Selecting Effective Umbrella Species
By Erica Fleishman, Dennis D. Murphy, & Robert B. Blair
Even a well-designed shortcut can miss its destination. The role of umbrella species in conservation planning is a case in point. In the face of limited time and resources, umbrella species offer an appealing shortcut to land managers. But they do not shorten every road.
The idea of umbrella species is straightforward: the conservation of some species is thought to provide a protective umbrella to numerous co-occurring species. It is often faster and less expensive to sample a few species than it is to survey the entire assemblage. Hence, umbrella species can reduce the investments in sampling that are necessary when prioritizing areas for competing land uses. However, there is little empirical evidence that this approach actually works. Furthermore, few researchers have offered guidance on identifying umbrella species and the contexts in which this approach work best.
In theory, selection of umbrella species is prospective; in practice, it has almost always been retrospective. Species have been suggested as umbrellas not on the basis of occurrence patterns or ecological traits, but simply because they are imperiled and have statutory protection. The northern spotted owl (Strix occidentalis) and California gnatcatcher (Polioptila californica) are well known examples of species proposed as umbrellas subsequent to their listing under the U.S. Endangered Species Act. As a result, ecologists and managers have been restricted to asking — after the fact — whether other species will benefit from the conservation of these and other listed species.
Using existing data sets for birds and butterflies, we found that umbrella species can be selected prospectively using objective ecological criteria. Here, we describe three key aspects of a species’ distribution and ecology that should be considered when selecting umbrella species. Using these factors, we developed a numerical index to measure the potential of each species to serve as an umbrella for its regional species assemblage. Because few data support the notion that conservation of species in one taxonomic group effectively protects species in other groups, we focused on umbrellas within a taxonomic group.
We also found that in many cases, umbrella species may not be the tool of choice. As such, we conclude by considering situations in which umbrellas likely can or cannot provide an effective shortcut.
Co-occurrence of Species
Arguably the most critical factor to weigh in selecting umbrella species is co-occurrence of a potential umbrella and other species. Co-occurrence refers to the number of species in the same taxonomic group that are present at locations where a species occurs. For example, if you wanted to evaluate whether conservation of side-blotched lizards (Uta stansburiana) in the Sonoran Desert would protect other lizards in the ecosystem, co-occurrence would be the measure of the number of Sonoran Desert lizard species that are present at each location where side-blotched lizards are present. Calculating co-occurrence is easy and requires only presence/absence data.
Co-occurrence of species rarely has been considered explicitly in attempts to earmark umbrellas. Instead, planners often assume that protecting species with large area requirements will conserve habitat for species that are more insular or sedentary. Grizzly bears (Ursus arctos), wolves (Canis lupus), and mountain lions (Felis concolor) are popular examples. But focusing on species with large home ranges can compromise conservation efficacy for at least two interconnected reasons.
First, species with large home ranges often are habitat generalists, and it may be impossible, and unnecessary, to protect all locations where they occur. Indeed, if we could protect sufficient habitat for grizzly bears in western North America, our conservation dilemma in that region essentially would be resolved. Unfortunately, this scenario is unlikely. Second, species richness tends to vary considerably on the local scale. Conservation of only a portion of the area occupied by a species with a large home range may miss locations with the greatest species richness. Selection of umbrella species should focus not on how many places a species occurs but rather on how species-rich those places are.
To illustrate this distinction, compare a wide-ranging raptor with a more narrowly occurring songbird. With respect to co-occurrence, which would make a better umbrella? The habitats used by the raptor might include both species-rich riparian corridors and comparatively species-poor uplands. If we could protect all of these habitats, we undoubtedly would protect many other species. But if we indiscriminately conserve only half of the habitats used by the raptor, we could leave many species-rich riparian communities unprotected. From a practical standpoint, the songbird that is restricted to riparian areas might be a more effective umbrella. Even if we could protect just a portion of the songbird’s habitat, we still would be concentrating efforts on locations with the greatest species richness.
To obtain reliable estimates of species richness (and, by extension, co-occurrence of species) across a planning area, it is important to obtain a representative sample from as many environmental gradients and vegetational communities as possible. For example, a manager responsible for developing land-use plans for ten watersheds in a mountain range might inventory birds along five 1-km stream reaches, at different elevations and aspects, within each watershed. These inventories yield three pertinent pieces of information: the number of species in each sampled location, the number of locations in which each species occurs, and the total number of species. These numbers are the basis for calculating co-occurrence of species.
Degree of Ubiquity
An ideal umbrella species candidate should be neither ubiquitous nor extremely rare but instead should strike a balance between these two extremes. Ubiquitous species are unlikely to serve as effective umbrellas because, as noted above, they may occur in many locations with relatively low species richness, and it is not feasible to protect all areas in which they are present. While rare species certainly should be protected on their own merits, they, too, are unlikely to be effective umbrellas precisely because they occur in so few locations. A species that occupies, for example, less than five percent of a planning area probably is not distributed across enough of the landscape to provide many other species with a protective umbrella.
Another reason why rare species make poor umbrellas is that rarity may not reflect the influence of environmental factors that managers realistically can control. Some species are rare because their resources have been degraded by human activities, but others are rare simply because their resources are patchily distributed. For example, the brown elfin (Incisalia augustinus) is one of the rarest butterflies in the mountains of central Nevada. It is uncommon not because its habitat is favored by livestock or hikers but because its larval hostplant (Ceanothus sp.) occurs at only a few isolated locations across the region.
Sensitivity to Human Disturbance
Our third consideration for selecting umbrella species recognizes that species respond differently to various disturbances. Because human land-use poses the greatest threat to most species, we concentrate on sensitivity to human disturbance as opposed to sensitivity to natural phenomena such as weather extremes or episodic disease outbreaks. We presume that sensitive species will provide a protective umbrella for others that are equally or less sensitive to human activities.
Because no two disturbances, landscapes, or taxa are identical, there is no “magic formula” for calculating disturbance sensitivity. Deforestation may be the dominant human disturbance in one ecoregion and livestock grazing in another. The particular life history-related characteristics that affect whether a species is vulnerable to the dominant disturbances also vary. For example, habitat fragmentation poses a considerable threat to birds in woodlands. Variables that affect whether a species of bird is vulnerable to fragmentation include nest height, territory size, and sensitivity to edge effects. The greatest threat to butterflies in an arid landscape, by comparison, might be loss of wetland habitat. Life history-related characteristics that affect the extent to which a given butterfly species is threatened by this disturbance include vagility, larval hostplant specificity, and dependence of adult butterflies on water.
The Best Tool for the Job
Like misuse of antibiotics that can be effective for bacterial infections but will not cure the flu, misuse has confounded practical application of the umbrella species concept. Umbrella species are useful in addressing some but not all conservation problems.
Our research suggests that a practical advantage of umbrella species is that they can help managers realize a degree of species protection in an efficient way. This was evident when we used data on bird and butterfly species from three different ecosystems to establish whether species identified with the umbrella index (see opposite page) were more effective as umbrellas than randomly selected species. We found that while umbrellas and randomly selected species protected an equal proportion of each species assemblage, umbrella species could be used to identify a smaller subset of locations for conservation and still achieve that level of protection. In other words, umbrella species can be an efficient shortcut for choosing where to locate competing land uses when the amount of land dedicated to conservation is limited. Because environmental variation can affect local distribution of many organisms in a given year, we recommend that a suite of umbrella species rather than a single species be employed in planning and management.
Umbrella species, however, may not prove to be effective in assessing the status of unusual habitats, such as an isolated wetland or uncommon plant community. Instead, we recommend that ecologists and managers target species whose viability is better tied to the integrity of those habitats. Thoughtful consideration of ecology and life history may be more useful than distribution studies for identifying such species, sometimes referred to as indicator species.
Indicator species should not be confused with umbrella species. According to our definition, an umbrella is simply a species whose conservation confers protection to co-occurring species in the same region. An indicator, by contrast, is a readily sampled species whose distribution, abundance, or population dynamics serves as a reliable and affordable surrogate measure of the status of other species or environmental parameters. Several authors have proposed thoughtful schemes for selecting complementary sets of “surrogate species.” These methods are sound theoretically but perhaps too data-intensive to be widely applicable. In reality, very few species can serve as umbrellas or indicators, let alone both.
Finally, use of umbrella species does not appear to be an effective way to maximize species richness of multiple taxonomic groups. It is often assumed that the species richness of one group of animals or plants can serve as a reliable index of the species richness of other groups. But the few studies providing evidence that members of one taxonomic group can serve as effective umbrellas for members of other taxonomic groups are the exception rather than the rule. For example, a recent study found that the California gnatcatcher, a de facto umbrella species for coastal sage scrub communities in southern California, probably would not serve to protect many Lepidoptera. Existing data show that at scales that frequently concern managers — stream reaches or grazing allotments, for instance — species richness of invertebrates, vertebrates, and plants frequently is not correlated.
We recognize, nevertheless, that if effective cross-taxonomic umbrella species existed, they could be extremely valuable. Accordingly, we tested the cross-taxonomic potential of umbrella species using data on birds and butterflies from the same set of locations in a broadleaf forest in the eastern United States. Our results indicated that the ability of umbrella species for one group to protect species in other groups is limited. We found that, on average, cross-taxonomic umbrellas protected 21 percent fewer species than did same-taxon umbrellas. Even more telling was our discovery that randomly selected species in the same taxonomic group actually would provide better protection than would cross-taxonomic umbrellas.
Honest motives and urgent conservation challenges have driven development of the umbrella species concept. Yet common sense alone reminds us that few species can function as reliable and affordable measurements of community-level or ecosystem-level variables. This situation underscores the importance of rigorously testing whether proposed shortcuts do indeed function as intended. We suggest that umbrella species can be useful for maximizing species protection while minimizing the area that must be protected — if selection is predicated upon objective ecological criteria as opposed to fond wishes.
There is no sound ecological reason to expect that a species’ charisma will enable it to function as an umbrella. Nor does endangerment guarantee umbrella potential, despite our hopes that legally mandated protection of threatened taxa will benefit other organisms. Additional species often do benefit from single-species conservation efforts, but this fact does not necessarily mean that even a small fraction of our currently protected species can serve as umbrellas for other species. In this light, we emphasize that no one species, or shortcut, can solve all conservation problems.
This article is adapted from:
Fleishman, E., D.D. Murphy, and P.F. Brussard. 2000. A new method for selection of umbrella species for conservation planning. Ecological Applications 10:569-579.
Fleishman, E., R.B. Blair, and D.D. Murphy. In Press. Empirical validation of a method for umbrella species selection. Ecological Applications.
The literature on umbrella species and other species that can be used to provide shortcuts for land-use planning and management (collectively referred to as “surrogate species”) is limited. Several papers have addressed surrogate species on a conceptual basis; the extent to which they include empirical examples varies. An important message emerging from this collection of papers is that unless shortcuts are chosen using objective criteria and data, they are likely to prove costly at best and ineffective at worst.
Andelman, S. J., and W. F. Fagan. 2000. Umbrellas and flagships: efficient conservation surrogates, or expensive mistakes? Proceedings of the National Academy of Sciences 97:5954-5959.
The authors review different criteria used to identify potential surrogate species. They examine the benefits (in terms of species protection) and the practical costs of using these species as shortcuts for conservation planning. Species proposed as surrogates often were judged ineffective because they did not co-occur with many other species.
Martikainen, P., L. Kaila, and Y. Haila. 1998. Threatened beetles in White-backed Woodpecker habitats. Conservation Biology 12:293-301.
One of few empirical studies demonstrating that protection of species in one taxonomic group (an endangered woodpecker in mature deciduous forest in Finland and Russian Karelia) might confer protection to species in a different taxonomic group (saproxylic beetles).
Rubinoff, D. 2001. Evaluating the California Gnatcatcher as an umbrella species for conservation of southern California coastal sage scrub. Conservation Biology, In Press.
Conservation planning for coastal sage scrub communities has assumed that vertebrate-based reserve designs effectively protect invertebrates. To the contrary, Rubinoff found that the presence of the California Gnatcatcher in remnant patches of coastal sage scrub was a poor indicator of the presence of three rare Lepidoptera.
Simberloff, D. 1998. Flagships, umbrellas, and keystones: is single-species management pass? in the landscape era? Biological Conservation 83:247-257.
Simberloff defines and compares the theoretical concepts of indicator, umbrella, and flagship species; examines whether data support the function of species proposed as shortcuts for management; and discusses the ramifications of single-species management for those proposed shortcuts.
Caro, T. M., and G. O’Doherty. 1999. On the use of surrogate species in conservation biology. Conservation Biology 13:805-814.
The authors define and distinguish several categories of species — including indicators, umbrellas, and flagships — that have been proposed as shortcuts for conservation and management. Caro and O’Doherty also examine conceptual criteria that have been used to select species within each surrogate category.
Lambeck, R. J. 1997. Focal species: a multi-species umbrella for nature conservation. Conservation Biology 11:849-856.
Lambeck presents a comprehensive approach for determining how to manage a landscape to meet the needs of its constituent species. He suggests that a set of “focal species” can be used to help define key landscape attributes and appropriate land uses.
Noss, R. F. 1990. Indicators for monitoring biodiversity: a hierarchical approach. Conservation Biology 4:355-364.
Noss explores how individual species might be used as surrogate measures of ecosystem composition, structure, and function at different levels of organization. He emphasizes that indicators must meet rigorous standards of scientific reliability and practicality. This is one of the seminal papers on “indicator species.”
Watt, A. D. 1998. Measuring disturbance in tropical forests: a critique of the use of species-abundance models and indicator measures in general. Journal of Applied Ecology 35:467-469.
Watt offers a concise commentary emphasizing that “indicator species” should be easier to sample than the variables of interest, and that different disturbances (types, extents, and intensities) vary in their effects on plants and animals.
Niemi, G. J., J. M. Hanowski, A. R. Lima, T. Nicholls, and N. Weiland. 1997. A critical analysis on the use of indicator species in management. Journal of Wildlife Management 61:1240-1252.
The authors examine the distribution, in terms of habitat and species co-occurrence, of birds proposed as “management indicators” or “sensitive” within a National Forest in northern Wisconsin. Few species consistently were associated with habitats for which they were proposed as “indicators,” and few sensitive species were positively or negatively associated with other species. Further, most of the surrogate species were either too rare or difficult to monitor in practice.