Todd E. Minchinton

Disturbance-mediated competition and the spread of phragmites australis in a coastal marsh

In recent decades the grass Phragmites australis has been aggressively in- vading coastal, tidal marshes of North America, and in many areas it is now considered a nuisance species. While P. australis has historically been restricted to the relatively benign upper border of brackish and salt marshes, it has been expanding seaward into more phys- iologically stressful regions. Here we test a leading hypothesis that the spread of P. australis is due to anthropogenic modification of coastal marshes. We did a field experiment along natural borders between stands of P. australis and the other dominant grasses and rushes (i.e., matrix vegetation) in a brackish marsh in Rhode Island, USA. We applied a pulse disturbance in one year by removing or not removing neighboring matrix vegetation and adding three levels of nutrients (specifically nitrogen) in a factorial design, and then we monitored the aboveground performance of P. australis and the matrix vegetation. Both disturbances increased the density, height, and biomass of shoots of P. australis, and the effects of fertilization were more pronounced where matrix vegetation was removed. Clear- ing competing matrix vegetation also increased the distance that shoots expanded and their reproductive output, both indicators of the potential for P. australis to spread within and among local marshes. In contrast, the biomass of the matrix vegetation decreased with increasing severity of disturbance. Disturbance increased the total aboveground production of plants in the marsh as matrix vegetation was displaced by P. australis. A greenhouse experiment showed that, with increasing nutrient levels, P. australis allocates proportionally more of its biomass to aboveground structures used for spread than to belowground struc- tures used for nutrient acquisition. Therefore, disturbances that enrich nutrients or remove competitors promote the spread of P. australis by reducing belowground competition for nutrients between P. australis and the matrix vegetation, thus allowing P. australis, the largest plant in the marsh, to expand and displace the matrix vegetation. Reducing nutrient load and maintaining buffers of matrix vegetation along the terrestrial-marsh ecotone will, therefore, be important methods of control for this nuisance species.

Disturbance by wrack facilitates spread of Phragmites australis in a coastal marsh

Abstract Disturbance is an important factor facilitating the spread of nuisance and invasive species. Over the past century, the grass Phragmites australis has been spreading aggressively throughout coastal marshes of North America and is now considered a nuisance species in many regions. In this study, I examine how natural disturbance by wrack (primarily dead plant material of the native grass Spartina alterniflora ) affects the local spread of stands of P. australis in a coastal marsh in southern New England, USA. A survey indicated that wrack covered 8.4% of the marsh and that a disproportionate amount of wrack (73% of wrack cover) is stranded at the highest tidal elevations of the marsh next to stands of P. australis . To test hypotheses that wrack inhibits or facilitates the seaward spread of P. australis , I did two experiments where I manipulated the presence or absence of wrack next to stands of P. australis and then monitored the performance of P. australis : (1) wrack was either removed or not removed for two growing seasons from areas where wrack had naturally stranded adjacent to stands of P. australis , and (2) wrack was either added or not added next to stands of P. australis without naturally stranded wrack for one growing season, and then the added wrack was removed before the next growing season. Removing naturally stranded wrack resulted in a substantial increase in the density of shoots of P. australis compared to areas with wrack. Similarly, areas where wrack was added for one growing season and then removed before the emergence of shoots in the next, had more than double the density of shoots compared to areas of natural marsh turf that did not undergo this disturbance. Moreover, these shoots were significantly taller and produced more inflorescences. Therefore, wrack stranded in the marsh can suppress the vegetative growth of P. australis , but once it decomposes or is removed by the tides, P. australis spreads dramatically. Wrack appears to facilitate the spread of P. australis by smothering the underlying marsh turf, creating patches of bare space with environmental conditions favourable to the spread of P. australis . The only areas where P. australis did not increase substantially were those continually occupied by marsh turf, indicating that undisturbed marsh turf is an important buffer resisting the local spread by P. australis . Because P. australis is currently spreading and the litter of P. australis contributes to the biomass of wrack, disturbance by wrack may be an increasingly important factor facilitating the spread of P. australis and influencing the ecology of coastal marshes.

Life on the edge: conspecific attraction and recruitment of populations to disturbed habitats

Abstract Variation in the recruitment of benthic marine invertebrates is often attributed to the interaction of the supply of new individuals to a habitat and the availability of space for colonisation when they arrive. Also important in determining variation in recruitment is the response of the larvae to the characteristics of the habitat. Larvae of many benthic marine invertebrates have shown great specificity of requirements in setting their limits of distribution at the time of selection of a habitat. The tubeworm Galeolaria caespitosa shows great variation in recruitment from place to place on rocky intertidal seashores and is a gregarious animal with larvae showing directed responses to conspecific adults on the substratum. I hypothesised that, if variation in recruitment of G. caespitosa were independent of conditions on the substratum, the magnitude of recruitment in patches of the same shape but different sizes cleared within continuous mats of conspecific adults would be directly related to the area available for colonisation in the patch. Alternatively, if variation in recruitment were due to the response of larvae to conspecific adults on the substratum, the magnitude of recruitment would be a function of the perimeter of the patch, which, given patches of the same shape, is a measure of the influence of conspecific adults in that patch. To distinguish between these alternatives, small (areau2009=u200925u2009cm2; perimeteru2009=u200920u2009cm) and large (areau2009=u2009225u2009cm2; perimeteru2009=u200960u2009cm) square patches were cleared within continuous mats of conspecific adults at four sites and recruitment of G. caespitosa was monitored over two seasons of recruitment. The density of recruits per unit area was, on average, almost three times greater in small than in large patches, indicating that recruitment of G. caespitosa is not directly related to the area of the patch. In contrast, the density of recruits per unit perimeter was not significantly different between small and large patches, indicating that recruitment of G. caespitosa is related to the proximity of conspecific adults in the patch. Therefore, at a given site, the perimeter of patches within mats of G. caespitosa is a better predictor of the relative magnitude of recruitment among patches than that provided by their areas. These results are contrary to many models of invertebrate recruitment that assume close linkage between available space on the substratum and settlement. Moreover, they highlight the importance of behavioural responses of animals at the time of selection of habitat in accounting for variation in recruitment. For populations of organisms that display gregarious behaviour at settlement, or conspecific attraction, this direct relationship between the perimeter of patches and recruitment could be used as a tool in restoring populations to disturbed habitats. The added benefit of such facilitative interactions in restoring populations is that they provide increasing returns to the population for a given supply of potential colonists to a habitat.

Frugivory by insects on mangrove propagules: effects on the early life history of Avicennia marina

This study investigates how herbivory by larval insects on fruit of the mangrove Avicennia marina affects the subsequent establishment, growth, and survivorship of its seedlings. Research was done in a temperate mangrove forest in Australia where the larvae of two species of insects were the dominant frugivores: the mangrove fruit fly Euphranta marina and the mangrove plume moth Cenoloba obliteralis. Larvae consumed the cotyledons of fruit, but not their embryonic axes. Damage to the cotyledons of fruit while on the tree was obvious in abscised propagules and the cotyledons of seedlings, indicating that larvae within fruit continue to consume the cotyledons and develop while the propagule disperses and the seedling establishes. We found that 53% of fruit, 69% of abscised propagules, and 80% of the cotyledons of seedlings had been attacked by larval insects. The degree of damage to the cotyledons was positively related to the densities of larvae within the fruit and the number of emergence or exit holes in the surface of the cotyledons. Consequently, the number of exit holes could be used as a proxy for the amount of damage to the cotyledons. An experiment in which we placed propagules with different levels of damage (zero, one, two, or three exit holes) on the forest floor revealed that frugivory did not influence the establishment of seedlings. In another experiment, we monitored the performance of newly established seedlings with cotyledons that had zero exit holes, one or more exit holes, or where we had removed their cotyledons. In both experiments, the growth of seedlings as measured by their heights and number of leaves was negatively related to the degree of frugivory. Consumption of the cotyledons by larvae apparently reduces the energy reserves available for the initial growth of seedlings. The early mortality of seedlings in both experiments was minimal and appeared to be independent of differences in size due to frugivory. Experimentally removing the cotyledons of seedlings produced extremely short seedlings with increased mortality, confirming the importance of cotyledonary reserves to the initial growth and survivorship of seedlings of A. marina. This study highlights how the early life history of plants may be impacted by conditions experienced by the propagules before they disperse from the parental plant.