Habitat Loss and Changes in the Species-Area Relationship

The species-area relationship (SAR) has been used successfully to predict extinction from extent of habitat reduction. These extinction estimates assume that species have uniformly distributed range requirements and a minimum abundance level required for persistence; how many species are lost depends solely on how much habitat is removed, not on where it is removed. We consider another limiting case in which range requirements, rather than abundances, determine extinctions. We used a new method for constructing SARs based on assumptions about geographic ranges of species. Our results show that habitat destruction can change the SAR and consequently the number of species predicted to be lost due to habitat destruction. Our method generates SARs that vary in shape according to the specific distributions of geographic range and occupancy but that have the common feature of being described by a power law with an exponent of , 1. When the geographic range of species was included in the SAR, the way habitat was lost became important. Although the SAR before habitat destruction is often used to predict species loss after habitat destruction, assumptions must be clearly stated. To predict the damage caused by habitat loss with our model, it is necessary to know the fraction of aggregated species, the distribution of geographic ranges, the form of habitat destruction, and the sampling protocol. The remaining theoretical challenge is to develop a full theory that links abundance and range. Pérdida del Hábitat y Cambios en la Relación Especie-Area Resumen: La relación especie-área (REA) ha sido utilizada exitosamente para predecir la extinción a partir de la extensión de la reducción del hábitat. Estas estimaciones de extinción asumen que las especies tienen requerimientos de rango uniformemente distribuidos y un nivel mínimo de abundancia requerido para persistir; cuantas especies sean perdidas depende únicamente de cuanto hábitat es removido, y no en base a donde el hábitat es removido. Consideramos otro caso limitante en el cual los requerimientos de rango, en lugar de la abundancia, determinan las extinciones. Usamos un método nuevo para construir REAs basadas en conjeturas sobre los rangos geográficos de las especies. Nuestros resultados muestran que la destrucción del hábitat puede cambiar la REA y consecuentemente el número de especies predecidas a ser perdidas debido a la destrucción del hábitat. Nuestro método genera REAs que varían en forma de acuerdo con las especificidades de las distribuciones del rango geográfico y la ocupación, pero tienen la característica común de estar descritos por una ley de poder con un exponente , 1. Cuando los rangos geográficos de la especie fueron incluidos en la REA, la pérdida del hábitat se volvió importante. A pesar de que el valor de REA antes de la destrucción del hábitat es utilizado como un predictor de la pérdida de especies después de la destrucción, uno debe ser claro con las conjeturas. Para predecir el daño ocasionado por la pérdida del hábitat usando nuestro modelo, uno necesita conocer la fracción agregada de especies, la distribución de los rangos geográficos, la forma de destrucción del hábitat y el protocolo de muestreo. El reto teórico remanente es el desarrollar una teoría completa que vincule la abundancia con el rango. ‡ Address correspondence to M. Mangel, email [email protected] Paper submitted March 31, 1998; revised manuscript accepted September 11, 1999. 894 Habitat Loss and the Species-Area Relationship Ney-Nifle & Mangel Conservation Biology Volume 14, No. 3, June 2000 Introduction The species-area relationship (SAR) is one of the cornerstones of modern ecological science (Rosenzweig 1995) and conservation biology. In general, the relationship between the number of species S and the area A of a habitat is S 5 CA z , where the parameter z is generally , 1, typically around 0.1–0.4 (McGuinness 1984; Rosenzweig 1995; He & Legendre 1996). Although they are usually treated as constants, it is recognized that the intercept C and exponent z may change as area changes (Preston 1962; MacArthur & Wilson 1963, 1967; Brown 1984; reviewed in Rosenzweig 1995). Because z , 1, the number of species increases with area, but it does so at a decreasing rate: larger areas hold proportionately fewer species than smaller areas. The standard species-area relationship has been used successfully in a number of cases to predict extinction based on habitat reduction (e.g., Pimm & Askins 1995; Pimm et al. 1995; Brooks et al. 1997; Pimm 1998). These extinction estimates assume that species have uniformly distributed range requirements and a minimum abundance level required for persistence. Thus, as the total number of all species is reduced during habitat destruction, an increasing fraction of species falls below the minimum number needed to persist. In this case, how many species are lost depends solely on how much habitat is removed, not on where the habitat is removed. Species abundances determine which species are lost and thus as area decreases the SAR can be used to predict species loss (Fig. 1). Assuming that species have essentially uniform ranges is a good approximation to the applications in which the SAR has been used to predict extinctions. It is a limiting case, however, in which abundance determines extinction. We considered another limiting case in which range requirements rather than abundances determine extinctions. We used a new method (Ney-Nifle & Mangel 1999) for constructing SARs from assumptions about the geographic ranges of species. Our results show that habitat destruction can change the SAR and consequently the number of species predicted to be lost due to habitat destruction.