Biological Invasions

Holsbeek, G.; Jooris, R.

doi: 10.1007/s10530-009-9427-2pmid: N/A

Globalization and increasing human impact on natural aquatic systems have facilitated the movement of species and the establishment of nonindigenous species enhancing hybridisation opportunities between naturally allopatric species. In this review, we focus on a special case of natural hybrid speciation and the consequences of recent anthropogenic hybridisation in the water frog complex (Pelophylax esculentus complex), which consists of two parental species, Pelophylax lessonae and Pelophylax ridibundus and a hybrid taxon. The hybrid water frogs reproduce hybridogenetically and eliminate the genome of the syntopic water frog species. Although the actual cause triggering chromosome exclusion remains elusive, it has been proposed that chromosome elimination takes place prior to meiosis and may involve enzymatic degradation of the discarded genome. Translocations of water frogs in Western Europe have become frequent the last decade leading to rapid expansion of the range of the marsh frog P. ridibundus. Subsequent hybridisation of the exotic P. ridibundus may dramatically affect the viability and maintenance of hybrid water frog populations throughout Europe. Interestingly, the impact of this introduced species may differ depending on their geographic origin, which defines the ability to induce genome elimination. This may result in fertile or sterile hybrids, making global conservation guidelines challenging. We predict a severe genetic and ecological impact of nonindigenous P. ridibundus prompting for strict conservation measures to reduce species translocations and for studies on the geographic origin of exotic frog species.

Markert, Alexandra; Wehrmann, Achim; Kröncke, Ingrid

doi: 10.1007/s10530-009-9425-4pmid: N/A

Since 1998 the non-indigenous Pacific oyster Crassostrea gigas (Thunberg 1793) has been invading the Wadden Sea of Lower Saxony, southern German Bight. C. gigas settles predominantly on intertidal Mytilus-beds (M. edulis L.) and subsequently create rigid reef-like structures. Both bivalve species are ecosystem engineers in sedimentary tidal flats. They provide hard substrate for sessile species, mobile organisms find refuge within the habitat matrix of dense suspension feeders, and biodeposits enrich the sediments with organic matter. The transformation of Mytilus-beds into Crassostrea-reefs gives rise to the question whether the invader may affect the native community. We investigated two parts of a changing bivalve bed in the backbarrier area of the island of Juist in March 2005. One part was still dominated by M. edulis whereas the other part was already densely colonized by C. gigas. Crassostrea-reefs compensate for the conceivable loss of Mytilus-beds in the intertidal of the Wadden Sea by replacing the ecological function of M. edulis. There was no indication of a suppression of indigenous species. This even applied to M. edulis, which persisted at the site invaded by C. gigas. The associated macrofaunal community showed increased species richness, abundance, biomass, and diversity in the Crassostrea-reef. The latter particularly favored sessile species like anthozoans, hydrozoans, and barnacles. Higher abundance and biomass for vagile epizoic species like the shore crab Carcinus maenas and the periwinkle Littorina littorea also occurred among oysters. Abundance of deposit feeding oligochaetes was enhanced by oysters as well. More opportunistic, facultative filter-feeding polychaetes occurred in the Crassostrea-reef.

Rodewald, Amanda; Shustack, Daniel; Hitchcock, Lauren

doi: 10.1007/s10530-009-9426-3pmid: N/A

Certain exotic plants may increase risk of nest predation, and, in this way, may act as ecological traps. We hypothesized that the greater vulnerability to predation was a consequence of either (1) reduced nest height due to architectural differences among plant species or (2) seasonal changes in the distribution of nests among forest strata. To test this, we examined temporal variation in nest survival of 888 nests of Northern Cardinal (Cardinalis cardinalis) in native substrates and two exotic shrubs (Lonicera maackii and Rosa multiflora) in Ohio, USA, 2001–2006. We evaluated evidence for an ecological trap by monitoring the annual reproductive productivity of 245 breeding pairs of cardinals. Only nests in Rosa experienced relatively constant survival rates across the season, whereas probability of survival increased over the season for nests in other substrates. Interestingly, the relative vulnerability of nests in different substrates varied across the season. Most strikingly, nests in Lonicera in early spring showed the lowest survival rates but exceeded survival rates of nests in native substrates late in the season. Nest height failed to explain seasonal changes in nest survival, as only nests in native plants significantly increased in height as the season progressed. Rather, predation risk seemed to be a function of the proportion of nests within each substrate, as illustrated by the decreased predation in Lonicera as the relative proportion of nests in native substrates increased. The patterns of temporal variation in predation risk that we detected show that impacts of Lonicera are not a function of plant architecture alone and may be related to leaf phenology, changes in nest density, nest site location, and/or nest synchrony. Examination of the reproductive productivity of cardinals showed that pairs that made their first nest attempt in Lonicera fledged 20% fewer cardinal young than birds that began the season using other substrates. Thus, we suggest that exotic plants may represent an ephemeral ecological trap for certain nesting birds, where negative effects persist only during certain periods.

Deudero, S.; Blanco, A.; Box, A.; Mateu-Vicens, G.; Cabanellas-Reboredo, M.; Sureda, A.

doi: 10.1007/s10530-009-9428-1pmid: N/A

Invasive epiphyte Lophocladia lallemandii macroalga induces changes in the erect bryozoan Reteporella grimaldii at shallow Posidonia oceanica meadows at a Mediterranean pristine location. Bryozoan densities at noninvaded seagrass plots (88.32 ± 3.11 colonies m−2) are higher than those at invaded plots (13.39 ± 1.09 colonies m−2) with a fourfold decrease in number of colonies. Activation of enzymatic pathways (catalase, superoxide dismutase, glutathione peroxidase) and increase in lipid peroxidation malondialdehyde (MDA) [0.80 ± 0.06 nmol/mg prot at Posidonia oceanica plots to 1.08 ± 0.04 nmol/mg prot at L. lallemandii (P < 0.05)] is observed on sessile bryozoans as response to anoxia caused by L. lallemandii. δ13C of bryozoan isotopic composition differed among treatments, covering a broad range (−19.30‰ invaded to −2.84‰ at noninvaded plots), suggesting modification of food sources. Induced shifts of a filter-feeding erect bryozoan by dense algal turfs at invaded seagrasses are demonstrated, highlighting the need to further address interaction across natural communities and alien species invaded systems before further cascade effects are driven.

Vohmann, Andreas; Borcherding, Jost; Kureck, Armin; bij de Vaate, Abraham; Arndt, Hartmut; Weitere, Markus

doi: 10.1007/s10530-009-9429-0pmid: N/A

The filter-feeding clam Corbicula fluminea has widely spread from its Asiatic origin into freshwater habitats on several continents, where it often has a considerable impact on ecosystem processes. The present study was stimulated by the observation that C. fluminea can experience mass mortality during warm summers, even when temperatures are still far below the lethal level. We hypothesized that starvation due to low food quantities during summer is a main factor in this context. In order to test nutritional conditions in an environment where summer mortality occurred, the clams’ body mass was tracked in river bypass systems installed at the Lower River Rhine (Germany and The Netherlands). Two food levels were adjusted in the bypass channels: one corresponding to the original chlorophyll level in the river (ambient food) and one with a chlorophyll a level reduced by about 50% (low food). The clams kept at the ambient food level increased their shell length during summer, although growth rates decreased at low food levels in the River Rhine in late summer. In contrast to shell length, body mass decrease in late summer cumulated in 94% reduction from August until October. This trend was enhanced by the experimental food reduction, i.e., clams kept in the low food level treatments weighed 60% less than the clams in the ambient food treatment at the end of summer. However, mortality was low in both treatments. The data demonstrate a high plasticity in the body mass of C. fluminea. The corresponding possibility to respire body mass can be seen as one strategy to survive longer starvation periods.

Dickson, Timothy; Wilsey, Brian; Busby, Ryan; Gebhart, Dick

doi: 10.1007/s10530-009-9430-7pmid: N/A

Non-native species are hypothesized to decrease native species establishment and cover crops are hypothesized to decrease non-native species abundance. Although many studies have compared invaded to non-invaded habitats, relatively few studies have experimentally added non-native species to directly examine their effects. In a greenhouse mesocosm experiment, we tested the effects of non-native forbs (Melilotus officinalis, Verbascum thapsus, and Lespedeza cuneata), a proposed C3 grass cover crop (Pascopyrum smithii), and a commonly seeded non-native C3 grass (Bromus inermis) on the establishment of target native C4 prairie grass species. All treatments contained the same seed density of target C4 species and were begun on bare soil collected from the field. The legume M. officinalis strongly decreased the abundance of all other species, species diversity, and light and soil moisture levels. Surprisingly, M. officinalis took up relatively large amounts of labeled nitrogen (15N) from the soil early in its development, but M. officinalis fixed nitrogen, thus increasing nitrogen in biomass nearly fivefold by the end of the study. We found few effects of either C3 grass species on non-native forbs or C4 target species, but seeded P. smithii did increase species diversity. Non-native plants therefore impeded native C4 grass establishment through long-lasting effects of target species seedbank depletion (death of most target seedlings) and altered nutrient availability. The effects of M. officinalis were not reduced by the presence of a cover crop.

Eales, J.; Thorpe, R.

doi: 10.1007/s10530-009-9431-6pmid: N/A

The brown anole, Anolis sagrei, is one of the most widespread and successful colonisers of the diverse Anolis genus, which comprises c. 400 species occurring naturally in Central and South America and the Caribbean. Based on extensive between and within population sampling from a previously published study (334 mitochondrial DNA sequences) and sampling for this study (37 mtDNA sequences), we reconstruct a phylogeny and produce a haplotype network to assign a recently introduced population in St Vincent, Lesser Antilles to its geographic origin. A single haplotype was present in the St Vincent population, which was identical to a haplotype from Tampa, FL. We show that genetic diversity within native range populations, combined with low frequencies of introduced haplotypes in native ranges, may impair attempts to identify source populations, even despite intensive sampling effort. The absence of mtDNA haplotype diversity suggests a significant genetic founder effect within the St Vincent population.

MacGregor-Fors, Ian; Morales-Pérez, Lorena; Quesada, Javier; Schondube, Jorge

doi: 10.1007/s10530-009-9432-5pmid: N/A

Invasive exotic species pose an important threat to biodiversity worldwide. However, there is little information on the effects that specific exotic bird species have on native biota. The House Sparrow is an excellent ecological model to evaluate the effect that an invasive exotic species has on native bird communities. Our study describes the relationship of the presence and abundance of House Sparrows with the structure, diversity, and composition of native bird communities in West Mexico. We used two approaches to compare House Sparrow invaded and non-invaded bird communities: (1) at a small geographic-scale that allowed us to evaluate shifts in avian communities with presence of the House Sparrow under similar environmental conditions; and (2) at the landscape-level to evaluate the effect of this species under a scenario of greater environmental heterogeneity. Results from both approaches show that areas invaded by House Sparrows have heavily-dominated avian communities with low species richness, while non-invaded areas exhibit highly-even and species-rich bird communities. Species turnover analysis indicates that the decrease in the number of bird species in House Sparrow invaded areas is caused by species loss, rather than a shift in species composition. Our results indicate that the invasion of an area by the House Sparrow, through synergistic interactions with human activities, determines the composition, structure, and diversity of native bird communities.

Kueffer, Christoph

doi: 10.1007/s10530-009-9433-4pmid: N/A

Invasive plants sometimes alter habitat conditions so as to promote further invasion, either by the same or by other non-native species. Such positive feedbacks often occur because the non-native species increases soil fertility, thereby favouring recruitment of non-native seedlings. This has been demonstrated in nitrogen-poor habitats invaded by nitrogen-fixing species, but it is unclear whether similar processes operate in habitats limited by phosphorus and other nutrients. I compared the growth of seedlings of Cinnamomum verum, an abundant invasive tree on phosphorus-poor soils in the Seychelles, in soils taken from beneath different tree species. I expected that soil phosphorus availability would be higher beneath stands of C. verum than beneath stands of either the native Northea hornei or the non-native nitrogen-fixing species, Falcataria moluccana. I therefore predicted that C. verum seedlings would grow faster in soil taken from beneath C. verum trees than in soil taken from beneath either of the other two species. To test this hypothesis, I performed a bioassay experiment with seedlings of C. verum grown in soils from stands of C. verum, F. moluccana and N. hornei. Different nutrient treatments (control, plus phosphorus (P), plus nitrogen (N), plus N and P, and plus complete fertilizer) were applied to investigate how nutrient availabilities modulate the effects of the trees. In the control treatment without added nutrients, there was a weak tendency for seedlings to perform better in the soils from beneath invasive than native trees. However, seedling growth in soils from beneath invasive species was markedly higher following the addition of phosphorus in the case of the F. moluccana soil, and complete fertilizer in the case of the C. verum soil. These results indicate that on very nutrient-poor soils, a low supply of nutrients other than N may reduce the risk of a soil-feedback by invasive trees on seedling regeneration.

Meyerson, Laura; Viola, David; Brown, Rebecca

doi: 10.1007/s10530-009-9434-3pmid: N/A

Interspecific hybridization can lead to the extinction of native populations and increased aggressiveness in hybrid forms relative to their parental lineages. However, interbreeding among subspecies is less often recognized as a serious threat to native species. Phragmites australis offers an excellent opportunity to investigate intraspecific hybridization since both native and introduced lineages occur in North America. Introduced Phragmites is a highly successful estuarine plant invader throughout North America, but native Phragmites populations are declining in the eastern US. Despite range overlaps, hybridization has not yet been detected between the native and introduced lineages in the wild, suggesting that phenological or physiological barriers preclude cross-pollination. We demonstrate, for the first time, that native and introduced populations of Phragmites can hybridize. There is substantial overlap in flowering period between native and introduced populations from the same geographic locations. We manually cross-pollinated native individuals with pollen from introduced Phragmites and recovered viable offspring. We then used microsatellite markers to prove that alleles unique to the pollen parent were transferred to progeny. Our results imply a mechanism for the further decline of native Phragmites in North America and a potential for the formation of aggressive hybrid offspring.