■ Abstract The literature on effects of habitat fragmentation on biodiversity is huge. It is also very diverse, with different authors measuring fragmentation in different ways and, as a consequence, drawing different conclusions regarding both the magnitude and direction of its effects. Habitat fragmentation is usually defined as a landscape-scale process involving both habitat loss and the breaking apart of habitat. Results of empirical studies of habitat fragmentation are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking apart of habitat after controlling for habitat loss. Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. Therefore, to correctly interpret the influence of habitat fragmentation on biodiversity, the effects of these two components of fragmentation must be measured independently. More studies of the independent effects of habitat loss and fragmentation per se are needed to determine the factors that lead to positive versus negative effects of fragmentation per se. I suggest that the term “fragmentation” should be reserved for the breaking apart of habitat, independent of habitat loss.

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Effects of Habitat Fragmentation on Biodiversity

https://www.environment.nsw.gov.au/resources/research/pt-scattered-trees-journal-article.pdf

Scattered trees are keystone structures – Implications for conservation

Conservation of genetic diversity, one of the three main forms of biodiversity, is a fundamental concern in conservation biology as it provides the raw material for evolutionary change and thus the potential to adapt to changing environments. By means of meta-analyses, we tested the generality of the hypotheses that habitat fragmentation affects genetic diversity of plant populations and that certain life history and ecological traits of plants can determine differential susceptibility to genetic erosion in fragmented habitats. Additionally, we assessed whether certain methodological approaches used by authors influence the ability to detect fragmentation effects on plant genetic diversity. We found overall large and negative effects of fragmentation on genetic diversity and outcrossing rates but no effects on inbreeding coefficients. Significant increases in inbreeding coefficient in fragmented habitats were only observed in studies analyzing progenies. The mating system and the rarity status of plants explained the highest proportion of variation in the effect sizes among species. The age of the fragment was also decisive in explaining variability among effect sizes: the larger the number of generations elapsed in fragmentation conditions, the larger the negative magnitude of effect sizes on heterozygosity. Our results also suggest that fragmentation is shifting mating patterns towards increased selfing. We conclude that current conservation efforts in fragmented habitats should be focused on common or recently rare species and mainly outcrossing species and outline important issues that need to be addressed in future research on this area.

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Genetic consequences of habitat fragmentation in plant populations: susceptible signals in plant traits and methodological approaches

Biodiversity conservation in forestry and agricultural landscapes is important because (1) reserves alone will not protect biodiversity; (2) commodity production relies on vital services provided by biodiversity; and (3) biodiversity enhances resilience, or a system's capacity to recover from external pressures such as droughts or management mistakes. We suggest ten guiding principles to help maintain biodiversity, ecosystem function, and resilience in production landscapes. Landscapes should include structurally characteristic patches of native vegetation, corridors and stepping stones between them, a structurally complex matrix, and buffers around sensitive areas. Management should maintain a diversity of species within and across functional groups. Highly focused management actions may be required to maintain keystone species and threatened species, and to control invasive species. These guiding principles provide a scientifically defensible starting point for the integration of conservation and production, which is urgently required from both an ecological and a long-term economic perspective.

Biodiversity, ecosystem function, and resilience: ten guiding principles for commodity production landscapes

The theoretical impacts of anthropogenic habitat degradation on genetic resources have been well articulated. Here we use a simulation approach to assess the magnitude of expected genetic change, and review 31 studies of 23 neotropical tree species to assess whether empirical case studies conform to theory. Major differences in the sensitivity of measures to detect the genetic health of degraded populations were obvious. Most studies employing genetic diversity (nine out of 13) found no significant consequences, yet most that assessed progeny inbreeding (six out of eight), reproductive output (seven out of 10) and fitness (all six) highlighted significant impacts. These observations are in line with theory, where inbreeding is observed immediately following impact, but genetic diversity is lost slowly over subsequent generations, which for trees may take decades. Studies also highlight the ecological, not just genetic, consequences of habitat degradation that can cause reduced seed set and progeny fitness. Unexpectedly, two studies examining pollen flow using paternity analysis highlight an extensive network of gene flow at smaller spatial scales (less than 10 km). Gene flow can thus mitigate against loss of genetic diversity and assist in long-term population viability, even in degraded landscapes. Unfortunately, the surveyed studies were too few and heterogeneous to examine concepts of population size thresholds and genetic resilience in relation to life history. Future suggested research priorities include undertaking integrated studies on a range of species in the same landscapes; better documentation of the extent and duration of impact; and most importantly, combining neutral marker, pollination dynamics, ecological consequences, and progeny fitness assessment within single studies.

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Genetic resource impacts of habitat loss and degradation; reconciling empirical evidence and predicted theory for neotropical trees

Tropical trees are particularly vulnerable to forest fragmentation because of their low densities, self-incompatibilty system, and high rates of outbreeding. Forest fragmentation is likely to decrease gene flow, increase endogamy, and eventually produce a high differentiation among remnant populations. Our objectives were to evaluate the effect of forest fragmentation on the reproductive success, progeny vigor, and genetic variation of the tropical dry-forest tree Samanea saman. We conducted our study in the dry forest of Costa Rica and compared two tree conditions to evaluate forest fragmentation: (1) isolated, individual trees more than 500 m from the nearest conspecific and surrounded by agricultural fields, pastures, or small remnant forest patches and (2) trees in continuous populations of 10 or more individuals per hectare and surrounded by undisturbed forest. We compared the probability of natural pollination, seed production, genetic variation, and progeny vigor of trees in isolation and in continuous populations. The probability of flowers receiving more than 23 pollen tubes at the base of styles (the flowers more likely to produce a mature fruit) was significantly greater for trees in continuous populations (2.1%) than for trees in isolation (0.4%). However, flowers from the two tree conditions received similar pollen loads on their stigmas. Fruits of trees from continuous populations produced similar numbers of seeds as isolated trees did and had a similar probability of seed abortion. The probability of seed predation by bruchid beetles was significantly greater in continuous populations (30%) than in isolated trees (20%). The number of undamaged seeds (  potentially viable) was similar for both tree conditions. A genetic analysis of progeny showed that levels of genetic diversity in trees in isolation and in continuous populations were comparable. The effective self-fertilization rate and inbreeding coefficient of the progeny were slightly higher for isolated trees than for trees in continuous populations. In addition, there was more genetic similarity in the progeny of isolated trees, within and between fruits. Seeds produced by different fruits within a tree were more likely to be related in an isolated tree than in a tree from continuous populations. Seeds produced by trees from continuous populations were more likely to germinate and to produce greater leaf area and biomass as seedlings than progeny from isolated trees. We concluded that the fragmentation of tropical dry forests affects the genetic variation and vigor of S. saman progeny. Isolated trees showed high reproductive capacity, however, in spite of their habitat condition.




Resumen: Los arboles tropicales son particularmente vulnerables a la fragmentacion de bosques debido a su baja densidad, su sistema de auto-incompatibilidad y altas tasas de exogamia. La fragmentacion del bosque posiblemente decrece el flujo de genes, incrementa la endogamia y eventualmente produce una alta diferenciacion entre las poblaciones remanentes. Nuestros objetivos fueron evaluar el efecto de la fragmentacion del bosque sobre el exito reproductivo, el vigor de la progenie y la variacion genetica del arbol de bosque tropical seco Samanea saman. Realizamos nuestro estudio en el bosque tropical seco de Costa Rica y comparamos dos condiciones de los arboles para evaluar la fragmentacion del bosque: (1) individuos aislados mas de 500 m del individuo conespecifico mas cercano y rodeados por campos agricolas, ganaderos o pequenos fragmentos de bosque y (2) arboles en poblaciones continuas de 10 o mas individuos por hectarea y rodeados de bosque no perturbado. Comparamos la probabilidad de la polinizacion natural, produccion de semillas, variacion genetica y vigor de la progenie en arboles aislados y en poblaciones continuas. La probabilidad de que las flores reciban mas de 23 tubos de polen en la base de los pistilos (  las flores con mayor probabilidad de producir un fruto maduro) fue significativamente mayor para arboles en poblaciones continuas (2.1%) que en arboles aislados (0.4%). Sin embargo, flores en arboles en las dos condiciones recibieron similares cargas de polen en sus estigmas. Los frutos de arboles de poblaciones continuas produjeron similares numeros de semillas y tuvieron similar probabilidad de abortar que los de arboles aislados. La probabilidad de depredacion de semillas por escarabajos bruquidos fue significativamente mayor en las poblaciones continuas (30%) que en arboles aislados (20%). El numero de semillas sin dano (  potencialmente viables) fue similar en arboles en las dos condiciones. El analisis genetico de la progenie mostro que los niveles de diversidad genetica de arboles aislados y en poblaciones continuas fueron comparables. La tasa de autofecundacion efectiva y el coeficiente de endogamia de la progenie fue ligeramente mayor en arboles aislados que en los de poblaciones continuas. Adicionalmente, hubo mayor similitud genetica en la progenie de arboles aislados, dentro y entre frutos. Semillas producidas por distintos frutos del mismo arbol tuvieron mas probabilidad de relacion que los de poblaciones continuas. Semillas producidas por arboles en poblaciones continuas tuvieron mayor posibilidad de germinar y producir mayor area foliar y biomasa que las plantulas de la progenie de arboles aislados. Concluimos que la fragmentacion de bosques tropicales secos afecta la variacion genetica y el vigor de la progenie de S. saman. Sin embargo, los arboles aislados tuvieron una alta capacidad reproductiva a pesar de la condicion de su habitat.

Eric A. Fuchs

Papers

Effects of dry tropical forest fragmentation on the reproductive success and genetic structure of the tree Samanea saman.