Spurious thresholds in the relationship between species richness and vegetation cover

Abstract

Aim Thresholds often exist in the relationship between species richness and the area of remaining habitat in human‐modified landscapes, prompting debate about the mechanisms responsible. We hypothesize that if species–area relationships differ with underlying factors such as landscape productivity, and such factors correlate with patterns of habitat clearance, then spurious thresholds can arise where the separate species–area relationships intersect. We assessed whether this phenomenon could explain landscape‐level species–area relationships for birds occupying 31 landscapes of 100 km2 in eastern Australia. Location Eastern Australia. Methods Landscape‐level species richness estimates were modelled as a function of the percentage of native vegetation remaining in the study landscapes. The performance of traditional species–area curves and continuous and discontinuous piecewise models was compared using an information theoretic approach. Separate models for high‐ and low‐productivity and high‐ and low‐fragmentation landscapes were examined to determine whether they implied different species–area relationships. Results The species–area relationship exhibited a rapid change‐point at approximately 40% vegetation cover, but this was most parsimoniously explained by two disjunct slopes rather than a continuous threshold model or a classic species–area curve. Exploration of models fitted separately to high‐ and low‐productivity landscapes suggested that such landscapes may differ in their characteristic species–area relationships. Main conclusions The observed pattern is consistent with the spurious threshold hypothesis, and opens a new avenue of enquiry into the processes behind apparent ecological thresholds. This hypothesis may be valid in other regions where clearing history is confounded by underlying factors such as landscape productivity, and demands further research. In such systems, real thresholds for different landscape types may occur at different levels of cover, or might not exist at all. If so, a simple space‐for‐time substitution may not be valid, and management prescriptions based on threshold values (e.g. 40%) will be flawed.

Shifting agriculture supports more tropical forest birds than oil palm or teak plantations in Mizoram, northeast India

“…Understanding interspecific variation in species' responses to human disturbances is important to enable effective conservation decisionmaking, for example, by informing habitat protection and restoration targets to maintain important ecological phenomena such as species-area thresholds (Bruner et al 2001, Bleher et al 2006, Maron et al 2012, Game et al 2013. Protected areas are frequently viewed as safeguarding ecological communities (Bruner et al 2001); however, particularly in developing countries, where funds and national strategies for conservation and protected areas are low, protected areas frequently fail to adequately achieve this aim.…”

Effects of habitat extent and forest disturbance on bird communities in lowland nepal

“…Landscapes with more forest cover support a larger species pool (Hu et al 2012, Taylor et al 2012) through both sampling effects (Wiens 1992, Whittaker andFernández-Palacios 2007) and because a greater forest extent offers habitat diversity (Radford et al 2005, Maron et al 2012. Several empirical studies have shown that sites in landscapes with more forest support higher densities of reptiles (McAlpine et al 2015), greater species richness and abundance of birds (Villard et al 1999, Mortelliti et al 2010, Martensen et al 2012, Taylor et al 2012, and greater richness of small mammals (McAlpine et al 2006, Estavillo et al 2013).…”

“…As habitat extent decreases, so do ecological functions and associated biodiversity in the landscape (Swift and Hannon 2010). Species richness may respond nonlinearly to habitat loss, and decline sharply when the extent of forest exceeds certain threshold levels (Maron et al 2012, Ochoa-Quintero et al 2015. For example, in the eastern Australia, Maron et al (2012) found a non-linear relationship between woodland birds and landscape-level forest extent.…”

“…Landscapes with more forest cover support a larger species pool (Radford and Bennett 2007, Haslem and Bennett 2008, Taylor et al 2012) through both sampling effects, and because a greater extent of forest cover in landscapes offers habitat diversity (Radford et al 2005, Maron et al 2012, and can serve source habitats for range of species (Pulliam 1988). Birds are a mobile taxon, and their presence at a particular site does not necessarily mean they are resident, nor that they solely use the resources within that site.…”

“…Although these relationships have typically been considered as general phenomena, the response of species to habitat extent may vary depending on land-use or soil type (Maron et al 2012), species sensitivity to forest cover change (Martensen et al 2012), and diversity of food resoures available in the landscape. For example, species richness of frugivorous birds in particular landscapes is likely to be driven by the diversity and availability of fruit resources (Kissling et al 2007), while, although at site level, abundance of insectivorous birds is affected by the abundance of prey resources such as arthropods in forest patches (Capinera 2011).…”