The number of interacting species

The first characteristic is the number of interacting species at any point in time. In some interactions this is static. For example, mutual dependence interactions (Figure 11.1) characterize mutualisms that become increasingly intimate and obligate over time. This has occurred most notably with the mitochondria and chloroplasts of eukaryotes, all three of which were

Table 11.2 The characteristics that differentiate between the modes of co-evolution.

Mode of

Number of

Dynamics

Dynamics

Dynamics of

co-evolution

interacting species

of species richness

of traits

antagonism

Diversifying

2

Radiation during the interaction

NS

NS

Escape-and-

(1), 2, > 2

Radiation in

Punctuated

Increasing

radiation

absence of interaction

equilibria

antagonism

Arms race

NS

NS

Dynamic, directional, local equilibria

Increasing antagonism

Co-evolutionary

(1), 2, > 2

Stable

Dynamic

Increasing

alternation

antagonism

Mutual

2

Stable

Stable

Reduced

dependence

antagonism

Co-evolutionary

NS

Stable

Stable

Reduced

successional

antagonism

cycles

Co-evolutionary

(1), 2

Cycles of

Dynamic

Stable

turnover

invasion and extinction

antagonism

Notes: NS indicates where the characteristic is not integral to the model definition. (1) refers to the case where the interaction ceases for one or more species.

Notes: NS indicates where the characteristic is not integral to the model definition. (1) refers to the case where the interaction ceases for one or more species.

previously independent-living bacteria. Now, however, eukaryotes cannot survive without their mitochondria or chloroplasts and neither can mitochondria or chloroplasts survive independently of eukaryotic cells.

Other modes of co-evolution, however, involve a change in the number of interacting species over time. Escape-and-radiation co-evolution (Figure 11.1) refers to the process that may have occurred in many plant-herbivore interactions, whereby a plant develops a novel defence against the herbivores which ceases the interaction temporarily. In the absence of herbivory, the plants diversify into a greater number of species, perhaps because incipient species can more readily establish. Eventually, a herbivore species evolves a counter-defence, and this can then speciate into the new niches provided by the previously resistant plants. Thus, the process involves first a decrease in the number of species, and later a compensating increase. The best evidence for this process comes from the Umbelliferae family and their lepidopteran herbivores (Berenbaum 1983) (Figure 11.2). A similar dynamic occurs in co-evolutionary alternation (Figure 11.1). This occurs when a host evolves defences against a parasite, and in response the parasite is selected to look for alternative unresistant hosts. Having

Co-evolution and the number of interacting species (represented by the different shapes connected by double headed arrows). In mutual dependence, this remains steady (a), In escape-and-radiation co-evolution (b), one of the interacting species goes, extinct, the other radiates and finally becomes host to another species. In co-evolutionary alternation (c), one species is added to the interaction as another is removed.

(b)

Niche

Speciation and extinction resulting from co-evolution. In diversifying speciation (a), such as between a host and a symbiont, interactions with alternative symbionts can lead to differentiation of the host. In co-evolutionary turnover (b) one species is driven extinct during the interaction (in this case via competition), but the resulting changes make the survivor vulnerable to extinction itself (in this case by subsequent competitors).

Fig. 11.3 A parsnip webworm caterpillar, Depressariapastinacella, feeding in the flower head of a wild parsnip plant, Pastinaca sativa. Parsnips and webworms represent an advanced stage in escape-and-radiation co-evolution, with very nasty plant and very specialized herbivore. The parsnip contains both linear and angular furanocoumarins, which the webworm is able to detoxify. Photo courtesy of May Berenbaum and Arthur Zangerl.

Fig. 11.3 A parsnip webworm caterpillar, Depressariapastinacella, feeding in the flower head of a wild parsnip plant, Pastinaca sativa. Parsnips and webworms represent an advanced stage in escape-and-radiation co-evolution, with very nasty plant and very specialized herbivore. The parsnip contains both linear and angular furanocoumarins, which the webworm is able to detoxify. Photo courtesy of May Berenbaum and Arthur Zangerl.

found one, it eventually switches hosts entirely. Over time the old host may eventually lose its previous resistance. This process has been suggested to occur in the European cuckoo, a brood parasite that lays its eggs into the nests ofpasserine bird species (Davies and Brooke 1989).Again,the number of interacting species changes for both host and parasite.

Co-evolutionary turnover (Figure 11.3) is another mode of co-evolution that involves a change in the number of interacting species. This occurs when a species invades a new habitat and finds itself superior to the native competition. The invader evolves convergently to resemble the native species, forcing divergent changes on the traits of the competitor, eventually sending it extinct. Eventually, this species may fall victim to a new invader.

The process has been suggested from observations on the Anolis lizards of the Caribbean islands, where body size is the trait under question (Roughgarden and Pacala 1989). Invading lizards arriving at an occupied island tend to have large bodies and outcompete the smaller competitors, evolving convergently towards a smaller body size that supports the highest carrying capacity. Eventually, the native lizard may be forced into small bodied niches that cannot support large populations and make it vulnerable to extinction. Again, the number of interacting species varies over time.

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