the ones left are quite difficult, mostly natural sites
Yes - and quite a lot of them are situated in the wider "Congo Basin" area and probably have the same (or similar) "answers". So I found myself trying to establish a "time period" for this entire area!!
I ran into a few difficulties and think it is worth trying to write down a number of "principles" for asigning a time period to natural sites. I make no claims to great knowledge about geological time periods and evolution etc so would be very happy to have some input from others who are more knowledgeable in these matters than I.
a. The first principle must be that we assign WHS to the period when the attributes which were regarded as giving them OUV occurred/arose
b. However, many sites will demonstrate geological features from several/many geological periods - so we need to choose.
c. In the case of many sites what we see today is inevitably the result of quite recent geomorphological events. We need however to guard against assigning every site to "Holocene" just because they have only looked "exactly" like they do today because of events in that period. To do so would be akin to assigning every European city to "20th Century" because of its roads and street furniture!!
d. On the other hand, there are many Natural WHS whose value, whilst underpinned by rocks/strata etc set down over many geological periods, is almost entirely the result of events in the Holocene period. Such sites are particularly those "created" by the latest Ice Age and its retreat, recent sea level changes (as a result of the former), recent "dessication" and climatic changes (weather patterns) etc. An interesting aspect of assigning time periods to natural sites for me has been the realisation of just how many Natural sites have only existed a relatively "short time" having been created by Holocene events - indeed this applies to most coastal sites and those in areas which experienced the last Ice Age.
e. Some sites have been inscribed because of some particular event e.g Stevns Klint or Sardona Thrust. In such cases we should clearly choose the period when the event occurred, not the period when the evidence of that event became exposed.
f. Natural sites can be viewed as having OUV which can be categorised as "Geological" or "Biological" - Every one of course comprises a mixture of both but, whilst some possess an "equal" mixture, most will tend to lean in one or other direction. So - to take African "Wildlife Parks" (covering botanical/zoological etc values) as an example - Whilst their "wildlife" depends of course on their wider "geology" it is not for their geological value that they were inscribed. So, we need to try to pin down the period in which that "wildlife" (in the fullest "ecological" terms) became established.
g. Just as some sites are in locations which have undergone major "short term" (in Geological time!!) changes as a result of Ice Age and Sea Level change, others exist in locations which have been rather more stable across a rather longer geological period. But, the further back in time one goes, the more that some sort of geological change is likely to have occurred across that very long period. So - to take the wider "Congo Basin" sites which started this train of thought - That part of Africa has been "reasonably" stable both in terms of its geology and in terms of its wildlife genera (if not its exact "species") since the Oligocene ( 33.9 to 23 million years ago). Since then there have been periods of drier and wetter climate, there has been faulting and uplift in East Africa and there have been variuos changes in water courses as rivers became "captured" by different systems. There have been periods when the wildlife we see now has had to retreat to "refuges" followed by periods of expansion etc etc. So - how do we choose one of these periods as "representing" the WHS of this area without reverting to the "everything is "Holocene" argument??
h. For sites which are inscribed because of a single species (E.g Okapi) we should presumably try to home in on the period when that particular species evolved or (if later) moved into that area. But are e.g Virunga and Bwindi not primarily inscribed for the Mountain Gorilla even if it isn't in their name? For sites inscribed for a wider ecosystem we can use less "precise" measures (though the answer may well be the same!) trying to establish when that sort of habitat became established in that particular area (even if it has undergone shrinking/expansion etc since then)
To apply such principles to the "Congo Basin" sites. (I have tried to to cover - Garamba, Okapi, Sangha, Dja, Salonga, Bwindi and Virunga and have revisited Rwenzori)
a. " The current course of the Congo River formed 1.5-2 million years BP, during the Pleistocene"
(Wiki) So "Early Pleistocene" would seem to be the "norm" for all these sites unless some "better" reason can be found on an individual site basis?
b. In the North of the Region (Garamba
) - " The Northern Congolian Forest Savanna Mosaic ecoregion") - "The transition from the equatorial forest to northern latitude savannas was most probably gradual throughout the early Pleistocene" http://www.worldwildlife.org/ecoregions/at0712
c. But, during that period there have been plenty of changes "Central Africa has experienced repeated climatic fluctuations that have caused rain forest and savanna expansion and contraction at least since the late Pleistocene tectonics defined the Congo Basin (Faniran and Jeje 1983). It has been speculated that significant climatic shifts have occurred more than 20 times in less than 10 million years. Plants and animals adapted, migrated or became extinct with each climatic oscillation. Widespread and highly adaptable organisms that survived in islands of habitat and adapted to spatial fluidity were favored . During dry periods, savanna communities invaded far into the Congo Basin. The relatively moist riparian forests become isolated from one another and formed forest island communities in the savanna matrix . Paleobotanical evidence suggests that rain forests grew more extensively on the deeper lowland and low slope soils during wetter periods"
. I would propse that we ignore all these "toings and froings" and use the earliest period that the Congo sites emerged with their current ecology unless there is a good reason to do otherwise.
d. On the eastern side of the Congo the impact of the African Rift would seem to become important - e.g What gives "Virunga
" particular value is its volcanism and its altitude ("Mountain Gorillas" etc) - but when exactly did that occur? See this on the Albertine Rift system - http://adsabs.harvard.edu/abs/2014EGUGA..16.5057S e.g "Late Pliocene (~ 3Ma) to Early Pleistocene (~ 2 Ma): rifting 2 - major uplift and growth of the Ruwenzori Mountains (5000 m of elevation"
I see that currently we have Rwenzori assigned to "Pliocene" quoting Wiki "The mountains formed about three million years ago in the late Pliocene as a result of an uplifted block of crystalline rocks ..... pushed up by tremendous forces originating deep within the earth's crust".
(Wiki). It doesn't seem unreasonable to assign Rwenzori based on its geology alone which puts it a bit earlier than the other WHS in the area?
e. But do we "assign" a time period to Virunga and Bwindi
on the basis of their Volcanic/Rifting "formation" or on the basis of the evolution of the Mountain gorilla - which was much later? On this matter Wiki states "The fossil record of the area where mountain gorillas live is particularly poor and so its evolutionary history is not clear. It was about 9 million years ago that the group of primates that were to evolve into gorillas split from their common ancestor with humans and chimps; this is when the genus Gorilla emerged. It is not certain what this early relative of the gorilla was, but it is traced back to the early ape Proconsul africanus. Mountain gorillas have been isolated from eastern lowland gorillas for about 400,000 years and these two taxa separated from their western counterparts approximately 2 million years ago"
. So do we stick with "Early Pleistocene" or move to "late Pleistocene" for the final emergence of the Eastern Mountain Gorilla? (several articles indicate that this period was the end of "Gene exchange" across the species)?
f. Regarding the Okapi reserve
- wiki states "The earliest members of Giraffidae first appeared in the Early Miocene in Africa, ........ Giraffids spread into Europe and Asia by the middle Miocene in a first radiation. Another radiation began in the Pliocene but was terminated by a decline in diversity in the Pleistocene. Several important primitive giraffids existed more or less contemporaneously in the Miocene (23-10 million years ago), including Canthumeryx, Giraffokeryx, Palaeotragus and Samotherium. ..., Samotherium split into Okapia (18 million years ago) and Giraffa (12 million years ago). However, another author J. D. Skinner argued that Canthumeryx gave rise to the okapi and giraffe through the latter three genera and that the okapi is the extant form of Palaeotragus. The okapi is sometimes referred to as an example of a living fossil, as it has existed as a species over a long geological time period, and morphologically resembles more primitive forms (e.g. Samotherium"
. So is that Miocene, Pliocene or Pleistocene?? I am inclined to go for Pliocene based on this "The end of the Pliocene epoch (2.5-6 million years ago) saw a number of long necked giraffids evolve, but largely unsuccessfully with only 2 surving to this day
" See http://www.giraffeconservation.org/giraffe_facts.php?pgid=51