Individuals die, populations disappear, and species become extinct. That is one view of the world. But another view of the world concentrates not so much on presence or absence as upon the numbers of organisms and the degree of constancy of their numbers. These are two very different ways of viewing the behavior of systems and the usefulness of the view depends very much on the properties of the system concerned. If we are examining a particular device designed by the engineer to perform specific tasks under a rather narrow range of predictable external conditions, we are likely to be more concerned with consistent nonvariable performance in which slight departures from the performance goal are immediately counteracted. A quantitative view of the behavior of the system is, therefore, essential. With attention focused upon achieving constancy, the critical events seem to be the amplitude and frequency of oscillations. But if we are dealing with a system profoundly affected by changes external to it, and continually confronted by the unexpected, the constancy of its behavior becomes less important than the persistence of the relationships. Attention shifts, therefore, to the qualitative and to questions of existence or not. Our traditions of analysis in theoretical and empirical ecology have been largely inherited from developments in classical physics and its applied variants. Inevitably, there has been a tendency to emphasize the quantitative rather than the qualitative, for it is important in this tradition to know not just that a quantity is larger than another quantity, but precisely how much larger. It is similarly important, if a quantity fluctuates, to know its amplitude and period of fluctuation. But this orientation may simply reflect an analytic approach developed in one area because it was useful and then transferred to another where it may not be. Our traditional view of natural systems, therefore, might well be less a meaningful reality than a perceptual convenience. There can in some years be more owls and fewer mice and in others, the reverse. Fish populations wax and wane as a natural condition, and insect populations can range over extremes that only logarithmic

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Resilience and Stability of Ecological Systems

This book describes the present state of knowledge regarding the impact of cyanobacteria on health through the use of water. It considers aspects of risk management and details the information needed for protecting drinking water sources and recreational water bodies from the health hazards caused by cyanobacteria and their toxins. It also outlines the state of knowledge regarding the principal considerations in the design of programmes and studies for monitoring water resources and supplies and describes the approaches and procedures used.
The development of this publication was guided by the recommendations of several expert meetings concerning drinking water (Geneva, December 1995; Bad Elster, June
1996) and recreational water (Bad Elster, June 1996; St Helier, May 1997). An expert meeting in Bad Elster, April 1997, critically reviewed the literature concerning the toxicity of cyanotoxins and developed the scope and content of this book. A draft manuscript was reviewed at an editorial meeting in November 1997, and a further draft was
reviewed by the working group responsible for updating the Guidelines for Drinkingwater Quality in March 1998.

/pdf/toxic-cyanobacteria-in-water-a-guide-to-their-public-health-2y64v55hv1.pdf

Toxic cyanobacteria in water: a guide to their public health consequences, monitoring and management.

1 It is useful to distinguish between the immediate effects of species richness on ecosystems and those which become apparent on a longer time scale, described here as filter and founder effects.

2 Relationships between plant diversity and ecosystem properties can be explored by classifying component species into three categories – dominants, subordinates and transients. Dominants recur in particular vegetation types, are relatively large, exhibit coarse-grained foraging for resources and, as individual species, make a substantial contribution to the plant biomass. Subordinates also show high fidelity of association with particular vegetation types but they are smaller in stature, forage on a more restricted scale and tend to occupy microhabitats delimited by the architecture and phenology of their associated dominants. Transients comprise a heterogeneous assortment of species of low abundance and persistence; a high proportion are juveniles of species that occur as dominants or subordinates in neighbouring ecosystems.

3 A ‘mass ratio’ theory proposes that immediate controls are in proportion to inputs to primary production, are determined to an overwhelming extent by the traits and functional diversity of the dominant plants and are relatively insensitive to the richness of subordinates and transients. Recent experiments support the mass ratio hypothesis and the conclusion of Huston (1997) that claims of immediate benefits of high species richness to ecosystem functions arise from misinterpretation of data.

4 Attribution of immediate control to dominants does not exclude subordinates and transients from involvement in the determination of ecosystem function and sustainability. Both are suspected to play a crucial, if intermittent, role by influencing the recruitment of dominants. Some subordinates may act as a filter influencing regeneration by dominants following major perturbations.

5 Transients originate from the seed rain and seed banks and provide an index of the pool of potential dominants and subordinates at specific sites. Where the landscape carousel operates against a background of declining diversity in the reservoir of colonizing transients, we may predict that a progressive loss of ecosystem functions will arise from the decline in the precision with which dominants can engage in the re-assembly and relocation of ecosystems.

https://besjournals.onlinelibrary.wiley.com/doi/epdf/10.1046/j.1365-2745.1998.00306.x

Benefits of plant diversity to ecosystems: immediate, filter and founder effects

Communities of microscopic plant life, or phytoplankton, dominate the Earth's aquatic ecosystems. This important new book by Colin Reynolds covers the adaptations, physiology and population dynamics of phytoplankton communities in lakes and rivers and oceans. It provides basic information on composition, morphology and physiology of the main phyletic groups represented in marine and freshwater systems and in addition reviews recent advances in community ecology, developing an appreciation of assembly processes, co-existence and competition, disturbance and diversity. Although focussed on one group of organisms, the book develops many concepts relevant to ecology in the broadest sense, and as such will appeal to graduate students and researchers in ecology, limnology and oceanography.

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The Ecology of Phytoplankton

We review the effects of fishing on benthic fauna, habitat, diversity, community structure and trophic interactions in tropical, temperate and polar marine environments and consider whether it is possible to predict or manage fishing-induced changes in marine ecosystems Such considerations are timely given the disillusionment with some fishery management strategies and that policy makers need a scientific basis for deciding whether they should respond to social, economic and political demands for instituting or preventing ecosystem-based management Fishing has significant direct and indirect effects on habitat, and on the diversity, structure and productivity of benthic communities These effects are most readily identified and last longest in those areas that experience infrequent natural disturbance The initiation of fishing in an unfished system leads to dramatic changes in fish community structure As fishing intensity increases the additional effects are more difficult to detect Fishing has accelerated and magnified natural declines in the abundance of many forage fishes and this has lead to reduced reproductive success and abundance in birds and marine mammals However, such donor-controlled dynamics are less apparent in food webs where fishes are the top predators since their feeding strategies are rather more plastic than those of most birds and mammals Fishers tend to target species in sequence as a fishery develops and this leads to changes in the composition of the fished communities with time The dramatic and apparently compensatory shifts in the biomass of different species in many fished ecosystems have often been driven by environmental change rather than the indirect effects of fishing Indeed, in most pelagic systems, species replacements would have occurred, albeit less rapidly, in the absence of fishing pressure In those cases when predator or prey species fill a key role, fishing can have dramatic indirect effects on community structure Thus fishing has shifted some coral reef ecosystems to alternate stable states because there is tight predator–prey coupling between invertebrate feeding fishes and sea urchins Fishing has reduced, and locally extirpated, populations of predatory fishes These reductions do not have a consistent effect on the abundance and diversity of their prey: environmental processes control prey populations in some systems, whereas top-down processes are more important in others By-catch which is discarded during fishing activities may sustain populations of scavenging species, particularly seabirds We conclude by identifying the circumstances in which new research is needed to guide managers and stress the importance of unfished control sites for studies of fishing effects We discuss the advantages and disadvantages of closed area management (marine reserves) and the conditions under which such management is likely to provide benefits for the fishery or ecosystem

/pdf/the-effects-of-fishing-on-marine-ecosystems-4zb9eqofxt.pdf

The effects of fishing on marine ecosystems

Various methods for the estimation of populations of algae and other small freshwater organisms are described. A method of counting is described in detail. It is basically that of Utermohl and uses an inverted microscope. If the organisms are randomly distributed, a single count is sufficient to obtain an estimate of their abundance and confidence limits for this estimate, even if pipetting, dilution or concentration are involved. The errors in the actual counting and in converting colony counts to cell numbers are considered and found to be small relative to the random sampling error. Data are also given for a variant of Utermohl's method using a normal microscope and for a method of using a haemocytometer for the larger plankton algae.

The inverted microscope method of estimating algal numbers and the statistical basis of estimations by counting

A Study of the Numbers, Biomass and Year-Class Strengths of Perch (Perca fluviatilis L.) in Windermere from 1941 to 1966

Estimates of the Numbers, Biomass and Year-Class Strengths of Perch (Perca fluviatilis L.) in Windermere from 1967 to 1977 and Some Comparisons with Earlier Years

1.

Recording thermographs were used to record the water temperatures in three small stony streams in northwest England. These were two Lake District streams, Black Brows Beck and King's Well Beck which rise at altitudes of 246 and 328 m.a.s.l. respectively, and Rough Sike a Pennine stream rising at 686 m.a.s.l.




2.

The annual mean water temperatures in Black Brows and King's Well becks were within the range 8 to 10°C but the annual mean in Rough Sike was only 6°C. Rough Sike had lower monthly means than the other two becks during winter and it had a midsummer temperature peak of 11 to 12°C which lasted about one month, whilst the other becks had summer temperature means of 13 to 15°C for three to four months.




3.

Rough Sike had an annual temperature sum of about 1700 degree-days above 0°C whilst a value of over 3,000 degree-days was obtained in the other two becks.




4.

The daily water temperature range was similar in all three becks. The monthly means of daily ranges were 1.5°C or less during winter and 5 to 7°C during summer, though on some days ranges of 10 to 12°C occurred.




5.

Macan (1958) found that in the stream he studied the highest summer water temperatures occurred on days when the sun shone after rain, rather than on sunny rainless days. This conclusion does not appear to be applicable to the becks studied in the present work.




6.

Evidence is presented to show that water temperature can be appreciably modified by passage through underground sections of stream.




7.

Some general problems of stream temperature measurement were discussed and some tentative calculations were made to illustrate the possible effects of stream temperature on trout metabolism and on the time taken for trout eggs to develop.

The temperature of three different small streams in northwest England

Windermere is a lake of 14 km2 divided into two glacially excavated basins. It is essentially mesotrophic but, about 1840, Asterionella became the dominant planktonic diatom and small changes towards eutrophication have recently occurred, all as a result of increases in population, water borne sewage, and agricultural fertilization in the catchment.Windermere has five salmonid taxa belonging to the three species: Salmo salar, S. trutta, and Salvelinus willughbii. Perca fluviatilis, Esox lucius, Anguilla anguilla, and Phoxinus phoxinus are abundant, and there are two other small fish species.A successful net fishery for the char, perch, pike, and trout operated for several centuries until overfishing for the char and an ascending interest in sport fishing for salmon and trout brought it to an end in 1921. There was then no significant exploitation until 1941.The Freshwater Biological Association has experimented with the fish populations since 1941 by intensive trapping of perch from 1941 to 1964 and gilln...

E. D. Le Cren

Papers

The inverted microscope method of estimating algal numbers and the statistical basis of estimations by counting

A Study of the Numbers, Biomass and Year-Class Strengths of Perch (Perca fluviatilis L.) in Windermere from 1941 to 1966

Estimates of the Numbers, Biomass and Year-Class Strengths of Perch (Perca fluviatilis L.) in Windermere from 1967 to 1977 and Some Comparisons with Earlier Years

The temperature of three different small streams in northwest England

Windermere: Effects of Exploitation and Eutrophication on the Salmonid Community