Ecosystems, Population Dynamics and Conservation

Trophic energy and niche concepts

Energy flows through an ecosystem in one direction, from producers to herbivores and then to successive carnivore levels, and only a small fraction passes from one trophic level to the next. Lindeman's law of trophic efficiency, often called the 10% rule, states that roughly 10% of the energy at one trophic level is converted into biomass at the next. To apply it, multiply the producer energy by 0.1 once for each transfer. For example, with 500 kJ at the producer level, herbivores store about 500 \times 0.1 = 50 kJ, primary carnivores about 50 \times 0.1 = 5 kJ, and secondary carnivores about 5 \times 0.1 = 0.5 kJ. The rest is lost mainly as heat through respiration, and some material is never eaten or digested.

Phenotypic plasticity allows one genotype to produce different forms under different environmental conditions. Inducible defences are morphological changes triggered by predators. For instance, Daphnia develops protective neck spines and head helmets when it detects chemical cues called kairomones released by invertebrate predators, and it does not develop these structures when predators are absent. Such a response is adaptive only because it is switched on by a predator cue.

The niche of a species is its functional role, including the resources it uses and the conditions it tolerates. The fundamental niche is the full set of conditions under which a species could survive and reproduce without competitors or predators, whereas the realized niche is the smaller set it actually occupies once biotic interactions are included. Competition almost always shrinks the realized niche, so the realized niche can never be larger than the fundamental niche. If a species occupies the same range in both situations, its two niches coincide and no competitor is restricting it.

Specialists use a narrow range of food or habitat, such as giant pandas eating bamboo almost exclusively, koalas eating eucalyptus leaves, and polar bears depending on sea ice and seal prey. Generalists, such as raccoons, tolerate many food types and habitats. Specialists are generally more vulnerable to environmental change because they cannot switch to alternative resources.

Speciation is the origin of new species, and its types are classified by the geographic setting in which gene flow is interrupted. In allopatric speciation a physical barrier separates the populations. In parapatric speciation populations are adjacent and diverge along a contact zone. In sympatric speciation divergence occurs within an overlapping range without any physical barrier. Sympatric divergence is often driven by niche differentiation, in which populations exploit different resources in the same area, reducing competition and promoting reproductive isolation.