most commonly used in early E. crassipes control programmes, and associated competitive release from the otherwise dominant E. crassipes (Eggler 1953; Weldon and Blackburn 1967). The high growth potential for Pistia in eutrophic and hydrologically altered waterways, where nuisance populations of E. crassipes tended to become most severe, was also noted as a possible factor in Pistia's emergence as a novel aquatic weed in Florida (Boyd 1971).

All of these historical details are important to note, as they strongly imply that ecological mechanisms alone may provide a sufficient explanation for observed Pistia overgrowth in Florida. In particular, factors such as release from the superior competition of E. crassipes, major nutrient pulses in the aftermath of E. crassipes control (e.g., Reddy and Sacco 1981), anthropogenic nutrient subsidy from ongoing land use intensification, and large-scale hydrologic disturbances (e.g., Boyd 1971) all suggest that Pistia was exposed to conditions of increased resource availability at the times it was originally observed to become invasive in Florida. Because similar conditions of increased resource availability are known to promote invasiveness among both non-native and native species (Davis et al. 2000), an assumption of non-native origins may therefore be regarded as formally unnecessary for explaining Pistia's invasive behaviour as historically observed in Florida.

Specific studies of aquatic plant ecology, including that of Pistia, bolster this argument in several ways. First, it is well established that certain native aquatic plants can become ecologically invasive in response to increased resource availability and habitat changes associated with modern human impacts. One example of this is the well-known relationship between nutrient-enrichment and hydrologic disturbance in the Florida Everglades with invasive overgrowth by the native plant, Typha domingensis (Miao et al. 2000; Richardson et al. 2008). Second, it is also well-established that altered conditions can specifically result in Pistia becoming ecologically invasive in areas where it is native. For example, severe outbreaks of Pistia overgrowth have been reported in association with nutrient enrichment and hydrologic disturbance in areas of Africa (Kasulo 2000) and South America (Bini et al. 1999) where the plant's native tenure is strongly indicated. Moreover, at least two studies (Vaithiyanathan and Richardson 1999; Ogden et al. 2005) have reported that Pistia overgrowth in the Everglades is almost exclusively associated with nutrient enrichment and hydrologic disturbances similar to those that promote T. domingensis. All of these examples are consistent with Pistia's invasive behaviour in Florida having a possible basis in increased resource availability. For these reasons, observations of Pistia's invasiveness alone may be regarded as an insufficient basis for inferring a non-native introduction into Florida.

Enemy release hypothesis

Although Pistia's ecological invasiveness is itself insufficient for assuming non-native status in Florida, an adjunct claim that has added significant power to the non-native hypothesis is that the plant's local invasiveness may be promoted by enemy release (e.g., Cordo et al. 1981; Dray et al. 1988; Dray et al. 1993; Habeck and Thompson 1997; Dray and Center 2002; Neuenschwander et al. 2009). As generally applied to plants, the term enemy release refers to the condition of being freed from specialist herbivores and/or other controlling pests, with the result often being the advent of uncontrolled invasive overgrowth. Because introduction of a plant into a new range provides an obvious geographic mechanism for escaping co-evolved pests, the demonstration of enemy release is generally regarded as a powerful foundation for inferring non-native status in a local area, especially when more direct information about the plant's introduction history may be unavailable or otherwise inconclusive (Liu and Stiling 2006). A proxy test for enemy release typically involves a detailed comparison of local pest and herbivore communities between various geographic areas where the plant is found (Keane and Crawley 2002). Extrapolating from the evolutionary axiom that more species will evolve to utilise specialised niches over time (e.g., Strong et al. 1984), the straightforward expectation is that plants will have a more diverse herbivore community in their native range (i.e., where specialists co-relationships have evolved over long periods of time) as compared to their non-native ranges (i.e., where there has been insufficient time for evolutionary specialisation).

The specific claim that Florida's Pistia is under a condition of enemy release has its basis in entomological comparisons of Pistia herbivore communities in Florida with those found in Chaco Province, Argentina (Dray et al. 1993; Neuenschwander et al. 2009). In particular, entomological research in Chaco has revealed at least thirteen species of herbivorous insects that specialise on Pistia (Dray et al. 1993). This high specialist insect diversity provides strong evidence of the plant's ancient tenure in this region (Cordo et al. 1981; Dray and Center 2002). By contrast, similar surveys among Florida's Pistia detected no locally unique specialist herbivores (Dray et al. 1993). Explicitly applying the logic of enemy release, Dray et al. (1993) argue that this relative paucity of local enemies is strongly suggestive of Florida's Pistia having non-native origins - a claim that is cited in at least two subsequent works (Dray and Center 2002; Neuenschwander et al. 2009).

When considered more closely, however, such a strong inference immediately runs into at least two quite serious biogeographic concerns. First, peninsular Florida's freshwater invertebrate fauna is considered impoverished when compared with other subtropical/tropical zones in terms of both absolute diversity and local endemism (Brown et al. 2006). This is thought to be associated with Florida's land mass creating an inherent 'peninsular effect' of island-like isolation from a northern frost line and water on all other sides (Webb 1990). Second, the Florida peninsula is also noted for having very flat topographic relief and concomitantly low habitat variety that together promote low rates of speciation (Means and Simberloff 1987). By contrast, Chaco is located within a neo-tropical region of South America known for extremely high insect diversity and endemism (Wilf et al. 2005).

As such, there is ultimately little reason to suspect that a simple comparison between herbivore assemblages in Chaco (which can be expected to have relatively high biodiversity) and Florida (which can be expected to have relatively low biodiversity) provides a sufficient basis for making strong inferences about Pistia's natural history in Florida.

Notably, Dray et al. (1993) explicitly acknowledge such biogeographic concerns. In an attempt to rebut these concerns, Dray et al. (1993) develop the following argument:

  1. Florida's Pistia contains a similar total number (19) of herbivore taxa (composed entirely of generalists) as compared to the total number (22) of herbivores (including the many local specialists) found in Chaco
  2. The similarity in total herbivores in (A) implies that the regional pool of generalist herbivores available for Pistia specialisation in Florida is similar to the regional pool of generalist herbivores found in Chaco Province
  3. Given both (B) and the large number of Pistia specialists in Chaco, it would be expected that Florida's Pistia would have accumulated some specialists if it had been locally present for a similar period as the Pistia in Chaco
  4. Because, contrary to the expectation set by (C), Florida's Pistia has no specialists, it can be concluded that Pistia is characterised by a local condition of enemy release consistent with a recent, non-native introduction into Florida.
This is a complex argument that rests upon a suite of assumptions and associated inferences, several of which remain quite problematic. For example, the foundational theoretical stipulation of the argument, as summarised by premise (B), suggests that total herbivore richness counts can be used to infer equivalent opportunities toward specialist evolution. This specific theoretical claim is, however, controversial at best, or, at worst, a rather significant overreach in terms of both biogeographic and ecological theory. An arguably more parsimonious theoretical explanation for the observed similarity in habitat-specific species richness indicated by premise A) is that niche saturation (see, e.g., Cornell and Lawton 1992; McPeek 2008) has effectively put an asymptotic limit on the amount of local herbivores that can sustainably coexist on Pistia habitat. Because richness saturation into a particular habitat niche may be reached over time irrespective of regional species pools, the idea that generalist pool equivalence between Chaco and Florida, as suggested by premise (B), can be inferred simply by an observation of similar total herbivore richness on Pistia is nonsensical from the standpoint of niche saturation theory. From an empirical standpoint, the very high overall insect richness in South American regions near Chaco (e.g., Wilf et al. 2005) and low richness observed in Florida (e.g., Brown et al. 2006) further argues against the veracity of the equivalent evolutionary opportunities stipulation in premise B). Consequently, there is no obvious reason to assume that the relative species pool for any particular habitat specialisation (including Pistia) would not also be similarly high for Chaco and low for Florida. By contrast, the empirical evidence of richness differences between Chaco and Florida provides no obvious contradiction to the applicability of niche saturation theory. In fact, the findings presented in (A) may alternatively suggest that Pistia-associated herbivores in both Chaco (including the many specialists) and Florida (as composed entirely of generalists) are both approaching an asymptotic local richness limit determined by the specific niche resources associated with the plant. For this reason, the overall richness similarity noted by (A) may be reasonably interpreted as suggestive of native status in both Chaco and Florida, rather than as the basis for an argument for non-native status in the manner constructed by Dray et al. (1993).

While these questions about biogeographic and evolutionary theory are serious, they do not pose the most formidable problem for the argument developed by Dray et al. (1993). In particular, a detailed examination of the logic and explanations that Dray et al. (1993) use to support the inference from premise (C) to the conclusion statement (D) indicates the apparent reliance upon an extremely unusual temporal baseline to define a 'native' species. Citing a review by Stoddard (1989), Dray et al. (1993) note that the earliest North American Pistia fossils are dated at approximately 35 million years BP. Extrapolating from this record, Dray et al. (1993: 1152) argue that a continuous tenure of 35 million years would have given Florida's Pistia "ample time to accumulate a few host specialists," especially given the "abundance of host specialists in South and Central America." From this basis, Dray et al. (1993) argue that the lack of Florida specialists affirms support for the hypothesis that Florida's Pistia is "not a native species" (Dray et al. 1993: 1146), but instead likely reinvaded into "North America following local extinctions of the genus during the Pleistocene glaciations" (Dray et al. 1993: 1152).

What inherently problematises this line of reasoning is an apparent stipulation that Pistia's native status in North America (or, more specifically, Florida) should be defined, as in premise (C), as equal tenure to other regions where it is presumed native (i.e., Chaco), or, more onerously, continuous local presence throughout vast reaches of epochal time (i.e., 35 million years). While defining what comprises native versus non-native tenure is ultimately somewhat subjective (see, e.g., Schrader-Frechette 2001; Larson 2007), it bears repeating that the most standard way in which species are identified native to Florida and other areas of North America is presence at the beginning of European colonisation in the late fifteenth century (Whitney et al. 2004; Stoddard et al. 2006).

In developing their evolutionary argument, Dray et al. (1993: 1147) themselves assert (following Strong et al. 1984) that "adaptation of specialists onto novel food plants" likely requires a minimum tenure of 10,000 years. Given this stipulation, the strongest conclusion (i.e., assuming high local evolutionary pressure) that Dray et al. (1993) can objectively draw from the finding of no Pistia specialists in Florida is a local tenure of 10,000 years or less. Such a temporal range-while perhaps broadly suggesting the possibility of a post-Pleistocene reinvasion into Florida (the strongest temporal claim specifically given by Dray et al. 1993)-does not rule out native tenure as typically defined, which formally only requires a minimum of ~500 years (pre-1492) and certainly includes other periods less than 10,000 years. For example, many tropical plants non-controversially regarded as native to southern Florida likely dispersed into the peninsula subsequent to the most recent Pleistocene glacial maximum (e.g., Alexander 1967; Webb 1990; Brown et al. 2006), generally from Caribbean islands where these species presumably had much longer continuous presence. Under the equal tenure criterion implicitly applied to Pistia in premise (C) and conclusion statement (D), all of these comparatively recent additions to the local flora would likewise have to be considered non-native. This line of reasoning represents an absurd and almost certainly unintended implication of the argument developed by Dray et al. (1993).

Later works (Dray and Center 2002; Neuenschwander et al. 2009) cite the Dray et al. (1993) paper to suggest that Florida's lack of Pistia specialists directly implies an introduction during the Spanish colonial period (~1565-1763 CE). While such an historical introduction would be consistent with non-native status as typically defined, the inferential reasoning underlying this claim is not supported by the evolutionary terms and associated interpretations originally published by Dray et al. (1993). To reiterate this point, the minimum of 10,000 years needed for specialist evolution onto Florida that Dray et al. (1993) posit simply does not exclude the possibility of a pre-Columbian (~ 500 years BP) dispersal of Pistia into places such as Florida where the plant is found to lack such specialists. For the sake of argument, if Pistia had naturally dispersed into Florida at ~5,000 BP (which would clearly provide native status according to traditional definitions), the formal expectation from the working evolutionary theory (i.e., Strong et al. 1984; Dray et al. 1993) is that the subsequent tenure would be insufficient for development of specialist herbivores. Once this underlying interpretive error is taken into account, the biogeographic assertions of both Dray and Center (2002) and Neuenschwander et al. (2009) can be regarded as simply begging the unresolved questions about a historical mechanism that might satisfactorily account for a Pistia introduction into Florida during the Spanish period. A map summary of Pistia's paleo-botany and present distribution in Florida is provided in [Figure 3] (map constructed from Berry 1917; Bartram 1955; Thompson 1968; Stuckey and Les 1984; Quillen et al. 2011; extent of most recent glacial maximum landmass, ~18,000 BP, is generally adapted from Webb 1990).


   Evidence of Pistia as Native to Florida Top


Given the noted weaknesses of the various claims developed in support of a post-Columbian origin for Florida's Pistia discussed in sections 3.1-3.3, the current non-native designation for the species is arguably unjustified on the grounds that the null hypothesis of native tenure, as suggested by the historic Bartram sightings, has never merited rejection. However, a synthesis of available literature provides an even stronger basis for a complete rejection of the non-native hypothesis in its current form. Such a review also provides some insight into local systems where Pistia may be most likely to have ancient tenure and associated natural history relationships.

Paleo-botanical records

One of the most compelling lines of material evidence suggesting that Pistia has pre-Columbian native tenure in Florida comes from an archival plant fossil (reported as P. spathulata Michx., a taxonomic synonym for P. stratiotes L.) collected at an archaeological site near Vero Beach, Florida (Berry 1917; also discussed in Weigel 1962). The stratigraphy that contains the Pistia fossil is described by Berry (1917) as a muck deposit, which is located directly above late-Pleistocene deposits containing bones of early human remains and various mega-fauna mammals. Weigel (1962) further describes the local stratigraphy as indicating ecological succession from a riparian marsh/swamp system containing Taxodium distichum (bald cypress), Anona glabra (pond apple), Brasenia purperea (water-shield), and Pistia into a mesic hammock forest in more recent periods. Given these descriptions, the earliest dating for the deposit containing the Pistia fossil can be inferred as approximately 12,000 BP, which recent investigations have confirmed as the earliest approximate period for human co-occurrence with mammalian mega-fauna at the Vero Beach site (Purdy et al. 2011). However, radiocarbon tests from a muck layer at the Vero site described as stratigraphically similar to that in which the Pistia fossil suggest a possible dating as late as 3,550 BP (Weigel 1962).

Even given the uncertain stratigraphic dating associated with the Pistia fossil in Berry's (1917) report, the approximate date range (~12,000-3,550 BP) for this record contradicts a longstanding belief that no post-Pleistocene fossils of the species have been recorded in temperate North America, including the Florida peninsula (e.g., Neuenschwander et al. 2009). Because there is no climatological basis (see, e.g., Grimm et al. 1993) for suspecting a cold-induced disappearance of Pistia from central to southern Florida in the late-Pleistocene to Holocene periods, this record is incongruous with the hypothesis that the species most recently reinvaded into Florida during the pre-Bartram colonial Spanish period. The location of the fossil report in Vero Beach, located near the headwaters of the St. Johns River, further suggests an upstream and warmer climate zone source for the Pistia reported by the Bartram explorers in the lower St. Johns River basin during the eighteenth century [Figure 3]. For all these reasons, a prolonged pre-Columbian tenure in the Florida peninsula offers the most parsimonious explanation for the geographic consonance between the Berry (1917) fossil and the historic Bartram reports.

It is important to note that neither the Pistia fossil record in Berry (1917), nor the later paleontological investigations of the Vero site and Pistia fossil listing by Weigel (1962), have been cited in any of the past discussions of Pistia biogeography in Florida (e.g., Cordo et al. 1981; Stuckey and Les 1984; Dray et al. 1988; Stoddard 1989; Dray et al. 1993; Habeck and Thompson 1997; Cordo and Sosa 2000; Dray and Center 2002; Evans et al. 2007; Evans 2008; Neuenschwander et al. 2009). A likely explanation for this omission is that these documents were only recently digitised, making their discovery through electronic keyword searches much easier than through traditional library research. The original Berry (1917) report was made available online by JSTOR on 11 May 2008 (Baumann pers. comm. 2011), while the Weigel (1962) report was made electronically available by the University of Florida in September 2010 ( http://ufdc.ufl.edu/UF00000476/00001/metadata ).

It may be plausibly argued that the Berry (1917) fossil report, if taken alone, provides a questionable basis for making very strong inferences about Pistia's Florida tenure. For example, the archaic stratigraphic methods employed by Berry (1917) may raise questions about the validity of the dating chronology extrapolated from much later work at the site (i.e., Weigel 1962; Purdy et al. 2011). The grounds for such an objection are arguably strengthened by the fact that a number of historic paleo-pollen studies in Florida lakes do not report Pistia in late-Pleistocene or Holocene sediment layers (Watts 1969; Watts 1975; Brenner et al. 2006). However, such doubts about the Berry (1917) work are considerably weakened by a very recent paleo-limnological report of Pistia pollen from early Holocene (7,000-11,000 BP) sediments at Lake Annie (Quillen et al. 2011), a sinkhole lake in south central Florida. The pre-Columbian timing and latitudinal range of the Lake Annie pollen report [Figure 3] are quite consonant with those associated with the Berry (1917) fossil, and thus provide an independent line of material evidence that places Pistia in southern Florida during the early Holocene. Taken together, these fossil records provide a compelling degree of support for the hypothesis that the Florida Pistia observed by the Bartram explorers was of pre-Columbian origins.

Ecological associates

The paleo-botanical evidence of Pistia's native tenure in Florida is complemented by entomological surveys that indicate several apparently unique local species with strong life cycle dependencies on the plant. For example, a long-term survey of Florida ceratopogonids resulted in the description of one new species, Forcipomyia (Euprojoannisia) dolichopodida that shows indications of Pistia specialisation throughout its entire life cycle (Chan and Linley 1989). Species descriptions for two ceratopogonids (Atrichopogon wirthi and Dasyhelea chani) and a saprophage (Rhysophora laffooni), both of which have to date been observed only in Florida, also note indications of Pistia specialisation during larval stages (Chan and Linley 1988; Chan and Linley 1990; Wirth and Linley 1990; Chan and Linley 1991; Deonier 1998). A long-term survey by Escher and Lounibos (1993) further reported that the most abundant Chironomadae on Florida's Pistia was an undescribed, potentially locally unique, species of the genus Stempellina.

The original species description for Aphaostracon pycnus, a highly local endemic Hydrobiidae snail, is also somewhat suggestive of a possible long-term natural history relationship with Pistia. Thompson (1968) describes collecting high numbers of A. pycnus exclusively from the roots of the floating plants Pistia and E. crassipes in north central Florida's Alexander Springs [Figure 3], which comprises the entirety of the known species range. Given the relative interchangeability in habitat function between Pistia and E. crassipes (e.g., Schmitz et al. 1993; Carr 1994; Rader 1994) and the unambiguously non-native status of Florida's E. crassipes, it may be speculated that affinity for floating plants by the highly local A. pycnus presumably would have developed in association with Pistia throughout recent evolutionary time. The close proximity of Alexander Springs, a tributary of the St. Johns River, to several of the historic Bartram sightings for Pistia (shown in [Figure 3] may provide indication of long-term Pistia presence in this local ecosystem.

Thermal streams as potential paleo-refugia and dispersal points

Although the direct Florida records of Pistia in the early Holocene are currently limited to a paleo-marsh (Berry 1917; Weigel 1962) and a sinkhole lake (Quillen et al. 2011) in the southern half of the peninsula, a variety of research and associated inferences suggest that thermal spring systems may have served as primary paleo-refugia and/or dispersal pathway into the central and even northern peninsula. A recent investigation by Sajna et al. (2007) for example, has documented the invasion and persistence of Pistia into an isolated spring-fed stream in the eastern European country of Slovenia. This finding is relevant because Pistia's overwintering in this temperate zone (local average January temperature < 0°C) stream is associated with the thermal mediation of the spring flow source, which discharges at a constant temperature of ~25°C (Sajna et al. 2007). Analogous Pistia overwintering in a thermal spring run has also been recently reported in southwestern Idaho, USA (Howard 2010), which has a temperate climate characterised by an average January temperature of ~0°C (Western Regional Climate Center 2005).

Extrapolating from these examples, logical attraction toward the possibility of a similar spring association for Florida's Pistia, potentially as early as the most recent Pleistocene glacial maximum climate period (~18,000 BP) is provided by several factors: 1) the very large concentration of thermally mediated freshwater spring systems found in the Florida peninsula (Scott et al. 2004); 2) paleo-climate studies (Watts 1980; Grimm et al. 1993) indicating that average January temperatures in northern Florida during the most recent glacial maximum were several degrees higher than the Slovenia and Idaho locations where Pistia is now over-wintering; 3) that the Bartram explorers noted significant Pistia communities around several spring runs (e.g., Manatee Springs in the Suwannee River basin and several springs feeding into the St. Johns River; [Figure 3]; 4) evidence for some degree of Pistia habitat affinity for the highly local A. pycnus at Alexander Springs (Thompson 1968); and 5) observations of major insect attacks that resulted in large-scale reductions of Pistia populations at Silver Springs (Odum 1957; stream location noted in [Figure 3], a phenomenon that may be more generally suggestive of long-term ecological associations. Although many contemporary Florida spring systems likely lacked artesian flow in the most recent glacial maximum due to much lower sea levels and associated regional groundwater tables (Webb 1990), historic reports of large freshwater springs in the Biscayne Bay system of extreme southern Florida (Langevin 2003; noted in [Figure 3] and several known examples of extant submarine springs in the Atlantic Ocean and Gulf of Mexico suggest the likelihood of many Pleistocene-epoch springs throughout now-submersed areas of the Florida shelf (Nordlie 1990; extent of submersed shelf shown in [Figure 3]. At the very least, this line of reasoning suggests support for detailed biogeographic and ecological research investigations into natural history associations that may characterise Pistia within Florida spring systems.


   Conclusions and Implications Top


Three arguments about Pistia biogeography have been advanced in this paper: 1) existing historic, ecological, and paleo-botanical evidence indicates an apparently ancient, cross-continental distribution for Pistia in neo-tropical America, northern Africa, and southern Asia; 2) claims advanced in support of the hypothesis that Pistia is non-native to the Florida peninsula are historically, ecologically, and/or logically insufficient for scientific acceptance; and 3) paleo-botanical, historical, and ecological literature provide a compelling basis of support for the hypothesis that Pistia meets the consensual definition of a native Florida species.

This case study joins several others in which native species demonstrating ecological invasiveness have been historically misidentified as invasive non-native species. For example, a recent paleo-pollen study in the Galapagos Islands indicates that at least six plant species historically identified as non-native or likely non-native were present many thousands of years before humans discovered the archipelago (van Leeuwen et al. 2008). Subsequent macrofossil investigations of the Galapagos have confirmed all six of the native plant identifications by van Leeuwen et al. (2008), and identified native tenure for three additional plant species previously suspected as non-native (Coffey et al. 2011). Similarly, a phylogenetic study of the yellowfin shiner (Notropis lutipinnis) in the Little Tennessee River indicates that the fish, which was previously suspected as a non-native invasive in this area due to recent population increases, has had a population presence in the river basin well before human colonisation of North America (Scott et al. 2009).

From a management perspective, the current non-native designation for Florida's Pistia is significant because it underpins a "maintenance control" programme that seeks to minimise the plant in all water bodies where it is established (FDEP 2008). Application of chemical herbicides is the most commonly employed Pistia control method, although bio-control organisms and various aquatic plant harvesting programmes are also sometimes utilised (FDEP 2008). By contrast, management of Florida's native aquatic plant species, including those that may display invasive or weedy characteristics in some circumstances, does not involve the goal of minimizing all populations, but instead is implemented as a case by case response to nuisance issues or other undesirable overgrowth conditions.

Similar to discussions introduced in all of the above case studies for previously misidentified species (i.e., van Leeuwen et al. 2008; Scott et al. 2009; Coffey et al. 2011), evidence of native tenure for Florida's Pistia raises important and difficult questions about the unintended consequences of current management interventions - particularly to the extent that these are based on an uncritical assumption of non-native status. For example, several studies (Bryan 1990; Rodgers et al. 2001; Corrao et al. 2006) have observed that the herbicidal control of Pistia may have potentially deleterious impacts on native apple snails (Pomacea paludosa), a primary food source for species of conservation concern such as Aramus guarana (limpkin) and Rostrhamus sociabilis (snail kite). In addition, a stream metabolism study at the spring-fed Wekiva River system specifically noted a highly significant depression of dissolved oxygen immediately following chemical control of mixed populations of Pistia and E. crassipes (Wetland Solutions, Inc. 2006). Such oxygen depression from herbicidal management suggests a specific conservation concern associated with control of Pistia at spring run systems that contain highly localised endemic Hydrobiidae snail species, including, for example, A. pycnus at Alexander Springs. A recent review by Shelton (2005) specifically notes that a large number of Florida's Hydrobiidae are imperiled due to degradation of local spring run habitats (for comprehensive species lists and distributions of Florida Hydrobiidae, see Thompson 1968; Shelton 2005), with depressed dissolved oxygen and exposure to herbicide compounds both listed as specific stressors for these taxa. Such management concerns are further amplified by the above arguments suggesting that spring runs could be among the most likely candidates for Pistia paleo-refugia in the Florida peninsula.

At the same time, it should be noted that apparent confirmation of native status does not imply that future control of Florida's Pistia should necessarily be avoided in all circumstances. Using the systematic plant invasion terminology presented by Pysek et al. (2004), native Pistia may in many contexts still be considered a "weed," in the sense that overgrowth can have environmental and/or economic impacts that are considered undesirable by humans. Consequently, selective Pistia control for improved drainage (Bini et al. 1999), mosquito suppression (Escher and Lounibos 1993), fisheries enhancement (Adams and Lee 2007), or local maintenance of plant communities more conducive to endangered species such as R. sociabilis (Sykes 1987; Rodgers et al. 2001) can and should be considered somewhat independently of biogeographic tenure. Put another way, it is clearly appropriate to consider benefits, costs, and harms associated with selective Pistia control for prescribed management purposes in the context of challenges facing specific Florida ecosystems. However, an a priori imperative to minimise Pistia in all contexts, as clearly fostered by the attribution of non-native status (see, e.g., Hager and McCoy 1998; Evans et al. 2008; Stromberg et al. 2009; Davis et al. 2011), is quite inappropriate in Florida given the compelling evidence for the plant's native tenure.

A final issue of some importance is that official identification of Florida's Pistia as a non-native species has resulted in no previous attention to the possibility of more recent (e.g., the twentieth century) cryptic hybridisation between local native Florida populations of the plant and other continental lineages (e.g., from South America, Africa, or Asia). While current literature indicates no direct evidence of more recent introduction pathways or associated cryptic lineage displacement, the example of Phragmites australis becoming highly invasive in North America subsequent to non-native hybridisation with native populations (Saltonstall 2002) does supply an analogue for some consideration. Detailed molecular phytogenetics studies (see, e.g., Renner 2005) would likely provide the most straightforward way for identifying Pistia biotypes unique to Florida, making biogeographic linkages with other locations in which the plant is found, and determining any extent to which non-native biotypes may have cryptically transgressed into the Florida population. Particular attention should be taken to include plant samples from spring runs, ancient lakes (e.g., Lake Annie) and across a variety of eutrophication and disturbance gradients as well as potential identification of local Pistia populations that may have had little herbicidal selection pressure. In addition to helping solve remaining biogeographic puzzles, such research would likely also provide useful guidance for watershed remediation and restoration programmes that propose to utilise Pistia for water quality purposes (e.g., SJRWMD 2009; Liu et al. 2010; Liu et al. 2011), as care may be taken to identify and preferentially utilise lineages with locally native tenure.


   Acknowledgements Top


Much of the research for this paper was performed with support from the E.T. York Presidential Fellowship through the University of Florida's School of Natural Resources and the Environment. Original ideas for the paper were inspired by conversations with the late H.T. Odum, in whose memory the published version is dedicated. Advisement and assistance through the research process were provided by M.T. Brown, J. Burkhardt, M.J. Cohen, R. Hamann, R.P. Haynes, S. Humphrey, and A.C. Wilkie. I also sincerely thank M. Brenner, J. Dame, J. Heffernan, S. Kingery, B. Knight, J. Miller, B. Morgan, H. Spivey, and several anonymous reviewers for additional comments and criticisms that greatly improved the final manuscript . Funding sources, advisors, and reviewers played no direct role in developing the paper and are not responsible for any of its content.[126]

 
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