Habitat Fragmentation

Habitat fragmentation is when a natural habitat is disrupted or divided, leaving an area of that habitat that is hard/impossible for certain species to cross.  Examples of these include rivers, roads, deforested areas, damns, crop fields etc.  Humans are constantly changing the natural environment by adding these features which then have a negative effect on species in the habitat.  Habitat fragmentation is especially important with migrating species (i.e. birds) and those species that have started to shift polewards due to climate change.  Shifting northwards is not always possible for species as they depend on food sources and certain habitat conditions, however those species that are able to adapt to other conditions are being prevented from moving north due to habitat fragmentation.

For example, the Bog turtle (America's smallest turtle) may be prone to extinction in the near future as it has very specific habitat requirements (bogs and wetlands) which are not only scarce but are often sandwiched in between main roads and areas of housing [1].  These act as barriers for the slow ground moving turtle, making it very hard to migrate to any other areas.

Another example is bird populations.  A study by Melles et al analysed the range expansion of bird species in the last 20 years.  It was found that fragmentation (such as deforestation and treeless areas) caused expansion rates to be slower than in areas without fragmentation.  The main reason for this lag in expansion is due to low breeding success and smaller populations (of species and their prey) existing in fragmented areas [2].

HOWEVER scientists have recognised this problem and there is a lot of research and conservation work being done on habitat fragmentation and assisted migration of species.  In California, mountain lions are being tagged in order to observe and understand their movements around fragmented areas (i.e. areas with roads, housing, water and concrete) [3].  A study like this can aid scientists with the knowledge on how to build wildlife corridors, allowing migration and range expansion of species in the light of climate change.  Currently there are lots of conservationists working on the idea of assisted migration as a means to save endangered species globally.

References:


[1] http://news.nationalgeographic.com/news/091211-ten-threatened-species-animals-global-warming-pictures/#/bog-turtle_10582_600x450.jpg
[2] Melles, S.J., M.J. Fortin, K. Lindsay and D. Badzinski (2011), 'Expanding northward: influence of climate change, forest connectivity, and population processes on a threatened species' range shift', Global change biology, 17, 17 - 31.
[3] http://www.newscientist.com/article/mg21028094.000-on-mountain-lion-patrol-in-the-california-wilds.html

Interglacial refugia?!

A couple of blogs ago I was talking about glacial refugia and how, during glacial periods, expanding ice cover forced species to migrate southwards in order to reach warmer climates. This is a natural phenomena and species have always adapted to climate change in the past. However, is there such thing as 'interglacial refugia'?! Do species in the sub-polar regions have to move closer to the poles during interglacial warm periods?!

The answer is - YES.

In the past, during glacial periods, cold adapted species such as the arctic fox had much larger populations than present. This was because as the climate became colder and ice cover increased, these species had larger areas to live and breed in and so their populations increased (expansion without rear contraction). Then, as ice started to melt and the earth shifted into an interglacial, these species could no longer survive in these warming areas and were pushed back towards the poles. The interglacial we are currently experiencing has reduced populations and contracted ranges of polar species.

Noone is quite sure when the current interglacial is going to end, some scientists say it should have already ended while others say it has around another 10,000 years to go. However, the number of species currently moving north due to climate change has no reflection on interglacial refugia. Present rates of northward movement are much higher than any known rates of expansion/contraction in the past.

References:

[1] Stewart et al (2011) 'Refugia revisited: individualistic responses of species in space and time', Proceedings of the Royal Society, 277, 661 - 671.

Tree line expansion?

A bit of late night journal reading led me to an article about tree line expansion due to climate change. The idea that areas currently covered in ice could become suitable environments for new trees and shrubs to grow in the near future is a positive impact of climate change! New trees = increased uptake of CO2!

The study was taken in Sweden near the tree line which marks the boundaries of the Arctic. The image on the left shows the shrub colonisation of the area in 1977 (top) compared with 2009 (bottom). As temperatures have increased here, we can see that increased amount of vegetation can grow here. Research has found that although models predict this expansion could happen along the entire Arctic boundary, some areas are showing no new vegetation despite ideal conditions...

However, surely this northward expansion of vegetation is only a good thing?! Except of course the decreased albedo due to ice melt...which I personally think can be solved by everyone painting their roofs white?!

References:

Rundqvist et al (2011), 'Tree and shrub expansion over the past 34 years at the tree-line near Abisko, Sweden', Journal of the human environment, 40, 683 - 692.

Bridging the Gap: The Holocene

The Holocene is an interesting time period to investigate as it is more like our present climate than during the Pleistocene where mass extinctions and migrations occurred between glacials and interglacials. During the Holocene, we experienced periods of cooling (little ice age) and periods of warming (medieval warming period). The warming periods can be useful in understanding as they are the closest to the climate change we are currently experiencing. Bruyn et al wrote a paper this year called ‘Faunal histories from Holocene ancient DNA’ which looks at the changes in species, distribution and migration during the Holocene. DNA can be used to calculate the genetic diversity of a species at any given time. A large diversity of a particular species suggests a larger population, perhaps allowed by migration and expansion without rear contraction. An example of this is the Elephant seal which colonised new southern areas during the Holocene and increased its population and genetic diversity.

However, the Holocene also gave rise to many species extinctions due to its variability in climate. During the late Holocene period this was most apparent as habitats were degraded due to climate variability, including the southern elephant seals’ newly found habitats. (Talking of elephant seals, did anyone see that graphic fight between them on Frozen Planet last week?!)

Although climate triggered migrations were apparent during the Holocene and Late Glacial Maximum, these migrations were often on a smaller and/or slower scale than any migratory movements seen within the last century due to climate change.

References:

1. Bruyn et al 2011, ‘Faunal histories from Holocene ancient DNA’, Trends in ecology and evolution, 26, 8, 405 – 413.

Glacial Refugia

Recent climate change may be causing species to migrate northwards but this is not the first time in the long history of our earth that changes in the climate have caused mass migrations. It seems that species have been able to adapt to climate variations in the past in the exact same way as now – by migrating to areas of the earth that are more suitable in terms of temperature and habitat. The most well-known example of this is Glacial Refugia. During the Last Glacial Maximum (LGM), many species in the Northern Hemisphere had to migrate southwards in order to escape the cold temperatures and increasing ice cover that was occurring in the higher latitudes [1]. Evidence for this migration can be found by investigating the biodiversity and genetic variation of species in refugia sites. Refugia sites were found in Europe and Asia, where temperatures were warmer, and the effects of glaciation were less apparent. These sites are sometimes referred to as ‘hybrid zones’ as migration of different species (previously in allopatry) allowed interbreeding and introduced hybrid species [1].

I’ve just finished reading an article on the glacial refugia of marine fish by Kettle et al. In the paper they have looked at migration during the LGM and at various other points throughout the Holocene in order to understand and predict future migration due to climate change. Identified fish remains, leftover from human consumption, is the main proxy used in this study in order to determine the past ranges of different species. Studies like this have great importance for the future of the fishing industry, however, like the science of climate change they are still very vague and migration is difficult to predict. Patterns from the past show possible migration to areas of low oxygen concentration and high sea ice despite these conditions being seen as too severe for certain population (i.e. Salmon and cod) to inhabit [2].

Investigations of past species migration are important to gain an understanding of climate induced movement – however, as with all studies that are based on proxies and occurred thousands of years ago, there is a large grey area of uncertainty which reflects the future of our planet through climate change. On top of this, migration during the LGM may have been easier as there were less areas of habitat fragmentation preventing movement.

References:

1. Feliner 2011, 'Southern European glacial refugia: A tale of tales', Taxon, 60, 365 - 372.
2. Kettle et al 2011, 'Refugia of marine fish in the northeast Atlantic during the last
glacial maximum: concordant assessment from archaeozoology and palaeotemperature reconstructions', Climate of the past, 7, 181 - 201.

Beetles, butterflies and how pole-ward migration works

The movement of species towards the poles is not as simple as individual organisms migrating further north. The actual phenomenon involves a decrease in population in the southern ends of a distribution of species (extinction due to warmer temperatures) and an increase in colonisation rates of the same species in the northern end of a distribution (including new areas which are now warm enough to host these species) [1]. This shows a gradual shift in the population of a species which correlates which global temperature rise. This pole-ward migration is likely to affect all types of species, however, airborne species are far more likely to be able to shift northward than slower land moving species due to habitat fragmentation. For example, animals such as birds are likely to be able to cross boundaries such as barren land (increasing with climate change!) and urban areas to habitats more suitable to live in. Slow moving creatures such as snails, for example, may find moving their distribution northwards a much more difficult task.

Butterflies are a good example as many species have already shown a great northward movement – however, other species may find it more difficult to move north due to how delicate and sensitive they are to habitat fragmentation (and not being able to fly long distances). In a study of 35 UK butterflies, 63% were found to have moved north significantly in the last century (35 – 240km) [1]. However, some scientists have been looking at ‘assisted colonisation’ of certain butterfly species. This term means that humans can help to relocate declining butterfly species into new habitats which are suitable for colonisation but they may not naturally be able to reach [2]. This is one way we may be able to reduce the loss of biodiversity from the impact of climate change over the next few decades.

Lastly, it is not such a bleak future for our bio diverse world. There are some species which will expand their range northward without contracting their southern range – increasing their distribution into new areas without their loss in others. An example of this is a species of ground beetle called Agonum viridicupreum which has been found to have expanded its range northward by about 100km in the last century without any loss of population in the southern range [3].

References:
[1] Parmesan et al 1999, 'Poleward shifts in geographical ranges of butterfly species associated with regional warming', Nature, 399, 579 – 583.
[2] http://planetearth.nerc.ac.uk
[3] Drees et al 2011, ‘Poleward range expansion without a southern contraction in the ground beetle Agonum viridicupreum (Coleoptera, Carabidae)’, ZooKeys, 100, 333-352.