1  Introduction

Fungus took a lichen to an alga…

Ecological effects of interactions at the species level

Ecology is concerned with understanding the distribution and abundance of living organisms, and focuses on the interactions that occur among organisms, and between organisms and their physical environment. A recurring theme in ecology is that the patterns of species distributions and abundances are shaped not only by edaphic and environmental factors, but also by interactions with other organisms. How one species interacts jointly with the environment and with another species has profound extended consequences for the composition of communities and potentially ecosystems (Whitham et al. 2003). A well-researched illustration of this can be found in the predator-prey relationship between bears (Ursus spp.) and salmon (Onchorynchus spp.). Salmon is a particularly important food source for brown bears (Ursus arctos), being positively correlated with female body mass, breeding capability, litter size and population density (Hilderbrand et al. 1999). During spawning season, salmon return from oceanic environments to the rivers they originally spawned from to continue their life cycle. Much of the energy obtained from the ocean is retained within migrating salmon populations and returned to the rivers they spawned from. There is a dramatic increase in body fat (up to 85% increase) and protein (up to 40% increase) from the moment salmon arrive at their destination to senescence several weeks later (Gende et al. 2004). Bears populating these rivers in order to feed have been found to select for larger, male salmon, thus potentially reducing the average body size of salmon that survive long enough to breed. Smaller salmon are generally less fecund, and in the case of no or partial spawning, there is a consequence of reduced paternity for offspring, potentially reducing genetic diversity (Reimchen 2000; Quinn and Buck 2001). This energy is either consumed by bear populations near the site with the remains decomposing into the water system, or used for spawning before death occurs.

The commonest way of exploring interactions among organisms and their effects on communities and ecosystems is to focus on interactions that occur between pairs of species. These interactions may involve competition, mutualism, herbivory, parasitism, mutualism or synergistic interactions and can occur between organisms at the same or different trophic levels. It has been recognised that individuals within species are not uniform, but differ genetically from one another. Genetic variation among individuals within a species can affect phenotypic traits that may be important in determining their interactions with other organisms. For instance genetic variation in secondary chemistry of plants influences their interaction with herbivores with important ecological consequences. A good example of this is provided by recent studies of the effects of monoterpenes in Scots pine (Pinus sylvestris) and a range of herbivores, such as slugs (Arion ater), capercaillie (Tetrao urogallus), bank voles (Myodes glareolus) and red deer (Cervus elaphus) (Iason et al. 2011). Grazing experiments were carried out on each herbivore type using a range of Scots pine trees that had been verified as having variable monoterpene concentration (high, medium, or low). Of the thirteen monoterpenes selected to test herbivore response, \(\alpha\)-pinene, the most abundant monoterpene in Scots pine and with strong genetic correlation to other monoterpenes, successfully deterred slugs and capercaillie. By avoiding those pines with higher concentrations of \(\alpha\)-pinene, slugs and capercaillie are selecting for higher abundance of a specific monoterpene.

The outcome of interactions is often to change the genetic composition of one species as a consequence of natural selection. Ultimately this may affect the population size of that species e.g. adaptation of pests to pesticides first brings about resistance evolution in the pest population, then an increase in the pest population as resistant individuals rise in frequency in the population. A well-known example of this is the case of myxomatosis-resistance in wild rabbit (Oryctolagus cuniculus) populations in Europe and Australia (Ross and Sanders 1977, 1984; Aparicio, Solari, and Bonino 2004). Originally introduced in the 1950s in Great Britain to control high numbers of wild rabbits, the myxoma virus was incredibly effective in killing an estimated 99% of the total rabbit population that were exposed. By the 1970s increasing resistance had been recorded in some wild British populations (Ross and Sanders 1984), with a greater level of resistance recorded at least a decade earlier in Australian populations (Aparicio, Solari, and Bonino 2004). Several strains of the original virus have evolved, but so too has rabbit resistance to the virus, demonstrating a host-parasite co-evolutionary process. This evolution of host resistance explained a rise in rabbit population sizes in both areas.