Introduction Of Exotic Predators Research Articles

Relative strength of top-down effects of an invasive fish and bottom-up effects of nutrient addition in a simple aquatic food web.

Introduction of exotic predators or runoff of fertilizers can alter aquatic food webs, in particular zooplankton communities, through top-down and bottom-up effects. In a mesocosm experiment, we manipulated the density of Western Mosquitofish (Gambusia affinis) and nutrient levels (nitrate and phosphate independently) and observed effects on zooplankton and phytoplankton in a fall, temperate zone system. If top-down regulation were important, we expected mosquitofish predation to reduce zooplankton abundance, which would indirectly benefit phytoplankton. If bottom-up regulation were important, we expected nutrient addition to increase both primary producers and zooplankton. Western Mosquitofish predation significantly decreased the abundance of several zooplankton taxa, resulting in a trophic cascade with increased chlorophyll a (i.e., primary productivity). This effect did not differ between mesocosms with 5 or 10 fish. Nutrient addition had no significant effects on zooplankton; however, chlorophyll a was positively affected by both nitrogen addition and phosphorus addition. Our results suggest weak bottom-up regulation in our experimental community, but strong top-down regulation, emphasizing the potential consequences of introducing non-native Western Mosquitofish to native aquatic ecosystems.

When the neighbourhood goes bad: can endangered black robins adjust nest‐site selection in response to the risk of an invasive predator?

AbstractThe introduction of exotic predators into new areas has had significant impacts on naïve prey species, which initially lack behavioural mechanisms to avoid predation. Understanding a species’ ability to respond appropriately to novel threats may inform in situ management options for threatened island populations. In the presence of nest predators, physical characteristics of the nest site can influence the likelihood that eggs or chicks are lost to predation. Some birds that evolved under high nest predation pressure show predictable changes in nest‐site selection following nest predation, but the ability of naïve island birds to alter nest‐site decisions based on experience with novel nest predators has not been well studied. We examined nest‐site choices of re‐nesting Chatham Island black robins Petroica traversi following nest predation by invasive European starlings Sturnus vulgaris. Robins whose first nests were depredated re‐nested in a new location that was significantly lower to the ground than pairs whose first nests were lost through other causes. As predation risk increases with nest height, the lower placement of re‐nests is consistent with a movement towards safer nest sites. The cause of nest failure did not influence the choice of substrate for a replacement nest, nor did individual pairs re‐nest further away from depredated nests. Changes in nest‐site selection did not carry over into subsequent nesting seasons. Our results suggest that some evolutionarily naïve species may be capable of assessing immediate risk through individual experience with novel predators, and modifying their nest‐site decisions accordingly.

Home Range and Habitat Use of New Zealand Falcons (Falco novaeseelandiae) in an Exotic Plantation Forest During the Breeding Season

The New Zealand Falcon (Falco novaeseelandiae) is a threatened endemic species whose numbers have been reduced by habitat loss and the introduction of exotic predators. The species has recently been discovered breeding in exotic plantation forests. Understanding how New Zealand Falcons use plantation forests is critical to its effective management in this habitat. To determine patterns of habitat selection and inform habitat management, we studied the home-range size and habitat use of 13 adult New Zealand Falcons in Kaingaroa pine plantation. This plantation comprises 180,000 ha of nonnative pine trees that are planted and harvested in discrete ‘stands’ that create a mosaic of pine stand ages across the landscape. We radio-tracked eight males and five females for 12 wk during the 2004 and 2005 breeding seasons. Mean home-range sizes were 9.2 ± 4.2 km2 for males and 6.2 ± 3.2 km2 for females (mean ± SD). To investigate habitat use, pine stands were categorized by tree age and further classified into stand edge and stand interior. Both sexes strongly favored the edges between pine stands where stands less than 4 yr old bordered those greater than 19 yr old. Females also preferentially used the interior of pine stands less than 4 yr old, whereas males were more often found along the edges of these young stands. We conclude that if forestry managers wish to enhance the suitability of their estates for the New Zealand Falcon, they should provide a mosaic of different aged stands, while ensuring that stands less than 4 yr old bordering stands greater than 19 yr old are consistently available.Falco novaeseelandiae es una especie endémica amenazada cuyos números poblacionales se han visto reducidos por la pérdida de hábitat y la introducción de depredadores exóticos. Recientemente se ha descubierto a la especie reproduciéndose en plantaciones forestales exóticas. Entender cómo F. novaeseelandiae utiliza los bosques implantados es crítico para su manejo efectivo en este tipo de hábitat. Para determinar los patrones de selección de hábitat y conducir el manejo de hábitat, estudiamos el tamaño del rango de hogar y el uso de hábitat de 13 individuos adultos de F. novaeseelandiae en la plantación de pinos de Kaingaroa. Esta plantación comprende 180,000 ha de pinos exóticos implantados y que son explotados en rodales discretos que crean un mosaico de rodales de pinos de distinta edad a través del paisaje. Localizamos con radio transmisores ocho machos y cinco hembras durante 12 semanas en las épocas reproductivas de 2004 y 2005. Los tamaños medios de rango de hogar fueron de 9.2 ± 4.2 km2 para los machos y de 6.2 ± 3.2 km2 para las hembras (media ± DE). Para investigar el uso de hábitat, los rodales de pino fueron categorizados por edad de árbol y clasificados en borde de rodal e interior de rodal. Ambos sexos mostraron una marcada preferencia por los bordes entre rodales de pino, donde los rodales de menos de cuatro años de edad bordearon aquellos de más de 19 años de edad. Las hembras también utilizaron preferentemente el interior de los rodales de pino de menos de cuatro años de edad, mientras que los machos fueron encontrados más a menudo a lo largo de los bordes de estos rodales jóvenes. Concluimos que si los encargados de la forestación desean mejorar la aptitud de sus plantaciones para F. novaeseelandiae, deberían mantener un mosaico de rodales de edades diferentes, asegurando que los rodales de menos de cuatro años de edad estén bordeando los rodales de más de 19 años de edad.

Predator recognition of native but not invasive turtle predators by naïve anuran tadpoles

The impact of alien predators on prey populations is well known by conservation biologists, but little attention has been paid to the negative effects that the introduction of exotic predators may have on native predators through competition for food. In the Iberian Peninsula, the red-eared slider, Trachemys scripta elegans, and other exotic freshwater turtles have been introduced, competing and displacing the native endangered terrapins (the European pond turtle, Emys orbicularis, and the Spanish terrapin, Mauremys leprosa). Although the nature of competitive interactions remains unclear, direct competition for food is likely to occur. Both native and invasive freshwater turtles are common predators of amphibian tadpoles. Naive amphibian tadpoles are known to recognize and respond to local predators with no prior experience, but tadpoles might not recognize new predatory species, since they have not shared a long evolutionary history with them. We examined the ability of four species of Iberian anuran tadpoles to recognize and respond to chemical cues from invasive and native freshwater predatory turtles. Three of the four tadpole species tested reduced their swimming activity when cues from native turtles were present in water, but not when cues belonged to exotic turtles. This inability of tadpole prey to respond innately to chemicals from introduced predatory turtles might be one of the causes that explains the displacement of native populations of Iberian terrapins by invasive exotic turtles, and may help clarify why alien predators sometimes prosper better in new habitats than locally adapted predators.

The decline of the endemic Fijian crested iguana Brachylophus vitiensis in the Yasawa and Mamanuca archipelagos, western Fiji

The endemic Fijian crested iguana Brachylophus vitiensis, categorized as Critically Endangered on the IUCN Red List, has been recorded from several islands in western Fiji. We conducted a survey for the crested iguana on 12 uninhabited and five inhabited islands in the Yasawa and Mamanuca archipelagos of western Fiji in September 2000. Night searches for sleeping iguanas along a total of 11.2 km of forest transects suggest that crested iguanas are either extremely rare or extinct on all of these islands. Although we collectively searched a total of 44 km of transect over 123 person hours, we located crested iguanas on only four islands: three small uninhabited islands (all <73 ha) and one large inhabited island (22 km). In July 2003 we resurveyed two islands identified as having the best potential for the long-term conservation of crested iguanas, and found that populations were continuing to decline. We suggest that the scarcity of crested iguanas on all islands surveyed is due to the combination of habitat loss and the introduction of exotic predators. All islands surveyed have free ranging goats, forest fires have occurred repeatedly over the last few decades, and feral cats are established on many islands. To reverse the population decline of this species immediate intervention is required on selected islands to halt continuing forest degradation and to clarify the effects of introduced predators.

Diurnal Versus Nocturnal Surveys for California Red-Legged Frogs

The United States Fish and Wildlife Service (USFWS) federally listed the California red-legged frog (Rana draytonii; Shaffer et al. 2004) as a threatened species in June 1996 (USFWS 1996). The listing justification stated that a variety of human activities, including urban encroachment, construction of reservoirs and water diversions, introduction of exotic predators and competitors, livestock grazing, and habitat fragmentation, had extirpated redlegged frogs from 70% of their former range and continue to be a threat (see also Fellers 2005). Because the Endangered Species Act of 1973 now protects this frog, biologists must conduct surveys prior to any federally permitted habitat modifications or any activity on federal lands that has the potential to affect this species. In February 1997, USFWS provided guidelines for conducting red-legged frog surveys, including general guidelines for conducting both diurnal and nocturnal surveys (USFWS 1997). The USFWS updated these guidelines in 2005 (USFWS 2005) and gave detailed requirements for both the qualifications of biologists conducting surveys and the techniques for carrying out field surveys. The goal of our study was to compare diurnal and nocturnal surveys for California red-legged frogs to determine whether there was a difference in detections. We conducted paired diurnal and nocturnal surveys for adult and subadult California redlegged frogs at sites in the California Coast Range and Sierra Nevada foothills.

Conservation of Mediterranean temporary ponds under agricultural intensification: an evaluation using amphibians

This study examined responses of amphibians breeding in Mediterranean temporary ponds to a gradient of agricultural intensification, aiming to identify land uses and management prescriptions favouring the conservation of these habitats in farmed landscapes. Larval amphibian assemblages and habitat attributes were sampled at 57 ponds, 10 of which had been converted into permanent irrigation reservoirs. Species richness increased with area and hydroperiod in temporary ponds, with the addition of rare species to ponds with long hydroperiods resulting in a tendency for the less widespread species (e.g. Triturus marmoratus and T. boscai) to occur in the most species-rich ponds, while species-poor ponds consisted predominantly of widespread species only (nested pattern). However, one species (Pelodytes punctatus) was largely restricted to the most ephemeral ponds, whereas permanent irrigation reservoirs were species-poor and lacked most species occurring in temporary waters. The strongest negative correlates of amphibian abundances were the intensification of agricultural land uses, the transformation of ponds into permanent reservoirs and the introduction of exotic predators (fish and crayfish) from the irrigation channels. The results suggest that conservation of temporary pond amphibian assemblages in Mediterranean farmland requires networks of ponds with diverse hydroperiods, where the natural hydrologic regimes, less intensive land uses and isolation from irrigation waters should be preserved.

Variation in Sound Levels Produced within a Sooty Tern Colony

Sooty Terns (Sterna fuscata) have been extirpated from some islands in the Seychelles through habitat modification, introduction of exotic predators and over-exploitation of their eggs. Recently, removal of exotic predators and habitat restoration have made some islands available for the re-introduction of or re-colonization by former breeding birds. Attempts will be made to encourage Sooty Terns to re-colonize formerly occupied islands that are now deemed suitable to support colonies, using decoy models and broadcasts of recorded vocalizations. As a prelude to these attempts, this study measured the sound levels produced within a Sooty Tern colony at different times of day and season, in order that broadcasts can reasonably well simulate the sounds produced naturally. Statistically significant diurnal and seasonal differences in sound levels were recorded, related to differing activities of the adult birds during the breeding season. Our results suggested that broadcasting equipment should be capable of producing Sooty Tern colony vocalizations at sound levels at least 85-90 dB, at a distance of about three m from the speakers, in the intended nesting area. Potential human health risks of prolonged exposure to vocalizations within Sooty tern colonies are highlighted.