Editing Habitat fragmentation (section)

=== Genetic risks ===
As the remaining habitat patches are smaller, they tend to support smaller populations of fewer species.<ref>{{cite book|last1=Simberloff|first1=Daniel|date=1 January 1998|chapter=Small and Declining Populations|title=Conservation Science and Action|language=en|pages=116–134|doi=10.1002/9781444313499.ch6|isbn=978-1-4443-1349-9}}</ref> Small populations are at an increased risk of a variety of genetic consequences that influence their long-term survival.<ref>{{cite book|title=Introduction to conservation genetics|last1=Frankham|first1=Richard|last2=Ballou|first2=Jonathan D.|last3=Briscoe|first3=David A.|date=2009|publisher=Cambridge University Press|isbn=978-0-521-70271-3|edition=2nd|location=Cambridge}}</ref> Remnant populations often contain only a subset of the genetic diversity found in the previously continuous habitat. In these cases, processes that act upon underlying genetic diversity, such as [[adaptation]], have a smaller pool of fitness-maintaining alleles to survive in the face of environmental change. However, in some scenarios, where subsets of genetic diversity are partitioned among multiple habitat fragments, almost all original genetic diversity can be maintained despite each individual fragment displaying a reduced subset of diversity.<ref name="Borrell2018">{{cite journal |last1=Borrell |first1=James S. |last2=Wang |first2=Nian |last3=Nichols |first3=Richard A. |last4=Buggs |first4=Richard J. A. |title=Genetic diversity maintained among fragmented populations of a tree undergoing range contraction |journal=Heredity |date=15 August 2018 |volume=121 |issue=4 |pages=304–318 |doi=10.1038/s41437-018-0132-8 |pmid=30111882 |pmc=6134035|bibcode=2018Hered.121..304B }}</ref><ref>{{Cite journal |last1=Mustajärvi |first1=Kaisa |last2=Siikamäki |first2=Pirkko |last3=Rytkönen |first3=Saara |last4=Lammi |first4=Antti |date=2001 |title=Consequences of plant population size and density for plant-pollinator interactions and plant performance: Plant-pollinator interactions |journal=Journal of Ecology |language=en |volume=89 |issue=1 |pages=80–87 |doi=10.1046/j.1365-2745.2001.00521.x|s2cid=84923092 |doi-access=free }}</ref>

==== Gene Flow and Inbreeding ====
[[Gene flow]] occurs when individuals of the same species exchange genetic information through reproduction. Populations can maintain genetic diversity through [[Animal migration|migration]]. When a habitat becomes fragmented and reduced in area, gene flow and migration are typically reduced. Fewer individuals will migrate into the remaining fragments, and small disconnected populations that may have once been part of a single large population will become reproductively isolated. Scientific evidence that gene flow is reduced due to fragmentation depends on the study species. While trees that have long-range pollination and dispersal mechanisms may not experience reduced gene flow following fragmentation,<ref>{{cite journal |last1=Kramer |first1=Andrea T. |last2=Ison |first2=Jennifer L. |last3=Ashley |first3=Mary V. |last4=Howe |first4=Henry F. |title=The Paradox of Forest Fragmentation Genetics |journal=Conservation Biology |date=August 2008 |volume=22 |issue=4 |pages=878–885 |doi=10.1111/j.1523-1739.2008.00944.x |pmid=18544089|bibcode=2008ConBi..22..878K |s2cid=1665248 }}</ref> most species are at risk of reduced gene flow following habitat fragmentation.<ref name="Lienert2004" />

Reduced gene flow, and reproductive isolation can result in [[inbreeding]] between related individuals. Inbreeding does not always result in negative fitness consequences, but when inbreeding is associated with fitness reduction it is called [[inbreeding depression]]. Inbreeding becomes of increasing concern as the level of [[homozygosity]] increases, facilitating the expression of deleterious alleles that reduce the fitness. Habitat fragmentation can lead to inbreeding depression for many species due to reduced gene flow.<ref name="Pavolva2017">{{cite journal |last1=Pavlova |first1=Alexandra |last2=Beheregaray |first2=Luciano B. |last3=Coleman |first3=Rhys |last4=Gilligan |first4=Dean |last5=Harrisson |first5=Katherine A. |last6=Ingram |first6=Brett A. |last7=Kearns |first7=Joanne |last8=Lamb |first8=Annika M. |last9=Lintermans |first9=Mark |last10=Lyon |first10=Jarod |last11=Nguyen |first11=Thuy T. T. |last12=Sasaki |first12=Minami |last13=Tonkin |first13=Zeb |last14=Yen |first14=Jian D. L. |last15=Sunnucks |first15=Paul |title=Severe consequences of habitat fragmentation on genetic diversity of an endangered Australian freshwater fish: A call for assisted gene flow |journal=Evolutionary Applications |date=July 2017 |volume=10 |issue=6 |pages=531–550 |doi=10.1111/eva.12484 |pmid=28616062 |pmc=5469170|bibcode=2017EvApp..10..531P }}</ref><ref>{{cite journal |last1=Wang |first1=W |last2=Qiao |first2=Y |last3=Li |first3=S |last4=Pan |first4=W |last5=Yao |first5=M |title=Low genetic diversity and strong population structure shaped by anthropogenic habitat fragmentation in a critically endangered primate, Trachypithecus leucocephalus |journal=Heredity |date=15 February 2017 |volume=118 |issue=6 |pages=542–553 |doi=10.1038/hdy.2017.2 |pmid=28198816 |pmc=5436025|bibcode=2017Hered.118..542W }}</ref> Inbreeding depression is associated with conservation risks, like local extinction.<ref>{{Cite journal|last1=Hedrick|first1=Philip W.|last2=Kalinowski|first2=Steven T.|date=November 2000|title=Inbreeding Depression in Conservation Biology|journal=Annual Review of Ecology and Systematics|language=en|volume=31|issue=1|pages=139–162|doi=10.1146/annurev.ecolsys.31.1.139|bibcode=2000AnRES..31..139H |issn=0066-4162}}</ref>

==== Genetic drift ====
Small populations are more susceptible to [[genetic drift]]. Genetic drift is random changes to the genetic makeup of populations and leads to reductions in genetic diversity. The smaller the population is, the more likely genetic drift will be a driving force of evolution rather than natural selection. Because genetic drift is a random process, it does not allow species to become more adapted to their environment. Habitat fragmentation is associated with increases to genetic drift in small populations which can have negative consequences for the genetic diversity of the populations.<ref name="Pavolva2017" /> However, research suggests that some tree species may be resilient to the negative consequences of genetic drift until population size is as small as ten individuals or less.<ref name="Borrell2018" />

===== Genetic consequences of habitat fragmentation for plant populations =====

Habitat fragmentation decreases the size and increases plant populations' spatial isolation. With [[genetic variation]] and increased methods of inter-population [[genetic divergence]] due to increased effects of [[Genetic drift|random genetic drift]], elevating [[inbreeding]] and reducing gene flow within plant species. While genetic variation may decrease with remnant population size, not all fragmentation events lead to genetic losses and different types of genetic variation. Rarely, fragmentation can also increase gene flow among remnant populations, breaking down local genetic structure.<ref>{{Cite journal|last1=Young|first1=Andrew|last2=Boyle|first2=Tim|last3=Brown|first3=Tony|date=1996|title=The population genetic consequences of habitat fragmentation for plants|journal=Trends in Ecology & Evolution|language=en|volume=11|issue=10|pages=413–418|doi=10.1016/0169-5347(96)10045-8|pmid=21237900|bibcode=1996TEcoE..11..413Y }}</ref>

==== Adaptation ====
In order for populations to evolve in response to natural selection, they must be large enough that natural selection is a stronger evolutionary force than genetic drift. Recent studies on the impacts of habitat fragmentation on adaptation in some plant species have suggested that organisms in fragmented landscapes may be able to adapt to fragmentation.<ref>{{cite journal |last1=Matesanz |first1=Silvia |last2=Rubio Teso |first2=María Luisa |last3=García-Fernández |first3=Alfredo |last4=Escudero |first4=Adrián |title=Habitat Fragmentation Differentially Affects Genetic Variation, Phenotypic Plasticity and Survival in Populations of a Gypsum Endemic |journal=Frontiers in Plant Science |date=26 May 2017 |volume=8 |pages=843 |doi=10.3389/fpls.2017.00843 |pmid=28603529 |pmc=5445106|doi-access=free |bibcode=2017FrPS....8..843M }}</ref><ref>{{cite journal |last1=Dubois |first1=Jonathan |last2=Cheptou |first2=Pierre-Olivier |title=Effects of fragmentation on plant adaptation to urban environments |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=5 December 2016 |volume=372 |issue=1712 |pages=20160038 |doi=10.1098/rstb.2016.0038 |pmid=27920383 |pmc=5182434}}</ref> However, there are also many cases where fragmentation reduces adaptation capacity because of small population size.<ref>{{cite journal |last1=Legrand |first1=Delphine |last2=Cote |first2=Julien |last3=Fronhofer |first3=Emanuel A. |last4=Holt |first4=Robert D. |last5=Ronce |first5=Ophélie |last6=Schtickzelle |first6=Nicolas |last7=Travis |first7=Justin M. J. |last8=Clobert |first8=Jean |title=Eco-evolutionary dynamics in fragmented landscapes |journal=Ecography |date=January 2017 |volume=40 |issue=1 |pages=9–25 |doi=10.1111/ecog.02537 |url=http://aura.abdn.ac.uk/bitstream/2164/9606/1/Legrand_et_al_2016_Ecography.pdf |hdl=2164/9606|doi-access=free |bibcode=2017Ecogr..40....9L }}</ref>

==== Examples of impacted species ====
Some species that have experienced genetic consequences due to habitat fragmentation are listed below: 
[[File:Macquarie perch.jpg|thumb|Macquarie perch]]

* ''[[Macquaria australasica]]''<ref name="Pavolva2017" /><ref>{{Cite web|url=http://fishesofaustralia.net.au/home/species/1594|title=Macquaria australasica|website=fishesofaustralia.net.au|language=en|access-date=2018-06-06}}</ref>
*''[[Fagus sylvatica]]''<ref>{{cite journal |last1=Jump |first1=A. S. |last2=Penuelas |first2=J. |title=Genetic effects of chronic habitat fragmentation in a wind-pollinated tree |journal=Proceedings of the National Academy of Sciences |date=12 May 2006 |volume=103 |issue=21 |pages=8096–8100 |doi=10.1073/pnas.0510127103 |pmid=16698935 |pmc=1472435 |bibcode=2006PNAS..103.8096J|doi-access=free }}</ref>
*''[[Betula nana]]''<ref name="Borrell2018" />
*''[[Rhinella ornata]]''<ref>{{cite journal |last1=Dixo |first1=Marianna |last2=Metzger |first2=Jean Paul |last3=Morgante |first3=João S. |last4=Zamudio |first4=Kelly R. |title=Habitat fragmentation reduces genetic diversity and connectivity among toad populations in the Brazilian Atlantic Coastal Forest |journal=Biological Conservation |date=August 2009 |volume=142 |issue=8 |pages=1560–1569 |doi=10.1016/j.biocon.2008.11.016|bibcode=2009BCons.142.1560D }}</ref>
*''[[Ochotona princeps]]''<ref>{{cite journal |last1=Peacock |first1=Mary M. |last2=Smith |first2=Andrew T. |title=The effect of habitat fragmentation on dispersal patterns, mating behavior, and genetic variation in a pika ( Ochotona princeps ) metapopulation |journal=Oecologia |date=24 November 1997 |volume=112 |issue=4 |pages=524–533 |doi=10.1007/s004420050341 |pmid=28307630 |bibcode=1997Oecol.112..524P |s2cid=2446276}}</ref>
*''[[Uta stansburiana]]''<ref name="Delany2010">{{cite journal |last1=Delaney |first1=Kathleen Semple |last2=Riley |first2=Seth P. D. |last3=Fisher |first3=Robert N. |last4=Fleischer |first4=Robert C. |title=A Rapid, Strong, and Convergent Genetic Response to Urban Habitat Fragmentation in Four Divergent and Widespread Vertebrates |journal=PLOS ONE |date=16 September 2010 |volume=5 |issue=9 |pages=e12767 |doi=10.1371/journal.pone.0012767 |pmid=20862274 |pmc=2940822 |bibcode=2010PLoSO...512767D|doi-access=free }}</ref>
*''[[Plestiodon skiltonianus]]''<ref name="Delany2010" />
*''[[Sceloporus occidentalis]]''<ref name="Delany2010" />
*''[[Chamaea fasciata]]''<ref name="Delany2010" />