Temporal Hierarchies of Variation and the Maintenance of Diversity

Abstract A general model shows how the long-term growth rate of a population can be partitioned into components representing various mechanisms of maintenance of species diversity. One component summarises the effects of fluctuation-independent mechanisms, which include classical resource partitioning and frequency-dependent herbivory. Two other components represent fluctuation-dependent mechanisms, the storage effect and relative nonlinearity of competition.
The general model shows how a community will track an equilibrium set by fluctuation-independent mechanisms and the environmental state when community dynamics are faster than the rate of environmental change. Fluctuation-dependent mechanisms can be important for diversity maintenance with or without such tracking, but on long timescales their effects are indistinguishable from those of fluctuation-independent mechanisms.
These considerations lead to a hierarchical view of mechanisms of diversity maintenance where the effects of different timescales are partitioned or merged depending on the timescale of observation. These issues are illustrated with model examples involving various combinations of resource partitioning, fluctuations in recruitment rates, variation in the timing of germination, and seasonality. The very long timescales associated with climate change contain many complexities but nevertheless many ideas applicable to shorter timescales may be useful in a modified form.     

The upstream/downstream movement paradox of lotic invertebrates: quantitative evidence from a Welsh mountain stream

• 1 This study attempts to determine the mechanisms by which the downstream displacement of lotic invertebrates is compensated by the upstream movements of immatures and adults. To this end, submerged and aerial traps were set up at three sites, each 100 m apart on a small mountain stream (Yr Ogof) in North Wales and operated for 12 months.
• 2 At Site 1 (the source), 19.3% of the benthic invertebrates lost as drift were replaced by the upstream movement of aquatic stages. At Site 2, the average upstream compensation was 51.1% (although there was a marked difference between the two traps at the site). At downstream Site 3 the average compensation was 55.4%. The net loss of benthos downstream over 1 year was estimated to be 261920 individuals from Site 1, 41891 from Site 2, and 40470 from Site 3.
• 3 At Sites 1 and 2, the numbers of invertebrates drifting were positively correlated with both current speed and water depth. At Sites 2 and 3, the numbers moving upstream were positively correlated with water temperature. At all three sites, the numbers of animals drifting were positively correlated with the numbers moving upstream, under water.
• 4 In none of the eight common species of stoneffy did females show a strong overall upstream flight preference—however, two species, Leuctra nigra and Nemoura erratica, showed a significant downstream preference.
• 5 As with the stonefly nymphs, most of the caddisfly larvae showed a longitudinal distribution. Agapetus fuscipes larvae were most abundant at Site 1, yet females showed no preferred flight direction—this would seem to be inappropriate for maintaining this larval distribution pattern. However, females caught moving upstream at Site 2 were bigger and contained twice as many eggs, which were also larger, than females moving upstream at Site 1. The flight patterns of the other Trichoptera also seemed inappropriate for maintaining their respective larval distributions. Immature insects moved upstream at certain sites and times, but trends were not consistent. There was no correlation between female flight direction and local wind speed and direction.
• 6 Downstream displacement of the most abundant non-insect species, Gammarus pulex, was quite considerable at all three sites. However, at several times during the year the numbers moving upstream approached or exceeded those drifting. Maintenance of benthic population densities in this species is thought to be through a combination of upstream movement (which was highly correlated with benthic density at Sites 1 and 3) and production of young over a long (10 month) reproductive period. Maintenance of the benthic populations of the immature stages of the majority of insect species in Yr Ogof is thought to be through oviposition by females in the vicinity of their emergence sites.

     

Fine Structure of Sarcocystis cruzi Schizonts

SYNOPSIS Schizogony of Sarcocystis cruzi Hasselmann (syn. S. fusiformis Railliet) takes place in vascular endothelial cells 26 to 33 days after cattle ingest sporocysts from dogs. Kidney cortex from a heavily infected, dexamethasone-treated bovine was fixed for electron microscopy to determine the method of schizogonie development. Schizogony takes place by endopolygeny characterized by marked enlargement of the parasite nucleus, formation of nuclear lobes, presence of numerous spindles with adjacent pairs of centrioles along the nucleus, and simultaneous formation of daughter merozoites in the cytoplasm adjacent to the spindle poles. Endopolygeny in S. cruzi differs from that in other Sporozoa in that merozoite anlagen are seen in the cytoplasm before any nuclei divide. The resultant merozoites continue development and, when mature, resemble other sporozoan zoites. Upon release from the host cell into capillaries, they travel to muscle tissue to continue the life cycle by forming sarcocysts.     

Phenotypic Variation and Systemic Cancer in the FAMMM Syndrome

Familial atypical multiple mole melanoma (FAMMM) syndrome, a cancer-associated genodermatosis, is a dominantly inherited heterogeneous disorder with variable expressivity of both its cutaneous and cancer phenotypes. By using a verified historical review technique of cancer documentation (idout patient health records, pathology reports/slides, autopsy reports/slides, and death certificates) of all anatomic sites in all members of a modified nuclear pedigree (first-degree relatives plus maternal and paternal grandparents, aunts, and uncles) over several generations, we showed that the FAMMM syndrome is similar to the majority of autosomal dominant inherited cancer-associated genodermatoses and has excessive risk for cancer of multiple anatomic sites. With respect to the FAMMM syndrome, these cancers involved the breast, respiratory tract, gastrointestinal system, the eye (intraocular melanoma), and the lymphatic system. These FAMMM pedigrees showed some of the following distinctive characteristics of hereditary cancer: 1) integral patterns of cancer within and between pedigrees; 2) early age of onset of cancer; 3) prolonged survival of some pedigree members with cancer; and 4) an excess of multiple primary melanomas and cancers of variable anatomic sites. The presence of these features indicates that these cancers of variable anatomic sites may be etiologically associated with the FAMMM syndrome. Heterogeneity should be investigated in FAMMM pedigrees with attention to consistent differences in size and distribution of atypical lesions, age at cancer onset, and pattern of tumor occurrences. The occurrence of FAMMM pedigrees in the general population or among pedigrees of probands with atypical nevi is not known. The occurrence of systemic cancers in these FAMMM pedigrees requires the development of cancer surveillance programs that are specifically modified to the particular cancer pattern of each pedigree.     

Reflections on Organisms: Examples from the Neurobiology of Insect Rhythmic Movements

Consideration of organisms is necessary for neurobiologistsinterested in the neural bases of behavior, because a behavioralact performed by an intact animal defines the problem to besolved and also constitutes the context in which the nervoussystem operates. In addition, observations of behaving organismscan introduce new preparations for study of general questions.Examples are given from rhythmic gill movements of Corydaluscornutus, insect flight, and development of the moth flightmotor. In a concluding section, questions are raised about possiblephilosophical and judgmental influences on the current tensionsbetween holistic approaches expressed in organismic biologyand reductionist approaches expressed in biotechnology.     

Seasonal decline in plant defence is associated with relaxed offensive oviposition behaviour in the viburnum leaf beetle Pyrrhalta viburni

1. Plant defence of Viburnum shrubs against oviposition by its specialist herbivore, the viburnum leaf beetle [VLB Pyrrhalta viburni(Paykull)], involves an egg‐crushing wound response in twigs. Although the response is variable among Viburnum species, it can have a strong impact on egg survivorship. Beetles typically aggregate egg masses with conspecifics along infested twigs, forming clusters that can overwhelm the twig response. It was investigated whether twig responses and beetle oviposition behaviour vary seasonally. 2. In a field experiment, twig defences decreased towards the end of the VLB oviposition period: wound response of the North American Viburnum dentatum L. and the European V. opulus L. was reduced by 100% and 54%, respectively, in September compared with the July to August period. 3. Oviposition trials demonstrated a corresponding behavioural change: VLB females displayed aggregative oviposition in August, but not in September. 4. Further tests revealed that late‐season VLB females reverted to aggregative oviposition after being kept on uninfested twigs, whereas females kept on heavily infested twigs did not. This behavioural change suggests that relaxation of aggregative oviposition originates from cues associated with high densities of egg masses. 5. Relaxation of aggregative oviposition may be adaptive (and beneficial for invasion) on shrubs with low levels of defences by reducing intra‐specific competition.     

Evaluating Residual Tree Patches as Stepping Stones and Short-Term Refugia for Red-Legged Frogs

Abstract: Temperate pond-breeding amphibians are vulnerable to forest fragmentation because they must access upland terrestrial sites during the nonbreeding season but are prone to desiccation in hot, dry environments without canopy cover. Harvesting techniques that retain live trees in the cut block are advocated for sustaining forest biodiversity, but the effects of these practices on amphibians are unknown. We studied red-legged frogs (Rana aurora) in movement trials to assess: 1) how short-term use of residual trees was affected by tree patch size, streams, and neighborhood features; 2) whether residual tree patches were used as stepping stones in negotiating cut blocks; 3) the effects of patch size and patch proximity in altering movement paths; and 4) the effects of retention level and patch size on interpatch distance. Residual tree patches were potentially valuable short-term refugia but their value was size dependent. Virtually all frogs released at the base of single trees or inside small tree clusters left within 72 hours, but the proportion leaving decreased curvilinearly with increasing patch size. Frogs were less likely to leave tree patches with a running stream or where neighborhood stream density was high. Residual tree patches did not systematically alter movement paths. Frogs intercepted residual tree patches mostly at random and had to be within 5-20 m of a tree patch before moving to it in greater proportions than expected by chance. However, amphibian movements were biased toward large (0.8 ha) patches and away from small (0.3 ha) patches 50 m away. Our results indicated that residual trees should not be retained singly but should be aggregated in groups between 0.8 ha and 1.5 ha, preferably in stream locations.