Indirect threats

While bryophytes may be directly threatened through harvest in specific areas for reasons detailed above, most serious threats originate indirectly from the destruction or degradation of their habitats. The causes are many and operate at both the global and local scales. At the global scale, threats include factors such as global warming, pollution and biological invasions. Invasive plants often constitute a threat to biodiversity and have consequently received much attention recently. Surveys of the vegetation of permanent plots in Dutch sand dunes demonstrated, for example, that Campylopus introflexus has continuously increased in abundance since the 1960s, outgrowing the native vegetation and finally forming a monotonous, dense moss carpet with an open canopy of the grass Corynephorus canescens (Biermann & Daniels 1997, Ketner-Oostra & Sykora 2004). At the local scale, deforestation, for both agriculture and timber harvest, at rates as high as 2.9 million ha per year in areas such as Brazilian Amazonia, is one of the most serious areas of concern (Laurance 1998). Such habitats, which were once continuous, have become increasingly fragmented. After intensive clearing, the fragments tend to be very small and isolated from each other by crop land, pasture, pavement, or even barren land. This process of fragmentation is one of the main issues in conservation (Heywood & Iriondo 2003).

In fact, it has been widely acknowledged, since McArthur and Wilson's (1967) equilibrium theory of island biogeography, that the number of species reflects a balance between immigration and extinction rates, which are both influenced by two main intrinsic island characteristics. On the one hand, island size impacts on immigration and extinction by influencing the likelihood of diaspore deposition and the availability and duration of suitable habitats. On the other hand, immigration is controlled by species dispersa-bility and thus by the distance separating areas to be colonized and source populations. As a result, species diversity is expected to be correlated with both island size and connectivity with continental source areas. This conceptual model has since been widely applied to predict species richness in both oceanic islands and terrestrial habitats (Whittaker 1998).

Increased extinction risk in isolates results from interactions between local and regional population dynamics. At the local scale, small fragments exhibit proportionally more degraded edge surfaces than large ones. Edges are characterized by high wind turbulence, light levels, rates of evaporation, ambient temperature and low humidity. These factors primarily impact on epiphytes. Epiphytic edge communities can be as rich or even richer than interior, undisturbed communities (Vanderpoorten et al. 2004a, Holz & Gradstein 2005), but the composition of their epiphytic assemblages markedly differs. In fact, epiphytes can be classified into 'sun' and 'shade' species. Shade epiphytes are characteristic of the understorey of dense primary forests (see Fig. 10.3). They are less desiccation-tolerant than sun epiphytes (Section 8.1.2) and generalists that developed a series of putative adaptations, such as papillose cell walls, which enhance the capillary absorption and speed up the process of rehydration (Section 4.1). Shade epiphytes are, therefore, highly sensitive to disturbance (Gradstein 1992a, b, Gradstein et al. 2001, Acebey et al. 2003) and the ratio of shade versus light epiphytes is used as a surrogate of 'naturalness' of forest stands (Box 10.2).

At the regional scale, increased insularity associated with fragmentation potentially reduces migration rates among isolated populations relative

Fig. 10.3. The laurel forests of the Canary Islands, Madeira and the Azores are the relicts of a sub-tropical forest that spread across Europe during the Tertiary Period and was decimated during the glaciations. Those of

Anaga on Tenerife (a) and the Garajonay National Park on La Gomera (b) (photo V. Hutsemekers), a UNESCO World Heritage Site, are noteworthy for their rich bryophyte assemblages (Gonzalez-Mancebo et al. 2004), including numerous endemics, such as the moss Echinodium prolixum (c) (photo J.-P. Frahm), which forms lush 'curtains' hanging on the branches of the laurels. See plate section for colour version.

Box 10.2

Epiphytic bryophytes as biodiversity indicators

The parameters defining the conservation value of forest stands are numerous and complex (e.g. tree composition, regeneration mode and structure, amount of dead wood and overmature trees). Biological indicators are therefore frequently used for assessing the conservation value in such complex ecosystems. Epiphyte bryophytes exhibit a series of features that make them ideal bioindicators of disturbance in forests. Because bryophytes are poikilohydric, their physiology is directly controlled by ambient air humidity (Section 8.1). In particular, shade epiphytes, which are characteristic of the understorey of dense primary forests, are considered to be less drought resistant than sun epiphytes and generalists. Shade epiphytes are therefore highly sensitive to deforestation (Gradstein 1992a, b, Gradstein et al. 2001, Acebey et al. 2003) and more likely threatened (Gradstein et al. 2001).

Furthermore, demographic processes are especially crucial in fugitive species such as epiphytes, for which substrate availability lasts for a limited amount of time. Thus, epiphytes with a low dispersal ability are most sensitive to disturbance because they do not have the capacity to respond quickly to a change in their environment (Boudreault et al. 2000, Cobb et al. 2001, Berg et al. 2002). Therefore, large old trees, which are available for a longer time for colonization, tend to harbour more species with low colonization abilities than young ones.

It is precisely this low resilience of shade epiphytes that is exploited for bioindication. For example, Drehwald (2005) presented a biomonitoring system of disturbance in tropical rainforests based on floristic studies, wherein indicators for primary undisturbed forests versus secondary forests and isolated trees are distinguished. Indeed, although primary and secondary forests display similar diversity patterns because harvested areas are rapidly invaded by sun epiphytes (Hyvonen et al. 1987), the composition of their epiphytic assemblages markedly differ. One third of primary forest species had not re-established in Costa Rican secondary forests after 40 years of succession, indicating that a long time is needed for the re-establishment of microhabitats and reinvasion of species and communities adapted to differentiated niches (Holz & Gradstein 2005). Bioindicators are therefore selected so that (i) they exhibit a narrow habitat specificity; (ii) they are taxonomically not problematic; (iii) their identification, often to the genus level only, is relatively easy; (iv) they are described in recent floras. The method involves a simple protocol, according to which the occurrence of indicative taxa at < 2 m from 10-20 trunks exhibiting well-developed bryophyte vegetation is recorded. From the list of taxa and their indicator value, an index of 'naturalness' can be derived (Box 10.2 Fig. 1).

primary logged socondary

Transect

Box 10.2 Fig. 1. 'Naturalness' indices of primary, logged, secondary Neotropical forests and isolated trees derived from their composition in indicator epiphytic bryophytes at five low montane and lowland rainforest areas in South America (reproduced from Drehwald 2005 with permission of the Hattori Botanical Laboratory).

primary logged socondary

Transect

Box 10.2 Fig. 1. 'Naturalness' indices of primary, logged, secondary Neotropical forests and isolated trees derived from their composition in indicator epiphytic bryophytes at five low montane and lowland rainforest areas in South America (reproduced from Drehwald 2005 with permission of the Hattori Botanical Laboratory).

to extinction rates, resulting in a non-equilibrium metapopulation dynamic (Hanski 1999). For example, Zartman and Shaw (2006) measured population size biannually over four years and tallied the number of colonization and extinction events of two epiphyllous liverworts on the leaves of 98 palm trees within an experimental design of lowland Amazonian forest patches of 1, 10 and 100 to 110 000 ha. They measured the rate of colonization as the fraction of initially vacant palm leaves subsequently colonized by either of the two focal epiphyll species within the four years of the experiment. Leaf patches in forest stands of > 100 ha experienced nearly twice the colonization rates observed in small stands (Fig. 10.4), suggesting that the cause of epiphyll species loss in small fragments (< 10 ha) (Fig. 10.5) is reduced colonization.

In the long-term, fragmentation can also have deleterious effects on levels of genetic diversity in impacted populations. Fragmentation is expected to result in a loss of genetic diversity in small, isolated populations by the random process of genetic drift. In a study on the impact of peatland

IH Colonization [_j Extinction

Forest fragments (> 100 ha}

Fig. 10.4. Colonization and extinction rates measured for populations of two epiphyll liverwort species on the leaves of 98 palm trees within an experimental design of lowland Amazonian forest patches of < 10 and > 100 ha. Population size and number of colonization and extinction events were recorded biannually over four years. Rate of colonization is defined as the fraction of initially vacant palm leaves subsequently colonized by either of the two focal epiphyll species within the four years of the experiment (redrawn from Zartman & Shaw 2006).

Abundance 6050 -403020100 -

0- 20- 0- 20- 40- 60- 80->100 0- ZQ- 40" 60- 80->100>100 20 40 20 40 60 80 100 20 40 60 80 100 Distance from edge (m)

Fig. 10.5. Epiphyll mean abundances ± SD (i.e. number of bushes <2m having >1 epiphyll colonies in 20 x 20 m plots) in 1 to 100 ha forest fragments and continuous forest as a function of proximity to forest border at the Biological Dynamics of Forest Fragments Project in Manaus, Brazil (reproduced from Zartman & Nascimento 2006 with permission of Elsevier).

1 30

Forest fragments (< 10 ha)

Fig. 10.4. Colonization and extinction rates measured for populations of two epiphyll liverwort species on the leaves of 98 palm trees within an experimental design of lowland Amazonian forest patches of < 10 and > 100 ha. Population size and number of colonization and extinction events were recorded biannually over four years. Rate of colonization is defined as the fraction of initially vacant palm leaves subsequently colonized by either of the two focal epiphyll species within the four years of the experiment (redrawn from Zartman & Shaw 2006).

fragmentation on the genetic diversity and structure of the peat moss Polytrichum commune, genetic diversity values from completely cut bogs were indeed found to be lower than those from uncut peatlands (Wilson & Provan 2003). Other studies, however, failed to demonstrate a significant impact of fragmentation on population genetic diversity and structure in Sphagnum (Thingsgaard 2001). These contrasting results suggest that the long-term consequences of fragmentation may be differently expressed in different taxa with contrasting life history strategies and dispersal ability.

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