Uninvaded Areas Research Articles

Assessing the impact of plant invasions on soil seed bank communities: use of univariate and multivariate statistical approaches

AbstractQuestions: Are soil seed banks affected by invasions of alien plants? How can we rigorously assess alterations in seed bank communities associated with invasive species and account for the high spatial variability of seed bank data? How do multivariate approaches compare with more traditional approaches based on analysis of variance?Location: Three riparian sites, Ireland.Methods: A protocol based on a combination of multivariate techniques was used to characterize soil seed bank communities associated with the herbaceous invasive species Heracleum mantegazzianum in May and October. Permutational multivariate analysis of variance (PERMANOVA) was used to test the effects of the factors “invasion”, “site”, “plot” and “depth” on the soil seed bank, while multivariate analysis of dispersion (PERMDISP) provided a measure of the variability of seed bank data at different spatial scales. Similarity percentages analysis (SIMPER) was used to identify the species that contributed most to the differences between invaded and uninvaded communities. A comparison between the results of PERMANOVA and ANOVA analyses was also made.Results: The composition of seed bank communities invaded by H. mantegazzianum differed significantly from that of uninvaded seed banks. Invaded seed banks were less diverse and had reduced abundance, and were dominated by only a few species, such as Urtica dioica and Juncus effusus. Such patterns were recorded at each of three depth categories, indicating that invasive plants can affect both the transient and the more persistent component of the soil seed bank. Seed bank variability was significantly higher within uninvaded areas, supporting the notion that invasions tend to lead to more homogeneous communities.Conclusion: The analytical protocol used in this study was effective in quantifying the effect of plant invasions, at different spatial scales, providing a statistically robust analysis of alterations in soil seed bank communities. Compared to ANOVA, this protocol provided more biological information and was more appropriate for analysis of the data. This approach is therefore recommended in soil seed bank and invasion ecological studies.

Sampling across space and time to validate natural experiments: an example with ant invasions in Hawaii

Measuring the impacts of invasive species on recipient biotic communities can be challenging because standard ecological field experiments may be impossible, undesirable or inaccurate due to limita- tions of scale. Many studies therefore rely on a com- parative observational approach, in which invaded areas are compared to similar nearby uninvaded areas, but this approach suffers from reduced inferential power because of the inability to randomly assign treatments. When possible, the best solution may be to incorporate comparative sampling over time, from before and after invasion, as a means of validating comparisons across space. We used this approach to assess the impacts of Argentine and big-headed ants on arthropod communities at two natural area sites in the Hawaiian Islands. Two sampling events, separated by 2 years, indicated that invaded plots at each site were significantly different from uninvaded plots in terms of overall community composition and distributions of certain indicator species. This compositional change associated with ant invasion was accompanied by a significant decrease in arthropod species richness at one site, but not at the second site. Changes in arthropod communities from before invasion to after invasion in separate before-after plots at each site were qualitatively consistent with the aforementioned changes measured across the invasion boundaries, but were smaller in magnitude because the invasion processes were still in early stages. Together, these findings support the inference that carefully planned studies of biological invasions that use a comparative observational approach can provide an accurate picture of resultant community level changes, and illustrate an effective method for validating such studies.

African sunbirds hover to pollinate an invasive hummingbird-pollinated plant

Why do hummingbirds hover while Old World nectar-feeding birds perch? A unique opportunity to explore this question is presented by the invasion into Africa of a plant adapted for pollination by hovering hummingbirds. Like other hover-pollinated plants of the New World, the flowers of the tree tobacco Nicotiana glauca lack perches and are oriented towards open space. We find that Old World nectarivores, especially the malachite sunbird, Nectarinia famosa, hover 80% of the time when taking nectar from these flowers. They hover for up to 30 s, and are able to sustain this hovering lifestyle in an area where native nectar plants are absent. Nicotiana glauca greatly increases the local abundance of sunbirds compared with uninvaded areas. In turn, flowers visited by sunbirds formed significantly more capsules and set significantly more seed than sunbird-excluded flowers, possibly facilitating the invasion. The results suggest a prominent role for plant, rather than bird traits in determining the occurrence of hover-pollination, begging the question of why plants adapted for hover pollination do not occur outside the New World.

African sunbirds hover to pollinate an invasive hummingbird‐pollinated plant

Why do hummingbirds hover while Old World nectar‐feeding birds perch? A unique opportunity to explore this question is presented by the invasion into Africa of a plant adapted for pollination by hovering hummingbirds. Like other hover‐pollinated plants of the New World, the flowers of the tree tobacco Nicotiana glauca lack perches and are oriented towards open space. We find that Old World nectarivores, especially the malachite sunbird, Nectarinia famosa, hover 80% of the time when taking nectar from these flowers. They hover for up to 30 s, and are able to sustain this hovering lifestyle in an area where native nectar plants are absent. Nicotiana glauca greatly increases the local abundance of sunbirds compared with uninvaded areas. In turn, flowers visited by sunbirds formed significantly more capsules and set significantly more seed than sunbird‐excluded flowers, possibly facilitating the invasion. The results suggest a prominent role for plant, rather than bird traits in determining the occurrence of hover‐pollination, begging the question of why plants adapted for hover pollination do not occur outside the New World.

THE ADAPTIVE VALUE OF REMNANT NATIVE PLANTS IN INVADED COMMUNITIES: AN EXAMPLE FROM THE GREAT BASIN

Changes in the species composition of biotic communities may alter patterns of natural selection occurring within them. Native perennial grass species in the Intermountain West are experiencing a shift in the composition of interspecific competitors from primarily perennial species to an exotic, annual grass. Thus traits that confer an advantage to perennial grasses in the presence of novel annual competitors may evolve in invaded communities. Here I show that such traits are apparent in populations of a native perennial grass, big squirreltail (Elymus multisetus M.E. Jones), exposed to cheatgrass (Bromus tectorum L.) competitors. Dormant big squirreltail plants were collected from cheatgrass-invaded and uninvaded sites near Bordertown, California, USA, a mid-elevation (1600 m) sagebrush community, and transplanted into pots in a greenhouse. Individual plants were split into equal halves. One half was grown with competition from cheatgrass, and the other half was grown without competition. Plants collected from invaded sites responded more quickly to watering, growing more leaves in the first 10 days after transplanting. In addition, big squirreltail plants collected from invaded areas experienced a smaller decrease in plant size when grown with competition than did plants collected from uninvaded areas. Accordingly, while there were fewer big squirreltail individuals in the invaded sites, they were more competitive with cheatgrass than were the more abundant conspecifics in nearby uninvaded areas. It is possible that annual grasses were the selective force that caused these population differences, which may contribute to the long-term persistence of the native populations. While it is tempting to restore degraded areas to higher densities of natives (usually done by bringing in outside seed material), such actions may impede long-term adaptation to new conditions by arresting or reversing the direction of ongoing natural selection in the resident population. If hot spots of rapid evolutionary change can be identified within invaded systems, these areas should be managed to promote desirable change and could serve as possible sources of restoration material or reveal traits that should be prioritized during the development of restoration seed material.

REGIONAL EXTENT OF AN ECOSYSTEM ENGINEER: EARTHWORM INVASION IN NORTHERN HARDWOOD FORESTS

The invasion of exotic earthworms into northern temperate and boreal forests previously devoid of earthworms is an important driver of ecosystem change. Earthworm invasion can cause significant changes in soil structure and communities, nutrient cycles, and the diversity and abundance of herbaceous plants. However, the regional extent and patterns of this invasion are poorly known. We conducted a regional survey in the Chippewa and Chequamegon National Forests, in Minnesota and Wisconsin, U.S.A., respectively, to measure the extent and patterns of earthworm invasion and their relationship to potential earthworm introduction sites. We sampled earthworms, soils, and vegetation in 20 mature, sugar maple-dominated forest stands in each national forest and analyzed the relationship between the presence of five earthworm taxonomic groups, habitat variables, and distance to the nearest potential introduction site. Earthworm invasion was extensive but incomplete in the two national forests. Four of the six earthworm taxonomic groups occurred in 55-95% of transects; however 20% of all transects were invaded by only one taxonomic group that has relatively minor ecological effects. Earthworm taxonomic groups exhibited a similar sequence of invasion found in other studies: Dendrobaena > Aporrectodea = Lumbricus juveniles > L. rubellus > L. terrestris. Distance to the nearest road was the best predictor of earthworm invasion in Wisconsin while distance to the nearest cabin was the best predictor in Minnesota. These data allow us to make preliminary assessments of landscape patterns of earthworm invasion. As an example, we estimate that 82% of upland mesic hardwood stands in the Wisconsin region are likely invaded by most taxonomic groups while only 3% are unlikely to be invaded at present. Distance to roads and cabins provides a coarse-scale predictor of earthworm invasion to focus stand-level assessments that will help forest managers better understand current and potential forest conditions and identify uninvaded areas that could serve as important refugia for plant species threatened by earthworm invasion.

Consumption of introduced prey by native predators: Argentine ants and pit-building ant lions

When populations of native predators are subsidized by numerically dominant introduced species, the structure of food webs can be greatly altered. Surprisingly little is known, however, about the general factors that influence whether or not native predators consume introduced species. To learn more about this issue, we examined how native pit-building ant lions (Myrmeleon) are affected by Argentine ant (Linepithema humile) invasions in coastal southern California. Compared to areas without L. humile, invaded areas contained few native ant species and were deficient in medium-sized and large bodied native ants. Based on these differences, we predicted that Argentine ants would negatively affect ant lion larvae. Contrary to this expectation, observational surveys and laboratory growth rate experiments revealed that Myrmeleon were heavier, had longer mandibles, and grew more quickly when their main ant prey were Argentine ants rather than native ants. Moreover, a field transplant experiment indicated that growth rates and pupal weights were not statistically different for larval ant lions reared in invaded areas compared to those reared in uninvaded areas. Argentine ants were also highly susceptible to capture by larval Myrmeleon. The species-level traits that presumably make Argentine ant workers susceptible to capture by larval ant lions—small size and high activity levels—appear to be the same characteristics that make them unsuitable prey for vertebrate predators, such as horned lizards. These results underscore the difficulties in predicting whether or not numerically dominant introduced species serve as prey for native predators.

Does invasion by an alien plant species affect the soil seed bank?

AbstractQuestions: How does invasion affect old‐field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old‐fields?Location: Menorca, Balearic Islands, Spain, western Mediterranean basin.Methods: We monitored seed germination in soils from old‐fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes‐caprae. We also added O. pes‐caprae bulbs to uninvaded soils to test O. pes‐caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation.Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes‐caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%).Conclusions: Despite invasion by O. pes‐caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.

Does invasion by an alien plant species affect the soil seed bank?

Questions: How does invasion affect old-field seed bank species richness, composition and density? How consistent are these effects across sites? Does the soil seed bank match vegetation structure in old-fields? Location: Menorca, Balearic Islands, Spain, western Mediterranean basin. Methods: We monitored seed germination in soils from old-fields that were both uninvaded and invaded (legacy effect) by the annual geophyte Oxalis pes-caprae. We also added O. pes-caprae bulbs to uninvaded soils to test O. pes-caprae interference with seedling emergence (competitive effect). We compared species composition in the seed bank with that of the vegetation. Results: Species richness in the seed bank and in the vegetation was not significantly different between invaded and uninvaded areas. Uninvaded areas did not have larger seed banks than invaded areas. More seedlings, especially of geophytes, emerged when O. pes-caprae bulbs were added to the soil. Species similarity between invaded and uninvaded areas was higher in the seed bank (74%) than in the vegetation (49%). Differences in species composition were as important as differences among sites. The degree of species similarity between the seed bank and the vegetation was very low (17%). Conclusions: Despite invasion by O. pes-caprae not affecting species richness, the variation in the seed bank species composition in invaded and uninvaded areas, and the differences between the seed bank and the mature vegetation, highlights that even if the invader could be eradicated the vegetation could not be restored back to the exact composition as found in uninvaded areas.

Do seed banks confer resilience to coastal wetlands invaded by Typha ×glauca?

Historically, seed banks conferred resilience to Laurentian Great Lakes coastal wetlands by providing propagules of many species to replace invasive plants, such as Typha ×glauca Godr. After flooding, the seed bank could allow recovery of wetland composition and structure as the water levels fall. Using the seedling emergence method to estimate seed density, species and guild richness, and floristic quality, we evaluated the resilience of five wetlands along the western coast of Green Bay, Lake Michigan by comparing seed bank attributes in areas invaded by Typha to those of neighboring uninvaded areas and to attributes of the extant vegetation. Resilience decreased from north to south among the five wetlands, mirroring a gradient of decreasing water quality. The invasive Lythrum salicaria L. dominated the seed bank of all five wetlands despite low relative abundance in the extant vegetation. The resilience of Green Bay coastal wetlands is threatened by the overwhelming presence of L. salicaria in the seed bank. Because the seed bank holds a wetland’s potential to be renewed by the natural hydrological cycle, seed bank assessment should be a routine measure of wetland resilience.

Shrub invasion into subalpine vegetation: implications for restoration of the native ecosystem

The ability of plant communities to recover after non-native species invasion will depend upon the nature of their soil seed bank and seed rain characteristics. This study assessed changes in the soil seed bank and seed rain associated with the invasion of the non-native shrub Cytisus scoparius in subalpine vegetation. Soil seed bank and seed rain composition, density and richness were investigated at three areas of different stages of invasion: (i) recent (8–10 years), (ii) mature (15–16 years) and (iii) long-term (25 years). There were few changes in seed bank composition or richness regardless of invasion stage. By contrast, the seed rain composition, richness and density was substantially different within long-invaded areas. Very few seeds were able to colonise the dense barrier characteristic of larger, more mature C. scoparius stands. Some prominent herbs from the native vegetation were under-represented or absent from the seed bank, both in invaded and uninvaded areas. Laboratory germination experiments demonstrated that most native species germinate easily, which may imply a transient seed bank, rather than a persistent one. The majority of herbaceous and shrub species were capable of resprouting vegetatively. Therefore, regeneration appeared more reliant on the bud and tuber bank than a persistent soil seed bank. The dominance of graminoid species and C. scoparius rather than other herbaceous, shrub or tree species suggests that the regenerating vegetation will be dominated by grass species and/or C. scoparius. Hence, in areas where long-invaded C.␣scoparius stands are present the recovery of native subalpine vegetation maybe difficult. Recovery may only be possible through wind dispersal from the surrounding intact vegetation or through actively reseeding the area. This study highlights the importance of early intervention in invasive species management.

Nitrogen availability and weed invasion in a remnant native woodland in urban Melbourne

Abstract The woodlands of Studley Park, and the greater Yarra Bend Park, are within 4 km of the centre of the city of Melbourne and total more than 200 ha. Weeds are major contributors to the loss of diversity of native species. We examined the hypothesis that an increase in the availability of nitrogen (N) may be responsible for the increase in weed abundance. In a small field trial, nitrogen availability was greater in areas invaded by weeds and weeds had greater nitrate reductase activity than native species. Both invaded and uninvaded areas were well‐supplied with weed seed, although the composition of the seed bank varied greatly between sites. Diaspores of the weed Romulea rosea were abundant in uninvaded sites while those of Ehrharta erecta and Ehrharta longiflora were abundant in invaded sites. The abundance of weed diaspores suggested that increased N availability may be a precondition for invasion. Formation of a dense litter layer by senescent weeds is likely to promote the capture and retention of N added from anthropogenic sources including air pollution and overland flow from nearby roads and drainage channels.

Mechanisms of exclusion of native coastal marsh plants by an invasive grass

Summary 1 Determining the mechanisms by which invasive species exclude natives is critical for conserving and restoring native populations in impacted habitats. In recent decades the grass Phragmites australis has been aggressively invading coastal marshes of North America, with monocultures often replacing diverse assemblages of plants. 2 Our objective was to quantify how P. australis modifies the abiotic (soil and light conditions) and biotic (litter and shoots) environment and to determine the mechanisms by which it excludes two common forbs, the annual chenopod Atriplex patula var. hastata and the perennial aster Solidago sempervirens, from the highest tidal elevations of a brackish marsh in southern New England, USA. 3 In a 3-year field experiment we added seeds of both forb species to stands of P. australis, where we manipulated shoots and litter in an orthogonal design, and to uninvaded marsh areas dominated by the rush Juncus gerardi, where we manipulated the shoots of the marsh vegetation. In general, seedling establishment and the number of plants surviving until the end of the growing season were substantially greater in areas not invaded by P. australis, and both shoots and litter limited the abundance of forbs within stands. 4 Forbs surviving within stands of P. australis grew larger and produced more seeds than those in uninvaded areas, indicating that changes to the soil resulting from invasion do not preclude the survival of established forbs. This was confirmed by a glasshouse study where the performance of forbs in soil collected from within stands of P. australis was better than in soil from areas dominated by J. gerardi. 5 Similar to many invasive grasses in terrestrial communities, P. australis excludes native forbs through competition, modifying the biotic environment of the marsh at both the ground (litter) and above-ground (shoots) levels. Our results suggest that successful invaders, such as P. australis, are likely to be the ones that can engineer habitats in multiple ways and limit populations of native species across several critical stages of their life history.

Homogenization of ant communities in mediterranean California: The effects of urbanization and invasion

In coastal California, the invasive Argentine ant ( Linepithema humile) displaces nearly all above ground foraging native ant species. The loss of native ants following invasion by Argentine ants homogenizes these faunas; natural habitats invaded by L. humile have lower beta diversity compared to comparable uninvaded areas. Argentine ant abundance in the seasonally dry mediterranean environments of this region correlates strongly and positively with soil moisture. For this reason, the displacement of native ants across natural and artificial moisture gradients often resembles an edge effect, the magnitude of which is inversely proportional to the suitability of the physical environment from the perspective of L. humile. The direct effects of Argentine ant invasions in natural environments are therefore amplified by inputs of urban and agricultural run off. Indirect ecological effects of these invasions arise from the loss of large-bodied ants, arid adapted ants, and behavioral repertoires unique to particular native ant species. Further research is needed to quantify how these aspects of functional homogenization affect invaded communities. The close association between L. humile and moist soils suggests that, at least in arid regions, control strategies might be aimed at reducing urban run off in order to maintain functionally diverse communities of native ants.

Does the Lack of Reference Ecosystems Limit Our Science? A Case Study in Nonnnative Invasive Plants as Forest Fuels

Abstract In forest experiments the problem of inadequate controls often arises. True controls might not be required in case studies, comparisons along an environmental gradient, or comparisons of multiple treated and untreated areas. In a recent characterization of fuels in invaded and uninvaded forest conditions for four forest types at 12 locations in Maine, Maryland, Massachusetts, New Jersey, New York, Vermont, and Virginia, high-quality reference stands usually were not available as true controls. We called the uninvaded areas “comparison areas,” and applied a modified planar sampling technique to quantify live and dead fuels. No overarching pattern emerged; fuels in fire-adapted pitch pine differed from the three other forest types in that stands invaded by black locust had fewer 1- and 10-hour fuels, but more forbs cover and higher basal area. Invasive shrubs increased fuel height and density across most forest types. Invasive grasses in forests present an underrecognized hazard fuel if drought ensues. The comparison stand study design enabled uncovering of significant differences between invaded and uninvaded stands, especially in hardwoods and mixed woods, and fuels in softwoods were less affected by invasive plants.

Spatial model for predicting the presence of cinnamon fungus (Phytophthora cinnamomi) in sclerophyll vegetation communities in south‐eastern Australia

Abstract The pathogen Phytophthora cinnamomi causes extensive ‘dieback’ of Australian native vegetation. This study investigated the distribution of infection in an area of significant sclerophyll vegetation in Australia. It aimed to determine the relationship of infection to site variables and to develop a predictive model of infection. Site variables recorded at 50 study sites included aspect, slope, altitude, proximity to road and road characteristics, soil profile characteristics and vegetation attributes. Soil and plant tissues were assayed for the presence of the pathogen. A geographical information systyem (GIS) was employed to provide accurate estimations of spatial variables and develop a predictive model for the distribution of P. cinnamomi. The pathogen was isolated from 76% of the study sites. Of the 17 site variables initially investigated during the study a logistic regression model identified only two, elevation and sun‐index, as significant in determining the probability of infection. The presence of P. cinnamomi infection was negatively associated with elevation and positively associated with sun‐index. The model predicted that up to 74% of the study area (11 875 ha) had a high probability of being affected by P. cinnamomi. However, the present areas of infection were small, providing an opportunity for management to minimize spread into highly susceptible uninvaded areas.

COMMUNITY-LEVEL CONSEQUENCES OF A PLANT INVASION: EFFECTS ON THREE HABITATS IN COASTAL CALIFORNIA

Biological invasion by nonnative species is a global phenomenon that has the capacity to dramatically alter native communities. However, surprisingly few studies have quantified the effects of exotic plant species on the communities they invade, or have considered how these effects vary among habitat types or seasons. Here, we used both comparative and experimental field studies to investigate the influence of Cape ivy (Delairea odorata; Asteraceae), an invasive evergreen vine native to South Africa, on three habitat types in coastal regions of northern California (coastal scrub, willow riparian, and alder riparian). In the comparative study, plots invaded by Cape ivy contained 36% fewer native plant species and 37% fewer nonnative taxa, and this pattern persisted across habitat types and seasons. The richness of grass and forb species was lower in invaded plots, whereas fern and shrub richness did not vary among zones. Native species richness was significantly lower with increasing cover of Cape ivy, but this was not the case for nonnative species. In addition, invasion by Cape ivy was associated with a 31% decrease in species diversity as well as an 88% decrease in the abundance of native seedlings and a 92% decrease in nonnative seedlings compared to uninvaded areas. After 2 yr, a Cape-ivy reduction experiment yielded similar results, with a 10% increase in the richness of native species compared to control plots, and a 43% increase in the richness of nonnative taxa. Forb species richness increased significantly when Cape-ivy cover was reduced, whereas shrub richness decreased slightly and no effects were detected for ferns and grasses. We also found that Cape-ivy reduction led to a 32% increase in plant species diversity, an 86% increase in the abundance of native seedlings, and an 85% increase for nonnative seedlings. In all cases, the effects of Cape-ivy reduction were consistent across habitat types. Collectively, our results indicate that this invader has significantly changed the composition of three different habitat types, and its control should be a major priority. However, our data also indicate that Cape ivy had negative effects on the richness of both native and nonnative plant species. Such findings suggest that a consequence of removing Cape ivy from invaded areas may be to facilitate the proliferation of other nonnative species.

Community-Level Consequences of a Plant Invasion: Effects on Three Habitats in Coastal California

Biological invasion by nonnative species is a global phenomenon that has the capacity to dramatically alter native communities. However, surprisingly few studies have quantified the effects of exotic plant species on the communities they invade, or have considered how these effects vary among habitat types or seasons. Here, we used both comparative and experimental field studies to investigate the influence of Cape ivy (Delairea odorata; Asteraceae), an invasive evergreen vine native to South Africa, on three habitat types in coastal regions of northern California (coastal scrub, willow riparian, and alder riparian). In the comparative study, plots invaded by Cape ivy contained 36% fewer native plant species and 37% fewer nonnative taxa, and this pattern persisted across habitat types and seasons. The richness of grass and forb species was lower in invaded plots, whereas fern and shrub richness did not vary among zones. Native species richness was significantly lower with increasing cover of Cape ivy, but this was not the case for nonnative species. In addition, invasion by Cape ivy was associated with a 31% decrease in species diversity as well as an 88% decrease in the abundance of native seedlings and a 92% decrease in nonnative seedlings compared to uninvaded areas. After 2 yr, a Cape-ivy reduction experiment yielded similar results, with a 10% increase in the richness of native species compared to control plots, and a 43% increase in the richness of nonnative taxa. Forb species richness increased significantly when Cape-ivy cover was reduced, whereas shrub richness decreased slightly and no effects were detected for ferns and grasses. We also found that Cape-ivy reduction led to a 32% increase in plant species diversity, an 86% increase in the abundance of native seedlings, and an 85% increase for nonnative seedlings. In all cases, the effects of Cape-ivy reduction were consistent across habitat types. Collectively, our results indicate that this invader has significantly changed the composition of three different habitat types, and its control should be a major priority. However, our data also indicate that Cape ivy had negative effects on the richness of both native and nonnative plant species. Such findings suggest that a consequence of removing Cape ivy from invaded areas may be to facilitate the proliferation of other nonnative species.

Prey Selection in Horned Lizards Following the Invasion of Argentine Ants in Southern California

Biological invasions can have severe direct and indirect impacts on natural communities and are one of the leading reasons behind the loss of biodiversity. We investigated the indirect effects of Argentine ant (Linepithema humile) invasions on coastal horned lizards (Phrynosoma coronatum) through an examination of prey selection in the field and laboratory. In non-invaded areas, coastal horned lizard diet consists predominately of ants (>94% by prey item in three reserves examined), particularly harvester ants in the genera Pogonomyrmex and Messor. In invaded areas, most native ants were displaced, and remaining horned lizards incorporated more non-ant arthropods and smaller ants into their diet. In addition, Argentine ants were never detected in horned lizard fecal pellets, suggesting that they are not a suitable replacement food source for the native species they are displacing. Analysis of prey selection in uninvaded areas reveals that ant species consumption by adult lizards is correlated with ant body size rather than ant abundance, foraging behavior, or defensive method. Differences in diet between adult and juvenile lizards suggest that a diversity of ant species is necessary to support both age classes. Laboratory experiments, consisting of paired presentations of four native ant species and the Argentine ant, confirmed patterns found in the field. Both adult and juvenile horned lizards preferred native ants to Argentine ants. Patterns of preference suggest that, in addition to L. humile's small size, other factors may contribute to the horned lizard's aversion to this species. Populations of coastal horned lizards have undergone severe declines in recent years. While habitat loss has been the primary concern, the invasion of Argentine ants into their remaining suitable habitat needs to be monitored. Reserves need to be managed to prevent the penetration of Argentine ants in order to minimize their direct and indirect effects on natural communities.

PREY SELECTION IN HORNED LIZARDS FOLLOWING THE INVASION OF ARGENTINE ANTS IN SOUTHERN CALIFORNIA

Biological invasions can have severe direct and indirect impacts on natural communities and are one of the leading reasons behind the loss of biodiversity. We inves- tigated the indirect effects of Argentine ant (Linepithema humile) invasions on coastal horned lizards (Phrynosoma coronatum) through an examination of prey selection in the field and laboratory. In non-invaded areas, coastal horned lizard diet consists predominately of ants (>94% by prey item in three reserves examined), particularly harvester ants in the genera Pogonomyrmex and Messor. In invaded areas, most native ants were displaced, and remaining horned lizards incorporated more non-ant arthropods and smaller ants into their diet. In addition, Argentine ants were never detected in horned lizard fecal pellets, sug- gesting that they are not a suitable replacement food source for the native species they are displacing. Analysis of prey selection in uninvaded areas reveals that ant species con- sumption by adult lizards is correlated with ant body size rather than ant abundance, foraging behavior, or defensive method. Differences in diet between adult and juvenile lizards suggest that a diversity of ant species is necessary to support both age classes. Laboratory exper- iments, consisting of paired presentations of four native ant species and the Argentine ant, confirmed patterns found in the field. Both adult and juvenile horned lizards preferred native ants to Argentine ants. Patterns of preference suggest that, in addition to L. humile's small size, other factors may contribute to the horned lizard's aversion to this species. Populations of coastal horned lizards have undergone severe declines in recent years. While habitat loss has been the primary concern, the invasion of Argentine ants into their remaining suitable habitat needs to be monitored. Reserves need to be managed to prevent the penetration of Argentine ants in order to minimize their direct and indirect effects on natural communities.