O A

O Cladocera O Cyclopoids A Rotifers Other

20 40 60

Time to first colonization

Figure 1 (a) Time to colonization (weeks) into each mesocosm for each of 26 zooplankton taxa. Taxa are listed in order of average time to colonization. Diamonds indicate cladocera; circles indicate copepods; triangles indicate rotifers; squares indicate all other taxa. Because colonization by individuals is difficult to observe, 'colonization' here also includes establishment of reproducing populations large enough to be detected in samples. (b) Negative correlation between the week each taxon was first found in the array, and the number of mesocosms that were invaded over the 2 years. The other category includes annelids, flatworms, water mites, and ostracods. Reprinted from Caceres CE and Soluk DA (2002) Blowing in the wind: A field test of overland dispersal and colonization by aquatic invertebrates. Oecologia 131: 402-408.

the total number of mesocosms colonized over the course of 2 years. In general, whether or not a particular organism colonizes a given new habitat depends on life-history characteristics, behavior, and environmental factors. Classically, the r-selected life-history strategy (see r-Strategist/K-Strategists) is associated with strong colonization ability through the production of many, low-quality propagules, often adapted to long-distance dispersal via wind or water. This strategy increases the probability that propagules will arrive in a patch because of the vast number of propagules released. However, strategies that increase the probability of establishment, rather than simply the probability of arrival, also enhance colonization success, including habitat selection and competitive ability. Many animals use habitat selection (see Habitat Selection and Habitat Suitability Preferences) to detect appropriate habitat, thereby increasing the probability of arrival and subsequent establishment. This is particularly true for specialist organisms with highly specific habitat requirements or organisms that utilize short-lived resources (e.g., rotting fruit or hydrothermal vents). Good competitors, especially those with clonal growth, often make good colonizers; few propagules may arrive, but those that do are likely to be successful.

Among herbaceous plants, an annual life-history strategy, rapid growth, and the production of many, small seeds are characteristic of the 'ruderal' strategy (sensu Grime; see section titled 'Further reading'). Ruderals (see Pioneer Species) rapidly colonize habitat opened by disturbance, allowing them to coexist with stronger competitors that colonize more slowly but ultimately exclude the ruderal species (see Competition and Competition Models and Plant Competition). Ruderals are r-strategists that increase colonization ability by increasing the probability of arrival at a new patch. Other life-history characteristics may enhance the probability of establishment after arrival, such as phenotypic plasticity and the ability to reproduce asexually. Phenotypic plasticity allows an organism to use the morphology or behavior that is appropriate to its new environment. The ability to reproduce asexually, whether by clonal growth or selfing, is clearly advantageous to establishment because only a single individual is necessary to produce a viable population. In their experimental mesocosms, however, Caceres and Soluk (2002) found that the colonization ability of freshwater zooplankton was not necessarily predicted well by mode of reproduction (see the section titled 'Further reading').

Among sessile marine invertebrates, there are two contrasting life-history strategies that may enhance colonization ability. Some taxa produce many, small larvae that feed during an extended larval period (weeks to months) in the plankton and are, therefore, transported over long distances. This long larval period results in long-distance transport far from their natal areas, enhancing the probability of arrival in uninhabited areas. Other taxa produce fewer, larger, larvae with enough yolk to survive a shorter (hours to days), non-feeding planktonic period or, more extreme, larvae that crawl away and have no planktonic transport at all. While these taxa are less likely to be transported far from their natal areas, when they do arrive they are more likely to colonize successfully and establish a new population. This increased colonization success is due to the increased survival of colonizing individuals but also to more rapid local population growth since subsequent reproduction is retained locally. These examples make it clear that there are two strategies for colonization success: produce many propagules to ensure arrival at new habitat or produce well-supplied or highly competitive propagules that are likely to succeed if they arrive.

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