Grass-to-tree facilitation in an arid grazed environment (Aïr Mountains, Sahara)

The scarcity of tree regeneration is a major concern for the long-term conservation of tree cover in the Aïr-Ténéré Nature Reserve (Sahara, Niger), and the causes of this shortage are poorly understood. We raised the hypothesis that livestock pressure is a key-factor in the mortality of tree seedlings and that facilitation processes among plants may enhance tree regeneration. We tested whether the tussock grass Panicum turgidum could facilitate the regeneration of a keystone tree species, Acacia tortilis var. raddiana, under a grazing regime. We transplanted 3-month-old seedlings of Acacia to plots representing different neighbourhood conditions (in areas without vegetation, amongst intact tussocks, among cropped tussocks, and under branches), which were either fenced or exposed to grazing by large herbivores. Survival and height of 160 transplants were recorded during one year. Acacia performance was much lower in the grazed plots, indicating that large herbivores had a strong negative impact on tree regeneration. Presence of Panicum tussocks had a positive effect on the height of seedlings, likely because they improved soil properties and water availability. No aboveground positive effect on Acacia was detected though the grass did reduce atmospheric aridity. We detected an indirect positive effect of Panicum on Acacia survival in the presence of herbivores when aridity and grazing pressure reached their maximum levels. We obtained evidence that indirect facilitation may occur in arid grazed areas, although it has been predicted to mainly exist in mesic competitive environments. Additionally, our results support the stress-gradient hypothesis against assumptions that competitive effects may dominate in extremely arid environments. Since most of the positive effect on Acacia survival is indirect (biotic grazing refuge) due to the presence of herbivores, transplantation of tree seedlings amongst Panicum may be a simple and low-cost technique for the restoration of Saharan ecosystems degraded by herbivore grazing.     

Grazing dynamics on a Caribbean reef-building coral

Corals in certain Caribbean coral reef habitats are constantly grazed-on due to the territorial marking behavior of the stoplight parrotfish Sparisoma viride. We studied the grazing dynamics on the Caribbean reef-building coral Montastraea annularis. We transplanted colonies to algae-dominated reefs (Rosario Islands, Cartagena, Colombia), where they encountered higher grazing pressure. We counted grazed polyps every month throughout a year. Over the course of a year, 4,101 different grazed polyps were counted on lobe-like M. annularis transplants ( n =23). Grazing was evaluated on a monthly basis as the probabilities of all the possible transitions among four grazing categories (0%, >0–1%, >1–5%, >5% grazed tissue), uncovering a dynamic process. Higher transition probabilities were always between 0 and 1% (coral tissue grazed) grazing states, indicating that grazing did not usually exceed 1% per coral per month. The probability of remaining uninjured in a month was 0.19, 0.17 of a change from 0–1% grazing state, 0.31 of remaining at 1%, and of full recovery from 1% grazing was 0.16. More than one month was usually required for complete recovery ( P<<1) probably due to both steady grazing pressure and slow regeneration rates. Since the marking behavior of the parrotfish was not as common on other zones of the reef no comparison on the grazing among environments was possible. In spite of this, it is possible to have stable transplanted populations of corals such as M. annularis on algae-dominated Caribbean reef environments due to their tolerance to the natural grazing pressure.Communicated by: K. S. Sealey     

Long- and short-term effects of reindeer grazing on tundra wetland vegetation

We studied long-term (50 years) and short-term (4 years) effects of summer grazing of reindeer on subarctic tundra wetland vegetation. The long-term effects of summer grazing were studied by comparing vegetation on Finnish and Norwegian sides of the fence line separating reindeer grazing regimes. The Finnish side was intensively grazed and trampled throughout the year, whereas the Norwegian side was grazed in winter. Experimental fences were erected to examine short-term effects of grazing exclusion. Both in the long- and short-term, summer grazing decreased the height of Salix lapponum whereas the short-term effects on willow cover were less clear than the long-term effects. In contrast, Carex spp. benefited from grazing. Long-term grazing had little effect on total bryophyte cover. Grazing had negligible effects on the nutrient content of leaves of S. lapponum and Eriophorum angustifolium. We conclude that tundra wetlands can withstand moderately high grazing pressure sustained over several decades.     

Herbivory of clonal populations: cattle browsing affects reproduction and population structure of Yucca elata

The hypothesis that cattle browsing on inflorescences of the soaptree Yucca elata reduces reproductive success was investigated by comparing recruitment and population structure in six populations protected from grazing, six grazed during the flowering season and five grazed outside the flowering season. Cattle consumed 98% of inflorescences, which were found to be highly nutritious. Reduced recruitment in flower-grazed populations could not be attributed to reduced flower survival, as recruitment in non-flower grazed populations was also reduced. Changes in population structure were due to cattle browsing small caudices, including both genets and ramets. An alternate hypothesis of limited germination in soils compacted by cattle was not supported. Cattle browsing of inflorescences reduced reproductive effort, which may be due to the inability of the plants to resorb nutrients after flowering. Browsing also increased branching, probably through lack of apical dominance, whereas physical trampling increased procumbency. Cattle browsing implies a lack of genetic recruitment, possible local extinction of the yucca moth Tegeticula yuccasella, the exclusive pollinator of Y. elata, and local reduction in insect and bird biodiversity.     

Spatial patchiness of epilithic biofilm caused by refuge-inhabiting high shore gastropods

Aggregations of Melarhaphe neritoides and Littorina saxatilis are common on the high shore in north-east England. These aggregations are frequently found in crevices which act as a refuge from desiccation. The aggregations are surrounded by patches of epilithic biofilm which are visibly lighter in colour than the biofilm present on the remainder of the high shore. Chlorophyll a levels were lower in regions of close proximity to aggregated littorinids (mean = 4.9 μg cm⁻²) as compared to areas >‰5 cm away from the edge of the visible colour change (mean = 15.9 μg cm⁻²). Manipulations of littorinid density showed that areas of high density had significantly lower levels of chlorophyll a than those where littorinids were excluded. This difference was mainly due to the presence of higher numbers of small grazed areas or an increase in size of grazed areas rather than a homogenous change in chlorophyll a levels across the entire manipulated area. These results are supported by observations showing that only 7% of the total foraging time of littorinids was spent outside visibly lighter patches of biofilm suggesting littorinids only control local biofilm levels through grazing.This paper has not been submitted elsewhere in identical or similar form, nor will it be during the first three months after its submission to Hydrobiologia     

The effects of phosphorus nutrition and soil pH on the growth of some ancient woodland indicator plants and their interaction with competitor species

The forest under-storey herbs Anemone nemorosa, Lamiastrum galeobdolon and Veronica montana are generally considered indicator species of old, broadleaved woodland sites where the soil fertility is often low. In a glasshouse bioassay, however, all three species not only showed large positive growth responses to supplied P concentrations (0–10mgL ^–1) solutions, but also tolerated high P concentrations (20–40mgL ^–1), well above those normally found in their natural habitat. Plants responded by raising the concentrations of P in their shoot and root tissues and increasing their biomass, resulting in an increased P uptake. A shade-tolerant competitor species, Urtica dioica, also grew vigorously across the full range of P concentrations, restricting the growth of the woodland species. This emphasises the difficulty of establishing semi-natural woodland vegetation in the presence of competitor species, for example in situations where new woodlands are planted on fertile ex-agricultural soils containing large residual concentrations of P. The influence of soil pH on the growth and nutrient relations of A. nemorosa, L. galeobdolon, V. montana, Poa trivialisandU. dioicawas determined in a separate experiment using an ex-arable soil as the growing medium with pH levels adjusted from 7.4 to 5.8 and 4.3 respectively. Acidifying the soil enhanced growth, but reduced the concentrations of N, P and K in the leaves of all three woodland species, probably due to dilution of these minerals in the increased dry matter production. The competitor species (P. trivialis and U. dioica) responded in similar manner to the woodland indicator species. These results suggest that manipulating soil pH as a means of facilitating the establishment of woodland indicator species in new farm woods is unlikely, in the short term, to be effective where competitor species are present.     

Responses of ground flora to coppice management in an English woodland – a study using permanent quadrats

Changes in the ground flora in sweet chestnut (Castaneasativa) woodland were studied over 11 years usingpermanent quadrats in two coppiced plots and one plot in a ride widened by treeremoval. Light and temperature were also recorded. The temperature range wasgreater in the ride than in the coppice plots. Light reaching the woodland floorin summer fell to less than 1% of incident radiation at later stages of thecoppice cycle. Species richness peaked in the second and third year aftercoppicing and then declined in both the coppice plots and the ride. A similarpattern was seen in total percentage frequency of cover. The dominant species,Anemone nemorosa, showed little response to the coppicecycle in terms of frequency, but showed a fourfold increase in floweringintensity in the second and third year following coppicing. Other speciesgenerally declined in frequency through the coppice cycle, and flowering waslargely suppressed in the later stages. The results are discussed in relation toalternative methods of woodland management.     

Impact of reindeer grazing on ground-dwelling Carabidae and Curculionidae assemblages in Lapland

Reindeer Rangifer tarandus L. grazing shapes forest vegetation, microclimate, and soil respiration in Lapland, especially due to grazing on lichens (Cladina). We studied how these changes and their magnitude affect ground‐dwelling species of beetle families Carabidae (predators) and Curculionidae (herbivores), by using pitfall traps to collect invertebrates from pairs of grazed and ungrazed study plots over a wide range of site types. Changes in abundance, composition, richness and diversity of beetle assemblage were tested in relation to magnitude of the impacts on vegetation. The species compositions of Carabidae and Curculionidae differed between grazed and ungrazed plots in all sites. The relative difference between grazed and ungrazed plots in the number of individuals increased linearly with the impact of reindeer on vegetation cover. Carabid beetles, as a family, were more common in grazed plots in all sites. Curculionid beetles were more common in ungrazed plots in the birch dominated sites. This difference was mainly due to the species that feeds on deciduous leaves. In the pine dominated sites with high Cladina cover and more changes in ground vegetation, the number of curculionids feeding on conifers was higher in grazed plots. Species richness and diversity (H’) of both families were higher in grazed plots. Of the total 27 species, 11 were found only in grazed plots, while not a single species was found only in ungrazed plots. The relative difference between plots in diversity and evennes (H’/H'max) had humped response to the difference in Cladina cover. The diversity values were greater in grazed plots at the intermediate levels of grazing impact, and only in sites with very low or extremely high Cladina cover difference was the diversity higher in ungrazed plots. The response of beetle diversity resembled the hypotheses suggested for the relationship between grazing and vegetation diversity: greatest positive effect at intermediate grazing intensity and negative effects at unproductive sites.     

Effects of grazing on the quantity and quality of freshwater Aufwuchs

Qualitative and quantitative measures of the Aufwuchs (scum flora) on artificial substrates in situ were used to evaluate the effects of grazing by freshwater pulmonate snails in a shallow pond in southeastern Michigan. Grazer densities of 216 snails/m² marked reduced standing crop so that after 45 days grazed substrata had 6.46 mg dry weight, 604 µg C and 4.18 µg chlorophyll a as compared to controls with 30.62 mg dry weight, 3699 µg C and 6.29 µg chlorophyll a, all on a per dm² basis.Grazing did not change carbon per mg dry matter but caused significant increases in both µg chlorophyll a (control, 0.206; and grazed, 0.649 µg chlorophyll a/mg dry weight, P < 0.01) and nitrogen (control, 8.3; and grazed, 24.2 µg N/mg dry weight, P < 0.001) after 45 days. Both abundance and diversity of the attached community was reduced by grazing from 24 taxa and 80,889 individuals/cm² on control to 8 taxa and 501 individuals/cm² on grazed substrates. Mean productivity of the Aufwuchs was significantly (P < 0.001) reduced by grazing from 76.3 µg C/(dm²·day) on control to 17.7 µg C/(dm²·day) on grazed substrata.Snails were very efficient at clearing smooth surfaces of living cells, detritus, and particulate inorganic matter. There was little evidence of selectivity except for an apparent inability to remove some of the smallest cells (e.g. Cocconeis sp.) probably for mechanical reasons.     

Time-scale effects in the interaction between a large and a small herbivore

In the short term, grazing will mainly affect plant biomass and forage quality. However, grazing can affect plant species composition by accelerating or retarding succession at longer time-scales. Few studies concerning interactions among herbivores have taken the change in plant species composition into account. In a salt-marsh system, the long-term effects of exclusion of a large herbivore (cattle) on the abundance of a small herbivore (hare) were studied. Excluding cattle grazing for 30 years resulted in large changes in vegetation composition. In general, the cover of tall-growing species increased in the absence of cattle grazing. These long-term changes negatively affected hare grazing intensity. Hares preferentially fed on Festuca rubra and negatively selected tall growing plants, such as Elymus athericus, both in cattle-grazed and long-term ungrazed areas. However, the intensity of hare grazing was not related to the cover of F. rubra. The cover of tall-growing plants (E. athericus, Atriplex prostrata and Juncus maritimus) appeared to be the best predictor and hare grazing intensity decreased sharply with an increase of the cover of tall plants. When cover of tall plants did not increase, hare grazing intensity was not affected. The study shows that the time-scale of the experiment is of prime importance in studying interactions between herbivores. Species that do not seem to influence the abundance of one another or are competing for the same resources on a short time-scale might well be facilitating each other when looking at larger time-scales while taking plant species replacement into account.     

A numerical study of successions in an abandoned,damp calcareous meadow in S Sweden

Summary Two sites at Örup, SE skåne, Sweden, have been investigated, viz. a grazed, unimproved, tussocky pasture on a calcareous moraine clay; and an originally similar, adjacent area that was abandoned about 1960. On the grazed site the vegetation is extremely rich in species. This vegetation type was formerly widespread, but nowadays it is rare and therefore it is important to try to conserve examples of it. In total, 80 0.1 m² quadrates distributed at random within 4 homogeneous plots were investigated. The species composition and the cover, the height of the vegetation, the position on a tussock or in a depression and the amount of litter were recorded. The cover data were ordinated (PCA). The differentiation between tussocks and depressions and the effects of ceasing grazing were clearly separated. The connections between the vegetation and the position of the quadrat, grazing intensity etc. were investigated. The vegetation of the ungrazed parts had become dominated by Filipendula ulmaria and Carex disticha; on an average there were 5.5 species/0.1 m². The grazed quadrats contained three times as many species and showed much higher spatial variation, important species being Carex panicea, C. flacca, Molinia caerulea, Festuca ovina, Potentilla erecta, Centaurea jacea and Serratula tinctoria.Nomenclature of vascular plants follows Lid (1974), for names of bryophytes see Nyholm (1954–69) and Arnell (1956).This paper is mainly based on a more detailed paper in Swedish (Regnéll 1979).I thank Dr. E. van der Maarel, Prof. Nils Malmer, Fil. lic. Anders Larsson, Fil. Dr. Eva Waldemarsson-Jensén and Fil. kand. Stefan Persson for valuable discussions and encouragement.     

Conservation of heathland by sheep grazing in Brittany (France): Importance of grazing period on dry and mesophilous heathlands

Heathlands are characteristic semi-natural ecosystems of western Europe. Nowadays they are threatened because of their lack of agricultural value. Management of the remaining heathland areas necessitates precise knowledge about different management tools that can be used, including grazing. The effects of sheep grazing on dry and mesophilous heathlands of Brittany were studied in relation to grazing season and duration. Management aims were to control shrubs and to maintain the dominance of heathers. The grazing pressures applied here were quite high, about 10–15 sheep ha⁻¹ yr⁻¹. Erica ciliaris appeared to be very sensitive to grazing at the end of summer. This implicates special care in the management of mesophilous heathland. In dry heathland, Erica cinerea did not show this sensitivity. Control of shrubs was achieved by most grazing regimes which led to a decrease of 11–22 cm in gorse height, except summer grazing which seemed to stimulate their growth.This study underlined the relevance of the use of sheep grazing as a heathland management tool but also the necessity of prior experiments to adapt these operations mainly in terms of grazing period to the type of heathland and the objectives put forward.     

Low vegetation recovery after short-term cattle exclusion on Robinson Crusoe Island

Livestock browsing and grazing are considered serious threats to the conservation of the Juan Fernández Islands’ flora, Chile. Nowadays the elimination of grazing by cattle is very difficult. In order to prevent the␣entrance of cattle into the native forests, an 8.3 km-long fence was established on the main island (Robinson Crusoe). The response of the vegetation was evaluated during 27 months of exclusion. The changes in abundances and composition of species were monitored in 12 permanent plots, each of 25 m², located in ungrazed and grazed areas. We expected an increase in plant height and total ground cover in the ungrazed area, and also a directional compositional change towards forest species. We found five different patterns of variation or non-variation for the 22 species observed: (i) one species (Acaena argentea) diminished in abundance in the ungrazed area; (ii) another one (Conium maculatum) increased in the ungrazed area; (iii) one species (Vulpia bromoides) increased in the grazing area; and some species displayed no net variation, but they showed (iv) a nearly constant occurrence, or (v) they appeared and disappeared sporadically. Contrary to our hypothesis, the vegetation showed no net differences in cover on both sides of the fence. As expected, plant height increased in the ungrazed area. In this area, we detected no noteworthy changes in floristic composition towards forest species. On the contrary, four new pastureland species appeared outside of the exclusion area during the last year of evaluation. Other ungrazed sectors of the island showed qualitative differences from grazed sectors, such as major height and density of plants, and lower intensity of browsing, grazing, and trampling, attributable to a reduction of herbivorous pressure. The modest responses reported in this study could be related to the short lapse of time since the exclusion, soil compaction, overgrowth by a single species (A. argentea), great seasonal variations in different species’ abundances, the reduction in the number of cattle grazing the area previous to the fence’s installation, and persistent herbivory by rabbits (Oryctolagus cuniculus L.). We conclude that the effect of grazing by cattle is␣less than expected, and that there are other factors that delay the recovery of the Juan Fernández pasturelands.     

Karlotoxin mediates grazing by Oxyrrhis marina on strains of Karlodinium veneficum

Karlodinium veneficum is a common member of temperate, coastal phytoplankton assemblages that occasionally forms blooms associated with fish kills. Here, we tested the hypothesis that the cytotoxic and ichthyotoxic compounds produced by K. veneficum, karlotoxins, can have anti-grazing properties against the heterotrophic dinoflagellate, Oxyrrhis marina. The sterol composition of O. marina (>80% cholesterol) renders it sensitive to karlotoxin, and does not vary substantially when fed different algal diets even for prey that are resistant to karlotoxin. At in situ bloom concentrations (10⁴–10⁵ K. veneficum ml⁻¹), grazing rates (cells ingested per Oxyrrhis h⁻¹) on toxic K. veneficum strain CCMP 2064 were 55% that observed on the non-toxic K. veneficum strain MD5. At lower prey concentrations typical of in situ non-bloom levels (<10³ cells ml⁻¹), grazing rates (cells ingested per Oxyrrhis h⁻¹) on toxic K. veneficum strain CCMP 2064 were 70–80% of rates on non-toxic strain MD5. Growth of O. marina was significantly suppressed when fed the toxic strain of K. veneficum. Experiments with mixed prey cultures, where non-toxic strain MD5 was fluorescently stained, showed that the presence of toxic strain CCMP 2064 inhibited grazing of O. marina on the co-occurring non-toxic strain MD5. Exogenous addition of a sub-lethal dose (100 ng ml⁻¹) of purified karlotoxin inhibited grazing of O. marina by approximately 50% on the non-toxic K. veneficum strain MD5 or the cryptophyte S. major. These results identify karlotoxin as an anti-grazing compound for those grazers with appropriate sterol composition (i.e., desmethyl sterols). This strategy is likely to be an important mechanism whereby growth of K. veneficum is uncoupled from losses due to grazing, allowing it to form ichthyotoxic blooms in situ.     

The population biology of Bromus tectorum in forests: effect of disturbance,grazing, and litter on seedling establishment and reproduction

Summary The effect of tree canopy, understory, herbivores, and litter depth on seedling establishment, survival, and reproduction of the alien grass, Bromus tectorum (cheatgrass), was examined in a series of experiments in four forest habitat types in western North America. Higher recruitment, survival, and reproduction on clearcuts, which would be expected if the overstory alone is limiting the distribution of cheatgrass in forests, were not observed. Removing the understory in an otherwise undisturbed Pinus ponderosa forest did, however, increase the emergence of B. tectorum, but plants in these experimentally-created openings were more vulnerable to grazing by small mammals. In contrast, removing the sparse understory in an Abies forest neither enhanced recruitment nor increased the incidence of grazing of B. tectorum seedlings. Regardless of the forest habitat, most grazed plants died before maturity; even fewer grazed plants produced seeds. Litter depth influenced both recruitment and biomass production: both the rate of germination and the size of resultant seedlings were lower on thick litter (6 cm) compared to results on thin litter (1.5 cm). In the more open Pinus ponderosa and Pseudotsuga menziesii forests, cheatgrass colonization may often occur in openings in the understory alone. Colonization in the more shady A. grandis and Thuja plicata forests is unlikely, however, unless the opening extends through both the understory and the overstory. As a result, cheatgrass is unlikely to increase in any of these forests unless the scale and incidence of disturbance increases substantially.     

Competition between two grass species with and without grazing over a productivity gradient

Soil nutrient-level and herbivory are predicted to have opposing effects on the allocation pattern of the competitive dominant plant species. Lower stem and higher leaf allocation are favoured when plants are grazed, whereas a higher stem allocation is favoured at high nutrient levels. Grazing by hares and geese can prevent invasion of the tall Elymus athericus, into short vegetation of Festuca rubra, at unproductive stages of salt-marsh succession but not at more productive stages. We hypothesise that the negative effect of herbivory on Elymus decreases due to increasing soil nitrogen levels and shifts the competitive balance towards this species. We tested how simulated grazing and nitrogen availability affected the competitive balance between adult plants of both grass species in a greenhouse experiment. Elymus had a higher above-ground biomass production, invested relatively more in stem and root tissue and had a larger shoot length than Festuca. The above-ground relative yield of Elymus in mixtures of both species increased with increasing nitrogen levels. This indicates that Elymus was the superior competitor at high soil fertility. Although clipping removed relatively more biomass from Elymus than from Festuca and exceeded the observed biomass removal in field conditions, it did not change the competitive balance between both species. Decreasing effects of herbivory due to increasing nitrogen levels are not a likely explanation for the invasion of Elymus in productive marshes. The results suggest that once Elymus has established it can easily invade vegetation dominated by Festuca irrespective of grazing by herbivores such as hares and geese. Herbivory by small herbivores may mainly retard the invasion of this plant by influencing establishment itself.     

Interaction between large herbivore activities,vegetation structure,and flooding affects tree seedling emergence

Tree establishment in grazed vegetation mosaics involves a series of early bottlenecks, including seed dispersal, germination, seedling emergence, survival and growth. In a field experiment, we studied seedling emergence of two species with contrasting recruitment strategies, Fraxinus excelsior and Quercus robur, in five structurally different vegetations: grazed and ungrazed grassland, ruderal pioneer vegetation, soft rush tussocks, tall sedge mats and bramble scrub. In a simulation experiment, we studied the interaction effects of pre-emergence flooding (3 weeks of inundation), trampling and grazing (simulated by clipping) of grassland vegetation on the emergence and early growth of both tree species in grass swards. Seedling emergence was enhanced in low swards and sparse vegetation types. Despite different recruitment strategies, the interaction of flooding and trampling of swards enhanced seedling emergence of both species. Grazing of soft rush and tall sedges enhanced emergence of F. excelsior. Clipping grass swards increased early growth of emerging Q. robur. Our results support the hypothesis that natural disturbances of soil and vegetation create microsites for seedling emergence and reduce above-ground competition. In grazed systems however, these results suggest a discordant relationship between successful seedling emergence and subsequent seedling growth/survival during the establishment process in structurally different vegetations.     

Large herbivores influence the composition and diversity of shrub-steppe communities in the Rocky Mountains, USA

It is widely believed that wild and domestic herbivores have modified the structure and composition of arid and semi-arid plant communities of western North America, but these beliefs have rarely been tested in long-term, well-replicated studies. We examined the effects of removing large herbivores from semi-arid shrublands for 40–50 years using 17 fenced exclosures in western Colorado, USA. Shrub cover was greater (F=5.87, P=0.0020) and cover (F=3.01, P=0.0601) and frequency (F=3.89, P=0.0211) of forbs was less inside the exclosures (protected) relative to grazed plots. However, we found no significant effects (minimum P=0.18) of protection from grazing on cover or frequency of grasses, biotic crusts, or bare soil. Although mean species richness and diversity were similar between treatments, protected areas had much higher dominance by fewer species, primarily sagebrush. Exclusion of herbivores changed the relationship between species richness and evenness. Consistent with theoretical expectations, species evenness was positively correlated with richness in protected plots (r ²=0.54). However, contrary to theory, evenness and richness were inversely related in grazed plots (r ² _adjacent=0.72, r ² _distant=0.84). We suggest that these differences resulted because grazing acts as a stressor promoting facilitative relationships between plant species that might compete for resources in the absence of grazing. We conclude that exclusion of grazing in the sites we studied caused minor changes in cover and diversity of herbaceous plants, but caused a clear increase in the cover of shrubs. Importantly, the exclusion of ungulates changed the relationship between evenness and richness.     

Effects of the estuarine dinoflagellate Pfiesteria shumwayae (Dinophyceae) on survival and grazing activity of several shellfish species

A series of experiments was conducted to examine effects of four strains of the estuarine dinoflagellate, Pfiesteria shumwayae, on the behavior and survival of larval and adult shellfish (bay scallop, Argopecten irradians; eastern oyster, Crassostrea virginica; northern quahogs, Mercenaria mercenaria; green mussels, Perna viridis [adults only]). In separate trials with larvae of A. irradians, C. virginica, and M. mercenaria, an aggressive predatory response of three strains of algal- and fish-fed P. shumwayae was observed (exception, algal-fed strain 1024C). Larval mortality resulted primarily from damage inflicted by physical attack of the flagellated cells, and secondarily from Pfiesteria toxin, as demonstrated in larval C. virginica exposed to P. shumwayae with versus without direct physical contact. Survival of adult shellfish and grazing activity depended upon the species and the cell density, strain, and nutritional history of P. shumwayae. No mortality of the four shellfish species was noted after 24 h of exposure to algal- or fish-fed P. shumwayae (strains 1024C, 1048C, and CCMP2089) in separate trials at ≤5 × 10³ cells ml⁻¹, whereas higher densities of fish-fed, but not algal-fed, populations (>7–8 × 10³ cells ml⁻¹) induced low (≤15%) but significant mortality. Adults of all four shellfish species sustained >90% mortality when exposed to fish-fed strain 270A1 (8 × 10³ cells ml⁻¹). In contrast, adult M. mercenaria and P. viridis exposed to a similar density of fish-fed strain 2172C sustained <15% mortality, and there was no mortality of A. irradians and C. virginica exposed to that strain. In mouse bioassays with tissue homogenates (adductor muscle, mantle, and whole animals) of A. irradians and M. mercenaria that had been exposed to P. shumwayae (three strains, separate trials), mice experienced several minutes of disorientation followed by recovery. Mice injected with tissue extracts from control animals fed cryptomonads showed no response. Grazing rates of adult shellfish on P. shumwayae (mean cell length ±1 standard error [S.E.], 9 ± 1 μm) generally were significantly lower when fed fish-fed (toxic) populations than when fed populations that previously had been maintained on algal prey, and grazing rates were highest with the nontoxic cryptomonad, Storeatula major (cell length 7 ± 1 μm). Abundant cysts of P. shumwayae were found in fecal strands of all shellfish species tested, and ≤45% of the feces produced viable flagellated cells when placed into favorable culture conditions. These findings were supported by a field study wherein fecal strands collected from field-collected adult shellfish (C. virginica, M. mercenaria, and ribbed mussels, Geukensia demissa) were confirmed to contain cysts of P. shumwayae, and these cysts produced fish-killing flagellated populations in standardized fish bioassays. Thus, predatory feeding by flagellated cells of P. shumwayae can adversely affect survival of larval bivalve molluscs, and grazing can be depressed when adult shellfish are fed P. shumwayae. The data suggest that P. shumwayae could affect recruitment of larval shellfish in estuaries and aquaculture facilities; shellfish can be adversely affected via reduced filtration rates; and adult shellfish may be vectors of toxic P. shumwayae when shellfish are transported from one geographic location to another.     

Factors affecting fruit set in Aizoaceae species of the Succulent Karoo

By reducing the number of flowers and fruits, intense grazing activities of domestic livestock are considered to have negative effects on the plant recruitment of perennial plants in the Succulent Karoo, South Africa. In the present study, the fruit set of six perennial species belonging to the Aizoaceae was investigated under two different grazing intensities. Two species (Cheiridopsis imitans and Leipoldtia schultzei) were examined more closely to determine whether factors other than grazing impact fruit production of these plants. Apart from reproductive output, the population structure of these two species was explored in heavily and slightly grazed areas.For all investigated species, heavy livestock grazing was identified as a major threat to fruit production. Deviations from this pattern were recorded in a drought year, when harsh weather conditions reduced herd sizes dramatically and the usually high grazing pressure was alleviated. However, detailed analyses revealed that also other biotic factors can cause severe fruit and flower losses even exceeding that caused by grazing. Caterpillars of the noctuid moth, Diaphone eumela, and two rodent species, Otomys unisulcatus and Rhabdomys pumilio, proved to be further important herbivores of flowers and fruits. Experiments with caged plants, excluding livestock from grazing, confirmed though that livestock grazing definitely accounts for a continuous reduction of fruit production. Insufficient pollination success was another parameter reducing fruit set; however, this was almost exclusively observed under low grazing pressure.