The Chile-California Invasion Connection
Mark A. Blumler
SUNY-Binghamton, Binghamton, NY, USA (email mablum@binghamton.edu)
Keywords: Invading species, Mediterranean ecosystems, multilocus genotypes
Plant species from the Mediterranean region have severely invaded grasslands and savannas in regions of similar climate.
California has suffered noticeably more serious invasion than Chile. Gulmon reported that many if not most of the major
invaders are shared by California and Chile – wild oat, Avena fatua, is an exception - giving very similar species-composition
at individual sites. Her explanation for the similiarity was ecological, but in recent years it has become increasingly
recognized that invasion is at least as much a geographical as an ecological phenomenon, i.e., that the historical geography
of invasion needs to be taken into account. Although most discussions of California invasion have emphasized dispersal
from Mexico or Spain, transport from Spain to Chile to California has been ignored. Early (mid-nineteenth century and
earlier) invasion into California was along 3 main routes: via Mexico, via Chile, and via the eastern US. In contrast, Chile
was reached only along one main route, direct from Spain, so it is not surprising that California received more alien, invading
species. Of the three routes to California, only that via Chile was Mediterranean, so it is also not surprising that the major
invaders into Chile tended to become major invaders into California too. Comparisons with other Mediterranean-type
regions, such as Australia, are needed to determine if the same suite of species was successful there too, or if there was a
different mix reflecting different routes of invasion.
An illuminating example is the slender wild oat, A. barbata, which was thoroughly studied by Allard’s group at Davis. In
California there are two main multilocus genotypes, the “xeric” and the “mesic”. Neither was found in Spain, though the xeric
differs only slightly from common types there. Allard proposed that both must have evolved in California in response to
environmental conditions there. Since the species is not found along the other two invasion routes, it most likely came to
California via Chile. Every S. American herbarium specimen that I have seen corresponds morphologically to the xeric
genotype. The mesic genotype appears to have arisen in situ in California as a hybrid between the xeric genotype and the
related species A. strigosa. The mesic genotype, and recombinants between it and the xeric genotype, are now replacing
the latter over much of its distribution.
A. strigosa occurs in Chile, but appears to be separated geographically from A. barbata. This leads to two predictions: 1) all
slender wild oat in Chile are xeric genotype electrophoretically as well as morphologically (i.e., if analyzed for the many loci
that Allard used to define the genotype). 2) If and when A. strigosa and A. barbata come in contact in Chile, hybrids similar
to the mesic genotype will arise.
The xeric genotype differs from typical xeric forms of annual species in that it is relatively late flowering. This might be
adaptive in central Chile, which has an unusually variable precipitation regime compared to California or the Mediterranean.
Thus, it may have evolved in Chile before spreading to California, where it was not as well adapted but was able to flourish
because no other genotypes of the species were present until the hybridization with A. strigosa. If this scenario is correct, it
would illustrate an interplay between contingency, adaptative evolution, and geography in the success of invading species.
Introduction
Invasion of alien species is a major, global concern today, threatening native species with extinction, as
well as producing detrimental economic impacts. In recent years, ecologists have attempted to develop
“invasion rules” to predict which alien species are likely to become invasive if introduced (Enserink,
1999; Kolar and Lodge, 2001); ideally, governments would then blacklist those species. The focus has
been on ecological invasion rules, but it has turned out that successful invaders are a diverse lot, with
few if any ecological commonalities (Blumler, 2007; n.d.) On the other hand, there is increasing
recognition of the importance of geographical factors, specifically: a) similarity in climate of the native
and introduced regions; and b) propagule pressure, i.e., number of introductions and number of
propagules disseminated with each introduction (Kolar and Lodge, 2001; Castro et al., 2005;
Lockwood et al., 2005; Blumler, 2007; n.d.) As Castro et al. (2005:183) concluded from their study of
the spread of invasives in Chile: “…historical factors are more important than biological ones in
determining the geographical extent of naturalized plants…Thus, caution should be exercised when
assigning value to biological attributes that may confer invasiveness to naturalized plants.” Kolar and
Lodge (2001) pointed out that the overseas dispersal phase is crucial, yet has been the least studied of
all the stages of the invasion process. There is a need for detailed study of the historical geography of
invasion (Kruger et al., 1986; Blumler, 1995; 2005; 2006; 2007; n.d.)
One of the most striking cases is the invasion of mostly annual plants from the Mediterranean and
Europe to overseas regions of mediterranean-type climate (Kruger et al., 1986; Blumler, 1995). This
invasion has been remarkably one-way, with very little return invasion. Introduced species dominate
grasslands and savannas in the analogue regions – California, central Chile, Cape region of South
Africa, and western and southern Australia. The invasion and replacement of indigenous grasslands
and oak woodlands in low-elevation California has been more drastic than that experienced by any
other large-scale ecosystem on a continental land mass (Blumler, 1995). It is not unusual for alien
species to constitute 80-90% of the cover in California’s Valley grassland (Heady, 1977; Bartolome,
1979). Kruger et al. (1986) compared the number of introduced species in the floras of the
mediterranean-type regions, and found that more species have invaded California than the other
regions.
Many scholars have speculated as to the reason(s) for the spectacular success of Mediterranean species
overseas, and a few have asked why Chile is less dominated by introduced species than California
(e.g., Gulmon, 1977). Their answers have tended to emphasize ecological factors rather than the
vagaries of dispersal (but see Castro et al., 2005). Gulmon (1977) studied species-composition in
Chilean and Californian grasslands, and found that the introduced species were highly similar, though
the Chilean samples lacked alien tall grasses such as common wild oat (Avena fatua) and ripgut brome
(Bromus diandrus). She related the similarity in species composition between California and Chile to
ecological convergence. An alternative hypothesis is that many of the characteristic introduced species
in California came there from Spain via Chile, and that the assortment of species present was
contingent upon successful dispersal along this route (Blumler, 2006; 2007). If so, one would expect
greater similarity between California and Chile than between either of them and South Africa or
Australia since the latter were never Spanish. Indeed, the data of Kruger et al. (1986) suggest that is
the case. This paper explores the role of the largely unrecognized Spain to Chile to California invasion
route.
Early Routes to California
Blumler (2006) pointed out that despite the frequent emphasis on direct dispersal from Spain in the
California invasion literature, in fact, no Spanish ships sailed to California other than those of the
Malaspina scientific expedition. Since Malaspina did not engage in commerce, he presumably carried
little in the way of agricultural goods amongst which alien species could have hidden and been
dispersed. In this modern era of air travel, it is little appreciated that California was the furthest
frontier of the Spanish empire (along with the Philippines). To reach it from Spain entailed sailing
around Cape Horn, past all the other New World Spanish possessions excepting those bordering the
Caribbean. During the periods of Spanish and, later, Mexican rule, ships came to California from the
Pacific Coast of Mexico, Chile, and Peru; New England and British whaling vessels also were frequent
visitors. The Gold Rush expanded the number of routes by which individuals came to California notably there was some trade with Australia - but on the whole it is fair to say that through much of the
nineteenth century alien species came to California by three main routes, each requiring successful
establishment in a staging area along the way (Fig. 1): 1) from Spain via the Mexican highlands; 2)
from Spain via Chile and neighboring regions; and 3) from Britain and west-central Europe via the
eastern US and then either overland or by boat to the Pacific Coast (Blumler, 2006; 2007).
Figure 1. Simplified depiction of the three main routes taken by early invaders of California and its
Pacific Coast neighbors, numbered in approximate chronological order (Blumler, 2007).
Chronologically, the route via Mexico was the earliest, while the eastern US route continued to
increase in prominence as the nineteenth century passed into the twentieth. For Mediterranean species
to disperse to California by either route 1 or 3, they would have had to establish in non-mediterranean
regions; in contrast, dispersal via Chile would have been possible without leaving the mediterranean
climate. Thus, one might expect that Mediterranean endemics, i.e., species that in the Old World do
not extend outside the Mediterranean zone to any appreciable extent, might be restricted to the Chilean
route. On the other hand, European winter annuals, or annuals that are common to both the
Mediterranean and parts of northern Europe, might have been able to disperse to California along
routes 1 and 3. Even if these species were abundant in the Mediterranean, the genotypes arriving along
routes 1 and 3 might have been only submediterranean in their adaptation. In contrast, Chile was
reached along only one main route, from Spain, so it is not surprising that California received many
more alien, invading species than Chile did (Blumler, 2006).
Drawing upon Hendry’s (1925; Hendry and Bellue, 1931) pioneering archaeobotanical studies, and
other sources, Robbins (1940) listed the species that he believed were introduced to California before
1860. Frenkel (1977) modified Robbins’ list, and divided the species according to whether in his
estimation they came in during the Spanish (prior to 1825), Mexican (1825-1848), or early American
(1848-1860) periods. I further modified Frenkel’s list, and dropped from consideration those
introduced species that are not invasive in low-elevation California. Then, I attempted to determine by
which route(s) each species came to California. This required attention to Old World distribution,
reported presence/absence in the staging areas of the 3 routes, and abundance in those areas. If a
species is present in both Mexico and Chile but much more abundant in the latter, then Chile might be
a more likely source for dispersal on to California. Tutin (1964-1980) is authoritative on European
distributions. Information on presence or absence and abundance in the staging areas, and dates of
introduction to those areas, is much less comprehensive, particularly for Mexico. Veblen’s (1975)
excellent treatment of the Guatemala Highlands aided in assessing Mexican introductions1. For Chile
and the eastern US information is better, though by no means ideal. Chile’s record of introductions
begins essentially with Gay (1845-1854), about three centuries after the initiation of Spanish
settlement. Philippi (1875) also is informative. There remain additional sources to examine, so the
conclusions drawn here should be considered highly tentative and preliminary.
Patterns of Dispersal and Invasion
My speculations are summarized in Table 1. Those species that became widespread in California very
early almost certainly came via Mexico. For instance, filaree (Erodium cicutarium) reached California
a few years before the first Spanish settlement (Mensing and Byrne, 1997), suggesting a dispersal
overland from Mexico, possibly from Baja California missions. Filaree also is present in Chile and the
eastern US; it is likely that there were subsequent introductions from these regions, which presumably
increased its genetic diversity, but it spread so rapidly that it is doubtful whether it really “needed” any
additional diversity. Similarly, common wild oat dominated a vast area in California very quickly
(Blumler, 1995; n.d.) but was not reported in Chile until the twentieth century (Castro et al., 2005). It
is certain in this case that there were many subsequent introductions, along route 3 and probably from
other sources: common wild oat is a crop seed mimic that prior to the advent of mechanical cleaning
devices was often harvested with the crop and sown along with it again; hence it was a characteristic
contaminant of grain and would have been introduced repeatedly and often into California with grain
shipments. This is suggested also by its very high morphological and genetic diversity there (Blumler,
1995; 2004).
1
European or Mediterranean species invasive in Guatemala most likely dispersed there via Mexico; ecological conditions
in its highlands are not dissimilar to those of central Mexico.
Table 1. Probable route(s) taken by successful early invaders of CA grasslands. More important
invaders listed first.
Via Mexico, at least initially
By 1825
Avena fatua
Erodium cicutarium
Medicago polymorpha
Brassica nigra
Hordeum murinum ssp. leporinum
Malva parviflora
Melilotus indica
Via Mexico and/or Chile
By 1825
Lolium multiflorum
Rumex crispus
Sonchus asper
Poa annua
Via Chile
By 1825
Centaurea melitensis
Madia sativa
By 1848
Vulpia myuros var. myuros
Marrubium vulgare
Raphanus sativus
Cynodon dactylon
By 1848
Vulpia myuros var. hirsuta
Silene gallica
Anagallis arvensis
Convolvulus arvensis
Erodium moschatum
Stellaria media
By 1860
Vulpia bromoides
Brassica rapa
Cerastium glomeratum
Lamarckia aurea
Malva nicaeensis
By 1848
Briza minor
Anthemis cotula
Gastridium ventricosum
Sisymbrium officinalis
Bromus trinii
By 1860
Bromus hordeaceus
Centaurea solstitialis
Avena barbata
Erodium botrys
Hypochoeris glabra
Torilis nodosa
The major early invaders were filaree, common wild oat, bur clover (Medicago polymorpha), and in
southern California, black mustard (Brassica nigra). All occur in Mexico, so it seems likely that all
spread from there originally (Table 1). On the other hand, a suite of species present in Chile but not,
apparently, in Mexico, that are Mediterranean endemics (or nearly so), probably came to California via
South America. These appeared in California shortly after the spread of the four species mentioned
above. Notable among the introductions by the Chilean route are slender wild oat (A. barbata), soft
chess (Bromus hordeaceus), yellow star thistle (Centaurea solstitialis) and another filaree (E. botrys),
which often dominate the vegetation today2. But there are others, such as tocalote (C. melitensis) and
2
The closely similar E. brachycarpum may also have come from Chile, where it may be present but confused with E.
botrys, just as it was in California until recently.
nitgrass (Gastridium ventricosum), which apparently were much more important in the vegetation in
the nineteenth century than they are today. In addition, the inconspicuous species, Logfia gallica and
Aira caryophyllea, may have arrived in California long before they were recognized, in which case
they also would be logical candidates for the Chilean route. Along with these Mediterranean species
came at least two native Chilean ones: Chilean tarweed (Madia sativa) and Chilean brome (Bromus
trinii)3. Other species, common in Spain but also in Mexico as well as Chile, may have reached
California via either route, or both.
As part of a series of investigations testing for ecological convergence in areas of similar climate on
different continents, Gulmon (1977) found that the invasive floras of California and Chile grasslands
are highly similar, despite site to site variation. But many of the aliens that she found in Chile most
likely dispersed on to California, thus it is not terribly surprising that the species-composition was
similar (Table 2).
Table 2. Alien species in Gulmon’s (1977) samples, and their most probable invasion route to
California, if there (some species are in California, but not in Gulmon’s samples).
In both Chilean and California samples
Old World species
Via Rt. 1
Via Rt. 2
24
5
9
Via 1 or 2
6
Via Rt. 3
0
Uncertain
4
In Chile samples only
Old World species
Via Rt. 1
6
1
Via Rt. 2
1
Via 1 or 2
1
Via Rt. 3
0
Uncertain
3
In California samples only
Old World species
Via Rt. 1
17
4
Via Rt. 2
1
Via 1 or 2
2
Via Rt. 3
6
Uncertain
4
To summarize these results, almost all important Chilean invaders also came to California, but there
were additional species that came to California by Route 3, and that reached Chile only much later or
not at all, and which therefore have not had nearly as much time as in California to invade.
These speculations need to be confirmed through genetic investigations of the sort carried out by
Novak and Mack (2001) on cheatgrass (B. tectorum) (vide infra), and it is almost certain that there will
be some surprises; but on the whole, it appears that Chile made a major contribution of adapted,
Spanish species to California. Without them, California would be significantly less dominated by
introduced species. Moreover, route 3 species have had significant success penetrating into moist
3
There is some question concerning the nativity of these two species: amphitropical distributions, in Chile and California,
are seen in a number of herbaceous species (Raven , 1963); determining whether the distributions preceded European
colonization, or resulted from subsequent introductions, is often problematic. Chilean brome is generally considered
introduced to California, Chilean tarweed is not. I believe both are introduced there.
microsites and regions within California, thus contributing to the overall greater replacement of the
native vegetation.
The example of Bromegrasses (Bromus spp.)
Annual bromegrasses from the Mediterranean and vicinity are among the most successful of invaders
overseas (Roy et al., 1991). However, they were not among the very first invaders of California, or
probably, Chile. Crop weeds tend to show up first in the California record, while the bromegrasses are
more adapted to pastures and roadsides. Blumler (2006) showed that the invasion of each Bromus
species into overseas mediterranean-type regions could be predicted by their distribution in the Old
World in relation to the early trade routes. Thus, Chile received Spanish species, South Africa
received species that range up to Britain or Holland, and so on. Because California was ruled first by
Spain and then by Anglos, more brome grass species invaded it than the other mediterranean regions.
Novak and Mack (2001) reviewed the genetic evidence they had collected on the origins of cheatgrass
populations in the Western US. Cheatgrass is the most serious invader of the Great Basin and other
interior, relatively cold winter areas. Novak and Mack expected that genotypes from Central Asia and
the Middle East would predominate, because of climatic similarity to the Great Basin and the
abundance of cheatgrass there. But they found no introduced plants from there and instead it was
central European material that predominated. I.e., cheatgrass came over route 3, which fits also with
the timing (late nineteenth century). It should not be surprising that no accessions from Central
Asia/Middle East were sampled, given that even today trade and transport from that part of the planet
is limited – though it would not be surprising if Afghanistani genotypes were to start showing up
around western military bases.
Novak and Mack (2001) also traced some California accessions to Spain. Since their samples from
California were limited, it is possible that Spanish material is more widespread than they assume.
Cheatgrass is not known from Mexico or Chile; but the California material suggests the possibility of
an early introduction to one or the other that subsequently disappeared, but not before dispersal on to
California.
The example of slender wild oat
Robert Allard and his colleagues at Davis gathered detailed electrophoretic data on the genetic makeup
of slender wild oat in California, in the context of a heated debate over the importance of natural
selection vs. random processes in evolution. Clegg and Allard (1972) documented the existence in
California of two multilocus genotypes, the “xeric” and the “mesic”, and argued that their distribution
must reflect natural selection. The argument rested upon the assumption that there were many,
genetically diverse introductions of the species from Spain, with selection winnowing the genetic
material down to two main genotypes. In contrast, Hedrick and Holden (1979) showed mathematically
that if the xeric and mesic genotypes were the only ones introduced, then they would be expected to
continue to predominate over recombinants for centuries given slender wild oat’s high rate of selffertilization. Resolving this debate rested upon determining the number of introductions of slender
wild oat, but neither Allard nor Hedrick attempted to investigate this question (Blumler, 1995; 2000;
n.d.) Allard did demonstrate that neither genotype seems to be present in Spain today (Garcia et al.,
1989; Perez de la Vega et al., 1991), though the xeric genotype differs only slightly from common
types there.
Two morphological characters clearly distinguish the xeric and mesic genotypes. I went through the
hundreds of California specimens to investigate history of introduction and spread (Blumler, 2005;
n.d.) Only the xeric genotype was present in California until the twentieth century; the mesic genotype
arose in the San Francisco Bay Area, almost certainly as the result of hybridization with another
introduced species, sand oat (A. strigosa). The mesic genotype, and several others resulting from
further hybridization and recombination with the xeric, are spreading, often rapidly, replacing the xeric
genotype in many places, and expanding the range of the species (i.e., its invasiveness) in others. For
instance, it is becoming a serious invader in western Oregon. When only the xeric genotype existed,
the species had only marginal presence there.
Thus, it appears that only a single genotype of slender wild oat was introduced to California! It spread
spectacularly, but now it finds itself beset by the new genotypes resulting from hybridization. Slender
wild oat is not found in Mexico, other than across the state line in Baja California, nor is it found in the
eastern US, so the logical dispersal route is via Chile. I have had the opportunity to examine about two
dozen herbarium specimens from South America, most of them Chilean. All correspond
morphologically to the xeric genotype. Thus, the xeric genotype may have 1) come to Chile direct
from Spain (given that the introduction could have occurred as early as the sixteenth century, other
genotypes may have since replaced it in Spain); 2) arisen in Chile in consequence of genetic drift after
introduction of a small number of genotypes from Spain; or 3) evolved adaptively in Chile from
genotypes introduced from Spain. It also is possible that although morphologically xeric, Chilean
material may be more genetically diverse than California xeric (which is entirely monomorphic for
almost every gene studied [Blumler, 1995; 2000]). Sand oat is also present in Chile, though
concentrated as it is in the Biobio watershed, it is geographically separated from the major slender wild
oat populations in the mediterranean region to the north (Matthei, 1995).
The xeric genotype differs from typical xeric forms of annual species in that it is relatively late
flowering (Hamrick and Allard, 1975; Blumler, 1995; 2000). This might be adaptive in central Chile,
which has a more variable precipitation regime than California or the Mediterranean (Gulmon, 1977,
was misinformed on this point), giving rise to an unusually high frequency of prolonged droughts
within the rainy season. Alternatively, it may represent a “weed” adaptation, to unpredictable
disturbance regimes in South America. If so, it may have evolved in Chile before spreading to
California, where it was not as well adapted but was able to flourish because no other genotypes of the
species were present until the hybridization with sand oat. If this scenario is correct, it would illustrate
an interplay between contingency, adaptive evolution, and geography in the success of invading
species.
Several hypotheses deserve testing: 1) all Chilean slender wild oat, extant and in herbarium collections,
will correspond morphologically to the xeric genotype of California (a caveat is that the mesic or other
genotypes might be recently introduced to Chile from California, in which case their distribution will
be highly restricted geographically); 2) Chilean slender wild oat will also correspond
electrophoretically to the xeric genotype; 3) if and when sand oat comes into contact with slender wild
oat in Chile, new genotypes similar to the mesic of California will arise.
Discussion/Conclusions
Comparisons with other mediterranean-type regions, such as Australia, are needed to determine if the
same suite of species was successful there too, or if there was a different mix reflecting different routes
of invasion. The information that Kruger et al. (1986) distilled from the literature suggests the latter is
likely. Kruger et al. also discuss climatic similarity as an influence on invasion success. Implicit in
the analysis reported here is the assumption that on the one hand, origin in a “homoclimate” gives a
species an inherent leg up in invasion, but that it also is true that adaptive evolution, especially through
hybridization and introgression, in the invaded region can dramatically increase invasiveness (Blumler,
2004). Slender wild oat illustrates both the homoclimate rule, and the value of hybridization after
introduction. Finally, Kruger et al. bemoaned the lack of information on the history of invasions in the
different mediterranean regions. While such information will never be as good as one would wish, the
research reported here illustrates that it should be possible to improve our understanding considerably,
with undoubted implications for invasion ecology and management.
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