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Can. Ent. 115: 1359-1364 (1983) A very simple model of normalized growth rates of temperate pasture legumes and grasses in response to temperature and moisture developed by Fitzpatrick and Nix for Australia was modified and usdd to examine the favorability of weather for the buildup of eight species of aphids in southeastern Australia. The work takes a physiological approach to describing the condition favorable for aphid population development. ResumeUn modkle trks simple de la croissance normalisee des 1Cgumineuses et des graminCes des p2turages ternperks en fonction de la tempkrature et de l'humidite, mis au point par Fitzpatrick et Nix pour l'Australie, a Ct C modifiC et utilisC pour Ctudier la convenance de la tempCrature pour l'accroissement de 8 esphces de pucerons dans le sud de 1'Australie. Une approche physiologique est utilisCe pour dkcrire les conditions favorisant la croissance des populations aphidiennes.

There is little research on processes limiting macroconsumer species in salt marshes. We investigated the significance of competition for space and food resources in three North Carolina salt marshes by altering densities of the molluscs Littorina irrorata and Geukensia demissa, and by manipulating Spartina litter. Morality and weight gain were the two principal responses measured. Enclosures (7.5 m2) were used to contain Littorina and marsh grass. Molluscan densities and Spartina litter levels were one—half, control, and double control levels. The field experiments ran from February 1975 through March 1977. The overall mortality and growth patterns of both species were similar within each marsh, but differed among marshes. The molluscs exhibited high mortality and slow growth in Tar Landing marsh, high mortality and rapid growth in Causeway marsh, and low mortality and slow growth in Calico marsh (enriched by sewage effluent). In Tar Landing survival and weight gain in both species were reduced by crowding. Increasing Spartina litter produced significant weight gain of medium—sized Littorina and small Geukensia. In Causeway marsh crowding decreased Littorina and Geukensia growth rates, but increased mortality only in the mussel population. Litter manipulations primarily affected weight gain in Littorina and mortality in Geukensia. In the enriched Calico marsh crowding depressed Geukensia survivorship and Littorina weight gain. Only in the mussel population was weight gain enhanced (1st yr only) by the addition of Spartina litter. We conclude that competition for food is an important limiting process, particularly for larger Littorina in Tar Landing marsh. In Causeway marsh competition for space appears more significant. In Calico marsh space limitation in Littorina populations and competition for food resources in Geukensia appear to be the principal processes. Variation in these competition—limiting mechanisms between two distant marsh consumers, a grazer and a filter feeder, may represent different procedures for dealing with the variable resource levels existing among salt marshes.

No abstract

Most studies of the effects of predation on the organization and structure of prey communities have ignored predator population size structure and density. In the present investigation, we designed a series of field experiments to test the effects of these predator population properties on the intertidal benthic invertebrates of a salt marsh. The common mummichog, Fudulus heteroclitus, was the primary predator in our experiments. Three size—classes of mummichogs (small, 40—50) mm Total Length [TL]: medium, 50—69 mm TL; large >70 mm TL) were maintained at each of three densities (1, 2 and 4 fish/m2) in 10—m2 enclosures on the salt marsh. Water—filled pits were included in each cage as low—tide refuges for the fish. Each fish treatment combination was replicated three times. In addition, there were three uncaged 10—m2 plots (open controls) and three caged plots from which Fundulus were excluded (closed controls). Three cylindrical core samples (0.005 m2) were extracted from the sediment to a depth of 10 cm in each of the 33 experimental plots 1, 4, and 10 mo after initiation of the experiment in June 1978. Densities of the small, surface—dwelling invertebrates which make up most of the benthic community (Manayunkia aestuarina, Streblospio benedicti, Leptochelia rapax, ostracods, hydrobiid snails, and Gemma gemma) were highest in enclosures containing large fish, and lowest in closed controls and in enclosures containing small fish. Intermediate invertebrate densities occurred in enclosures containing medium—sized mumichogs. The effects of fish size were enhanced at high fish densities (2—4 fish/m2), although fish density alone had little effect on invertebrate abundance. We considered several hypotheses that could explain the findings. Small epibenthic predators—disturbers like the grass shrimp, Palemonetes pugio, were commonly observed in the experimental enclosures. The gut contents of fish taken from enclosures indicated that the shrimp were ingested only by medium and large size—classes of F. heteroclitus. We suggest that infaunal densities increased in response to the control of small epibenthic predator—disturbers (i.e., P. pugio) by the larger F. heteroclitus.