Ecological Mechanisms of Extinction

Commentary

Ecological mechanisms of extinction

Steven R. Beissinger*

Ecosystem Sciences Division, Department of Science, Policy, and Management, University of California, Berkeley, CA 94720-3110

cological theory offers predictions, families using phylogenetic comparative duced predators were associated with Esometimes conflicting, about the eco- methods. Birds are one of the few taxa birds that had large body size and long logical characteristics of species that cor- whose species are well enough described, generation times. This result is entirely relate with their risk of extinction. It is whose phylogeny has been widely investi- expected, because rate of population generally agreed that risk should be higher gated (14), and whose ecology is suffi- change (r) of long-lived and slowly repro- for species with small populations, small ciently known to permit a global analysis. ducing species and lineages is especially geographic ranges, and poor dispersal The outstanding scholarship of Collar et sensitive to small perturbations to adult ability than for their ecological counter- al. (15), who compiled life history ac- survival (18). For example, factors that parts (1–3). How suites of life history counts of all 1,111 species of birds in the lowered the survival of long-lived adult characteristics affect risk of extinction is world thought to be at risk, made possible California condors (Gymnogyps califor- less clear. Species with high variance in the this and similar analyses (16). nianus) or albatrosses would have a far intrinsic rate of population increase (r), Multiple factors may interact to greater impact on population change than which is often associated with high fecun- threaten species. About one-third of the proportional changes in their reproductive dity, moderate to low survival rates, short world’s threatened bird species are at risk success, which is limited by a clutch size of generation times, and small body size, are from direct mortality because of human one egg annually. Thus, slow lifestyle spe- predicted to be more susceptible to ex- persecution, including harvesting, poison- cies and lineages should be more suscep- tinction because they are prone to large ing, egg collecting, and capture for trade, tible to human persecution and intro- stochastic population fluctuations (4). Al- and by predation from introduced preda- duced predators if the impact primarily ternatively, species with a low r (because tors, which has been especially devastating affects survival, compared to their fast- of low fecundity, high survival, and long to fauna and flora on island ecosystems. lifestyle counterparts. generation times) are predicted to be at These factors directly reduce survival This phenomenon is illustrated effec- increased risk, because they would recover and͞or reproduction, to result in popula- tively by the differential vulnerability of slowly from a severe reduction in population declines. Birds are primarily threat- parrots and finches harvested as adults for tion size and remain threatened longer by ened by habitat loss because of habitat the international pet trade (Table 1). Be- demographic and genetic stochasticity (5, destruction and habitat degradation from tween 1,600,000 and 3,200,000 birds were 6). Such species are typically large. Thus, agricultural practices and water manage- taken annually from wild populations for it is unclear whether the ‘‘fast lifestyle’’ ment, which affects over two-thirds of the the live bird industry in the 1990s (19). associated with small body size and short threatened species. For birds, habitat loss Finches of the families Passeridae and generation times or the ‘‘slow lifestyle’’ may not result in direct mortality unless Fringillidae composed 70% of the trade, represented by large organisms with long the impacts destroy active nests. However, and parrots (Psittacidae) accounted for generation times makes species and lin- mortality may occur after habitat has been 25% of the volume. The body mass of a eages more or less likely to become ex- lost through starvation, accidents, and typical finch is nearly 10 times smaller tinct. Empirical studies of island fauna predation caused when birds must dis- than the mass of a modal parrot. Despite yielded contradictory conclusions about perse in search of unspoiled areas to live the large numbers of finches traded, par- the effects of body size and lifestyle on the in and from crowding into remaining hab- rots suffer over three times the rate of risk of extinction and produced conflicting itats (17). The impacts of habitat loss, threat that finches incur from trade (Table explanations to account for the mecha- however, are likely to be different for less 1), making Psittacidae among the most nisms underlying the patterns (5, 7–11). vagile or smaller animals and plants, which threatened families of birds (6). Many Resolution of such issues transcends aca- may suffer more immediate mortality. Al- parrots are threatened by a combination demic debates, as governments and con- though habitat loss and human persecu- of trade and habitat destruction (20), but servation organizations struggle to apply tion͞introduced predators can occur si- trade may be more threatening because laws, like the United States Endangered multaneously to drive a species toward often species persist in a variety of dis- Species Act, and decide how to rank extinction, Owens and Bennett (13) found turbed habitats (21). Life history differ- threats and allocate funds among taxa that that they often acted independently on ences partly explain why parrots are more may differ in risk (12). lineages, as there was no correlation be- susceptible to overharvesting than finches Now Owens and Bennett (13) present tween the percentage of species within a (Table 1). Annual fecundity of parrots is new evidence that the ecological mecha- family threatened by one force or the much less than that of finches by virtue of nisms underlying extinction may differ for other. smaller clutch sizes and fewer broods per lineages of birds threatened by habitat loss Could differences among species in ex- year. Finches do not require specialized and for lineages threatened by human tinction risk be caused by differential vul- structures for nesting, whereas parrots persecution and introduced predators. nerability of lineages to habitat loss vs. typically nest in tree cavities, which are The study both supports and challenges persecution͞predation as a result of the often in short supply. This results in large current thinking in extinction theory and differing ecological pathways that these raises a number of intriguing issues. The forces affect? Owens and Bennett’s (13) authors tested predictions about extinc- results suggest they could. Extinction risks See companion article on page 12144. tion theory on a database of 95 avian through human persecution and intro- *E-mail: [email protected].

11688–11689 ͉ PNAS ͉ October 24, 2000 ͉ vol. 97 ͉ no. 22 Downloaded by guest on September 29, 2021 Table 1. Comparison of levels of threat and ecology between parrots (Psittacidae) and tinction risk from habitat loss dispropor- ﬁnches (Passeridae and Fringillidae), the two most traded groups of birds tionately affected birds that were small, Category Comparison Parrots Finches had short generation times, and were habitat specialists. The latter trait was to be Threat No. of species 358 1,379 expected, but the mechanisms of habitat No. of endangered and threatened species 103 90 destruction that would promote extinction % endangered and threatened species 28.7 6.5 of small and short-lived birds over large % endangered and threatened affected by trade 47.5 13.3 and long-lived lineages remain to be elu- Ecology Clutch size small large cidated. Conservation biologists have typ- No. of broods per year single multiple ͞ ically worked under the assumption that Nest type cavity open cup hanging habitat destruction will first extirpate Age of ﬁrst breeding delayed rapid large species because they require large Adult survivorship rate high intermediate home ranges and occur at lower densities. Endangered and threatened species are those listed by IUCN–The World Conservation Union as critically The ability to traverse landscapes may endangered, endangered, or vulnerable (19). assist large species to escape harm and find new homes when their habitats are destroyed. However, many small birds mi- proportions of nonbreeding individuals others persist by the slimmest of margins. grate thousands of kilometers and possess parrots (22) that could create a surplus for The island swiftlet (Aerodramus vaniko- the ability to search for remaining suitable harvesting, but also contributes to low rensis bartschi) survives by nesting in caves habitat. It is notable that the families of rates of population growth. Furthermore, that snakes do not penetrate, and the small birds identified by Owens and Ben- Aplonis opaca medium- and large-sized parrots do not Micronesian starling ( ) per- nett as affected only by habitat destruction breed until 2–5 years of age, whereas sists in urban areas where snake densities are primarily composed of frugivores or finches usually mature within a year. Fi- are low. Practically the only native bird nectivores, which could be more suscepti- nally, most parrots are long lived com- remaining in the forest is the Aga or ble to changes in the spatial arrangement pared to finches. Thus, high reproductive Mariana Crow (Corvus kubaryi), the larg- effort and moderate survival make finches est forest bird. The Aga survived because of habitats because they depend on less susceptible to overharvesting than it typically grows too big for the snake to ephemeral and specialized food resources. parrots. eat! Down to less than one dozen individ- Because the role of body size and life A classic example of the impact of an uals, the Aga has produced only two young history traits in determining extinction introduced predator on island birds also in the wild over the past dozen years (24). risk varies, future research might be provides an interesting exception to the This example illustrates the importance of more fruitful if it focused on the inter- rule that larger slowly reproducing species understanding how threats act on demog- action between threats and diverse but become extinct faster than smaller more raphy and the ecological mechanisms of specific ecological variables. The result- fecund ones. The brown tree snake (Boiga extinction if we are to predict successfully ing elucidation of risk to species and irregularis) was accidentally introduced to the differential vulnerability of species lineages would contribute a theoretical COMMENTARY Guam, the largest island of Micronesia, and lineages. component that the ‘‘declining popula- around 1950 (23). It is a voracious pred- The most surprising result of Owens and tion paradigm,’’ which identifies and ator and is mainly responsible for the Bennett’s work was that lineages threat- ameliorates threats, currently lacks and extirpation of 9 of 13 native forest birds, ened principally by habitat loss exhibited would complement the burgeoning the- having eaten its way through eggs, nest- ecological correlates that differed from ory of genetic and demographic risks of lings, and adults. One species became lineages suffering because of human per- smallness, which composes the ‘‘small extinct before the snake arrived and three secution and introduced predators. Ex- population paradigm’’ (25).

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