Rarity
To determine what species or community is in dire
need of conservation, we need to understand what
rarity is.
Deborah Rabinowitz (1981, 1986) described a
general pattern of species abundances, in which there
were 7 different ways that species could be rare.
What follows is a reconstruction of the table she
developed:
Geographic range
Somewhere
large
Population
Size
Everywhere
small
Large
Common
Locally
abundant over
a large range
in a specific
habitat type
Constantly
sparse over
a large
range and in
several
habitats
_______
Broad
Small
Locally
Locally
abundant in
abundant in
several
specific
habitats, but habitat, but
restricted
restricted
geographically geographically
Constantly
Constantly
Constantly
sparse in a
sparse and
sparse and
specific
geographically geographically
habitat, but restricted in
restricted in
over a large
several
a specific
range
habitats
habitat
________ ________
________
Restricted
Broad
Restricted
Habitat Specificity
Rabinowitz and her colleagues checked the patterns
of distribution and local abundance for the species
sufficiently well described in the flora of the British
Isles.
From maps of individual species, it was possible to
determine which species had large geographic ranges
and which small.
From descriptions of habitats where species had been
collected, fellow scientists were asked to decide
whether the species were habitat specialists or
generalists, as well as whether species were at least
somewhere locally abundant.
Descriptions for some species make the categories
clear:
1.Marshes, fens, heaths, woodlands, and waste
ground. A common weed of arable land. – this is a
description clearly for a habitat generalist, at least
some places locally abundant.
2.Restricted to soil-filled crevices in scree slopes. –
this is clearly the description of a habitat specialist.
3.Scarce where present. Occurs as widely scattered
individuals. – this is equally clearly the description
of a species that is everywhere scarce.
Rabinowitz worked by consensus. A statistical test
indicates the three 'traits' are statistically independent.
Think about that one!
Common sense ecology would likely suggest that
habitat generalists ought to be widespread in
distribution, but there are habitat generalists with
narrow (small) range.
On the next slide is a table of her results.
Geographic range
Large
Somewhere
large
58
Small
71
6
14
Population Size
Everywhere
small
2
_____
Broad
6
0
______
______
Restricted
Broad
Habitat Specificity
3
______
Restricted
Species rarity could occur in three different ways:
1) restricted geographic distribution;
2) narrow habitat distribution;
3) low local population abundance.
Of the British flora analyzed, 39% had no component
of rarity (i.e. they were not rare in any way), while
among the 61% that were rare in one or more ways:
59% had narrow habitat specificity;
15% had small geographic range;
7% had low population size.
There has been at least one other analysis using this
scheme. Pitman et al. (1999) analyzed trees
in Peruvian Amazon forest, and set 1 ind./ha as the
abundance dividing line, 1 vs. 2 or more forest type
occurrences for habitat breadth, and a political
boundary (Madre de Dios) for geographic
distribution.
None of the trees were geographically confined (no
small geographic ranges); only 13% were locally rare.
The rarest trees occurred at 1 stem per 36 ha; over the
forest this 'rare' species would encompass 200,000
stems!
One important consequence of this result is, however,
that conservation of truly rare species will require
preservation of a lot of land (Ricklefs 2000).
Much of the difference between Rabinowitz’s and
Pittman’s results may relate to the spatial scale used.
Consideration of spatial scale is obligatory when
assessing rarity.
The lack of apparent habitat specialization by these
tropical trees runs in contrast to prevailing views.
They also stand in contrast to Rapaport's Rule that
high tropical diversity is associated with habitat
specialists and small geographic ranges.
Why is the separation of forms of rarity important to
conservation biology?
Think about how the limited resources available for
conservation should be spent. Whole countries
identified as centers of diversity cannot be wholly
conserved.
Within such countries, what should be protected?
Clearly, the best approach is to preserve habitats and
ecosystems. But which ones?
If information about rarity is available (or can be
developed) for species of various sorts (not just
plants), then the form of rarity can be very useful.
Species rare due to limited geographic distribution,
but with fairly generalized habitat requirements and
locally abundant are good candidates for introduction
into areas outside their current distribution.
Species with wide distributions and high local
abundance, but narrow habitat requirements, probably
cannot be successfully moved.
Species that are locally sparse, but widely distributed
and generalized in habitat requirements probably
need no protection.
In sum, species restricted to narrow niche
characteristics in two or more of the three traits are
those most likely to need help.
Kunin and Gaston (1993), in a review of rare and
common species, found that rare species (i.e. locally
rare and geographically restricted) differ from more
common species. Rare species (plants) have lower
levels of self-incompatability, a greater tendency to
asexual reproduction, lower overall reproductive
effort and poorer dispersal abilities. In a sense, they
tended to make the best of a bad situation.
Groups such as the World Conservation Union
(IUCN) use indicators to determine which species are
rare and at risk of extinction. Typical indicators used
for this purpose include:
1) rarity;
2) rate of decline (high rates being bad);
3) degree of population fragmentation.
In the IUCN model, species are considered critically
endangered if the extent of occurrence is <10km2,
endangered if they occupy <500km2, and vulnerable
if they occupy <20,000km2 .
Rate of decline is measured by assessing population
size over at least 2 points in time. Four possible
measures are possible using only range size and
population number (declines in range but not
population number, declines in population number but
not range, both, neither). Scale has been shown to
make a difference in these results.
The IUCN method considers population structure
with respect to dispersal potential (i.e. fragmentation
extent). However, severe fragmentation (many small,
isolated populations) and lack of fragmentation (a
single or few subpopulations) can be considered as
indicative of increased risk of extinction.
References (Rarity)
Harte, J. and E. Hoffman. 1989. Possible effects of acidic deposition on a Rocky
Mountain population of the tiger salamander Ambystoma tigrinum. Conservation
Biology 3:149-158.
Hartley, S. and W. Kunin. 2003. Scale dependency of rarity, extinction risk, and
conservation priority. Conservation Biology 17: 1559-1570.
Kunin, W.E. and K.J. Gaston. 1993. The biology of rarity: patterns, causes and
consequences. TREE 8:298-301.
Pitman, N.C.A. et al. 1999. Tree species distributions in an upper Amazonian forest.
Ecology 80:2651-2661.
Rabinowitz, D. 1981. Seven forms of rarity. pp. 205-217 in The Biological aspects of
rare plant conservation. Ed. by H. Synge. Wiley.
Rabinowitz, D., S. Cairns and T. Dillon. 1986. Seven forms of rarity and their
frequency in the flora of the British Isles. In M.E. Soule; (ed.) Conservation
Biology: The Science of Scarcity and Diversity. pp. 182-204. Sinauer.
Ricklefs, R.E. 2000. Rarity and diversity in Amazonian forest trees. TREE 15:83-84.