Ecology, like chemistry and physics, can be explained as the interaction of particles. The particles in this case are individual plants and animals ( and I do include fungi and microbes ). True, they do not look or act like billiard balls but they do have measurable properties that allow us to predict their behaviour both as pure collections of the same kind of particle and as mixtures. For this overview, I would like to focus on four properties of plants and animals : size, variety, spatial distribution and diet (The “diet” of plants requires some explaining – but go with me on this).
Perhaps the most important theory in ecology is that body size is destiny. Large animals eat more food and use more energy just to stay alive. Oddly, larger animals are more efficient about this, using less energy per body weight than small animals. Initial explanations of this efficiency pointed out that small animals have more surface area for their body weight and would lose more energy to the world around them. If this were the only factor, then larger animals would be more efficient than they actually are. Dawkins gives an amusing explanantion for this phenomenon in his book “The Ancestor’s Tale”. Not only do larger animals have less surface area per weight but they have more support structures and plumbing per weight . They need these structures just to hold things together and to give the cells in the centre of the body the same contact with oxygen, food and water that microbes enjoy. The net result is a rather precise ratio of increased energy consumption to increased weight of 3:4. Though body size relationships are not usually applied to plants, Dawkins uses the lowly cauliflower to illustrate the need for more plumbing and support structures as a species aspires to larger body sizes.
Body size allows us to predict a number of physiological rates (heart rate, excretion rate, nutrient consumption rate, etc) that are relevant to ecology and a couple of more ecological characteristics, like the number of individuals in a given area or the size of the individual’s prey. Dodds (2009; p.159) rejects the idea of a general theory of body size in ecology as there is still much unexplained variation. I suppose I place more value on a theory that makes quantitative estimates than one that applies in every single instance.
Energy creates diversity
The variety of ecological particles, or plants and animals, in a given area is a collective property. It seems clear, as we discussed in my posting on biodiversity, that, taken at the scale of thousands of square kilometers, variety can be predicted by the amount of energy available for the species to use; I called this Wallace’s Rule. It is interesting to consider whether this biogeographic prediction has application at the hectare or square kilometer scale that ecologists tend to work at.
Diversity creates stability
We have also discussed how collections of ecological particles with variety tend to be stable. This goes to the heart of ecology’s central question: “Do we use nature sustainably?” The answer lies, in part, in how well we maintain variety. Ecologists, however, are impatient to go beyond the easily measured aspect of stability in small-scale experiments (reliable yield). Ideally, we would be measuring resilience, the ability of an ecosystem to return to the variety and abundance of ecological particles that were present before a disturbance. It is, as suggested by the name, the ability of an ecosystem to “bounce back”.
Landscapes have a tipping point
The spatial distribution of ecological particles is another collective property, which can describe pure groups (populations) or mixed collections of particles (communities). The ability of particles to move across a landscape or a connected set of water bodies is important for maintaining individual populations as well as promoting variety and its stabilizing effects. This ability declines dramatically when less than half the area is suitable for a species’ survival. Below 50% , the spatial arrangement of the remnant patches plays an increasingly important role in determining the movement of particles. This tipping point was first identified through computer modelling but a recent study shows that fewer species survive in landscapes with less than 50% natural habitat.
Ecosystems are what they eat
Ecological particles consume each other. Their diet , or preferred pattern of consumption, is their most biological property. Though diet is a characteristic of individuals, the collective diet of mixtures of particles is an important predictor. Plants (and some microbes) consume mineral compounds and make their own carbohydrates. They have very similar diets and this leads to precise predictions when a single nutrient is limiting (e.g. phosphorus in fresh water ecosystems). The network of dietary connections in an ecosystem is known as a foodweb, literally a list of who eats who. The main characteristics of a food web (e.g. % top predators, length of food chains, etc.) can be predicted with a model based on just two properties: the variety (or number of species) and diet (or the proportion of possible dietary links that occur) of the particles in the food web. Food web characteristics link back to other ecological properties, as they are partly determined by the size, variety and spatial distribution of particles and they provide the basic mechanisms for resilience (e.g. negative feedback loops).
Some physicists dream of the day when all their theories can be summarized by a single equation that would fit on a t-shirt. Ecology is far from being ready to print such a t-shirt. However, I am pretty sure that ecology’s ultimate equation would predict the amount of resilience in a given ecosystem. It is worth dreaming about.