Arguments published, in order:
Design of vertebrate eye
Ecosystems: how systematic are they?
A pond, a meadow and a forest, in Poland. Photo credit Wikipedia
One hundred years ago several people enquiring about how the living world works developed an idea of an ecological system. The system part was analogous to machines with defined flows of materials, energy, and information. These are cybernetic things, amenable to the techniques of systems analysis.
A problem arose when ecosystems were promoted as literal organisms, somehow equivalent with organisms like worms or whales. Some researchers disagreed, pointing out that unlike organisms, ecosystems have no distinct boundaries, are not autonomously self-reproducing, and without plausible genetic mechanism to evolve by natural selection. These objections went unheeded, and the idea of ecosystem-as-organism became popular. Many people thought of ecosystem as a defined thing in which numerous plants and animals lived closely interconnected and generally cooperating for the good of the system. However, this concept poses difficult questions. What are its boundaries in space and time? How many of these inter-dependencies are true mutualisms, or looser non-obligate symbioses, or non-existent? The co-evolution of two species is well known and understood, but just three or more species together becomes difficult to explain.
Vast assemblages of organisms are daunting to understand, but gaining pragmatic knowledge makes a start. Despite vague boundaries, and random variability of these assemblages they are greater than the sum of their parts. The plants depend on soil, herbivores depend on plants and predators depend on herbivores, all channelling interconnected flows of energy and materials. The dynamics of birth and death of different populations are interlinked. Evolution of these species responded to competition, herbivory, predation and parasitism. So an assemblage needs to be studied at the appropriate hierarchical level where its emergent properties can be measured.
Will these assemblages collapse if we disturb them too much? If we hunt the wolves in a large forest so none are left then the deer of the forest will increase in numbers. They in turn will eat so many seedling trees that eventually the forest will disappear, to be replaced by scrubby grassland. In a similar forest there might be a small population of a beautiful species of woodpecker. Rare because it is at the edge of the natural climatic range of the species of tree that it most needs for nesting and feeding. If the woodpecker disappears from that forest would any ecologist detect any change in the rest of the forest?
Recent large studies of where many separate sets of results about ecosystems are analysed as a whole, a meta-analysis, show a positive relationship between diversity and stability. But when it comes to identifying whether a species that is rare and getting rarer is a keystone species, then the complexity of the links overwhelms our ability to predict. So far: but now our understanding steadily increases as theories of ecosystems are put the empirical test. In the meantime most ecologists recommend the precautionary principle. In the lack of sufficient empirical and experimentally tested knowledge we should manage a natural resource or nature reserve to maintain as much diversity of species as possible.
For the full version of this essay, complete with references, download here.
Good or bad design of the eye of vertebrates?
Human eye. Credit: Petr Novák, Wikipedia
The eye is the most important sense organ for us humans. It is very well studied for medical reasons and eye has been important in the development of ideas about how life evolved. One aspect of studies on the evolution of our eyes and those of vertebrate animals generally is the proposition that the design is poor when compared to cameras. If the evolved design is poor, then it cannot be the work of an intelligent designer. Not only is the vertebrate eye compared unfavourably to cameras but also to the eye of squids and octopuses.
Seemingly the problem with our eyes is that the light sensor, the retina, is the wrong way round! The photoreceptor cells, face away from the incoming light. Bad design? Moreover, they are covered with layers of nerve cells and blood vessels. An octopus has eyes with the photoreceptor layer facing towards the lens, and with the nerve cells going beneath this layer as they lead from the retina to the brain.
Our eyes work well because of the inverted retina, not despite it. The problem of obstruction by nerve fibres and blood vessels is solved at the spot where light is naturally focussed. Here the layer of nerves is thin and the blood vessel layer is reduced. Our retinas have the highest demand for oxygen of any tissue in our bodies. To supply the retina there is a special blood system as network of small vessels that envelope the outside of the eyeball and press against the outermost retinal layer providing a very high rate of exchange with the retina. This entire adaptation for vision of high information content is physically possible only with an inverted retina.
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Are the world's forests really expanding?
Forest cover in part of North America mapped using satellite data; yellow to blue = least to most.
Newspaper and television reports of deforestation and forest fires in the context of the climate crisis are dramatic and depressing. But is this the whole story or is there more happening to forests at the global scale that is cause for optimism and incentive to action? Yes is the short answer, but as often with anything ecological the story contains accounts of reliable research that seem contradictory. This essay presents the case for optimism when these varying studies are balanced against each other for a global perspective based on three mechanisms by which forests can expand.
Growth of plants depends on photosynthesis which requires light, carbon dioxide, water and mineral nutrients. The CO2 is a scarce nutrient for plants but increasing in concentration in the atmosphere. This increase is equivalent to a fertilizer for growth and can be demonstrated experimentally and measured as extra biomass as wood per unit area. CO2 is also a greenhouse gas that heats the atmosphere and changes seasonal climates. When leaf-bud in a deciduous forest starts earlier and leaf-fall starts later there is a greening of the forests that can be mapped using data from satellites. The longer the season the greater the annual growth and this can be estimated as mass of wood per unit area per year. A third way in which forests expand is by regeneration on land they formerly occupied or by spread into new areas of land by the hectare or square kilometre.
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