The effect of suspended sediments on Lake Texoma Daphnia: field distributions and in situ incubations

1. We measured the abundance and eggs per female of four Daphnia species in turbid and relatively clear regions of Lake Texoma (Oklahoma‐Texas, U.S.A.) on 12 dates over the course of 5 years. 2. Two species, Daphnia lumholtzi and Daphnia parvula, occurred and reproduced in turbid locations, but two other species, Daphnia mendotae and Daphnia pulicaria, occurred almost exclusively in relatively clear conditions. 3. To test the hypothesis that interference with foraging excludes clear‐water Daphnia species from turbid locations, we incubated adult D. mendotae at both a clear and a turbid site. In three successive experiments D. mendotae individuals incubated at the turbid site carried as many or more eggs than individuals incubated at the clear site.     

Impact of larval Dorosoma predation on Daphnia parvula dynamics

• 1 In reservoirs of the south-eastern United States, Daphnia typically decline in abundance during the summer months. Current understanding suggests that adult shad (Dorosoma spp.), the dominant planktivore in these reservoirs, plays a limited role in Daphnia midsummer declines. The impact of shad larvae is beginning to be documented.
• 2 To examine the impact of larval shad on Daphnia seasonal dynamics, larval dynamics were studied over two seasons and stomach contents were analysed during one season.
• 3 Initial Daphnia panwla declines correlated with abundance of shad larvae. During the decline, shad stomach contents consisted of 97—100%D. parvula. After D. parvula declined, Bosmina and Diaphanosoma accounted for up to 97% of larval diets. When shad diets included other cladocerans, D. parvula abundance increased in the reservoir.
• 4 When this predation pattern was incorporated into a predictive model, larval shad predation accounted for an initial, temporary decline in D. parvula abundance. However, when other crustaceans were incorporated into the shad diet, D. parvula populations increased.
• 5 These results suggest that larval shad have a temporary, strong influence on the structure and abundance of D. parvula populations in south-eastern reservoirs of the United States.

     

Aluminium Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of the Root Cap

The primary symptom of aluminium (Al) toxicity in higher plantsis inhibition of root growth. In this study, we investigatedthe spatial sensitivity of maize (Zea mays L.) roots to Al.A divided-chamber technique indicated that only exposure ofthe terminal 10 to 15 mm of the root to Al resulted in inhibitionof growth. Application of Al to all but this apical region ofthe root had little or no effect on growth for 24 h and causedminimal damage to the root tissue. Small agar blocks infusedwith Al were then applied to discrete areas of the apex of maizeroots to determine which section (root cap, meristem or elongationzone) was more important to Al-induced inhibition of growth.The terminal 20 to 30 mm of root (root cap and meristem) mustbe exposed to Al for inhibition. Application of Al to the 30mm of root proximal to this terminal zone (elongation zone)resulted in damage to the root tissue but no significant inhibitionof growth. Therefore, the visible injuries incurred by rootsduring Al-stress are not associated directly with the inhibitionof root growth. Furthermore, removal of the root cap had noeffect on the Al-induced inhibition of root growth in solutionexperiments and argues against the root cap providing protectionfrom Al stress or serving an essential role in the mechanismof toxicity. We suggest that the meristem is the primary siteof Al-toxicity. Key words: Aluminium, toxicity, root growth, root cap     

Plant coexistence in coastal heaths: Between- and within-habitat effects of competition,disturbance and predation in the post-fire environment

Abstract Previous experiments on post-fire establishment of two obligate seeders and two resprouting species suggested that initial establishment processes differ between wet- and dry-heath habitats. Disturbance, interspecific competition between seedlings and between seedlings and adults, and access for potential predators were manipulated in the field after a fire to identify mechanisms to explain between- and witm'n-habitat species coexistence. When soil surfaces are disturbed or seeds are buried, dry-heath species can establish in the wet heath. Under natural conditions, however, wet heath is rarely disturbed by animal foragers and lack of safe sites may preclude establishment of dry-heath species. In contrast, dry heath is often disturbed by ground foragers such as bandicoots and safe sites are plentiful for establishment of all heath species. Nevertheless, while wet-heath species can establish in dry heath, their seedlings are apparently unable to survive there due to the drier conditions. No evidence for competition among species was detected in either type of habitat during the first 3 years of the experiment, even at elevated seedling densities. Lack of early competition and unsaturated seed-banks may promote coexistence of species. Seedling predation from vertebrates was low and did not appear to influence patterns of coexistence within or between habitats. In terms of community theory we suggest that several interacting mechanisms are responsible for community structure in heaths after a fire. Chance and microsite disturbance are important during dispersal, while resources are critical during establishment and early survival of seedling populations. In these heath systems where fires burn extensive areas, seed-banks may be more important in temporal storage of species than patch or spatial storage as a mechanism for maintaining coexistence and plant diversity.