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Reproductive and Recruitment Dynamics of Invasive Hybrid Cordgrasses (S. alterniflora x S. foliosa) in San Francisco Bay Tidal Flats

  • Author(s): Sloop, Christina M.
  • et al.

Maritime cordgrasses in the genus Spartina (S. alterniflora and S. anglica) are potent invaders of naturally open intertidal mudflats worldwide. Successful colonization appears to be due to large open tidal mudflats providing regeneration sites for seedlings, and the ability of large cordgrasses to survive in this harsh environment. In Willapa Bay, WA, for instance, the rate of S. alterniflora spread is exponential, and an expected endpoint to this invasion, without control, would be extensive meadows of cordgrass, as are seen within the native range of this species. In San Francisco Bay (SFB), the Spartina invasion is different in that the introduced species, S. alterniflora, hybridized with a native species, S. foliosa. Subsequent introgressive hybridization has left only a small number of extant SFB S. alterniflora, and has produced a diverse array of hybrid genotypes, now spreading at a rate exceeding exponential growth.

Due to their invasive histories and extreme and accelerating spread rates, Spartina cordgrasses are model organisms for studying biological invasions, and are so the subjects of many contemporary ecological and evolutionary investigations, including Spartina population genetics and systematics, and the hybridization between S. alterniflora and S.foliosa in SFB. In order to increase the low number of 11 available microsatellite markers, I describe an additional 24 disomic microsatellite loci in Spartina alterniflora in Chapter 1. All new loci were polymorphic and amplified in Pacific and Atlantic S. alterniflora, and in a swarm of S. alterniflora x S.foliosa hybrids. All loci also amplified in S. joliosa, but most were monomorphic. A subset of loci successfully amplified in S. densiflora, and showed some polymorphism. I established several species-specific alleles for S. densiflora and S.foliosa. Levels of allelic variation in these 24 new loci were again lower in invasive populations of S. alterniflora in pacific estuaries than in native east coast populations.

Hybridization has been proposed as a stimulus to invasiveness. The contemporary invasion of hybrid cordgrass, introduced Spartina alterniflora x native S. foliosa in SFB, serves as a model system for the role of hybridization in increasing invasiveness. In Chapter 2, I investigate the possibility that increased self-fertility has heightened the spread rate of hybrid Spartina in this system, characterized by salt marsh estuaries and open, unvegetated tidal flats. My results showed both parent species to be largely self­ incompatible, with pollen availability limiting seed set. Hybrids in situ set on average 13% (range: 0% to 76%) self-fertilized seed, while neither parent did so in SFB. As well, some of the most recent descendents of this ca 30 year-old hybrid swarm have colonized and reproduced successfully in slightly deeper, open, unoccupied mud in tidal flats; they were more self-fertile than their progenitors in a nearby cordgrass meadow along the shore. Their survival in the harsh mudflat environment indicates that selfing interacts with other transgressive survival traits in these colonizers. Allee effects can limit invasion rates, and pollen limitation at the low-density leading edge has reduced the rate of invasive S. alterniflora in Willapa Bay, WA. In contrast, self-fertilization ensures sexual reproduction at the leading edge of hybrid Spartina. In the absence of inbreeding depression, colonizing plants can so contribute vast numbers of seeds, establishing as robust transgressive seedlings in the open mud or float elsewhere. Hybrid cordgrasses in SFB have spread at greater than exponential rate, and I propose that increasing frequency of a few hybrids possessing transgressive traits, including overcoming pollen limitation, have allowed colonization and reproduction at low density and are keys to this invasion success.

Hybrid cordgrasses (Spartina alterniflora x S. foliosa) have invaded SFB and are not only spreading within native Spartina marshes; isolated individuals, capable of self­ pollination, have also established in the vast, un-vegetated SFB tidal flats. This establishment, if unchecked, will cover these tidal flats, and result in the loss of valuable shorebird foraging habitat, seriously impacting migrating birds on the Pacific Flyway route. As local recruitment intensity is one factor underlying the rate and direction of spatial spread of invasive organisms, in Chapter 3, I examine SFB hybrid genetic structure, hybrid colonization history, potential source populations for regional tidal flat recruitment, and the local fine-scale hybrid seedling recruitment dynamics at three hybrid cordgrass populations along the eastern SFB shoreline. These tidal flat populations are linked by tidal creeks to a nearby Spartina hybrid meadow in the south, and directly adjacent to one in the north. I combined molecular microsatellite marker and geographical (GPS/GIS) data, via multivariate ordination, Bayesian clustering, parentage/sib-ship studies, and spatial genetic analysis. I found the geographically most distant northern population to be genetically distinct from two southern, sympatric and closely related populations. At each site, meadow and tidal flat adult plants were genetically related. In the northern population, the majority of tidal flat seedlings were a product of adults from the adjacent meadow, while at both southern locations, tidal flat hybrids produced most seedlings. At both tidal flat populations, a single isolated individual contributed up to 55% of all surveyed seedlings, the majority of which were inbred.

Tidal flat seed dispersal follows the stepping stone model and most recruitment occurs non-randomly and locally. Positive spatial genetic structure up to 200 meters and negative structure above 440 or 540 meters showed seedling recruitment to be spatially non-random, and suggests a large number of seedlings to establish close to their mothers in sheltered 'safe sites.' All genotyped seedlings were hybrid and seedling recruitment increased on average 6-fold from 2003 to 2004. Seedling size and shoot number surveys show 9 %, 12 %, and 45 % seedling survival per site to one-year or greater. Rapid evolutionary changes in this hybrid Spartina swarm have resulted in a few extremely fit individuals responsible for the majority of locally recruiting seedlings at the forefront of the tidal flats invasion. By producing large numbers of inbred, highly fit propagules in isolation, these transgressive plants ensure further colonization of the tidal flats habitat they have adapted to, and heighten the likelihood of long-distance dispersal. This successful recipe for invasion, coupled with pollen swamping close to native marshes, may explain greater than exponential hybrid spread, and if unchecked, will assure the continuing invasion of SFB by hybrid Spartina.

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