Darwin’s famous naturalization hypothesis states that the probability of an invasive species successfully introducing itself to a new environment decreases due to competition if closely related species are already present. Though this has proven true for many invasives, the opposite appears true for frogs. Researchers at the University of Sydeny, Australia, recently published a study examining a suite of successful and unsuccessful amphibian invasions across 162 species. It turns out that chance of a successful invasion increases significantly as the number of related species increases. Invader success is also higher on islands than mainlands, and higher in areas with abiotic conditions similar to the invader’s natural habitat.
Why would invaders be more likely to succeed if closely related species, who undoubtedly use similar resources and occupy similar habitats, are present? The “preadaptation hypothesis” suggests that the very attributes of an ecosystem that allow an invader’s relatives to thrive allow the invader to establish itself as well- that is to say, the resource competition between relatives is not as important as the suitability of the habitat for both species. (Could humans learn a thing or two from frogs?) Preadaptation is known to be true for some invasive plants, but these findings represent the first example of support for the preadaptation hypothesis in an animal. For conservationists interested in endangered frogs, this information will no doubt be valuable in searching for suitable new habitats.
Hybridization of closely related species to due land use and climate changes is accelerating. While hybridization often results in offspring sterility and thus reduces the fitness of parent species, in some cases viable hybrids are produced and the benefit or cost of this to the original species must be judged on a case by case basis. One of the world’s rarest birds, the New Zealand black stilt or kaki, has recently begun breeding with a self-introduced cogener, the pied stilt or poaka.
A recent molecular analysis of hybrid DNA confirmed that birds classified as hybrids on the basis of adult plumage are indeed genetically related to both parent species. Hybridization is both extensive and bidirectional- both sexes of both species will breed with the opposite sex of the other species.
An important question for scientists studying hybridization is whether introgression is occurring. Introgression, the introduction of genes from one distinct species into another, occurs via the backcrossing of hybrids to individuals of the parent species. To make this concept clearer, imagine two farmers from two different towns. One farmer specializes in growing cucumbers, just like everyone else in his town. The other farmer, in the tradition of his town, grows only pumpkins. Now, if the children of these two farmers meet in college and decide to get married, they may well decide to start their own farm and choose to raise both cucumbers and pumpkins. Some years later, one of the children from this “hybrid” farm decides to start her own farm, but realizes that she’s really only interested in growing pumpkins (they make such tasty pies!). However, the cucumber-growing town has a much better craft beer selection, so of course she must live there. This third generation child has, in a sense, caused an introgression event to occur- she has brought a skill only available in one town into another due to the fortuitous marriage of her parents.
Why does introgression matter? In species that engage in hybridization, it can be an important mechanism for the transfer of new genes into either original species, thus maintaining a healthy level of genetic variation. In fact, “genetic rescue” of an endangered species through intentional hybridization has become a population conservation strategy for preventing imminent extinction. Hybridization between individuals in a small population with reduced genetic diversity and a population of unrelated individuals increases the “genetic load” of the small, low diversity population.
Now, back to the mysterious kaki bird. Despite widespread evidence of hybridization, there is no evidence of introgression between hybrids and either parent species. This may be the first study to document a general lack of introgression, despite a history of extensive hybridization.
Okay, so a few weird hybrids aren’t backcrossing to the parent kaki species, so what?
Some conservationists think that introgression may be just what is needed to keep this rare species alive. Once abundant and widely distributed across New Zealand, the kaki is now critically endangered and currently restricted to the Upper Waitaki River Basin of the South Island. Intensive efforts to conserve the iconic bird have caused the population to nearly quadruple, from a scant 23 adults in 1981 to 98 adults in 2010. This population size is, however, far from ideal, and introduced mammalian predators are now representing a major threat to further species recovery.
Given that introgression may provide the fresh genetic load needed to keep the kaki species alive, researchers studying the kaki began searching for an explanation for this unusual lack of backcrossing. It turns out that the survival rate of fledgings born from hybrid female x kaki male pairs is significantly lower than the survival rate of either the normal kaki x kaki pairing, or hybrid x hybrid crosses. Coupled with an extremely small hybrid population size, which leads to less predictability in survival rates, the reduced fitness of hybrid female x kaki male pairings is not terribly surprising.
If genetic rescue will not offer relief to the kaki, are we nearing the end of the road for this marginalized species? The authors of this study don’t think so. The flip side to intentional hybridization is reduced breeding opportunities between kaki birds themselves (maybe they don’t need our help after all!). Although inbreeding and reduced genetic diversity are expected due to the low kaki population size, there is little genetic evidence for high inbreeding among kaki birds. Kaki exhibit relatively high fitness when compared to other New Zealand endemics that have suffered comparable reductions in population size. Rather than encourage further hybridization, the authors advocate active conservation strategies that promote the formation of “pure pairs” and maintain a balanced adult sex ratio in order to keep survival chances high.
Steeves et al. 2010. Genetic analysis reveals hybridization but no hybrid swarm in one of the world’s rarest birds. Molecular Ecology19: 5090-5100
Globally, industrialization is causing human populations to migrate into cities. As a result, urbanization is among the fastest growing land use changes. The necessity to see urban environments as true ecosystems in order to understand what these environments can provide for displaced species is a topic of growing importance.
Ornithologists are now attempting to determine what life history traits can predict a particular avian species’ probability of surviving an urban transition. To identify traits that make a species vulnerable to urban development, one group of researchers used a data set comprising avian responses to towns and cities for a <3,000 square kilometer, highly urbanized British region.
Previous research has found that migrating birds tend to disappear more quickly than permanent residents. Migrants may be disadvantaged when competing for more limited urban resources with permanent residents that have established territories. Migrants also seem to be more susceptible to mismatches in the timing of breeding and peak food availability. Such mismatches are becoming more common due to climate warming and increased climate variability, and are exacerbated in urban regions due to urban heat-island effects.
This recent study determined two new factors driving survival patters. The first important ‘survival factor’ is diet. Birds that feed on plant material have higher survival rates in urban environments than insectivores. High levels of “supplementary” vegetarian food supplies, including seeds and processed plant material, often characterize urban regions. Species that can exploit an entirely vegetarian diet may therefore be at an advantage over insect-dependent species.
The second important survival factor determined in this study is nesting location. Birds that nest above ground fare much better in urban environments than ground nesters. Obvious reasons for this include increased danger of ground nest destruction due to pedestrians, bicycles and automobiles. Ground nesters may also be at much higher risk of predation, due principally to the fact that they tend to produce broader, more open-cupped and vulnerable nests.
Studies quantifying traits that increase an organism’s survival chances due to urbanization are important not only in predicting which species will survive but in developing more conservation-friendly urban landscapes. For example, this study indicates that maximizing the availability of insect food sources, which could be achieved by increasing urban green space, may increase avian diversity. In addition to increased green space, the implementation or larger patches of green space may reduce intense resource competition that occurs on small, fragmented habitat patches. Improved suitability for ground-nesting species may be more challenging to implement, but is an important goal for city planners to keep in when designing new urban developments.
Evans et al. 2001. What makes an urban bird? Global Change Biology 17: 32-44.
microbes, carbon, energy, the planet. life is a struggle for order in a disorderly world.