Effects of Habitat Fragmentation on Dispersal Behavior

Deforestariorz i.r associutetl 11.itl1 hc-retr.~ed erz\~iro~lme~~tal heteroge~zeih ~ir~predic tabi l ih arid stress that mcrj ~nodi f i ptrtrerrz,s of bel l~l~, ior in r~zarnwzaliavl yop~~lrrtiolls. In particular; habitcrt distcrrbalzce ant1 orher anthropogenic effhcts appear to be increasirlg the inciclerzcr cfbisesutrl dispersal. These e13ents 11i.c. cc~lnl!'zecl ill terrtzs of berzefits and costs njdispersal to i ~ ~ d i ~ l i d u a l s of either se.u. I~nplicntions jot. conset~~crtiorz are Jiscl~.rsed. Loss of suitable habitat is one of the greatest threats to biodiversity (Primack 1993: Brokau 1999 ) . When continuous forest is cut. fragments often remain that yield a significantly different physical and biotic environment than existed before deforestation. possibly modifying the patterns of stimuli evoking behavioral responses from remaining plants and animals. Fragmentation increases environmental heterogeneity. patchiness, and unpredictability. and may increase stress on the individual organism (Hofer and East 1998). In this paper, I address the effects of habitat fragmentation on behavior. in particular, dispersal by both sexes of a species from their natal or resident groups. Most of the examples herein will be taken from the primate literature, but the arguments are expected to apply to other taxa. The study of dispersal is the study of why and under what circumstances individuals leave their residences. usually their natal groups (Johnson and Gaines 1990). Often. individuals leave home to escape the negative consequences of living with kin (e.g., inbreeding or resource competition), and scientists have identified reliable patterns of animal dispersal. For most species of birds, females are the dispersing sex whereas, among mammals, including primates, males are more likely to disperse (Green\vood 1980). While these differences are not well understood. researchers agree that the dispersing sex is the sex least likely to benefit from interactions with relatives, particularly interactions that lead to inbreedingand limited access to mates (Greenwood 1980: Pusey 1980; Pusey and Packer 1987: Johnson and Gaines 1990). Habitat fragmentation has the potential to significantly limit a species capacity to move to habitable areas because fragmentation changes the spatial configuration of a landscape (Primack 1983: Olivieri and Goyon 1997; Van Vuren 1998 ). Most importantly. interpatch distances may increase. Some insect. bird. and mammal species will only move short distances between patches (e .g. . Bierregaard et al. 1992). and "stepping-stone" or shortto medium-distance dispersal appears to be the norm for primates (Glander 1992; Moore 1992 ). Deforestation may result in patches too far apart for individuals of many species to navigate, decreasing likelihoods of successful dispersal and colonization and increasing likelihoods of extinction through inbreeding within groups in patches from which individuals are prevented from emigrating (Van Vuren 1998). Although dispersal is expected to entail high risk, such as exposure to predation, starvation. or to the costs of high energy expenditure borne by movement between fragments (Johnson and Gaines 1990). researchers assume that, where dispersal occurs, ultimate benefits from leaving one's group will, on average, outweigh its costs (Johnson and Gaine:, 1990). Until the 1980s it was generally thought that dispersal by both sexes of a species mias rare. There have been. however, documented cases of female dispersal. Wrangham ( 1980). for example. was the first to point out that primates demonstrating female dispersal are often leaf-eating species. suggesting an association between stressful conditions caused by food of poor quality and dispersal behavior. In other words. as food quality decreases. female dispersal increases. Feeding on mature leaves, in particular. may create stressful conditions because they are often low in carbohydrates and "are not generally eaten for ready energy," and leaves may be high in phenolic content (Milton 1980). Individuals may leave home in search of food of higher quality, such as fruit or flowers or patches of more nutritious leaves. Primate research strongly supports Wrangham's conclusions because bisexual dispersal has been documented in howler monkeys (Alouut to spp.), colobus monkeys (Colohlls s u t a ~ ~ u s and C. 70 Endangered Species UPDATE Vol. 16 No. 4 1999 Vol. 16 No. 4 1999 Endangered Species UPDATE 71 polykomos), langurs (Presbytis spp.), model. It is expected that, regardless and the mountain gorilla (Gorilla of sex, an individual will disperse gorilla beringei)-all folivorous priwhen conditions away from home mates (Moore 1984, 1992, 1993 ; confer greater survival and reproducMoore and Ali 1984) -and the relative success than conditions at home, tionship has recently been supported Recent reports from the field demonby studies of three grass-eating spestrate that primates living in disturbed cies of zebra (Equus burchelli, E. zehabitat often demonstrate bisexual bra, and E. grevyi) that display bidispersal, supporting the view that sexual dispersal (Hack and loss of habitat may have significant Rubenstein 1998). Moore (1984, consequences for a species' behav1993) extended Wrangham's work to ioral repertoire. A preliminary test of show that bisexual dispersal for reathis relationship was recently reported sons other than inbreeding avoidance for common chimpanzees (Pan trois more common than previously asglodytes), a species typically demonsumed. Individuals may "decide" to strating female dispersal and male leave groups because of a failure to philopatry, similar to that of many locate mates or food or because they bird species. Y. Sugiyama, a Japaare expelled aggressively by neseprimatologist, observed bisexual groupmates, including kin. dispersal among chimpanzees in disThe studies by Wrangham and turbed habitat in Bossou, Guinea Moore are consistent with recent re(Normile 1998). Similarly, Sterck ports by Emlen (1994, 1995). Emlen (1998) has argued that female disstudied cooperative breeding in birds, persal in langurs (Presbytis spp.) is developing a general model for the linked to "human disturbance. " Furevolution of family structure. This thermore, bisexual dispersal is found model predicts when offspring will in mantled howler monkeys (Alouatta leave or remain in their natal groups. palliata) (Jones 1978,1995a) who are Emlen expects individuals to disperse thought to have evolved in tropical when the reproductive benefits of dry forest, heterogeneous conditions leaving home outweigh the benefits similar to those found where forests of staying. Individuals are more are fragmented (Jones 1995b). likely to disperse when breeding vaClarke et a1 . ( 1 998) reported that cancies exist away from home, a conmantled howlers are more likely to dition unlikely to be found in habiemigrate from natal groups in lowtats of high quality where population quality conditions. If these obserdensity is high and space is saturated. vations identify significant relaDispersing from a natal group on a tionships between habitat quality poor habitat, then, may have higher and patterns of behavior, it is expotential benefits to individuals bepected that bisexual dispersal will cause of the chances to locate an unbecome more common as habitat saturated group in better habitat or to fragmentation increases. colonize uninhabited landscape. A recent in Emlen's work demonstrates that enground squirrels (Spermophilus vironmental variations may explain beldingi) (Nunes et al. 1998) suggests variations in behavior and that indimechanisms that might trigger disviduals make decisions based on lopersal under the conditions that cal conditions such as physical conEmlen's model predicts. In particudition or rates of interaction with lar, Nunes et al. (1998) find evidence groupmates. to support the view that dispersal is a Wrangham's and Moore's concluresult of (1) an organism's physical sions are consistent with Emlen's condition, especially fat reserves and Endangered Species UPDATE Aforumfor information e.xchange on endangered species issues hlYIAueust 19w vol, 16 No, M. McPhee ...,.............Managing Andrew R. Hayes ...................... Associate Editor TervL. Root ,,.,.,,.,,,.,,,,.,,.,.,.,,,,, Advisor RichardBbck Advisow Board Santa Barbara Zoological Gardens Susan Haig Forest and Rangeland Ecosystem Science Center, USGS Oregon State University ChrisHOwes Chicago Zoological Society ,V,mmM!,ers International Consultant in Environment and Development Patrick O'Brien Chevron Ecological Services Hal Salwasser U.S. Forest Service, Boone and Crockett Club lmmctiomforAut'wrs: The Endangered Species UPDATE welcomes articles, editorial comments, ,, annoUnCemen6 protection, For further information contact the editor. s s c p t i o n I n f o t i o n The species UPDATE^^ published sk times per the School of Natural Resources and Envimnment at The University of Michigan. Annual rates are s28 sub~riptions~ and s23 for and seniorcifizens (add$5 forpostage outside the US). Sendcheckormoneyorder(~a~*letone University of Michigan) to: Endangered Species UPDATE School of Natural Resources and Environment The University of Michigan Ann Arbor, MI 48109-11 15 (734) 763-324; fax (734) 936-2195 E-mail: [email protected] http:l/www,umich,edu/-esupdate The views expressed in the Endangered Species UPDATEmaynotnecessaril~ reflectthoseof the U.S. Fish and Wildlife Service or The Michigan, The Endangered Species UPDATE was made possible in part by the David and Lucile Packard Foundation, Turner Foundation, Boone and C r ~ k e t t Club, ~hevron Corporation, and the U.S. FWS Division of Endangered Species Region 3. Moore, J. 1984. Female transfer in primates. International Journal of Primatology 5:537-589. Moore, J. 1992. Dispersal, nepotism, and primate social behavior. International Journal of Primatology 13:361-378. Moore, J. 1993. Inbreeding and outbreeding in primates: What's wrong with "the dispersing sex"? Pages 392-426 in N.W. Thornhill, ed. 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