Systematics and endangered species conservation

Why worry about systematics?

Systematics and endangered species conservation


Why worry about systematics? 

Systematics is the science of diversity, and if we are concerned about the loss of diversity one might think that it would be a major contributor to the theory and practice of conservation biology. After all, before you can conserve anything, you have to be able to identify what it is you intend to conserve. The U.S. Endangered Species Act ( specifies, for example, that

§3.D.15 The term ``species'' includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species or vertebrate fish or wildlife which interbreeds when mature.
We've already seen some important ways that ecology and genetics contribute to conservation biology, and we'll see more through the remainder of this course. Part of the reason is that much conservation effort is expended on ecosystems or habitats rather than individual species. Nonetheless, systematics plays an important role, and there are some important ways in which it should contribute more in the future. Soltis and Gitzendanner [10] identify four of them.
  1. Species concepts
  2. Identifying lineages worthy of conservation
  3. Setting conservation priorities
  4. The effects of hybridization on the biology and conservation of rare species



(About this document)

These notes are licensed under the Creative Commons Attribution-NonCommercial-ShareAlike License.

Copyright © 1993, 1994, 1995, 1996, Nikos Drakos, Computer Based Learning Unit, University of Leeds.
Copyright © 1997, 1998, 1999, Ross Moore, Mathematics Department, Macquarie University, Sydney.

The translation was initiated by Kent Holsinger on 2005-09-25

Species concepts

Biologists have been arguing about what species are for as long as they have been grouping organisms into species, and I don't propose to solve the problem. Fortunately, we don't necessarily have to agree with one another. The ``hybrid policy'' proposed by the U.S. Fish & Wildlife Service in 19961 notes that

The [Endangered Species] Act does not attempt to define ``species'' in biological terms, and thus allows the term to be applied according to the best current biological knowledge and understanding of evolution, speciation, and genetics.

The biological species concept2 has been the most widely accepted and influential species definition for most of the last sixty years. In the last twenty years, however, systematists are increasingly inclined to define species in phylogenetic terms, either as minimal (or at least very small) monophyletic clades (the history-based conception) or as population systems with fixed, diagnosable differences (the character-based conception).

In one sense, it might not seem to matter how we define species. In fact, many conservation biologists are now focusing on the protection of ``evolutionarily significant units''3 precisely because systematists can't agree on how to define what species are. Remember, however, that the U.S. Endangered Species Act states specifically that

the term ``species'' includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species or vertebrate fish or wildlife which interbreeds when mature.
That means that ``evolutionarily significant units'' without a formal latinized name can be protected only if they are vertebrates, e.g., grizzly bear, timber wolf, and certain salmon runs in the Pacific northwest. Plants and invertebrates can be protected only if they have latinized names that can be applied to them. Of course, it doesn't matter whether they are recognized as subspecies (or varieties in plants), so long as they are formally recognized. It does mean, however, that if you work with plants or invertebrates and identify an evolutionarily significant unit worthy of protection, you (or a systematist friend of yours) will have to put a formal name on it if you want it to receive protection under the U.S. Endangered Species Act.

Moreover, Collar [3] points out that the species concept we adopt could have a large impact on the conservation decisions we make. In birds, whose taxonomy is better understood than that of any other group of animals or plants, using a phylogenetic species concept instead of a biological species concept could double or triple the number of species recognized [3, p. 131]. The result would undoubtedly be a large increase in the number of bird species that we recognize as threatened, which may or may not be a good thing. Similarly, the ``Conservation Forum'' on the status of the black sea turtle that appeared in volume 13 of Conservation Biology4 makes it clear that deciding not to recognize a population as evolutionarily distinct may have important conservation consequences - that population may no longer receive special conservation attention.

Evolutionarily significant units

Ryder [9] proposed the evolutionarily significant unit (ESU)5 as the minimal unit of conservation management. It is an attractive idea because it avoids problems associated with species definitions - or at least it seems to. An ESU is simply

This captures the idea that in most groups of plants and animals there are ``units'' of some sort above the level of individuals and populations that are worthy of concern. But you can probably see the difficulty with this definition already.

Well, the answer to the first question is: ``It depends.''6 Pennock and Dimmick [7] argue, for example, that many population segments of vertebrates currently protected under the ESA would not be protected if population segments were defined as ESUs. Protected populations of grey wolf and grizzly bear in the lower 48, for example, don't seem particularly different either genetically or morphologically from those just across the border in Canada, nor do they seem likely to have an independent evolutionary history. In response, Waples [11] argues that use of ESUs to define population segments in fish7 is precisely what the ESA intends when it directs that listing decisions be based ``solely on the basis of the best scientific and commercial data availabls'' (§4(b)(1)(A)). Dimmick et al. [5] respond by arguing that ``any unit of conservation defined solely in terms of adaptation is likely to underestimate biological diversity.

Questions for discussion

What about determining whether populations are historically distinct from one another? That's a little more straightforward, at least in principle. Consider the case of the dusky seaside sparrow.

Setting conservation priorities

We'll come back to this topic and discuss it in more detail later in the course. For now just notice that it might make more sense to devote more conservation attention to taxa that are the last surviving representative of their lineage than to those that are part of a large and speciose group.

Clark and May [2] argue that the allocation of research effort is very uneven. Based on a review of 32,000 entries in the Zoological Record, invertebrates account for about 79% of known species worldwide, but only 11% of research articles on conservation while vertebrates account for only about 3% of known species and about 69% of conservation research articles. While this suggests a considerable bias in research effort, Czech et al. [4] argue that allocation of benefits by the U.S. Endangered Species Act follows what would be expected from a political science model based on a combination of public attitudes toward particular taxa (favorable: plants, birds, mammals, fish; unfavorable: reptiles, amphibians, invertebrates) and the number of non-governmental organizations (NGOs) with an interest in those particular taxa. The large amount of resources devoted to birds, mammals, fish, and tortoises reflects both positive public attitudes and a lot of NGO interest.


The ``hybrid policy'' ( proposed by the U.S. Fish & Wildlife Service in 1996 specifically notes that hybrids may be worthy of protection.

The Services believe the responsibility to conserve endangered and threatened species under the Act extends to those intercross progeny if (1) the progeny share the traits that characterize the taxon of the listed parent, and (2) the progeny more closely resemble the listed parent's taxon than an entity intermediate between it and the other known or suspected non-listed parental stock. The best biological information available, including morphometric, ecological, behavioral, genetic, phylogenetic, and/or biochemical data, can be used in this determination.

The questions involving identification and protection of hybrids are fundamentally in the domain of systematics

In the end the question of whether to protect the product of becomes the question, ``Is this entity an evolutionarily significant unit?'' In that sense, hybrids pose no particular problem for endangered species protection. For example, the U.S. Fish and Wildlife determined

Recent evidence indicates that Lloyd's hedgehog cactus is not a distinct species but rather a hybrid or cross which is not evolving independently of its parental species. Therefore, E. lloydii no longer qualifies for protection under the Act. (
In that case a taxonomic decision led to delisting of a species. In another recent case taxonomic work confirmed that a species of hybrid origin was distinct and worthy of protection.
Heiser was able to produce hybrids between Pecos sunflower and both common sunflower and prairie sunflower, but these hybrids were of low fertility. These results support the validity of Pecos sunflower as a true species. In 1990, Rieseberg et al. published the results of molecular tests on the hypothesized hybrid origin of Pecos sunflower, using electrophoresis to test enzymes and restriction- fragment analysis to test ribosomal and chloroplast DNA. This work identified Pecos sunflower as a true species of ancient hybrid origin with the most likely hybrid parents being common sunflower and prairie sunflower. (

Hybridization can, however, threaten the persistence of endangered species. Of the eleven individuals of Catalina Island mountain mahogany that remained in the wild in 1995, five were hybrids [8].



1  J. C. Avise and W. S. Nelson. Molecular genetic relationships of the extinct dusky seaside sparrow. Science, 243:646-648, 1989.


2  J. A. Clark and R. M. May. Taxonomic bias in conservation research. Science, 297:191-192, 2002.


3 N. J. Collar. Taxonomy and conservation: chicken and egg. Bulletin of the British Ornithological Council, 117:122-136, 1997.


4 B. Czech, P. R. Krausman, and R. Borkhataria. Social construction, political power, and the allocation of benefits to endangered species. Conservation Biology, 12:1103-1112, 1998.


5 W. W. Dimmick, M. J. Ghedotti, and D. S. Pennock. The importance of systematic biology in defining units of conservation. Conservation Biology, 13:653, 1999.


6 E. Mayr. Speciation phenomena in birds. American Naturalist, 74:249-278, 1940.


7 D. S. Pennock and W. W. Dimmick. Critique of the evolutionarily significant unit as a definition for ``distinct population segments'' under the U.S. Endangered Species Act. Conservation Biology, 11:611-619, 1997.


8 L. H. Rieseberg and D. Gerber. Hybridization in the catalina island mountain mahogany ( Cercocarpus traskiae): RAPD evidence. Conservation Biology, 9:199-203, 1995.


9 O. A. Ryder. Species conservation and systematics: the dilemma of subspecies. Trends in Ecology & Evolution, 1:9-10, 1986.


10 P. S. Soltis and M. A. Gitzendanner. Molecular systemics and the conservation of rare species. Conservation Biology, 13:471-483, 1999.


11 R. S. Waples. Evolutionarily significant units, distinct population segments, and the endangered species act: Reply to Pennock and Dimmick. Conservation Biology, 12:718-721, 1998.



Figure 1: Distribution of seaside sparrow (Ammodramus maritimus) subspecies in eastern North America. The dusky seaside sparrow is subspecies nigrescens (from [1], © 1989 American Association for the Advancement of Science).

figure 1


Figure 2: Haplotypes found in analysis of seaside sparrow DNA. The haplotype of the last male dusky seaside sparrow is 1. That haplotype is also found in subspecies maritima and macgillivraii (from [1], © 1989 American Association for the Advancement of Science).

figure 2