From Humboldt's Observation to Modern Conservation Science
When Alexander von Humboldt explored the mountains and forests of South America in the early 19th century, he noticed a consistent pattern: the more area he surveyed, the more species he found. This seemingly obvious observation became one of ecology's most powerful laws — the species-area relationship — and ultimately shaped modern conservation planning.
The relationship is quantified by the equation log S = log C + Z log A, where S is species richness, A is the geographic area, Z is the regression coefficient (slope of the log-log plot), and C is the y-intercept. The non-linear power function S = C × becomes a straight line when both axes are logarithmically transformed, making the pattern clear and mathematically tractable. The critical parameter is Z: it measures how steeply species richness responds to changes in area.
For large, connected continental regions, Z ranges from 0.1 to 0.2 — a relatively gentle slope. The gentle slope reflects the immigration rescue effect: when a species disappears from one patch of a continuous forest, individuals from adjacent areas can recolonise. Continental landscapes act as a reservoir of potential colonists, buffering local extinctions from becoming permanent losses.
For oceanic islands, Z ranges from 0.6 to 1.2 — a dramatically steeper slope. Islands are geographically isolated, so when a species goes locally extinct on an island, there is no nearby source pool for recolonisation. The extinction is permanent. As island area decreases, species richness declines much more rapidly than it would on an equivalent mainland area. This is why small islands have impoverished faunas, and why even modest reductions in island area cause substantial species loss.
The practical conservation implication is profound: habitat fragments surrounded by agricultural land behave ecologically like islands. A forest fragment isolated by farmland loses species at island-like rates (high Z) rather than at continental rates (low Z). This insight directly motivates the conservation principle of maintaining large, connected habitat blocks rather than allowing fragmentation into small, isolated patches of equivalent total area.
The species-area relationship also underpins the biodiversity hotspot concept. Norman Myers (1988) proposed identifying regions where conservation investment would save the most irreplaceable species. He developed a two-criterion qualification system: a region must have at least 1,500 endemic vascular plant species (representing at least 0.5% of the world's total) AND must have already lost at least 70% of its original habitat. The first criterion identifies regions that are irreplaceable — where species exist nowhere else and would be permanently lost if the remaining habitat is destroyed. The second criterion identifies regions where urgency is highest — where the destruction is so advanced that remaining habitats are critically threatened.
Both criteria must be simultaneously met. A region can have extraordinary endemism but remain relatively intact, meaning it is important but not yet at crisis point. Conversely, a highly degraded region with few endemic species may be in crisis but not worth prioritising because species there occur elsewhere too. It is the combination of irreplaceability and urgency that justifies focusing scarce conservation resources on hotspots.
India contains four of the world's 36 recognised hotspots: the Western Ghats & Sri Lanka, the Himalayas, Indo-Burma, and Sundaland (Nicobar Islands). Together, these four hotspots cover a small fraction of India's land area but harbour a disproportionate share of its endemic biodiversity. The Western Ghats alone has over 5,000 endemic plant species and hundreds of endemic vertebrates. The hotspot approach directs conservation investment to these high-return areas, protecting maximum biodiversity per unit of effort.