Comparative ecology of Gammarus pulex (L.) and Asellus aquaticus (L.) II: fungal preferences

This paper continues to explore niche differentiation in Gammarus pulex and Asellus aquaticus by analysis of their food preferences. Individuals from both species discriminated between leaf discs colonized by different fungal species and exhibited strong preferences for Anguillospora longissima and Heliscus lugdunensis. Fungal preferences were not correlated with the relative abundance of fungi in the field and there was considerable intra-population variability in food preferences — both between individuals and for the same individual through time. Niche overlap between animals from all four study populations was high and there was no evidence of differences in the potential trophic niches of animals from sympatric and allopatric populations.     

Ecological compensation--a complication for testing life-history theory

Mortality, growth and birth rates cannot vary independently in stable populations, environmental change of one variable must be accompanied by compensatory variation of another. Ecological compensation is recognized if the stable populations are genetically identical. Ecological compensation, if it operates, constrains the direction of evolutionary change, and predictions that ignore it may be in error.     

Can ecosystems be healthy? Critical consideration of concepts

Health, it is argued, implies that a system has an optimum state that can be defended. For organisms and populations this can be understood objectively and generally in terms of neo-Darwinian principles. Similar reasoning cannot be applied to ecosystems. The possible advantages and difficulties of applying this concept of health to ecosystems are critically considered.     

Physiological costs of combating chemical toxicants: ecological implications.

1. Evidence is presented that combating the poisoning effects of toxic chemicals is metabolically costly. 2. This has implications for relating physiological stress responses observed at the level of individual organisms to population effects, and needs to be incorporated explicitly into models making this link. 3. The cost hypothesis also has implication for the evolution of stress resistance either as a fixed or facultative (inducible) response. Optimization models incorporating these ideas are reviewed and discussed.     

The food of Ancylus fluviatilis (Müll.), a littoral stone-dwelling,herbivore

Summary A. fluviatilis is a herbivore which usually ingests periphyton. Evidence derived from both field observation and laboratory experiments suggests that the limpet prefers diatoms and that within this group Gomphonema is apparently the most attractive genus. These food preferences were most clearly defined in satiated individuals.Snails had no apparent effect on algal community structures in the field situation and it is suggested that this may either result from adaptability of the periphyton with respect to grazing disturbances, or from low snail density and thus low grazing intensity.The food preferences of A. fluviatilis in particular, and other algal exploiters in general, are discussed in terms of plant defense mechanisms. It is argued that an animal's ability to cope with algal cell walls (either in a mechanical or chemical sense) may be one of the major determinants in the food preference behaviour shown by herbivores feeding on algae.     

A new radiotracer technique involving 14C and 51Cr,for estimating the assimilation efficiencies of aquatic,primary consumers

Summary Gravimetric, radiotracer, and indicator methods currently available for estimating assimilation efficiencies, have been reviewed and their associated limitations have been discussed. It was concluded that the basic assumption implicit to gravimetric and indicator techniques, i.e. that all material contained within the faeces is derived from the food, does not generally hold. Radiotracer techniques are not based on this assumption but are time consuming. Consequently a new radiotracer technique analogous to indicator methods has been developed. In this technique the concentration of a non-absorbed indicator is expressed in terms of a radiotracer, ¹⁴C, which can be absorbed but which, at least initially, is only present in the food, rather than expressing it in terms of dry weight. ⁵¹Cr has been used as the nonabsorbed indicator.Use of these two isotopes in conjunction not only enables a distinction to be made between faecal material derived from food, and that derived from metabolic secretions but also facilitates estimation of assimilation efficiences fromsmall samples of faeces only. The new technique requires simply, measurement of the ratio ¹⁴C:⁵¹Cr in samples of both food and faeces.The applicability of conditions necessary for operation of the new technique has been tested on two species of freshwater gastropod, one feeding on epilithic algae, the other on bacteria, and its effectiveness has been tested by reference to results obtained from another, more conventional method involving ¹⁴C only.