©

Commentary - doi: 10.3832/ifor0496-002

iForest – Biogeosciences and Forestry

cases studied would be of too recent origin to have produced recognizable genetic signals. As a consequence, they put 75 forward some suggestions: 1) since forest species do not show similar responses to fragmentation, future studies should care­ Piotti A 78 fully take into account species-specific eco­ logical realities; 2) ecological consequences Habitat fragmentation is one of the most serious threats to plant biodiversity at of fragmentation, such as reduced pollen and the within-population level. Growing attention on the genetic effects of habitat 81 seed production or recruitment failure, can fragmentation is reflected in the 2008 publication of several review papers. In frequently be more urgent to assess than re­ general, fragmentation showed a negative effect on the genetic variability of duction of genetic variability. plant populations. However, for forest trees the genetic signal of fragmenta­ 84 So why this discrepancy? A closer look at tion seems less clear. Here I discuss the development of less explored issues the results shows that Kramer et al. (2008) that can help to clarify some unresolved questions about tree responses to did not claim to have carried out a rigorous fragmentation. In particular, the understudied effect of delay in sexual matur­ 87 literature review. Their conclusions are in­ ity and the need for accurate estimates of gene flow are taken into account. I tentionally and intelligently challenging. On finally underline the potential role of the Italian peninsula as an open-sky the other hand, less than 20% of the plant 90 species analyzed by Aguilar et al. (2008) are laboratory for forest fragmentation studies. trees, even though the vast majority of re­ Keywords: conservation genetics, range limits, genetic diversity, gene flow viewed studies were on long lived species 93 (perennial herbs). Results by Eckert et al. (2008) were obtained analyzing studies on Much attention has been paid in recent 30 In a recent paper, Aguilar et al. (2008) re­ both plants and animals, when they con­ years to the effect of habitat fragmentation viewed 101 studies on the effects of frag­ 96 sidered only conifers the percentage of stud­ 3 on plant population genetics. Searching the mentation on plant species and found a signi­ ies showing a reduction in genetic variability ISI Web of Knowledge database using a 33 ficant reduction in genetic variability in rem­ decreased from 68% to 54%. Therefore the combination of “fragmentation” and “genetic nant populations. They draw the conclusion 99 truth lies somewhere in the middle. In fact, 6 diversity” (or “gene flow”) and “plant*” (or that habitat fragmentation decreases the ge­ what it is really persistent is this fifty-fifty “tree*”) I obtained 442 publications since 36 netic diversity of plant populations. This trend that suggests that trees might not be­ 1992 (cited 7388 times), more than a half trend, more pronounced for common and 102 have like herbaceous plant and animal: their 9 published after 2005. In 2008 some enlight­ outcrossing species, seems strongly depend­ responses seem somehow more variable. ening review papers were published, report­ 39 ent on the time elapsed from fragmentation: Trees can certainly buffer, or at least delay, ing apparently controversial results on the the older the fragmentation event the higher105 the effects of fragmentation because of typ­ 12 relationship between fragmentation and ge­ the loss of genetic variability. ically high gene flow and long life cycles, as netic diversity (Aguilar et al. 2008, Eckert et 42 A previous review on the “central-marginal discussed by Kramer et al. (2008). But given al. 2008, Kramer et al. 2008). hypothesis” by Eckert et al. (2008) also108 the high variability of forest tree responses to 15 Genetic consequences of fragmentation are found reduced variability in fragmented pop­ habitat fragmentation these two key-factors suggested by the classical theory of popula­ 45 ulations: in 68% of the considered studies deserve a deeper analysis, focusing on what tion genetics. At the population level, the peripheral populations (typically smaller and111 differentiate trees from other plants. 18 fragmentation process reduces population more fragmented than central ones) experi­ In addition to a very long life span many size and increases isolation determining ge­ 48 enced a decline in genetic diversity. Ohsawa tree species are characterized by a marked netic bottlenecks. Remnant populations ex­ & Ide (2008) review of the distribution of 114 delay in sexual maturity that can determine a 21 perience increased genetic drift, elevated in­ genetic variability along altitudinal transects peculiar behavior in ecological processes. breeding and limited gene flow from sur­ 51 confirms that a reduction of genetic variabil­ For instance, due to an extended juvenile rounding populations. This is expected to ity in peripheral plant populations is quite117 phase, in recently colonized areas, estab­ 24 lead to decreasing genetic variability and in­ common. In half of the studies high altitude lished individuals do not contribute for dec­ creasing differentiation among remnant pop­ 54 populations, usually more scattered, are ades to regeneration. Hence gene flow from ulations. The smaller the population size the characterized by lower genetic variability 120 outside represents the only source of new 27 greater these effects on genetic structure than medium altitude ones (whereas the op­ colonizers. Austerlitz et al. (2000) showed 57 posite result was found in only 20% of (Young et al. 1996). that this can markedly reduce the effect of 123 genetic bottlenecks in subsequent coloniza­ cases). Apparently quite different results were tion events during tree migrations, explain­ Department of Environmental Sciences, 60 found by Kramer and colleagues (2008). Re­ ing the low among-population differentiation University of Parma, v.le Usberti 11/A, Iviewing some studies on genetic structure 126 that usually characterized forest trees. Delay 43100 Parma (Italy) and levels of gene flow in fragmented tree in sexual maturity can therefore further in­ 63 populations they hypothesized that forest crease the “genetic resilience” to fragmenta­ @ Andrea Piotti ([email protected]) trees might be an exception to theoretical ex­129 tion of some tree species, with respect to Received: Jan 12, 2009 - Accepted: Mar 31, pectations. They assert that empirical sup­ what Kramer et al. (2008) hypothesized. 66 port for expected effects is scarce and as a 2009 But probably gene flow plays the lead in consequence they proposed that a “paradox 132 this intricate plot. The study of gene flow or, Citation: Piotti A, 2009. The genetic of forest fragmentation genetics” exists. with a brand new definition, of the ’move­ consequences of habitat fragmentation: the case of forests. iForest 0: 0-0 [online 0000- 69 They primarily ascribe this lack of evidence ment ecology’ of a species in a fragmenta­ to extensive gene flow characteristic of 135 tion context is crucial to understand how 00-00] URL: http://www.sisef.it/iforest/show.php? forest trees and to a methodological flaw: the really isolated is a geographically isolated

The genetic consequences of habitat fragmentation: the case of forests

72 fragmentation

id=496

© SISEF http://www.sisef.it/iforest/

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iForest (2009) 0: 0-0

Piotti A - iForest 0: 0-0 population (Aguilar et al. 2008, Nathan 2008). It is known that forest trees show ex­ 3 tensive gene flow, especially when the wind is the dispersal vector for pollen or seeds. If a remnant population, hundreds of kilomet­ 6 ers apart from the nearest one, has an immig­ ration rate sufficiently high to counterbal­ ance genetic drift, it can maintain its genetic 9 variability. Until now gene flow in fragmen­ ted population has been studied by means of indirect genetic methods (i.e. FST-based 12 methods). The results obtained with this ap­ proach do not tell us what is the current gene flow pattern, they only summarize what the 15 history of a population has determined (e.g. how much gene flow has occurred over the last n generations). On the contrary, direct 18 methods (i.e. parentage analysis) furnish ac­ curate estimate of contemporary gene flow (e.g. how many foreign seeds have immig­ 21 rate and established, say, during the last 10 years). Such estimates allow to detect in­ stantaneous signals produced by fragmenta­ 24 tion. There is a “clear gap in the literature of plant population genetics that precluded us making further generalization” stated 27 Aguilar et al. (2008) particularly referring to parentage studies. The assessment of gene flow in fragmented population at distribution 30 edges, for example, will be crucial to solve one among the most intriguing ecological topics: why does adaptation fail at range 33 margins? As Brindle & Vines (2006) pointed out, two contrasting hypotheses can be ad­ vanced. If fragmented peripheral populations 36 experimented high gene flow rate, alleles with adaptive value could be lost because of immigration of locally deleterious alleles

iForest (2009) 0: 0-0

39 from

central populations. If gene flow is re­ susceptible signals in plant traits and methodolo­ stricted adaptation can on the contrary be gical approaches. Molecular Ecology 17: 5177hampered by the rapid decrease of genetic 78 5188. - doi: 10.1111/j.1365-294X.2008.03971.x 42 variability because Allee effect and genetic Austerlitz F, Mariette S, Machon N, Gouyon PH, drift prevail. In such scenario it will be im­ Godelle B (2000). Effects of colonization pro­ portant to explore whether asymmetries in 81 cesses on genetic diversity: differences between 45 emigration and immigration rates between annual plants and tree species. Genetics 154: large (usually central) and small (peripheral) 1309-1321. [online] URL: http://www.genetic­ populations can influence the adaptive po­ 84 s.org/cgi/content/abstract/154/3/1309 48 tential in fragmented populations. Brindle JR, Vines TH (2006). Limit to evolution at Experiments designed for the comparison range margins: when and why does adaptation among increasing levels of fragmentation 87 fail? Trends in Ecology and Evolution 22: 14051 can help to understand how and on what 147. - doi: 10.1016/j.tree.2006.11.002 geographical scale gene flow patterns vary, Eckert CG, Samis KE, Lougheed SC (2008). Ge­ and, eventually, to highlight ecologically rel­ 90 netic variation across species’ geographical 54 evant thresholds a fragmented population ranges: the central-marginal hypothesis and bey­ should not go over. Accurate estimates of ond. Molecular Ecology 17: 1170-1188. - doi: gene flow in fragmented populations might 93 10.1111/j.1365-294X.2007.03659.x 57 give a boost to the comprehension of current Kramer AT, Ison JL, Ashley MV, Howe HF and future responses of forest trees to frag­ (2008). The paradox of forest fragmentation ge­ mentation and certainly will help to design 96 netics. Conservation Biology 22:878-885. - doi: 60 species-specific conservation strategies. The 10.1111/j.1523-1739.2008.00944.x Italian peninsula served as glacial refugium Nathan R (2008). An emerging movement ecology where much genetic variability has been 99 paradigm. Proceedings of the National Society of 63 conserved during past glaciations, it repres­ Science 105:19050-19051. doi: ents an important European hot spot for 10.1073/pnas.0808918105 biodiversity and nowadays is the rear edge of 102 Ohsawa T, Ide Y (2008). Global patterns of genet­ 66 the distribution of many forest tree species. ic variation in plant species along vertical and Italy represents an open-sky laboratory for horizontal gradients on mountains. Global Eco­ forest fragmentation studies. Nevertheless, 105 logy and Biogeography 17:152-163. - doi: 69 little efforts have been done up to now for 10.1111/j.1466-8238.2007.00357.x testing the hypotheses suggested by theoret­ Young A, Boyle T, Brown T (1996). The popula­ 108 tion genetic consequences of habitat fragmenta­ ical and experimental evidence. 72

References

Aguilar R, Quesada M, Ashworth L, Herrerias-111 Diego Y, Lobo J (2008). Genetic consequences 75 of habitat fragmentation in plant populations:

2

tion for plants. Trends in Ecology and Evolution 11:413-418. - doi: 10.1016/0169-5347(96)100458

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Piotti A - The genetic consequences of habitat ...

Ide (2008) review of the distribution of ... http://www.sisef.it/iforest/show.php? id=496 ... Here I discuss the development of less explored issues that can help to ...

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