Ecological Fitting

Ecological fitting is "the process whereby organisms colonize and persist in novel environments, use novel resources or form novel associations with other species as a result of the suites of traits that they carry at the time they encounter the novel condition". It can be understood as a situation in which a species' interactions with its biotic and abiotic environment seem to indicate a history of coevolution, when in actuality the relevant traits evolved in response to a different set of biotic and abiotic conditions.

The simplest form of ecological fitting is resource tracking, in which an organism continues to exploit the same resources, but in a new host or environment. In this framework, the organism occupies a multidimensional operative environment defined by the conditions in which it can persist, similar to the idea of the Hutchinsonian niche. In this case, a species can colonize new environments (e.g. an area with the same temperature and water regime), form new species interactions (e.g. a parasite infecting a new host), or both, which can lead to the misinterpretation of the relationship as coevolution, although the organism has not evolved and is continuing to exploit the same resources it always has. The more strict definition of ecological fitting requires that a species encounter an environment or host outside of its original operative environment and obtain realized fitness based on traits developed in previous environments that are now co-opted for a new purpose. This strict form of ecological fitting can also be expressed either as colonization of new habitat or the formation of new species interactions.

Origin

The evolutionary ecologist Daniel H. Janzen began to explicate the idea of ecological fitting with a 1980 paper that observed that many instances of ecological interactions were inferred to be the result of coevolution when this was not necessarily the case, and encouraged ecologists to use the term coevolution more strictly. He observed that the existing defense traits of plants were likely produced by co-evolution with herbivores or parasites that no longer co-occurred with the plants, but that these traits were continuing to protect the plants against new attacks.

He expanded this idea in a 1985 paper written while visiting Santa Rosa National Park in Costa Rica. While there, he observed that almost all of the species in the park occupied large geographic ranges, and despite the heterogeneity of habitats across these ranges, individuals were mostly identical across locations, indicating that little local adaptation had taken place. He described the cyclical life history pattern he believed responsible for this pattern: a species begins as a small population occupying a small area with little genetic variation, but then over the course of a few generations grows to occupy a large area, either because of the emergence of a genotype successful over a wider range, or because of the removal of a geographic barrier. This large interconnected population is now subject to many contradictory selection pressures and thus remains evolutionarily static until a disturbance separates populations, restarting the cycle.

This cyclic life history pattern is dependent on three premises: that the ancestral range of most species is smaller than the ones now occupied, that biological communities have porous borders and are thus subject to invasion, and that species possess robust genotypes that allow them to colonize new habitats without evolution. Thus, many biological communities may be made up of organisms that despite their complex biological interactions have very little evolutionary history with each other.

Contrasting views

Ecological fitting represents a contrasting view to, and null hypothesis for, the hypothesis that current species interactions are evidence of coevolution. Coevolution occurs when each species in a relationship imposes evolutionary selection on the other(s). Examples could include mutualisms or predator-prey systems. The traditional view of plant–insect, host–parasite, and other tightly associated species, explained by Ehrlich and Raven (1964), defines coevolution as the primary mechanism for these associations. In his 1980 paper, Janzen gives a response to these adaptationist explanations of why a phenotype or species might exist in a particular environment, and expressed his concern with what he perceived as an overuse of coevolutionary explanations for current species associations. He stated that it would be difficult to distinguish between coevolution and ecological fitting, leading ecologists to potentially spurious explanations of current species associations. It is difficult to determine whether a close relationship is the result of coevolution or of ecological fitting because ecological fitting is a sorting process in which only associations that 'fit', or increase fitness (biology), will be maintained. When trying to determine which process is at work in a particular interaction, species can only come into contact through biotic expansion and ecological fitting, followed by adaptation or coevolution. Thus, both processes are important in shaping interactions and communities.

Mechanisms

Ecological fitting can occur by a variety of mechanisms, and can help to explain some ecological phenomena. Resource tracking can help to explain the parasite paradox: that parasites are specialists with narrow environmental ranges, which would encourage host fidelity, yet scientists commonly observe parasite shifts onto novel hosts, both in the phylogenetic record and in ecological time. Ecological fitting can explain the frequency of this phenomenon: similar to the expansion phase of the cyclic life cycle described by Janzen, a species undergoes taxon pulses, usually in a time of ecological disturbance, and expands its range, disperses, and colonizes new areas. For parasite–host, insect–plant, or plant–pollinator associations, this colonization is facilitated by the organism tracking an ancestral resource, and not tracking a particular species. The probability of this is increased when the tracked resource is widespread, or when specialization on a certain resource is a shared trait among distantly related species. This resource tracking has been demonstrated for both insect–plant and parasite–host systems in which sister species are capable of surviving on each other's hosts, even if they were never associated in nature.

When operating under the more strict definition of ecological fitting, in which traits must be exapted for a new purpose, several mechanisms could be operating. Phenotypic plasticity, in which an organism changes phenotype in response to environmental variables, allows for individuals with existing genotypes to obtain fitness in novel conditions without adaptation occurring. Correlated trait evolution can encourage ecological fitting when direct selection on one trait causes a correlated change in another, potentially creating a phenotype that is pre-adapted to possible future conditions. Phylogenetic conservatism is the latent retention of genetic changes from past conditions: for instance, historical exposure to a certain host may predispose it to colonization in the future. Finally, fixed traits such as body size may lead to entirely different biotic interactions in different environments; for example, pollinators visiting different sets of flowers.

Examples

Studies of introduced species can provide some of the best evidence for ecological fitting, because species invasions represent natural experiments testing how a new species fits into a community. Invasion ecology teaches us that changes in geographic range can occur quickly, as is required by the Janzen model for ecological fitting, and ecological fitting provides an important mechanism whereby new species can fit into an existing community without adaptation. These natural experiments have often shown that communities dominated by invasive species, such as those on Ascension Island, can be as diverse and complex as native communities. Additionally, phylogenetic studies show evidence for ecological fitting when lineages of the associated species do not correlate over evolutionary time; that is, if host–parasite or other interactions are as tightly coevolved as was previously believed, parasites should not be switching to unrelated hosts. This kind of host switching has been shown many times: in insect–plant relationships where oligophagy

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